Superconducting and structural properties of δ-MoC0.681 cubic molybdenum carbide phase

The superconducting and lattice properties of δ-MoC0.681 were studied by electromagnetic measurements, synchrotron X-ray diffraction, neutron diffraction, and electron diffraction. The superconducting properties (Tc=12 K) of δ-MoC0.681 were well characterized by a weak coupling model. The carbon vacancies present in the host cubic structure were found to be robust, although the material was synthesized from stoichiometric carbon and Mo powder under a high-pressure of 6 GPa. A thermodynamically-stable structure with ordered vacancies did not account for the robust features of δ-MoC0.681 since the vacancies are unlikely to be ordered in long range in the host structure. A model based on inherent phonon instability theoretically predicted for a stoichiometric MoC phase might be responsible for the robust features of δ-MoC0.681.