Trends in mechanical, anisotropic, electronic, and thermal properties of MAX phases: a DFT study on M2SX phases

Abstract

Based on the density functional theory, the structural, mechanical and thermal properties of 27 S-based M2SX (M=Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W; X = B, C, N) phases have been investigated systematically. The calculated formation enthalpies (ΔH) reveal that W2SC and W2SN are thermodynamically unstable phases due to the positive ΔH, and the V2SN, Nb2SN, and Mo2SC are mechanically unstable phases with C12 > C11. The calculated phonon spectrum indicates that 16 phases are dynamically stable phases except M2SN (M=V, Nb, Ta), M2SX (M=Cr, W; X = B, C, N), Mo2SC, and Mo2SN phases. Among the 16 stable phases, the Ti2SB, V2SB, Ta2SB and Ta2SC are predicted in this work, for the other 12 stable phases, the physical properties have been reported more or less. From the calculated results, V2SC, Nb2SC, and Ta2SC are resistant to compression and thermal shock, and which exhibit potential application in thermal barrier coatings. All the stable phases are more compressible along the a-axis, which means that c-axis is stiffer owing to strong atomic bond. Electronic structures indicate the stable phases are all metallic, and the Mo2SB and Ta2SC can be applied in thermoelectric material with very low lattice thermal conductivity. All the present results could motivate exploration and research on novel S-based MAX phases in the future.