Effect of magnetism and temperature on the stability of (Crx,V1−x)2AlC phases

The stability of $\text{(}{\text{Cr}}_x\text{,}{\text{V}}_{1\ensuremath{-}x}{\text{)}}_{\text{2}}\text{AlC}$ 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 stabilized by configurational entropy at temperatures well below the values used for synthesis. The temperature ${T}_s$ 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 ${T}_s$ for $\text{(}{\text{Cr}}_{\text{0.5}}\text{,}{\text{V}}_{\text{0.5}}{\text{)}}_{\text{2}}\text{AlC}$ with chemically disordered Cr and V atoms is estimated to be between 516 and 645 K depending on the level of theory, while if constrained to spin paired, ${T}_s$ drops to $\ensuremath{\sim}142$ K. Antiferromagnetic spin arrangements are found to be favored. The combination of antiferromagnetic frustration and configurational disorder should give rise to interesting spin textures at low temperatures.