Raman-active phonons in A15 compounds with differing superconducting transition temperatures

The E/sub g/- and F/sub 2g/-Raman spectra of V/sub 3/Ge and of the low-T/sub c/ compounds Nb/sub 3/Sb,Cr/sub 3/Si, and the temperature dependence of the F/sub 2g/ peak for Nb/sub 3/Sn have been measured. The intensity ratio of the E/sub g/ to the F/sub 2g/ peak turns out to be small in the low-T/sub c/ compounds, whereas it is strong in V/sub 3/Ge and the previously investigated high-T/sub c/ compounds V/sub 3/Si and Nb/sub 3/Sn. The E/sub g/ linewidth, which is particularly small for Nb/sub 3/Sb, seems to follow the same trend as the corresponding values for N(0), the electronic density of states at the Fermi level. The E/sub g/-peak frequency increases with decreasing temperature in the low-T/sub c/ compounds Nb/sub 3/Sb and Cr/sub 3/Si, whereas it decreases in the high-T/sub c/ compounds V/sub 3/Si and Nb/sub 3/Sn in which a cubic-to-tetragonal transformation may occur. In V/sub 3/Ge the E/sub g/ frequency is temperature independent. The F/sub 2g/-peak frequency which is related to the elastic modulus c/sub 44/ shows a considerable softening with decreasing temperature for Nb/sub 3/Sn. The E/sub g/- and F/sub 2g/-frequency squares of the compounds V/sub 3/Si, Nb/sub 3/Sn, V/sub 3/Ge, and Nb/sub 3/Sb scale well withmore » the masses of the transition-metal atoms in the high-temperature limit.« less