Structural investigation of the SrAl2O4-BaAl2O4 solid solution system with unstable domain walls

Abstract

We investigated the structural phase transition of the Ba1−xSrxAl2O4 solid-solution system (x⩾0.6) via in situ powder X-ray diffraction and transmission electron microscopy. The sequence of structural phase transitions P6322↔P63(3ap)↔P21 occurs in a composition window of x=0.8–1.0. Ba substitution suppresses the P63 intermediate and low-temperature P21 phases, which disappear at x=0.7 and 0.6, respectively. The P21 phase boundary exhibits first-order characteristics, at which the cell parameters change discontinuously and the cell volume slightly contracts as the temperature increases. In the P21 phase, unstable twin walls are observed; the shapes and the crystal axes of each twin domain change as the temperature increases. This instability can be attributed to the latent instabilities that both the BaAl2O4 and SrAl2O4 compounds possess.