Investigation of Phase Transitions in Molecular Solid Solutions Using an Organic Ferroelectric Model System

Despite the importance of electronic materials that undergo thermally-induced solid-state structural transitions for modern technologies, the diversity of factors influencing the emergence of phase transitions has not yet been translated into general principles for the discovery of novel materials. Tuning performance through substitutional replacement is an alternative approach that is routine in the field of inorganic electronics but, while substitutional replacement is becoming more common in organic materials, studies correlating the effects of substitutional replacement with changes in phase transition behavior in organics are scarce. The ferroelectric charge-transfer (CT) cocrystal formed between acenaphthene (AN) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is an ideal model system in which to investigate the role of molecular dopants on thermally-induced phase transitions. In AN-F4TCNQ, the ferroelectric-to-paraelectric phase transition is attributed to thermally-induced whole-molecule rotation of AN. In this work, we investigate the structural, electronic, and thermal responses of the AN-F4TCNQ model system to the substitutional replacement of AN with two molecular dopants. Critically, we find that the degree of CT can be increased upon dopant incorporation, but in both cases of substitutional replacement, the phase transition temperature is lowered. A combination of X-ray diffraction measurements and computational studies is used to explain this behavior.