Pressure induced phase transition, electronic and optical properties of LiBeX (X=As, Sb and Bi)

In this contribution, we report the results of theoretical calculation on the pressure induced phase transitions, structural, electronic and optical properties of the lithium based ternary LiBeX (X = As, Sb, Bi) compounds. These calculations are carried out using the full potential linearized augmented plane wave method. Our results show that these compounds undergo first order phase transitions. LiBeAs transforms from the reported semiconducting tetragonal Cu₂Sb-type structure (P4/nmm) to the semiconducting polar hexagonal LiGaGe-type structure (P6₃mc) at 3.95 GPa, then to the metallic Ni₂In (P6₃/mmc) structure at 66.62 GPa. In LiBeSb, only one transition occurs at 63.95 GPa from the semiconducting LiGaGe type-structure to the metallic Ni₂In one. LiBeBi exhibits two phase transitions, the first one from the semiconducting LiGaGe phase to the MgSrSi one at 50 GPa, then to the metallic Ni₂In at 61 GPa. Our calculated structural parameters with the modified generalized gradient approximation (PBEsol) functional are in very good agreement with other experimental and theoretical values available in the literature. The band structure and density of states of the studied compounds in different phases are calculated using PBEsol functional and Tran-Blaha modified Becke-Johnson (mBJ) exchange potential. Studied materials in their different semiconducting phases have indirect band gaps, which are higher with the mBJ potential than those obtained with PBEsol functional. The structures and peaks in the optical spectra are related to the transitions in the band structure. The mean value ofɛ(0) increases in this sequence LiBeAs-LiBeSb-LiBeBi. The variation of the dielectric constants with pressure is consistent with the variation of the direct band gaps and shows a discontinuity at the transition. The absorption coefficients,α_xxandα_zz, are the highest for the LiBeBi in the LiGaGe structure in the visible regime. LiBeSb and LiBeBi are promising candidates for application in the visible regime.