Spin-polarized density functional theory (DFT) calculations have been performed on the structure and stability of Fe2C. It is found that orthorhombic Fe2C is more stable than hexagonal Fe2C by 0.16eV on the basis of the computed cohesive energies. The structures and stability of the orthorhombic-Fe2C low index surfaces have also been investigated at the same level and the low index surfaces have the decreased stability order of (011) > (110) > (100) > (101) > (001). Comparison of the most stable Fe3C, Fe4C and Fe2C surfaces shows that there is no linear correlation of surface energy and carbon content. And comparison of their most stable surfaces with the body-centered cubic Fe shows that these carbide surfaces have lower surface energies than the most stable (110) surface of body-centered cubic Fe, indicating that the surface thermodynamics favor carburization at Fe surfaces.%采用自旋极化的密度泛函理论(DFT)对正交与六方的Fe2C晶体体相与表面性质进行了研究,计算了晶胞的聚合能、磁矩以及低指数晶面的表面能.研究结果表明,两种晶型Fe2C 的磁性质相似,但正交堆积的Fe2C比六方堆积的Fe2C更稳定.正交晶系Fe2C低指数晶面的稳定性以 (011) > (110) > (100) > (101) > (001) 顺序降低.对一系列碳化程度不同的碳化铁最稳定表面(Fe2C(011)、Fe3C(001)和Fe4C(100))表面能的比较显示,碳化铁表面的相对稳定性与碳化度非线性相关.另外,与面心立方(BCC)铁最稳定表面(110)相比,Fe2C、Fe3C及Fe4C晶体最稳定表面具有较低的表面能,表明铁表面碳化在热力学上是有利的.
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