基于第一性原理的Si掺杂磁铁矿稳定性研究

磁铁矿中出现的杂质元素及其赋存形式具有很好的环境指示意义。Si是磁铁矿中常见的杂质之一，尽管Si与Fe在离子半径、化合价、置换能量效应等方面均存在较大差别，但无论磁铁矿的原位成分数据还是湿法化学分析数据均表明，磁铁矿中含有一定数量(热液环境中生成的磁铁矿晶体的SiO₂的质量分数可达6.19%)的Si。为探究Si在磁铁矿中是否能稳定存在及其具体存在位置，在VASP中建立Si在磁铁矿中不同位置的模型，然后利用密度泛函理论第一性原理计算Si在磁铁矿晶体中不同位置的形成能，以此判断Si在磁铁矿晶体中的稳定性。结果表明，Si替代磁铁矿八面体中心的Fe的掺杂形成能为-4.13 eV,替代四面体中心的Fe的掺杂形成能为-3.85 eV。值得注意的是，1个磁铁矿原胞内2个Fe³⁺(四面体中心)全部被Si替代的形成能为-8.87 eV。从掺杂形成能的角度来看，Si在磁铁矿四面体及八面体的中心位置均可存在，但Si代替四面体中心位置的全部Fe³⁺得到的结构最稳定。

Stability study on Si doped magnetite based on the first principles

The impurity elements and their occurrence forms in magnetite have good environmental indication significance. Si is one of the common impurities in magnetite. Although there are great differences between Si and Fe in ionic radius, valency, energy effect of replacement and other aspects, both the in-situ composition data of magnetite and the wet chemical analysis data show that, magnetite contains a certain amount of Si (the mass fraction of SiO₂ of magnetite crystals generated in hydrothermal environment can reach 6.19%). In order to explore whether Si can statically exist in magnetite and its specific form, the first principles of density functional theory was used to calculate the formation energy of Si in different positions of magnetite crystal, and the stability of Si in magnetite crystal was studied. The results show that the doping formation energy of replacing octahedral Fe with Si in magnetite is -4.13 eV, and the doping formation energy of replacing Fe in the tetrahedron center is -3.85 eV. It is noteworthy that the formation energy of both the two Fe³⁺ (tetrahedron center) in a magnetite cell are replaced is -8.87 eV. From the perspective of doping formation energy, Si can exist in the center of tetrahedron and octahedron of magnetite, but the structure obtained by replacing all Fe³⁺ in the center of tetrahedron with Si is the most stable.