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 La 2 Mo 2 - y Cr y O 9 ( y ≤ 0.5) and La 2 Mo 2 - y W y O 9 ( 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] anti-tetrahedra, is proposed in order to better account for the transport properties of these materials.