High-Pressure Behavior of Nickel Sulfate Monohydrate: Isothermal Compressibility, Structural Polymorphism, and Transition Pathway

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.


■ INTRODUCTION
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 MgSO 4 ·nH 2 O (0 ≤ n ≤ 11) series. 7−10 However, it can also involve incongruent melting 11 or polymorphism with associated structural phase transitions. 12 The Ni-enrichment in solid phases 5 in the ternary system NiSO 4 −MgSO 4 −H 2 O 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 also known as the rare terrestrial hydrate minerals morenosite (n = 7), retgersite (n = 6), and dwornikite (n = 1). 14 The monohydrates of divalent cations have received particular attention, since they apparently represent considerably stable solids. They have been observed on Mars and Jovian moons Ganymede and Callisto, preferably as the monohydrated magnesium sulfate mineral kieserite. Their large structural flexibility leads to the existence of various endmember representatives (M 2+ SO 4 ·H 2 O with M 2+ = Mg, Mn, Fe, Co, Ni, Zn) 15,16 and their solid solutions. 17−19 This also includes selenates, 20 and in rare cases even phosphates such as Mn 3+ PO 4 ·H 2 O. 21,22 The structure type has already been determined in space group C2/c by Leonhard and Weiss. 23 It can be described as a three-dimensional heterodesmic framework, which consists of continuous octahedral chains of corner-sharing [MO 4 (H 2 O) 2 ] 6− units running along the [001] direction, and which are cross-linked through the tetrahedral SO 4 units. Within the given C2/c symmetry, the M and S atoms each occupy a single crystallographic site. The barycenter of the water molecule (Ow) is also located on a unique site. 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 contacts, not involved in the H-bonding scheme under ambient conditions. Apart from the kieserite aristotype compounds with C2/c symmetry, CdSO 4 ·H 2 O was found to crystallize in a different monoclinic space-group symmetry, 26 i.e., in the maximal nonisomorphic subgroup P2 1 /n, which was attributed to the relatively larger size of the Cd 2+ atoms located on the octahedral site. Moreover, the Jahn−Teller effect of Cu 2+ was found to be responsible for a cooperative lattice distortion resulting in the triclinic space group P1̅ in CuSO 4 ·H 2 O. 16,25 Recent investigations on MgSO 4 ·H 2 O and FeSO 4 ·H 2 O under hydrostatic compression revealed a ferroelastic phase transition following the translationsgleiche C2/c-P1̅ group−subgroup relation and being apparently second order in its thermodynamic character. 27,28 Any of the symmetry changes follow a pattern of atom displacements related to the changes within the hydrogen bonding systems, whereas the topology of the heterodesmic frameworks remains unchanged, apart from distortion originating from symmetry breaking. In this context, synthetic NiSO 4 ·H 2 O single crystals are investigated in this study with respect to the occurrence of an equivalent structural polymorphism. In particular, we examine how the critical size and the electronic configuration of the cations on the M site influence the stability of the low-symmetry polymorphs to better understand the mechanism and pathway of structural transitions. Investigation of the high-p behavior of the Ni end member of this type of sulfate is essential from the cosmochemical point of view, given the potential presence and distribution of hydrated sulfates inside icy planetary bodies.

■ MATERIALS AND METHODS
Synthesis. Sample crystals were grown under low hydrothermal conditions using a similar technique to that described by Talla and Wildner 29 within a temperature range between room temperature and 210°C at moderate autogenous vapor pressures using polytetrafluoroethylene (PTFE)-lined autoclaves. Subeuhedral crystals formed from a supersaturated aqueous solution of sulfuric acid (∼0.25−0.75 mL concentrated H 2 SO 4 (98%) + ∼0.5 mL H 2 O) added to ∼0.3 g of NiSO 4 ·7H 2 O (Alfa Aesar, p.a.) inside the PTFE reaction chambers with ∼3−4 cm 3 volume capacity. It was kept at T = 220°C for 14 days and then cooled down to RT. The solid products were finally washed with pure H 2 O and ethanol and then dried overnight at T = 75°C (cf. Supporting Information for more data).
High-Pressure Sample Environment. Selected crystals were loaded into ETH-type 30 diamond-anvil cells (DAC) equipped with either standard brilliant-cut or Boḧler Almax-type 31 type-I diamond anvils using culet faces of ⌀ = 600 μm. Pressure chambers with ⌀ ≈ 250 μm were prepared using stainless steel gaskets preindented to ∼90−100 μm thickness. A 4:1 methanol−ethanol mixture was used as a p-transmitting medium for the single-crystal X-ray diffraction (scXRD) investigations, while in situ vibrational spectroscopy was carried out on crystals pressurized in cryogenically loaded argon. Pressures were determined using conventional quartz and ruby standards, following the calibration of Scheidl et al. 32 and Jacobsen et al., 33 respectively. The estimated standard deviations (ESDs) of the p values obtained by the internal quartz standard were determined from the ESDs of the measured p-dependent unit-cell volume (V). The ESDs of the p obtained from the R 1 -line shift of the ruby luminescence spectra were estimated to be ±0.06 GPa after averaging repeated measurements.
Raman Spectroscopy. In-situ high-p Raman spectroscopy was carried out on several crystals (40 × 60 × 15 μm 3 , 110 × 95 × 20 μm 3 , 80 × 80 × 40 μm 3 , 40 × 50 × 40 μm 3 ), together with ruby spheres as pressure calibrants, 34,35 all compressed in dense liquefied Ar. Spectra were collected by means of a confocal Horiba Jobin Yvon LabRAM-HR 800 spectrometer using a red He−Nd laser source emitting a beam with a wavelength of 632.8 nm and 20 mW, an Olympus BX41 microscope, and a mercury−cadmium−telluride detector. An Olympus LMPlanFL N 50× objective with a long working distance of 10.6 mm enabled focus of the laser beam onto the sample crystal inside the p chamber. A diffraction grating with 600 lines per millimeter was used for the measurements. High-p spectra were acquired using the software LabSpec 6 (HORIBA Scientific) in the spectral range from the 35 to 1190 cm −1 shift with an exposure time of 60 s and two accumulations. The recorded spectra were fitted with the Gauss-Lorentz-area method after background subtraction achieved using the program Peakf it 4.1.2 (Systat Software Inc.).
Precise Measurement of Lattice Parameters. Sample crystals were compressed in a conventional methanol−ethanol mixture together with a quartz single crystal serving as the p reference. XRD Bragg peak positions were measured on a Stoe AED II and Huber 5042 diffractometer using a conventional scintillation counter and nonmonochromatized Mo radiation, with the peak centering procedure being controlled through the SINGLE 36 software. For each hydrostatic p point, 16 to 23 individual Bragg reflections were measured in the 8-position centering mode 37 to refine the respective unit-cell volumes. Lattice parameters were refined in a first step without any symmetry constraints in order to identify possible deviations in metrics and were finally constrained to monoclinic symmetry, if applicable. The equations of state (EoS) were fitted by applying weighted least-squares techniques using the software EoSFit7. 38 Normalized pressure-strain analyses were carried out in order to determine the relevant order of truncation of the Birch−Murnaghan (BM) EoS.
High-Pressure Crystallography. The scXRD intensity data collections were performed on a sample crystal (∼140 × 70 × 40 μm 3 ) loaded in a methanol−ethanol mixture (see Supporting Information, Figure S1). The data sets were measured on a Stoe StadiVari X-ray diffractometer using a DECTRIS Pilatus 300 K detector with a 450 μm silicon layer and an air-cooled Incoatec IμS molybdenum high-brilliance microfocus tube source (50 kV, 1 mA, beam size ≈ 110 μm) at p = 0.6(1), 2.3(1), 4.5(1), and 8.4(1) GPa. Frames were collected with angular steps of 0.5°in ω rotational mode with the χ circle fixed to a value ranging from 0°to 90°. Data were collected on both sides of the DAC as achieved by a 180°rotation of the φ circle after each run. The exposure time was set to 100 s per frame. Intensities were integrated using the X-area 1.72 (STOE& Cie GmbH) software and corrected for absorption through the sample and DAC components by using the ABSORB 39 code. Indexing of the reconstructed reciprocal space yielded monoclinic lattice metrics for the measurements at 0.6(1) and 2.3(1) GPa, while at 4.5(1) and 8.4(1) GPa triclinic metrics were found. Initial positional parameters for the refinements were assigned according to those reported by Meusburger et al. 28 All refinements were carried out using scattering curves from the International Tables for Crystallography 40 and isotropic displacement parameters for all atoms using SHELXL 41 44 The high-p spectra reveal the typical average blueshifts with Δν ̅ /Δp values ranging between 2.42(7) and 7.45(8) cm −1 GPa −1 (Figure 3). The spectral evolution and the lack of drastic changes with respect to band positions, numbers, and intensities of modes suggest that the structural topology should be largely retained across the recorded p range. Nevertheless, subtle and more or less pronounced changes in the Δν ̅ /Δp rates can be observed, which suggests two intervals of potential structural changes, i.e., between 2.30(6) and 2.94(6) GPa ( Figure 1) and between 6.36(6) and 7.41(6) GPa ( Figure 2). Apart from an emerging shoulder followed by a significant splitting of the ν 1 (SO 4 ) mode located at 1021 cm −1 (at 1 bar), new weak modes in the low-frequency range similar to those reported for MgSO 4 ·H 2 O and FeSO 4 ·H 2 O 27, 28 have not been observed. This can be simply explained as the arbitrary orientation of samples does not necessarily allow all modes to be recognizable with comparable intensities. Compared to Meusburger et al., 27,28 in NiSO 4 ·H 2 O there is evidence for a second potential structural change within the experimentally accessible p regime. The first is represented by a potential soft mode, which is just within the measurable range around 58 cm −1 (at 1 bar, Figure 1). Taking a closer look at the splitting of the Raman band attributed to the ν 1 (SO 4 ) mode indicates that there is more than one crystallographic independent sulfate group in the structure. The careful analysis of the spectra ( Figure 2) shows that the second band component starts emerging at p ≥ 7.41(6) GPa, and thus these spectral changes coincide with the second supposed discontinuity as described above. Considering that for the earlier reported C2/c-P1̅ transition in isostructural compounds 27,28 the structures of both polymorphs exhibit just one single S position, the second structural change must be accompanied by continued symmetry breaking, leading to the diversification of more than one S site. The current observation may thus be a possible indication of another structural change, which has not yet been recognized so far. In decompression, the second Raman band component seems to vanish at p ≤ 5.60(6) GPa ( Figure S4), which means that the second potential transformation in contrast to the first one would show a hysteresis of about 1.8 GPa.
A detailed analysis of the fitted pressure-dependent Raman band shifts ( Figure 3) in addition to a correlation matrix of these band positions especially in the region between 240 and 525 cm −1 (see the Supporting Information, Figure S3) support the observation of two distinct structural transitions in NiSO 4 ·    Figure S3, top right). This technique was for example previously successfully applied on Raman and powder X-ray diffraction data for the detection of subtle second order phase transitions in lawsonite. 45 However, only the second phase transition between 6.36(6) and 7.41 (6) GPa could clearly be identified by a subsequent hierarchical cluster analysis (see dendrogram in the Supporting Information, Figure S2). The first two clusters, separated between 2.94(6) and 4.30(6) GPa, are most likely influenced by the relatively larger pressure steps facilitated under low pressure conditions. Tentative clusters were shown in the PCA plot ( Figure S3, top right) using three ellipses. In some Raman band shift correlations, however, this separation is more obvious as manifested by a sudden discontinuity located between 2.30(6) and 2.94(6) GPa.
Lattice Properties under Isothermal Compression. Precise unit-cell parameters and unit-cell volumes were determined at 18 individual pressure points within the pressure range 10 −4 to 8.67(1) GPa by means of single-crystal diffraction and applying unconstrained refinements of metric parameters (see Table S1 in the Supporting Information). In order to facilitate the comparison between monoclinic and triclinic lattices, the lattice parameters are provided in addition to the standard monoclinic C2/c setting in that of the reduced cell (with a red = b red ≠ c red , α red = β red ≠ γ red for a monoclinic metric, see Supporting Information, Table S2). Plotting the cell parameters ( Figure 4) and lattice angles ( Figure 5) as a function of p, there is clear evidence of an obvious lattice distortion from monoclinic (a red = b red , α red = β red ) to triclinic symmetry (a red ≠ b red , α red ≠ β red ) at ∼2.66(1) GPa. The character of lattice distortion is identical to what has been observed for the isostructural compounds of Mg 28 and Fe, 27 and hence it can be assumed that it is the equivalent C2/c−P1̅ transformation. The evolution of all three lattice angles α, β, and γ with p ( Figure 5) provides again a clear proof in α and β for this first discontinuity, which starts between 2.38(1) and 2.66(1) GPa. This is less obvious for the refined base vector c (Figure 4). While a and b reveal a clear disproportionation of their values in the setting of the reduced cell, the c axis appears to shrink continuously even across the first discontinuity following the typical trend of nonlinear length change with pressure.
As the Raman spectra reveal several hints for the occurrence of a second subtle structural change, the evolution of the lattice parameters was carefully inspected with respect to changes in the p interval around the second critical pressure between 6.36 (6) and 7.41(6) GPa. There is no conspicuous deviation or any significant discontinuity recognizable in the p dependencies of the base vectors. It requires a very careful inspection to recognize very subtle deviations as exemplified in Figure 6. This plot shows the differences of the c-axis values in the two possible settings of the reduced cell, thus exhibiting a distinct change in the Δc/Δp rate of the linear trends with both lines intersecting around 2.4 GPa. The second linear trend line, as fitted to the data in the p interval 2.7−6 GPa, matches perfectly the linear relationship for the data points above the first discontinuity up to a pressure around 6.7 GPa. At this point, the last four data points clearly deviate by following another third linear trend at a deliberately lower Δc/ Δp rate, thus falling slightly below the second trendline. The onset point for the second change (6.7 GPa) perfectly coincides with the pressure range between 6.36(6) and 7.41(6) GPa as determined from Raman spectroscopy, at which the splitting of the ν 1 (SO 4 ) band has been observed.   Inorganic Chemistry pubs.acs.org/IC Article A similar deviation can be observed for evaluating the spontaneous strain components e 1 and e 2 , which can be expected to be proportional to the order parameter for a continuous second order phase transition from monoclinic to triclinic symmetry (Figure 7). The values of the spontaneous strain components e 1 , e 2 , and e 3 were calculated from the lattice parameters of the triclinic reduced cell using the extrapolated fits shown in Figures 4 and 5 and derived from eqs 1−3) discussed in Carpenter et al. 46 for phase transitions between two triclinic forms (with β* and β 0 * being the corresponding reciprocal lattice angles).
The plot in Figure 7 shows nonzero values starting at 2.66(1) GPa, which clearly follow a nonlinear Δ ∝ |p−p crit | β trend. The critical exponent β was found to be very close to 1/ 2, such as found for both isostructural phases MgSO 4 ·H 2 O and FeSO 4 ·H 2 O, thus suggesting the C2/c−P1̅ being proper second order from the point of view of Landau theory. 27,28 The equivalent fits onto the spontaneous strain values with β constrained to 1/2 reveal clear deviations starting around 6.5 GPa. The values for the critical pressure p c obtained from the fits (= 2.47 and 2.46 GPa) confirm the pressure of the monoclinic-to-triclinic transition with an uncertainty of 0.05− 0.01 GPa. The critical pressure for the second transition at 6.5 GPa can only be specified with a considerably greater inaccuracy of ±0.5 GPa.
Comparative Lattice Elasticities of Polymorphs.   Table S3 in the Supporting Information), it is possible to determine accurate values for the parameters of the empirical equations of state (EoS). Therefore, Birch− Murnaghan (BM) equations of state 47 were fitted to V, trying to fit third-and second-order truncations. Evaluating the p−V data (Figure 8a), it is noticeable that despite the two suspected phase transformations, the evolution of V does not show any discontinuity, thus evidently excluding any of the transitions being first order in character. The fit for the pressure interval relevant for the monoclinic phase (hereinafter referred to as α-NiSO 4 ·H 2 O) revealed a bulk modulus K 0 = 65.9 ± 6.7 GPa and its pressure derivative K′ = 2.3 ± 4.6 for a third-order BM-EoS. Considering the error of K 0 and K′ as too large, due to the limited number of data points, and K′ being too small due to elastic softening effects, K 0 turns to 63.4 ± 1.0 GPa on fitting   Inorganic Chemistry pubs.acs.org/IC Article the data to second-order BM-EoS. Extrapolating the obtained EoS beyond the 2.5 GPa, it can clearly be seen that the V data at higher p deviate between −0.5 Å 3 and +1.3 Å 3 (cf. Figure  8a). The values for the β-NiSO 4 ·H 2 O polymorph in the range 2.5 to 6.0 GPa correspond to K 0 = 44.5 ± 9.4 GPa and K′ = 8.2 ± 3.2, with a relatively high value for K′ due to obvious elastic softening effects as reported for the isostructural phases. 27,28 When the p range is restricted between 4.0 and 6.0 GPa, the fit yields a 500% error value for K 0 and K′ for BM-3 EoS and  Table 1) and might be indicative for the structural instability of the phase at even higher pressures beyond the maximum p within this experimental series. High-Pressure Crystal Structures. The scXRD intensity data were collected at p = 0.6(1), 2.3(1), 4.5(1) ,and 8.4(1) GPa in order to extract structural information for all three potential phases. The results of the refinements are given together with details of the data collection in Table 2, and the resulting parameters (fractional coordinates and isotropic displacement parameters) of the structure models are summarized in the Supporting Information (Tables S4). The refinement at 0.6(1) GPa was performed in C2/c U iso constrained only for the two H atoms, at 2.3(1) GPa in addition, the U iso 's for the three O atoms were constrained. For the refinements in P1̅ at 4.5(1) and 8.4(1) GPa, the H positions were constrained to fixed values in addition to the fixed U iso 's. An attempt for a refinement in P1 (at 8.4(1) GPa) was performed with positional constraints according to local 1̅ symmetry for all atoms. This symmetry was stepwise released for each atom, testing for split positions. However, there was no displacement above the three-sigma limit, apart from O2B, which was then accounted into the disordered P1̅ structure (Table S4).

Changes of individual lattice parameters and the unit-cell
Applying the three-dimensional reconstruction of the reciprocal space from a series of collected frames, the data collections corresponding to α-NiSO 4 ·H 2 O reveal an untwinned single reciprocal lattice of monoclinic symmetry with systematic Bragg peak extinctions corresponding to C2/c symmetry. Above 2.5 GPa, the reciprocal lattice points show a distinct splitting (cf. Figure 9), which is characteristic of a nonmerohedral twinning corresponding to the symmetry reduction from monoclinic to triclinic lattice metrics. Integration of Bragg peak intensities was achieved either for untwinned single-domain samples or, if twinned, at higher angles where peak separation was large enough. In addition, weak diffraction peaks (Figure 9), which were forbidden in C2/c, were observed when the samples transformed to triclinic symmetry, confirming the assumed symmetry change. While the changes for the α-to-β transition around 2.5 GPa are obvious and the reflection conditions support the equivalent C2/c-to-P1̅ transition, such as reported for FeSO 4 ·H 2 O, 27 there is no apparent evidence from the diffraction pattern for the supposed β-to-γ transition. With respect to hints from the Raman spectra that the S atoms could be located at different crystallographically independent sites, the reciprocal space of the 8.4(1) GPa measurement was deliberately investigated for the existence of superstructure reflections or satellites as would be expected for a commensurately or incommensurately modulated structure, e.g., Ende et al. 49 Despite adequate attention and careful inspection, neither additional Bragg peaks nor satellite reflections could be detected, despite having applied adequate exposure times for recording the diffraction patterns.
Once any kind of superordinate modulation of the structure could be ruled out, all that remains to be considered is a transformation to acentric P1 related to a nonisosymmetric Figure 9. Layers of reconstructed reciprocal space derived from measured frames using the C2/c cell setting. Arrows indicate the appearance of diffraction peaks, which are forbidden in space group C2/c. Moreover, the reconstructed hk5 layer of the measurement at 4.5 GPa shows split components of individual twin domains following nonmerohedral twinning accompanying the monoclinic-to-triclinic transition.
Inorganic Chemistry pubs.acs.org/IC Article structural change. A refinement of a structural model in P1 was achieved by stepwise releasing the symmetry constraints of the inversion center of individual atoms between corresponding atoms in P1̅ . The refinement of various soft-constrained models finally leads to a significant improvement of the reliability indices (Table 2) only if the O2B was split into two components (O2B1 and O2B2) as done for a proposed P1̅ structure with disordered O2B position. According to the findings from vibrational spectroscopy, the structure is retained without any changes in the bonding topology of the heterodesmic framework, which is composed of octahedral NiO 6 and tetrahedral SO 4 units as described by Giester. 25 The evolution of bond distances is independent of symmetry and does not provide any significant change that could be responsible for the transitions (Tables 3 and 4). This is in complete agreement what has been reported for the C2/cto-P1̅ transition from the ambient αto the high-p βpolymorph of FeSO 4 ·H 2 O. 27 In accordance with the displacive changes observed in this isostructural compound, a tendency of the framework to fold up can be anticipated for NiSO 4 ·H 2 O as well, as could be observed on the bond angles on the bridging oxygen atoms. There is a general decrease of Ni−O− Ni and Ni−O−S angles as a consequence of the compacting folding, together with the diversification of Ni−O2−S angles (126°vs 133°at 4.5 GPa) in the same manner as reported for FeSO 4 ·H 2 O upon exceeding the α-to-β transition point. Looking more closely at the development at the second critical point around 6.5 GPa, it can be seen that two resulting sulfate tetrahedra actually reveal different bond-length distortions with respect to their individual S−O bond lengths (e.g., S1−O2B1 = 1.56(4) Å, S1−O2B2 = 1.44(3) Å at 8.4(1) GPa). However, the true crystallographic symmetry of the main framework is yet not beyond doubt P1̅ . The shown model has to be assumed to be an approximated model. One can prove beyond doubt the development of crystallographically different sites, which is just most pronounced in the case of the O2B positions. Within the obtained uncertainties from the structure refinement, the symmetry lowering recognizable from the Raman spectra beyond 7 GPa can be confirmed.
Hydrogen Bonding under Pressure. While the transformations do not cause virtually any changes with respect to the main structural units of the framework, with the exception of some polyhedral tilting leading to higher compaction, the hydrogen bonding system reveals notable changes at the critical transition points. Due to the loss of the local 2-fold axis  (1) GPa) thus becoming potentially involved in the hydrogen-bonding system ( Figure 10). These changes confirm the mechanism reported for the α-to-β transition in the analogous ferrous compound with the development of three short O3···O contacts suitable as relevant hydrogen bonds with O3 being the donator oxygen. As found in the isostructural compounds, the O3···O1 contacts show a significant diversification with respect to their interatomic distances, i.e., the significant shortening of O3···O1B from 3.296(5) Å (at 2.3(1) GPa) to 3.016(7) Å (at 4.5(1) GPa) while the O3···O1A distance even increases in length (to 3.436(8) Å at 4.5(1) GPa) upon crossing the critical transition point around 2.5 GPa. As the voids, in which the hydrogen atoms are positioned, are oriented along the a and b axes, their different pressure-dependent compression behavior starting around 2.5 GPa is manifested in noticeably different void sizes (O2···O2 distances) visible in Figure 10.
Of particular interest are the changes in the hydrogenbonding system when the second transformation point is exceeded around 6.5 GPa. There, the relevant O···O distances are subject to only minor changes, and the bond-length differences are only relatively small (e.g., O3···O2 = 2.53(4) and 2.56(2) Å). It is therefore obvious that the second structural change is not primarily due to the hydrogen-bonding system and apparently only adapts to the small changes arising from the framework as discussed above. However, similar to the first transition, the second one is mostly affecting the size of the voids hosting the hydrogen atoms ( Figure 10). It even affects them in the same fashion by further increasing the long O2···O2 distances (which are oriented along the a axis), while the short O2···O2 distances (which are oriented along the baxis direction) decrease (O2A···O2B2: 3.306(24) Å vs O2A··· O2B1: 3.115(24) Å and O2B1···O2A: 4.032(23) Å vs O2B2··· O2A: 3.859(22) Å, Figure 10).

■ CONCLUSIONS
The in situ high-p investigations on nickel sulfate monohydrate reveal two apparent structural instabilities accompanied by transformations under isothermal hydrostatic compression. While the first transition observed in NiSO 4 ·H 2 O occurs at 2.47(1) GPa, showing all features equivalent to the ferroelastic transformations reported in isostructural MgSO 4 ·H 2 O (p c = 2.72 GPa) and FeSO 4 ·H 2 O (p c = 6.15 GPa), this study reveals a second subtle structural change in NiSO 4 ·H 2 O at 6.5(5) GPa. The ferroelastic C2/c-to-P1̅ transition at 2.47 GPa is identical in its character to those described so far for the isostructural compounds. In particular, the type of lattice changes as described by the settings in C2/c and its reduced cell are more or less identical, such as the interatomic distances of the O···O contacts within the hydrogen-bonding system, which change in the same fashion. Comparing the high-p behavior of the three known kieserite-type representatives and their critical transition pressures p c (2.47, 2.72, 6.15 GPa), the unit-cell volumes (342.3, 355.5, 365.2 Å 3 ), and the bulk moduli   When comparing the compressibilities along the different lattice directions, it is striking that the a-axis shows a noticeable negative compressibility (M′ = −3.9 ± 2.9, BM-3 all data points above 2.5 GPa) for the β-phase of NiSO 4 ·H 2 O, which has not been observed for the other two compounds. Negative values for the pressure derivatives of axial moduli have been found in many cases to indicate a structural instability, which in turn might act as the precursor of the β-to-γ transition around 6.5 GPa. In this respect, it is not surprising that a further transformation was indeed observed. The second transition is evident from results of independent analytical techniques, on the one hand by the splitting of the Raman band attributed to the ν 1 (SO 4 ) mode and on the other hand by observing higher values for the reliability indices for the structure refinement of data sets measured beyond 6.5 GPa. Although weak satellite reflections could not be observed within the data sets of this study, the true symmetry of the γphase and the existence of a superstructure cannot be completely ruled out, especially if it is based only on slight changes in the oxygen and hydrogen positions. Therefore, the relevant Bragg peak intensities are possibly far below the detectable signal-to-noise ratio in our DAC measurement. Nevertheless, the presented model of the structure at 8.4(1) GPa reveals pseudocentrosymmetry of the heavy atoms when refining in P1 global symmetry, where the symmetry coupling of centrosymmetric positions was released stepwise for each pair of atoms. Only the oxygen atoms indicated deviations from centrosymmetry with the effect being most significant for the O2 positions. The differences between a proper refinement in P1̅ and with local symmetry constraints in P1 are evident ( Table 2) and provide an additional proof for the existence of a third polymorph. The transition pathway is determined by the symmetry relationship with monoclinic C2/c (α-NiSO 4 ·H 2 O) and triclinic P1̅ (β-NiSO 4 ·H 2 O) and a disordered O2B position in P1̅ (γ-NiSO 4 ·H 2 O).
Polymorphism of M 2+ SO 4 ·H 2 O is an important topic in astrophysics and planetary geology with respect to the monohydrate sulfates being an abundant component on the surface of icy moons (cf. Comodi et al. 51 and McCord et al. 52 ). α-MgSO 4 ·H 2 O and β-MgSO 4 ·H 2 O have recently been considered for the interior of Callisto, 28 where the maximum pressure corresponds to around 5 GPa. Considering the significant Ni contents in the soluble sulfate fraction in C1/C2 carbonaceous chondrites believed to be chemically similar to the rocky cores of the icy moons (e.g., Ivuna and Orgueil) and meteorites with up to 13.6 wt % NiO, 13 NiSO 4 ·H 2 O can be considered as an important component in natural monohydrate sulfates. According to previously reported findings, there is a tendency of the enrichment of Ni relative to Mg during the crystallization of solid nickel−magnesium sulfate from aqueous Mg−Ni-containing solutions. 5 As the first critical transition in NiSO 4 ·H 2 O occurs at even lower pressure than in kieserite itself, the occurrence of the high-p β-form is therefore very likely wherever Ni substitution occurs.
Refined triclinic and monoclinic lattice parameters and unit-cell volumes; fractional atomic coordinates of the α, β, and γ-crystal structures as well as isotropic displacement parameters; photograph of a NiSO 4 ·H 2 O sample crystal inside the pressure chamber compressed in a dense methanol/ethanol mixture at 10.8 GPa; dendrogram of a hierarchical cluster analysis and subsequent principle cluster analysis of pressure induced Raman shifts (PDF)