Pressure-induced homothetic volume collapse in silicon clathrates

Abstract

The high-pressure properties of the Ba-doped silicon clathrate Ba8Si46 have been investigated combining X-ray diffraction and X-ray absorption spectroscopy. A pressureinduced isostructural phase transition associated with an important volume collapse takes place at 11.5-14 GPa. This transformation is characterized by the homothetic contraction of the silicon cages containing the Ba atoms. This transition is preceded by a change in the electronic structure at 5 GPa in good agreement with Raman spectroscopy observations (T. Kume et al., Phys. Rev. Lett., 90 (2003) 155503) that it is also of isostructural nature. The cage structure is preserved through the phase transitions allowing to obtain tetrahedral silicon with record interatomic distances as low as 2.13 Å. At the highest studied pressure of 49 GPa, the structure becomes irreversibly amorphous. The physical origin of the homothetic isostructural transitions is discussed.

Pressure-induced phase transformations in theBa8Si46clathrate

“…Although the large observed volume compression was not reproduced, the calculated trends of the atomic coordinates are in agreement with the experimental data reported here and in a previous study. 18 Specifically, the lack of pressure dependence of the x coordinate of Si16i and the y coordinate of Si24k and the slight increase of the z coordinate of Si24k above 20 GPa are reproduced correctly. The theoretical phonon band structures of Ba 8 Si 46 at 0, 7, 35, and 42 GPa are shown in Fig.…”

“…This is confirmed by the analysis of the Ba L I,III near edge x-ray absorption spectra which show an increase in the intensity due to p → d transitions. 18 Although the theory succeeded in predicting an anomaly, the predicted change in the cell parameter is much smaller than the observed. To explain this discrepancy, other possible mechanisms must be considered.…”

“…The more positively charged Ba atoms is consistent with the observed 0.4 eV shift to high binding energy of the Ba L II,III edge. 18 Around 7 GPa the Raman peaks at 50-60 cm −1 was reported to be missing. 15 Theoretical calculations have shown that this Raman band is entirely due to the vibrations of Ba located in the large cage ͑i.e., Ba6d͒.…”

“…15͒ measurements on Ba 8 Si 46 , however, is not as clear. A more recent XRD and x-ray absorption study 18 attributed the transition to the rehybridization of the Ba atoms. This suggestion, however, does not explain the large volume reduction accompanying the phase transition.…”

Electronic structures and mechanical properties of boron and boron-rich crystals (Part 2)

“…If there is no such phase near the original phase, or if the temperature is so low that the atomic relaxation is too slow, amorphization occurs. In fact, irreversible transformation to amorphous phases was observed in clathrate compounds: Ba 8 Si 46 (at 40 GPa) [37], K 8 Si 46 (32 GPa) [38], and Ba 24 Si 100 (23 GPa) [39]. The amorphization of β-rhombohedral boron can be classified as this category with a higher pressure.…”