Electronic structure of superconducting layered zirconium and hafnium nitride

The electronic energy-band structures for beta-ZrNCl and beta-HfNCl, which can be superconducting by intercalation, have been calculated by using the scalar-relativistic full-potential linearized augmented-plane-wave method w ithin the local-density approximation. For beta-ZrNCl, we have calculated t he electronic structure for the four proposed crystal structures, and obtai ned two kinds of qualitatively different energy bands. For the first two st ructures, the calculations show that this compound is a band insulator with a gap of similar to 0.2 eV, which is significantly smaller than the experi mental value. For the last two structures, this band gap is increased to si milar to 1.6 and similar to 0.8 eV, which is closer to the experiment. A co mparison between other experimental data and the calculated density of stat es also supports the crystal structure of the recent neutron-diffraction ex periment. A tight-binding analysis has revealed that direct d-d hopping is important for the gap formation of the mother material. We have compared th e experimental T-c gap calculated band gap, and density of states for sever al intercalation-derived superconductors, and found some empirical trends.