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One step closer to solving a mystery – structural and theoretical study of the thermosalient phenomenon

Imagine the surprise on one’s face when, during the microscopic evaluation of the crystals’ habits, these crystals start to literally jump and fly of the stage providing a vivid example of thermosalient effect. Thermosalient materials are the ones that during heating/cooling undergo an energetic phase transition which is so sudden and sharp that the crystals are ballistically projected to heights of several hundred times larger than their own dimensions. Apart from providing visually extremely attractive phenomenon, these materials have tremendous technological potential as the future self-actuation device (nanoswitches, thermal sensors, artificial muscles, etc.)1. Several exciting experimental studies on the origin of the thermosalient effect were published resulting in astonishing wealth of information about the effect, but still we are far from the full understanding of the peculiarities and all the interplays and forces that are governing it. It is now obvious that experimental investigations alone are not able to provide the much needed elucidation of the effect and it is necessary to accompany experimental knowledge with the theoretical studies, involving the most modern state-of-the-art DFT methods. Here we present a combined experimental and theoretical study of the thermosalient effect in the N-2-propylidene-4- hydroxybenzohydrazide, which shows not one but three thermosalient phase transitions previously unreported in the literature. This is, to the best of our knowledge, first attempt to theoretically decipher the jumping crystals effect. As in most of the thermosalient systems, immense negative thermal expansion seems to be the most likely candidate for the driving force behind the phenomenon. Our results show that it is the direct consequence of the elastic properties of the crystal which exhibits uniaxial negative compressibility. Furthermore, these properties are also determining the reversibilities/irreversibilities of the phase transitions in this system. And in conclusion, we propose a new mechanism of the thermosalient effect in this system, based on the excitations of the low-energy phonons which provide energy needed for the crystal to overcome the energy barrier between the phases. 1 J. Am. Chem. Soc., 2010, 132 (40), pp 14191–14202

thermosalient effect, DFT, crystal structure

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