Abstract
Single crystals of synthetic nickel sulfate monohydrate, α-NiSO4·H2O (space-group symmetry C2/c at ambient conditions), were subject to high-pressure behavior investigations in a diamond-anvil cell up to 10.8 GPa. By means of subtle spectral changes in Raman spectra recorded at 298 K on isothermal compression, two discontinuities were identified at 2.47(1) and 6.5(5) GPa. Both transitions turn out to be apparently second order in character, as deduced from the continuous evolution of unit-cell volumes determined from single-crystal X-ray diffraction. The first structural transition from α- to β-NiSO4·H2O is an obvious ferroelastic C2/c–P1̅ transition. It is purely displacive from a structural point of view, accompanied by symmetry changes in the hydrogen-bonding scheme. The second β- to γ-NiSO4·H2O transition, further splitting the O2 (hydrogen bridge acceptor) position and violating the P1̅ space-group symmetry, is also evident from the splitting of individual bands in the Raman spectra. It can be attributed to symmetry reduction through local violation of local centrosymmetry. Lattice elasticities were obtained by fitting second-order Birch–Murnaghan equations of state to the p-V data points yielding the following zero-pressure bulk moduli values: K0 = 63.4 ± 1.0 GPa for α-NiSO4·H2O, K0 = 61.3 ± 1.9 GPa for β-NiSO4·H2O, and K0 = 68.8 ± 2.5 GPa for γ-NiSO4·H2O.
Highlights
Hydrated sulfate salts and their behavior under extreme conditions are the subject of intensive research, as they are widespread on surfaces of planets, their moons, and presumably even exoplanets.[1−6] The extreme temperature (T) oscillations on planetary surfaces and the extraordinary pressures (p), along with high-p/high-T variations inside the icy satellites, may lead to structural instabilities and phase transitions
The Ni-enrichment in solid phases[5] in the ternary system NiSO4−MgSO4−H2O together with high Ni concentrations in the sulfate-rich soluble fraction of natural meteorite samples[13] emphasize the particular importance of nickel sulfate components, which are known as the rare terrestrial hydrate minerals morenosite (n = 7), retgersite (n = 6), and dwornikite (n = 1).[14]
The structure type has already been determined in space group C2/c by Leonhard and Weiss.[23]
Summary
Hydrated sulfate salts and their behavior under extreme conditions are the subject of intensive research, as they are widespread on surfaces of planets, their moons, and presumably even exoplanets.[1−6] The extreme temperature (T) oscillations on planetary surfaces and the extraordinary pressures (p), along with high-p/high-T variations inside the icy satellites, may lead to structural instabilities and phase transitions This may include changes of the hydration state, as exemplified by the p-induced dehydration reactions and described for the MgSO4·nH2O (0 ≤ n ≤ 11) series.[7−10] it can involve incongruent melting[11] or polymorphism with associated structural phase transitions.[12] The Ni-enrichment in solid phases[5] in the ternary system NiSO4−MgSO4−H2O together with high Ni concentrations in the sulfate-rich soluble fraction of natural meteorite samples[13] emphasize the particular importance of nickel sulfate components, which are known as the rare terrestrial hydrate minerals morenosite (n = 7), retgersite (n = 6), and dwornikite (n = 1).[14]. Due to the site symmetry of the Ow atoms on this position, the hydrogen bonding system exhibits 2-fold symmetry with two Ow-H···O hydrogen bonds of medium strength,[15,24,25] as well as two further intermolecular O···O
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