Structure and properties ofNb3SnHx

The effect of alloying H with ${\mathrm{Nb}}_3$Sn has been studied as a function of composition out to ${\mathrm{Nb}}_3$SnH. Superconductive transition temperatures and lattice constants of the $\beta -\mathrm{W}$ phase were measured over the entire range, and the absence of the low-temperature cubic-tetragonal lattice transformation confirmed for $x>\mathrm{}0.1\mathrm{}$. $T_c$ falls to below 4.2 °K at $x\sim 1$ in a manner resembling that for Sb doping, and the rigid-band-approximation implications of this similarity are briefly discussed. Neutron diffraction reveals that the H atoms lie at the alternative Nb positions of the $\beta -\mathrm{W}$ lattice, cross linking orthogonal Nb chains. The low-temperature specific heat of lightly hydrided ${\mathrm{Nb}}_3$Sn is strikingly different than for the same sample with the H removed, the density of states being higher, but the normal-state specific heat lower. This difference is related, through x-ray measurements of a temperature-dependent tetragonal strain near $T_c$ in pure single-crystal ${\mathrm{Nb}}_3$Sn, to the martensitic transformation.