Negative linear compressibility of molecular and ionic-molecular crystals

The compressibility of crystalline tetrabromophthalic anhydride (TBPA) and 1-ethyl-3-methylimidazolium nitrate (EMN) was studied based on density functional theory including dispersion interactions at pressures below 1 GPa. It is found for the first time that EMN demonstrates negative linear compressibility (NLC) up to ∼0.15 GPa, whereas TBPA shows significant NLC at pressures higher than ∼0.2 GPa. Mechanisms of the negative linear compressibility of TBPA and EMN at the microscopic (molecular) level have been found for the first time. It was shown that NLC correlates with a baric change of spatial orientation (rotation) and linear dimensions of molecular structural units relative to crystallographic axes, as well as with a baric increase of intermolecular distances along the NLC direction. Quantum topological analysis of electron density was used to study intermolecular interactions. It has been established that TBPA and EMN crystals are optically transparent for visible light at pressures up to 1 GPa.