Binary and Ternary Solid-Solutions of Ionic Plastic Crystals, and the Modulation of the Plastic Phase Transitions

Plastic crystals are solids characterized by disorder−order transitions that are often exploitable for the realization of functional materials. A major challenge in the design of such materials lies in the difficulty of both predicting and tailoring the phase stability and the relative transitions. Herein we describe a simple approach for the modulation of the plastic transition in the ferroelectric salts (R)-3- hydroxlyquinuclidinium chloride and bromide, [QH]Cl and [QH]Br, based on the formation of solid solutions. Mixed crystals of formula [QH]ClxBr1−x could be prepared over the entire compositional range. Additionally, the iodide analogous [QH]I was synthesized showing an ordered phase not isostructural with those of [QH]Cl and [QH]Br. In spite of the structural differences, binary solid solutions [QH]IzCl1−z and [QH]BryI1−y could also be obtained, as well as the ternary solid solutions [QH]ClxBryIz. All solids were characterized by a combination of single-crystal and powder X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis techniques in order to map the composition-dependent field of stability of the phases. It was observed that the [QH]ClxBr1−x solutions show a reversible order−disorder transition, while [QH]IzCl1−z and [QH]BryI1−y undergo an unusual first-order transition from the plastic phase to a glassy low-temperature phase. The ternary solid solutions, on the other hand, displayed a more complex behavior, influenced in turn by the prevailing component. Finally, the results are rationalized on the basis of the structural differences between the components, providing a simple criterion for the engineering of plastic phase transitions.