Pressure-Dependent Lattice Dynamics and Electronic Structure of Scheelite-Type AgReO₄

Abstract

The properties of scheelite-type AgReO 4 were systematically investigated using first-principle calculations based on density functional theory (DFT). Structural, mechanical, electronic, optical, and vibrational properties were computed as a function of pressure up to 12.45 GPa, offering insights into the behavior of this compound under external pressure. A systematic comparison of calculated and experimental properties was performed showing that the previously reported equation of state might be hindered by experimental artifacts highlighting the need for new experiments. Our study also shows that compared to the lattice modes, the internal vibrational modes of the ReO 4 tetrahedra are only weakly pressure-dependent, indicating that the ReO 4 tetrahedra in AgReO 4 are almost incompressible. Calculations of elastic constants showed that the Born criteria of stability are satisfied and that C 11 > C 33 , which suggests a stiffer lattice along the a-axis than along the c-axis. Phonon dispersion curves confirm the dynamic stability of the material across the entire pressure range. On the other hand, an analysis of charge density plots indicates that the bonding in AgReO 4 is predominantly ionic and does not change with pressure. Finally, we obtained information on electronic properties. Employing the HSE06 hybrid functional, our electronic structure calculations suggest that AgReO 4 is a wide-gap semiconductor with a bandgap of 3.33 eV under ambient conditions that narrows to 2.28 eV at 12.45 GPa. The bandgap narrowing is a result of the shift in the hybridized Re 5d and O 2p states toward the Fermi level. Furthermore, the resulting static values of the refractive indices for AgReO 4 , η 100 = 2.37 and η 001 = 2.31, indicate birefringence in the crystal. The calculated refractive index increases with increasing pressure, suggesting enhanced crystal binding. The shift in the absorption energy toward the visible region, with an increase in its absorption coefficient with increasing pressure, signifies an increase in the electron response to the incident photon energy.