Effects of partial substitution of Mo6+ by Cr6+ and W6+ on the crystal structure of the fast oxide-ion conductor structural effects of W6+

The crystal structure of the solid solutions La2MO2-yCryO9 (y <= 0.5) and La2Mo2-yWyO9 (y <= 1.4) has been studied using X-ray and neutron powder diffraction. These two series of lanthanum molybdates, which belong to the LAMOX family of fast oxide-ion conductors, exhibit a different structural behavior depending on the substituting element. The Cr series follows a regular Vegard-type evolution of crystallographic parameters. However, the behavior of the W series is different, the lattice constant varying in a nonlinear fashion with substitution level, resulting in a smaller cell volume for higher tungsten contents, despite the larger ionic radius of tungsten. Two main structural effects are evidenced: a variation in the distribution of ligands around tungsten, made apparent through the changing balance of oxygen O2 and O3 site occupancies along the series, and a nonlinear evolution of interatomic distances and angles involving the O1 oxygen site. An alternative structural description, based on [O1La(3)Mo] antitetrahedra, is proposed in order to better account for the transport properties of these materials.