Nanoscale Engineering of Polymorphism in Cu2Se-Based Composites

Crystal polymorphism selection during synthesis is extremely challenging. However, promoting the formation of a specific metastable polymorph enables modulation of the functional properties of phase-change materials through alteration of the relative abundance of various polymorphs. Here, we demonstrate the stabilization of the superionic β-Cu2Se phase under ambient conditions and the direct control over the relative ratio between the α-Cu2Se and β-Cu2Se polymorphs in (x)CuGaSe2/(1-x)Cu2Se composites using CuGaSe2 nanoseeds. We found that the small lattice mismatch between β-Cu2Se (cubic) and the ab plane of tetragonal CuGaSe2 nanoseeds promotes the formation of low-energy coherent CuGaSe2/β-Cu2Se interfaces, leading to preferential stabilization of β-Cu2Se. Astonishingly, the hierarchical microstructure of the resulting composites enables a remarkable decoupling of charge and heat transport, which is manifested by a breakdown of the Wiedemann-Franz law.