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We investigate here the internal structure of zinc ferrite nanoparticles designed and prepared by a soft chemistry method to elaborate magnetic nanocolloids. The strategy used to avoid acid dissolution modifies the chemical composition of the surface of the nanoparticles, which are described as a core of stoichiometric zinc ferrite surrounded by a maghemite shell. Measurements of X-ray absorption nearedge spectroscopy, extended X-ray absorption fine structure, and X-ray diffraction are undertaken to investigate the local structure of nontreated nanocrystals and of surface-treated ones as a function of their sizes. The qualitative analysis of X-ray absorption results indicates a nonequilibrium cation distribution among the interstitial sites of the zinc ferrite nanocrystals core. Ab-initio calculations of theoretical photoelectron backscattering phases and amplitudes give, by fitting Fourier transformed EXAFS data at both Zn and Fe K-edges, an average inversion degree of 0.34. This value well matches the result of Rietveld refinement of X-ray diffraction data. Magnetization measurements performed on dilute aqueous nanocrystal dispersions, liquid at room temperature and frozen at low temperatures, are carried out in order to test the obtained results.

We investigate the local structure of nanoparticles based on a manganese ferrite core surrounded or not by a maghemite layer obtained after hydrothermal surface treatment. Results of X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) measurements are crossed with those of infield Mossbauer spectroscopy and X-ray absorption spectroscopy (XANES/EXAFS) to study the valence state of Mn ions and the cation distribution at interstitial sites of the core−shell nanoparticle structure. Linear combination fitting of XANES data clearly indicates the existence of mixed valence states of Mn cations in the Mn ferrite phase. As a direct consequence, it induces nonequilibrium cation distributions in the nanoparticle core with the presence of a large amount of Mn cations at octahedral sites. The quantitative results of the inversion degree given by NPD, Mossbauer spectroscopy measurements, and EXAFS are in good accordance. It is also shown that both the proportions of each oxidation degree of Mn ions and their location at tetrahedral or octahedral sites of the spinel nanocrystal core can be modified by increasing the duration of the surface treatment. a χ M is the molar fraction of manganese ions obtained by ICP and AAS techniques, ⟨a⟩ is the average lattice parameter deduced from Bragg's law, ϕ c / ϕ is the volume fraction of the core, and t sh is the thickness of the surface layer. The particle sizes (D XR ) were obtained by Scherrer's equation.

The method of perturbed angular correlation (PAC) was applied to selected MAX phases with 211 stoichiometry. Radioactive (111)In ions were implanted in order to measure the electric field gradients (EFG) in the key compounds Ti(2)InC and Zr(2)InC to determine the strength and symmetry of the EFG at the In-site. Further PAC studies in the In-free MAX phases Ti(2)AlN, Nb(2)AlC, Nb(2)AsC and Cr(2)GeC were performed to confirm that the In probes occupy the A-site as well. The strength of the EFG, with a quadrupole coupling constant ν(Q) between 250 and 300 MHz in these phases, is quite similar to the ones found in Ti(2)InC with ν(Q) = 292(1) MHz and in Zr(2)InC with ν(Q) = 344(1) MHz, respectively. Different annealing behavior was observed whereas in all cases a linear decrease of ν(Q) with increasing measuring temperatures was found. The experimental results are also in excellent agreement with those predicted by ab initio calculations using the APW+lo method implemented in the WIEN2k code. This study shows in an exceptional manner that (111)In → (111)Cd atoms are suitable probes to investigate the local surrounding at the A-site in 211-MAX phases.