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Micro-Brilouin scattering study of field cooling effects on ferroelectric relaxor PZN-9%PT single crystals Jae-Hyeon Ko 1 *, Do Han Kim 2 , Seiji Kojima 2, D. C. Feng 3 1 Department of Physics, Hallym University, Chuncheon, Gangwondo, Korea 2 Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki, Japan 3 Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China * Corresponding author: hwangko@hallym.ac.kr

I. What is relaxor ferroelectrics? � Diffused, rounded and frequency-dependent dielectric constant (high dielectric constant near room temperature) � Existence of nanopolar clusters at high temperatures � No macroscopic change of the symmetry in many compounds � Dipolar glass model / random field model PbMg 1/3 Nb 2/3 O 3

Examples of Ferroelectric Relaxors � Complex Perovskites B-site complex Lead magnesium/zinc niobate PbMg 1/3 Nb 2/3 O 3 , PbZn 1/3 Nb 2/3 O 3 Lead scandium/magnesium tantalate PbSc 1/2 Ta 1/2 O 3 , PbMg 1/2 Ta 1/2 O 3 (cf: BaMg 1/2 Ta 1/2 O 3 ) A-site complex Lead lanthanum zirconate titanate (Pb 1-x La x )(Zr y Ti 1-y )O 3 (PLZT100(x/y/1-y)) � Tungsten bronze structure compositions Strontium barium niobate Sr 1-X Ba X Nb 2 O 6

Complex Perovskite Relaxors � Relaxor-based complex perovskite ferroelectrics: � Pb[(Zn 1/3 Nb 2/3 ) 1-x Ti x ]O 3 (PZN-x%PT) � Pb[(Mg 1/3 Nb 2/3 ) 1-x Ti x ]O 3 (PMN-x%PT) � outstanding piezoelectric properties when the electric field is along non-polar direction like [001] - strain level ~ 1.7 % - electromechanical coupling constant > 90% � promising materials for electromechanical applications like actuators, transducers… � superior to PZT due to the single crystal form

Phase Diagram of PZN-x%PT • What changes can we expect from field cooling studies on PZN-9%PT rather than 8% composition? • Comparison between PZN-8%PT and PZN-9%PT is necessary for our better understanding of the morphotropic phase boundary(MPB).

II. Experimental Details: Tandem multi-pass Fabry-Perot interferometer 1. The conventional scanning-type tandem multipass Fabry-Perot Interferometer is characterized by high contrast and resolution. 2. The combination of tandem FPI and a microscope made it possible to examine elastic properties of very small samples whose sizes are only a few microns. 3. The phonon propagating direction was along [001] of PZN- 9%PT at a backward scattering geometry, which was the same direction of the applied DC bias field. Incident polarization was [010], and no analyzer was used for the scattered light.

III. Results (1) – temperature dependence of Brillouin spectra of PZN-9%PT under ZFC ZFH process o C 21 • Typical Brillouin spectra consisted of one longitudinal acoustic (LA) mode, Intensity (arb. unit) one weak transverse acoustic (TA) mode and a central peak (CP), where o C 85 the TA mode is noticeable only in the low-temperature rhombohedral phase below 73 o C. o C 90 • From a symmetrical point of view since the TA mode is not allowed at the present scattering geometry in both -60 -40 -20 0 20 40 60 cubic and tetragonal phases. Frequency (GHz)

III. Results (2) – Comparison of Brillouin data between PZN-4.5% and 9%PT PZN-4.5%PT PZN-9%PT 46 43 Cooling cooling Heating 45 heating Frequency (GHz) Brillouin Shift (GHz) 42 44 41 43 40 42 41 300 350 400 450 500 550 0 200 400 600 800 Temperature (K) Temperature (K) • Clear hysteresis can be seen from the Brillouin shift measured during heating and cooling in both components. • It may indicate complex dynamics related to the formation of microdomains and glassy dynamics at low temperatures in case of PZN-4.5%PT and first-order character of the successive phase transitions in case of PZN-9%PT .

III. Results (3) – Brillouin frequency shift and hypersonic damping of 9%PT 43 6 • Two abrupt step-like changes in frequency shift and FWHM are very Frequency (GHz) 42 4 ) 4 significant at two phase transition ε ' (x 10 temperatures from cubic-to-tetragonal 41 and tetragonal-to-rhombohedral 2 40 phases(T C-T and T T-R ). • The differences of frequency shift 0 and FWHM in both phases below T C-T , ZFC ZFH 3 observed at the same measured ε ' FWHM (GHz) point of PZN-9%PT during heating and cooling processes, may reflect 2 the microheterogeneity which is inherent in ferroelectric relaxors near MPB. 300 350 400 450 500 550 Temperature (K)

III. Results (4) – Field cooling effects on the Brillouin spectra of PZN-9%PT 43 LA mode Frequency (GHz) (1) The temperature range of the E//[001], q//[001] 42 tetragonal phase is significantly 41 extended into both high- and low- temperature sides under the electric ZFC 40 1.1 kV/cm field along the [001] direction. 39 2.2 kV/cm (2) The discontinuity at T C − T under 4.4 kV/cm 38 ZFC process is smeared out during the 6.7 kV/cm 3.5 Phonon damping (GHz) FC process as the amplitude of the 3.0 biasing electric field increases. (3) The phonon damping has been 2.5 greatly suppressed by the application 2.0 of the poling field probably due to the 1.5 marked decrease of scattering at 1.0 domain walls. 0 50 100 150 200 250 o C) Temperature (

III. Results (5) – Bias-field dependence of the Brillouin data at constant temperatures • At temperatures far above T C − T ~162 o C, LA induced by the biasing field gradually E//[100], q//[100] 42.5 approaches the value of the tetragonal LA mode Frequency Shift (GHz) phase. However, as the temperature 42.0 approaches closer to T C − T , the change of 41.5 the frequency shift becomes more drastic. o C 230 41.0 • At temperatures of 180 and 170 o C above o C ∆ν 210 o C 205 T C − T , the smallest applied fields of 1.1 40.5 o C 190 kV/cm was enough for making the o C 40.0 180 o C frequency shift equal to the value of a 170 39.5 o C 110 tetragonal phase. o C 70 39.0 • On the other hand, a tetragonal phase is o C 50 o C induced from a rhombohedral a phase at 50 30 38.5 0 1 2 3 4 5 6 7 8 91011121314 o C by applying an electric field of 6.7 kV/cm. Electric Field (kV/cm) o C Only at 30 a low-temperature rhombohedral phase can be stable under the biasing field up to 6.7 kV/cm.

III. Results (6) – Tentative E-T phase diagram of PZN-9%PT 8 E // [001], q // [001] Electric Field (kV/cm) 6 4 C R T 2 or X 0 0 50 100 150 200 250 o C) Temperature ( • A E-T phase diagram of PZN-9%PT can be constructed from the present study by observing the changes of the frequency shift as a function of the temperature under the constant biasing field or of the biasing field at a constant temperature. • The change of the Curie temperature (Tc) under the applied field gives the values of dTc/dE ~ 7.8 × 10 − 3 K cm/V and -5.8 × 10 − 3 K cm/V for T C-T and T T-R phase boundaries, respectively.

Conclusions • Variation of phase transition have been examined in PZN-9%PT under the electric field along the [001] direction by the high-resolution micro- Brillouin scattering. Very sharp step-like changes in both LA mode frequency and damping factor have been observed in ZFH and ZFC processes. The significant thermal hysteresis was observed in cubic-to-tetragonal and tetragonal-to-rhombohedral phase transitions. The absolute values of frequency shift and damping factor depend on the thermal history, which may reflect the microheterogeneity of relaxor ferroelectrics. • The first-order nature of the cubic-to-tetragonal phase transition seems to disappear at the poling field of 6.7 kV/cm along the [001] direction, while the sharp step-like transition from tetragonal to low-temperature phase still remained. The temperature range of a tetragonal phase of PZN-9%PT has been significantly widened under the electric field along [001] into both low-temperature and high-temperature sides, which is in contrast to the situation of PZN-8%PT. A new electric field-temperature phase diagram of PZN-9%PT has been determined based on the changes of the phase transition temperatures.

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