MS35-P01 Negative compressibility of a metal-organic framework?Fundamental thermodynamics requires that all materials reduce their volume at high pressure. Owing to specific anisotropic features, originating from various structural motifs, some compounds can elongate in one or two directions, which is termed as negative linear and area compressibility.1 The volume compression restriction applies to constant composition materials (i.e. closed systems). It was evidenced that zeolites and other porous materials can uptake the guest molecules from gaseous and liquid surrounding, which can increase the crystal volume despite the increasing pressure.2 These are so called open systems. In those high-pressure experiments, various simple molecular fluids (water, methanol, ethanol, etc.) and their mixtures were used. The transport of guest molecules can be eliminated by compressing the sample in liquids composed of molecules much larger than the dimensions of pores, for example in oils.
We have performed a series of such experiments for a newly synthesized 2D metal-organic framework Ni(hip)(bipy)(H2O)2 · H2O · CH3OH · DMF, denoted as AMU-2 (where AMU abbreviates Adam Mickiewicz University). Its flexible structure is built of 4,4’-bipyridines (bipy) and 5-hydroxyisophthalic acid anions (hip) into grids further connected by H-bonds. The compression of AMU-2 revealed its unpreceded elastic and sorption-elastic properties. Hydrostatic compression performed in three different large-molecule liquids (oils Daphne 7373 and NVH or Fluorinert FC-77) result in a significant volume expansion of about 120 Å3 at 0.2 GPa. This counterintuitive effect mimicking the negative volume compression was observed by X-ray diffraction for several samples. We have shown that this porous material, when compressed in oils, partially collapses and becomes amorphous, which triggers a transport of guest molecules present in the pores to the crystalline parts of the sample. This mechanism of transport of gests induced be external stimulus of pressure has been described as a ‘zone-collapse’ effect.3References:
 Cairns, A. B. & Goodwin, A. L. (2015). Phys. Chem. Chem. Phys. 17, 20449–20465.
 Coudert, F. X. (2015). Chem. Mater. 27, 1905–1916.
 Sobczak, S. & Katrusiak, A. (2018). Cryst. Growth Des. 18, 1082–1089.
Keywords: metal-organic framework, high-pressure, volume compression, amorphization