Mechanical stability and optoelectronic behavior of BeXP2 (X = Si and Ge) chalcopyrite

We have conducted a first-principles study on the structural, electronic, optical and elastic properties of BeSiP(2 )and BeGeP2 chalcopyrite compounds. Using the density functional theory (DFT), implemented in both full potential linear muffin-fin orbital (FP-LMTO) and Vienna Ab initio simulation (VASP) packages. The FP-LMTO is used for the determination of the structural, electronic and optical properties, while the VASP is used to determine the elastic constants that give indications about the material stability. The obtained equilibrium structural parameters are in good agreement with available results. An investigation of the band gap indicates that our compounds possess a semiconductor behavior with direct band gap for BeSiP2 and with an indirect band gap for BeGeP2. The energy band gaps decreased by changing Be atoms from Si to Ge. We have calculated the dielectric function epsilon(omega). The obtained results show that these materials are promising semiconductors for photovoltaic applications. For the elastic properties, the single-crystal elastic constants C-ij, shear anisotropic factors A, as well as polycrystalline bulk, shear and Young's modulus (B, G and E) and Poisson's ratio v have been predicted. The generalized elastic stability criteria for a tetragonal crystal are well satisfied, indicating that BeSiP2 and BeGeP2 are mechanically stable in the chalcopyrite structure.