Structure of two disordered molybdates, (Li2MoO3)-O-IV and (Li4Mo3O8)-O-IV neutron scattering

The structures of the disordered lithium molybdates Li2MoO3 and Li4Mo3O8 have been investigated using total neutron scattering from polycrystalline powders. Rietveld analysis of the Bragg scattering is used to determine the average structures. Shortcomings in this method of analysis are demonstrated by comparing the total correlation function, T(r), determined from total neutron scattering, with those calculated from the structures determined from Rietveld analysis. Much more satisfactory models for these materials are derived from the structurally related ordered material LiZn2Mo3O8, using information from Mo K-edge extended X-ray absorption fine-structure spectroscopy (EXAFS). These models include metal-metal-bonded Mo3O13 clusters [d(Mo-Mo) = 2.58 Angstrom in Li2MoO3 and 2.56 Angstrom in Li4Mo3O8] not present in the average structure determined from Rietveld analysis [d(Mo-Mo) = 2.88 Angstrom in Li2MoO3]. In contrast to EXAFS studies neutron diffraction yields information on all the pair correlations in the material, not merely those involving molybdenum, and allows, for example, the location of Lithium. Remaining discrepancies between our models and the experimental T(r)'s give an insight into the disorder in the two materials.