2. Computational methods and details
In this work, we set up the theoretical models of chemically ordered MAX phase compositions Ti3AlB2 and two Ti2ZrAlB2 structures based on the structure of Ti3AlC2 reported by Pietzkaet al [27]. The predicted structural parameters of these three new compounds are presented in the Supplement Material. Then first-principles calculations were carried out by VASP software to study the structural stability, electronic, elastic and thermophysical properties of the ternary ceramic Ti3AlB2 and two quaternary ceramic Ti2ZrAlB2. The crystal structures of these ceramic compounds were optimized by the conjugate gradient (CG) algorithm. The plane-wave pseudopotential method within the PBE-GGA designed by Perdew, Burke, and Ernzerhof [28-29] is employed in the calculations. The cut-off energy for the plane wave basis set was configured as 520 eV. A 12×12×2 Monkhorst-pack grid [30] of k-point sampling was set for the first electronic Brillouin zone (BZ). In the structural optimizations, the calculated systems are relaxed until the total energy change and the band structure energy change between two steps are both smaller than 1×10−6 eV in the relaxation of the electronic degrees of freedom, and the forces of all atoms are smaller than 0.02 eV/Å in the ionic relaxation. Phonon dispersions were calculated in the Phonopy software based on the density functional linear-response method [31].