Magnetocaloric and barocaloric effects of metal complexes for solid state cooling: Review, trends and perspectives

Abstract Solid state refrigeration is a viable alternative for the conventional gas-compression technology due to the environmental friendliness of its materials, energy efficiency, and low noise. Research in this field is focused on the development of advanced prototypes and smart materials. This Review focuses on a special family of quantum materials: metal complexes. We introduce the fundamentals of caloric effects and magnetism of these complexes, discussing their applications at different ranges of temperature, based on different physical mechanisms. At cryogenic temperatures (close to temperature of liquid He), some metal complexes present a huge value of magnetic entropy change, ranging from c.a. 10 J/kgK to c.a. 70 J/kgK (for 7 T of magnetic field change). These values make some metal complexes appealing as cryogenic coolant materials. We also present a comprehensive collection of results from the literature, organized on a chart as a function of time, for different classes of metal complexes; those with 3d-3d magnetic interactions, 3d-4f coupling, and 4f-4f interactions. We observed that those materials that achieved the maximum value of entropy change, i.e., the spin-only value, follow an exponential scaling law with time. This result helps to predict a new class of metal complexes and further outcomes for the field. On the other hand, for a small amount of applied pressure, these materials produce large barocaloric effect around the spin crossover transition (this transition occurs in a wide range of temperature, even close to room temperature). Thus, we introduce the SCO mechanism and comprehensively review this topic, along with the recent theoretical models and experimental results. The recent results of barocaloric effect are considered enormously significant (56 J/kgK for 0.9 kbar of pressure change, close to room temperature), even in comparison with traditional metallic barocaloric materials. We also provide perspectives for this subject, with discussions about new mechanisms for the models (as the Jahn-Teller distortion and orbital contribution).