Revealing the effect of indium on the ordering of PdCu in PdCu/PdIn heterostructure

The promotion of intermetallic compound (IMC) formation by a third element has emerged as a promising yet largely underexplored research topic; however, the specific regulation mechanism of the third element on IMC ordering and the influence of its doping content remain unclear. Herein, we comprehensively elucidate the intricate mechanism of indium (In)-induced PdCu ordering in PdCu/PdIn heterostructures. A unique “volcano-type” relationship between the ordering degree of Pd₄Cu₆In_x-300 (the annealed product of the Pd₄Cu₆In_x) and In doping content is reported, which originates from the competition between the kinetic promotion and thermodynamic inhibition effects induced by In doping during high-temperature annealing. The annealing process involves distinct sequential stages: Pd-In bond cleavage generates diffusible In atoms, and the high diffusivity of In atoms subsequently induce vacancy formation to accelerate the ordered rearrangement of Pd/Cu atoms. Notably, a critical and previously overlooked insight is that only diffusible In species can effectively modulate PdCu ordering, with a clear negative correlation between relative bonding strength of Pd-In and annealing-induced ordering degree. This study provides novel insights into the regulation of Pd-based IMCs formation by third-element doping, and provides vital guidance for the rational design of high-performance IMC materials and the optimization of their ordered microstructures.