High-pressure transitions in MgAl{sub 2}O{sub 4} and a new high-pressure phase of Mg{sub 2}Al{sub 2}O{sub 5}
- Department of Chemistry, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588 (Japan)
- Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10, W8, Kita-ku, Sapporo 060-0810 (Japan)
- Advanced Materials Laboratory, National Institute of Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan)
Phase transitions in MgAl{sub 2}O{sub 4} were examined at 21-27 GPa and 1400-2500 deg. C using a multianvil apparatus. A mixture of MgO and Al{sub 2}O{sub 3} corundum that are high-pressure dissociation products of MgAl{sub 2}O{sub 4} spinel combines into calcium-ferrite type MgAl{sub 2}O{sub 4} at 26-27 GPa and 1400-2000 deg. C. At temperature above 2000 deg. C at pressure below 25.5 GPa, a mixture of Al{sub 2}O{sub 3} corundum and a new phase with Mg{sub 2}Al{sub 2}O{sub 5} composition is stable. The transition boundary between the two fields has a strongly negative pressure-temperature slope. Structure analysis and Rietveld refinement on the basis of the powder X-ray diffraction profile of the Mg{sub 2}Al{sub 2}O{sub 5} phase indicated that the phase represented a new structure type with orthorhombic symmetry (Pbam), and the lattice parameters were determined as a=9.3710(6) A, b=12.1952(6) A, c=2.7916(2) A, V=319.03(3) A{sup 3}, Z=4. The structure consists of edge-sharing and corner-sharing (Mg, Al)O{sub 6} octahedra, and contains chains of edge-sharing octahedra running along the c-axis. A part of Mg atoms are accommodated in six-coordinated trigonal prism sites in tunnels surrounded by the chains of edge-sharing (Mg, Al)O{sub 6} octahedra. The structure is related with that of ludwigite (Mg, Fe{sup 2+}){sub 2}(Fe{sup 3+}, Al)(BO{sub 3})O{sub 2}. The molar volume of the Mg{sub 2}Al{sub 2}O{sub 5} phase is smaller by 0.18% than sum of molar volumes of 2MgO and Al{sub 2}O{sub 3} corundum. High-pressure dissociation to the mixture of corundum-type phase and the phase with ludwigite-related structure has been found only in MgAl{sub 2}O{sub 4} among various A{sup 2+}B{sup 3+}{sub 2}O{sub 4} compounds. - Graphical abstract: MgAl{sub 2}O{sub 4} dissociates into a mixture of Al{sub 2}O{sub 3} corundum and a new high-pressure phase of Mg{sub 2}Al{sub 2}O{sub 5} at high pressures and temperatures. The Mg{sub 2}Al{sub 2}O{sub 5} phase represents a new structure type with orthorhombic symmetry, showing the projection to (0 0 1) plane. The structure consists of edge- and corner-shared (Mg, Al)O{sub 6} octahedra, and contains chains of edge-shared octahedra running along the c-axis.
- OSTI ID:
- 21212204
- Journal Information:
- Journal of Solid State Chemistry, Vol. 182, Issue 2; Other Information: DOI: 10.1016/j.jssc.2008.11.015; PII: S0022-4596(08)00577-X; Copyright (c) 2008 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA); ISSN 0022-4596
- Country of Publication:
- United States
- Language:
- English
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