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

Acoustic properties of PLZT ceramics studied by Brillouin scattering

Jae-Hyeon Ko1∗, Do Han Kim2, and Seiji Kojima2

1Department of Physics, Hallym University, Chuncheon,

Gangwondo 200-702, Korea

2Institute 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-

• ptic 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

PbMg1/3Nb2/3O3

(3) Examples of Ferroelectric Relaxors

Complex Perovskites B-site complex Lead magnesium/zinc niobate PbMg1/3Nb2/3O3, PbZn1/3Nb2/3O3 Lead scandium/magnesium tantalate \ PbSc1/2Ta1/2O3, PbMg1/2Ta1/2O3 (cf: BaMg1/2Ta1/2O3) A-site complex Lead lanthanum zirconate titanate (Pb1-xLax)(ZryTi1-y)O3 (PLZT100(x/y/1-y)) Tungsten bronze structure compositions Strontium barium niobate Sr1-XBaXNb2O6

• 2. Experimental

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.

• A

special 90O scattering geometry was used, by which sound velocity and elastic stiffness coefficients can be

• btained

without the knowledge of the refractive index. A DPSS single mode laser (532nm) was used to excite the sample.

(1) Brillouin spectroscopy

(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. (Pb1-xLax)(ZryTi1-y)O3 (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

• f

PLZT-10/65/35 shows typical relaxor behaviors. The dielectric maximum temperature decreases

• n

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 selected temperatures Temperature dependences of 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, 490 K, and 370 K at the (V, V+H) scattering geometry. The VV and VH components of the Brillouin spectrum at 370 K.

* VV: polarized component VH: depolarized component

Temperature dependence of the integrated intensity of the central peak

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

• n 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 the formation

• f

PNRs. The marked growth of the intensity might reflect the increase of the size and density of PNRs below the Burns temperature.

(4) Field effect on acoustic properties of PLZT

• 200
• 100

100 200 50 100 150 200 250 300 350 400 450 500

VH Intensity Brillouin Shift (GHz)

FSR=150GHz,488nm 0 V/mm 462 V/mm

• 200
• 100

100 200 100 200 300 400 500

VV

DC bias field effect on the central peak in both VV and VH geometries at room temperature. 295 K

• Both the frequency shift and the full-width-at-half-maximum
• f 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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References