title: Comparative study of structures, thermal stabilities and dielectric properties for a ferroelectric MOF [Sr(μ-BDC)(DMF)]∞ with its solvent-free framework.
abstract: 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 BDC(2-) = 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 Sr(2+) 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 Sr(2+) ion, namely, the coordinate bond between the closed-shell Sr(2+) ion and the bridged BDC(2-) 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.
authors.affiliation: State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, P R China.
authors.forename: Ping-Chun
authors.initials: PC
authors.lastname: Guo
authors.forename: Zhenyu
authors.initials: Z
authors.lastname: Chu
authors.forename: Xiao-Ming
authors.initials: XM
authors.lastname: Ren
authors.forename: Wei-Hua
authors.initials: WH
authors.lastname: Ning
authors.forename: Wanqin
authors.initials: W
authors.lastname: Jin