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The magnetism of Fe 2 MnAl and Mn 2 FeAl compounds are studied by first principles. Evolutions of magnetic moment of Fe 2 MnAl display distinct variation trends under pressure, showing three different slopes at different pressure intervals, 0~100 GPa, 100~250 GPa, 250-400 GPa, respectively, and the moment collapses finally at 450 GPa. The magnetic moment of Mn 2 FeAl shows an increasing tendency below 40 GPa and decreases subsequently with pressure, and collapses ultimately at about 175 GPa. Such non-continuing decrease of Fe 2 MnAl originates from the unusual charge transfer of Fe and Mn and bond populations rearrangement of Fe-Fe and Mn-Fe, whereas the distinct moment evolution of Mn 2 FeAl is attributed to the complicated distributions of bond populations. The half-metallicity of the compounds can be maintained at low pressure, below about 100 GPa in Fe 2 MnAl and 50 GPa in Mn 2 FeAl. The magnetic moment collapse process didn't induce volume and bond length anomalies in the two compounds, the unique anomaly is the elastic softening behaviour in elastic constant c 44 and shear (G) and Young's (E) moduli of Fe 2 MnAl at 270 GPa, where the second moment collapse occurs.

The electronic structure and thermodynamical properties of Cr 2 TiAlC 2 are studied by first principles under pressure. The obtained results observed that the ferromagnetic order is the most stable ground state and the magnetic moment will collapse at about 50 GPa. As a result, the lattice a axis becomes stiffer above about 420 GPa, ultimately presenting the same axial compressibility trends with those of nonmagnetic compounds Mo 2 TiAlC 2 and hypothetical Cr 2 TiAlC 2 . The elastic constants and phonon dispersion curves demonstrate the structural stability during the disappearance of magnetic moment and occurrence of axial alternative compressibility. The density of states and energy band calculations confirmed the existence of magnetic moment of Cr 2 TiAlC 2 at 0 GPa and disappearance at high pressures above 50 GPa. Evolutions of magnetic moment collapse with pressure are confirmed by a variety of properties. The obtained grüneisen parameter and thermal expansion coefficients show the maximum value among the known MAX phases, to date and to the author's knowledge.