Theoretical investigation of the effects of impurity on the properties of Nb2GeC

Abstract

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 irradiation-induced 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 and 1.985 Å 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 Å respectively. The formation energies of He substitution present the sequence: Ef(Hesub-C) > Ef(Hesub-Nb) > Ef(Hesub-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.