Binary and Ternary Solid Solutions of Ionic Plastic Crystals, and Modulation of 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.