Linear-combination-of-atomic-orbitals-coherent-potential-approximation studies of carbon vacancies in the substoichiometric refractory monocarbides NbCx, TaCx, and HfCx

Abstract

We have calculated the electronic densities of states and Bloch spectral functions of the refractory monocarbides $\mathrm{Nb}{\mathrm{C}}_x$, $\mathrm{Ta}{\mathrm{C}}_x$, and $\mathrm{Hf}{\mathrm{C}}_x$, $0.7\le x\le 1.0$, using the coherent-potential approximation in a linear-combination-of-atomic-orbitals (LCAO) basis. The LCAO Hamiltonian is derived from self-consistent augmented-plane-wave calculations performed for the stoichiometric materials, $x=1.0\mathrm{}$. We discuss the bonding and charge transfer in these compounds and show that the latter is crucial for interpreting the Fermi-level motion of the substoichiometric materials. In all cases the charge transfer is from the transition metal to carbon. We present results of the smearing of the Fermi surfaces for $x<\mathrm{}1.0\mathrm{}$ due to lifetime broadening of the electron quasiparticles, and we argue that the superconducting and phononic properties are greatly affected by this. Further comparisons are made between our results and Hall-effect, resistivity, specific-heat, and photoelectron-spectra measurements.