Pseudopolymorphic Phase Engineering for Improved Thermoelectric Performance in Copper Sulfides

Abstract 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 CuBr 2 doping. A peak ZT value of 1.25 is obtained at 773 K in Cu 1.8 S + 3 wt% CuBr 2 , 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.