Superconductivity and ordering in the transition metal carbides Ti2C and Nb6C5
Tatyana G Utkina
The influence of carbon sublattice ordering on the electric properties of compounds has been investigated. Heat treatment of the transition metal carbides Ti2 and Nb6C5 alters the state of the carbon sublattice and causes a significant change in the superconducting transition temperature. It was found that the disordered phase of the carbides was superconducting (Tc/K ~ 3.2 for Ti2C, maximum Tc/K ~ 8.6 for Nb6C5), whereas the ordered phase did not demonstrate superconductivity down to 1.27 K. The electron pairing in this case might be due to an interaction of the conduction electrons with the local modes of tunnelling configurations which arise upon local restructuring of a small group of carbon atoms. The study of the superconductivity of the Nb6C5 samples, quenched from different temperatures (13500 < T/K < 1500), provided a linear dependence of the superconducting transition temperature, Tc, on quenching temperatures. It may be suggested that this linear dependence of Tc is defined by a change of the short-range ordering scale with temperature.