The Rise of 212 MAX Phase Borides: DFT Insights into the Physical Properties of Ti2PB2, Zr2PbB2, and Nb2AB2 [A = P, S] for Thermomechanical Applications

In this article, ab initio calculations of unexplored Ti2PB2, Zr2PbB2, and Nb2AB2 [A = P, S] were performed wherein Ti2PB2 along with its 211 boride phase Ti2PB was predicted for the first time. The stability was confirmed by calculating the formation energy, phonon dispersion curve, and elastic stiffness constants. The obtained elastic constants, elastic moduli, and Vickers hardness values of Ti2PB2, Zr2PbB2, and Nb2AB2 [A = P, S] were found to be significantly larger than those of their counterparts 211 borides and carbides. The studied compounds are brittle, like most MAX and MAB phases. The electronic band structure and density of states revealed the metallic nature of the titled borides. Several thermal parameters were explored, certifying the suitability of Ti2PB2, Zr2PbB2, and Nb2AB2 [A = P, S] to be used as efficient thermal barrier coating materials. The response of Ti2PB2, Zr2PbB2, and Nb2AB2 [A = P, S] to the incident photon was studied by computing the dielectric constant (real and imaginary parts), refractive index, absorption coefficient, photoconductivity, reflectivity, and energy loss function. In this work, we have explored the physical basis of the improved thermomechanical properties of 212 MAX phase borides compared to their existing carbide and boride counterparts.