Carta Daniela, Casula Maria Francesca, Mountjoy Gavin, Corrias Anna
Dipartimento di Scienze Chimiche and INSTM, Università di Cagliari, S.P. Monserrato-Sestu Km 0.700, I-09042, Monserrato, Cagliari, Italy.
Phys Chem Chem Phys. 2008 Jun 7;10(21):3108-17. doi: 10.1039/b800359a. Epub 2008 Apr 10.
Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques at both Fe and Mn K-edges were used to investigate the formation of MnFe(2)O(4) nanoparticles embedded in a silica aerogel matrix as a function of calcination temperature (at 450, 750 and 900 degrees C). Up to 450 degrees C, two separated highly-disordered phases of iron and manganese are present. With increasing the temperature (to 750 and 900 degrees C), the structure of aerogel nanoparticles becomes progressively similar to that of the spinel structure MnFe(2)O(4) (jacobsite). Quantitative determination of cations distribution in the spinel structure shows that aerogels calcined at 750 and 900 degrees C have a degree of inversion i = 0.20. A pure jacobsite sample synthesised by co-precipitation and used as a reference compound shows a much higher degree of inversion (i = 0.70). The different distribution of iron and manganese cations in the octahedral and tetrahedral sites in pure jacobsite and in the aerogels can be ascribed to partial oxidation of Mn(2+) to Mn(3+) in pure jacobsite, confirmed by XANES analysis, probably due to the synthesis conditions.
利用铁和锰K边的扩展X射线吸收精细结构（EXAFS）和X射线吸收近边结构（XANES）技术，研究了嵌入二氧化硅气凝胶基质中的MnFe₂O₄纳米颗粒的形成与煅烧温度（450、750和900℃）的关系。在450℃以下，存在两个分离的高度无序的铁和锰相。随着温度升高（至750和900℃），气凝胶纳米颗粒的结构逐渐变得与尖晶石结构MnFe₂O₄（铁锰尖晶石）相似。对尖晶石结构中阳离子分布的定量测定表明，在750和900℃煅烧的气凝胶的反位度i = 0.20。通过共沉淀合成并用作参考化合物的纯铁锰尖晶石样品显示出更高的反位度（i = 0.70）。纯铁锰尖晶石和气凝胶中八面体和四面体位置的铁和锰阳离子的不同分布可归因于纯铁锰尖晶石中Mn²⁺部分氧化为Mn³⁺，XANES分析证实了这一点，这可能是由于合成条件所致。
