(Color online) Symbolic representation of face-sharing dodecahedron, open cage, and distorted cube as building unit of type-III clathrates. There are three different sites for the guest Ba atoms: Ba1, Ba2, and Ba3 are enclosed inside small , cubic , and open cages, respectively. Blue and white spheres are four- and three-bonded Ge atoms, respectively: 68 out of 100 Ge atoms in the unit cell are 4-bonded and the remaining 32 Ge atoms are 3-bonded.
Raman spectrum of at 1 bar and room temperature with that of clathrate for comparison. The numbering of the peaks on their spectra indicate Raman peak wave numbers. The Raman frequency at the top of is scaled by the square root of atomic mass ratio of Si to Ge, 1/1.61. Four dotted lines indicate the same modes assigned by the corresponding spectra between framework vibrations of and . Vertical broken lines of d-Ge and d-Si show the typical vibrational frequencies for strong Raman peaks of diamond-type Ge and Si crystals, respectively.
(Color online) Raman spectra of at various pressures up to 26 GPa and room temperature. Two pressure-induced phase transitions can be seen at about 3.2 and 22 GPa. Black, blue, and red lines show their typical spectra in each phase.
Pressure dependence of Raman frequency shifts of at room temperature. Open circles show the weak Raman peaks. Vertical dotted lines indicate phase transition pressures.
Raman spectra at 1 bar for amorphous Ge (a-Ge) and for amorphous decompressed from (an amorphous phase) to ambient pressure. Horizontal arrows indicate the frequency differences of TA- and TO-like phonon bands between a-Ge and amorphous .
Summary of high-pressure phase transitions of , (Refs. 20 and 21), (Ref. 38), and (Refs. 17 and 40) at room temperature. It is noted that only type-I clathrate shows the isostructural phase transition at about 15 GPa.
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