Structure of Two Disordered Molybdates, Li₂Mo^IVO₃ and Li₄Mo₃^IVO₈, from Total 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 Å in Li₂MoO₃ and 2.56 Å in Li₄Mo₃O₈] not present in the average structure determined from Rietveld analysis [d(Mo-Mo) = 2.88 Å 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.