The pyrochlore family – a potential panacea for the frustrated perovskite chemist

Abstract

Many known complex oxides of general formula A₂B₂X₇ adopt the pyrochlore structure, a key structure-type that has been shown to demonstrate a vast range of useful physical properties. Areas currently of much interest with respect to pyrochlores, include metal–insulator transitions, magnetic frustration/spin ices, magnetoresistance, superconductivity, ferroelectrics, O/F ionic conductivity, mixed conductivity, pigments and catalysis. We present some recent results on three types of pyrochlore materials that show unusual magnetic, optical and electronic behaviours associated with subtle structural and compositional changes. High-resolution powder neutron diffraction studies of the superconducting Cd₂Re₂O₇ and the ferroelectric Cd₂Nb₂O₇ have been undertaken on material cooled below room temperature. Both Cd₂Re₂O₇ and Cd₂Nb₂O₇ exhibit small structure distortions, in each case involving a distortion from a cubic unit cell, on cooling below ∼180 K and possible models that can be used to describe the low-temperature structures and associated atomic displacements are developed and described in this article. A range of materials of the general formula Ca_1−xLn_xTaO_2−xN_1+x, x = 0.5 and x = 1, Ln = La–Yb have been synthesised and shown to adopt pyrochlore and/or perovskite structures. The absorption spectra of these materials are discussed in terms of their structures and compositions.