A model for high-spin/low-spin transitions with an interpretation of thermal hysteresis effects

A model for high-spin/low-spin transitions is discussed. It can explain the temperature dependence of the high-spin fraction of the compound Fe(4, 7-(CH₃)₂-1,10-phenanthroline) (NCS)₂ and allows to interpret hysteresis effects at T_c = 120 K as a result of the existence of two minima for the free energy. The transition is assumed to take place by nucleation. The smoothness of the transition (1.5 K wide) is due to a distribution of the nucleation rate within the microcrystals of the powder sample. The distribution of the T_c is smaller 0.1 K.