Synthesis, crystal structures, phase transition characterization and thermal decomposition of a new dabcodiium hexaaquairon(II) bis(sulfate): (C6H14N2)[Fe(H2O)6](SO4)2

The crystal structures of 1,4-diazabicyclo[2.2.2]octane (dabco)-templated iron sulfate, (C6H14N2)[Fe(H2O)6](SO4)2, were determined at room temperature and at -173 deg. C from single-crystal X-ray diffraction. At 20 deg. C, it crystallises in the monoclinic symmetry, centrosymmetric space group P21/n, Z=2, a=7.964(5), b=9.100(5), c=12.065(5) A, β=95.426(5) deg. and V=870.5(8) A3. The structure consists of [Fe(H2O)6]2+ and disordered (C6H14N2)2+ cations and (SO4)2- anions connected together by an extensive three-dimensional H-bond network. The title compound undergoes a reversible phase transition of the first-order at -2.3 deg. C, characterized by DSC, dielectric measurement and optical observations, that suggests a relaxor-ferroelectric behavior. Below the transition temperature, the compound crystallizes in the monoclinic system, non-centrosymmetric space group Cc, with eight times the volume of the ambient phase: a=15.883(3), b=36.409(7), c=13.747(3) A, β=120.2304(8)o, Z=16 and V=6868.7(2) A3. The organic moiety is then fully ordered within a supramolecular structure. Thermodiffractometry and thermogravimetric analyses indicate that its decomposition proceeds through three stages giving rise to the iron oxide. - Graphical abstract: The new dabcodiium hexaaquairon(II) bis(sulfate), (C6H14N2)[Fe(H2O)6](SO4)2, was prepared and characterized. It exhibits a supramolecular structure and undergoes a reversible order-disorder phase transition at -2.3 deg. C