Total neutron diffraction: a route to the correct local structure of disordered LaMo2O5 and its application to the model compound Zn2Mo3O8

Analysis of Bragg diffraction is the normal route to the structure of crystalline materials. Here we demonstrate the use of total neutron diffraction in determining the local structure in the disordered lanthanum molybdate LaMo2O5. An average structure with space-group symmetry P63/mmc accounts for the Bragg scattering and shows that the compound contains the rare Mo6O18 cluster and a unique type of Mo—Mo bonded sheet. However, this gives an incomplete picture of the structure, since it does not reveal how the sites with fractional occupancy are occupied at a local level. Two models describing possible local structures are constructed by removing symmetry elements present in the average structure. Total correlation functions, T(r), calculated from these structures, with space-group symmetry P63 mc and P3¯m1, are compared with the experimental T(r) to show the validity of these local structures. The close relationship between the T(r)'s of the component structures gives an insight into why disorder occurs in LaMo2O5. The calculated and experimental T(r)'s for a model compound, Zn2Mo3O8, are compared to show the agreement expected from an ordered crystalline material. Remaining discrepancies between our model and the experimental T(r) give an insight into the origin of additional disorder in LaMo2O5.