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 ${\mathrm{NiMoO}}_{\mathrm{4}}$ and ${\mathrm{MgMoO}}_{\mathrm{4}}$ 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 (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 ${\mathrm{NiMoO}}_{\mathrm{4}}$ is ∼9 kcal/mol more stable than the β-phase. On the other hand, in the case of magnesium molybdate, an $\mathrm{\alpha -}{\mathrm{NiMoO}}_{\mathrm{4}}$ -type phase is ∼13 kcal/mol less stable than $\mathrm{\beta -}{\mathrm{MgMoO}}_{\mathrm{4}}.$ These trends in stability probably result from variations in the metal–metal repulsion within the α-phases of the compounds. For the $\mathrm{\alpha \to \beta }$ transition in ${\mathrm{NiMoO}}_{\mathrm{4}},$ 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 ${\mathrm{MgMoO}}_{\mathrm{4}}$ is larger than that in ${\mathrm{NiMoO}}_{\mathrm{4}}.$ 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 ${\mathrm{NiMoO}}_{\mathrm{4}}$ more chemically active than ${\mathrm{MgMoO}}_{\mathrm{4}}.$ A similar type of correlation is found between the electronic and chemical properties of ${\mathrm{NiMoO}}_{\mathrm{4}}\mathrm{,\hspace{0.5em}}{\mathrm{CoMoO}}_{\mathrm{4}},$ and ${\mathrm{FeMoO}}_{\mathrm{4}}.$ The DFT results and Mo $L_{\mathrm{II}}$ -edge XANES spectra show big differences in the splitting of the Mo $\text{4d}$ 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 $\text{4d}$ orbitals in the Mo $L_{\mathrm{II}}$ -edge (i.e., mainly O $\text{1s→}\mathrm{Mo}$ $\text{4d}$ electronic transitions). The Mo $L_{\mathrm{II}}$ - and O K-edges in XANES can be very useful for probing the local symmetry of Mo atoms in mixed-metal oxides.