Theoretical study on structural and mechanical properties of Si-containing ternary transition metal nitrides M(0.5)Si(0.5)N (M = Ti, Zr, Hf)

Si-containing transition-metal nitrides Ti(0.5)Si(0.5)N, Zr(0.5)Si(0.5)N and Hf(0.5)Si(0.5)N with conventional rock salt B1 structure exhibit superior hardness, strength and oxidation resistance. However, the potential phases of the ternary systems at various pressures remain unexplored. In this work, we firstly studied the potential structures of Ti(0.5)Si(0.5)N, Zr(0.5)Si(0.5)N and Hf(0.5)Si(0.5)N in pressures of 0–100 GPa. A hexagonal phase with P6(3)/mmc symmetry was uncovered and verified to be quenchable in the ambient conditions. The structural, mechanical and electronic properties were systematically studied and compared with the well-known ordered B1 structure. We surprisingly found that Ti(0.5)Si(0.5)N within this hexagonal phase displayed much improved ideal indentation shear strength from about 10 GPa for a B1 structure to 30 GPa. The estimated hardness based on the empirical formula is up to 38 GPa, greatly exceeding that of the B1 structure. By the detailed electronic analysis, the underlying atomic mechanism for the outstanding mechanical properties was also studied.