The effect of the phase transition on the optical properties of the lanthanum monopnictide compounds

We report results of the optical properties for LaP and LaAs compounds in three phases: rocksalt (B1), CsCl-type (B2) and primitive tetragonal (PT) using the full potential linearized augmented plane wave (FP-LAPW). The local-density approximation (LDA) and the Engel?Vosko-generalized gradient approximation (EV-GGA) were used. We notice that below 19 and 17?GPa respectively, LaP and LaAs maintain the rocksalt structure (B1). Then at high pressure a crystallographic phase transition occurs. The high pressure stabler phase is the tetragonal structure. We found that using EV-GGA for the B1 phase led to both compounds being semiconductors with an indirect band gap. While for the same phase (B1) the LDA leads to closure of the gap at the Fermi energy making the both compounds metallic in agreement with the previous calculation. For the PT and B2 phases both LDA and EV-GGA show these compounds as metallic. We study the effect of the high pressure and using different approximations on the optical properties of these compounds. We have calculated the optical conductivity and reflectivity spectra for LaAs compounds and compare it with the available experimental data; reasonable agreement was found.