Illustrating Extreme Negative Linear Compressibility in Thermosalient Molecular Crystals

We report a high-pressure single-crystal X-ray diffraction study of the thermosalient molecular crystal N’-2-propylidene-4hydroxybenzohydrazide. We find that it features one of the largest negative linear compressibilities recorded, with a coefficient of −39 TPa^–1. With increasing pressure, it converts to a positive linear compressibility and at even higher pressures again to a negative linear compressibility. This switchable behavior arises from the pressure-induced straightening of the hydrogen-bonded zigzag chains, which is eventually counteracted by direct bond compression. To capture this mechanism, we trained a machine-learning interatomic potential and performed quasi-harmonic free-energy calculations, thereby obtaining atomistic insight across the whole pressure range. These findings broaden the still-scarce catalog of organic crystals that combine thermosalient activity with experimentally verified, pressure-tunable NLC.