Pressure induced phase transition in metallic LaB6: a Raman study

We report high pressure Raman studies on single crystals of metallic LaB6 up to a pressure of 16.4 GPa. Raman spectra shows three lines at 680 cm-1 (T2g), 1120 cm-1(Eg) and 1258 cm-1 (A1g), associated with the internal modes of B6 molecule. The T2g mode shows an asymmetric Fano line shape, arising from the interference between the phonon line and the electronic continuum. The line is fitted with I(ω) = Io (q + ε2)/(1 + ε2) where ε = (ω - ωo)/Γ, ωo is the phonon frequency renormalized due to electron- phonon self energy corrections, Γ is the width parameter proportional to the square of the matrix element of the electron-phonon interaction potential. The parameter q signifies the strength of interference. Most interestingly, our pressure data for the T2g mode shows a significant change in the slope of the mode frequency with pressure dωo/dP and Γ at 9.5 GPa. This clearly indicates that LaB6 undergoes a subtle phase transition at 9.5 GPa within the metallic phase. (author)