Thermal expansion and phase transitions of Na2MoO4

Many simple AB2O4-type compounds exhibit a surprising degree of polymorphism as a function of pressure and temperature; between room temperature and its melting point at 960 K, sodium molybdate forms four different crystal structures. Despite many years of study using X-rays, the diffraction pattern of the beta-phase (stable from 720 - 860 K) has been indexed with two very different orthorhombic unit cells and one other group could not match their data with either of these two interpretations. We hope to resolve the identity of the beta-phase using neutron powder diffraction, to refine the structures of the other phases to 10x higher precision than has been achieved previously, to better characterise the disorder in the delta phase, and to measure the thermal expansion of the three higher temperature polymorphs for the first time.