From zirconia to yttria: Sampling the YSZ phase diagram using sputter-deposited thin films Thomas Götsch, Wolfgang Wallisch, Michael Stöger-Pollach, Bernhard Klötzer, and Simon Penner Citation: AIP Advances 6, 025119 (2016); doi: 10.1063/1.4942818 View online: http://dx.doi.org/10.1063/1.4942818 View Table of Contents: http://aip.scitation.org/toc/adv/6/2 Published by the American Institute of Physics , AIP ADVANCES 6, 025119 (2016) From zirconia to yttria: Sampling the YSZ phase diagram using sputter-deposited thin films Thomas Götsch,1 Wolfgang Wallisch,2 Michael Stöger-Pollach,2 Bernhard Klötzer,1 and Simon Penner1,a Institute of Physical Chemistry, University of Innsbruck, Innrain 80/82, A-6020 Innsbruck, Austria University Service Center for Transmission Electron Microscopy (USTEM), Vienna University of Technology, Wiedner Hauptstraße 8–10, A-1040 Vienna, Austria (Received 30 November 2015; accepted 12 February 2016; published online 22 February 2016) Yttria-stabilized zirconia (YSZ) thin films with varying composition between mol% and 40 mol% have been prepared by direct-current ion beam sputtering at a substrate temperature of 300 ◦C, with ideal transfer of the stoichiometry from the target to the thin film and a high degree of homogeneity, as determined by X-ray photoelectron and energy-dispersive X-ray spectroscopy The films were analyzed using transmission electron microscopy, revealing that, while the films with mol% and 20 mol% yttria retain their crystal structure from the bulk compound (tetragonal and cubic, respectively), those with mol% and 40 mol% Y2O3 undergo a phase transition upon sputtering (from a tetragonal/monoclinic mixture to purely tetragonal YSZ, and from a rhombohedral structure to a cubic one, respectively) Selected area electron diffraction shows a strong texturing for the three samples with lower yttria-content, while the one with 40 mol% Y2O3 is fully disordered, owing to the phase transition Additionally, AFM topology images show somewhat similar structures up to 20 mol% yttria, while the specimen with the highest amount of dopant features a lower roughness In order to facilitate the discussion of the phases present for each sample, a thorough review of previously published phase diagrams is presented C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4942818] I INTRODUCTION Yttria-stabilized zirconia (YSZ) is one of the most widely-used and studied materials due to its high ionic conductivity at elevated temperatures,1 its high chemical inertness and thermal stability, as well as its hardness.2 Thus, it is often used as electrolytes in solid oxide fuel cells,3 but also in chemical sensors,4 as well as in coatings for thermal barriers,5 and to create optical devices like switchable mirrors or filters.6 However, the influence the amount of yttria in the solid solution on the relevant properties, such as crystal structure and conductivity is often neglected By doping zirconium oxide, ZrO2, with yttrium oxide, Y2O3, a tetravalent ion (Zr4+) is substituted by a trivalent one (Y3+) Due to charge neutralization, oxygen vacancies are formed, increasing the ionic conductivity These vacancies also play a role in stabilizing the often desired tetragonal or cubic structures.7 YSZ has been proposed to exist in various crystal structures, some of which are shown in Figure In Figure 1(a), monoclinic ZrO2/YSZ is shown (for YSZ, the only difference is that some of the Zr are substituted by Y and some O are missing, and that the lattice parameters will thus differ).8 In this structure, edge-sharing polyhedra are formed by seven-fold coordinated Zr atoms For tetragonal zirconia,9 displayed in Figure 1(b), the coordination number of Zr increases by a Electronic mail: Simon.Penner@uibk.ac.at 2158-3226/2016/6(2)/025119/20 6, 025119-1 © Author(s) 2016 025119-2 Gưtsch et al AIP Advances 6, 025119 (2016) FIG Various crystal structures of YSZ as found in several phase diagrams Based on Refs 8–13 one, resulting in distorted cubes Cubic zirconia (Figure 1(c)), exhibiting the CaF2 structure, also features cubically coordinated zirconium atoms (undistorted).10 It is to be noted that the images in Figures 1(a) to Figure 1(c) depict ZrO2, but are also valid for YSZ, where a certain fraction of the Zr atoms are simply replaced by Y (and the occupancies of the O sites are reduced as well) These three structures comprise the majority of the phase diagram at the zirconia-rich side (see below for a discussion of the phase diagrams) There have been several other proposals in the past, most describing ordered structures, such as Zr3Y4O12, in a rhombohedral structure,11 as depicted in Figure 1(d) This structure is made up of a mixture of edge-sharing octahedrally and seven-fold coordinated zirconia/yttria polyhedra This structure results from an ordering of the vacancies in the defective fluorite structure.11 Additionally, the existence of a cubic pyrochlore (Figure 1(e)) has been suggested, with the composition Zr2Y2O7,12 consisting of alternating layers of edge-sharing distorted [ZrO6] octahedra and [YO8] cubes In Figure 1(f), the structure of pure yttria (Y2O3) is shown This substance crystallizes in a body-centered cubic structure and contains edge-sharing distorted [YO6] octahedra In order to investigate the influence of the yttria-content on the crystal structure, the morphology and surface topology, as well as the epitaxial growth properties, we decided to employ our model thin film systems with the goal of establishing a thin film “phase diagram” of YSZ, forming the required basis for future investigations regarding other properties of these thin films, and, by aiming for epitaxially-grown films, preparing model systems for future studies as eventual catalysts The thin film phase diagram is especially important since there is a vast number of applications of YSZ thin films, as outlined above It appears that, while the number of publications dealing with zirconia is large, many of them only focus on pure ZrO2,14,15 or present results on just one stoichiometry of yttria-stabilized zirconia.16,17 In many cases, supported films and not free-standing ones (as in our case) are used.16 To the best of our knowledge, there have been no systematic studies regarding the Y-influence over a wider stoichiometry range, but only for limited compositional variations such as in Ref 18 For the present study, we hence restricted ourselves to the low-yttria side, featuring sample compositions ranging from mol% yttria up to 40 mol%, so as to just focus on the technologically-relevant materials Additionally, to facilitate the discussion of the compositions and crystal structures obtained in our study, an overview of often-used concentration quantities, as well as 025119-3 Götsch et al AIP Advances 6, 025119 (2016) a thorough discussion of existing phase diagrams, going more into detail than existing publications such as the one by Chevalier et al.,19 are presented in the next sections II YSZ STOICHIOMETRY QUANTITIES There are various ways to present the composition of solid mixtures such as yttria-stabilized zirconia, such as the molar percentage of one of the constituents, the atomic percentages or the Zr/Y ratio The practically most useful quantity probably is the molar percentage of Y2O3 (mol% Y2O3) as it is directly related to the preparation procedure Also often used is the atomic percentage of yttrium (at% Y), for it is the quantity that is obtained from elemental composition analyses such as XPS or EDX Another composition often found, especially in phase diagrams, is the molar percentage of YO1.5 (mol% YO1.5) It should be noted that, against common belief, this value is not simply twice that of mol% Y2O3 because n(Y2O3) and n(YO1.5) are found in the numerator, but also in the denominator of the equations for the molar percentages, respectively In Table I, the conversions between all mentioned concentration quantities are listed Each equation describes the calculation of the property in the left column from that in the top row III DISCUSSION OF EXISTING PHASE DIAGRAMS A large amount of phase diagrams concerning the zirconia-yttria system are reported in literature, but these not all agree with each other and, in fact, contain many contradictory examples Hence, we strive to give a brief overview of the published proposals in order to facilitate the discussion of our findings later The focus of this small review will thus be put on those yttria-concentrations that were also used in our study: mol%, mol%, 20 mol% and 40 mol% Y2O3 These samples will be referred to as 3YSZ, 8YSZ, 20YSZ and 40YSZ, respectively, with the number denoting the amount of yttria (in mol% Y2O3) present One of the first phase diagrams of this system was published by Duwez et al.20 in 1951, featuring no ordered phases such as Zr3Y4O12 Also, according to this diagram, no pure cubic zirconia could exist Rather, the monoclinic polymorph would transform into its tetragonal counterpart at approximately 1000 ◦C, which then would melt at about 2700 ◦C The first occurrence of the cubic form of the yttria-doped variant would be at mol% Y2O3 At low temperatures, a miscibility gap between cubic and monoclinic YSZ would exist, which would react towards the tetragonal polymorph upon reaching the respective eutectoidic temperature between 400 ◦C and 500 ◦C Thus, 3YSZ would be either monoclinic or tetragonal, depending on whether the latter was frozen in a metastable state, and all other samples of interest would be pure cubic YSZ (since that region extends from 7.5 mol% to more than 50 mol% Y2O3, where a two-phase area between cubic YSZ and bcc yttria starts) In 1963, Fan et al.21 published an incomplete version of the low-temperature region of the phase diagram, differing strongly from that by Duwez et al by containing a compound of the composition Zr2Y2O7 at 33.3 mol% Y2O3, exhibiting a cubic pyrochlore structure Also, the zirconia-rich region of the diagram features dissimilarities in the progression of the phase boundaries, featuring no cubic TABLE I Conversion between commonly-used concentration values for yttria-stabilized zirconia y always refers to the quantity in the left column, x to that in the top row vert.: y, horiz.: x mol% Y2O3 mol% Y2O3 — mol% YO1.5 2x y = 1+x 2x y = 2x+3 2x y = 1+x y = 1−x 2x at% Y (incl O) at% Y (excl O) Zr/Y Ratio mol% YO1.5 y= x 2−x — y= 2x 6−x y=x y= 1−x x at% Y (incl O) y= y 3x 2−2x 6x = x+2 at% Y (excl O) y= x 2−x y=x — y= 6x y = x+2 y = 2−5x 6x y= 2x 6−x — 1−x x Zr/Y Ratio y= 1+2x y = 1+x y = 6x+5 y = 1+x — 025119-4 Götsch et al AIP Advances 6, 025119 (2016) YSZ at lower temperatures, but rather suggesting the existence of solid solutions of zirconia and yttria, respectively, in the pyrochlore, as well as the stability of monoclinic solid solutions until yttria concentrations corresponding to the pyrochlore Also, the pyrochlore and Y2O3 show a miscibility gap between approximately 47 mol% and 33 mol% Y2O3 On the basis of this phase diagram, each of our samples would contain the pyrochlore structure, as all the specimens up to 20YSZ would be a mixture between either monoclinic or tetragonal YSZ and Zr2Y2O7, and 40 YSZ would consist of only the pyrochlore structure, which would exhibit a certain degree of flexibility regarding the exact composition However, no further study was able to confirm the existence of the Zr2Y2O7 phase At the end of the 1960s, another systematic study was performed by mixing ZrO2 and Y2O3 in mol% steps and heating the mixture with a CO2 laser.22–24 The pyrochlore suggested by Fan et al was not found, with cubic YSZ seemingly taking its place (also including a region of immiscibility between cubic YSZ and yttria, but at higher concentrations), and the zirconia-rich region shows similarities to Duwez’ version In contrast to the proposal by Fan et al.,21 monoclinic YSZ is only found up to a very small amount of yttria, with the cubic polymorph becoming the stable phase at room temperature already below 10 mol% Y2O3 It has to be noted, though, that the data below 500 ◦C is sparse in this study This may be the reason for the omission of the eutectoidic decomposition of the tetragonal phase, as seen in Duwez’ version (and many later diagrams), for instance At mol%, 20 mol% and 40 mol% Y2O3, a cubic structure should be found and, for the sample with the lowest yttria content, namely mol%, tetragonal YSZ would be the stable form, with small amounts of monoclinic YSZ at low temperatures A slightly different low-yttria region was found by Srivastava et al.,25 who, like Duwez and co-workers, observed a eutectoidic transformation at around mol% Y2O3 at 565 ◦C, below which tetragonal YSZ would decompose into the monoclinic and cubic variants, which is where 3YSZ would be located Otherwise, no large differences from the previous diagram are to be found, with the cubic YSZ/Y2O3 two-phase region starting only at 80 mol% instead of already at a bit more than 40 mol% in the case of the previously mentioned study, but also comprising all other three concentrations investigated in this work Also in the early 1970s, Rouanet26 could not validate the existence of a Zr2Y2O7 compound either, but instead located a new ordered phase: Zr3Y4O12, with a hexagonal crystal structure This new phase, also called the δ phase (derived from the zirconia-scandia system),11 is located at 40 mol% Y2O3, which would correspond directly to our 40YSZ sample, and is stable up to 1250 ◦C, where it decomposes peritectoidically At the low-yttria side of the diagram, no eutectoid is visible in the diagram, which is due to the omission of the low-temperature part (only data above 1000 ◦C are shown) Judging from the limited information available, 3YSZ would be tetragonal, 8YSZ a mixture between tetragonal and cubic polymorphs, and 20YSZ would be found in its cubic structure The eutectoid, however, was published again by Scott,27 with the two-phase region below the eutectoid extending to 10 mol% Y2O3 The ordered phase, Zr3Y4O12, on the other hand, is not featured in this diagram Instead, two cubic bcc-phases are found between 80 mol% and 95 mol% Y2O3, which, according to later work of the same author,28 are due to contaminations causing non-equilibrium effects Scott also examined the concentration regions where the various polymorphs could be kept in a metastable state at room temperature, which results in the information that only tetragonal YSZ is possible between mol% and mol% Y2O3 — at lower concentrations, the monoclinic analogue is obtained, and, at higher concentrations, cubic YSZ becomes more stable (with small regions of overlap between the polymorphs) This would mean that 3YSZ would be tetragonal, 8YSZ, 20YSZ and 40YSZ already cubic Gorelov investigated the low-yttria content region more closely,29 and found four ZrO2 polymorphs (and, hence, also four YSZ solid solutions) instead of the usual three In addition to the monoclinic one, and the already known tetragonal analogue, which is stable between 1200 ◦C and 2300 ◦C, another tetragonal phase was observed between 2300 ◦C and 2500 ◦C, where it transforms to cubic zirconia This new tetragonal phase is primarily distinguished by differences in lattice parameters: the c axis is compressed and a is increased Both tetragonal phases decompose eutectoidically upon cooling, with the high-temperature one forming cubic and the low-temperature tetragonal compound, and the latter yielding monoclinic and cubic YSZ Due to the limitation on concentrations below 10 mol%, no conclusion can be drawn for 20YSZ or 40YSZ, but 8YSZ should be cubic according to 025119-5 Götsch et al AIP Advances 6, 025119 (2016) this diagram, and 3YSZ could be either a mixture between monoclinic and cubic YSZ, or one of the tetragonal polymorphs, depending on the ability to keep them metastable at low temperatures Stubican and coworkers found a eutectoidic decomposition of cubic YSZ into monoclinic zirconia and Zr3Y4O12 at temperatures as low as 400 ◦C,30–32 which could have an influence on 3YSZ, 8YSZ and 20YSZ However, looking at the higher-temperature reading, the two former could also be within the tetragonal/cubic two-phase region, and 20YSZ in the cubic area Their version of the phase diagram also contains the peritectoidically transforming Zr3Y4O12 phase, again at 40 mol% In the diagram published by Pascual and Duran,33 on the other hand, this ordered phase would transform dystectoidically into cubic YSZ at 1375 ◦C However, they also proposed the existence of a second phase, for which they coined the term “1:6 phase” as it complies with the stoichiometry of ZrY6O11, which supposedly crystallizes hexagonally, transforms dystectoidically at 1700 ◦C and is found at 75 mol% Y2O3 What they could not verify was the miscibility gap between monoclinic ZrO2 and Zr3Y4O12— that is, there is no eutectoid for cubic YSZ The sample with mol% Y2O3 would be tetragonal at elevated temperatures, 8YSZ would exhibit the cubic polymorph, and 20YSZ either the cubic one too, or a mixture between the cubic and the δ phase In 1984, Ruh et al.34 again focused on the high-ZrO2 region of the YSZ phase diagram Their findings not deviate drastically from other proposals What is, however, interesting, is that the cubic phase starts already for yttria-contents below mol% Also, according to this diagram, the tetragonal phase would only be stable up to about mol%, which would mean that 3YSZ would already be a mixture between the tetragonal and cubic structures, even at relatively low temperatures Again, as already found in other references,25,29 the tetragonal variant undergoes a eutectoidic decomposition Other papers by Yoshikawa, Suto et al.35,36 also discussed the low-yttria region, and their findings correspond well with those of Ruh et al., with the cubic YSZ region starting below mol% Y2O3 too, with the same conclusions regarding our investigated samples being drawn as for Ruh’s work Some of these results are confirmed by another publication from Stubican in 1988,37 although the cubic region of the phase diagram starts only well above mol% Y2O3 at lower temperatures (also due to the eutectoid already published earlier30–32), meaning that both 3YSZ and 8YSZ would be mixtures between tetragonal and cubic structures, except for high temperatures in the case of mol% Y2O3, where it would be purely cubic They mention that this deviation from the work of Ruh et al could, however, also be due their use of a hydrothermal method since it was difficult ot obtain equilibrium at lower temperatures This time, in contrast to their previous proposal, they found that the rhombohedral, ordered phase, Zr3Y4O12, transforms dystectoidically at 1382 ◦C to the cubic analogue What they couldn’t confirm was the ZrY6O11 compound proposed by Pascual and Duran The 1980s also brought about the advent of calculated phase diagrams For instance, Degtyarev and Voronin constructed such diagrams based on the calculated thermodynamic properties of all the phases.38–40 This included the rhombohedral Zr3Y4O12 phase, which transforms dystectoidically to cubic YSZ Also, this diagram features an eutectoidic decomposition of tetragonal, as well as the cubic structure (into monoclinic YSZ and the ordered phase) They also computed the diagrams for an increased pressure, where the region of stability for the cubic polymorph is enlarged and the monoclinic form is generally less stable.40 At ambient pressures, 3YSZ is located within the tetragonal/cubic mixture area, and 8YSZ at intermediate temperatures as well 20YSZ and 40YSZ should be inside the cubic region, even though none of their diagrams actually shows 40 mol%, as no hints of the δ phase are visible in the partial diagrams they show The two eutectoids were also found by the calculations of Du et al,41 who did additional experiments that confirmed their computations A year later, the same group published another version,42 with the main difference being in the high Y2O3 region They, however, employed two different sets of model parameters, which showed drastic differences in the eutectoidic region between the monoclinic zirconia and Zr3Y4O12 In another revision of this diagram by Jin and Du,43 another miscibility gap was introduced at the low-temperature end below 479 ◦C, where the ordered phase decomposes into the monoclinic solid solution and Y2O3 This would suggest that, at low temperatures, a demixing of the solid solutions would be favoured If high-temperature phases are quenched and brought to room temperature, 3YSZ and 8YSZ would be tetragonal or cubic (if initial temperatures exceeded 2000 ◦C), and 20YSZ cubic 40YSZ would crystallize in the rhombohedral Zr3Y4O12 structure — at least for a limited temperature range between 479 ◦C and 1376 ◦C 025119-6 Götsch et al AIP Advances 6, 025119 (2016) TABLE II Summary of the YSZ phase diagrams found in literature Abbreviations: cub = cubic, tetr = tetragonal, dystect = dystectoidic, peritect = peritectoidic, eutect = eutectoidic Year 1951 1963 1968 1971 1974 1975 1978 1982 1983 1984 1987 1988 1987 1990 1992 1995 1996 1997 2005 Citation Duwez et al.20 Fan et al.21 Ruh, Rouanet, Skaggs22–24 Rouanet26 Srivastava et al.25 Scott27 Gorelov29 Stubican et al.30–32 Pascual, Duran33 Ruh et al.34 Suto, Yoshikawa et al.35,36 Stubican37 Degtyarev, Voronin38–40 Du et al.41,42 Jin, Du43 Suzuki44 Yashima45 Suzuki46 Fabrichnaya et al.47 Exp./Theo Ordered Phase(s) Main Features exp exp exp exp exp exp exp exp exp exp exp exp theo theo theo exp exp exp theo — Zr2Y2O7 — Zr3Y4O12 — — — Zr3Y4O12 Zr3Y4O12, ZrY6O11 — — Zr3Y4O12 Zr3Y4O12 Zr3Y4O12 Zr3Y4O12 Zr3Y4O12 — Zr3Y4O12 Zr3Y4O12 no cub ZrO2, eutect tetr YSZ no cub YSZ peritect Zr3Y4O12 eutect tetr YSZ two cub phases two tetr phases, two eutect peritect Zr3Y4O12, eutect cub YSZ dystect ordered phases cub YSZ for < mol%Y2O3 cub YSZ for < mol%Y2O3 dystect Zr3Y4O12 dystect Zr3Y4O12 dystect Zr3Y4O12 dystect Zr3Y4O12 peritect Zr3Y4O12 no equilibrium < 1200 ◦C peritect Zr3Y4O12 dystect Zr3Y4O12 In an experimental phase diagram, created by Suzuki,44 only minor differences to other proposals can be discerned For instance, Zr3Y4O12 transforms peritectoidically at 1360 ◦C This diagram also features the eutectoids for the cubic and tetragonal YSZ species and the miscibility gap between monoclinic zirconia and Zr3Y4O12, and the crystal structures of our samples would be the same as for the previously discussed versions Yashima et al.,45 on the other hand, took a closer look at the low-yttria content region (up to around 20 mol% Y2O3) They found it impossible to reach thermodynamic equilibrium below 1200 ◦C, and, hence, their diagram does not show equilibrium lines for those temperatures However, like Scott,27 they managed to determine the regions of metastability for each of the solid solutions, according to which cubic YSZ is obtainable for concentrations above 11 mol% Y2O3 (e.g 20YSZ), and the tetragonal structure is obtained above mol% Y2O3 (hence, it being the stable phase for 3YSZ and 8YSZ) Suzuki later published another (partial) phase diagram for YSZ,46 investigated using conductivity measurements This version also is in good agreement with other publications Fabrichnaya et al.47 reported a theoretical diagram in 2005, which differs significantly from other recent proposals in our region of interest, in that, for a certain range in Y content, the cubic polymorph (for example for 20 mol% Y2O3) would not decompose at all upon lowering the temperature (i.e there is no eutectoid) The remaining regions resemble previous diagrams, such as the dystectoidically transforming δ phase, with other implications on our samples being that 3YSZ would crystallize tetragonally and 8YSZ both, cubically and tetragonally Table II summarizes these diagrams, allowing for a quick overview of the major points of each of them, where the evolutionary nature of the phase diagram proposals can be witnessed, especially with regard to the ordered compounds — at first, none had been reported, then there were different suggestions, and, soon it became accepted and almost every diagram contained Zr3Y4O12 IV EXPERIMENTAL DETAILS A Thin film deposition The thin films have been deposited on NaCl(001) single crystals at elevated temperatures of 623 K to facilitate crystallization and possibly epitaxy using a custom-made sputter-gun (see below for details) in a modular high-vacuum apparatus with a base pressure in the low 10−7 mbar regime 025119-7 Götsch et al AIP Advances 6, 025119 (2016) FIG Overview of the redesigned sputter device, featuring a modular setup with an easily removable shielding (at the top), onto which the filament is mounted, allowing for a significant speed-up in filament changes In addition to a higher mechanical stability, this new design enables the sputtering of insulating targets due to the higher temperatures reached because of the closer proximity to the glowing filament The sputtering was performed in × 10−5 mbar Ar, and as targets, pellets of the different YSZ samples (commercial powders with mol%, mol%, 20 mol% and 40 mol% Y2O3, by Sigma Aldrich) were used These targets were prepared by pressing the powders onto a spirally-shaped Ta wire at approximately 20 kN in a KBr pellet press used for infrared spectroscopy studies By submerging the coated sodium chloride crystals in water, the thin films can be floated off, and afterwards collected using TEM gold grids to yield unsupported thin films with nominal thicknesses of 25 nm While there were successes in sputtering YSZ with mol% yttria using our previously published direct-current ion beam sputter device,48,49 a redesign was required in order to suit more insulating targets, such as some of the oxides used in this study, without having to resort to radio-frequency power supplies, as is often the case for commercial sputtering systems The main changes, displayed in Figure 2, encompass a new, modular head for the device, where the target can be brought in closer proximity to the hot filament, causing the target to reach higher temperatures, decreasing the resistivity Also, not visible in the illustration, to limit the heat conductance away from the oxide, the target mounting was altered to split up the structural connection (now using ceramics) from the electric connection (via a very thin Ta wire) The new design also brings about a higher mechanical stability compared to previous versions, resulting in a better stability of the deposition process due to less vibrations being transmitted to target and the gun head, allowing for the growth of more homogeneous films Additionally, the revised shielding (needed to block the line of sight between the target and the substrate in order to avoid evaporated or sputtered tungsten (oxide) contaminations) now features a conical center part instead of a cylindrical one, and is lower in height, maximizing the substrate area that is coated Another benefit relating to the shielding stems from the new filament mountings, which are now directly attached to the removable shielding, meaning that the whole filament can now be swapped much quicker than before B Characterization of the films The unsupported films were investigated with respect to their crystallographic properties, structure and homogeneity using a FEI Tecnai F20 S-TWIN (high-resolution) analytical (scanning) transmission electron microscope (200 kV), equipped with an EDAX Apollo XLT2 silicon-drift detector for energy-dispersive X-ray spectrometry (EDX) and a GIF Tridiem electron energy-loss spectrometer For the X-ray photoelectron spectroscopy (XPS) compositional analyses, a Thermo Scientific MultiLab 2000 spectrometer (with a base pressure in the high 10−11 mbar to low 10−10 mbar range), fitted with a monochromated Al-Kα X-ray source, an Alpha 110 hemispehrical sector analyzer and an ion gun for sputter-depth profiling (operated at kV using argon), was utilized For these investigations, additional samples had been prepared by depositing the oxides on silicon wafers 025119-8 Götsch et al AIP Advances 6, 025119 (2016) The atomic force microscopy (AFM) surface topology images of the unsupported specimens, from which the surface roughness values were calculated, were recorded using a Veeco Digital Instruments Dimension 3100 in tapping mode For this, Veeco RTESPW silicon cantilevers with force constants between 20 N m-1 and 80 N m-1 as well as resonance frequencies from 256 kHz to 317 kHz were employed C Characterization of the target materials In order to gain an understanding about the change of crystallographic structure from the target to the thin film, X-ray diffraction studies have been performed on the target powders using a Siemens D5000 diffractometer under ambient conditions, while recording a 2θ range from 20◦ to 70◦ with a step size of 0.02◦ V RESULTS AND DISCUSSION A Composition To check the purity and composition of the thin films, various methods were employed: using X-ray photoelectron spectroscopy, sputter depth profiles were recorded, energy-dispersive X-ray (EDX) spectra were taken both in TEM, as well as in STEM mode (spectrum imaging), and electron energy-loss spectrometry was utilized as well Figure displays the surface-sensitive XP spectra (Figure 3(a)) and EDX spectra (Figure 3(b)), which, due to the larger mean free path of X-rays in contrast to electrons of the same energy, corresponds to an integrated composition over the whole thickness range These two sets of plots show that the samples are contamination-free In Figure 3(a), the main peaks visible are the O 1s (at 532 eV), the corresponding O KLL auger peak at around 1000 eV binding energy, the Zr 3d and 3p (180 eV and 331 eV, respectively), as well as the Y 3d (158 eV) and 3p peaks (301 eV) Going from 3YSZ to 40YSZ, the relative change of Zr/Y peak intensities can be observed nicely For the thinner films, namely 3YSZ and 40YSZ (and, to a smaller extent, 20YSZ), the Si 2p peak at 99 eV and the Si 2s one (149 eV) from the underlying substrate (XPS samples were deposited directly on silicon wafers) begin to be visible at the surface spectrum already At 245 eV, the Ar 2p peak can be seen, which is due to incorporated argon in the thin FIG Surface-sensitive X-ray photoelectron spectra (a) and energy-dispersive X-ray spectra recorded in the TEM (b) both show that the films are impurity-free (note that the NaCl does not originate from the deposition process, but rather from the substrate, and that it can be removed by rinsing with water) and that the stoichiometry is the same as in the target 025119-9 Götsch et al AIP Advances 6, 025119 (2016) films, most likely due to implanted Ar+ ions into the target during the sputter-depositing process For 40YSZ, two very small peaks are found at 1070 eV and 508 eV, which are Na 1s and Na KLL peaks, originating from the sample preparation process when transferring it into the XPS chamber This can be confirmed by sputtering the film for a few seconds, after which the sodium contamination vanishes Hence, it was only on the top of the surface and does not arise from the thin film deposition process In the EDX spectra (Figure 3(b)), the peaks from other elements than Zr, Y or O can readily be explained as well: the very small Na and Cl signals are remnants of the sodium chloride substrates (and, hence, not from the deposition process itself) and can be minimized by introducing additional cleaning steps simply using water Au stems from the TEM gold grid, upon which the films are placed, Cu from the specimen holder, and since the pole piece of the lens contains Fe and Co, these consequently yield X-ray fluorescence peaks This fluorescence is due to the measurement positions being chosen close to the grid bars in order to minimize charging effects The proximity to the gold grid causes the high-energy Au X-ray lines to be emitted, in turn resulting in the emission of X-ray fluorescence from the pole pieces for two of the samples (3YSZ and 40YSZ) Thus, it can be concluded that the films are of high purity, and the composition is the same as in the target as well, as the data in Table III show There, the stoichiometries, as obtained from various methods, are shown: the mean of an XPS depth profile, the quantification results from the EDX spectra shown in Figure 3(b), the integrated EDX spectra from spectrum images, and the quantification of the Zr and Y L3 edges The EDX spectra have been quantified using the Cliff-Lorimer method with the software Digital Micrograph by Gatan, using the k-factors contained therein Calculating the mean of all these values, one can see that the yttria content in the thin film correlates well with that in the target: for 3YSZ, 3.3(7) mol% Y2O3 is measured, for 8YSZ 8(2) mol%, for 20YSZ 17(2) mol%, and 40YSZ contains 38(3) mol% yttria Looking at the separate values, it immediately comes to mind that the EELS quantification is off the most, for example only yielding 13.8 mol% Y2O3 for 20YSZ This is due to the difficulties arising when quantifying EELS edges, which, in contrast to for instance the Gauss peaks in EDX spectroscopy, is not as straightforward Also, the L3 edges are found at 2080 eV energy-loss (yttrium) and 2222 eV (zirconium), respectively, where the intensity in the spectrum is already extremely low All other methods are in good agreement with the target values, with the EDX results in TEM mode corresponding better than those in STEM, because a larger area on the specimen was sampled, allowing us to use a higher beam current without destroying the thin films due to charging and, thus, obtain a better signal-to-noise ratio Because not only the composition is relevant, but also its depth and spacial homogeneity, XPS depth profiling and EDX spectrum imaging were employed (Figure 4) The XPS depth profile (Figure 4(a)), which was normalized to the film thickness (determined by taking the inflection point of a sigmoidal fit of the Si 2p peak intensity) in order to allow for easier comparison of the profiles if the films not have the same thicknesses, shows that the composition does not change drastically The slight increase in yttria-content at higher etch depths comes from the increased silicon content, which results in the other peak intensities diminishing, making the quantification less reliable For 40YSZ, this plot shows that the XPS measurements yield too low Y2O3 concentrations, as already seen in Table III For the other specimens, the values correspond well with the compositions from the targets, indicated by the dashed lines TABLE III Composition of the thin films, as determined by various methods The mean of these values correlates with the target compositions target XPS depth profile 20 40 3.8(12) 6.8(3) 17.7(30) 33.7(9) mol% Y2O3 EDX TEM 3.2 8.3 17.8 41.0 EDX STEM EELS Mean 4.0(1) 9.5(2) 18.5(2) 39.0(5) 2.4 5.8 13.8 38.5 3.3(7) 8(2) 17(2) 38(3) 025119-10 Götsch et al AIP Advances 6, 025119 (2016) FIG (a) Results from the XPS depth profiles, showing the yttria-concentration as a function of sample depth In (b), an extracted EDX spectrum from the spectrum image of 3YSZ is shown exemplarily, featuring the Y and Zr Kα peaks Prior to quantification of the respective map, the principal component has been selected by principal component analysis, thus removing the noise The mean concentration value as calculated from the EDX map for this compound is 4.0(1) mol% yttria (see Table III) The very small standard deviation confirms the high homogeneity of our samples In Figure 4(b), the results from a corresponding EDX analysis are shown for the case of 3YSZ The spectrum was derived from a mapping generated by a principal component analysis, removing the noise sufficiently to allow for a reliable quantification The spectrum image, recorded over a 35 nm × 35 nm region reveals a very high homogeneity despite the short dwell times, with the yttria content being 4.0(1) mol% Y2O3,and a negligibly small standard deviation (see also Table III) For the other three specimens, the same procedure has been applied, yielding similarly high degrees of homogeneity Figure 5(a) displays an exemplary chemical state depth profile for the Zr 3d peak of the 20YSZ thin film It can be observed that, at the surface, the film is fully oxidized, as is typical for sputtered thin films prepared from oxidic targets With increasing depth, small amounts of “suboxide A” (blue) and another state, denoted as “suboxide B”, arise The latter state is just another more reduced type of suboxide, closer to the metallic state, for the occurrence of pure metallic zirconium in these samples is unlikely These reduced states arise from the depth profiling process itself because the oxygen is preferentially sputtered Thus, even though the fully oxidic signal drops to about 80 %, the film most likely still is completely oxidized Closer to the substrate interface (Si), a larger suboxide portion is observed, which stems from the fact that the silicon signal is becoming dominant, reducing the intensity of the zirconium peak, rendering the fitting procedure less reliable An example of such a fit is shown in Figure 5(b) This region was obtained for a depth corresponding to the point that is marked by an arrow in the plot in Figure 5(a) For the fit, products of Gaussian/Lorentzian peaks were fitted to the spectrum, with restraints on the peak ratios (determined from the fully oxidized surface) and widths It can be seen that the model of three distinct Zr states describes the experimental data very well B Crystallographic properties Before starting any discussion regarding the crystal structure of the thin films, a determination of the phases present in the sputtering target materials is appropriate Figure shows the X-ray diffractograms of the respective source materials What can immediately be seen is that 3YSZ, i.e the sample with the lowest yttria-content, features more peaks than any of the other samples In fact, the reflexes visible can be attributed to both monoclinic ZrO2/YSZ as well as tetragonal YSZ.8,51 Thus, the starting material for mol% thin films would agree with all those phase diagrams that predict a corresponding two-phase region at this concentration.22–24 For 8YSZ, the diffractogram looks a lot simpler, with peaks assignable to either tetragonal or cubic YSZ,51 which already shows a drawback when determining the phases via diffraction techniques: tetragonal and cubic diffractograms are almost impossible to distinguish as both structures feature lattice planes with the same spacing For instance, the tetragonal (101) spacings are the same 025119-11 Götsch et al AIP Advances 6, 025119 (2016) FIG (a) An exemplary chemical state depth profile for the Zr 3d peak of 20YSZ That small amounts of suboxides, in the form of two distinct, differently reduced states (suboxide A and B), are visible, stems from the sputtering process during depth profiling (b) Zr 3d region for the point indicated with an arrow in (a) including the fitted peaks as those of the cubic (111) planes, and the t(002) and c(200) reflexes also are located at the same position in the diffractogram However, during previous investigations, the crystal structure of this powder has been determined to be tetragonal.50 The plot corresponding to 20YSZ thus strongly resembles that of 8YSZ, making it difficult to determine whether they feature different crystal structures That the cubic phase was assigned to 20YSZ was based on the published phase diagrams discussed above, for none of them feature a tetragonal phase at 20 mol% Y2O3 It has to be noted that the two peaks between 40◦ and 50◦ that are pronounced for 20YSZ actually arise from the sample holder (marked with “SH” in the plot), and are visible to a lesser extent in all diffractograms The sample with 40 mol% Y2O3 gives rise to a slightly different-looking diffractogram, which is characteristic for Zr3Y4O12 (i.e the ordered δ phase).11 The existence of this phase would invalidate all phase diagrams not containing it.20–25,27 It has to be considered, however, that the ordered compound could either not be thermodynamically stable, or the temperatures required during the sample pretreatment in these studies could have exceeded the limit of stability for this phase (at approximately 1300 ◦C), hence causing it to be overlooked Such could have been the case, for example, for the early studies by Ruh, Rouanet and Skaggs,22–24 where the samples were heated with a CO2 laser In Figure 7, TEM bright field images are shown to give an overview of the morphology of the thin films From Figure 7(a) it can be deduced that the thin film prepared by sputtering the 3YSZ target consists of small nanocrystallites (about 10 nm in diameter) Strong diffraction contrast arises from those particles in ideal Bragg orientation, and, as Moiré patterns are visible on some of them, the grains also seem to overlap The crystallites not feature a common, regular shape: while there 025119-12 Götsch et al AIP Advances 6, 025119 (2016) FIG X-ray diffractograms of the powders, from which the targets were produced Some of the major peaks are indexed, with SH denoting reflexes originating from the sample holder The assignment of the tetragonal phase to 8YSZ was done according to earlier investigations,50 and 20YSZ is assumed to be cubic based on the published phase diagrams are some that look approximately rectangular, most of them are in the form of rounded, irregular particles For 8YSZ (Figure 7(b))), the morphology resembles that of 3YSZ, with some of the crystallites being larger Again, crystallinity is confirmed by the presence of Bragg contrast within the grains 20YSZ in Figure 7(c), too, looks very similar to 3YSZ, albeit the particles are even a bit smaller Like for the mol% specimen, Moiré patterns indicate that there are multiple crystallites stacked on each other, from which information regarding the growth process of the films can be gathered since a columnar growth (i.e single grains stretching from one surface to the other through the whole film), as is often obtained by magnetron sputtering,52 can be excluded This could be advantageous for several applications, such as electrolytes, because the non-columnar microstructure would allow for the preparation of pinhole-free films much easier since, as long as the film is thick enough, the probability that the pinholes are covered by new particles, is much higher than for adjacent columns In Figure 7, some thin areas are still visible between the grains, but the films are also a lot thinner than what would usually be the case for electrolyte applications The overview micrograph of 40YSZ, seen in Figure 7(d), looks vastly different from the other ones as no defined particles are visible Rather, the whole film looks rather corrugated and irregular Also, while there is strong contrast in places, no particles with either Bragg contrast or Moiré patterns are discernible 025119-13 Götsch et al AIP Advances 6, 025119 (2016) FIG Bright field overview images of the specimens with varying Y2O3 content While the first three micrographs look similar, the one for 40 mol% yttria is distinctly different The selected area electron diffraction (SAED) patterns of the various samples, as deposited at 300 ◦C, are displayed in Figure (preparations were carried out at room temperature as well, but the films were all amorphous and, thus, are not shown here) At first glance, a strong ordering for all films except for that with 40 mol% becomes apparent from the pronounced texturing in FIG Selected area electron diffraction patterns for the samples show a large degree of ordering except for 40YSZ For the assignment of tetragonal/cubic structures, see Table IV Note that the t(200)/t(110) peaks could also be assigned to t(002)/t(112) 025119-14 Götsch et al AIP Advances 6, 025119 (2016) TABLE IV The lattice parameter c, as calculated from the diffraction patterns This parameter is the same, regardless whether the crystal structure is cubic or tetragonal, since the (002) reflexes in both cases are located at the same positions in the pattern mol% Y2O3 c / nm 20 40 0.51 0.52 0.52 0.53 the ring-patterns Only 40YSZ features a Debye-Scherrer like ring pattern typical of disordered polycrystalline materials Taking a closer look at the diffraction patterns, the same problem as for the XRD analysis occurs: it is basically impossible to distinguish between tetragonal and cubic YSZ simply by looking at the SAED patterns As mentioned, this stems from the fact that the tetragonal and cubic structures both have lattice planes with the same spacings, even though the unit cell dimensions are different Before assigning any lattice spacings to each diffraction pattern, an evaluation of the crystal structure is required For this, intensity profiles were generated by angular integration of each diffraction pattern using the PASAD software.53 From the peak positions of four spots, the lattice constant c (which is the same as a in the case of the cubic system) was calculated This was chosen because the tetragonal (002) and cubic (200) planes feature the same spacings and, hence, yield spots at the same distance from the direct beam.51 The calculation of c was done by a simple crystallographic relation between the lattice spacing, d hk l , the miller indices (h, k and l), as well as the the lattice parameter, c (denoted by this letter to better illustrate the connection to the tetragonal c parameter), c = d hk · (h2 + k + l 2) (1) l The lattice constants computed this way are listed in Table IV It can be observed that there is a strong increase of the unit cell dimensions upon increasing the yttria-content from mol% to mol%, namely from 0.51 nm to 0.52 nm This is expected behavior since the ionic radius of Y is larger than that of Zr However, by substituting even more Zr with Y (i.e reaching 20 mol% Y2O3), c remains approximately constant (at now 0.52 nm) within the margin of error, before increasing again towards 40 mol% yttria (to 0.53 nm — this specimen is here assumed to be cubic instead of rhombohedral as well, for details regarding this, see the discussion below) This would be counter-intuitive, as an increased fraction of larger ionic species will inevitably cause the unit cell to be scaled up appropriately Hence, this retention of the lattice parameter can only be explained by a phase transformation to the cubic polymorph, upon which the unit cell volume would increase due to the larger a and b dimensions And, judging from the existing phase diagrams discussed in Section III, the samples with lower Y2O3 content will be tetragonal, and 20YSZ as well as 40YSZ cubic The texturing in the 3YSZ pattern in Figure 8(a) can be used to assist in determining the lattice planes that cause the ordered spots to appear: the innermost ring (0.29 nm) can be ascribed to the tetragonal (101) plane (these planes and spots will from now on be called t(101), where the letter in front indicates whether the cubic (c) or tetragonal (t) structure is referenced), which stems from crystallites that are not grown epitaxially, and, thus, exhibit a rather large disorder, causing the more ring-like appearance of the signals from this plane The next spots at 0.26 nm can, in principle, derive from the t(110) or t(002) planes, as they both feature the same spacings The assignment, again, is difficult, but can be done by looking at the next diffraction spots (0.18 nm), which, too, can originate from two lattice planes, namely t(200) and t(112) Looking at the expected angles between each pair of spots, for t(002) and t(200), one would expect 90◦, which is not the case as the measured angle between these spots is 45(1)◦ The distinction between either t(002)/t(112) (45.67◦), t(110)/t(200) (45◦) and t(110)/t(112) (44.33◦) is not unambiguous since all three angles fall within the standard deviation of the measured angle However, the zone axis would differ between the 025119-15 Götsch et al AIP Advances 6, 025119 (2016) ¯ ¯ and t(110)/t(200) would assignments: for t(002)/t(112), it would be [110], for t(110)/t(112) [110], yield a [001] zone axis A zone axis, resulting from an YSZ{110}/NaCl(001) interface, could show epitaxy, depending on whether the YSZ surface is O- or Zr-terminated, for the latter would form a square surface unit cell (with a = 0.359 nm), and the oxygen atoms a rectangular one (0.503 nm × 0.206 nm), with a certain corrugation in the O positions For the (001) face, both atom types arrange in the same square lattice with a periodicity constant of 0.355 nm Even though the assignment is still not absolutely clear, the spots are labeled with t(110) and t(200), respectively, for, in addition to the surface arrangement, an epitaxial growth along one of the principal directions of the crystal structure seems more likely A very similar diffraction pattern is obtained for the specimen with mol% of yttria in Figure 8(b), with slightly increased lattice spacings for each set of planes: 0.30 nm for the t(101), 0.26 nm and 0.19 nm for t(110) and t(200), respectively Thus, the same discussion as for 3YSZ also applies for 8YSZ The degree of texturing in the SAED pattern is approximately the same, meaning that this thin film was grown with a high amount of epitaxy as well The spots in 20YSZ (Figure 8(c)) can be assigned to the cubic polymorph, as already discussed Here, the innermost, disordered ring-like arrangement of spots at 0.30 nm stems from the c(111) planes The c(200) signals are found at 0.26 nm and show a pronounced ordering In fact, two sets of spots are discernible, suggesting two preferred orientations in terms of rotation around the growth direction axis The angle to the c(220) spots, which are found at 0.18 nm, is measured to be 45.2(5)◦, correlating nicely with the theoretical value of 45◦ From these two planes, the zone axis of this film can be determined to be along the [001] direction This ordered growth along the unit cell axis corroborates the theory that, for the tetragonal analogues, the same is more likely than the growth along one of the directions Taking a closer look at the 40YSZ diffraction pattern (Figure 8(d)), a much worse ordering than for the other samples can be identified; however, there is a very faint texturing visible within the rings, indicating a not complete disordering The spacings of these rings can all be attributed to cubic YSZ, in contrast to the target’s rhombohedral structure (Zr3Y4O12): at 0.30 nm, there is the c(111) signal, at 0.27 nm the c(200) one, and the c(220) rings are located at 0.19 nm Still further to the outside, the ring stemming from the c(311) planes is visible at 0.16 nm While the rhombohedral ordered phase would have similar spacings (e.g 0.30 nm for the (003) planes), the δ phase would have a lot more rings in the diffraction pattern.54 This suggests that a phase transformation has taken place during the sputtering process This could be the result of the sputtered clusters traveling in close proximity to the hot filament when leaving the sputtering device (see Figure 2) since the glowing filament is at a temperature well above the stability limit of the δ phase, which, according to most phase diagrams, is around 1300 ◦C, where the compound transforms either peritectoidically or dystectoidically Another reason could be the effect of the substrate: if depositing a thin film on the cubic sodium chloride (100) facets, the growth in a cubic structure will most likely be favored due to a template-effect However, if this was the case, one would assume a larger degree of ordering since epitaxy would be forced due to this effect Regarding the substrate, it thus seems more likely that small amounts of NaCl have been incorporated into the crystal structure of the thin film, as these ions also have the ability to stabilize cubic zirconia and YSZ.50 It is, of course, also possible that, during the sputtering process, some argon ions have been implanted in the lattice of the target, which will stabilize the cubic polymorph.55 Furthermore, the grain size (which, judging from the diffraction rings and the overview image is very small in the case of this sample) has a big influence on the crystal structure as well, as demonstrated by Drazin et al.,56 even though their assessment of the extent of the cubic region clashes with our results regarding the tetragonal structure for 8YSZ It is challenging to determine which of these processes is the cause for the phase transformation Nevertheless, some conclusions can be drawn that give an indication as to which reasons are likely and which are not First, as already mentioned, if this transformation was the cause of a substrate-template effect, a higher degree of ordering would be expected, which is not the case; hence, this effect can be excluded Similarily, if implanted Ar+ were the reason, the transformation would already occur in the target — then, a cubic target would be sputtered, due to which one would again expect similar results to, for example, 20YSZ The disorder, thus, cannot be explained by a template effect or an argon-induced target transformation If the phase transition was 025119-16 Götsch et al AIP Advances 6, 025119 (2016) to occur during the time of flight of the sputtered clusters due to thermal reasons, one could again argue that it’s the same case as for 20YSZ, with cubic particles arriving at the substrate However, it is not inconceivable to imagine that, if clusters that are already cubic are heated up by the filament, they arrive in a “hot” state at the surface If a rhombohedral-cubic transition has to occur first, on the other hand, these clusters could lose some heat due to the transformation Thus, maybe the effective temperature of the material deposited at the substrate plays a role in the ordering since the particles have to possess a certain mobility to rearrange on the surface, which will be higher at elevated temperatures The effect of Na/Cl impurities in the lattice would only affect the first layer of clusters arriving at the surface, which could also cause an ordering to happen because to incorporate these ions into the lattice, a large degree of mobility is required, which also allows for the rearrangement of the YSZ-atoms itself, although all further grains that have no contact with the substrate could only be cubic due to epitaxy of the already present crystallites, which would then lead to a high ordering again As for the crystallite size effect, this could be possible, but one has to ask why these crystallites are smaller than for the other targets in the first place; this sounds more like a secondary effect instead of the cause of this transformation To shed more light on the crystallography of these specimens, high-resolution TEM (HRTEM) images containing lattice fringes are given in Figure In Figure 9(a), a micrograph of 3YSZ containing multiple crystallites is shown In the grain on the left, lattice spacings of 0.29 nm can be ¯ lattice planes The angle between measured in two directions These correspond to t(101) and t(101) ◦ them was measured to be 70.0 From theoretical calculations, a reference value of 69.3◦ is obtained for these two planes, which is in good agreement with the experimental value It is to be noted that this particle is not viewed in a [001] zone axis for, otherwise, these fringes would not be visible The high-resolution image of 8YSZ (Figure 9(b)) contains one particle (bottom) also exhibits t(101) planes (0.29 nm), while, above, there are t(110) fringes with d = 0.25 nm observable Since both fringes originate from different grains, no conclusion can be drawn regarding the zone axis, and no angle-analysis is possible In the case of 20YSZ, in Figure 9(c), a particle in [001] zone axis could be imaged (in the center) This contains both c(200) and c(020) fringes, with a spacing of 0.26 nm each The angle between them is measured 90◦, as expected from theory For 40YSZ, the disorder and nanocrystallinity can immediately be seen in the image (Figure 9(d)): the Fourier transform shows a ring-like FIG HRTEM images of the specimens The scale bar is 10 nm in all cases 025119-17 Götsch et al AIP Advances 6, 025119 (2016) pattern, even for this small (20 nm × 20 nm) image, and the crystallites visible are smaller than for the other specimens In the grain with the markings, two spacings of 0.30 nm can be measured, with ¯ fringes that should span an angle of 70.4◦ between them This corresponds to the c(111) and c(111) ◦ 70.5 , which correlates nicely with the measured values, confirming the assignment of the cubic structure to this thin film C Surface topography Figure 10 shows topography and phase images (insets, àm ì àm each) of the unsupported thin films, as taken from AFM measurements In all images, the vacuum side (i.e not the side exposed to the NaCl during the growth) is displayed In Figure 10(a), the surface of 3YSZ is shown to contain very regular-looking, rectangular structures These are also visible in the phase image With the exception of the one on the top right, all of them feature 90◦ angles (the distorted-looking one is probably skewed due to the film hanging through between the grid bars) These most likely are crystallites, like small single crystals grown on top of the surface In fact, the same kind of structure can also be seen within the surface (see also the phase image), like platelets layered above one another It seems that the crystallites visible on top are just the beginnings of a new layer being deposited The surface of 8YSZ (Figure 10(b)), in principle, looks similar to that of the mol% Y2O3 specimen, in that it contains the same plates, at least to a certain extent The surface generally is more irregular, also featuring more round disk-like grains stacked on top of each other And 20YSZ (Figure 10(c)) looks even more distinct from the other two surfaces, for it showcases flake-like, very irregular grains, in a layered structure So there is a trend in surface topography when going from 3YSZ to 20YSZ, in that the microstructure at the surface becomes less ideal and more irregular and seemingly disordered (even though the electron diffraction experiments showed that the ordering of the films is comparable) 40YSZ (Figure 10(d)) looks drastically different, with the surface appearing much smoother, which would be in line with the observed disordering and nanocrystallinity (i.e the crystallites are too small to be imaged using the AFM tip) While no platelets are visible, what can be seen are the pits already observed previously for 8YSZ thin films when either depositing them at higher FIG 10 àm ì àm AFM surface topography (main panels) and phase images (insets) of the unsupported thin films 025119-18 Götsch et al AIP Advances 6, 025119 (2016) TABLE V The RMS surface roughness values calculated from the images in Figure 10 mol% Y2O3 Sq / nm 20 40 6.3 11.2 6.3 3.7 substrate temperatures or after post-annealing.48 They have been attributed to a high mobility in the vicinity of vacancies, which could very well be the case for 40YSZ here, too, as this stoichiometry brings about the highest concentration of oxygen vacancies This also suggests that the atoms in the growing thin film have enough mobility to rearrange in order to form such pits Table V lists the RMS surface roughness values as calculated from the àm ì àm images in Figure 10 40YSZ has a drastically lower roughness than the other samples, as could already be concluded by looking at the surface topography, with 3YSZ and 20YSZ being comparable (again, see the similar surface structure), and 8YSZ has a higher roughness, most likely due to the big trenches in the film In summary, the films are very similar, and of low roughness, with the exception of 40YSZ, which features a completely different morphology with a higher degree of smoothness VI CONCLUSION Impurity-free thin films of various YSZ samples with different yttria-content could be prepared with excellent transfer of stoichiometry to the thin film by several modifications to our home-built ion beam sputtering device Prior to the thin film deposition, the targets have been analyzed regarding their crystal structures using XRD, and the results showed that the commercially-available 3YSZ sample was a mixture of monoclinic YSZ and tetragonal YSZ, while 8YSZ was cubic 20YSZ showed a cubic crystal structure and 40YSZ was found to be the rhombohedral δ phase corresponding to the compound Zr3Y4O12 In the thin film, different phases are found for some of the compositions, as obtained by analysis of the lattice parameters calculated from the electron diffraction patterns: 3YSZ is not heterogeneous any longer, but a single phase, namely tetragonal YSZ, while 8YSZ retains its tetragonal structure 20YSZ is cubic, just like in the target, and 40YSZ is also cubic This means that for two stoichiometries, phase transitions have occurred during the deposition This could be due to an effect from the substrate acting as a deposition template, which is plausible in the case of 3YSZ, because a tetragonal structure will be favoured over the monoclinic one on a cubic substrate, provided similar lattice spacings are present For 40YSZ, the diffraction pattern shows strong disorder, 3YSZ grows epitaxially (as the other two samples) There seems to be a high mobility within the 40YSZ film, however, as the surface, which features smaller crystallites than the other specimens, shows pits that have previously been seen on 8YSZ thin films,48 as well as on single crystals of YSZ (with mol% yttria).57 They are thought to originate from high mobility around point defects in the lattice This would favour theories regarding a rearrangement after the deposition, while, at the same time, one would expect a higher ordering then There is also the possibility of a phase transition during the time of flight of the sputtered clusters due to the limited thermal stability of the Zr3Y4O12 phase and the close proximity to the hot filament Figure 11 summarizes the correlations between target and thin film crystal structures in a schematic way In this graphic, the symbols relate to the crystal structure as follows: square means cubic, skewed square monoclinic, the oblong rectangle corresponds to the tetragonal phase, and the hexagon to the rhombohedral one The squiggly arrows denote a phase transition, which occur for 3YSZ (monoclinic/tetragonal → tetragonal) and 40YSZ (rhombohedral → cubic), and, for the ordered thin films, the directions at the top indicate the zone axes 025119-19 Götsch et al AIP Advances 6, 025119 (2016) FIG 11 Schematic thin film phase diagram for YSZ The symbols denote the crystal structure: the square means cubic, the skewed square monoclinic, the oblong rectangle tetragonal, and the hexagon depicts the rhombohedral structure Straight arrows mean a retention of crystal structure has occurred, while sinusoidal arrows mark a phase transition during the thin film deposition process The directions above the thin film phases label the zone axes These data will, thus, help with the choice of the composition of YSZ for various applications where a certain crystal structure is desired, since the ionic conductivity is dependent on the crystal structure Also, this lays a foundation for further work in this area, as we have been able to prepare model thin films with a high degree of ordering (at least for stoichiometries between mol% and 20 mol% yttria), being ideally suitable e.g for further studies using impedance spectrosopy to further elucidate the suitability as electrolytes, 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