Higher lanthanum molybdates: Structures, crystal chemistry and properties

The synthesis, structure and properties of all the compounds known to date in the phase diagram La₂O₃–MoO₃ are reviewed. Special attention is given to the most studied oxide-ion conductor La₂Mo₂O₉, and to fluorite-type La_6-xMoO_12-3x/2 phases with in-depth analysis of crystallographic interrelation and evolution as a function of the Mo:La ratio. Structural relationships between these fluorites and non-stoechiometric scheelite-type La₆Mo₈O₃₃ and La₂Mo₃O₁₂ are also analyzed. The crystal chemical peculiarities of all these phases are reported, together with their chemical and physical characteristics, as well as possible application fields. Aside ionic conduction, catalysis, luminescence and giant electrostriction are some of the many properties displayed by lanthanum molybdates. Fostered by their uncovering, the renewed interest in this phase diagram led to the recent identification of a few additional, more structurally isolated phases, with higher Mo amount. Their structures are also presented, even though their properties have not yet been fully explored. The richness of the explored system, both in terms of existing structural characteristics and properties, makes it an exciting area to dig and unveil. As far as atomic distribution and oxide-ion conduction are concerned, both global trends and singular features are depicted, which might promote insightful investigations about the specificity or universality of the surveyed behaviours.