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Crystal structure of CaMg₂Al₆O₁₂, a new Al-rich high pressure form

¹ Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10,W8, Kita-ku, Sapporo 060-0810, Japan
² Department of Chemistry,Gakushuin University, Mejiro, Toshima-ku, Tokyo 171-8588, Japan
³ Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106-8666, Japan

Correspondence: ^* E-mail: hiro{at}ep.sci.hokudai.ac.jp

The crystal structure of CaMg₂Al₆O₁₂, a new high-pressure phase synthesized at 21.8 GPa and 1200 °C using a multi-anvil apparatus, was solved by a newly developed structure program and refined by Rietveld analysis of the powder X-ray diffraction profile. The structure is hexagonal with cell constants a = 8.7616(2) and c = 2.7850(1) Å, and space group P6₃/m. The structure of this phase contains double chains of edge shared AlO₆ octahedra running along the c axis. Three double chains share corners to form sixfold positions in which octahedrally coordinated Mg atoms reside. The large Ca atoms are randomly distributed at ninefold sites with half-occupancy in the hexagonal tunnel. Previously reported Al-rich silicate phases could possibly have the same structure. This structure could thus qualify as one of the possible major host phases for aluminum in the lower mantle.

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