Multistep solution-mediated formation of AuCuSn₂: Mechanistic insights for the guided design of intermetallic solid-state materials and complex multimetal nanocrystals

Understanding how solids form is a challenging task, and few strategies allow for the elucidation of reaction pathways that are useful for designing new solids. Here, we describe an unusual multistep reaction pathway that leads to the formation of AuCuSn₂, a new ternary intermetallic compound that was discovered as nanocrystals using a low-temperature solution route. The formation of AuCuSn₂ using a modified polyol process occurs through a multistep pathway that was elucidated by taking aliquots throughout the course of the reaction and studying the products using a variety of techniques. The reaction proceeds through four distinct steps: (a) formation of Au nanoparticles at or near room temperature, mediated by a galvanic reaction between Au ³⁺ and Sn²⁺ (forming Au⁰ and Sn⁴⁺, precipitated as SnO₂ that forms a shell around the nanoparticles), (b) formation of NiAs-type AuSn nanoparticles, along with Cu and Sn, upon addition of NaBH₄, (c) aggregation and thermal interdiffusion to form AuCu_xSn_y alloy nanoparticles, and (d) nucleation of intermetallic AuCuSn₂, which has an ordered NiAs-derived structure. The proposed mechanism was tested by starting the reaction with the AuSn intermediate. AuSn nanoparticles were synthesized separately and reacted with Cu and Sn nanoparticles, and ordered AuCuSn₂ formed as expected. Elucidation of this reaction pathway has important implications for guiding the design of new intermetallic solids, as well as for controlling the synthesis of complex multimetal nanocrystals.