Development of magnetocaloric coordination polymers for low temperature cooling

Caloric materials have attracted significant interest as replacements for conventional refrigeration, which is becoming increasingly important in our daily lives, yet poses issues for sustainability due to both energy consumption and loss of refrigerants into the atmosphere. Among caloric materials, which are key to solid state cooling technologies, those exhibiting the magnetocaloric effect (MCE), an entropy-driven phenomenon under cycled applied magnetic fields, are promising candidates for cryogenic cooling. These have potential to replace conventional cryogenics, particularly liquid He-an increasingly scarce and expensive resource. Amongst magnetocalorics, coordination polymers containing polyatomic ligands have been shown to be very promising materials due to their large entropy changes at low temperatures. One of the contributing factors to this peformance is their unique structural flexibility, as they can adopt a wide range of structures usually not accessible for conventional materials, such as close-packed metal oxides. The most researched materials for magnetocaloric applications are those containing Gd as their magnetic centre, as the combination of structure and the weakly interacting 4f orbitals of Gd3+ in these materials enables the fabrication of promising magnetocalorics that contain a high density of cations and thus exhibit a high entropy change as a function of their weight and volume at ultra-low cryogenic temperatures. Alongside this, there is a growing interest in magnetocaloric coordination polymers with their magnetocaloric effect optimised for lower applied fields that can be generated using permanent magnets through incorporating other magnetic cations, including lanthanides with greater magnetic anisotropy. When combined with tailored magnetic interactions this leads to promising entropy changes above 4 K, a typical base temperature for many cryogenic applications. This review discusses the most promising magnetocalorics among coordination polymers and MOFs, highlighting their structural characteristics, and concluding with a brief perspective on the future of this field.