Raman scattering investigation of RB6 (R=Ca, La, Ce, Pr, Sm, Gd, Dy, and Yb)

概要

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The excitation-energy, polarization, and temperature dependeces of Raman scattering spectra have been measured for the RB6 crystals (R=Ca 2+, La3+, Ce3+, Pr3+, Sm 2.6+, Gd3+, Dy3+, and Yb2+). In the divalent crystals of CaB6 and YbB6, the Eg phonon shows doublet, while the line shape is a broad single peak for the trivalent case. In the trivalent crystals, two kinds of the extra peaks have been clearly observed at around 200 cm-1 and at around 1400 cm -1 in addition to the Raman-active phonons and crystal electric-field (CEF) excitations. The peak intensities at around 200 cm -1 show the anomalous decrease with decreasing temperature, and this temperature dependence correlates with that of the mean-square displacement of the R ion. Furthermore, their energies decrease with the increase of the cage space due to B6. These peaks are originated from the second-order excitation of acoustic phonons. The broad peak at around 1400 cm-1 is assigned as the second-order excitation of the T2g phonon. For the T2g phonon, the energy difference and its line-shape change between the trivalent and divalent crystals suggests the existence of electron-phonon interaction. We have observed the CEF excitation in PrB 6 and have obtained the Lea, Leask, and Wolf parameters with x=0.94 and W=9.2 K.
American Physical Societyの論文
2003-12-30