Structural characterisation of the high-pressure phase CaAl4Si2O11

Single crystals of CaAl4Si2O11 were synthesised at 1,500 degrees C and 14 G Pa in a multi-anvil press, and the structure of the phase determined by sin gle-crystal X-ray diffraction at room conditions. The structure-type is tha t of the "hexagonal barium ferrites". The space group of the average struct ure is P6(3)/mmc and the cell parameters are a = 5.4223(4) Angstrom, c = 12 .7041(6) Angstrom, V = 323.28(5) Angstrom(3), with Z = 2, and its density i s 3.905 g cm(-3) which is reasonable for a high-pressure alumino-silicate p hase. The 22 oxygen and two calcium atoms within the unit-cell form an appr oximate hexagonal-close-packed array. Ten of the twelve octahedral intersti ces within this array that have only oxygen atoms for apices are filled wit h Si and/or Al. Ml octahedra share edges to form a spinel-like sheet of oct ahedra. The average bond length <M1-0> = 1.833 Angstrom suggests mixed occu pancy by Si and Al. The Ml octahedral sheets are linked by shared corners t o pairs of face-sharing M2 octahedra containing Al, with <M2-O> = 1.918 Ang strom. The remaining two cations of the unit-cell contents statistically oc cupy four tetrahedrally-coordi nated interstices, which occur as face-shari ng pairs. The average bond length for these sites (1.742 Angstrom) 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 o ccupied, with the possibility of some short-range ordering of such occupanc ies. 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 bei ng comprised of merohedral (0 0 0 1) twins of P6(3)mc symmetry.