Etude des propriétés physiques de quelques composés intermétalliques ternaires à base de carbone: M2AX; M=V, Nb, A= P, As et X=C. Effet de la pression hydrostatique

Abstract

The family of ternary Carbide and/or Nitride (MAX phases) presents new class of solids. These phases are nanolaminate compounds with interesting properties between metals and ceramics. The present work is based on the CASTEP code using the density functional theory (DFT), the formalism of pseudopetentials and the plane wave basis (PP-PW). The exchange correlation energy was estimated under the gradient generalized approximation (GGA-PW91). We have investigated the pressure effect on the structural properties of unexplored M2AC compounds, relevant to the so called MAX phases, with M=V or Nb and A= P or As. Our results for the lattice parameters a, c and the equilibrium volume V are in good agreement with the available experimental data. Relative changes of the volume, lattice parameters, M−C and M−A bonds length are studied up to 40 GPa. The bulk modulus was deduced from the Birch-Murnaghan equation of state. A systematic study on the effect of both M and A elements on the bulk modulus B, shows that the bulk modulus is more affected by the substitution of A atoms (P→ As) than by the substitution of M atoms (V→ Nb). Thermodynamic properties were calculated using GIBBS program which is based on E-V data as the only input parameters. The effect of temperature (up to 1200 K) at various pressure values on the bulk modulus, volume expansion, heat capacities and Debye temperature was discussed. Band structure and bonding properties of these compounds were studied at 0 GPa. We have also found that the band structure of the investigated compounds shows a metallic character. From the total and partial density of states, we could notice that the conductivity is mostly assured by the d electrons of the transition metal (M= V, Nb). The charge densities are presented and the chemical bonding are discussed and compared to the available data in the literature.