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Title: Experimental and theoretical characterization of ordered MAX phases Mo{sub 2}TiAlC{sub 2} and Mo{sub 2}Ti{sub 2}AlC{sub 3}

Herein, we report on the phase stabilities and crystal structures of two newly discovered ordered, quaternary MAX phases—Mo{sub 2}TiAlC{sub 2} and Mo{sub 2}Ti{sub 2}AlC{sub 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 °C for 4 h under Ar flow. In general, for m ≥ 2 an ordered 312 phase, (Mo{sub 2}Ti)AlC{sub 2}, was the majority phase; for m < 2, an ordered 413 phase (Mo{sub 2}Ti{sub 2})AlC{sub 3}, was the major product. The actual chemistries determined from X-ray photoelectron spectroscopy (XPS) are Mo{sub 2}TiAlC{sub 1.7} and Mo{sub 2}Ti{sub 1.9}Al{sub 0.9}C{sub 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{sub 2}TiAlC{sub 2} and Mo-Ti-Ti-Mo-Al-Mo-Ti-Ti-Mo for Mo{sub 2}Ti{sub 2}AlC{sub 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{sub n+1}X{sub n} blocks. At 331 GPa and 367 GPa, respectively, the Young's moduli of the ordered Mo{submore » 2}TiAlC{sub 2} and Mo{sub 2}Ti{sub 2}AlC{sub 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.« less
Authors:
 [1] ;  [2] ; ; ; ; ;  [3] ;  [1] ;  [4] ; ; ; ;  [1] ;  [1] ;  [5]
  1. Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104 (United States)
  2. (United States)
  3. Thin Film Physics Division, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping (Sweden)
  4. (IFM), Linköping University, SE-581 83 Linköping (Sweden)
  5. (Israel)
Publication Date:
OSTI Identifier:
22494828
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 118; Journal Issue: 9; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; CARBON; CRYSTAL STRUCTURE; DENSITY OF STATES; ELECTRON MICROSCOPY; FERMI LEVEL; HEATING; MIXING; MOLYBDENUM COMPOUNDS; PHASE STABILITY; POWDERS; PRESSURE RANGE GIGA PA; SYNTHESIS; TRANSITION ELEMENTS; X-RAY DIFFRACTION; X-RAY PHOTOELECTRON SPECTROSCOPY