The [Fe(etz) 6 ](BF 4 ) 2 Spin‐Crossover System—Part Two: Hysteresis in the LIESST Regime

In the [Fe(etz) 6 ](BF 4 ) 2 spincrossover system the iron(II) complexes occupy two nonequivalent lattice sites, sites A and B. Complexes on site A show a thermal high‐spin (HS) → low‐spin (LS) transition at 105 K, whereas complexes on site B remain in the HS state down to 10 K. Complexes on both sites exhibit light‐induced spin state conversions (LIESST) at 20 K: LS → HS on site A with λ = 514.5 nm, and HS → LS on site B with λ = 820 nm. The relaxation processes subsequent to the HS ⇌ LS conversion on site B reveal a light‐induced HS⇌LS bistability for the complexes on site B at 70 K. The bistability as well as the absence of a thermal spin transition on site B are attributed to a thermal hysteresis for the B‐site complexes with a critical temperature T ↑ c ≈77 K on heating. This hysteresis can be interpreted in terms of strong cooperative effects of elastic origin, which, in addition, cause characteristic deviations of the relaxation on site B from first‐order kinetics (self‐acceleration). In contrast, the HS → LS relaxation at 60 K on site A after irradiation with λ = 514.5 nm shows an unusual self‐retardation.