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Change in compressibility of -AlOOH and -AlOOD at high pressure: A study of isotope effect and hydrogen-bond symmetrization

¹ Department of Earth and Planetary Materials Science, Tohoku University, Sendai 980-8578, Japan
² Institute for Solid State Physics, The University of Tokyo, Kashiwa 277-8581, Japan
³ Geochemical Laboratory, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan
⁴ Department of Natural History of Sciences, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
⁵ National Institute for Materials Science, Tsukuba 305-0044, Japan

The compression behaviors of -AlOOH and -AlOOD were investigated under quasi-hydrostatic conditions at pressures up to 63.5 and 34.9 GPa, respectively, using results from synchrotron X-ray diffraction experiments conducted at ambient temperature. Because of the geometric isotope effect, at ambient pressure, the a and b axes of -AlOOD, which define the plane in which the hydrogen bond lies, are longer than those of -AOOH. Under increasing pressure, the a and b axes of -AlOOH stiffen at 10 GPa, although the c axis shows no marked change. Identical behavior was found in -AlOOD, but the change in compressibility was observed at a slightly higher pressure of 12 GPa. Axial ratios a/c and b/c first decrease rapidly with increasing pressure, then begin to increase at pressures >10 GPa in -AlOOH and >12 GPa in -AlOOD. At these pressures, the pressure dependence of a/b also changes from increasing to decreasing. The unit-cell volumes of -AlOOH and -AlOOD become slightly less compressible at high pressures. Assuming K₀' = 4, the calculated bulk moduli of -AlOOH below and above 10 GPa are 152(2) and 219(3) GPa, respectively. Those of -AlOOD below and above 12 GPa are 151(1) and 207(2) GPa, respectively.

Key Words: -AlOOH • -AlOOD • hydrogen bond • symmetrization • high pressure • isotope effect