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

• Published in: Crystal growth & design Vol. 19; no. 11; pp. 6266 - 6273
• Main Authors: d’Agostino, Simone, Fornasari, Luca, Braga, Dario
• Format: Journal Article
• Language: English
• Published: American Chemical Society 06.11.2019
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/acs.cgd.9b00751

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]­Cl x Br1–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]­I z Cl1–z and [QH]­Br y I1–y could also be obtained, as well as the ternary solid solutions [QH]­Cl x Br y I z . 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]­Cl x Br1–x solutions show a reversible order–disorder transition, while [QH]­I z Cl1–z and [QH]­Br y I1–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.