Electronic and structural properties of NaZnX(X = P,As, Sb): an ab initio study

Abstract

The first-principles tight-binding linear muffin-tin orbital method within the local densityapproximation (LDA) has been used to calculate the ground-state properties,structural phase stability and pressure dependence of the band gap of NaZnX(X = P,As, Sb). All three compounds are found to crystallize in the tetragonalCu2Sb-type (C38) structure. NaZnAs is also found to crystallize inthe zinc-blende-type related structure, i.e. the MgAgAs (orderCaF2)-type structure. By interchanging the position of the atoms in the zinc-blende structure, three phases(α,β and γ) are formed. The energy–volume relations for these compounds have been obtained in theCu2Sb-typeand cubic α, β andγ phasesof the zinc-blende-type related structure. Under ambient conditions these compounds are more stable inthe Cu2Sb-type structure and are in agreement with experimental observations. At highpressure, these compounds undergo a structural phase transition from the tetragonalCu2Sb-type tocubic α (or β) phase, and the transition pressures were calculated. The equilibrium latticeparameter, bulk modulus and the cohesive energy for these compoundshave also been calculated and are compared with the available results. In theCu2Sb-type structure, NaZnP is found to be a direct-band-gap semiconductor, NaZnAsshows a very small direct band gap and NaZnSb is found to be a metal. In theα and β phases, NaZnP is found to be a direct-band-gap semiconductor,whereas NaZnAs and NaZnSb are found to be semi-metallic. In theγ-phase, all three compounds are found to exhibit metallic behaviour. However, this phase isenergetically unfavourable.