A variable temperature single crystal X-ray diffraction study revealed an unusual thermal expansion property of an organic salt, imidazolium 4-hydroxybenzene carboxylate, which exhibits colossal negative and positive axial thermal expansion along the crystallographic b axis and approximately along the a axis, respectively. The hydrogen bonded, two-dimensional square grid type of the flexible network in the crystal structure of the salt resembles a fencing structure that undergoes scissor-like motion resulting the abnormal thermal behavior. Thermal expansion induced by a scissor motion of the hydrogen bonded network in a multicomponent crystalline organic compound has not been reported before, although this mechanism is mentioned to elucidate colossal thermal expansion in some inorganic framework materials.

Switching from positive to negative axial thermal expansion in pure organic materials is reported for the first time. This rare phenomenon has been rationalized based on the packing of molecules in crystal structures and transverse thermal vibrations of atoms in the molecule. Unique packing of the molecules in the crystal structure contributes to the restricted movement of molecules along the c axis. Subsequently, contraction of molecular dimensions with increasing temperature, due to transverse vibrations of some atoms, assists with the switch from Positive Thermal Expansion (PTE) to Negative Thermal Expansion (NTE).

Rare area negative thermal expansion of a new mixed metal mixed organic MOF has been described using an “elevator-platform” analogy induced by O–H⋯O hydrogen bonding.

Thermal expansion of an organic salt, prepared by solvent-drop grinding of terephthalic acid and imidazole in a 1:2 ratio, has been studied over a wide range of temperature 100–430 K. Restricted motion of the components in the hydrogen-bonded network resembles a fence-like structure led to a uniaxial negative thermal expansion (NTE). Interestingly, N–H···O hydrogen bonding distances remain almost unchanged over the temperature of study. Large transverse vibration of atoms involved in the hydrogen bonding plays an unprecedented role to keep the distances constant.

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