Band Gap Engineering and Metastable Phase Discovery in Cu₂BaSnS_4–<i>x</i>Se_<i>x</i> Nanocrystals via Topotactic Anion Exchange

Earth-abundant multinary inorganic chalcogenides, such as Cu₂BaSnS_4-<i>x</i>Se_<i>x</i> (CBTSSe), have emerged as promising absorbers for next-generation solar cells. However, accessing colloidal CBTSSe nanocrystals, including metastable polymorphs with more favorable optoelectronic properties, remains a critical synthetic challenge. Herein, we report the first use of topotactic anion exchange to access metastable crystal structures in soft chemistry syntheses. By varying the stoichiometry of selenium precursor, we directly synthesize Cu₂BaSnS_4-<i>x</i>Se_<i>x</i> (0 ≤ <i>x</i> ≤ 4) nanocrystals with a broad band gap tunability of 1.51-2.04 eV. At <i>x</i> = 4, a new metastable trigonal phase of Cu₂BaSnSe₄ is isolated. This phase arises via an <i>in situ</i> topotactic anion exchange from sulfur-rich Cu₂BaSnS_4-<i>x</i>Se_<i>x</i> intermediates, while retaining both the cation sublattice and overall nanocrystal morphology. The trigonal Cu₂BaSnSe₄ exhibits a nearly direct band gap that is 280 meV lower than its thermodynamically stable orthorhombic counterpart, aligning more closely with the optimal band gap for single-junction solar cells. In contrast, a postsynthetic anion exchange route leads to the thermodynamically preferred orthorhombic polymorph, pointing to the critical nature of the <i>in situ</i> transformation. Our findings open a versatile pathway for both polymorphic and morphological control in colloidal nanocrystals, expanding the synthetic design space for new optoelectronic materials.