The thermal expansion of an organic salt, 4-methylimidazolum-4-hydroxybenzoate (4-MIMD-HBC) has been studied by variable temperature single-crystal X-ray diffraction (VT-SCXRD). The salt exhibits unusual biaxial negative thermal expansion (NTE) and uniaxial large positive thermal expansion (PTE), whereas imidazolium-4-hydroxybenzoate (IMD-HBC) shows uniaxial NTE and biaxial PTE. Although the crystal structures of IMD-HBC and 4-MIMD-HBC are almost similar, there is a significant difference of the thermal expansion property between these two salts. The scissor-like motion of the hydrogen bonded networks is responsible for uniaxial NTE and PTE in both IMD-HBC and 4-MIMD-HBC salts along two principal axes, X1 and X3. IMD-HBC shows PTE along the other principal axis X2, where 4-MIMD-HBC exhibits NTE. The difference in thermal expansion between the two salts along X2 arises due to the methyl substituent, which induces HBC moieties in 4-MIMD-HBC to come closer to each other as the temperature increases, resulting contraction along X2.

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Plastic and elastic bending is known to many organic and few inorganic molecular crystals. Intermolecular interactions such as hydrogen bonding and halogen bonding play an important role towards these interesting...