Rattling Vibrations Observed by Means of Single-Crystal X-ray Diffraction in the Filled Skutterudite ROs₄Sb₁₂ (R = La, Ce, Pr, Nd, Sm)

We prepared a series of high-quality single crystals of the filled skutterudite R Os 4 Sb 12 ( R = La, Ce, Pr, Nd, Sm) and performed precise X-ray structural analyses to investigate the relation between the crystal structure and rattling vibration of the guest R atoms. The results revealed that in a cage made of Sb, with increasing guest free space from La to Sm, the thermal displacement parameter of the R ion increases and the Einstein temperature, which represents the characteristic energy of the rattling, decreases. Anisotropic distribution of the electron density was observed, in particular for small R ions, which indicates that there is large anharmonicity along the [111] direction that reflects the specific shape of the Sb cage.