Raman scattering study of rare-earth hexaboride

The excitation-energy, polarization, pressure, and temperature dependences of Raman scattering spectra have been measured for the RB6 crystals (R=Ca2+, La3+, Ce3+, Pr3+, Sm2.6+, Gd3+, Dy3+, and Yb2+). In the Raman spectra, the extra peaks have been clearly observed at around 200cm-1, except for the Raman-active phonons and CEF excitations. The extra peaks show anomalous behavior in the temperature, polarization, and lattice parameter dependences. These anomalous behaviors originate from the vibration of the rare-earth ions in octahedral B6 cage, and the extra peaks can be assigned as the second-order Raman excitations of T1u phonon at Brillouin zone boundary. In the results of pressure dependence, the valence of all Sm ions in SmB6 becomes trivalent above 20 GPa at room temperature.