Temperature-driven phase transformations and microstructure evolution in the thermoelectric colusite Cu26V2Sn6S32. An in situ synchrotron diffraction study

Abstract

Understanding the structural stability and thermal behavior of thermoelectric materials is crucial for optimizing their long-term performance under operating conditions. In this work, we present an in situ high-resolution synchrotron X-ray powder diffraction (HR-SXRPD) study of the phase transformations occurring in the colusite-type compound Cu₂₆V₂Sn₆S₃₂. Two samples, synthesized by mechanochemical routes and subsequently annealed at 873 K and 1023 K, were investigated after several months of air exposure to elucidate the relationship between synthesis temperature, microstructural evolution, and phase stability. Our results reveal that both specimens contain colusite- and sphalerite-type phases, thereby ruling out the previously proposed interpretation involving two distinct colusite phases, named exsolution. In addition, minor amounts of copper sulfate were detected in the air-aged samples, indicating surface reactivity under ambient conditions. Upon heating, the colusite phase exhibits discontinuous variations in its lattice parameters, reflecting changes in crystallinity and site occupancies, whereas the sphalerite phase follows a linear thermal expansion behavior. The structural evolution of the colusite phase is closely related with the decomposition of the sulfate phase, underscoring the critical influence of oxygen exposure on the stability of these materials. These findings provide fundamental insights into the thermal stability and structural resilience of Cu₂₆V₂Sn₆S₃₂, offering valuable guidance for mitigating degradation processes and improving the long-term reliability of colusite-based thermoelectric devices.