Dielectric anomaly and relaxation natures in a Zn-Cr pillar−layered metal−organic framework with cages and channels
A bimetallic metal–organic framework (MOF) with the formula [Zn{sub 3}btc{sub 2}(Cr{sub 3}O(isonic){sub 6}(H{sub 2}O){sub 2}(OH))]·(DMF){sub 15.5}(H{sub 2}O){sub 8} (H{sub 3}btc=1,3,5-benzenetricarboxylic acid; isonic=isonicotinicate) shows a pillar-layered structure. The monolayer consists of hexagon-like rings formed by the [Zn(isonic){sub 2}(btc){sub 2}] tetrahedral and the consecutive monolayers are pillared by trigonal–prismatic clusters of [Cr{sub 3}O(isonic){sub 6}(H{sub 2}O){sub 2}(OH)]through the remaining binding sites of the Zn{sup 2+} ions. DMF and water molecules are confined in the cages and channels. TGA indicates that the lattice DMF and water molecules begin to be released at temperatures above 363 K. Dielectric measurements were carried out in the range of 173–363 K and 1–10{sup 7} Hz for three successive thermal cycles. The dielectric spectroscopy obtained in the first thermal cycle was different from that observed in the next two thermal cycles, while the dielectric spectra in the last two thermal cycles were almost identical. The dielectric nature of this MOF is discussed in detail for each thermal cycle. Since MOFs are unique host–guest systems in which the structure of the host framework is designable and the guests are exchangeable, it is no doubt those MOFs are materials with a variety of dielectric natures. This study gives a fresh impetus to achieve MOFs–based dielectric materials. - Graphical abstract: The bimetallic MOF [Zn{sub 3}btc{sub 2}(Cr{sub 3}O(isonic){sub 6}(H{sub 2}O){sub 2}(OH))]·(DMF){sub 15.5}(H{sub 2}O){sub 8}1, shows a pillar-layered open-framework structure. The dielectric spectra of 1 are almost identical in the last two thermal cycles, whereas significantly different from that observed in the first thermal cycle. The novel dielectric anomaly associated with a stacked structure transformation of the disordered guests. - Highlights: • A bimetallic metal-organic framework shows a pillar-layered structure. • The MOF displays novel dielectric anomaly and relaxation behaviors. • The dielectric anomaly arises from the stacking structure transformation of guests. • The dielectric relaxation is related to the dipole dynamics of guests.