THE [FE(ETZ)(6)](BF4)(2) SPIN-CROSSOVER SYSTEM .2. HYSTERESIS IN THE LIESST REGIME

In the [Fe(etz)(6)](BF4)(2) spin-crossover system the iron(II) complex es 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 stale conversions (LIESST) at 20 K:LS --> HS on site A with lambda = 514.5 nm, and HS - -> LS on site B with lambda = 520 nm. The relaxation processes subsequ ent to the HS --> LS conversion on site B reveal a light-induced HS re versible arrow LS bistability for the complexes on site B at 70 K, The bistability as well as the absence of a thermal spin transition on si te B are attributed to a thermal hysteresis for the B-site complexes w ith a critical temperature T-c(up arrow) approximate to 77 K on heatin g. This hysteresis can be interpreted in terms of strong cooperative e ffects of elastic origin, which, in addition, Cause characteristic dev iations of the relaxation on site B from first-order kinetics (self-ac celeration). In contrast, the HS --> LS relaxation at 60 K on site A a fter irradiation with lambda = 514.5 nm shows an unusual self-retardat ion.