High-pressure behaviour of lawsonite: a phase transition at 8.6 GPa

The structural behaviour of lawsonite CaAl2Si2O7(OH)2 . H2O, has been studi ed under quasi-hydrostatic conditions in a diamond-anvil cell to 18 GPa at room temperature, using angle-dispersive X-ray powder diffraction and Raman spectroscopy. With increasing pressure, we observe a phase transition at P = 8.6(3) GPa, characterized by (1) the splitting of diffraction lines, (2) the emergence of new Raman bands, and (3) significant changes in the frequ ency shifts of the hydroxyl O-H stretching modes. The transition is displac ive and fully reversible, without any detectable hysteresis. The high-press ure phase, referred to here as lawsonite III can be indexed into a monoclin ic unit-cell, with a = 5.6833(3), b = 8.5944(4), c = 12.8773(5) Angstrom, g amma= 91.42(4)degrees and V= 628.80(4) Angstrom(3) at P = 10.6 GPa. The spa ce-group of lawsonite III is likely to be C112(1)/m which is the unconventi onal representation of P2(1)/m related to the low-pressure Cmcm symmetry. A ssuming the change in space-group from Cmcm to C112(1)/m, the resulting com ponents of the spontaneous strain tensor are analyzed in terms of the chang e in point group from mmm to 2/m. The pressure-dependence of the fourth pow er of the symmetry-breaking component e(6)(4) is linear, indicating a tricr itical character for the transition. Both X-ray diffraction and Raman spect roscopy indicate that the overall aluminosilicate framework of lawsonite is retained through the transition. However, monoclinic lawsonite III is simi lar to 40% less compressible than the low-pressure orthorhombic polymorph. The Raman spectroscopic results, in good agreement with recent infrared one s reveal that this decrease in compressibility is likely to be related to t he increase in the hydrogen bond strength involving the hydroxyl groups in the structure, whereas the hydrogen bond system around the water molecule d oes not appear to be modified at the transition.