We report on recent progress in the control of dilution of photo-induced magnetic structures, by using thermal activation of the steric relaxation of the photo-excited electronic state. Experimental data relative to Rb- and Cs- based Prussian Blue Analogues are reported in terms of dynamic phase diagrams : Curie temperature vs concentration in photo-excited species. The building-up of correlations, due to steric interactions during the thermal relaxation process, is discussed.