Guest Ion Motion in Cage Structure Crystals Investigated by Raman Scattering

Abstract

Last five years we have mainly investigated phonons in the following cage compounds by Raman scattering: RT4X12, A8Ga16Ge30, La3Pd20Ge6, RB6, KOs2O6. From this systematic investigation we have clarified the following dynamical properties of the guest ion trapped in the large cages. The vibration of the guest ion is highly anharmonic due to weak atomic interaction between the guest ion and cage atom. This anharmonicity universally decreases the vibration energy of the guest ion with decreasing temperature. The existence of carriers decreases the interaction between the guest atoms and changes the dispersion curve of the guest ion mode to flat, which is the experimental evidence of the weak interaction between the guest atoms. For thermoelectricity, it is found in A8Ga16Ge30 that the off-center rattling decreases lattice thermal conductivity.

The anharmonic vibration of Li in lithium amide

“…We also note that the characteristic of the vibration we report here has similarity with that observed in the cage type structures 21-23 except that each Li3 vibration has single degree of freedom unlike the three degrees of freedom of each guest atom (Einstein oscillator) in the cage structure. Following these studies, where the rattling motion of guest atoms in the guest-host type structure has been claimed to be a universal phenomenon, 27 one can readily appreciate that the vibration frequency of Li3 atoms may increase with the temperature. [24][25][26][27][28] This property of large amplitude anharmonic vibration provides FIG.…”