Drastic lattice softening in mixed triazole ligand iron(II) spin crossover nanoparticles

A series of spin-crossover (SCO) coordination nanoparticles (ca. 60 nm) with the general formulae [Fe(Htrz)1+y-x(trz)2-y(NH2trz)x](BF4)y·nH2O (x=0, 0.1, 0.2 and 0.3) were synthesized in concentrated solutions without using any surfactant or polymer. The nanoparticle powders were investigated by transmission electron microscopy, powder X-ray diffraction, magnetometry, calorimetry, Raman/IR spectroscopies, elemental analysis and 57Fe Mossbauer spectrometry. Remarkably, the latter revealed a large decrease of the lattice stiffness when incorporating a small amount of amino-triazole ligand, reflected by the drop of the Debye temperature from 285 K (x=0) to 205 K (x=0.3). This collapse of the lattice cohesion was attributed to a reorganization of the supramolecular interactions between the Fe-triazole chains. This effect on the SCO properties is also discussed.