High resolution x ray diffraction study of phase and domain structures and thermally induced phase transformations in PZN (4 5 9)%PT 1

HIGH-RESOLUTION X-RAY STUDY OF PHASE AND DOMAIN STRUCTURES AND THERMALLY-INDUCED PHASE TRANSFORMATIONS IN PZN-(4.5-9)%PT CHANG WEI SEA NATIONAL UNIVERSITY OF SINGAPORE 2009 HIGH-RESOLUTION X-RAY STUDY OF PHASE AND DOMAIN STRUCTURES AND THERMALLY-INDUCED PHASE TRANSFORMATIONS IN PZN-(4.5-9)%PT CHANG WEI SEA (B.Sc.(Hons.), UTM; M.Sc., NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2009 Acknowledgements I would like to express my heartfelt gratitude to my supervisor, Assoc Prof Lim Leong Chew for his constant encouragement and invaluable advice and guidance throughout this research work My gratitude also goes to Prof Tu Chih-Shun and his research group for being supportive, caring, and helpful in numerous ways during my visit to Fu Jen Catholic University, Taipei I have also received a great deal of support and been fortunate to work with Dr Yang Ping, Miao Hua, and Prof Herbert O Moser at Singapore Synchrotron Light Source; Dr Ku Ching-Shun and Dr Lee Hsin-Yi at National Synchrotron Radiation Research Center, Taiwan Dr Yang Ping deserves a special mentioning for ensuring smooth operation in diffraction experiments Special thanks to Prof Amar S Bhalla for his invaluable suggestion on the initial experiment My thanks and appreciation to technical staffs in Materials Science Lab, namely, Thomas Tan, Ng Hon Wei, Abdul Kalim, and Maung Aye Thein; technical staffs in Mechanical Engineering Fabrication Support Centre, namely Lam Kim Song, Low Boo Kwan, and T Rajah for their help in machining work My appreciation also goes to Microfine staffs Dr Jin Jing, Dr K K Rajan, Paul Lim, Lenson Lim, and Joy Chuah for providing a good support in this work Thanks to my friends for being there through good times and the bad and for i all the great memories we have shared over these years Finally, I am especially indebted to my parents for their love and continuous moral support Without them, this work would never have been completed This work was supported by Ministry of Education (Singapore) and National University of Singapore, via research grants nos R-265-000-221-112, R-265-000-257-112, R-265-000-261-123/490 and R-265-000-257-731 ii Table of Contents _ Acknowledgements Table of Contents Summary List of Figures List of Tables List of Symbols Chapter Introduction Chapter Literature Survey 2.1 Background 2.2 Ferroelectrics 2.2.1 Properties o Ferroelectrics 2.2.1.1 Piezoelectric Effect 2.2.1.2 Electro-optic Effect 2.2.2 Perovskite-oxide Type 2.3 Paradigms of Relaxor Ferroelectric Single Crystals Thus Far 2.3.1 Bulk Phase Transformation and Domain Studies 2.3.2 Surface Layer and Dual Phases Chapter Statement of Present Research 3.1 Objective of Present Work iii 3.2 Scope of Present Work 3.3 Organization of Remaining Chapters Chapter Experimental Details 4.1 Sample Cut and Dimensions 4.2 Sample Preparation for Surface Layer Study 4.2.1 4.2.2 4.3 Mechanical Polishing Fracturing Technique Surface Layer Identification Methods 4.3.1 4.3.2 High-resolution Synchrotron Radiation 4.3.3 4.4 Normal X-ray Diffraction Polarized Light Microscopy Phase Transformation Studies 4.4.1 Polarization Characterization Methods 4.4.1.1 4.4.1.2 4.4.2 Dielectric Permittivity Thermal Current Density Structural Studies 4.4.2.1 High-resolution Synchrotron Radiation 4.4.2.2 Polarized Light Microscopy Chapter Surface Layer in Relaxor Ferroelectric PZN-PT Single Crystals 5.1 Introduction iv 5.2 Polished Surface vs Fractured Surface at Room Temperature 5.2.1 Effect of Polished Surface on X-ray Diffraction Results 5.2.2 Effect of Polished Surface on Polarized Light Microscopy Results 5.3 Stability of the Polishing-induced Surface Layer 5.3.1 Thermal Stability 5.3.2 Electrical Resistance 5.4 Summary of Main Observations Chapter Tetragonal Micro/Nanotwins and Thermally-induced Phase Transformations in Unpoled PZN-9%PT 6.1 Introduction 6.2 Theoretical Considerations of Diffractions from (002) planes of Perovskite Crystals 6.2.1 Monoclinic Diffractions 6.2.2 Tetragonal Micro/Nanotwin Diffractions 6.2.3 Crystal Group Theory of Phase Transformation 6.3 Evidence of Tetragonal Micro/Nanotwins in PZN-9%PT at Room Temperature 6.4 Thermally-induced Phase Transformations in Unpoled PZN-9%PT 6.4.1 Temperature Dependent Polarization Characteristics 6.4.2 Structural Studies 6.5 Summary of Main Observations v Chapter Rhombohedral Micro/Nanotwins and Thermally-induced Phase Transformations in UnPoled PZN-4.5%PT 7.1 Introduction 7.2 Theoretical Considerations of Rhombohedral Micro/Nanotwin Diffractions 7.3 Evidence of Rhombohedral Micro/Nanotwins in PZN-4.5%PT at Room Temperature 7.4 Thermally-induced Phase Transformations in Unpoled PZN4.5%PT 7.4.1 Temperature Dependent Polarization Characteristics 7.4.2 Structural Studies 7.5 Summary of Main Observations Chapter Rhombohedral and Tetragonal Micro/Nanotwins Mixture and Thermally-induced Phase Transformations in Unpoled PZN-(6-8)%PT 8.1 Introduction 8.2 Room Temperature Phases of PZN-(7-8)%PT 8.3 Nature of Rhombohedral and Tetragonal Micro/Nanotwin Mixture in PZN-(7-8)%PT at Room Temperature 8.4 Thermally-induced Phase Transformations in Unpoled PZN-8%PT 8.4.1 Temperature Dependent Polarization Characteristics 8.4.2 Structural Studies vi 8.5 Thermally-induced Phase Transformations in Unpoled PZN-7%PT 8.5.1 Temperature Dependent Polarization Characteristics 8.5.2 Structural Studies 8.6 Summary of Main Observations Chapter Revised Phase Diagram for PZN-PT and Other Observations 9.1 Revised Phase Diagram of PZN-PT System 9.2 Room Temperature Phase of PZN-PT Single Crystals of Different PT Contents 9.3 Effect of Poling on Rhombohedral and Tetragonal Domains in PZN- PT 9.4 Depolarization Phenomenon in Poled PZN-PT Single Crystals 9.5 Summary of Main Observations Chapter 10 Conclusions Chapter 11 Recommendations for Future Work References vii Summary Extensive (002)pc reciprocal space mappings have been performed on annealed (unpoled) relaxor ferroelectric PZN-(4.5-9)%PT single crystals by means of high-resolution synchrotron x-ray diffraction (HR-XRD) To avoid undesired surface effects produced by mechanical polishing, a fracturing technique has been devised to expose the relatively stress free crystal bulk for the HR-XRD study Evidence for rhombohedral and tetragonal micro/nanotwins could be detected in the crystals For PZN-(4.5)%PT, the room temperature rhombohedral phase exhibits an extremely broad diffraction in most instances, being the convoluted peak of {100}-type and {110}-type rhombohedral micro/nanotwin diffractions, while respective micro/nanotwin diffractions could be resolved in a number of samples The increased growth and transformation stresses in PZN-(6-8)%PT promote the coexistence of rhombohedral and tetragonal micro/nanotwins at room temperature in these crystals The tetragonal phase in this case is metastable stabilized by the residual stress in the crystal, which partly transforms to the stable rhombohedral phase when the residual stress in the surface layer is relieved by fracturing This accounts for the absence of (001)T diffraction in the exposed surface layer of the annealed crystals At room temperature, PZN-9%PT consists predominantly of {110}-type tetragonal micro/nanotwins which behave in a coordinated manner upon heating The fine details viii 4.3 Surface layer identification methods 4.3.2 Normal x-ray diffraction The normal x-ray diffraction (XRD) technique was used to determine the phases present in the samples The x-ray radiations used were Cu Kα1 and Cu Kα2 with a scanning angular range of 2θ = 41° to 2θ = 48° Pure silicon powder, of which the (220) peak is located at 2θ = 47.30°, was used as the internal reference The x-ray profiles have been corrected for broadening due to Cu Kα2 contribution and due to the instrument using the build-in software It should be noted that due to the high absorbing coefficient of lead-based crystals, the x-ray only detects the structure of a thin surface layer, being a few to low tens of micron in thickness [16, 63, 74] 4.3.2 High-resolution synchrotron radiation HR-XRD measurements were performed at the Singapore Synchrotron Light Source (SSLS) X-ray beam of 8.048 keV was used The layout of the beamline is as shown in Figure 4.3(a) The diffractometer used was the Huber 4-circle system 90000-0216/0, with high-precision 0.0001° step size for omega (ω) and two-theta (2θ) circles The distance from the entrance slit (S1) to the sample centre was 688 mm and that from the sample centre to the detector slit (S4) was 680 mm (adjustable) The storage ring, Helios 2, runs at 700 MeV, produces a stored electron beam current of 38 S4 (a) S e-beam Detector S1 S SR X-ray S θ θ Helios Sample Mirror Si (111) Channel-cut monochromator (b) ∆ω 2θ Figure 4.3 (a) Schematic of beamline of high-resolution x-ray diffractometry in SSLS and (b) differential movement of the rocking curve and detector to produce a RSM 39 300 mA The x-ray beam was conditioned to select photon energy by the Si (111) channel-cut monochromator It was blocked to be 0.30 mm in vertical direction and 3.50 mm in horizontal direction by the collimating mirror and the slit system (S1 and S4) for reciprocal space mapping (RSM) The set-up yielded x-ray beam of about 0.01° in divergence, with the best divergence being 0.006° and energy resolution (∆E/E) is 8.1 x 10-4 No crystal analyzer was used A series of rocking (∆ω) scans at a range of 2θ as illustrated in Figure 4.3(b) were carried out to form the (002) RSM Step size of 0.04° with counting time 0.5 s for every rotating step was used The ω-2θ RSM gives the in-plane (ω = 0°) and out-of-plane (∆ω ≠ 0º) diffraction patterns showing the domain distribution in the sample at different reciprocal lattice vectors 4.3.3 Polarized light microscopy Using polarized light microscope by means of the focusing technique, domain structures across the thickness of the PZN-PT single crystals were studied The PZN-PT single crystals of 400 µm thickness were prepared by mechanical polishing for this study The top surface layer was smoothened to near-mirror-finish as described in Section 4.2.1 40 4.4 Phase Transformation Studies 4.4.1 Polarization characterization methods 4.4.1.1 Dielectric permittivity The 7L×3W×1T mm3 and 7L×3W×1.5T mm3 PZN-PT single crystals were used for temperature and frequency dependent dielectric permittivity measurements All the samples were annealed at 257 ºC for h prior to any measurements Note that different prehistories (i.e., electrical and/or thermal treatment) which the samples have experienced would have influenced the resultant domain structures of the test samples upon structural investigations [66] Gold electrodes were deposited onto the 7L×3W faces by dc sputtering A Wayne-Kerr analyzer PMA3260A was used to obtain the real part of dielectric permittivity, ε’ A Janis CCS-450 cold-head was used with a Lakeshore 340 controller for temperature-dependent measurements For the unpoled samples, “zero-field-heating (ZFH)” process was used in which the data were taken upon heating without any applied E-field at a heating rate of 1.5 ºC/min They are hereafter referred to as the ZFH samples For poled samples, prior poling (PP) was performed at room temperature with a dc E-field along the [001]pc thickness direction The poled samples were subsequently tested under ZFH condition at a heating rate of 1.5 ºC/min They are hereafter referred to as the PP-ZFH samples Subsequent cooling of the ZFH and PP-ZFH samples to room temperature was conducted in the absence of 41 any E-field, and this cooling rate used was the same at 1.5 ºC/min The experimental set-up for ZFH ε’ measurement is shown in Figure 4.4 The basic principle of dielectric permittivity is to measure the response of a material to the frequency of the applied E-field The dielectric properties of a crystal reflect its crystal symmetry The dielectric permittivity measurement carried out in this work can be best modeled by a parallel plate capacitor for which the permittivity and capacitance are related by: C= εr εo A d (4.1) where C is the capacitance, A the area of the parallel plates, d the distance between the plates, εr the relative permittivity and εo the absolute permittivity The complex relative permittivity is defined as: C* ε = Co (4.2) * r where C* is the complex capacitance and Co the absolute capacitance By replacing the complex capacitance in Eq (4.2) with C * = C - i , where f = frequency and R = πfR resistance, the complex relative permittivity is given by: ε* = r i C C o C o πfR = ε’ + iε’’ (4.3) 42 Temperature controller ZFH ε’ curve Janis cold head Wayne-Kerr analyzer Figure 4.4 A picture of the ZFH ε’ measurement 43 The real part, ε’ determines the capacitance through ε ' = imaginary part, ε’’ determines the dielectric loss through ε’’ = C and the Co i C o πfR 4.4.1.2 Thermal current density Thermal current density (J) measures the pyroelectricity of a material and is closely related to changes in polarization and in crystal symmetry of the material under study upon heating Thermal current density is defined as: J = - where ∂ Ps ∂ T ∂ T ∂t (4.4) ∂Ps ∂T is the change in spontaneous polarization with temperature and the ∂t ∂T change in temperature per unit time The 7L×3W×1T mm3 and 7L×3W×1.5T mm3 PZN-PT single crystals were used for this study Gold electrodes were deposited onto the 7L×3W faces by dc sputtering ZFH J measurements were performed on both the unpoled and [001]-poled samples using a Keithley 6517A electrometer A Janis CCS-450 cold-head was used with a Lakeshore340 controller for temperature dependent measurements The heating rates used were the same at 1.5 ºC/min The experimental set-up for measuring the ZFH J is given in Figure 4.5 44 Electrometer Temperature controller ZFH current curve Figure 4.5 A picture of the ZFH J measurement 45 4.4.2 Structural studies 4.4.2.1 High-resolution synchrotron radiation To check for possible accompanied phase transformations, the 3L×1W×7T mm3 and 3L×1.5W×7T mm3 samples were used for the HR-XRD studies The unpoled PZN-PT single crystals were first fractured along the (001) plane to expose the strain-free bulk material The fracturing technique can be referred to Figure 4.1 A similar procedure was used to prepare the poled PZN-PT single crystals except that the samples were given a prior poling treatment at room temperature with a dc E-field along the [001] thickness direction All the HR-XRD measurements were taken from the (001) fractured surfaces only This was to avoid undesired surface effects produced by mechanical polishing Further details will be discussed in Chapter The details of the HR-XRD set-up can be found in Section 4.3.2 Step size of 0.02° and 0.03° with counting time 0.5 s for every rotating step was used When higher resolution of the diffractions was required, such as when we seek to resolve suspected convoluted diffractions, the vertical slits (S3 and S4) were reduced to 0.10 mm and the step size was decreased to 0.005° Angular resolution better than 0.007° could be obtained with the latter arrangement Temperature and E-field dependent HR-XRD measurements were employed to study the phase transformations in PZN-PT single crystals The temperature 46 (a) X-ray Detector Heating stage Temperature controller (b) X-ray Detector E-field Sample holder Figure 4.6 (a) Temperature and (b) E-field dependent HR-XRD measurements in SSLS 47 dependent phase transformation was carried out over the range of temperature from room temperature 25 °C to above the TC, with a heating rate of 1.5 °C/min For E-field dependent phase transformation, a dc E-field of increasing strength was applied on the fractured surface of the bulk samples in the [001] direction The experimental set-ups for temperature and E-field dependent measurements are shown in Figures 4.6(a) and (b), respectively 4.4.2.2 Polarized light microscopy Domain structures were observed by using a Nikon E600POL polarized light microscope (PLM) To avoid superposition of domain layers, the PZN-PT sample thickness was kept at about 50-100 µm The angles of the crossed polarizer/analyzer (P/A) pair quoted in this work were with referenced to the [010]pc sample edge direction Sample heating was achieved via a Linkam Model THMS600 heating/cooling stage mounted on the PLM with a heating rate of 1.5 °C/min The experimental set-up for PLM studies is shown in Figure 4.7(a) The basic principle of PLM is based on the behavior of plane-polarized light when propagating through a medium Since PLM is fitted with a rotating stage and plane-polarizing filter below (polarizer) and above (analyzer) the crystal, the crystal on the rotating stage can be rotated between the crossed plane-polarizing filters The 48 (a) PLM Heating stage Temperature controller cross polarizer light (b) analyzer anisotropy crystal plane-polarized light polarizer non-polarized light Figure 4.7 (a) Temperature dependent measurement for PLM studies (b) The interaction of plane-polarized light and the anisotropy crystal 49 intensity of the cross polarized light varies as the crystal on the rotating stage is rotated between the crossed plane-polarizing filters The intensity of the light passes through maxima brightness and minima Figure 4.7(b) illustrates the interaction between the incident plane-polarized light and the optically anisotropic crystal The above mentioned however does not apply to isotropic crystals because their refractive index is independent of the direction of the incident light For anisotropic crystals (or birefringent crystals), incident plane-polarized light will decompose into orthogonal polarized rays: ordinary (o) and extraordinary (e) rays with different phase velocities, resulting in double refraction Table 4.1 summarizes the optical and crystallographic properties of the seven crystal systems The minima position at which the intensity passes through is called extinction position, which occurs when (1) the wave vector lies parallel to an optic axis, or (2) if the wave vector is not lying parallel to the optic axis, the e ray must lie along the microscope stage Therefore, optical extinction displays by PLM provides important information on the crystal structure and in phases of the material being studied The details for using optical extinction to determine the various phases on a (001)-cut PZN-PT single crystal can be found in Figure 4.8 Table 4.2 lists the spontaneous direction of R, T, and O PZN-PT single crystals and the corresponding extinction angles 50 Table 4.1 The optical and crystallographical properties of the seven crystal systems Crystal system Optical symmetry Unit-cell geomerty Cubic Isotropic a = b = c; α = β = γ = 90° Tetragonal Uniaxial a = b ≠ c; α = β = γ = 90° Rhombohedral a = b = c; α = β = γ ≠ 90° Hexagonal a = b ≠ c; α =β = 90°, γ = 120° Orthorhombic Biaxial a ≠ b ≠ c; α = β = γ = 90° Monoclinic a ≠ b ≠ c; α = β = 90°, γ ≠ 90° Triclinic a ≠ b ≠ c; α ≠ β ≠ γ ≠ 90° 51 ˆ y [110 ] [111] [110] [111 ] MB MC MA ˆ x [001 ] : Rhombohedral : Tetragonal : Orthorhombic Figure 4.8 The (001)-projection of the corresponding crystal polarization [85] The extinction angles of the corresponding phases are given in Table 4.2 Table 4.2 Optical extinction angles of various phases along (001)-projection Crystal structure Rhombohedral (R) Tetragonal (T) Orthorhombic (O) Polarization direction {111} {100} {110} Optical extinction angle 45° 0° and 90° 0°, 45°and 90° 52 ... PZN -x% PT: single crystal (S) or powder (P) P: 5