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.

Inorganic fast ionic thermoelectric (TE) materials (IFITEMs) exhibit excellent TE capabilities due to the special carrier of delocalized ions. Optimization of the TE performance of an IFITEM, however, is limited by a conflict between its electrical conductivity (σ) and its Seebeck coefficient (S). It remains challenging to regulate σ and S in IFITEMs because they are mainly only stable under high temperature. In this work, σ and S of α-Ag2S (semiconductor) and β-Ag2S (fast ionic conductor) are modulated by the in situ measurement under high pressure and high temperature. It uncovered that pressure increases the electrical conductivity with improving the carrier concentration in α-Ag2S, but increased pressure hinders ion transfer and thus reduces conductivity in β-Ag2S. These results show that the pressure responses of σ and S in α-Ag2S and β-Ag2S are distinctly opposite. Nevertheless, pressure can optimize the power factor (PF) and estimated thermoelectric figure of merit (ZT) in both α-Ag2S and β-Ag2S, with optimum values of 1.97 × 10−4 W/m K2 and 0.122 (3.3 GPa, 447 K), and 2.93 × 10−4 W/m K2 and 0.18 (2.2 GPa, 574 K), respectively. The pressure effect has improved about 4.5 and 3.6 times in PF and ZT of β-Ag2S comparing with α-Ag2S at 0.8 GPa 436 K. This work provides a way to optimize TE performance in fast ionic conductors by altering the pressure, which will help in the production of high-powered TE materials.

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