Acoustic properties of PLZT ceramics studied by Brillouin - PowerPoint PPT Presentation

Acoustic properties of PLZT ceramics studied by Brillouin scattering Jae-Hyeon Ko 1 ∗ , Do Han Kim 2 , and Seiji Kojima 2 1 Department of Physics, Hallym University, Chuncheon, Gangwondo 200-702, Korea 2 Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan ∗ Corresponding author: hwangko@hallym.ac.kr

1. Introduction (1) Background � Lanthanum lead zirconate titanate (PLZT) ferroelectric ceramics are famous transparent materials for various applications such as electro- optic modulators and shutters, electrostrictive devices, phase retarders, etc. � PLZT-x/65/35 with x>5 shows relaxor behaviors characterized by frequency-dependent dielectric maximum, a broad distribution of relaxation times, and existence of polar nano-regions at temperatures far above the diffuse phase transition point. � The present study aims at the investigation of the temperature dependence of the central peak in the Brillouin spectrum of PLZT- 10/65/35 in a wide temperature range, which would give us insights into the nature of complex relaxational behaviors of ferroelectric relaxors.

(2) What is relaxor ferroelectrics? � Diffused, rounded and frequency-dependent dielectric constant (high dielectric constant near room temperature) � Existence of polar nano regions 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

(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

2. Experimental (1) Brillouin spectroscopy � The conventional scanning- type 3+3 pass tandem multipass Fabry-Perot Interferometer characterized by high contrast and resolution is used to record the Brillouin spectrum of PLZT ceramics. � 90 O A special scattering geometry was used, by which sound velocity and elastic stiffness coefficients can be obtained without the knowledge of the refractive index. � A DPSS single mode laser (532nm) was used to excite the sample.

(2) Sample preparation � Lead lanthanum zirconate titanate ceramics with the composition of x=0.1 and y=0.65 were prepared by hot- pressing method in an oxygen atmosphere. (Pb 1-x La x )(Zr y Ti 1-y )O 3 (PLZT100(x/y/1-y)) � PLZT-10/65/35 (x=0.1, y=0.65) � Specimens were carefully polished to optical quality and loaded into the furnace for temperature control. � For temperature variation, the sample was first annealed at 700 K for one hour and then was cooled to room temperature slowly.

3. Results and Discussion (1) Dielectric Constant � The complex dielectric constant of PLZT-10/65/35 shows typical relaxor behaviors. � The dielectric maximum temperature decreases on lowering the probe frequency. � Frequency-independent dielectric loss at low temperatures implies that the distribution of relaxation time is extremely broad.

(2) Longitudinal and transverse acoustic modes Brilouin spectra of PLZT at Temperature dependences of selected temperatures longitudinal and transverse acoustic modes. Acoustic damping for the longitudinal mode and imaginary part of dielectric constant are also shown.

• The acoustic mode can couple to the dynamics of polar nano regions through electrostrictive coupling, where the strain is coupled to the square of the local polarization through electrostrictive coefficient. • Due to the electrostrictive coupling, both acoustic modes show significant softening below the so-called Burns temperature ~ 620 K along with an increase of the acoustic damping. • Both acoustic modes are overlapped with a broad quasi- elastic central component called central peak.

(3) Central peaks in PLZT Brillouin spectra measured at 670 K, The VV and VH components of 490 K, and 370 K at the (V, V+H) the Brillouin spectrum at 370 K. scattering geometry. * VV: polarized component VH: depolarized component

� The central peak appears below the Burns temperature in both VV and VH scattering geometries. � The integrated intensity grows on cooling, shows a maximum at ~ 370 K and then begins to decrease on further cooling. � The close correlation between the Burns temperature and the temperature below which the central peak appears seems to suggest that the origin of the central peak in PLZT is related to Temperature dependence of the the formation of PNRs. The integrated intensity of the central marked growth of the intensity peak might reflect the increase of the size and density of PNRs below the Burns temperature.

(4) Field effect on acoustic properties of PLZT 500 500 FSR=150GHz,488nm VV VH 0 V/mm 450 462 V/mm 400 400 295 K 350 300 300 Intensity 250 200 200 150 100 100 50 0 0 -200 -100 0 100 200 -200 -100 0 100 200 Brillouin Shift (GHz) DC bias field effect on the central peak in both VV and VH geometries at room temperature.

• Both the frequency shift and the full-width-at-half-maximum of longitudinal and transverse acoustic modes did not show any marked changes in the electric field range up to 7 kV/cm at room temperature. • The overall shape and width of the central peak did not also change in the same electric field range. • It might mean that the PNRs in PLZT is effectively frozen in the Brillouin frequency window (or the density of PNRs active in this frequency range becomes negligible) such that the DC bias of 7 kV/cm might not induce any observable change.

4. Conclusion • The quasi-elastic light scattering spectrum of PLZT- 10/65/35 has been investigated by a high-resolution Brillouin spectroscopy. The Brillouin spectrum consisted of acoustic modes overlapped with a broad central peak. • The central peak was observed in both polarized and depolarized spectra below ~600 K at which polar nano regions begin to appear. The integrated intensity grew on cooling and reached a maximum at ~ 370 K. These two temperatures are in good agreement with previous results and seem to reflect the change in the dynamics of PNRs. • DC bias of 7 kV/cm did not induce any observable change in both acoustic modes and the central peak at room temperature.

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