Isotypism and phase transitions of (NH(4))M(HSO(4))(SO(4))(H(2)O)(2) (M = Fe, Co and Ni) compounds.

A systematic crystallization study of {(NH(4))[M(HSO(4))(SO(4))(H(2)O)(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 NH(4)HSO(4) and the corresponding MSO(4) sulfate, whereas for M = Mn, Cu, Zn different products were obtained under these conditions. The (NH(4))M(HSO(4))(SO(4))(H(2)O)(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 [MO(4)(OH(2))(2)] octa-hedra are linked by distinct [SO(3)(OH)] and [SO(4)] tetra-hedra into chains. Adjacent chains are linked by very strong hydrogen bonds (O⋯O ≃ 2.5†à ) 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 (NH(4))M(HSO(4))(SO(4))(H(2)O)(2) structures (M = Mg, Fe, Co, Ni) at 296†K and 100†K, respectively.