Cubic to tetragonal phase transition of Tm3+ doped nanocrystals in oxyfluoride glass ceramics Cubic to tetragonal phase transition of Tm3+ doped nanocrystals in oxyfluoride glass ceramics Yiming Li, L[.]
3+ Cubic to tetragonal phase transition of Tm doped nanocrystals in oxyfluoride glass ceramics , , Yiming Li, Lijuan Zhao , Yuting Fu, Yahui Shi, Xiaoyu Zhang, and Hua Yu Citation: AIP Advances 6, 025001 (2016); doi: 10.1063/1.4941442 View online: http://dx.doi.org/10.1063/1.4941442 View Table of Contents: http://aip.scitation.org/toc/adv/6/2 Published by the American Institute of Physics AIP ADVANCES 6, 025001 (2016) Cubic to tetragonal phase transition of Tm3+ doped nanocrystals in oxyfluoride glass ceramics Yiming Li,1 Lijuan Zhao,1,2,a Yuting Fu,1 Yahui Shi,1 Xiaoyu Zhang,1 and Hua Yu1,a The Key Laboratory of Weak-Light Nonlinear Photonics, Ministry of Education, School of Physics, Nankai University, Tianjin 300071, China Applied Physics School of TEDA, Nankai University, Tianjin 300457, China (Received June 2015; accepted 18 September 2015; published online February 2016) Tm3+ ions doped β-PbF2 nanocrystals in oxyfluoride glass ceramics with different doping concentrations and thermal temperatures are prepared by a traditional meltquenching and thermal treatment method to investigate the structure and the phase transition of Tm3+ doped nanocrystals The structures are characterized by Xray diffraction Rietveld analysis and confirmed with numerical simulation The phase transitions are proved further by the emission spectra Both of the doping concentration and thermal temperature can induce an Oh to D4h site symmetry distortion and a cubic to tetragonal phase transition The luminescence of Tm3+ doped nanocrystals at 800 nm was modulated by the phase transition of the surrounding crystal field 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/10.1063/1.4941442] Nowadays, fluorescence optical in vivo imaging technologies are compelling analytical and therapeutic methods for human disease.1–4 The NIR luminescent of Tm3+ ions at 800 nm can lead to a minimum of absorption in biological tissues,5–7 which makes Tm3+ doped nanocrystals very potential in vivo fluorescent probes in the future Luminescence of Thulium (Tm3+) ions in nanomaterials has strong dependence on its local crystal field It is promising to modulate luminescent properties of Tm3+ by studying structures of Tm3+ doped nanocrystals Phase transitions play a crucial part in the structure study, which has already been reported in many other materials Phase transitions can be achieved by varying particle sizes, inducing temperature, pressure and etc Tsunekawa et al found a tetragonal to cubic transition in pure ZrO2 nanopowders when decreasing the grain size.8 Kwon et al studied a temperature-induced cubic to tetragonal phase transition and tetragonality enhancement by thermal treatment in nano-sized barium titanate powder.9,10 Guo et al observed a pressure-induced cubic to tetragonal distortion for the first time in the cold-compressed Pd nanocubes.11 In this paper, we studied a cubic to tetragonal transition in our Tm3+ doped oxyfluoride glass ceramics by changing doping concentration and inducing temperature The phase transitions and structures of Eu3+ doped and Er3+/Yb3+ co-doped lead fluoride nanocrystals in oxyfluoride glass ceramics have been researched in our previous work.12,13 However, for the modification of Tm3+ photoluminescence, we still lack systematic study of the site occupancy and structure distortions in Tm3+ doped nanocrystals To have a thorough study of structures of Tm3+ doped fluoride nanocrystal and phase transition processes for optimization of Tm3+ luminescence in the future, Tm3+ doped oxyfluoride glass ceramics with different doping concentrations and inducing temperatures were prepared Materials Studio (MS) Program was employed to simulate possible structure models; furthermore, X-ray Diffraction (XRD) characterization with Rietveld refinement method was used to verify the models obtained by simulation Finally, fluorescence spectra were measured to give a further proof for the structures of Tm3+ doped fluoride nanocrystal and phase transition processes a Corresponding Authors: zhaolj@nankai.edu.cn, yuhua@nankai.edu.cn 2158-3226/2016/6(2)/025001/7 6, 025001-1 © Author(s) 2016 025001-2 Li et al AIP Advances 6, 025001 (2016) Precursor oxyfluoride glasses with composition formula (mol%) of (45-x)SiO2-5Al2O340PbF2-10CdF2-xTm2O3 (x = 1, 2, 3, 5) were prepared by traditional melting-quenching method.14 This initial composition comes from Wang and Ohwaki,15 while the composition in our samples has been optimized for better optical properties Glass ceramics (GCs) were obtained by thermal treatment for hours at a certain temperature Based on our previous work, rare earth ions (RE3+) doped oxyfluoride glass ceramics are multiphase materials, where RE3+ doped lead fluoride nanocrystals are embedded in the oxide glassy matrix.14,16 XRD (D/Max-2500, Rigaku using CuKα line) measurements were used to characterize the crystal structures of Tm3+ doped nanocrystals A spectrograph equipped with 150 W xenon lamps were used to record the emission spectra These GCs are labeled as xTm@t according to the Tm3+ mole fraction x and the inducing temperature t And xTm-GC means all those x mol% Tm3+ doped GCs obtained by thermal treatment from precursor glasses with composition (mol%) of (45-x)SiO2-5Al2O3-40PbF2-10CdF2-xTm2O3 at the temperature t β-PbF2 crystal has a face-centered-cubic symmetry When Tm3+ ions enter into a β-PbF2 lattice, a distorted β-PbF2 nanocrystal structure will be formed Assuming Pb2+ ions are partially substituted by Tm3+ ions, which will induce extra positive charges, the structure has to reduce Pb2+ or introduce F- to balance the electrical charge It has been proved that F- interstitial mechanism is more stable and favorable for charge balance.16,17 Thus, F- interstitial mechanism is adopted in the following discussion Besides, a doped fluorite crystal unit cell calculated by F- interstitial mechanism has a general formula Pb1−x RE x F2+x in the PbF2-RE lattice.17 All possible models were analyzed by Diamond program18 when Tm3+ ions substitute for different numbers of Pb2+ in the β-PbF2 lattice According to the former work,12,14 we simulated these models by optimizing geometric configuration and calculating binding energy, using the Dmol3 software package in Materials Studio program (Accelyrs Inc.).19 Finally, two possible models with lower binding energy (-65.17773 eV for a cubic Pb3TmF9 and -73.55117 eV for a tetragonal PbTmF5) are chosen to describe structures of Tm3+ doped nanocrystals in our materials, which is presented in Table I To characterize the crystal symmetry, typical XRD patterns of the GCs and standard β-PbF2 are showed in Fig Diffraction peaks of GCs shift towards a larger angle compared with standard β-PbF2, which confirms Tm3+ ions are incorporated into β-PbF2 nanocrystals Significant diffraction peak shift in Fig indicates inner changes of cell parameters for nanocrystals inside different GCs The first diffraction peaks of all the XRD patterns are labeled and presented in Fig 2(a) The inducing temperature and doping concentration enlarge the diffraction peak shift From Fig 2(a), those GCs are divided into two groups using the black boundary line according to the diffraction peak shift Besides, it has been found that the concentration of RE3+ has a linear relationship with the PbF2-RE cell parameters.16,20 Fedorov et al give a general formula that can be used to estimate the mole fraction x of RE3+ for Pb1−x RE x F2+x fluorite-type composition in solid solutions,21,22 where a = 5.940 + [2.48(r-1.456) + 0.3694]x[Å] (1) a and r are the cell parameter and the ion radium According to the formula, the x values is 0.25 for Pb3REF9 and the x values is 0.50 for PbREF5 If we get cell parameter a and corrected Tm3+ TABLE I Possible structural models of Tm3+ doped fluoride nanocrystals Tm3+ substitute for Pb2+ Tm3+ substitute for Pb2+ Space group Pm-3m (NO 221) Chemical formula Pb2TmF9 Point symmetry Oh (m-3m) Space group P4/mmm (NO 123) Chemical formula PbTmF5 Point symmetry D4h (4/mmm) 025001-3 Li et al AIP Advances 6, 025001 (2016) FIG XRD patterns of typical GCs and standard β-PbF2 ionic radius r 23, it is simple to get Tm3+ mole fraction x contained in nanocrystals A computational a-x plot of the GCs is shown in Fig 2(b) A part of x value is less than 0.35 and the others is large than 0.375 Two groups of GCs are obviously separated by green boxes in Fig 2(b) The mole fractions x of Tm3+ for GCs in left box are close to x = 0.25, while those in right box are close to x = 0.5 From Table I, we can judge that when x = 0.25, Pb1−x Tm x F2+x corresponds to a cubic phase Pb3TmF9 structure, with an Oh (m-3m) point symmetry And when x = 0.5, Pb1−x Tm x F2+x corresponds to a tetragonal phase PbTmF5 structure, with a D4h (4/mmm) point symmetry Using the XRD characterization method mentioned above, a concentration induced phase transition can be found In Fig 2(a), 1Tm-GC, 3Tm-GC and 5Tm-GC are on the opposite sides of FIG (a) First diffraction peaks and inducing temperature scatter picture: Each point stands for a GC sample, which is used to group GCs (b) Lattice parameter and Tm3+ molar fraction plot: Each point stands for a GC sample, which is used to group GCs 025001-4 Li et al AIP Advances 6, 025001 (2016) FIG (a) A complete description of the concentration induced phase transition: the phase transformation from cubic to tetragonal phase in lowly and highly doped nanocystals, respectively (b) A complete description of the temperature induced phase transition: the phase transformation from cubic to tetragonal then elongated tetragonal phase in low, middle and high temperature induced nanocrystals, respectively the black boundary line, while only 2Tm-GC crosses the boundary We divide those GCs into three groups, low doping group (1Tm-GC), middle doping group (2Tm-GC) and high doping group (3, 5Tm-GC) From 2(b), low doping group can be matched with cubic Pb3TmF9 (x = 0.25) and high doping group is close to tetragonal PbTmF5 (x = 0.5) Thus we present a Tm3+ ion concentration induced cubic to tetragonal phase transition in Fig 3(a) At low doping level, Tm3+ in a β-PbF2 cubic cell substitutes for one Pb2+ ions and an extra interstitial F- ion is introduced, forming the cubic Pb3TmF9 phase, with a Pm-3m (NO 221) space group As Tm3+ doping concentration increases, more Tm3+ ions could substitute for more Pb2+ ions When Tm3+ ions substitute for two Pb2+ ions, the structure transforms from cubic phase Pb3TmF9 to tetragonal phase PbTmF5, with a P4/mmm (NO 123) space group To further confirm the proposed structure models, Rietveld method with a Fullprof program24,25 was employed for the XRD data refinement, where XRD data were extended to X-ray diffraction (EXRD) patterns in the range of 5◦ to 135◦ Figures 4(a) and 4(b) show the refinement results of 1Tm@460 and 5Tm@460 The corresponding refinement factors Rp (= 4.02%) and Rwp (= 5.08%) of 1Tm@460 indicate that the cubic Pb3TmF9 model is accurate The corresponding factors Rp (= 4.99%) and Rwp (= 6.57%) of 5Tm@460 also fit well between the experimental and the calculated tetragonal PbTmF5 structure The calculated cell parameters of tetragonal PbTmF5 in 5Tm@460 sample are a = b = 4.071Å, c = 5.755Å The ratio of c/a is √ 5.755/4.071 which is close to 2, which indicates that this tetragonal PbTmF5 is originated from a ‘pseudo-cubic’ cell.13 In conclusion, the proposed models are suitable to analyze the Tm3+ doped nanocrystals in GCs and a concentration induced cubic to tetragonal phase transition happens in our materials In Fig 2(b), 2Tm-GC can be divided into another two groups as a low temperature group (400 ◦C, 440 ◦C, 460 ◦C) and a middle temperature group (480 ◦C) As presented in Fig 2(b), the low temperature group could be matched with the cubic Pb3TmF9 (x = 0.25) and the middle temperature group is close to tetragonal PbTmF5 (x = 0.5) In the middle doping group, although Tm3+ ions are relatively rich, part of the Tm3+ ions could not enter the β-PbF2 lattice due to lack of 025001-5 Li et al AIP Advances 6, 025001 (2016) FIG XRD Rietveld refinements and the corresponding error curve of 1Tm@460 (a), 5Tm@460 (b), and 5Tm@520 (c) Theoretical XRD simulation using MS program is presented for comparisons (d) energy Thus, Tm3+ ions substitute for one Pb2+ ion in the low temperature group, forming a cubic Pb3TmF9 phase When inducing temperature increases, more energy is provided and more Tm3+ ions can enter the lattice Then, Tm3+ substitutes for two Pb2+ ions in the middle temperature group, forming a tetragonal PbTmF5 phase Furthermore, the increase of inducing temperature can cause a decrease of total Gibbs energy for the unit cells of different phases in the RE3+ doped lead fluoride nanocrystals in our previous work.14 Tetragonal phase is a phase of stable state, while cubic group is metastable When thermal treatment temperature increases, the metastable cubic phase will transit to the stable tetragonal phase , which presenting a temperature induced phase transition process In addition, the XRD pattern of 2Tm@500 splits quite obviously in Fig 1, and a high temperature group is added due to the difference of its splitting peaks in XRD pattern Similar splitting peaks can also be found in XRD patterns of 5Tm@520 sample in Fig This new phenomenon can be easily simulated with MS program and comes from the increase of c/a ratio in tetragonal PbTmF5 structure, which is presented in Fig 4(d) Splitting peaks appear when c/a ratio is obviously larger √ than This c/a ratio is an indicator of tetragonality which presents a derivation from a cubic phase.26 The conclusion that tetragonality increases as temperature increases has been reported in many other materials.11,27 This elongated tetragonal structure can be further confirmed by Rietveld refinement with the 5Tm@520 sample The corresponding factors Rp (= 3.61%) and Rwp (= 4.62%) fit well between the experiment and elongated tetragonal PbTmF5 structure in Fig 4(c) Then a temperature induced cubic to tetragonal phase transition is presented in Fig 3(b) It is confirmed that a temperature induced cubic to tetragonal then elongated tetragonal phase transition happens in our materials Different site symmetries of Tm3+ ions will cause different energy level splitting of Tm3+ ions and will finally lead to different Stark splits in the emission spectra To further prove the structures of Tm3+ doped fluoride nanocrystals and phase transition processes, emission spectra were measured Fig displays the transitions of 3H4 to 3H6 in typical samples with two different site symmetries of Tm3+ (cubic Oh symmetry and tetragonal D4h symmetry) under 687 nm excitations The maximum intensity of the emission bands in all the spectra is centered at around 800 nm The major peak broadens and two shoulder peaks appears on both longer and shorter wavelength sides when Tm3+ sites inside the elongated tetragonal ligands instead of cubic samples, which indicates that the luminescence of Tm3+ could be modulated by the crystal phase transition of surrounding 025001-6 Li et al AIP Advances 6, 025001 (2016) FIG Emission spectra of the 3H4→ 3H6 transitions with the excitation at 687 nm in cubic samples (a) and elongated tetragonal samples (b) crystal field On the other hand, the cubic to tetragonal phase transitions of Tm3+ doped nanocrystals in oxyfluoride GC is proved by the emission spectra In summary, both of the doping concentration and thermal temperature can induce a cubic to tetragonal phase transition and an Oh to D4h site symmetry distortion in Tm3+ doped oxyfluoride GCs It is promising to use the structures of Tm3+ doped fluoride nanocrystals and phase transition processes to modulate the luminescent of Tm3+ ions in the future This will broaden the applications of Tm3+ doped fluoride 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Oh to D4h site symmetry distortion in Tm3+ doped oxyfluoride GCs It is promising to use the structures of Tm3+ doped fluoride nanocrystals and phase transition processes to modulate the luminescent