First-Principles Calculations of the Dynamical and Thermodynamic Properties of Yttrium Compounds YX (X = N, P, As and Sb)

The structural, dynamical and thermodynamical properties of the yttrium pnictides YN, YP, As and YSb compounds in the NaCl (B1) stable phase and the CsCl (B2) unstable phase structures are studied by performing ab initio calculations with the help of the generalized gradient approximation (GGA). The computed lattice parameters; lattice constants, static bulk modulus and corresponding first-order pressure derivative of the bulk modulus are obtained in the two structures NaCl and CsCl and compared with other theoretical and experimental values. The transition pressure from B1 type to B2 type phases is presented and it is about 171, 72, 75 and 34 GPa for YN, YP, YAs and YSb compounds respectively. To obtain the dispersion relation curves and density of states we used the linear-response approach to the density functional theory in both structures. The values of phonon frequencies are used to obtain the thermodynamic quantities such as entropy and heat capacity as function of temperature at different pressures.