Coso4 Center Dot H2o And Its Continuous Transition Compared To The Compression Properties Of Isostructural Kieserite-Type Polymorphs

The kieserite-type compound cobalt(II) sulfate monohydrate, CoSO4 center dot H2O, has been investigated under isothermal (T = 295 K) hydrostatic compression up to 10.1 GPa in a diamond anvil cell by means of single-crystal X-ray diffraction and Raman spectroscopy. The monoclinic alpha-phase (space group C2/c) undergoes a second-order ferroelastic phase transition at P-c = 2.40(3) GPa to a triclinic beta-phase (space group P (1) over bar). Lattice elasticities derived from fitting third-order Birch-Murnaghan equations of state to the pressure dependent unit-cell volume data yield V-0 = 354.20(6) angstrom(3), K-0 = 53.0(1.7) GPa, K-0 = 5.7(1.8) for the alpha-phase and V-0 = 355.9(8) angstrom(3), K-0 = 45.2(2.6) GPa, K-0 = 6.6(6) for the beta-phase. Crystal structure data of the high-pressure polymorph were determined at 2.98(6) and 4.88(6) GPa. The most obvious structural feature and thus a possible driving mechanism of the phase transition, is a partial rearrangement in the hydrogen bonding system. However, a comparative analysis of pressure-induced changes in the four kieserite-type compounds investigated to date suggests that the loss of the point symmetry 2 at the otherwise rather rigid SO4 tetrahedron, allowing symmetrically unrestricted tetrahedral rotations and edge tiltings in the beta-phase, could be the actual driving mechanism of the phase transition.