杂质对Nb2GeC性质影响的第一性原理研究

Theoretical investigation of the effects of impurity on the properties of Nb_2GeC

为进一步研究Nb_2GeC在辐照环境中的稳定性,本文研究了O,H和He杂质在Nb_2GeC中的稳定情况.所有杂质的研究都是从替代和间隙两个方面来进行的,计算得到了替代和间隙的形成能,存在替代和间隙时Nb_2GeC的晶格常数,以及单胞体积,并且与完美的晶胞进行了比较.此外,通过电荷密度分布和Mulliken布居,分析了O,H,He杂质对Nb_2GeC的电子性质的影响.

MAX phases are potential future materials used in the nuclear industry.Recently,a new MAX phase Nb2GeC is predicted as the most stable compound,and confirmed by thin film synthesis.In the operation of fusion reactor,the accumulation and aggregation of helium and hydrogen produced from transmutation reactions would induce bubble formation and void swelling and further result in embrittlement and irradiation-induced hardening of the materials.High solubility and permeability of tritium and solubility of interstitial impurities like O,C,and N can also lead to embrittlement.In order to further investigate the characters of Nb2Ge in irradiation environment,ab initio calculations are performed on the energetics of O,H and He impurities in Nb2Ge.The study of all the impurities is carried out in two ways,substitutionally and interstitially.Formation energies due to substitution and interstitial are calculated,lattice parameters and unit cell volume of Nb2GeC with substitutional or interstitial impurities are obtained,and its electronic property is analysed by Mulliken population and electron charge density.The formation energies of H substitution are lower than those of O substitution and He substitution,hence H atoms are trapped more easily by some irradiationinduced vacancies.The formation energies of O subtitution indicate the sequence Ef（Osub-Nb） 〉 Ef（Osub-Ge） ≈ Ef（Osub-C）,which is related to the strength of bonds.Analysis on electron charge density and Mulliken population shows that C O bond is stronger than Nb O and Ge O bond,and the bond lengths of C O,Nb O and Ge O are 3.256,2.118 and1.985 A respectively.Due to the interaction of O atom with Nb,Ge and C atoms in Nb2Ge,the O atom would deviate from the vacancy,and goes to the deformed sites in the crystal structure.As for H substitution,the formation energies of substitution show the sequence Ef（Hsub-Nb） 〉 Ef（Hsub-Ge） 〉 Ef（Hsub-C）.C H and Nb H are ionic bond and covalent bond respectively,and their bond lengths are 3.131 and 2.706 A respectively.The formation energies of He substitution present the sequence：Ef（Hesub-C） 〉 Ef（Hesub-Nb） 〉 Ef（Hsub-Ge）,and suggest that the He atom is the easiest to be trapped by C vacancy.All O,H and He interstitials make lattice parameter a increase,c decrease and unit cell V shrink.Besides,the results of substitution and interstitial formation energies show that O,H and He impurities prefer to stay on octahedral sites.These results could provide initial physical picture for further understanding the accumulation and bubble formation of impurities in Nb2GeC.