Synthesis, structure and properties of compounds stabilized in the wolframite structure: (AA′)(BB′)O8; A = Zn2+, Mg2+, Co2+, Ni2+, or Cu2+, A′ = Fe3+, Ga3+, In3+, Mn3+, or Sc3+, B = Nb5+ or Ta5+, and B′ = W6+

Abstract

A series of compounds with the formula (AA′)(BB′)O₈, stabilized in the monoclinic wolframite structure, were prepared and characterized. Rietveld refinement confirmed that the structures belong to the wolframite family of compounds. XPS analysis established the different oxidation states of the ions in the compounds. The substitution of transition elements at the Zn site enabled the preparation of new colored compounds. The optical absorption behavior (UV-visible spectroscopy) was understood based on the metal-to-metal charge transfer (MMCT) and allowed d–d transitions. NIR reflectivity studies suggested that the synthesized parent white compounds and colored compounds exhibit good NIR reflectivity values, comparable to those of TiO₂. Magnetic studies indicated antiferromagnetic interactions. Temperature-dependent dielectric studies revealed their high dielectric constant values with low dielectric losses at low frequencies. DFT calculations supported the experimental findings of the band gap and provided pointers about the participating orbitals at and near the Fermi level. The compound CoFeNbWO₈ was found to be a useful electrocatalyst for the oxygen evolution reaction (OER).