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 [Zn₃btc₂{Cr₃O(isonic)₆(H₂O)₂(OH)}]·(DMF)_15.5(H₂O)₈ (H₃btc=1,3,5-benzenetricarboxylic acid; isonic=isonicotinicate) shows a pillar-layered structure. The monolayer consists of hexagon-like rings formed by the [Zn(isonic)₂(btc)₂] tetrahedral and the consecutive monolayers are pillared by trigonal–prismatic clusters of [Cr₃O(isonic)₆(H₂O)₂(OH)]through the remaining binding sites of the Zn²⁺ 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 1–10⁷ 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.