Experimental and theoretical characterization of ordered MAX phases Mo₂TiAlC₂ and Mo₂Ti₂AlC₃

Herein, we report on the phase stabilities and crystal structures of two newly discovered ordered, quaternary MAX phases—Mo₂TiAlC₂ and Mo₂Ti₂AlC₃—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 °C for 4 h under Ar flow. In general, for m ≥ 2 an ordered 312 phase, (Mo₂Ti)AlC₂, was the majority phase; for m < 2, an ordered 413 phase (Mo₂Ti₂)AlC₃, was the major product. The actual chemistries determined from X-ray photoelectron spectroscopy (XPS) are Mo₂TiAlC_1.7 and Mo₂Ti_1.9Al_0.9C_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 Mo₂TiAlC₂ and Mo-Ti-Ti-Mo-Al-Mo-Ti-Ti-Mo for Mo₂Ti₂AlC₃, 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+1X_n blocks. At 331 GPa and 367 GPa, respectively, the Young's moduli of the ordered Mo₂TiAlC₂ and Mo₂Ti₂AlC₃ 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.