Synthesis, crystal structure, phase transition and thermal behaviour of a new dabcodiium hexaaquanickel(II) bis(sulphate), (C6H14N2)[Ni(H2O)6](SO4)2

A new dabcodiium-templated nickel sulphate, (C₆H₁₄N₂)Ni(H₂O)₆](SO₄)₂, has been synthesised and characterised by single-crystal X-ray diffraction at 20 and −173 °C, differential scanning calorimetry (DSC), thermogravimetry (TG) and temperature-dependent X-ray powder diffraction (TDXD). The high temperature phase crystallises in the monoclinic space group P2₁/n with the unit-cell parameters: a = 7.0000(1), b = 12.3342(2), c = 9.9940(2) Å; β = 90.661(1)°, V = 862.82(3) Å³ and Z = 2. The low temperature phase crystallises in the monoclinic space group P2₁/a with the unit-cell parameters: a = 12.0216(1), b = 12.3559(1), c = 12.2193(1) Å; β = 109.989(1)°, V = 1705.69(2) Å³ and Z = 4. The crystal structure of the HT-phase consists of Ni²⁺ cations octahedrally coordinated by six water molecules, sulphate tetrahedra and disordered dabcodiium cations linked together by hydrogen bonds. It undergoes a reversible phase transition (PT) of the second order at −53.7/−54.6 °C on heating-cooling runs. Below the PT temperature, the structure is fully ordered. The thermal decomposition of the precursor proceeds through three stages giving rise to the nickel oxide.