Lanthanide-Based Layer-Type Two-Dimensional Coordination Polymers Featuring Slow Magnetic Relaxation, Magnetocaloric Effect and Proton Conductivity

Three lanthanide-based two-dimensional (2D) coordination polymers (CPs), [Ln(L)(H₂ O)₂ ]_n , {H₃ L=(HO)₂ P(O)CH₂ CO₂ H; Ln=Dy³⁺ (CP 1), Er³⁺ (CP 2)} and [{Gd₂ (L)₂ (H₂ O)₃ }^. H₂ O]_n , (CP 3) were hydrothermally synthesized using phosphonoacetic acid as a linker. Structural features revealed that the dinuclear Ln³⁺ nodes were present in the 2D sheet of CP 1 and CP 2 while in the case of CP 3, nodes were further connected to each other forming a chain-type arrangement throughout the network. The magnetic studies show field-induced slow magnetic relaxation property in CP 1 and CP 2 with U_eff values of 72 K (relaxation time, τ₀ =3.05×10^-7 s) and 38.42 K (relaxation time, τ₀ =4.60×10^-8 s) respectively. Ab-initio calculations suggest that the g tensor of Kramers doublet of the lanthanide ion (Dy³⁺ and Er³⁺ ) is strongly axial in nature which reflects in the slow magnetic relaxation behavior of both CPs. CP 3 exhibits a significant magnetocaloric effect with -ΔS_m =49.29 J kg^-1 K^-1 , one of the highest value among the reported 2D CPs. Moreover, impedance analysis of all the CPs show high proton conductivity with values of 1.13×10^-6 S cm^-1 , 2.73×10^-3 S cm^-1 and 2, 6.27×10^-6 S cm^-1 for CPs 1-3, respectively, at high temperature (>75 °C) and maximum 95 % relative humidity (RH).