On the polymorphism of La2SiMoO8: New metastable high temperature forms and NTE (negative thermal expansion) phase transitions

Abstract

Following a previous study on scheelite LT-La2SiMoO8, a new high temperature polymorph was discovered after metastabilization down to room temperature (HT). An attempt at structural determination by neutron diffraction led to large atomic displacement parameters of the Mo cation and/or of some of its oxygen neighbors, suggesting structural instability. It was confirmed by thermal analysis and X-ray thermodiffraction, showing the existence of a phase transition at 440 ​°C between two very close polymorphs, probably mixed in the HT sample. The structural derivation of the two HT forms from the baryte-type BaSO4 structure was identified, with alternating [LaMoO4]+ and [LaSiO4]- layers. The differentiation between these two forms most likely comes from a slightly different arrangement of their respective [LaMoO4] layers. The anisotropic thermal expansion is accompanied by a volume contraction of −0.56% between the two HT forms, and a much larger one (−5.3%) at higher temperatures when the LT form stabilizes.