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Phys. Rev. B 74, 245209 (2006) [6 pages]

Pressure-induced phase transformations in the Ba8Si46 clathrate

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L. Yang1, Y. M. Ma2, T. Iitaka3, J. S. Tse4,*, K. Stahl5, Y. Ohishi6, Y. Wang1, R. W. Zhang1, J. F. Liu1, H.-K. Mao1,7, and J. Z. Jiang1,†
1Laboratory of New-Structured Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, People’s Republic of China
2National Laboratory of Superhard Materials, Jilin University, Qianwei road 10, Changchun, 130012, People’s Republic of China
3Computational Astrophysics Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama, 351-0198 Japan
4Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Canada S7N 5E2
5Department of Chemistry, Building 207, Technical University of Denmark, DK-2800 Lyngby, Denmark
6SPring8, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan
7Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington DC 20015, USA

Received 18 April 2006; published 13 December 2006

The nature of isostructural transformations of a type-I Ba8Si46 clathrate has been studied by in situ high-pressure angle-dispersive x-ray powder diffraction using liquid He as pressure transmitting medium. The good quality of the diffraction data permitted refinement of structural and thermal parameters from Rietveld analysis. The results show that the first transition at 7 GPa is caused by the displacement of the Ba atoms in the Si24 cages. The cause of the second transition at 15 GPa, characterized by a dramatic reduction of cell volume, is not as clear. Theoretical calculations predicted an electronic topological transition of Fermi surface at this pressure. Analysis of the anomalously large Si thermal parameters suggested a highly disordered Si framework. This disordering is probably static and may be due to the presence of Si vacancies. The latter hypothesis is supported by electronic calculations on model disordered systems.

© 2006 The American Physical Society

URL:
http://link.aps.org/doi/10.1103/PhysRevB.74.245209
DOI:
10.1103/PhysRevB.74.245209
PACS:
61.50.Ks, 64.70.Kb, 71.20.−b

*Author to whom correspondence should be addressed. Email address: jst634@mail.usask.ca

†Author to whom correspondence should be addressed. Email address jiangjz@zju.edu.cn

 
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