AB-INITIO ELECTRONIC BAND-STRUCTURE CALCULATIONS FOR CALCIUM MONOCHALCOGENIDES

The first principles tight-binding linear muffin-tin orbital method wi thin the local density approximation was used to calculate the electro nic band structures and the total energies of CaS, CaSe and CaTe in Na Cl-type and CsCl-type structures. The total energies were used to calc ulate the ground state properties such as lattice parameter, bulk modu lus and the structural phase stability of these compounds. The transit ion pressure at which these compounds undergo the structural phase tra nsition from NaCl-type to CsCl-type structure were calculated. The gro und state properties, the transition pressures and the transition volu mes are found to agree with the experimental and other theoretical res ults. The energy band gap at ambient condition in the NaCl-type struct ure were calculated and compared with the experimental results availab le for CaS and CaSe. For CaTe the experimental values of energy gap ar e not available. The valence-band width and the pressure coefficient o f energy gap were also calculated. The closure of band gap at transiti on in CsCl structure for CaSe, and CaTe were explained by comparing th e band structures of BaSe and SrSe in this phase.