CoSO₄·H₂O and its continuous transition compared to the compression properties of isostructural kieserite-type polymorphs

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.

X-ray diffraction reveals two structural transitions in szomolnokite

“…DOI: https://doi.org/10.2138/am-2022-8147. http://www.minsocam.org/ Revision 1 Pardo et al, 2022 American Mineralogist to occur in the Mg-endmember kieserite (MgSO 4 H 2 O) at lower pressures (Meusburger et al 2020), the Ni-endmember dworknikite (NiSO 4 H 2 O) (Ende et al 2020), and the Co-endmember (CoSO 4 H 2 O) (Wildner et al 2021).…”