Changes in Polycrystalline Barium-Strontium Titanate at its Transition Temperature

Abstract

THE changes in a single crystal of potassium dihydrogen phosphate passing through its transition point have been described in a recent letter by Ubbelohde and Woodward¹. Changes of a rather similar character occur in barium-strontium titanate, as I have been able to observe. The material examined (a solid solution of BaTiO₃ and SrTiO₃ in a molar ratio of about 7:1) has, at room temperature, a tetragonal perovskite-type structure like barium titanate² with an axial ratio c/a=1.0072, and changes at about 80° C. to an ideal cubic perovskite type. The change from cubic to tetragonal involves a simple uniform extension of the unit cell along one cube axis, which becomes the c-axis. There are three ways of selecting this axis for a given cubic crystal, and therefore three aspects (with c-axes mutually at right-angles) in which a tetragonal structure may be derived from the cubic with very little actual rearrangement of the atoms.