  The stability of Cr, V, Al carbide MAX phases, materials of interest for a
variety of magnetic as well as high temperature applications, has been studied
using density-functional-theory first-principles calculations. The enthalpy of
mixing predicts these alloys to be unstable towards unmixing at 0 K. The
calculations also predict, however, that these phases would be thermally
stabilised by configurational entropy at temperatures well below the values
used for synthesis. The temperature Ts below which they become unstable is
found to be quite sensitive to the presence of magnetic moments on Cr ions, as
well as to the material's magnetic order, in addition to chemical order and
composition. Allowing for magnetism, the value of Ts for helf V and half Cr
with chemically disordered Cr and V atoms, is estimated to be between 516 K and
645 K depending on the level of theory, while, if constrained to spin-paired,
Ts drops to 142 K. Antiferromagnetic spin arrangements are found to be favoured
at low temperatures, but they are most likely lost at synthesis temperatures,
and probably at room temperature as well. However, the combination of
antiferromagnetic frustration and configurational disorder should give rise to
interesting spin textures at low temperatures.
