Density-functional-theory predictions of mechanical behaviour and thermal properties as well as experimental hardness of the Ga-bilayer Mo₂Ga₂C

Mo₂Ga₂C is a new MAX phase with a stacking Ga-bilayer as well as possible unusual properties. To understand this unique MAX phase structure and promote possible future applications, the structure, chemical bonding, and mechanical and thermodynamic properties of Mo₂Ga₂C were investigated by first-principles. Using the "bond stiffness" model, the strongest covalent bonding (1162 GPa) was formed between Mo and C atoms in Mo₂Ga₂C, while the weakest Ga-Ga (389 GPa) bonding was formed between two Ga-atomic layers, different from other typical MAX phases. The ratio of the bond stiffness of the weakest bond to the strongest bond (0.33) was lower than 1/2, indicating the high damage tolerance and fracture toughness of Mo₂Ga₂C, which was confirmed by indentation without any cracks. The high-temperature heat capacity and thermal expansion of Mo₂Ga₂C were calculated in the framework of quasi-harmonic approximation from 0 to 1300 K. Because of the metal-like electronic structure, the electronic excitation contribution became more significant with increasing temperature above 300 K.