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

Abstract

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 3d 8 (t 2g 6 e g 2) 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.

Electronic, Vibrational, and Structural Properties of the Natural Mineral Ferberite (FeWO4): A High-Pressure Study

“…The only slight difference between calculations and experiments is that according to calculations the unit-cell parameters are slightly less compressible than in experiments. Such discrepancies are within the typical discrepancies between experiments and computational results in other wolframites . It might be possible to overcome them using different exchange-correlation functionals in calculations, but a systematic study of the optimum functional for FeWO 4 is beyond the scope of this study.…”

“…Due to their bulk modulus, an external pressure of 10 GPa causes a 10% change in the unit-cell volume. Consequently, the electronic, magnetic, vibrational, and elastic properties can be dramatically altered. , However, it is known that several AWO 4 wolframite-type compounds do not undergo phase transitions up to pressures of around 20 GPa. ,,,− …”

“…In the stability range of the wolframite structure, compression is anisotropic, with the symmetry of the structure decreasing with increasing pressure. , The structural changes cause a decrease in the electronic band gap energy of NiWO 4 , MnWO 4 , and CoWO 4 , ,, which has been attributed to an increase in hybridization between the 3d electrons of the divalent A cation and the 2p electrons of the oxygen atoms. However, the influence of the pressure on the properties of FeWO 4 has not been investigated.…”

Exploring the crystal structure and properties of ytterbium orthoantimonate under high pressure

“…4(c)), we have found that it can be described by a third-order Birch-Murnaghan equation of state (BM EoS). 39 According to the fits, the experimental unit-cell volume at zero pressure, bulk modulus, and its pressure derivative are V 0 = 290.5(2) Å 3 , B 0 = 155(6) GPa, and B′ 0 = 6.6 (7), respectively, while the corresponding theoretical values are V 0 = 294.8(2) Å, B 0 = 143(5) GPa and B′ 0 = 4.4 (6). The…”

“…The high-pressure (HP) behaviour of MTO 4 bimetal oxides has been the focus of research interest during the last two decades. 1,2 The main objects of study have been phosphates, 3,4 vanadates, 5,6 tungstates, 7,8 molybdates, 9,10 chromates, 11,12 niobates, 13,14 and tantalates. 15,16 On the other hand, a group of compounds that have been barely studied under HP are orthoantimonates, among which only BiSbO 4 has been studied.…”