The high‐spin and low‐spin crystal structures of [Fe(Htrz)2(trz)](BF4) (Htrz = 1H‐1,2,4‐triazole, trz– = deprotonated triazolato ligand) were determined and refined on the basis of X‐ray diffraction data obtained from a high‐quality crystalline powder. Noteworthy differences to the previously reported structural hypothesis are obtained, which includes a revision of the space group to orthorhombic Pnma. Notably, the distinction between the positions of the Htrz and the trz– ligand along the chains reveals their respective roles in the formation of direct interchain interactions. The latter are also mediated by the anions. In addition, the pair‐distribution‐function (PDF) method was applied to investigate the potential modification of the crystal structure by a reduction of the coherent‐domain size from 50 nm to 10 nm. First, the PDF investigation confirms the validity of the crystal structures presented here. Furthermore, in a first approach, it reveals that the crystal structure description remains suitable for the whole range of coherent‐domain sizes investigated.

This paper describes the synthesis of iron(II) spin-crossover nanoparticles prepared by the reverse micelle technique by using the non-ionic surfactant Lauropal (Ifralan D0205) from the polyoxyethylenic family. By changing the surfactant/water ratio, the size of the particles of [Fe(NH2-trz)3]Br2.3H2O (with NH2trz=4-amino-1,2,4-triazole) can be controlled. On the macroscopic scale this complex exhibits cooperative thermal spin crossovers at 305 and 320 K. We find that when the size is reduced down to 50 nm, the spin transition becomes gradual and no hysteresis can be detected. For our data it seems that the critical size, for which the existence of a thermal hysteresis can be detected, is around 50 nm. Interestingly, the change of the particle size induces almost no change in the temperature of the thermal spin transition. A systematic determination of coherent domain size carried out on the nanoparticles by powder X-ray diffraction indicates that at approximately 30 nm individual particles consist of one coherent domain.

International audienceVariable-temperature studies of the electronic structures of four different Fe(II) spin crossover molecules, [Fe(H2B(pz)2)2(bipy)] (pz = pyrazol-1-yl, bipy = 2,2′-bipyridine), [Fe(H2B(pz)2)2(phen)], [Fe(phen)2(NCS)2] (phen = 9,10-phenantroline), and [Fe(PM-AzA)2(NCS)2] (PM-AzA = 4-phenyldiazenyl-N-(pyridin-2-ylmethylene)aniline) by X-ray absorption spectroscopy (XAS), combined with electrical properties studies of the [Fe(PM-AzA)2(NCS)2] single crystal are presented. We show that both the XAS signature of the spin state of powdered samples and the dielectric permittivity of the [Fe(PM-AzA)2(NCS)2] single crystal change at significantly lower temperatures than the magnetometry, structure, and resistivity indicators of a spin crossover transition. The changes in electronic structure are in agreement with the expectations from density functional theory (DFT) results for the different molecular electronic structures associated with the high-spin and low-spin states. These findings suggest that the electronic structure phase ordering process does not simply follow the spin transition