Re-examination of the hydrogarnet structure at high pressure using neutron powder diffraction and infrared spectroscopy

Time-of-flight neutron powder data and synchrotron infrared absorption spectra were collected for katoite hydrogamet [Ca3Al2(O4D4)(3)] at pressures to 9.4 and 9.8 GPa, respectively. The phase transition from space group Ia3d to I43d was observed in the neutron spectrum at similar to 7.5 GPa, as indicated by the presence of two weak reflections (730 and 530) that violate the hkl conditions (hk0, h not equal 2n) imposed by the a-glide operation. However, attempts to refine the high-pressure structure in space group I43d did not significantly improve the fit and produced a chemically unreasonable O-D bond length at the second D position. Structure refinements in Ia3d indicate that (1) the O-D bond length, corrected for the effects of thermal motion, remains essentially constant (similar to 0.95 angstrom) with increasing pressure; (2) hydrogen bond lengths shorten with increasing pressure; however, the variation in O-(DO)-O-... angles indicates a preferential strengthening of H bonds; and (3) the compression mechanism is characterized by bond shortening rather than bond bending. The new results are in excellent agreement with both high-pressure X-ray diffraction experiments and ab initio calculations, and illustrate the need to eliminate peak broadening in high-pressure neutron powder experiments. IR spectra collected for the same sample showed discontinuities in both O-H and O-D vibrational frequencies at similar to 5 GPa, suggesting that deuteration does not significantly affect the pressure of the transition. The higher pressure observed for the transition in the neutron data is probably due to lower signal-to-noise levels, which mask the weaker, symmetry-forbidden reflections at lower pressure.