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
CJ Howard et al., Phys. Rev. B 76 (2007) 054108.