Space group and hydrogen sites of δ-AlOOH and implications for a hypothetical high-pressure form of Mg(OH)2

The space group and hydrogen positions of delta-(Al0.84Mg0.07Si0.09)OOH are investigated using a single crystal synthesized using a multi-anvil apparatus under conditions of 1000degreesC and 21 GPa. The space group determined by single-crystal X-ray diffraction is to Pnn2, with unit-cell parameters of a=4.6975(8) Angstrom, b= 4.2060(6) Angstrom, c=2.8327(4) Angstrom, and V=55.97(1) Angstrom(3). Partial occupancy of the Al site by Mg and Si suggests the possibility of a limited solid solution between delta-AlOOH, stishovite, and a hypothetical CaCl2-type Mg(OH)(2) that is 16% denser than brucite. Difference-Fourier maps reveal two small but significant Fourier peaks attributable to hydrogen atoms. Atomic distances and angles around the first peak indicate a hydrogen bond with O...O distances of 2.511 Angstrom, while those around the second peak are suggestive of a bifurcated hydrogen bond with O...O distances of 2.743 and 2.743 Angstrom.