Resistivity, superconductivity, and order-disorder transformations in transition metal carbides and hydrogen-doped carbides

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Abstract
Electron transport in transition metal carbides was investigated, with special attention to ordering effects in substoichiometric specimens and to the influence of hydrogen doping. Hydrided carbides specimens showed reduced resistivity values, a result attributed to the reduction of defect scattering as the carbon vacancies were filled with substitutional hydrogen ions. The effect of the order-disorder transformation NbC0.83 = Nb6C5 at 1030 °C was also to reduce defect scattering as the carbon vacancies became incorporated into the periodic potential of the ordered phase. As another illustration of vacancy scattering, the resistivities of non-stoichiometric HFCx and stiochiometric WC were compared. The superconducting transition temperatures of partially ordered Nb6C5 and hydrided NbCx decreased from those of the original specimens, disordered NbC0.83 and NbCx, respectively. The normal state resistivity of Nb6C5 displayed the bulge in its temperature dependence characteristic of high Tc superconducters. New data on the normal state resistivity and superconducting transition of single crystal NbC0.98 and TaC0.98 are presented, and a theorectical relation linking normal state resistivity and superconductivity is developed.