Raman scattering in intermediate valent SmB6

The Raman spectra of intermediate valent (IV) SmB6 has been measured and compared to LaB6 and EuB6. Beside the three high energy Raman active phonons, additional features in the spectra were found. Most prominent is a peak at 172 cm-1 for SmB6, at 214 cm-1 for LaB6 and at approximately 220 cm-1 for EuB6. The spectra are analysed in terms of defect induced phonon excitations yielding a weighted phonon density of states. The softening of the line in IV SmB6 compared to the other hexaborides is unusual since even in the trivalent MB6 compounds the Raman active phonon modes normally are at lower frequencies than in semiconducting, divalent samples. This phonon softening is explained in analogy with the phonon anomalies in other IV compounds like Smsub(l-x)Ysub(x)S and TmSe. (author)