Soft mode behavior in the orthorhombic to tetragonal transition in the high T c superconductor YBa2Cu3O7−δ

Abstract
The Young’s modulus and rigidity modulus of the high Tc superconductor YBa2Cu3O7−δ is measured from 300 to 1125 K using a 40‐kHz ultrasonic resonant composite oscillator. Marked soft mode behavior occurs in both moduli in the vicinity of the orthorhombic to tetragonal transition, the softening being associated with orthorhombic shear fluctuations. The deep soft mode cusp allows the transition to be precisely determined as a function of oxygen partial pressure. A mechanical damping peak similar to that in Y‐ZrO2 occurs at 800 K presumably arising from oxygen ion hopping.