Phase transformations and electronic properties in mixed-metal oxides:: Experimental and theoretical studies on the behavior of NiMoO4 and MgMoO4

Mixed-metal oxides play a relevant role in many areas of chemistry, physics, and materials science. We have examined the structural and electronic properties of NiMoO4 and MgMoO4 by means of synchrotron-based time-resolved x-ray diffraction (XRD), x-ray absorption near-edge spectroscopy (XANES), and first-principles density functional theory (DFT) calculations. Nickel molybdate can exist in two phases (alpha and beta). Mo is in a near tetrahedral environment in the beta-phase, whereas in the alpha-phase the metal exhibits a pseudo-octahedral coordination with two very long Mo-O distances (2.3-2.4 Angstrom). The results of DFT calculations indicate that the alpha-phase of NiMoO4 is similar to 9 kcal/mol more stable than the beta-phase. On the other hand, in the case of magnesium molybdate, an alpha-NiMoO4-type phase is similar to 13 kcal/mol less stable than beta-MgMoO4. These trends in stability probably result from variations in the metal-metal repulsion within the alpha-phases of the compounds. For the alpha -->beta transition in NiMoO4, the DFT calculations predict an energy barrier of similar to 50 kcal/mol. An apparent activation energy of similar to 80 kcal/mol can be derived from the time-resolved XRD experiments. The degree of ionicity in MgMoO4 is larger than that in NiMoO4. The nickel molybdate displays a large density of states near the top of the valence band that is not observed in the magnesium molybdate. This makes NiMoO4 more chemically active than MgMoO4. A similar type of correlation is found between the electronic and chemical properties of NiMoO4, CoMoO4, and FeMoO4. The DFT results and Mo L-II-edge XANES spectra show big differences in the splitting of the Mo 4d orbitals in the alpha- and beta-phases of the molybdates. The line shape in the O K-edge essentially reflects the behavior seen for the 4d orbitals in the Mo L-II-edge (i.e., mainly O 1s --> Mo 4d electronic transitions). The Mo L-II- and O K-edges in XANES can be very useful for probing the local symmetry of Mo atoms in mixed-metal oxides. (C) 2000 American Institute of Physics. [S0021-9606(00)70702-1].