Syntheses, structures and properties of some osmates(iv,v) adopting the pyrochlore and weberite structures

Abstract

Samples of the complex osmates Ca₂Os₂O₇, Tl₂Os₂O_{7 − x} and Pb₂Os₂O_{7 − x} have been obtained by direct reaction of the component oxides in sealed tubes. The structures and compositions of these materials have been investigated by powder neutron and X-ray diffraction at room temperature and through variable temperature studies. Ca₂Os₂O₇ adopts a stoichiometric, orthorhombic weberite structure but the distortion of the OsO₆ octahedra and eightfold Ca–O co-ordinations decrease with increasing temperature indicating that this material lies close to the weberite/pyrochlore phase boundary. The lead and thallium phases adopt oxygen deficient, cubic pyrochlore structures under the synthesis conditions employed though no evidence was found of cation non-stoichiometry. Preliminary measurements of the electronic properties of weberite Ca₂Os₂O₇ are also reported and discussed in comparison with the lead, thallium, cadmium and mercury osmate pyrochlores.