Gd‐based molecular coolants: Aggregating for better magnetocaloric effect

Abstract Two series of 3d‐Gd mixed‐metal phosphonate complexes with either only two gadolinium centers such as {Gd 2 }, {Ni 2 Gd 2 }, {Co 4 Gd 2 }, {Co 8 Gd 2 }, {Fe 6 Gd 2 }, and {Fe 17 Gd 2 } or more than two gadoliniums such as {Co 8 Gd 4 }, {Mn 8 Gd 4 }, {Co 4 Gd 6 }, {Mn 4 Gd 6 }, {Co 6 Gd 8 }, {Ni 5 Gd 8 }, {Ni 6 Gd 6 }, {Co 8 Gd 8 }, and {Mn 9 Gd 9 } have been solvothermally prepared and magnetothermally studied. The nearly identical environments of the Gd(III) dimer in the first series allow us to qualitatively analyze the effect of magnetic exchange coupling on the magnetocaloric effect (MCE). By doubling, tripling, or quadrupling of the Gd(III) centers, the second series of 3d‐Gd mixed‐metal complexes was built to further test the other effects of exchange couplings on MCE in more complicated circumstances. For the antiferromagnetic coupling cases, the results are nearly identical but diversify when topological spin frustrations are created, whose massive low‐lying excited spin states help enhance MCE. For presumably ferromagnetically coupled ones, albeit are rare in phosphonate complexes, they do exhibit excellent MCE. Meanwhile, the complexes with weakly coupled metal centers serve as excellent examples for studying the effect of molecular mass on MCE when its magnitude is expressed in the unit of Joule per kilogram, from which we can see the values are directly proportional to the percentage of the Gd(III) ions in molecular weight.