Brillouin scattering, ultrasonic and theoretical studies of acoustic anomalies in crystals showing Jahn-Teller phase transitions

Abstract

DyVO₄ and TbVO₄ undergo a Jahn-Teller induced crystallographic phase change at 14 and 34 K respectively, and elastic constants of these materials behave anomalously at the transition temperature T_D. Brillouin scattering measurements on DyVO₄ show that the elastic constant (c₁₁ - c₁₂) falls to zero at T_D. The electron lattice interaction which drives the transition in this material is predominantly of short range and a cluster theory is used to interpret the measurements. The elastic constant c₆₆ of TbVO₄ also has an anomaly. It falls to zero at T_D when measured at ultrasonic frequencies but only falls to a finite value when measured at the higher frequencies used in Brillouin scattering. This frequency dependence of the elastic constants is explained using both thermodynamic and microscopic theories.