Compression mechanism and amorphization of portlandite, Ca(OH)2: structural refinement under pressure
In order to investigate compression mechanism and the pressure-induced amorphization of portlandite, Ca(OH)2, the crystal structure has been refined up to 9.7GPa using Rietveld analysis. Angular-dispersive synchrotron X-ray powder diffraction experiments were performed using a diamond anvil cell and an imaging plate at BL-18C in the Photon Factory at KEK, Japan. Compression behavior is highly anisotropic and the c axis is approximately 2.5 times as compressible as the a axis (βa=0.004, βc=0.011GPa-1). Because the refined fractional coordinate, z, of the O atom increases linearly with pressure, compression along the c axis is due to the shortening of the interlayer spacing. The compression mechanism shows no change up to the amorphization pressure and is basically the same as that of brucite, Mg(OH)2, observed below 10 GPa. The octahedral regularity of CaO6 approaches a regular configuration with pressure. The interlayer O...O distance is expected to be about 2.75 Å at the amorphization pressure and should affect hydrogen bonding.