High-pressure transitions in MgAl 2O 4 and a new high-pressure phase of Mg 2Al 2O 5

Abstract

Phase transitions in MgAl 2O 4 were examined at 21–27 GPa and 1400–2500 °C using a multianvil apparatus. A mixture of MgO and Al 2O 3 corundum that are high-pressure dissociation products of MgAl 2O 4 spinel combines into calcium–ferrite type MgAl 2O 4 at 26–27 GPa and 1400–2000 °C. At temperature above 2000 °C at pressure below 25.5 GPa, a mixture of Al 2O 3 corundum and a new phase with Mg 2Al 2O 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 2Al 2O 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) Å, b=12.1952(6) Å, c=2.7916(2) Å, V=319.03(3) Å 3, Z=4. The structure consists of edge-sharing and corner-sharing (Mg, Al)O 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 6 octahedra. The structure is related with that of ludwigite (Mg, Fe 2+) 2(Fe 3+, Al)(BO 3)O 2. The molar volume of the Mg 2Al 2O 5 phase is smaller by 0.18% than sum of molar volumes of 2MgO and Al 2O 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 2O 4 among various A 2+B 3+ 2O 4 compounds.