The disentangling of hysteretic spin transition, polymorphism and metastability in bistable thin films formed by sublimation of bis(scorpionate) Fe( II ) molecules

We investigate the sizeB dependent bistability of a Fe II [HB(3,5B (Me)2Pz)3]2 spinB transition compound in the form of thin films, the thickness of which varies from 130 to 8500 nm. From combined structural, magnetic and optical measurements, we demonstrate that the materials prepared by sublimation consist in crystalline grains of the triclinic phase exhibiting the firstB order spin transition coexisting with those of a new highB spin metastable tetragonal polymorph in a quite large proportion (ca. 42%). Accordingly, we show that the asB sublimed thinB film of a 130 nm thickness retains a remarkable 17 K width hysteresis (T1/2 = 152 K) while the spin transition becomes quantitative by the thermal annealing of the metastable phase at 373 K. Consistently with the calorimetric data, the structural analysis based on singleB crystal xB ray diffraction measurements provides evidences for a thermal hysteresis resulting from successive crystallographic phase transitions, which account for the asymmetric shape of hysteresis and the variation of cooperativity observed in the magnetic and optical data and that, whatever the material dimension.