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First-principles calculation and valence electron structure analysis of diamond crystal growth from Fe-Ni-C system at high temperature and high pressure
Xiaohong Fan, Bin Xu, Jing Li and Xiaofei Guo

First principle was used to investigate the lattice constants of graphite at high temperature and high pressure (HTHP) firstly. The results computed by density-functional theory with the generalized gradient approximation (GGA) and the local density approximation (LDA) respectively, were agreed well with the experimental ones. After comparing the accuracy of the calculation results, the GGA method were used to compute the lattice constants of diamond and Fe3C at HTHP, while LDA method was used to compute those of graphite. Based on the empirical electron theory of solid and molecules (EET), the valence electron structures (VESs) of diamond, graphite and Fe3C under the condition of diamond synthesis temperature and pressure (5.3GPa, 1623 K) were then constructed. The relative electron density differences (REDDs) of the diamond/graphite and diamond/Fe3C interfaces were also calculated. It was found that the graphite/diamond interfaces were not continuous under 5.3GPa, 1623 K, while the Fe3C/diamond interfaces were continuous at the first order of approximation, indicating that the carbon atoms, required for diamond growth, could possibly come from the decomposition of Fe3C, but not directly from the graphite.

Keywords: Ab initio calculations, interfaces, carbides, electronic structure.

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