The dispersion of transverse-acoustic phonons along the [101] direction (${C}_{44}$ mode) in the tetragonal BaTi${\mathrm{O}}_{3}$ has been obtained by means of double-axis Brillouin scattering at room temperature. The ${C}_{44}$ mode strongly couples with the $E$-symmetry soft optic phonons through a piezoelectric interaction. We have measured the hypersonic dispersion of the phase velocity of the coupled ${C}_{44}$ mode. The observed hypersonic dispersion is well explained by the frequency dependence of the dielectric function in the 10-GHz region (Brillouin region). The dielectric function of BaTi${\mathrm{O}}_{3}$ in the 10-GHz region has been determined from the Brillouin data without any adjustable parameter.
Lattice vibrations of LaRu2Zn20, and LaIr2Zn20 have been investigated by the local density approximation. Although these compounds have a cage structure in which a La atom is surrounded by Zn, the calculated results show that La vibrations have large dispersion and relatively high frequencies. In both compounds, it is found that the vibrations of Zn at 16c site in the plane perpendicular to the three-fold axis have very low frequencies and can be unstable. This result suggests that the transitions in LaRu2Zn20 at 150 K and in LaIr2Zn20 at 200 K are caused by these unstable modes.
Lattice dynamics of La-filled skutterudite compounds, LaT 4 X 12 (T = Fe, Ru, and Os, and X = P, As, and Sb), has been investigated from first-principles calculations. The calculations show that distortions of the cage comprised of 12 pnictogens are important for La vibrations. The distortions decrease the energies of the La vibrations, and become large from the phosphorous compounds to the antimonide compounds. The La vibrations in the antimonide compounds can affect electronic states through the accompanying distortions. Although the La vibrations display no sign of Kohn anomaly, three pnictogen vibrations indicate Kohn anomaly, i.e., they show anomalous decrease of their energies around the wavevector q =(1, 0, 0) in a few compounds. One of the modes is non-degenerate mode, and the other two are degenerate. If the non-degenerate one becomes unstable, the resulting structure has the space group Pm 3 . The structure is the same as the low-temperature structure in PrRu 4 P 12 .
Polarization property of Raman spectra of simple fluids 3 He, 4 He, Ne, and Ar has been measured. We have clearly found the polarized spectra in the large energy shift region for 3 He and 4 He. For 4 He, we have estimated the s-wave contribution and have found that the ratio of the s-wave to the d-wave increases with an increase of pressure for both super fluid and normal fluid phases. We have also observed the polarized spectra in the large energy shift region for liquid Ne, but the amount of this contribution to the total scattering intensity is small. For liquid Ar, the polarized part is very weak but feasible. The observation of the polarized part suggests that the repulsive interaction due to the electron overlap effect is important in the Raman process of the simple fluids and we also found that the polarized part decreases with a decrease of a quantum nature of these fluids.
Temperature and polarization dependencies of Raman scattering spectra of YB 6 have been measured in the temperature region from room temperature to 2.4 K. All Raman active modes have been observed and well assigned by the polarization dependence. The observation of the doublet structure of T 2g and E g peaks suggests the crystallographic symmetry of P 4/ m m m for YB 6 , not P m 3 m from group theoretical consideration. In the lower energy region than 200 cm -1 , the peaks due to the Y vibration have been observed. Within the present experimental accuracy, any anomalies of phonon spectra have not been observed at the superconducting transition temperature.
To reveal dynamical property of a guest ion in type-I clathrate compound, $n$- and $p$-type clathrates ${\text{Ba}}_{8}{\text{Ga}}_{16}{\text{Ge}}_{30}$ have been investigated by Raman scattering. It is found that the guest ion in a 6d-site cage (6d-cage) rotationally moves for both $n$- and $p$-type since the additional guest mode ${E}_{\text{g}}(\text{A})$ has been observed regardless of its carrier. The potential-energy difference between [100] and [110] directions in the 6d-cage is proportional to the off-center distance of the guest-ion position from the cage center and this off-center distance for $p$-type is much larger than that for $n$-type ${\text{Ba}}_{8}{\text{Ga}}_{16}{\text{Ge}}_{30}$. In addition, the Raman intensity of the cage vibration at a 6c site for $p$-type is weaker than that for $n$-type. Thus, the amplitude of the vibration at the 6c site becomes small for $p$-type, and this small amplitude induces a large movable space for the guest ion, i.e., this vibrational amplitude of the 6c-site atom works as the barrier for the off-center position. For both systems, the guest ion in the 6d-cage shows an anharmonic vibration, judging from the anomalous energy decrease in the guest ion with decreasing temperature. The energy difference between ${T}_{2\text{g}}$ and ${T}_{1\text{u}}$ [T. Mori et al., Phys. Rev. B 79, 212301 (2009)] of the guest mode clearly supports the theoretical prediction of an interacting dipoles picture that explains the glasslike properties of the off-centered clathrate. It is concluded that the off-center rattling plays an important role to suppress a lattice thermal conductivity.
