In Situ Mechanistic Studies of Two Divergent Synthesis Routes Forming the Heteroanionic BiOCuSe

Heteroanionic materials are a burgeoning class of compounds that offer new properties via the targeted selection of anions. However, understanding the design principles and achieving successful syntheses of new materials in this class are in their infancy. To obtain mechanistic insight and a panoramic view of the reaction progression from beginning to end of the formation of a heteroanionic material, we selected BiOCuSe, a well-known thermoelectric compound, and utilized in situ synchrotron powder diffraction as a function of temperature and time. BiOCuSe is a layered material, which crystallizes in a common mixed anion structure type: ZrSiAsFe. Two reactions of starting materials (Bi₂O₂Se + Cu₂Se and Bi₂O₃ + Bi + 3Cu + 3Se) were studied to determine the effect of precursors on the reaction pathway. Our in situ investigation shows that the ternary-binary Bi₂O₂Se + Cu₂Se reaction proceeds without intermediates to directly form BiOCuSe, while the binary-elemental Bi₂O₃ + Bi + 3Cu + 3Se reaction generates many intermediates before the final product forms. These intermediates include CuSe, Bi₃Se₄, Bi₂Se₃, and Cu₂Se. While the stoichiometric loading of the precursors necessarily dictates the identity of the first intermediates, kinetics also plays a contributing role in stabilizing unexpected intermediates such as CuSe and Bi₃Se₄. Understanding and establishing a link between the selection of precursors and the reaction pathways improves the potential for rational synthesis of heteroanionic materials and solid-state reactions in general.