The quasi-static nature of a light induced thermal hysteresis was studied on the spin-transition compound [Fe(ptz)₆](BF₄)₂, by means of optical spectroscopy and magnetic measurements in the temperature interval between 10 and 80 K. Various experimental procedures are discussed in relation to the competition between the two processes considered, namely the photoexitation and the high-spin→low-spin relaxation. A detailed discussion of the experimental parameters, which should be considered in order to avoid erroneous interpretations of LITH, is given.

The propagation of the high-spin (HS) → low-spin (LS) relaxation at 53 K in a single crystal of the iron (II) spin-crossover compound [Fe(ptz)₆](BF₄)₂ was followed by photography, after inducing the local photoexcitation to the metastable HS state at 20 K using the single wavelength (457 nm Ar^± ion laser) irradiation. The photoinduced formation of the HS—LS patterns with a characteristic diameter of some 0.1 mm was observed to occur inhomogeneously at a macroscopic scale already during photoexcitation. The contrast between the HS (transparent) and the LS (purple) regions was amplified during relaxation. The effect is described in terms of a transient instability, for which a microscopic model in the mean-field approximation is proposed. The mechanism for the development of patterns at the macroscopic scale is discussed.