C2O42−-templated [Ln2Ni] heterometallic compounds for enhanced magnetocaloric effects at low fields

Abstract

In the history of magnetochemistry development, lanthanide-transition (3d–4f) heterometallic compounds have been considered an attractive candidate for magnetic refrigerants. Herein, a series of heterometallic compounds have been designed and templated by C₂O₄²⁻ anions, that is, {[Ln₂Ni(L)(C₂O₄)₂(H₂O)₂]·H₂O}_n [Ln = Gd^III (abbr. Gd₂Ni) = Sm^III (abbr. Sm₂Ni) = Tb^III (abbr. Tb₂Ni), H₂C₂O₄ = oxalic acid; H₄L = ethylene diamine tetraacetic acid]. The structural analysis reveals that Ln₂Ni compounds feature a C₂O₄²⁻-templated wavy-shaped two-dimensional layer. Then, adjacent wavy-shaped layers are further aggregated by C₂O₄²⁻ anions, resulting in an innovative three-dimensional metal–organic framework. Magnetization analysis revealed that Gd₂Ni exhibits a favorable −ΔS^max_m of 38.0 J kg⁻¹ K⁻¹ at 2.0 K for 7.0 T. In addition, the effect of weak ferromagnetic exchange interactions in Gd₂Ni contributes to a decent magnetocaloric effect at low fields (−ΔS_m = 25.1 J kg⁻¹ K⁻¹ at 2.0 T for 2.0 K and −ΔS_m = 14.5 J kg⁻¹ K⁻¹ at 1.0 T for 2.0 K), outperforming most 3d–4f heterometallic compounds reported previously.