Metal-to-Metal Charge Transfer in AWO₄ (A = Mg, Mn, Co, Ni, Cu, or Zn) Compounds with the Wolframite Structure

Using a combination of UV–visible spectroscopy and electronic structure calculations, we have characterized the electronic structures and optical properties of AWO₄ (A = Mn, Co, Ni, Cu, Zn, or Mg) tungstates with the wolframite structure. In MgWO₄ and ZnWO₄, the lowest energy optical excitation is a ligand to metal charge transfer (LMCT) excitation from oxygen 2p nonbonding orbitals to antibonding W 5d orbitals. The energy of the LMCT transition in these two compounds is 3.95 eV for ZnWO₄ and 4.06 eV for MgWO₄. The charge transfer energies observed for the other compounds are significantly smaller, falling in the visible region of the spectrum and ranging from 2.3 to 3.0 eV. In these compounds, the partially occupied 3d orbitals of the A²⁺ ion act as the HOMO, rather than the O 2p orbitals. The lowest energy charge transfer excitation now becomes a metal-to-metal charge transfer (MMCT) excitation, where an electron is transferred from the occupied 3d orbitals of the A²⁺ ion to unoccupied antibonding W 5d states. The MMCT value for CuWO₄ of 2.31 eV is the lowest in this series due to distortions of the crystal structure driven by the d⁹ configuration of the Cu²⁺ ion that lower the crystal symmetry to triclinic. The results of this study have important implications for the application of these and related materials as photocatalysts, photoanodes, pigments, and phosphors