Microstructure dynamics in orthorhombic perovskites

Anelastic loss mechanisms associated with phase transitions in BaCeO₃ have been investigated at relatively high frequency ~1 MHz and low stress by resonant ultrasound spectroscopy (RUS), and at relatively low frequency ~1 Hz and high stress by dynamic mechanical analysis (DMA). Changes in the elastic moduli and dissipation behavior clearly indicate phase transitions due to octahedral tilting: Pnma↔Imma↔R3̄c↔Pm3̄m structures at 551 K, 670 K, and 1168 K, and strain analysis shows that they are tricritical, first-order, and second-order phase transitions, respectively. Structures with intermediate tilt states (R3̄c and $Imma$ structures) show substantial anelastic softening and dissipation associated with the mobility of twin walls under applied stress. The $Pnma$ structure shows elastic stiffening which may be due to the simultaneous operation of two discrete order parameters with different symmetries. In contrast with studies of other perovskites, BaCeO₃ shows strong dissipation at both DMA and RUS frequencies in the stability field of the $Pnma$ structure. This is evidence that ferroelastic twin walls might become mobile in $Pnma$ perovskites and suggests that shearing of the octahedra may be a significant factor.