Effects of High Pressure on the Bandgap and the d–d Crystal Field Transitions in Wolframite NiWO₄

Abstract

The pressure effects on the optical and structural properties of NiWO4 have been studied experimentally and theoretically. The fundamental bandgap decreases with a pressure coefficient of −12.0 ± 0.2 meV/GPa. Meanwhile, the Ni2+ d–d transition energies increase at a rate of 7.4–14.8 meV/GPa. Therefore, the energy differences between the fundamental band and the Ni2+ d–d transition bands gradually decrease under pressure, which is beneficial to improve its optical performance. These optical phenomena are associated with structural variations. The shrinkage of the WO6 octahedron enhances the hybridization between the W 5d and O 2p orbitals, resulting in bandgap reduction. The pressure-induced enhancement of the NiO6 octahedral symmetry increases the crystal field splitting, thereby yielding increases in the Ni2+ d–d intraband transition energies. Besides, a pressure-induced structural phase transition is also observed around 20.0 GPa by both angle-dispersive synchrotron X-ray diffraction (ADXRD) and Raman experiments. This study provides valuable insight into the electron–lattice coupling of NiWO4 under compression and an effective way to modulate the electronic structure and optical properties of isomorphic wolframite materials.

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

“…In Figure a, we present a selection of XRD patterns measured at different pressures. In agreement with ref , we have found that NiWO 4 does not undergo any phase transition in the pressure range covered by our studies, i.e., the crystal structure can be described as isomorphic to wolframite up to 20 GPa. This is supported by the results of Rietveld refinements shown in Figure a at 1.5 and 18.7 GPa.…”

“…The pressure dependence of E gap can be well described by a linear function as shown with a red solid line in Figure b. The pressure coefficient of this function, d E gap /d P , is −13(1) meV/GPa, which is in excellent agreement with previous results . The pressure dependence obtained from experiments also agrees with that obtained from our calculations (see Figure b).…”

“…As it can be seen in the inset of Figure a, the band-gap energy at 0 GPa is estimated to be 3.00(5) eV. Comparing with previous experiments, ,− our band-gap energy best agrees with the result reported by Ye et al . From our experiments, we have determined the pressure dependence of the band-gap energy ( E gap ) which is shown in Figure b.…”

“…The values of K 0 and K 0 ′ obtained from our experiments and calculations agree within uncertainties, which shows that calculations accurately describe the changes induced by pressure in the crystal structure. In the previous study, it was reported that according to experiments, V 0 = 129.12(1) Å 3 , K 0 = 146.05(2) GPa, and K 0 ′ = 14.40(1). We consider that the overestimation of the pressure derivative of the bulk modulus is an artifact caused by the abrupt change of compressibility that occurs in previous experiments at 7 GPa .…”

“…It is isomorphic to the mineral wolframite - (Fe,Mn)­WO 4 – sharing the structure with several tungstates, including CoWO 4 , CdWO 4 , MgWO 4 , MnWO 4 , and ZnWO 4 . High-pressure studies have contributed to the understanding of the structural and electronic properties of wolframites. − High pressure (HP) can substantially modify the structural and electronic properties of materials favoring a deep understanding of them . NiWO 4 is one of the wolframites that has been less studied under compression, there being only one article recently published .…”