Superconductivity and atomic ordering in neutron-irradiated Nb3Al

The technique of neutron-induced disorder was used to study the effect of atomic ordering on the superconducting transition temperature T/subc/ of the A-15 compound Nb3Al. Large depressions in T/subc/ are observed when Nb3Al is irradiated with high-energy neutrons (E > 1 MeV) at 1400C. For samples whose value of T/subc/ before irradiation was 18.7 K no superconductivity was oberved to 1.4 K after irradiation to a fluence of 1.4 times 1019 n/cm2. The widths of the transition temperatures remain constant as T/subc/ is depressed and T/subc/ is recovered by annealing at 7500C after irradiation. The Bragg-Williams long-range-order parameter S was determined by least-squares analysis of neutron-diffraction data from powdered samples both before and after irradiation. A reduction in S of 18% is observed for samples irradiated to a fluence of 1.2 times 1019 n/cm2. For the same fluence T/subc/ is depressed to 52% of its value before irradiation. The lattice parameter a0 of the A-15 phase expands upon irradiation, increasing from 5.183 A for unirradiated samples to 5.200 A for samples irradiated to 5.0 times 19 n/cm2. The results are discussed in terms of A-site occuptation in the A3B A-15 structure and extrapolations of T/subc/ for complete order are considered