Dielectric switching in correlation with the structural phase transitions in tetrapropylammonium perchlorate

The crystals of the tetrapropylammonium perchlorate ([(CH₃CH₂CH₂)₄N]ClO₄, TePrAClO₄) compound undergo two reversible phase transitions: at ca. T₁ = 284 K and at ca. T₂ = 445 K. The observed phase transitions and distinct dielectric and relaxation effects are due to the dynamic motions of the organic cations and anionic framework. The crystals become ordered at low temperatures, then disordered at room temperature (propyl chains of the organic part as well as perchlorate ions are disordered over the mirror plane at c = 1/4 and 3/4) and highly disordered at high temperatures. The comparable changes in the wavenumber and FWHM shifts (IR and Raman spectroscopy) in the case of tetrapropylammonium and perchlorate ions in the phase transition at T₁ and slightly more significant changes for organic cations (juxtaposed with perchlorate ions) in the phase transition at T₂ lead to a conclusion that the phase transition at T₁ is equally driven by motions of the two ions, while the phase transition at T₂ is more influenced by the motions of organic cations. The phase transition at T₂ with its large entropy change resembles the behavior found in liquid crystals. The dielectric function values can be switched and tuned in the low- and high-dielectric states, which may indicate the potential application of this material in sensors or actuators.