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Raman and x-ray diffraction studies of cationic type-I clathrate : Pressure-induced phase transitions and amorphization
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Image of FIG. 1.
FIG. 1.

The electron transfer for fulfilling the electroneutrality for type-I cationic and anionic clathrates at ambient conditions. The ionic radius of guest is larger than that of .

Image of FIG. 2.
FIG. 2.

Raman spectrum of at ambient conditions with our results for and for comparison.

Image of FIG. 3.
FIG. 3.

High-pressure Raman spectra of on compression and decompression processes. Horizontal arrows indicate remarkable points for spectral changes. Small open circles indicate guest-I rattling vibrations, which show the frequency softening at pressures above 16 GPa.

Image of FIG. 4.
FIG. 4.

The deconvoluted Raman spectra of at 7.0, 15.8, and 26.7 GPa. Solid circles represent data points, the thick (solid) curve is the sum of the fitted peaks, and the colored (thick) curves show individual Raman peaks separated by the fit.

Image of FIG. 5.
FIG. 5.

Pressure dependence of Raman frequency shifts of at room temperature. Vertical arrow points out the pressure for a remarkable spectral change.

Image of FIG. 6.
FIG. 6.

Observed XRD patterns of in the range from 4° to 22° at high pressures of up to 50 GPa. The XRD patterns of the sample disappeared at pressures of around 50 GPa. Main diffraction lines were consistently indexed as indicated.

Image of FIG. 7.
FIG. 7.

Pressure dependence of the normalized volume for as indicated by solid circles, that for by open squares (Ref. 28), and that for (◻ denotes framework vacancy) by open circles. Vertical arrows indicate the isostructural phase transition points for and . For defect-clathrate , there exists no phase transition (Ref. 23). Solid curves show the results of theoretical fitting with the Murnaghan equation of state.


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Table I.

Comparison between Raman frequencies and Einstein temperatures for guest rattler vibrations of type-I clathrates.

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Table II.

Isothermal bulk moduli and its pressure derivative obtained by the Murnaghan equation of state.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Raman and x-ray diffraction studies of cationic type-I clathrate I8Sb8Ge38: Pressure-induced phase transitions and amorphization