Cooperative Large-Hysteresis Spin-Crossover Transition in the Iron(II) Triazolate [Fe(ta)2] Metal-Organic Framework.
2020
The metal–organic
framework [Fe(ta)2] (Hta =
1H-1,2,3-triazole) containing Fe(II) ions and 1,2,3-triazolate
ligands shows a reversible phase transition while retaining the cubic
crystal symmetry and space group Fd3m (no. 227). The phase transition between room temperature
(RT-[Fe(ta)2]; a = 16.6315(2) A, V = 4600.39(8) A3) and high temperature
(HT-[Fe(ta)2]; a = 17.7566(4) A, V = 5598.6(1) A3) phases occurs at a temperature
above 290 °C, whereas the phase transition between HT- and RT-[Fe(ta)2] starts at a temperature below 210 °C. Both [Fe(ta)2] polymorphs have identical bond topologies, but they differ
by a large increase of the unit cell’s volume of 22% for HT-[Fe(ta)2]. The compounds are characterized by powder X-ray diffraction,
differential scanning calorimetry, and thermogravimetric analyses.
Additionally, Mossbauer spectroscopy, magnetic studies, and
the electronic structure of both phases are discussed in detail with
respect to the spin-crossover transition from the low-spin (RT-[Fe(ta)2]) to the high-spin phase (HT-[Fe(ta)2]).
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