First-principles calculations of the structural, elastic, electronic and optical properties of Si2P2O and Ge2P2O at high pressures

The structures of Si2P2O and Ge2P2O with the space group Cmc21 are derived. The structural, mechanical, elastic anisotropy, electronic and optoelectronic properties are calculated by first principles calculations based on density functional theory (DFT) with generalized gradient approximation (GGA) at high pressure for Si2P2O and Ge2P2O. By using the elastic stability criteria, it is shown that their structures are all stable. The phonon dispersion spectra are researched throughout the Brillouin zone as implemented in the CASTEP code, which indicates that the optimized structures are stable dynamically. The brittle/ductile behaviors are assessed in the pressures from 0 GPa to 50 GPa. Our calculations present that the performances of them become ductile with pressure rise. Moreover, the anisotropies of them are discussed by the Young's moduli at different pressure, and the results indicate that the anisotropies of them are obvious. The direct band structures of Ge2P2O and the indirect band gap of Si2P2O show that Si2P2O and Ge2P2O present semiconducting character at 0 GPa and 50 GPa. The band structures of Si2P2O are changed obviously with the increase of pressure. The total DOS originate mainly from O 's' states, O 'p' states, P 's' statesand P 'p' states and M 'p' states (M = Si, Ge). The trends of DOS for Si2P2O and Ge2P2O display many similarities, and the change of DOS is obviously affected by the pressure for Si2P2O. The optoelectronic properties of them are researched. The calculated static dielectric constants, epsilon(1)(0), are 3.2 at 0 GPa and 6.4 at 50 GPa for Si2P2O, and the values of Ge2P2O are 10.2 and 9.2 at 0 GPa and 50 GPa.