Thermal Expansion Anomalies in Spinels: CdCr2O4 and Beyond

Although most materials tend to expand when heated, a select few, such as water ice, elastic bands, and invar alloy, display an anomalous phenomenon known as negative thermal expansion (NTE). Recently, it was discovered that CdCr2O4, a chromium spinel oxide, exhibits NTE within its ordered half-magnetization plateau phase. This phenomenon was also observed in an effective spin-lattice coupled bond-phonon model on the pyrochlore lattice [1].

In this study, we revisit CdCr2O4 using a more realistic site-phonon model that includes intra- and inter-tetrahedra interactions on the pyrochlore lattice [2]. We construct order parameters from the irreducible representations of the possible ground state spin configurations to map the temperature and magnetic field phase diagram through classical Monte Carlo simulations.

Our results demonstrate the presence of NTE in the half-magnetization plateau phase of the site-phonon model, thereby confirming the entropy-driven NTE mechanism presented in [1]. We further investigate the model's ability to explain other types of NTE observed in spinels, including at zero field.

[1] L. Rossi et. al., Physical Review LeAers 123, 027205 (2019).

[2] K. Aoyama et. al., Physical Review B 104, 184411 (2021).