| Abstract
| - The crystal structure of the solid solutions La2Mo2-yCryO9 (y ≤ 0.5) and La2Mo2-yWyO9 (y ≤ 1.4) hasbeen 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 behaviordepending on the substituting element. The Cr series follows a regular Vegard-type evolution ofcrystallographic parameters. However, the behavior of the W series is different, the lattice constant varyingin a nonlinear fashion with substitution level, resulting in a smaller cell volume for higher tungstencontents, despite the larger ionic radius of tungsten. Two main structural effects are evidenced: a variationin the distribution of ligands around tungsten, made apparent through the changing balance of oxygenO2 and O3 site occupancies along the series, and a nonlinear evolution of interatomic distances andangles involving the O1 oxygen site. An alternative structural description, based on [O1La3Mo] anti-tetrahedra, is proposed in order to better account for the transport properties of these materials.
- Contrary to other substitutions of Mo6+ in anion conductor La2Mo2O9, W6+ has a nonlinear effect on the crystal cell volume. It is due to a nonlinear evolution of interatomic distances and angles involving one oxygen site and to a lower coordination number of tungsten relative to molybdenum.
|
