Pressure-Induced Collapse of the Guest Eu Off-Centering in Type-I Clathrate Eu8Ga16Ge30

我々は、クラスレート化合物の圧力効果の研究を行い、 タイプ-IクラスレートEu8Ga16Ge30において、ケージのオフセンター位置を占めていたEu原子が、13.1万気圧の加圧によりケージの中心位置を占めるという、圧力誘起構造変化を起こすことを明らかにした。また、これらは、高圧下EXAFS実験により明らかにしたものである。 We have revealed the pressure-induced collapse of the guest Eu off-centering in type-I clathrate Eu8Ga16Ge30 by the measurements of extended X-ray absorption fine structure (EXAFS) at the Ge K-edge in a temperature range of 10–300 K. Our analysis of the EXAFS spectra at ambient pressure confirmed that the guest Eu is located in the off-centered positions at Δoffset = 0.39(1) Å away from the center of the Ga/Ge tetradodecahedron cage (14-cage). However, with applying pressure to 13.1 GPa, Δoffset decreases to zero, and the atomic density of Eu is concentrated near the center of the 14-cage with a broad distribution. The present result demonstrates a close relationship between Δoffset and the size of the cage: Δoffset decreases when the 14-cage shrinks by applying pressure. With further compression, we have observed a drastic change in the EXAFS spectra due to amorphization. In the amorphous phase, the first nearest neighbor Ge–Ga/Ge bond is longer than in the crystal phase, whereas the guest Eu atom is located randomly with respect to the Ge atoms.