Fortes, AD; Knight, KS; Gibbs, AS; Wood, IG; (2018) Crystal structures of NiSO4·9H2O and NiSO4·8H2O: magnetic properties, stability with respect to morenosite (NiSO4·7H2O), the solid-solution series MgxNi1−x)SO4·9H2O. Physics and Chemistry of Minerals , 45 (8) 10.1007/s00269-018-0956-z.

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Abstract

Since being discovered initially in mixed-cation systems, a method of forming end-member NiSO 4 ·9H 2 O and NiSO 4 ·8H 2 O has been found. We have obtained powder diffraction data from protonated analogues (with X-rays) and deuterated analogues (using neutrons) of these compounds over a range of temperatures, allowing us to determine their crystal structures—including all H-atoms—and to characterise the transitions on warming from 220 to 278 K; glass → 9-hydrate → 8-hydrate + ice → 7-hydrate + ice → partial melt (7-hydrate + liquid). NiSO 4 ·8D 2 O is triclinic, space-group (Formula presented.), Z = 2, with unit cell parameters at 150 K, a = 6.12463(8) Å, b = 6.8401(1) Å, c = 12.5339(2) Å, α = 92.846(1)°, β = 97.822(1)°, γ = 96.627(1)° and V = 515.58(1) Å 3 . The structure consists of two symmetry-inequivalent Ni(D 2 O) 6 octahedra on sites of (Formula presented.) symmetry. These are directly joined by a water–water H-bond to form chains of octahedra parallel with the c-axis at x = 0. Two interstitial water molecules serve both to bridge the Ni(D 2 O) 6 octahedral chains in the b–c plane and also to connect with the SO 4 2− tetrahedral oxyanion. These tetrahedra are linked by the two interstitial water molecules in a reticular motif to form sheets perpendicular to c. NiSO 4 ·9D 2 O is monoclinic, space-group P2 1 /c, Z = 4, with unit-cell parameters at 150 K, a = 6.69739(6) Å, b = 11.8628(1) Å, c = 14.5667(1) Å, β = 94.9739(8)° and V = 1152.96(1) Å 3 . The structure is isotypic with the Mg analogue described elsewhere (Fortes et al., Acta Cryst B 73:47‒64, 2017b). It shares the motif of H-bonded octahedral chains with NiSO 4 ·8D 2 O, although in the enneahydrate these run parallel with the b-axis at x = 0. Three interstitial water molecules bridge the Ni(D 2 O) 6 octahedra to the SO 4 2− tetrahedral oxyanion. The tetrahedra sit at x ≈ 0.5 and are linked by two of the three interstitial water molecules in a pentagonal motif to form ribbons parallel with b. A solid-solution series exists between Mg and Ni enneahydrate end-members where we observe preferential partitioning of Ni 2+ into the octahedral sites on the 2c Wyckoff positions rather than the 2a sites. The solution is slightly non-ideal, as indicated by the small positive excess volume of mixing. Measurements of the DC magnetisation of quenched NiSO 4 solutions reveal anomalies in the molar susceptibility on warming through the region from 221 to 225 K, probably due to devitrification of the (assumed) glassy specimen into a mixture of NiSO 4 ·9H 2 O + ice Ih. Further temperature-dependent measurements on repeated warming and cooling provide no evidence of magnetic ordering and indicate a weak ferromagnetic coupling between neighbouring Ni 2+ ions, likely via super-exchange through the H-bond between neighbouring Ni(H 2 O) 6 octahedra.

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