Reversibility in a Plastically Flexible Coordination Polymer Crystal: A High-Pressure Study

Single crystals which exhibit mechanical flexibility are promising materials for advanced technological applications. Before such materials can be used, detailed understanding of the mechanisms and structural effects of bending are needed. Coordination polymer single crystal represent a fascinating class of mechanically flexible material; their bending contradicts existing models. Using single crystal X-ray diffraction and microfocus Raman spectroscopy, we study in atomic detail the high-pressure response of the plastically flexible coordination polymer [Zn(μ‐Cl)₂(3,5‐dichloropyridine)₂]_n. In stark contrast to three-point bending, the quasi-hydrostatic compression of the single crystal is completely reversible, even following compression to over 9 GPa. A structural phase transition is observed at ca. 5 GPa. Ab initio DFT calculations show this transition to result from the pressure-induced softening of low frequency vibrations. This phase transition is not observed during three-point bending. Our combined experimental and theoretical high-pressure investigation propose slight compression at low levels of bending. However, our studies provide the first indication of overall disparate mechanical responses of bulk flexibility and quasi-hydrostatic compression. We suspect this to be a general feature of mechanically plastic materials.