Crystal Structure and Elastic Properties of Hypothesized MAX Phase-Like Compound (Cr_2Hf)_2Al_3C_3

摘要

The term "MAX phase" refers to a very interesting and important class of layered ternary transition-metal carbides and nitrides with a novel combination of both metal and ceramic-like properties that have made these materials highly regarded candidates for numerous technological and engineering applications. Using (Cr_2Hf)_2Al_3C_3 as an example, we demonstrate the possibility of incorporating more types of elements into a MAX phase while maintaining the crystallinity, instead of creating solid solution phases. The crystal structure and elastic properties of MAX phase-like (Cr_2Hf)_2Al_3C_3 are studied using the Vienna ab initio Simulation Package. Unlike MAX phases with a hexagonal symmetry (P6_3/mmc, #194), (Cr_2Hf)_2Al_3C_3 crystallizes in the monoclinic space group of P2_1/m (#11) with lattice parameters of a = 5.1739 A, b = 5.1974 A, c = 12.8019 A; α = β = 90°, γ = 119.8509°. Its structure is found to be energetically much more favorable with an energy (per formula unit) of -102.11 Ev, significantly lower than those of the allotropic segregation (-100.05 Ev) and solid solution (-100.13 Ev) phases. Calculations using a stress versus strain approach and the VRH approximation for polycrystals also show that (Cr_2Hf)_2Al_3C_3 has outstanding elastic moduli.