title: Reversible high-temperature phase transition of a manganese(II) formate framework with imidazolium cations.
abstract: A new metal-formate framework, poly[1H-imidazol-3-ium [tri-μ2-formato-manganese(II)]], {(C3H5N2)[Mn(HCOO)3]}n, was synthesized and its structural phase transition was studied by thermal analysis and variable-temperature X-ray diffraction analysis. The transition temperature is around 435 K. The high-temperature phase is tetragonal and the low-temperature phase is monoclinic, with a β angle close to 90°. The relationship of the unit cells between the two phases can be described as: a(HT) = 0.5a(LT) + 0.5b(LT); b(HT) = -0.5a(LT) + 0.5b(LT); c(HT) = 0.5c(LT). In the high-temperature phase, both the framework and the guest 1H-imidazol-3-ium (HIm) cations are disordered; the HIm cations are located about 2mm sites and were modelled as fourfold disordered. The Mn and a formate C atom are located on fourfold rotary inversion axes, while another formate C atom is on a mirror plane. The low-temperature structure is ordered and consists of two crystallographically independent HIm cations and two crystallographically independent Mn(2+) ions. The phase transition is attributable to the order-disorder transition of the HIm cations.
authors.affiliation: School of Chemistry and Chemical Technology, Faculty of Light Industry, Hubei University of Technology, Wuhan, Hubei 430068, People's Republic of China. wangbq@sjtu.org
authors.forename: Bi-Qin
authors.initials: BQ
authors.lastname: Wang
authors.forename: Hai-Biao
authors.initials: HB
authors.lastname: Yan
authors.forename: Zheng-Qing
authors.initials: ZQ
authors.lastname: Huang
authors.forename: Zhi
authors.initials: Z
authors.lastname: Zhang