Raman studies ofA15compounds

Abstract

The Raman spectra of transforming and nontransforming ${\mathrm{V}}_3$Si crystals and of nontransforming ${\mathrm{Nb}}_3$Sn were measured between 400 and 20 K. For transforming ${\mathrm{V}}_3$Si the frequency of the $E_g$ Raman peak decreases down to 120 K, but increases below this temperature. On the other hand, the width of the peak increases down to 90 K, but decreases at lower temperatures. No frequency increase and no decrease in width at low temperatures is found for the nontransforming ${\mathrm{V}}_3$Si sample. For ${\mathrm{Nb}}_3$Sn the peak frequency increases only little at low temperatures. These phenomena are interpreted as resulting from two Raman peaks, the first deriving from the cubic, the second from the tetragonal lattice structure. At $T>\mathrm{}{T}_m$ (martensitic transition) the existence of the second peak is believed to indicate the presence of tetragonal domains. These domains are thought to be present in the transforming ${\mathrm{V}}_3$Si sample even at temperatures above 100 K. Practically no or little tendency for forming tetragonal structure is found in the nontransforming ${\mathrm{V}}_3$Si and ${\mathrm{Nb}}_3$Sn samples, respectively. The surface versus bulk question is discussed.