Proper and improper ferroelastics with perovskite-derived structures

Abstract

Based on the combined group-theoretical and crystallographic approach, the possible perovskite-derived structures of proper and improper ferroelastics were established. The crystallographic analysis of the Wyckoff position splitting in the low-symmetry proper ferroelastic phases shows the absence of any non-ferroelastic degrees of freedom that reduces the possibilities of structural relaxation. Group-theoretical analysis reveals that tilts of anionic octahedra, as the most common type of structural distortions in perovskites, are closely related to ferroelastic strains, except for the phase with tilt pattern a + a + a +. For such kind of improper ferroelastics, the simplest classification was suggested based on the relationships between the coupled order parameter (tilts of octahedra) and the lattice strain components.