HIGH-SPIN (5T2) REVERSIBLE LOW-SPIN (1A1) TRANSITION IN SOLID DITHIOCYANATOBIS(2,2'-BI-2-THIAZOLINE)IRON(II) - HYSTERESIS EFFECTS, DEBYE-WALLER FACTORS AND CRYSTALLOGRAPHIC CHANGES

The almost discontinuous high-spin (5T2) ⇌ low-spin(1A1) transition in solid [Fe(bt)2(NCS)2] (bt = 2,2′-bi-2-thiazoline) has been studied in detail by 57Fe Mössbauer spectroscopy and X-ray diffraction. At the transition temperature Tc, the ground states involved are characterized by ΔEQ(5T2) = 3.00 mm s-1, δIS(5T2) = +0.97 mm s-1 and ΔEQ(1A1) = 0.54 mm s-1, δIS(1A1) = +0.30 mm s-1. A pronounced hysteresis of ΔTc = 9.5 K has been observed, the transition being centered on T>c = 181.86 K for rising and on T>c = 172.33 K for falling temperatures. The observations are compared with a Bragg and Williams tyupe theory and with the independent domain model. Debye-Waller factors show at Tc a discontinuity of Δftotal∼35.4% corresponding to the change between -ln fs and -ln f1. The X-ray diffraction patterns for the 25T2 and 1A1 phases show characteristics differences. The temperature dependence for ts/ttotal of the Mössbauer effect and for the relative intensities ts/Itotal of individual peak profiles of X-ray diffraction is equivalent. The observations provide evidence for a crystallographic phase change which is associated with the (S = 2) ⇌ (S = 0 spin transformation. The existence of crystal domains of finite dimension is required for both 5T2 and 1A1 ground state molecules. © 1979.