First-principles study on the band-gap changes of Zn2GeO4 under high pressure

Abstract

In this paper, the properties of electronic structure and band-gap change of Zn2GeO4 under high pressures are investigated using the first principles method based on the density functional theory (DFT). We demonstrate that the density functional theory calculations performed with the local density approximation (LDA) allows for a significantly better reproduction of lattice constants, the unit cell volume and the band gap of Zn2GeO4 than those performed with the generalized gradient approximation (GGA), so the electronic structure and the band-gap changes of Zn2GeO4 under high pressures can be systematically investigated by LDA. Result of the state density without application of pressures shows that Zn2GeO4 is a wide direct-band-gap semiconductor, and the top of the valence band is mainly composed of Zn 3d and O 2p states, while the conduction band is dominated by the Zn 4 s and Ge 4p. Calculated results about the energy band structure of Zn2GeO4 show that the band gaps of Zn2GeO4 first increase and have a peak at around 9.7 GPa, and then gradually decrease with increasing pressure. The Mulliken charge populations and the value of net charges of Zn2GeO4 at different pressures reveal that the charge distribution of O atoms does not change obviously, while the s and p orbital charges of Zn and Ge atom distributions have obviously charge transfer above 9.7 GPa, and result in an increase of Zn and Ge atom net charges. Analysis of the state density, the Mulliken charge populations, and the electronic density difference of Zn2GeO4 in (210) plane at different pressures indicate:in the low-pressure region (0PP>9.7 GPa), the delocalization phenomenon becomes dominant due to the fact that the delocalization action exceeds the force between the bonding state and anti-bonding state, which induces the decrease of the band gap. These results will not only help to understand the germanate crystal structures in Zn2GeO4 materials under high pressures and the unique characteristics and laws, and may provide a reference for the design of electronic devices of Zn2GeO4 crystals.