Structural characterisation of the high-pressure phase CaAl4Si2O11
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Single crystals of CaAl4Si2O11 were synthesised at 1,500 °C and 14 GPa in a multi-anvil press, and the structure of the phase determined by single-crystal X-ray diffraction at room conditions. The structure-type is that of the “hexagonal barium ferrites”. The space group of the average structure is P6 3 /mmc and the cell parameters are a = 5.4223(4) Å, c = 12.7041(6) Å, V = 323.28(5) Å3, with Z = 2, and its density is 3.905 g cm−3, which is reasonable for a high-pressure alumino-silicate phase. The 22 oxygen and two calcium atoms within the unit-cell form an approximate hexagonal-close-packed array. Ten of the twelve octahedral interstices within this array that have only oxygen atoms for apices are filled with Si and/or Al. M1 octahedra share edges to form a spinel-like sheet of octahedra. The average bond length <M1-O> = 1.833 Å suggests mixed occupancy by Si and Al. The M1 octahedral sheets are linked by shared corners to pairs of face-sharing M2 octahedra containing Al, with <M2-O> = 1.918 Å. The remaining two cations of the unit-cell contents statistically occupy four tetrahedrally-coordinated interstices, which occur as face-sharing pairs. The average bond length for these sites (1.742 Å) suggests that they are occupied by Al, although Si occupancy cannot be excluded by the data. It is proposed that only one interstice of each pair is locally occupied, with the possibility of some short-range ordering of such occupancies. Complete long-range order leading to the acentric space group P6 3 mc is excluded by the data, as is the possibility of the average structure being comprised of merohedral (0 0 0 1) twins of P6 3 mc symmetry.
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