Superconductivity and atomic ordering in neutron-irradiatedNb3Al

Abstract

We have utilized the technique of neutron-induced disorder to study the effect of atomic ordering on the superconducting transition temperature ${T}_{c}$ of the $A\ensuremath{-}15$ compound ${\mathrm{Nb}}_{3}$Al. Large depressions in ${T}_{c}$ are observed when ${\mathrm{Nb}}_{3}$Al is irradiated with high-energy neutrons ($Eg1$ MeV) at 140 \ifmmode^\circ\else\textdegree\fi{}C. For samples whose value of ${T}_{c}$ before irradiation was 18.7 K no superconductivity was oberved to 1.4 K after irradiation to a fluence of $1.4\ifmmode\times\else\texttimes\fi{}{10}^{19} \frac{n}{{\mathrm{cm}}^{2}}$. The widths of the transition temperatures remain constant as ${T}_{c}$ is depressed and ${T}_{c}$ is recovered by annealing at 750 \ifmmode^\circ\else\textdegree\fi{}C 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 $1.2\ifmmode\times\else\texttimes\fi{}{10}^{19} \frac{n}{{\mathrm{cm}}^{2}}$. For the same fluence ${T}_{c}$ is depressed to 52% of its value before irradiation. The lattice parameter ${a}_{0}$ of the $A\ensuremath{-}15$ phase expands upon irradiation, increasing from 5.183 \AA{} for unirradiated samples to 5.200 \AA{} for samples irradiated to $5.0\ifmmode\times\else\texttimes\fi{}{10}^{19} \frac{n}{{\mathrm{cm}}^{2}}$. The results are discussed in terms of $A$-site occuptation in the ${A}_{3}B A\ensuremath{-}15$ structure and extrapolations of ${T}_{c}$ for complete order are considered.