DFT insights into new B-containing 212 MAX phases: Hf2AB2 (A = In, Sn)

Highlights: • Physical properties of Hf₂AB₂ (A = In, Sn) have been studied using first-principles technique. • Hf₂AB₂ (A = In, Sn) compounds have higher mechanical strength and Vickers hardness compared to those of Hf₂AC (A = In, Sn). • The charge density mapping discloses the existence of very strong B‒B covalent bonds in Hf₂AB₂ (A = In, Sn). • Hf₂AB₂ (A = In, Sn) have superior thermal properties compared to Hf₂AC (A = In, Sn). • Hf₂AB₂ (A = In, Sn) has the potential to be used as reflecting coating to diminish solar heating. -- Abstract: 212 MAX phase borides are new additions to the MAX phase family with enhanced thermo-mechanical properties. In this article, first-principles calculations have been carried out to explore the mechanical properties, Vickers hardness, elastic anisotropy, thermal properties, and optical properties of predicted thermodynamically stable MAX compounds Hf₂AB₂ (A = In, Sn). The structural properties are compared with the available data to assess the validity of calculations. The mechanical stability of Hf₂AB₂ (A = In, Sn) compounds is established with the help of the computed stiffness constants (C_ij). The possible reason for enhanced mechanical properties and Vickers hardness of Hf₂SnB₂ is explained based on the analysis of bonding strength, followed by the electronic density of states. Higher mechanical strength and Vickers hardness of Hf₂AB₂ (A = In, Sn) compared to Hf₂AC (A = In, Sn) are also indicated in the light of charge density mapping. The values of Pugh ratio, Poisson's ratio and Cauchy pressure predict brittle character of the studied compounds. Besides, the anisotropic nature of the titled borides is investigated by 2D and 3D plots of elastic moduli along with some well established anisotropy indices. Thermal properties were investigated by calculating the Debye temperature, minimum thermal conductivity, Grüneisen parameter, and melting temperature. The thermal properties of Hf₂AB₂ (A = In, Sn) are also superior to Hf₂AC (A = In, Sn). The optical constants such as real and imaginary parts of the dielectric function, refractive index, extinction coefficient, absorption coefficient, photoconductivity, reflectivity, and loss function are investigated.