The coherent-domain morphology was determined for a crystalline powder of the 1D polymeric spin-crossover [Fe(Htrz)2(trz)](BF4) {Htrz = 1H-1,2,4-triazole, trz = deprotonated triazolato(–) ligand} compound. This morphology is in adequacy with the crystal packing and proved that the long axis of the domain corresponds to the iron–triazole chains axis, which gives information on the crystal-growth process. Furthermore, thermal cycling clearly demonstrated that the coherent-domain length strongly and continuously decreased as a function of the number of spin-crossover cycles undergone. The degradation in the crystalline quality corresponds to a break in the [Fe(Htrz)2(trz)]+ chains. This structural fatigability was confirmed by similar observations on a close spin-crossover compound, [Fe(NH2trz)3](NO3)2·H2O, showing that a few hundreds of thermal spin-crossover cycles could destroy the crystalline aspect of the sample.