δ-Al(OD)3 powders were synthesized from Al(OD)3 bayerite at 4 GPa and 523 K using a cubic press apparatus. Neutron powder diffraction analyses of δ-Al(OD)3 at ambient conditions revealed that the crystals are orthorhombic with space group P212121, not Pnma as reported previously based on X-ray diffraction data. The P212121 δ-Al(OH)3 structure contains seven independent atoms in the asymmetric unit, including one Al, three O, and three H atoms. The initial lattice parameters and the atomic positions of both Al and O were taken from previous X-ray structural analyses of the Pnma structure, while the positions of H were determined in the present study using ab initio calculations to (1) give the least energy among trial structural models for P212121 δ-Al(OH)3, (2) accurately reproduce the measured lattice parameters of δ-Al(OD)3, and (3) show reasonable energetic relations between the Al(OH)3 polymorphs; namely, gibbsite is stable at ambient pressure, δ-Al(OH)3 has the lowest enthalpy at pressure greater than 1.1 GPa, and both bayerite and η-Al(OH)3 are metastable over the entire pressure range. Furthermore, we found that the structure of δ-Al(OH)3 obtained from ab initio calculations is in good agreement with that derived from a Rietveld refinement of δ-Al(OD)3, based on the present powder neutron diffraction data. The δ-Al(OH)3 structure possesses one relatively long and two short O-H···O hydrogen bonds. Ab initio calculations also reveal that δ-Al(OH)3 with space group P212121 transforms to another high-pressure polymorph with space group Pnma at around 67 GPa, and that the two short hydrogen bonds in δ-Al(OH)3 become both symmetric through the P212121 to Pnma transformation, in which the protons are located at the midpoints of the O···O hydrogen bonds.

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