Raman-active phonons inA15compounds with differing superconducting transition temperatures

The $E_g$- and $F_{2g}$-Raman spectra of ${\mathrm{V}}_3$Ge and of the low-$T_c$ compounds ${\mathrm{Nb}}_3$Sb,${\mathrm{Cr}}_3$Si, and the temperature dependence of the $F_{2g}$ peak for ${\mathrm{Nb}}_3$Sn have been measured. The intensity ratio of the $E_g$ to the $F_{2g}$ peak turns out to be small in the low-$T_c$ compounds, whereas it is strong in ${\mathrm{V}}_3$Ge and the previously investigated high-$T_c$ compounds ${\mathrm{V}}_3$Si and ${\mathrm{Nb}}_3$Sn. The $E_g$ linewidth, which is particularly small for ${\mathrm{Nb}}_3$Sb, seems to follow the same trend as the corresponding values for $N\left(0\right)\mathrm{}$, the electronic density of states at the Fermi level. The $E_g$- peak frequency increases with decreasing temperature in the low-$T_c$ compounds ${\mathrm{Nb}}_3$Sb and ${\mathrm{Cr}}_3$Si, whereas it decreases in the high-$T_c$ compounds ${\mathrm{V}}_3$Si and ${\mathrm{Nb}}_3$Sn in which a cubic-to-tetragonal transformation may occur. In ${\mathrm{V}}_3$Ge the $E_g$ frequency is temperature independent. The $F_{2g}$-peak frequency which is related to the elastic modulus $c_{44}$ shows a considerable softening with decreasing temperature for ${\mathrm{Nb}}_3$Sn. The $E_g$- and $F_{2g}$-frequency squares of the compounds ${\mathrm{V}}_3$Si, ${\mathrm{Nb}}_3$Sn, ${\mathrm{V}}_3$Ge, and ${\mathrm{Nb}}_3$Sb scale well with the masses of the transition-metal atoms in the high-temperature limit.