title: The interplay of iron(II) spin transition and polymorphism.
abstract: The mononuclear compound (1) [Fe(II)(L)(2)](BF(4))(2) (L = 4-ethynyl-2,6-bis(pyrazol-1-yl)pyridine) was prepared and structurally as well as magnetically characterised. The crystallisation revealed the formation of two polymorphs--the orthorhombic 1A and the tetragonal form 1B. A third, intermediate phase 1C was found exhibiting a different orthorhombic space group. Reversibility of the phase transition between 1A and 1C was studied by variable-temperature single-crystal and powder X-ray diffraction studies, while an irreversible phase transition was observed for the transition of 1B→1C. The magnetic studies show that the 1A↔1C transition is accompanied by a very abrupt spin transition (ST) with 8 K hysteresis width (T(1/2)(↓) = 337 K, T(1/2)(↑) = 345 K). The ST was confirmed by Mössbauer spectroscopy as well as by DSC studies. In contrast, the 1B polymorph remained low-spin up to 420 K. In conclusion, a full cycle of intertwined phase- and spin-conversions of three polymorphs could be proven following the general scheme 1B→1C↔1A.
authors.affiliation: Institut für Nanotechnologie, Karlsruher Institut für Technologie, Postfach 3640, 76021, Karlsruhe, Germany.
authors.forename: Ivan
authors.initials: I
authors.lastname: Šalitroš
authors.forename: Olaf
authors.initials: O
authors.lastname: Fuhr
authors.forename: Andreas
authors.initials: A
authors.lastname: Eichhöfer
authors.forename: Robert
authors.initials: R
authors.lastname: Kruk
authors.forename: Ján
authors.initials: J
authors.lastname: Pavlik
authors.forename: Lubor
authors.initials: L
authors.lastname: Dlháň
authors.forename: Roman
authors.initials: R
authors.lastname: Boča
authors.forename: Mario
authors.initials: M
authors.lastname: Ruben