engleski

TUNING OF THERMOSALIENT PROPRIETIES OF MOLECULAR CRYSTALS, A STUDY BY NON-AMBIENT POWDER DIFRACTION.

The thermosalient crystals are newly researched class of adaptive crystalline solids that are thought of as candidates for active actuating elements in all-organic micromachines. Molecular crystals that exhibit thermally activated motion are currently hot topic in the materials science. Such thermosalient materials undergo an energetic phase transition during heating/cooling which is so swift that the crystals are ballistically projected to heights of several hundred times larger than their own dimensions. These locomotive materials possess tremendous technological potential as the future self-actuating devices (nanoswitches, thermal sensors, artificial muscles, etc.) [1]. The majority of thermosalient crystals are organic compounds and several cases have been studied in detail. Upon heating, oxitropium bromide (OXTB) crystals exhibit an endothermic phase transition from form A (OXTB-A) to form B (OXTB-B), occurring at 331.0 K. A detailed thermal study of oxitropium bromide and structure determination of both phases revealed that the jumping of the crystals is a macroscopic manifestation of a highly anisotropic change in the cell volume. [1] Crystals of 1, 2, 4, 5- tetrabromobenzene (TeBB) jump several centimeters during the phase transition from the low-temperature β-form to the high-temperature γ-form. Hopping of the crystals happens at 45.0 °C for single crystals and at 45.5 °C for twinned crystals [2]. We explored the thermosalient effect in in 1, 2, 4, 5- tetrabromobenzene alloys formed with organic isostructural compounds (for example, 1, 2, 4, 5- tetrachlorobenzene) and the impact of the variation of composition of the alloys on the thermosalient effect. The alloy exhibit similar structural characteristics at room temperature ; they all crystallize in β form, and upon heating convert to γ. The phase transition temperature changes with alloy composition and coincides with the temperature of jumping. We have observed a linear correlation between temperature of jumping and alloy composition, with the presence of a threshold concentration for the activation of thermosalient effect. We study the how the structural modification on molecular level affect macroscopically the thermosalient phase transition studying some derivatives of oxitropium bromide. This study produced rather interesting results showing that small changes could have an important impact on thermosalient proprieties bringing to vanishing of phase transition. [3] X-ray diffraction has revealed the mechanism of formation of mixed crystals showing how the Br an Cl atoms are integrated in the alloy structure. The non-ambiental xrd measurement provided a look inside the structure transformations of oxitropium bromide derivatives. References: [1] Skoko, Ž. ; Zamir, S. ; Naumov, P. ; Bernstein, J. J. Am. Chem. Soc. 2010, 132 (40), 14191 [2] Lieberman, H. F. ; Davey, R. J. ; Newsham, D. M. T. Chem. Mater. 2000, 12 (2), 490 [3] Klaser, T. ; Popović J. ; Fernandes, J. A. ; 1, Tarantino, S. C. ; 3, Zema, M. ; Skoko, Ž. Crystals, 2018, 8(7), 301

Thermosalient ; non-ambiental xrd ; phase transition

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