A series of infrared-active optical phonons have been detected in type-I clathrate Ba8Ga16Ge30 by terahertz time-domain spectroscopy. The conductivity spectra with the lowest-lying peaks at 1.15 and 1.80 THz are identified with so-called rattling phonons, i.e., optical modes of the guest ion Ba 2+ (2) with T1u symmetry in the oversized tetrakaidecahedral cage. The temperature dependence of the spectra from these modes are totally consistent with calculations based on a one-dimensional anharmonic potential model that, with decreasing temperature, the shape becomes asymmetrically sharp associated with a softening for the weight to shift to lower frequency. These temperature dependences are determined, without any interaction effects, by the Bose-factor for optical excitations of anharmonic phonons with the nonequally spaced energy levels.PACS numbers: 63.20. Ry, During the past decade, thermoelectric materials such as clathrates and filled-skutterudites have renewed an interest in phonons [1]. These conductors are formed by polyhedral building blocks, where each polyhedral cage accommodates a guest ion like an alkali-earth or rareearth element. If the cage is oversized, the guest ions can vibrate with large amplitude around an on-center or off-center site in the cage potential. These Einsteinlike local modes have been called rattling phonons. The renewed interest above has been paid to interactions of these rattling phonons with acoustic phonons propagating through the cage network and carrying heat entropy, and, equally or more interestingly, with conduction electrons. However, these issues are still far from being well understood that even the charge dynamics of a single ion in rattling vibration have been little known.This paper reports on the optical conductivity of rattling phonons detected in a type-I clathrate Ba 8 Ga 16 Ge 30 (hereafter abbreviated as BGG), featuring the anharmonicity effects on the conductivity spectra in comparison with model calculations.This compound belongs to a family of ternary type-I clathrates having cubic crystal structure with space group P m3n [2]. The unit cell of the host framework consists of 6 tetrakaidecahedra and 2 dodecahedra. The latter smaller cages around 2a sites occupy the bodycentered-cubic sites, while the former oversized cages around 6d sites line up along the principal axes by sharing both the pentagonal faces with the dodecahedral cages and the hexagonal faces with neighboring tetrakaidecahedra. Every cage encapsulates one Ba 2+ ion, and the guest ions in the smaller (2a) and oversized (6d) cages are defined as Ba 2+ (1) and (2), respectively. These guest ions satisfy the Zintl rule for charge compensations that the more electro-positive guest atom donates electrons to the more electro-negative cage; 16 electrons are transferred from 8 Ba atoms to 16 Ga atoms of the cage. With finely tuned Ga/Ge concentration, therefore, the system can be a heavily doped semiconductor, in general, having both the charge-carrier's sign and density controlled.So far the electric...

We present optical conductivity studies of the type-I clathrate Ba8Ga16Sn30, using a terahertz time-domain spectrometer (0.3-3.0 THz). The lowest-lying spectral peak at 0.72 THz due to the Ba(2) ion's off-center vibration in the oversized cage shows a drastic and anomalous temperature dependence. Below about 100 K, the single broad peak splits into two subpeaks, and with further lowering of the temperature, the spectral shape of this so-called rattling phonon shows non-Boltzmann broadening to the point that the linewidth becomes comparable to the peak frequency. Whereas the initial splitting can be understood by assuming a multiwell anharmonic potential, the strong linewidth broadening toward low temperature cannot, since the Boltzmann factor generally sharpens the low-temperature spectra. The observed behavior suggests strong interaction between the local anharmonic phonons and other excitations.

The optical conductivity spectra of the rattling phonons in the clathrate Ba8Ga16Ge30 are investigated in detail by use of the terahertz time-domain spectroscopy. The experiment has revealed that the lowest-lying vibrational mode of a Ba(2) 2+ ion consists of a sharp Lorentzian peak at 1.2 THz superimposed on a broad tail weighted in the lower frequency regime around 1.0 THz. With decreasing temperature, an unexpected linewidth broadening of the phonon peak is observed, together with monotonic softening of the phonon peak and the enhancement of the tail structure. These observed anomalies are discussed in terms of impurity scattering effects on the hybridized phonon system of rattling and acoustic phonons.

We Review Our Dynamical Property Measurements of Rattling Phonons in Type-I Clathrate Compounds, the Quasi-on-Center Ba8Ga16Ge30 (BGG) and Off-Center Ba8Ga16Sn30 (BGS), Using a Terahertz Time-Domain Spectrometer. The Lowest-Lying Vibrational Modes of Rattling Ba Ions in the Oversized Cage Show Anomalous Temperature Dependence in their Spectra. For BGG, the Temperature Dependence Is Mostly Consistent with a Local Anharmonic Potential Approximation that Predicts Softening towards Low Temperature. On the other Hand, for BGS, a Single Broad Peak of Off-Center Rattling Phonons Splits into Two Subpeaks below 120 K, and, with Further Lowering Temperature, the Spectra Show Anomalous Broadening. While the Splitting Can Be Understood by the Double Well Potential, the Linewidth Broadening Must Be Attributed to some Relevant Interactions with such Excitations as Acoustic Phonons and Doped Carriers.