RESUMEN
Magnetic refrigeration based on the magnetocaloric effect (MCE) in metal-organic frameworks (MOF) is regarded as an attractive approach to create more sustainable cooling systems with higher efficiency than traditional ones. Here, we report a study of the MCE in a series of rare-earth-based MOFs. We have considered the selection of the rare-earth cation by investigating materials belonging to the α-rare-earth polymeric framework-4 (α-RPF-4) MOF family, synthesized with different rare-earth cations, and observed that paramagnetic moment and saturation magnetization play an important role in enhancing the magnetic entropy change ΔSM. The effect of structural parameters has also been considered by investigating three classes of metal-organic Gd materials built up from different types of inorganic secondary building units, including clusters (as in Gd-UiO-66), one-dimensional (as in α-RPF-4), and layered (as in Gd-LRH) conformations. Moreover, the analysis of the hydrostatic pressure influence reveals a significant increase in the -ΔSM and relative cooling power (RCP) with values between 4.3 and 16.3 and 121-509 J/kg. Specifically, the RCPmax found was â¼683 J/kg for Gd-UiO-66, which is higher than the one recently observed for Gd2SiO5 (649.5 J/kg). The present study demonstrates that the engineering of metal-organic framework systems based on high Gd densities may favor enhancing of magnetocaloric responses even at low pressures, thus promoting a new design strategy for efficient cooling devices.
RESUMEN
The incorporation of multiple metal atoms in multivariate metal-organic frameworks is typically carried out through a one-pot synthesis procedure that involves the simultaneous reaction of the selected elements with the organic linkers. In order to attain control over the distribution of the elements and to be able to produce materials with controllable metal combinations, it is required to understand the synthetic and crystallization processes. In this work, we have completed a study with the RPF-4 MOF family, which is made of various rare-earth elements, to investigate and determine how the different initial combinations of metal cations result in different atomic distributions in the obtained materials. Thus, we have found that for equimolar combinations involving lanthanum and another rare-earth element, such as ytterbium, gadolinium, or dysprosium, a compositional segregation takes place in the products, resulting in crystals with different compositions. On the contrary, binary combinations of ytterbium, gadolinium, erbium, and dysprosium result in homogeneous distributions. This dissimilar behavior is ascribed to differences in the crystallization pathways through which the MOF is formed. Along with the synthetic and crystallization study and considering the structural features of this MOF family, we also disclose here a comprehensive characterization of the magnetic properties of the compounds and the heat capacity behavior under different external magnetic fields.
