Abstract
While the MXene Ti3C2 is well known for its extraordinary material properties with wide applications, it is demonstrated here that it is not the most stable 2D titanium carbide. Evolutionary search and first-principles calculations are employed to predict for Ti3C3 a novel structure prototype with P4/mmm symmetry and tetragonal sandwich structure. The cohesive energy, phonon dispersion, and melting point demonstrate high stability of Ti3C3. The mechanical properties are found to be even better than those of graphene in terms of Young's modulus and fracture strength. The metallicity of Ti3C3 indicates potential in metal-ion batteries. The diffusion barriers for Li, Na, K, and Ca atoms are found to be as low as 0.15, 0.04, 0.002 (record among the known 2D materials), and 0.14 eV, respectively, suggesting the possibility to realize fast charge and discharge. Importantly, the discovered structure prototype gives rise to a whole family of 2D materials. For example, six thermally and dynamically stable materials with metallic properties, Ti3X3 (X = B, Si, Ge, N, P, and As) are identified. The family is promising not only in the fields of nano-mechanics and metal-ion batteries but also can guide the search for further 2D structure prototypes.