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 size-dependent bistability of a Fe-II[HB(3,5-(Me)(2)Pz)(3)](2) spin-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 of crystalline grains of the triclinic phase exhibiting the first-order spin transition coexisting with those of a new high-spin metastable tetragonal polymorph in quite a large proportion (ca. 42%). Accordingly, we show that the as-sublimed thin-film of 130 nm thickness retains a remarkable 17 K width hysteresis (T-1/2 = 152 K) while the spin transition becomes quantitative by the thermal annealing of the metastable phase at 373 K. The structural analysis based on single-crystal X-ray diffraction measurements provides evidence for a thermal hysteresis resulting from successive crystallographic phase transitions that is consistent with the calorimetric data. This accounts for the asymmetric shape of the hysteresis and the variation of cooperativity observed in the magnetic and optical data whatever the material dimension is.