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Published online on May 1, 2008, 10.1073/pnas.0711925105
PNAS | May 6, 2008 | vol. 105 | no. 18 | 6526-6530


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PHYSICAL SCIENCES / GEOPHYSICS
Prediction of an U2S3-type polymorph of Al2O3 at 3.7 Mbar

Koichiro Umemoto* and Renata M. Wentzcovitch

Minnesota Supercomputing Institute and Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, MN 55455

Edited by Ho-kwang Mao, Carnegie Institution of Washington, Washington, DC, and approved February 27, 2008 (received for review December 18, 2007)

We predict by first principles a phase transition in alumina at {approx}3.7 Mbar and room temperature from the CaIrO3-type polymorph to another with the U2S3-type structure. Because alumina is used as window material in shock-wave experiments, this transformation should be important for the analysis of shock data in this pressure range. Comparison of our results on all high-pressure phases of alumina with shock data suggests the presence of two phase transitions in shock experiments: the corundum to Rh2O3(II)-type structure and the Rh2O3(II)-type to CaIrO3-type structure. The transformation to the U2S3-type polymorph is in the pressure range reached in the mantle of recently discovered terrestrial exoplanets and suggests that the multi-megabar crystal chemistry of planet-forming minerals might be related to that of the rare-earth sulfides.

alumina | first-principles calculation | high-pressure phase transition | postperovskite | rare-earth sulfide structure

Author contributions: K.U. designed research; K.U. performed research; K.U. and R.M.W. analyzed data; and K.U. and R.M.W. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/cgi/content/full/0711925105/DCSupplemental.

{dagger} The NdYbS3-type structure was first reported to have the B2212 symmetry which is a subgroup of the Cmcm group of the CaIrO3-type structure (42, 43). However the NdYbS3-type structure refined in the B2212 symmetry is nearly indistinguishable from the CaIrO3-type structure. The difference between them is very tiny; in the B2212 NdYbS3-type structure, the cation at the B site and one kind of sulfur are just slightly shifted from the 4a and 8f Wyckoff positions in the Cmcm CaIrO3-type structure. A recent x-ray diffraction study suggested the NdYbS3-type structure should have the Cmcm symmetry and be the same as the CaIrO3-type structure (44).

*To whom correspondence should be addressed. E-mail: umemoto{at}cems.umn.edu

© 2008 by The National Academy of Sciences of the USA


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