Structural Key of the Thermal Expansion and the Oxide Ionic Conduction in Derivatives of La2Mo2O9: a Temperature-Controlled Neutron Diffraction Study of β-La1.7Bi0.3Mo2O9

A thorough temperature-controlled powder neutron diffraction study was carried out on a Bi-substituted LAMOX (derivatives of La2Mo2O9) oxide ion conductor β-La1.7Bi0.3Mo2O9. It allowed us to identify the subtle structural distortions at the origin of, or correlated to, the peculiarities of the ionic conduction process, which were misinterpreted in previous studies on related compounds. The main distortions concern the morphology of the anti-tetrahedral building units of the cationic framework hosting conducting anions, rather than the tilt/rotation of such units as previously postulated. An analysis of the ionic displacements and the temperature factors within these [O1(La,Bi)3Mo] units upon heating suggests that the successive expansions of [(La,Bi)3] and [(La,Bi)2Mo] triangular faces enables some O1 oxide ions to escape from their initial position toward partially occupied O2 and O3 sites, respectively. The increase of ionic conductivity/mobility observed above about 450 °C and ascribed to a transition toward a Vogel−Tammann−Fulcher (VTF)-type assisted transport regime would consequently be due to the opening up of new conduction paths rather than to an intrinsic increase in mobility of O2 and O3 ions on their low temperature conduction paths.