Microstructure dynamics in orthorhombic perovskites

Abstract

Anelastic loss mechanisms associated with phase transitions in ${\text{BaCeO}}_{3}$ have been investigated at relatively high frequency $\ensuremath{\sim}1\text{ }\text{MHz}$ and low stress by resonant ultrasound spectroscopy (RUS), and at relatively low frequency $\ensuremath{\sim}1\text{ }\text{Hz}$ and high stress by dynamic mechanical analysis (DMA). Changes in the elastic moduli and dissipation behavior clearly indicate phase transitions due to octahedral tilting: $Pnma\ensuremath{\leftrightarrow}Imma\ensuremath{\leftrightarrow}R\overline{3}c\ensuremath{\leftrightarrow}Pm\overline{3}m$ 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 ($R\overline{3}c$ 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, ${\text{BaCeO}}_{3}$ 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.