Phase transition of LaX (X = P, As, Sb and Bi) at high pressure: Theoretical investigation of the structural and electronic properties

The electronic and structural properties of the Lanthanum pnictides LaX (X = P, As, Sb and Bi) in the two phases: rock-salt (B1), and primitive tetragonal (PT) have been studied using the full-potential linearized augmented plane wave (FP-LAPW) method. We used both the local-density approximation (LDA) and the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel–Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. We notice that below 19 GPa and around 17 GPa, 9 GPa and 8 GPa, respectively, LaP, LaAs, LaSb and LaBi maintain the rock-salt structure (B1). At high pressure there occurs a crystallographic phase transition. In all compounds, the high pressure phases are tetragonal structure. The ground state properties of these compounds such as equilibrium lattice constant, bulk modulus are calculated and compared with available literature. The calculated band structure for B1 phase of LaX (X = P, As, Sb and Bi), using EVGGA, show that these compounds are semiconductors with an indirect band gap ( Γ → X ) of about 0.56 eV, 0.43 eV, 0.15 eV and 0.07 eV, respectively. We note that for the same phase (B1) the LDA leads to close the gap at Fermi energy making all compounds as metallic in agreement with the previous calculation. For PT phase both LDA and EVGGA show these compounds as metallic.