Frustrated lanthanide oxides with dense magnetic lattices are of fundamental interest for their potential in cryogenic refrigeration due to a large ground state entropy and suppressed ordering temperatures but can often be limited by short-range correlations. Here, we present examples of frustrated fcc oxides, Ba 2 GdSbO 6 and Sr 2 GdSbO 6 , and the new site-disordered analogue Ca 2 GdSbO 6 ([CaGd] A [CaSb] B O 6 ), in which the magnetocaloric effect is influenced by minimal superexchange ( J 1 ∼ 10 mK). We report on the crystal structures using powder X-ray diffraction and the bulk magnetic properties through low-field susceptibility and isothermal magnetization measurements. The Gd compounds exhibit a magnetic entropy change of up to −15.8 J/K/mol Gd in a field of 7 T at 2 K, a 20% excess compared to the value of −13.0 J/K/mol Gd for a standard in magnetic refrigeration, Gd 3 Ga 5 O 12 . Heat capacity measurements indicate a lack of magnetic ordering down to 0.4 K for Ba 2 GdSbO 6 and Sr 2 GdSbO 6 , suggesting cooling down through the liquid 4-He regime. A mean-field model is used to elucidate the role of primarily free-spin behavior in the magnetocaloric performance of these compounds in comparison to other top-performing Gd-based oxides. The chemical flexibility of the double perovskites raises the possibility of further enhancement of the magnetocaloric effect in the Gd 3+ fcc lattices.

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Frustrated lanthanide oxides are promising candidates for cryogen-free magnetic refrigeration due to their suppressed ordering temperatures and high magnetic moments. While much attention has been paid to the garnet and pyrochlore lattices, the magnetocaloric effect in frustrated face-centered cubic (fcc) lattices remains relatively unexplored. We previously showed that the frustrated fcc double perovskite Ba 2 GdSbO 6 is a top-performing magnetocaloric material (per mol Gd) because of its small nearestneighbor interaction between spins. Here we investigate different tuning parameters to maximize the magnetocaloric effect in the family of fcc lanthanide oxides, A