Evidence for Symmetry Reduction in Ti₃(Al_1-δCu_δ)C₂ MAX Phase Solid Solutions

Ti₃[Al_1-δCu_δ]C₂ MAX phase solid solutions have been synthesized by sintering compacted Ti₃AlC₂-Cu composites produced by mechanical milling. Using X-ray and neutron diffraction techniques, it is demonstrated that the Cu mixing into the Al site is accompanied by lattice distortion, which leads to symmetry reduction from a hexagonal to a monoclinic structure. Such symmetry reduction likely results from this mixing through deviation of the A-site position from the special (0, 0, ¹/₄) position within the P6₃/mmc space group of the original Ti₃AlC₂ structure. Moreover, it is demonstrated that the Cu admixture into the A site can be adjusted from the composition of the reactant mixture. The lattice parameter variation of the solid solution compounds, with 10-50 atom % Cu in the A site, is found to be consistent with Vegard's law.