RECo-containing metal-organic frameworks based on 3-hydroxyisonicotinic acid: Proton conduction and magnetism

Three heterometallic metal-organic frameworks, namely, {RE₂Co₃L₆(H₂O)₆}_n (RE = Gd (1), Dy (2), Y(3)), have been hydrothermally synthesized based on rare-earth (RE) and transition metal ions and the organic ligand of 3-hydroxyisonicotinic acid (H₂L). Single-crystal X-ray diffraction reveals that compounds 1‒3 are isomorphic and features three-dimensional (3D) porous structure. In compound 1, the Gd³⁺ ion is located in the C₃ axis, and every two propeller-like {GdCo₃} units are capped up and down to a {Gd₆Co₆} ring through twelve L^2‒ ligands, giving rise to the formation of the cavities. Notably, three compounds display high thermal stability determined by thermogravimetric analysis (TGA), and good water stability proved by the well-matched of PXRD curves before and after soaked three samples in water for three days. AC impedance measurements exhibit the proton conductivities of compounds 1‒3 with powder samples are distinctly temperature and humidity dependent, and the values can reach 3.38×10^‒5, 1.91×10^‒5, and 2.23×10^‒5 S cm^‒1 at 98% RH and 338 K, respectively. Moreover, compounds 1‒3 display antiferromagnetic behaviours, and the magnetocaloric effect (MCE) study indicates that the entropy change (−ΔS_m) of compound 1 is 7.1 J kg⁻¹ K⁻¹ at 7 K and 7 T. Compound 2 exhibits the slow magnetic relaxation behaviour.