Solid Solution Engineering of [(C3H7)4N]x[(C4H9)4N]1-x[FeBrCl3] Plastic Crystal Phase-Change Materials

Abstract

Plastic crystals are supramolecular materials with promising properties for solid state phase-change material technologies. The [(C₃H₇)₄N]_x[(C₄H₉)₄N]_1-x[FeBrCl₃] binary system was used to study the effects of cation size in tetraalkylammonium-based plastic crystals. Materials were synthesized by evaporation crystallization and annealing in the liquid phase, and then characterized using in-situ synchrotron X-ray diffraction and differential scanning calorimetry. A single phase solid solution was confirmed for x < 0.2 at ambient temperatures, with an extended region of miscibility in the high–temperature plastic crystal phase. By introducing a degree x of smaller cations into the host lattice, the solid–solid phase transition temperature was seen to reduce consistently from 112 °C to 100 °C with increasing x, while the melt transition temperature also reduced from 132 °C to 98 °C. A phase diagram of the systems is also presented and will serve to increase understanding of compositional engineering in plastic crystal binary systems.