NaGdS2: A Promising Sulfide for Cryogenic Magnetic Cooling

The development of effective and eco-friendly cooling technology demands an investigation of new magnetocaloric materials. Compounds containing gadolinium are one of the best candidates due to the large spin-only magnetic moment of Gd3+ ions. This work reports on the magnetocaloric properties of the AGdS2 family (A = Li, Na, K, Rb) in relation to the crystal chemistry of these compounds. These sulfides crystallize in two different structure types: NaCl (A = Li) and α-NaFeO2 (A = Na, K, Rb). Although one would expect the larger magnetocaloric effect to be associated with LiGdS2 due to its higher magnetic/non-magnetic mass ratio, our study demonstrates that the NaGdS2 member leads to the best properties among the investigated series. The change in structure from the three-dimensional (3D) NaCl structure of LiGdS2 to the layered α-NaFeO2 structure of NaGdS2 drastically improves the magnetocaloric properties. Hence, thanks to its structural features associated with negligible exchange interactions, NaGdS2 exhibits a magnetic entropy change up to 54 J kg–1 K–1 at 2.5 K for μ0ΔH = 5 T, which is comparable to that of the top-ranked inorganic Gd-based materials operating in the cryogenic temperature range. These magnetocaloric figures of merit provide evidence that Gd-based sulfides are promising materials for magnetic refrigeration, and, more broadly, this highlights the potential of sulfides in that field.