Twin-to-shift transformation in B-site deficient hexagonal perovskites: Physical pressure and chemical substitution effects

Twinn-to-shift structure transformation of Ba 8 NiNb 6 O 24 hexagonal perovskite was achieved under high temperature and high pressure. • The high-pressure shifted phase remains stable at ambient pressure from room temperature to 1100 °C. • B-site cation size difference is a key factor governing the critical pressure for the structure transition. • Pressure-driven polymorphism provides a new strategy for structural regulation of hexagonal perovskites. Ba 8 NiNb 6 O 24 , adopting an eight-layer twinned hexagonal perovskite structure, transformed to shifted hexagonal perovskites under high pressure and high temperature. This pressure-induced phase transformation is interpreted by comparison with the phase transformations induced by previously reported chemical substitutions. In the shifted structure, B-site vacancy ordering completely empties the central layers of the face-sharing octahedral trimers. This removes the strong B–B electrostatic repulsion within the FSO units and is the main driving force for the pressure-induced stabilization of the shifted structure. In contrast, the differences in the chemical bonding character of d 0 Nb 5+ , d 0 Ta 5+ , and d 10 Sb 5+ drive the twin-to-shift transformation induced by chemical substitution in Ba 8 MNb x Ta 6− x O 24 and Ba 8 MNb x Sb 6− x O 24 (M = Zn, Co). High pressure induces a twin-to-shift structural transformation in B-site deficient hexagonal perovskite Ba 8 NiNb 6 O 24 , revealing B-site cation size difference as the key factor governing the critical transition pressure. Download: Download high-res image (395KB) Download: Download full-size image