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 (α and β). Mo is in a near tetrahedral environment in the β-phase, whereas in the α-phase the metal exhibits a pseudo-octahedral coordination with two very long Mo-O distances (2.3-2.4 Å). The results of DFT calculations indicate that the α-phase of NiMoO₄ is ∼9 kcal/mol more stable than the β-phase. On the other hand, in the case of magnesium molybdate, an α-NiMoO₄-type phase is ∼13 kcal/mol less stable than β-MgMoO₄. These trends in stability probably result from variations in the metal-metal repulsion within the α-phases of the compounds. For the α→β transition in NiMoO₄, the DFT calculations predict an energy barrier of ∼50 kcal/ mol. An apparent activation energy of ∼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 α- and β-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.