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

We report an extensivestudy of the optical and structural propertiesof NiWO4 combining experiments and density functional theorycalculations. We have obtained accurate information on the pressureeffect on the crystal structure determining the equation of stateand compressibility tensor. We have also determined the pressure dependenceof the band gap finding that it decreases under compression becauseof the contribution of Ni 3d states to the top ofthe valence band. We report on the sub-band-gap optical spectrum ofNiWO(4) showing that the five bands observed at 0.95, 1.48,1.70, 2.40, and 2.70 eV correspond to crystal-field transitions withinthe 3d (8) (t (2g) (6) e (g) (2)) configurationof Ni2+. Their assignment, which remained controversialuntil now, has been resolved mainly by their pressure shifts. In additionto the transition energies, their pressure derivatives are differentin each band, allowing a clear band assignment. To conclude, we reportresistivity and Hall-effect measurements showing that NiWO4 is a p-type semiconductor with a resistivity thatdecreases as pressure increases.