Effects of High Pressure on the Bandgap and the d–d Crystal Field Transitions in Wolframite NiWO<sub>4</sub>
Meiyan Ye, Yiyang Zhou, Tianyin Shao, Haozhe Liu, Qiang Tao, Xin Wang, Ruilian Tang, Huijuan Yue, Yan Li, Pinwen Zhu
Abstract
The pressure effects on the optical and structural properties of NiWO 4 have been studied experimentally and theoretically. The fundamental bandgap decreases with a pressure coefficient of −12.0 ± 0.2 meV/GPa. Meanwhile, the Ni 2+ 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 Ni 2+ 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 WO 6 octahedron enhances the hybridization between the W 5d and O 2p orbitals, resulting in bandgap reduction. The pressure-induced enhancement of the NiO 6 octahedral symmetry increases the crystal field splitting, thereby yielding increases in the Ni 2+ 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 NiWO 4 under compression and an effective way to modulate the electronic structure and optical properties of isomorphic wolframite materials.