Pseudopolymorphic Phase Engineering for Improved Thermoelectric Performance in Copper Sulfides

Polymorphism (and its extended form - pseudopolymorphism) in solids is ubiquitous in mineralogy, crystallography, chemistry/biochemistry, materials science, and the pharmaceutical industries. Despite the difficulty of controlling (pseudo-)polymorphism, the realization of specific (pseudo-)polymorphic phases and associated boundary structures is an efficient route to enhance material performance for energy conversion and electromechanical applications. Here, this work applies the pseudopolymorphic phase (PP) concept to a thermoelectric copper sulfide, Cu_{2- x} S (x ≤ 0.25), via CuBr₂ doping. A peak ZT value of 1.25 is obtained at 773 K in Cu_1.8 S + 3 wt% CuBr₂ , which is 2.3 times higher than that of a pristine Cu_1.8 S sample. Atomic-resolution scanning transmission electron microscopy confirms the transformation of pristine Cu_1.8 S low digenite into PP-engineered high digenite, as well as the formation of (semi-)coherent interfaces between different PPs, which is expected to enhance phonon scattering. The results demonstrate that PP engineering is an effective approach for achieving improved thermoelectric performance in Cu-S compounds. It is also expected to be useful in other materials.