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Lanthanide-Based Porous Coordination Polymers: Syntheses, Slow Relaxation of Magnetization, and Magnetocaloric Effect
journal contribution
posted on 2018-05-16, 13:54 authored by Chinmoy Das, Apoorva Upadhyay, Kamal Uddin Ansari, Naoki Ogiwara, Takashi Kitao, Satoshi Horike, Maheswaran ShanmugamTwo lanthanide-containing structurally
analogous porous coordination polymers (PCPs) have been isolated with
the general molecular formula [Ln2(L1)2(H2O)4(ox)]n.4nH2O (where L1 = fumarate, ox = oxalate;
Ln = Dy (1), Gd (2)). Thermogravimetric
analysis (TGA) and TG-MS measurements performed on 1 and 2 suggest that not only the solvated water molecules in the
crystal lattice but also the four coordinated water molecules on the
respective lanthanides in 1 and 2 are removed
upon activation. Due to the removal of the waters, 1 and 2 lost their crystallinity and became amorphous, as confirmed
by powder X-ray diffraction (PXRD). We propose the molecular formula
[Ln2(L1)2(ox)]n for the amorphous phase of 1 and 2 (where Ln = Dy (1′), Gd (2′)) on the basis of XANES, EXAFS, and other experimental investigations.
Magnetization relaxation dynamics probed on 1 and 1′ reveal two different relaxation processes with effective
energy barriers of 53.5 and 7.0 cm–1 for 1 and 45.1 and 6.4 cm–1 for 1′, which have been rationalized by detailed ab initio calculations.
For the isotropic lanthanide complexes 2 and 2′, magnetocaloric effect (MCE) efficiency was estimated through detailed
magnetization measurements. We have estimated −ΔSm values of 52.48 and 41.62
J kg1– K–1 for 2′ and 2, respectively, which are one of the largest values
reported for an extended structure. In addition, a 26% increase in
−ΔSm value in 2′ in comparison to 2 is achieved by simply removing the
passively contributing (for MCE) solvated water molecule in the lattice
and coordinated water molecules.