Lattice images of dislocations in the ferroelectric material Ba₂Bi₄Ti₅O₁₈

Abstract

THE compound Ba₂Bi₄Ti₅O₁₈ is a ferroelectric^1,2 member of the family of layered bismuth compounds first described by Aurivillius³. These compounds are built up by regular inter-growth of perovskite layers with ‘Bi₂O₂’ sheets. Between the Bi₂O₂ sheets are m layers of corner-shared octahedra containing the B-cations (Ti⁴⁺, Nb⁵⁺ or Ta⁵⁺) and (m–1) layers of the A-cations (Ba²⁺, Sr²⁺ or Bi³⁺ and so on) in 12-coordinate interstices. A schematic representation of Ba₂Bi₄Ti₅O₁₈, for which m = 5, is shown in Fig. 1. As the ferroelectric properties of these materials are connected with their microstructure it is useful to examine them as closely as possible to elucidate the nature of any disorder present. We report here for the first time the direct observation by lattice imaging of dislocation cores in Ba₂Bi₄Ti₅O₁₈.