Properties of high-Tc,A−15Nb3Si: An extrapolation

We have measured the low-temperature specific heats of two samples of $A-15\mathrm{}$ Nb-Si prepared by chemical vapor deposition. From the specific-heat data, low-$T_c$, $A-15\mathrm{}$ Nb-Si is found to have a $\gamma$ of 15.2 ± 1.5 mJ/mole ${\mathrm{K}}^2$ and a Debye temperature, ${\Theta }_D$, of 319 ± 10 K, for a mole containing four atoms. In addition, the low-temperature specific heat of arc-melted ${\mathrm{Nb}}_{0.82}$ ${\mathrm{Ge}}_{0.18}$ ($A-15\mathrm{}$ structure) has been measured. The $\gamma$ was found to be 15.2 ± 1.0 mJ/mole ${\mathrm{K}}^2$ and ${\Theta }_D$ was 345 ± 10 K. Using these data, the properties of stoichiometric (high-$T_c$) ${\mathrm{Nb}}_3$Si are extrapolated from the substoichiometric Nb-Si samples measured, by analogy to the Nb-Ge system. Stoichiometric, single-phase $A-15\mathrm{}$ ${\mathrm{Nb}}_3$Si, if preparable, should have a $\gamma$ of 30 mJ/mole ${\mathrm{K}}^2$ and a ${\Theta }_D$ of 275 K. By calculating $\lambda$, the electron-phonon coupling constant, and treating $T_c$ as a variable (15-30 K), the electronic density of states, $N\left(0\right)\mathrm{}$, is extracted from this extrapolated $\gamma$ and is found to be low, varying from 1.38 states/ (eV atom) at $T_c=15\mathrm{} \mathrm{K}\mathrm{to} 0.91$ states/ (eV atom) at $T_c=30\mathrm{}$ K. The $N\left(0\right)\mathrm{}$ calculated for the low-$T_c$ ${\mathrm{Nb}}_{0.82}$ ${\mathrm{Ge}}_{0.18}$ is 1.01 states/ (eV atom) and is remarkably close to the 1.08 states/ (eV atom) calculated for the high-$T_c$ ${\mathrm{Nb}}_3$Ge.