High-spin->low-spin relaxation in [Fe(bpp)(2)](CF3SO3)(2)center dot H2O after LIESST and thermal spin-state trapping - Dynamics of spin transition versus dynamics of phase transition

The iron(II) complex [Fe(bpp)(2)]-(CF3SO3)(2) . H2O (bpp = 2,6-bis(pyrazolyl-3-yl)pyridine) shows a thermal spin transition associated with a hysteresis of approximately 140 K width. The transition temperatures T-1/2 (where the fraction of HS species gamma(HS) = 0.5) are 147 K and approximate to 285 K in the cooling and heating directions, respectively, The compound shows the LIESST and reverse-LIESST effects at low temperatures. The relaxation of the metastable HS states generated by LIESST was observed quantitatively at temperatures between 77.5 and 85 K by Mossbauer spectroscopy. Metastable HS states can also be generated by rapid cooling of the sample. The relaxation of the metastable HS states formed by thermal spin-state trapping was monitored at temperatures between 104 and 118 K by magnetic susceptibility measurements. The relaxation mechanisms of the HS states generated by LIESST and thermal spin-state trapping are completely different. We suggest that the HS --> LS relaxation after thermal spin-state trapping is triggered by an additional structural phase transition of the system.