Designing Spin-Crossover Systems to Enhance Thermopower and Thermoelectric Figure-of-Merit in Paramagnetic Materials

Thermoelectric materials, capable of converting temperature gradients into electrical power, have been traditionally limited by a trade-off between thermopower and electrical conductivity. This study introduces a novel, broadly applicable approach that enhances both the spin-driven thermopower and the thermoelectric figure-of-merit (zT) without compromising electrical conductivity, using temperature-driven spin crossover. Our approach, supported by both theoretical and experimental evidence, is demonstrated through a case study of chromium doped-manganese telluride, but is not confined to this material and can be extended to other magnetic materials. By introducing dopants to create a high crystal field and exploiting the entropy changes associated with temperature-driven spin crossover, we achieved a significant increase in thermopower, by approximately 136 μV K⁻¹, representing more than a 200% enhancement at elevated temperatures within the paramagnetic domain. Our exploration of the bipolar semiconducting nature of these materials reveals that suppressing bipolar magnon/paramagnon-drag thermopower is key to understanding and utilizing spin crossover-driven thermopower. These findings, validated by inelastic neutron scattering, X-ray photoemission spectroscopy, thermal transport, and energy conversion measurements, shed light on crucial material design parameters. We provide a comprehensive framework that analyzes the interplay between spin entropy, hopping transport, and magnon/paramagnon lifetimes, paving the way for the development of high-performance spin-driven thermoelectric materials.