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(pyr azolyl-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 an d approximate to 285 K in the cooling and heating directions, respecti vely, 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 generat ed by rapid cooling of the sample. The relaxation of the metastable HS states formed by thermal spin-state trapping was monitored at tempera tures between 104 and 118 K by magnetic susceptibility measurements. T he relaxation mechanisms of the HS states generated by LIESST and ther mal 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.