Infrared detection of the hysteresis in the thermally induced spin-crossover in bis(4,4′-bis-1,2,4-triazole)bis(thiocyanato-N)iron(II) monohydrate

By the use of VT infrared spectroscopy, the thermally induced spin transition in the complex [Fe(btr)2(NCS)2]·H2O has been analyzed. The abbreviation btr stands for 4,4′-bis-1,2,4-triazole. This complex was reported earlier to exhibit an abrupt spin transition with a large hysteresis effect. The spin-transition curve calculated from the change in the absorption of the CN-stretching vibrations of the thiocyanate ligand is similar to the curve found by other techniques. The transition temperatures were found to be Tc↓=115±5 K and Tc↑=141±5 K, leading to a hysteresis of 26 K. These values for the same compound, deduced from magnetic and calorimetric measurements and Mössbauer spectroscopy are Tc↓=121 K and Tc↑=145 K. The spin-transition curve was analyzed thermodynamically, leading to the following parameters: ΔH=9.7 kJ mol−1, ΔS=79 J mol−1 K−1 and Γ=3.15 kJ mol−1 in agreement with earlier determined values.