The structural and magnetic properties of nanocrystalline manganese, cobalt, and nickel spinel ferrites dispersed in a highly porous SiO 2 aerogel matrix were studied. X-ray diffraction and high-resolution transmission electron microscopy indicate that single crystalline ferrite nanoparticles are well dispersed in the amorphous matrix. The cation distribution between the octahedral and tetrahedral sites of the spinel structure was investigated by X-ray absorption spectroscopy. The analysis of both the X-ray absorption near edge structure and the extended X-ray absorption fine structure indicates that the degree of inversion of the spinel structure increases in the series Mn, Co, and Ni spinel, in accordance with the values commonly found in the corresponding bulk spinels. In particular, fitting of the EXAFS data indicates that the degree of inversion in nanosized ferrites is 0.20 for MnFe 2 O 4 , 0.68 for CoFe 2 O 4 , and 1.00 for NiFe 2 O 4 . Magnetic characterization further supports these findings.

Here we report on new tris(haloanilato)metallate(III) complexes with general formula [A]3[M(X2An)3] (A = (n-Bu)4N(+), (Ph)4P(+); M = Cr(III), Fe(III); X2An = 3,6-dihalo derivatives of 2,5-dihydroxybenzoquinone (H4C6O4), chloranilate (Cl2An(2-)), bromanilate (Br2An(2-)) and iodanilate (I2An(2-))), obtained by a general synthetic strategy, and their full characterization. The crystal structures of these Fe(III) and Cr(III) haloanilate complexes consist of anions formed by homoleptic complexes formulated as [M(X2An)3](3-) and (Et)3NH(+), (n-Bu)4N(+), or (Ph4)P(+) cations. All complexes exhibit octahedral coordination geometry with metal ions surrounded by six oxygen atoms from three chelate ligands. These complexes are chiral according to the metal coordination of three bidentate ligands, and both Λ and Δ enantiomers are present in their crystal lattice. The packing of [(n-Bu)4N]3[Cr(I2An)3] (5a) shows that the complexes form supramolecular dimers that are held together by two symmetry related I···O interactions (3.092(8) Å), considerably shorter than the sum of iodine and oxygen van der Waals radii (3.50 Å). The I···O interaction can be regarded as a halogen bond (XB), where the iodine behaves as the XB donor and the oxygen atom as the XB acceptor. This is in agreement with the properties of the electrostatic potential for [Cr(I2An)3](3-) that predicts a negative charge accumulation on the peripheral oxygen atoms and a positive charge accumulation on the iodine. The magnetic behaviour of all complexes, except 5a, may be explained by considering a set of paramagnetic non-interacting Fe(III) or Cr(III) ions, taking into account the zero-field splitting effect. The presence of strong XB interactions in 5a are able, instead, to promote antiferromagnetic interactions among paramagnetic centers at low temperature, as shown by the fit with the Curie-Weiss law, in agreement with the formation of halogen-bonded supramolecular dimers.

X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques at both Fe and Co K-edges were used to investigate the formation of CoFe 2 O 4 nanoparticles embedded in a silica aerogel matrix as a function of calcination temperature and CoFe 2 O 4 content. In particular, nanocomposite aerogels containing relative CoFe 2 O 4 amounts of 5 and 10 wt % and calcined at 450, 750, and 900°C were studied. The evolution of the nanophase with calcination temperatures depends on the composition. In the sample containing 10 wt % of nanophase, results indicate that CoFe 2 O 4 nanocrystals were formed after calcination at 750°C, whereas in the sample containing 5 wt % of nanophase, they were obtained only after calcination at 900°C. Quantitative determination of the distribution of the iron and cobalt phase in the octahedral and tetrahedral sites of the spinel structure shows that cobalt ferrite prepared by sol-gel has a partially inverted spinel structure with a degree of inversion around 0.70.