Experimental and theoretical characterization of ordered MAX phases Mo_2TiAlC_2 and Mo_2Ti_2AlC_3

摘要

Herein, we report on the phase stabilities and crystal structures of two newly discovered ordered, quaternary MAX phases-Mo_2TiAlC_2 and Mo_2Ti_2AlC_3-synthesized by mixing and heating different elemental powder mixtures of mMo:(3-m)Ti:1.1Al:2C with 1.5≤m≤2.2 and 2Mo: 2Ti:1.1Al:2.7C to 1600℃ for 4h under Ar flow. In general, for m≥2 an ordered 312 phase, (Mo_2Ti)AlC_2, was the majority phase; for m＜2, an ordered 413 phase (Mo_2Ti_2)AlC_3, was the major product. The actual chemistries determined from X-ray photoelectron spectroscopy (XPS) are Mo_2TiAlC_(1.7) and Mo_2Ti_(1.9)Al_(0.9)C_(2.5), respectively. High resolution scanning transmission microscopy, XPS and Rietveld analysis of powder X-ray diffraction confirmed the general ordered stacking sequence to be Mo-Ti-Mo-Al-Mo-Ti-Mo for Mo_2TiAlC_2 and Mo-Ti-Ti-Mo-Al-Mo-Ti-Ti-Mo for Mo_2Ti_2AlC_3, with the carbon atoms occupying the octahedral sites between the transition metal layers. Consistent with the experimental results, the theoretical calculations clearly show that M layer ordering is mostly driven by the high penalty paid in energy by having the Mo atoms surrounded by C in a face-centered configuration, i.e., in the center of the M_(n+1)X_n blocks. At 331 GPa and 367 GPa, respectively, the Young's moduli of the ordered Mo_2TiAlC_2 and Mo_2Ti_2AlC_3 are predicted to be higher than those calculated for their ternary end members. Like most other MAX phases, because of the high density of states at the Fermi level, the resistivity measurement over 300 to 10 K for both phases showed metallic behavior.