The first principles method is used to study the intrinsic vacancy of Cr, Al, and C in Cr2AlC materials with formation energies of 1.89, 1.95, and 1.07 eV, respectively. It has been proven in previous research that Al layers could be easily removed from Cr2AlC, and the formation energies of Cr2AlC are, therefore, calculated in this study after removing two layers of Al atoms to form Cr2C. The formation energies of the H, He, and O atoms that replace the Al atoms are also calculated to be −2.83, 90.73, and −47.81 eV, respectively. It shows that under irradiation or a high temperature environment, Cr2AlC is easily oxidized to form Cr2C materials. Furthermore, the density of states of Cr2AlC with an Al layer substituted by H, He, and O atoms, as well as the phonon properties of Cr2AlC and Cr2C, are calculated. The results show that the Cr–C metal bond is dominant in Cr2AlC materials to maintain the stability of the structure. The calculation results of mechanical properties show that the presence of Al atoms enhances the plasticity of Cr2AlC.
First-principles calculations are performed to study the effects of defect on the structure and electronic properties of Ti3SiC2. The calculations show that the formation energy of Si vacancy is minimal compared with the Ti or C vacancies in Ti3SiC2. The defects of Si layer also can be formed under high-temperature or irradiation environments. The C-layers or Ti-layers are almost impossible to form. If the Si vacancy or Si layers are formed, they prefer to be substituted by the O and H atoms to form the MXene structure, and the unit cell of Ti3SiC2 lattice constant decreases in c-direction. However, it has quite slight effect on electronic properties of Ti3SiC2. The He impurities are almost impossible to occupy the Si vacancies, because the formation energy are 50.860 eV for one layer of Si atoms substituted by the He atoms. This type of defect leads to the lattice constant of Ti3SiC2 in c-direction increasing considerably. Therefore, Ti3SiC2 is a suitable candidate for nuclear materials because of the high-formation energies of He impurities under irradiation environment.
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