Exploring Entropy-Driven Thermodynamics in Chromium Spinels

While most materials expand when heated, a few exceptional cases−such as water ice, elastic bands, and invar alloy−exhibit a phenomenon known as negative thermal expansion (NTE). In these instances, heating causes the materials to contract rather than expand. Recently, it was discovered that CdCr2O4, a chromium spinel oxide, exhibits NTE within its ordered half-magnetization plateau phase [L. Rossi et. al.]. NTE was also observed in an effective spin-lattice coupled bond-phonon model on the pyrochlore lattice.

In this study, we revisit CdCr2O4 and the associated bond-phonon model to investigate how the entropy linked to magnetic frustration contributes to its unconventional thermodynamic properties. We find that entropy gradients within the half-magnetization plateau resemble those of an ideal paramagnet, resulting in both an enhanced magnetocaloric effect (MCE) and NTE. These results indicate the potential to develop a universal framework for understanding unconventional thermodynamics in chromium spinels.