We investigate the local structure of nanoparticles based on cobalt ferrite core with and without a hydrothermal surface treatment of variable duration of time that promotes a coating shell by crossing information obtained from atomic absorption spectroscopy (AAS), X-ray powder diffraction (XRD), neutron powder diffraction (NPD), and X-ray absorption spectroscopy (XAS) measurements. Chemical titration results show that the surface layer thickness increases as the duration of the surface treatment increases. The cation distribution obtained by Rietveld refinement of NPD patterns allows splitting the core and shell structural parameters obtained from Rietveld refinement of XRD data of the samples well described in a core-shell model. We follow the local structural evolution with the surface treatment time analyzing both XANES and EXAFS regions of the absorption spectrum. As a result, we observe that the surface treatment is not energetically strong enough to provoke changes on the mean oxidation states of the cations in the spinel structure; indeed, the interatomic distances are similar to those found in bulk materials and the inversion degree remains the same after the surface treatment, in good accordance with the cation distribution obtained by Rietveld refinement of NPD data.

中文翻译：

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核壳CoFe2O4@γ-Fe2O3纳米粒子的局部结构研究

我们通过交叉原子吸收光谱 (AAS)、X 射线粉末衍射 (XRD)、中子粉末衍射 (NPD) 和 X 射线吸收光谱 (XAS) 测量。化学滴定结果表明，表面层厚度随着表面处理时间的增加而增加。通过 NPD 图案的 Rietveld 精修获得的阳离子分布允许拆分从核壳模型中充分描述的样品的 XRD 数据的 Rietveld 精修获得的核和壳结构参数。我们通过分析吸收光谱的 XANES 和 EXAFS 区域的表面处理时间来跟踪局部结构演变。结果，我们观察到表面处理在能量上不足以引起尖晶石结构中阳离子平均氧化态的变化；事实上，原子间距离与散装材料中的原子间距离相似，表面处理后的反转程度保持不变，这与通过 NPD 数据的 Rietveld 细化获得的阳离子分布非常一致。