The double solid solution (Zr, Nb)₂(Al, Sn)C MAX phase: a steric stability approach

The addition of Nb and Sn to Zr₂AlC is investigated, targeting the synthesis of a Zr-rich bulk MAX phase free of ZrC. The 211 phase formation in the two quaternary Zr-Nb-Al-C and Zr-Al-Sn-C systems is evaluated. Solubility over the entire compositional range in (Zr, Nb)₂AlC and Zr₂(Al, Sn)C is observed. In terms of effectiveness, the addition of Sn is preferred over the addition of Nb, as the former is selectively incorporated into the 211 structure. A combinatorial approach results in the formation of phase-pure (Zr_0.8, Nb_0.2)₂(Al_0.5, Sn_0.5)C. The effect of the added solutes on the microstructure and crystallographic parameters is investigated. The addition of Nb and Sn reduces the distortion parameter of the trigonal prism compared to pure Zr₂AlC. Therefore, an attempt is made to establish a more general stability criterion for the M₂AC structure based on the steric relationship between the atoms in the M₆A trigonal prism. Inspired by the Hume-Rothery rules, it is suggested that comparable atomic radii of the M- and A-atoms provide a good starting point to obtain a stable 211 MAX phase.