A systematic crystallization study of {(NH4)[M(HSO4)(SO4)(H2O)2]} n com-pounds, catena-poly[ammonium [[di-aqua-metal(II)]-μ-(hydrogen sulfato)-μ-sul-fato]], revealed that crystals of the M = Fe, Co and Ni members could be grown by evaporation of mixed equimolar aqueous solutions of NH4HSO4 and the corresponding MSO4 sulfate, whereas for M = Mn, Cu, Zn different products were obtained under these conditions. The (NH4)M(HSO4)(SO4)(H2O)2 compounds (M = Fe, Co, Ni) show a similar behavior to the magnesium analogue, i.e. a reversible structural phase transition from an ordered triclinic crystal structure at T = 100 K (Z = 2) to a disordered triclinic structure at T = 296 K (Z = 1). The symmetry relationship between the structure at 296 K and the superstructure at 100 K is of the isomorphic type with index 2. At 100 K, the [MO4(OH2)2] octa-hedra are linked by distinct [SO3(OH)] and [SO4] tetra-hedra into chains. Adjacent chains are linked by very strong hydrogen bonds (O⋯O ≃ 2.5 Å) between the two types of sulfate tetra-hedra into layers. These layers are held together by hydrogen-bonding inter-actions of medium-to-weak strength between the ammonium cations and water mol-ecules. At 296 K, the H atoms of the ammonium tetra-hedron and the H atom between two symmetry-related sulfate groups are disordered. Qu-anti-tative structural comparisons are made between the isotypic (NH4)M(HSO4)(SO4)(H2O)2 structures (M = Mg, Fe, Co, Ni) at 296 K and 100 K, respectively.
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