Large Negative Linear Compressibility Triggered by Hydrogen Bonding

Abstract

Materials expanding in one or more directions under hydrostatic pressure are extremely rare, whereas such exceptional mechanical properties are desired for numerous technological applications. This study shows that pressure-induced intermolecular H-bonding interactions can change mechanical properties of crystalline solids dramatically, leading to negative linear compressibility (NLC). Two hybrid organic-inorganic crystals, dabco sulfamate (DSN, dabco = 1,4-diazabicyclo[2.2.2]octane) and dabco sulfamate monohydrate (DSNW), both characterized by the structures built of the one-dimensional supramolecular motifs, have been studied by high-pressure single-crystal X-ray diffraction. Despite the isomorphic structures, the crystals exhibit different mechanical responses to uniform pressure. The lattice parameters of DSN are monotonically contracted up to 4 GPa at least, whereas DSNW shows a large NLC between 1.3 and 2.5 GPa, where it transforms to the high-pressure phase. The structural mechanism of unusual behavior of DSNW is unique among the all known NLC materials. The crystal compression leads to the formation of interionic CH···N hydrogen bonds, which act similar to the springs imposing a nonzero torque on the H-bonded polyanionic ribbons. Strengthening of these bonds with increasing pressure causes a rotation of the ribbons, and as a result, the crystal expands along the b axis. The lack of similar effects in DSN is rationalized by the disorder of the sulfamate anions, which persists in the whole studied pressure range.