Isotypism and phase transitions of (NH)(HSO)(SO)(HO) ( = Fe, Co and Ni) compounds.

A systematic crystallization study of {(NH)[(HSO)(SO)(HO)]} com-pounds, -poly[ammonium [[di-aqua-metal(II)]-μ-(hydrogen sulfato)-μ-sul-fato]], revealed that crystals of the = Fe, Co and Ni members could be grown by evaporation of mixed equimolar aqueous solutions of NHHSO and the corresponding SO sulfate, whereas for = Mn, Cu, Zn different products were obtained under these conditions. The (NH)(HSO)(SO)(HO) compounds ( = Fe, Co, Ni) show a similar behavior to the magnesium analogue, a reversible structural phase transition from an ordered triclinic crystal structure at = 100 K ( = 2) to a disordered triclinic structure at = 296 K ( = 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 [O(OH)] octa-hedra are linked by distinct [SO(OH)] and [SO] 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 (NH)(HSO)(SO)(HO) structures ( = Mg, Fe, Co, Ni) at 296 K and 100 K, respectively.