Vegard’s law deviating Ti ₂(Sn _<i>x</i>Al _{1− <i>x</i>})C solid solution with enhanced properties

Abstract

The achievement of chemical diversity and performance regulation of MAX phases primarily rely on solid solution approaches. However, the reported A-site solid solutions are undervalued due to their expected chemical disorder and compliance with Vegard’s law, as well as discontinuous composition and poor purity. Herein, we synthesized high-purity Ti₂(Sn<em>_x</em>Al_1-<em>x</em>)C (<em>x</em> = 0⁠–⁠1) solid solutions by the feasible pressureless sintering, enabling us to investigate their property evolution upon A-site composition. The formation mechanism of Ti₂(Sn<em>_x</em>Al_1-<em>x</em>)C was revealed by thermal analysis, and the crystal parameters were determined by Rietveld refinement of X-ray diffraction. Their lattice constant <em>a</em> adheres to Vegard’s law, while lattice constant <em>c</em> and internal free parameter <em>z</em><em>_M</em> have noticeable deviations from the law, which is caused by the significant nonlinear distortion of Ti₆C octahedron as Al atoms are substituted by Sn atoms. And the deviation also results in nonlinear changes in their physicochemical properties, which means that the solid solutions often exhibit better performance than the end members, such as hardness, electrical conductivity, and corrosion resistance. This work offers insights into the deviation from Vegard’s law observed in A-site solid solutions and indicates that solid solutions with enhanced performance may be obtained via tuning A-site composition.