Phase transformations and electronic properties in mixed-metal oxides: Experimental and theoretical studies on the behavior of NiMoO{sub 4} and MgMoO{sub 4}

Journal Article · Sat Jan 08 04:00:00 EST 2000 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.480619· OSTI ID:20215049

Rodriguez, Jose A ^[1]; Hanson, Jonathan C ^[1]; Chaturvedi, Sanjay ^[1]; Maiti, Amitesh ^[2]; Brito, Joaquin L ^[3]

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1. Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973 (United States)
2. Molecular Simulations Inc., 9685 Scranton Road, San Diego, California 92121 (United States)
3. 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, Journal Name: Journal of Chemical Physics Journal Issue: 2 Vol. 112; ISSN JCPSA6; ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ACTIVATION ENERGY
ELECTRONIC STRUCTURE
EXPERIMENTAL DATA
IONIC COMPOSITION
MAGNESIUM OXIDES
MOLYBDENUM OXIDES
NICKEL OXIDES
PHASE TRANSFORMATIONS
THEORETICAL DATA
X-RAY DIFFRACTION