Observation of pressure-induced phase transition of δ-AlOOH by using single-crystal synchrotron X-ray diffraction method

Pressure-induced phase transition of delta-AlOOH was confirmed between 6.1 and 8.2 GPa by using a single-crystal synchrotron X-ray diffraction method. The phase transition is reversible and unquenchable. Results from analysis of the distribution of X-ray diffraction intensities at 8.2 GPa reveal an additional systematic, absence of k + l odd for 0kl in comparison with h + l odd for h0l observed prior to the phase transition (space group, P2(1) nm). The space group of the post-transition phase should be Pnnm or Pnn2 to satisfy the systematic absence rule. Crystal structure refinements of the post-transition phase conducted for the three models (Pnnm, Pnn2, and P2(1) nm) indicate that the space group of the post-transition phase is Pnnm. The O-O distance of hydrogen bond in the post-transition phase at 8.2 GPa is 2.439(6) a"<< and is significantly longer than the predicted distance (2.366 a"<<) of the hydrogen bond symmetrization in delta-AlOOH. The H distribution in the post-transition phase would display a fully disordered hydrogen bond pattern.