Reduced lattice parameters for samples of La0.6Sr0.1TiO3 with layered ordering (filled symbols) or random occupation (open symbols) on the perovskite A-site. Schematics of the ordered structures are included.
Phase transitions in perovskites and their variants, of general formula ABX3, are important for technological applications and also of great interest in the earth sciences.
Those with partial occupation of the A site, such as La2/3TiO3, have useful dielectric properties and show potential as ionic conductors too. We have prepared samples in which layers of cation A sites are alternately fully and partially occupied and, by quenching, different samples at the same composition in which the A sites are occupied at random. We have used HRPD to make precise measurements on the transition associated with the tilting of BX6 octahedra. The samples with cation ordering show a tetragonal metric above the transition, and become orthorhombic as the strain associated with tilting takes effect. Those with disordered cations show a cubic metric above the transition as expected but, surprisingly, the tilting induces no measurable strain. We believe cation disorder inhibits the development of long-range strain fields, and that the frequent occurrence of near cubic metrics in perovskites can be a consequence of similar effects.
CJ Howard, Z Zhang (ANSTO, Australia), MA Carpenter (University of Cambridge), KS Knight (ISIS)
Research date: December 2007
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