Electronic structure and properties ofNiSi2andCoSi2in the fluorite and adamantane structures

An electronic-band-structure study of ${\mathrm{NiSi}}_2$ and ${\mathrm{CoSi}}_2$ is performed by means of the linear-muffin-tin-orbital method for both the fluorite and the hypothetical adamantane structures, i.e., silicon with tetrahedral interstitial metal atoms. Energy bands along symmetry lines, densities of states, charge densities, and total energies are presented. In addition, the equilibrium lattice constant, bulk modulus, and cohesive energies are obtained from total-energy and pressure calculations as a function of volume. Special attention is given to the relative stability of both structures. The experimentally observed fluorite structure is found to be lower in energy by more than 1 eV in both cases. The total-energy difference is analyzed and discussed in terms of the electronic structure. The electrostatic energy due to the charge transfer is found to play a significant role, as well as the tendency to form Ni—Si covalent bonds in the fluorite case.