Acoustic anisotropy in uniaxial tungsten bronze ferroelectric single crystals studied by Brillouin light scattering

Abstract

Acoustic properties were investigated for four tungsten bronze (TB) uniaxial ferroelectric crystals, i.e., (Sr0.61 Ba0.39) 5 Nb10 O30 (SBN61), Cu-doped (K0.5 Na0.5) 1.0 (Sr0.75 Ba0.25) 4.5 Nb10 O30 (KNSBN:Cu), K5.80 Li3.82 Nb10.12 O30, and K4.74 Li3.07 Nb10.44 O30 of which the spontaneous polarization is directed along the polar c axis. Large acoustic anisotropy between the two elastic constants C11 and C33 have been observed from all samples. C33 exhibits a significant softening on approaching the diffuse phase transition temperature from higherature side while C11 does not show any substantial change in the same temperature range. This softening is accompanied by substantial growth of hypersonic damping, appearance and growth of central peak (CP), and slowing down of the relevant dynamics of CP represented by the reducing half width. All these results indicate that the lattice motions along the c axis couple strongly to the one-component order parameter of the polar nanoregions (PNRs) or precursor polar clusters which form and grow below a certain temperature in the paraelectric phase. The inverse dielectric constant measured along the c axis of SBN61 and KNSBN:Cu can be described by two linear regions divided by a crossover temperature at which the change in the magnitude of dipole moments and the strength of the dipole couplings are expected due to the formation of PNRs. C33 of SBN61 becomes continuously softened upon cooling even when the temperature crosses the Burns temperature TB at which PNRs begin to appear. It may suggest that additional relaxation process other than that of PNRs may exist at high temperatures above TB and couple to the longitudinal acoustic waves propagating along the polar axis. Recent observation of a single-particle relaxation at high temperatures and its transformation into a collective relaxation of PNRs by dielectric spectroscopy [Belous, J. Appl. Phys. 102, 014111 (2007)] might be related to the anomalous acoustic behavior of SBN61 observed above TB. © 2008 American Institute of Physics.