HYPERSOUND ANOMALIES AND ELASTIC-CONSTANTS IN SINGLE-CRYSTAL PBMG1 3NB2/3O3 BY BRILLOUIN-SCATTERING/

The longitudinal (LA) and transverse (TA) Brillouin spectra along [001 ] phonon direction have been measured as a function of temperature (50 -475 K) in single-crystal PbMg1/3Nb2/3O3 With scattering angles theta( s)=180 degrees and 32.5+/-0.2 degrees. The Brillouin frequency shift w ith decreasing temperature shows a broad softening anomaly for both LA and TA phonon modes. For theta(s)=180 degrees, a gradual growth in da mping with maximum near 270 K is observed and is attributed to order p arameter fluctuations. An additional Landau-Khalatnikov maximum is als o observed at T-c similar to 212 K. This anomaly implies a rapid growt h of ferroelectric ordering near T-c and is consistent with the earlie r linear birefringence results reported by Westphal et al.,The elastic stiffness and compliance constants, C-11(E), C-44(E), and s(44)(E) ar e also determined between 200 and 370 K. (C) 1995 American Institute o f Physics.