The heat capacities of Nb3Al, Nb3Sn, and Nb3Sb were measured between 2 and 400 K. Their superconducting transition temperatures Tc are 18.5, 17.9, and 0.2 K, respectively. The higher-temperature entropy was analyzed, and it was found that phonon-mode softening occurs on cooling for the two high-Tc compounds, but not for Nb3Sb. Geometric mean phonon frequencies ωg are also evaluated for these compounds and for V3X, where X=Si, Ga, Ga0.5Sn0.5, and Sn. As is shown, ωg is an appropriate phonon average for evaluating McMillan's expression for the electron-phonon mass enhancement. Average phonon properties cannot reliably be used to calculate Tc values for these A-15 materials. It is suggested that, for the higher electronic density-of-states materials, select phonon modes strongly couple to the electronic system and influence the magnitude of Tc to a greater extent than average-phonon correlations would indicate. This view is supported by the observation that the higher-density-of-states materials exhibit the more pronounced phonon-mode softening on cooling. © The American Physical Society.

We report new measurements of the normal-state electrical resistances of several high-Tc (∼ 20 K) A-15 structure superconductors. It is found that the resistances are linear functions of T2 from Tc to 40 K. Analysis of available lattice heat-capacity data on the same materials shows that it also varies as T2 over the same temperature interval. These observations suggest that the T2 resistivity is due to electron-phonon scattering. This interpretation is supported by the results of a model calculation for the temperature dependence of the resistivity and the lattice-heat capacity of Nb3Sn. © 1977 The American Physical Society.