Synthesis of phase-pure highly-doped MAX-phase (Cr_1-xMn_x)₂AlC

Mn doped (Cr_1-xMn_x)₂AlC compounds constitute a family of MAX-phases of great interest due to the possibility of tuning the magnetic properties they possess. Their synthesis, however, is not an easy task. Two main issues make its preparation difficult. First is the strong tendency secondary phases have to segregate when manganese is added to the parent compound and, secondly, the resulting doping level which remains relatively low due to the poor incorporation of Mn atoms into the MAX-phase hexagonal structure [Cr₂AlC-type]. In the present work the highest dopant concentration so far reported for bulk materials of 18.3 at.% Mn has been obtained through the exploitation of the arc-melting technique. At the same time, the use of the Cr₃C₂, as a precursor from the initial stage of the synthesis procedure, led to the preservation of the phase content in the Mn-doped samples. Finally, chemical post-treatment in acid solutions (HF, HCl) proved to be effective and helpful to dissolve the remaining secondary intermetallic phases; the final material resulting in a pure MAX-phase powder (>99 vol%). However, the doping level reached seems to be the limit for the arc fusion technique used; additional manganese no longer dissolves into the MAX-phase structure but tends to form some new Mn-based secondary phases, such as the perovskite Mn₃AlC and the carbide MnC₂. After the chemical etching, pure MAX-phase powders can be compacted into bulk using spark plasma sintering technique. However, while this method is useful for the non-doped Cr₂AlC, it needs further optimization for (Cr_1-xMn_x)₂AlC as it leads to a partial decomposition of Mn-containing MAX-phase.