Supercolossal Uniaxial Negative Thermal Expansion in Chloranilic Acid Pyrazine, CA-Pyz

There has been significant recent interest in exploiting the large dimension changes that can occur in molecular materials as a function of temperature, stress, or under optical illumination. Here, we report the remarkable thermal expansion properties of chloranilic acid pyrazine co-crystals. We show that the compound shows uniaxial negative thermal expansion over a wide temperature range with a linear contraction coefficient as low as (-)1500 X 10-6 IC' at 250 K. The corresponding 10% contraction between 200 and 300 K is an order of magnitude larger than in the so-called colossal contraction materials. We adopt a symmetry-inspired approach to describe both the structural changes that occur (using rotational symmetry modes) and the thermal expansion (using strain modes). This allows an extremely compact description of the phase transition responsible for this unusual behavior and gives detailed understanding of its atomic origins. We show how the coupling of primary and secondary strain modes in materials showing extreme expansion and contraction can lead to unusual reversals in the temperature dependence of cell parameters.