Y 5 Si 2 B 8 : A theoretically predicted new damage‐tolerant MAB phase with layered crystal structure

Layer structured MAB phases (M=transition metal, A=III A or IV A group element, B is boron) are promising ternary borides for high and ultrahigh‐temperature applications. Herein, a new MAB phase Y 5 Si 2 B 8 consisting of alternative stacking of YB 4 and Y 3 Si 2 slabs along the [001] direction is investigated. Density functional theory (DFT) calculations on the electronic structure and chemical bonding reveal that this new MAB phase has diverse chemical bonding and properties similar to MAX phases. The strong covalent bonds in the two‐dimensional B network on (001) plane in the YB 4 slab and between Si atoms on (002) plane in the Y 3 Si 2 ‐slab warrant the high stiffness ( E x =288 GPa) of Y 5 Si 2 B 8 in the ab plane and the weak Y2‐Si and Y1‐B2 bonds that connecting the YB 4 and Y 3 Si 2 slabs underpin the low Young's modulus in [001] direction ( E z =200 GPa). The low shear deformation resistance is due to the presence of the metallic bond and the weak bond within the B 6 octahedral. The possible slip systems are {001}< 100> and {110}< 111> . Based on the low shear modulus ( G = 104 GPa) and Pugh's ratio G / B , Y 5 Si 2 B 8 is predicted as a damage tolerant MAB phase. Y 5 Si 2 B 8 is also predicted electrically conductive and the conductivity is higher in directions parallel to ab plane. In addition, temperature‐dependent phonon and electron heat capacity are predicted based on the electron and phonon density of states analysis.