Investigation Of The Effects Of Point Defects On Properties Of Nb₂GeC And Ti₂GaC

The MAX phases have been considered as high-temperature structural materials with variety of potential applications, especially in the nuclear industry. In this paper, First principles calculations are performed on the energetics of monovacancy, O, H and He impurities in Nb₂ GeC and Ti₂ GaC, and the effects of them on structural and electronic properties are also investigated.The monovacancy formation energies of each element Nb, Ge and C in the MAX phase Nb₂ GeC are obtained. It is noted that the vacancy of each element has a slight effect on the hybridization bonding. If there is a vacancy of either Ge or C, the hybridization bonding will be slightly reduced. However, if there is a Nb vacancy, the hybridization bonding will be increased between the d-orbital and p-orbital. In contrast,the vacancies can substantially reduce the conductivity of the MAX phase Nb₂ GeC. It is also noted that if there is a vacancy of either Ge or C, the hardness of the Nb₂ GeC decreases, while it is increased if there is a Nb vacancy.In order to further investigate the characters of Nb₂ GeC in irradiation environment,theoretical calculations are performed on the energetics of O, H and He impurities in Nb₂ GeC. 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 Nb₂ GeC with substitutional or interstitial impurities are obtained, and its electronic property is analysed by Mulliken population and electron charge density.We also calculated the mono-vacancy formation and migration energies of each element Ti, Ga, and C in the MAX phase Ti₂ GaC, and formation energies of oxygen substituting for Ti, Ga, and C and two formation energies of oxygen interstitial in different sites. The results show that the formation energy of oxygen substituting for Ti is the highest, and the formation energies of the O substitution for Ga atoms decrease as the oxygen concentration increases. The two different formation energies of one oxygeninterstitial show that the stable site for the oxygen interstitial is at the center of the triangle composed by three Ga atoms. The effects of vacancy, oxygen substitution, and the interstitial on the electronic properties of Ti₂ GaC are also discussed in light of the density of states and the electron charge density.