Band-Gap Energy and Electronic d-d Transitions of NiWO₄ Studied under High-Pressure Conditions

We report an extensive study of the optical and structural properties of NiWO₄ combining experiments and density functional theory calculations. We have obtained accurate information on the pressure effect on the crystal structure determining the equation of state and compressibility tensor. We have also determined the pressure dependence of the band gap finding that it decreases under compression because of the contribution of Ni 3d states to the top of the valence band. We report on the sub-band-gap optical spectrum of NiWO₄ showing that the five bands observed at 0.95, 1.48, 1.70, 2.40, and 2.70 eV correspond to crystal-field transitions within the 3d⁸ (t_2g⁶e_g²) configuration of Ni²⁺. Their assignment, which remained controversial until now, has been resolved mainly by their pressure shifts. In addition to the transition energies, their pressure derivatives are different in each band, allowing a clear band assignment. To conclude, we report resistivity and Hall-effect measurements showing that NiWO₄ is a p-type semiconductor with a resistivity that decreases as pressure increases.