Effect of Lattice Instabilities on Superconducting and Other Properties inTaCx

Tunneling measurements performed on ${\mathrm{TaC}}_x$ single crystals and recently published neutron-scattering data show that stoichiometric TaC is in the vicinity of a lattice instability. This leads to a strong dependence of many physical properties on carbon content. It is shown that ${\mathrm{TaC}}_x$ can be well described by a very simple model in which the band structure is considered composition independent while the vibrational spectrum strongly depends on carbon content. In particular, the effect of the anomalous phonon spectrum on superconductivity is discussed. Phillips's ideas about the relation between the instability and superconductivity are supported. The lattice instability is discussed in terms of a hard-sphere model. A consistent picture relating the unusual superconducting, electrical, and mechanical properties in TaC is found.