Abstract
Abstract The kieserite-type compound cobalt(II) sulfate monohydrate, CoSO4·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 α-phase (space group C2/c) undergoes a second-order ferroelastic phase transition at P c = 2.40(3) GPa to a triclinic β-phase (space group P 1 ‾ $‾{1}$ ). 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) Å3, K 0 = 53.0(1.7) GPa, K′ = 5.7(1.8) for the α-phase and V 0 = 355.9(8) Å3, K 0 = 45.2(2.6) GPa, K′ = 6.6(6) for the β-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 β-phase, could be the actual driving mechanism of the phase transition.
Highlights
Hydrated sulfates and their properties have most recently gained increased interest after reports of extraterrestrial occurrences of individual representatives, for example on the surface of Mars or Jupiter’s and Saturn’s icy moons [1,2,3,4,5,6,7,8,9,10]
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 β-phase, could be the actual driving mechanism of the phase transition
Selected crystals of CoSO4·H2O were loaded into ETH-type [21] diamond-anvil cells (DAC) equipped with either standard brilliant-cut or Böhler Almax-type [22] type-I diamond anvils
Summary
Hydrated sulfates and their properties have most recently gained increased interest after reports of extraterrestrial occurrences of individual representatives, for example on the surface of Mars or Jupiter’s and Saturn’s icy moons [1,2,3,4,5,6,7,8,9,10] This includes the monoclinic kieserite-type monohydrate salts α-M(II)SO4·H2O (space group C2/c) of Mg (i.e. kieserite) and divalent transition metal cations and their intermiscibility within continuous solid solution series [11,12,13,14,15]. The recorded spectra were fitted with the GaussLorentz-area method after background subtraction achieved using the program Peakfit 4.1.2 (Systat Software Inc.)
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