Comparative study of structures, thermal stabilities and dielectric properties for a ferroelectric MOF [Sr(μ-BDC)(DMF)]_∞ with its solvent-free framework

A ferroelectric MOF with a formula [Sr(μ-BDC)(DMF)]_∞ (1) was transformed into [Sr(μ-BDC)(CH₂Cl₂)_x]_∞ (2) using a solvent exchange approach, where DMF = N, N-dimethylformamide and BDC²⁻ = benzene-1,4-dicarboxylate. The lattice solvents, CH₂Cl₂ molecules, in 2 were removed by heating to give the solvent-free metal–organic framework [Sr(μ-BDC)]_∞ (3) and the crystal-to-crystal transformation is reversible between 1 and 3. The release of DMF molecules from 1 results in the metal–organic framework of [Sr(μ-BDC)]_∞ expanding a little along the a- and b-axes. The crystal structure optimizations for 1 and 3 disclosed that the lattice expansion is associated with the alternations of the bond distances and angles in the Sr²⁺ ion coordination sphere along the a- and b-axes directions. The metal–organic framework 3 collapses at temperatures of more than 600 °C; such an extremely high thermal stability is related to the closed-shell electronic structure of the Sr²⁺ ion, namely, the coordinate bond between the closed-shell Sr²⁺ ion and the bridged BDC²⁻ ligands does not have a preferred direction, which is favored for reducing lattice strains and is responsible for the higher thermal stability. The comparative investigations for the dielectric and ferroelectric behaviors of 1 and 3 confirmed that the motion of the polar DMF molecules, but not the [Sr(μ-BDC)]_∞ framework, is responsible for the ferroelectric properties of 1.