The baddeleyite-type high pressure phase of Ca(OH)(2)

A phase transition from Ca(OH)(2) I (portlandite) to Ca(OH)(2) II at high pressure and temperature has been confirmed, using in situ x-ray diffraction in a multianvil high pressure device (DIA). The structure was determined at 9.5 GPa and room temperature from data collected after heating the sample al 300 degrees C at 7.2 GPa in a diamond anvil cell. Both the Le Bail fit and preliminary Rietveld refinement suggest that the new phase, which reverts to Ca(OH)(2) I during pressure release, has a structure related to that of baddeleyite (ZrO1); it is monoclinic (P2(1)/c) with a = 4.887(2), b = 5.834(2), c = 5.587(2), beta = 99.74(2)degrees. The coordination number of Ca increases from six to seven (5 + 2) across the transition. At 500 degrees C, the phase boundary is bracketed at 5.7 +/- 0.4 GPa by reversal experiments performed in the DIA.