A Thorough Investigation of Electronic, Optical, Mechanical, and Thermodynamic Properties of Stable Glasslike Sodium Germanate under Compressive Hydrostatic Pressure: Ab Initio Study.

In this paper, we have tried to elucidate the variation of structural, electronic, and thermodynamic properties of glasslike Na 2 GeO 3 under compressive isotropic pressure within a framework of density functional theory (DFT). The result shows stable structural (orthorhombic → tetragonal) and electronic (indirect → direct) phase transitions at P ∼ 20 GPa. The electronic band gap transition plays a key role in the enhancement of optical properties. The results of the thermodynamic properties have shown that Na 2 GeO 3 follows Debye's low-temperature specific heat law and the classical thermodynamic of the Dulong-Petit law at high temperature. The pressure sensitivity of the electronic properties led us to compute the piezoelectric tensor (both in relaxed and clamped ions). We have observed significant electric responses in the form of a piezoelectric coefficient under applied pressure. This property suggested that Na 2 GeO 3 could be a potential material for energy harvest in future energy-efficient devices. As expected, Na 2 GeO 3 becomes harder and harder under compressive pressure up to the phase transition pressure (∼20 GPa) which can be read from Pugh's ratio ( k H ) > 1.75, however, at pressures above 20 GPa k H < 1.75, which may be due to the formation of fractures at high pressure.