Specific heat of A-15 and bcc Mo/sub 0.4/Tc/sub 0.6/

The specific heat of the A-15 phase Mo/sub 0.4/Tc/sub 0.6/ and of the allotropic bcc (A-2) phase of the same composition prepared by a rapid-quench technique has been measured between 4 and 20 K. The A-15 allotrope has an electronic-specific-heat coefficient γ of 3.3 mJ/(g-atom K2) and a Debye temperature of 271 K. The bcc phase has a γ of 6.8 mJ/(g-atom K2) and a softer lattice, with a Debye temperature of 258 K. The energy gaps of both structure are appreciably higher than those of any other known superconductors, with 2Δ/kT/sub c/ values of 6.5 for the A-15 and 6.2 for the bcc compared with the previously known maxima of 4.8 for Nb3Sn and 4.6 for Hg. In addition, the specific-heat discontinuity at T/sub c/ for the A-15 Mo/sub 0.4/Tc/sub 0.6/ is the largest yet observed, with the electronic superconducting specific heat C/sub e s/ equal to 5.4γT/sub c/