Optical conductivity spectral anomalies in the off-center rattling system β-Ba8Ga16Sn30

Detection of boson peak and fractal dynamics of disordered systems using terahertz spectroscopy

The boson peak is a largely unexplained excitation found universally in the terahertz vibrational spectra of disordered systems; the so-called fracton is a vibrational excitation associated with the self-similar structure of monomers in polymeric glasses. We demonstrate that such excitations can be detected using terahertz spectroscopy. In the case of fractal structures, we determine the infrared light-vibration coupling coefficient for the fracton region and show that information concerning the fractal and fracton dimensions appears in the exponent of the absorption coefficient. Finally, using terahertz time-domain spectroscopy and low-frequency Raman scattering, we experimentally observe these universal excitations in a protein (lysozyme) system that has an intrinsically disordered and fractal structure and argue that the system should be considered a single supramolecule. These findings are applicable to amorphous and fractal objects in general and will be valuable for understanding universal dynamics of disordered systems via terahertz light.

Low-frequency vibrational properties of crystalline and glassy indomethacin probed by terahertz time-domain spectroscopy and low-frequency Raman scattering

In order to clarify the intermolecular vibrations, the low-frequency modes of the glassy and crystalline states of model pharmaceutical indomethacin have been studied using broadband terahertz time-domain spectroscopy and low-frequency Raman scattering. In the crystalline γ-form, the center of symmetry was suggested by the observation of the exclusion principle of the infrared (IR) and Raman selection rules in the frequency range between 0.2 and 6.5 THz. In addition, a boson peak of the glassy state was observed in both IR and Raman spectra and their frequency showed apparent discrepancy. The intermediate correlation length of the glassy structure was estimated to be about 2.5 nm. The existence of hydrogen bonded cyclic dimers in a glassy state was suggested by the observation of the infrared active intermolecular vibrational mode of the hydrogen bonded cyclic dimers as a broad peak at 3.0 THz in the IR spectrum.

K and Ba distribution in the structures of the clathrate compounds K(x)Ba(16-x)(Ga,Sn)136 (x = 0.8, 4.4, and 12.9) and K(x)Ba(8-x)(Ga,Sn)46 (x = 0.3)

Studies of the K-Ba-Ga-Sn system produced the clathrate compounds K(0.8(2))Ba(15.2(2))Ga(31.0(5))Sn(105.0(5)) [a = 17.0178 (4) Å], K(4.3(3))Ba(11.7(3))Ga(27.4(4))Sn(108.6(4)) [a = 17.0709 (6) Å] and K(12.9(2))Ba(3.1(2))Ga(19.5(4))Sn(116.5(4)) [a = 17.1946 (8) Å], with the type-II structure (cubic, space group Fd3m), and K(7.7(1))Ba(0.3(1))Ga(8.3(4))Sn(37.7(4)) [a = 11.9447 (4) Å], with the type-I structure (cubic, space group Pm3n). For the type-II structures, only the smaller (Ga,Sn)24 pentagonal dodecahedral cages are filled, while the (Ga,Sn)28 hexakaidecahedral cages remain empty. The unit-cell volume is directly correlated with the K:Ba ratio, since an increasing amount of monovalent K occupying the cages causes a decreasing substitution of the smaller Ga in the framework. All three formulae have an electron count that is in good agreement with the Zintl-Klemm rules. For the type-I compound, all framework sites are occupied by a mixture of Ga and Sn atoms, with Ga showing a preference for Wyckoff site 6c. The (Ga,Sn)20 pentagonal dodecahedral cages are occupied by statistically disordered K and Ba atoms, while the (Ga,Sn)24 tetrakaidecahedral cages encapsulate only K atoms. Large anisotropic displacement parameters for K in the latter cages suggest an off-centering of the guest atoms.