We have followed the photo-transformation, the light-induced thermal hysteresis, and the light-induced phase separation (LIPS) into like-spin phases on a [Fe(ptz)6](BF4)2 single crystal, in the quenched rhombohedral phase, through the analysis of neutron Laue diffraction patterns. In agreement with the analysis of previous magnetic measurements, phase separation was observed in the close vicinity of the static light-induced instability, but not otherwise. The positions of the Laue spots were analysed in terms of variation in the a/c lattice-parameter ratio which is well correlated with the variation of the high-spin fraction predicted by a simple homogeneous mean-field model. Using a mapped version of the same model, we analyzed the line shapes of the Laue spots during photo-excitation and relaxation experiments so as to determine the width of the distribution of irradiation intensities, a crucial parameter in phase separation. The predictions of the mapped model failed to reproduce the rapid initial kinetics of the phase separation process, but were in agreement with the further evolution. These features are attributed to a nucleation and growth process, for which the crossover from microscopic to macroscopic regimes has been observed.
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