Humidity and temperature driven transformations in ferroelectric quinuclidine-based chlorocobaltate(ii) complex salt: bulk and thin films with preferred orientation

Abstract

Ferroelectricity remains a fascinating phenomenon in materials science due to the constant uncovering of novel ferroelectric hybrid materials and new features of the phenomenon, related to its potential applications for the development of new technologies. Herein, preparation of a new inorganic–organic hybrid material composed of 3-quinuclidinone cations and a tetrachlorocobaltate(II) anion is reported, by utilization of a green chemistry approach without the use of solvents through a mechanochemically driven reaction. Two forms of the same compound; anhydrous and hydrate, can be prepared depending on the reactants used in the synthesis. Alternately two compounds can be transformed into one another by exposure to humid conditions or a dry atmosphere. In addition to humidity-driven compositional transformation, the prepared material undergoes multiple temperature-induced phase transformations. For the anhydrous phase, low-, room- and high-temperature phases were detected, belonging to the ferroelectric space group. Multiple transformations that occur in this material are an indication of the soft crystal packing driven by non-covalent interactions. Due to structural flexibility, this system is an ideal candidate for deposition on a substrate, for which the solution-based dip-coating method was used. The obtained metal–organic thin films grow in a specific crystal direction on Si(100) and ITO-coated glass substrates, forming crystalline and uniform films. The self-assembly between cations and anions is discussed, along with the energetic driving forces that lead to a particular thin film structure. Ferroelectric testing through remanent polarization experiments under ambient conditions confirms the uniformity and purity of this metal–organic thin-film system.