Phase transformations and electronic properties in mixed-metal oxides: Experimental and theoretical studies on the behavior of NiMoO{sub 4} and MgMoO{sub 4}
- Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973 (United States)
- Molecular Simulations Inc., 9685 Scranton Road, San Diego, California 92121 (United States)
- Centro de Quimica, Instituto Venezolano de Investigaciones Cientificas (IVIC), Apartado 21827, Caracas 1020-A, (Venezuela)
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 NiMoO{sub 4} and MgMoO{sub 4} 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 Aa). The results of DFT calculations indicate that the {alpha}-phase of NiMoO{sub 4} is {approx}9 kcal/mol more stable than the {beta}-phase. On the other hand, in the case of magnesium molybdate, an {alpha}-NiMoO{sub 4}-type phase is {approx}13 kcal/mol less stable than {beta}-MgMoO{sub 4}. These trends in stability probably result from variations in the metal-metal repulsion within the {alpha}-phases of the compounds. For the {alpha}{yields}{beta} transition in NiMoO{sub 4}, the DFT calculations predict an energy barrier of {approx}50 kcal/mol. An apparent activation energy of {approx}80 kcal/mol can be derived from the time-resolved XRD experiments. The degree of ionicity in MgMoO{sub 4} is larger than that in NiMoO{sub 4}. 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 NiMoO{sub 4} more chemically active than MgMoO{sub 4}. A similar type of correlation is found between the electronic and chemical properties of NiMoO{sub 4}, CoMoO{sub 4}, and FeMoO{sub 4}. The DFT results and Mo L{sub 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{sub II}-edge (i.e., mainly O 1s{yields}Mo 4d electronic transitions). The Mo L{sub 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.
- OSTI ID:
- 20215049
- Journal Information:
- Journal of Chemical Physics, Vol. 112, Issue 2; Other Information: PBD: 8 Jan 2000; ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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