Switching from Positive to Negative Axial Thermal Expansion in Two Organic Crystalline Compounds with Similar Packing

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).