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Pressure induced phase transformation of clathrate studied by x-ray diffraction and Raman spectroscopy
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Image of FIG. 1.
FIG. 1.

Synchrotron powder XRD patterns of in the range from 3° to 23°. The pressure medium was helium. In (a), the lower and upper panels correspond to the compression and decompression processes, respectively. The peaks denoted with are from the Re gasket and ruby. In (b), three diffraction data picked up for discussion of the crystal symmetry of HP phase are presented. The calculated position under simple-cubic and simple tetragonal lattices are indicated with thick marks. The peaks denoted with and are, respectively, from the Re gasket and ruby , of which the calculated positions (Refs. 31–33) are denoted with thick marks.

Image of FIG. 2.
FIG. 2.

Raman spectra of obtained for various pressures up to 33 GPa on compression and decompression processes. The perfect reversibility is found on the spectral change. Solid circles correspond to the guest Ba vibration in the large cage. For arrows denoting the spectra, see text. The pressure medium was argon.

Image of FIG. 3.
FIG. 3.

Typical results of Rietveld analyses performed up to 33 GPa. Solid circles denote the experimental data; solid lines, the calculated patterns; the thick marks, the calculated peak position; the bottom curves, the differences between the observed and calculated pattern. The angle regions in which the strong gasket peak appears were omitted from the fitting range.

Image of FIG. 4.
FIG. 4.

Pressure dependence of internal atomic coordinate parameters obtained by Rietveld refinements.

Image of FIG. 5.
FIG. 5.

Cage structure of . The unit cell is represented by a cube drawn by solid lines. The Wyckoff sites (6c, 16i, and 24k) of the host atoms are indicated. Arrows shows the displacements for the 24k site induced by pressurization.

Image of FIG. 6.
FIG. 6.

Pressure dependence of unit-cell volumes of , (Ref. 26), and (Ref. 21) normalized with those under the ambient condition. Symbols correspond to the experimental data, and lines the result of theoretical fitting with Murnaghan’s EOS.

Image of FIG. 7.
FIG. 7.

Raman frequencies of as a function of pressure.

Image of FIG. 8.
FIG. 8.

The vibration frequencies of Ba encapsulated in the large cages of the Ge clathrates against the guest-host distance. Solid squares and circles correspond to and , respectively. The straight line is a fitted line under the assumption of a linear function of the distance of the guest-host distance, using both the data in the range higher than 0.362 nm.


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Table I.

Typical results of Rietveld refinements for under high pressures. The space group is and atom positions are Ba(2a) 0,0,0; Ba(6d) 1/4,1/2,0; Ge/Ga(6c) 1/4,0,1/2; Ge/Ga(16i) ; and Ge/Ga(24k) .

Generic image for table
Table II.

Isothermal bulk moduli and its pressure derivative obtained by Murnaghan’s EOS.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Pressure induced phase transformation of Ba8Ga16Ge30 clathrate studied by x-ray diffraction and Raman spectroscopy