Negative Linear Compressibility in the Elastically Flexible Crystal of Copper(II) Acetylacetonate

Abstract

The coordination compound copper(II) acetylacetonate, [Cu(acac)₂], exhibits remarkable elastic flexibility due to the reversible rotation of molecules when the crystal is twisted or bent without loss of its crystallinity. The effects of external stimuli, including heat and strain, have been intensively studied on this material, but its mechanical response to the pressure has not yet been reported. Using the high-pressure single-crystal X-ray diffraction method, we show the herringbone-type [Cu(acac)₂] demonstrates remarkable structural changes in molecular orientations toward the (001) plane under compression, without the occurrence of phase transitions. The alterations in molecular packing arrangements result in the emergence of unusual negative linear compressibility (NLC) of −6.2(6) TPa^–1 along the b axis over the pressure range of 0.1 MPa–6.19 GPa. Moreover, synchrotron powder X-ray diffraction and Raman spectroscopy measurements demonstrate that the ambient-pressure phase is robust and can be stabilized at least to 10 GPa. In addition, the bandgap exhibits a slight reduction of ∼0.2 eV as the pressure is increased to 10.10 GPa, and the sample returns to its original state upon complete pressure release. Our findings suggest that the NLC behavior may be present in a diverse range of unidentified herringbone-type molecular crystals, which differs from the extensively researched hinged wine-rack frameworks.