Abstract
Solvothermal reactions of 3-(4-pyridyl)-benzoic acid (Hpba) with a series of transition metal ions yielded isostructral metal-organic frameworks [M(pba)2]·2DMA (MCF-52; M = Ni2+, Co2+, Zn2+, Cd2+, or mixed Zn2+/Cd2+; DMA = N,N-dimethylacetamide) possessing two-dimensional fence-like coordination networks based on mononuclear 4-connected metal nodes and 2-connected organic ligands. Variable-temperature single-crystal X-ray diffraction studies of these materials revealed huge positive and negative thermal expansions with |α| > 150 × 10−6 K−1, in which the larger metal ions give the larger thermal expansion coefficients, because the increased space not only enhance the ligand vibrational motion and hinged-fence effect, but also allow larger changes of steric hindrance between the layers. In addition, the solid-solution crystal with mixed metal ions further validates the abundant thermal expansion mechanisms of these metal-organic layers.
Highlights
Most solids expand slightly as temperature increases (0 < α < 20 × 10−6 K−1, α for axial thermal expansion coefficients, α = ∂l/∂T × 1/l), which is known as thermal expansion or positive thermal expansion (PTE)
Powder Xray diffraction (PXRD) patterns were collected on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, λ = 1.5418 Å)
Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) images were recorded on a Quanta 400 field-emitted SEM device
Summary
Materials with abnormal thermal expansion behaviors, such as zero thermal expansion (ZTE, |α| ≈ 0 × 10−6 K−1), negative thermal expansion (NTE, α < 0 × 10−6 K−1), or very large thermal expansion (|α| > 100 × 10−6 K−1), are scarce (Mary et al, 1996; Chapman et al, 2005; Goodwin et al, 2008; Das et al, 2010; Zhou et al, 2015). NTE compounds may be applied to compensate the thermal expansion of a “normal” PTE material, so that it can remain functional in extremely high or low temperatures without degradation (Mary et al, 1996; Chapman et al, 2005; Rowsell et al, 2005; Wu et al, 2008; Zhou et al, 2016). Designing and controlling thermal expansion behaviors of materials are still great challenges
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