Reversible Thermosalient Effect of N'-2-Propylidene-4-hydroxybenzohydrazide Accompanied by an Immense Negative Compressibility: Structural and Theoretical Arguments Aiming toward the Elucidation of Jumping Phenomenon

The temperature-induced reversible phase transition of N'-2-propylidene-4-hydroxybenzohydrazide from the polymorphic Form II to Form III, and vice versa, is accompanied by the dramatic change of the macroscopic dimensions of the crystal which resulted in the pronounced mechanical motion (jumping) during the phase transition. Prior to the phase transition, the extremely large uniaxial negative thermal expansion along one crystal axis (b axis) was observed, together with the positive thermal expansions along the other two crystal axes. Form III of N'-2-propylidene-4-hydroxybenzohydrazide exhibits the thermal expansion alpha(c) = 360 x 10(-6) K-1, which is the largest value ever noticed in any organic or metal organic crystal. From the structural point of view, a thermosalient effect is escorted by the springlike behavior of the zig-zag molecular assemblies along the c axis. First-principles electronic structure calculations show that negative thermal expansion arises from the elastic properties of the crystal which show uniaxial negative compressibilities, NLC. Form III exhibits the negative compressibility along the 001 direction beta(3) = 28 TPa-1, which is 1 order of magnitude larger than that of any organic compound and, in fact, is comparable to compressibilities of molecular frameworks showing the most pronounced NLC behavior. Elastic properties are also the reason for the reversibility of Form II to Form III transition in contrast to the irreversible Form I to Form II transition. Low energy springlike phonons are easily thermally excited and can assist in the overcoming of the energy barrier between the two phases that precedes thermosalient transition.