Polymorphism of Ti3SiC2 ceramic:: First-principles investigations -: art. no. 144108

The Nanolaminate Ti3SiC2 ceramic exhibited unique mechanical properties, such as high modulus, low anisotropic hardness to modulus ratio and microscale ductility etc. The planar close-packed Si atoms were expected to play a dominant role in deducing these properties. By performing first-principles total energy calculations, we demonstrated that a reversible polymorphic phase transition occurred when shear strain energy was large enough to close an energy barrier. The phase transition path was described as the Si atoms sliding between 2b and 2d Wyckoff positions on the (11(2) over bar 0) plane. The electronic band structure, lattice dynamics, and structure stability were discussed for the two polymorphs, respectively. We demonstrated that the alpha-Ti3SiC2 was more stable than beta-Ti3SiC2 by comparing the ground-state total energy and ab initio Gibbs free energy. Raman and infrared active phonon modes were illustrated for feasibly identifying the two phases in experimental spectra. The results were used to assign peaks in the experimental Raman spectrum with distinct vibrational modes, and to clarify the origin of the uncertain peak. The calculated heat capacity and volume thermal expansion coefficient agreed with experimental values well. The elastic mechanical parameters of the polymorphs were presented and compared with respect to various strain modes. Based on electronic band structure discussions, we clarified the mechanism of anisotropic hardness of Ti3SiC2, which attributed to different covalent bonding strengths involved in kink migration.