Factors controlling the physical properties of an organic ionic plastic crystal

Organic ionic plastic crystals (OIPCs) containing organic cations and inorganic anions are gaining tremendous attention as an unconventional type of crystalline material. They usually possess one or more solid-solid phase transitions due to different levels of thermodynamic molecular motion, and also display diverse ionic conductivity and plasticity. Until today, we have not fully understood the main determinants of these properties, which are critical to designing the material to meet requirements for practical applications, such as solid-state battery electrolytes. In this work, we conducted a comprehensive experimental and computational investigation on a recently reported ammonium-based OIPC, which possesses only a single solid-solid phase transition before melting. This material maintains a very organized structure orderly at temperatures up to the melting point. The volume expansion along three sides of the crystal structure during heating is anisotropic, mainly on the a-side, controlled by different interionic forces between adjacent ions in each direction. The c-side of the crystal lattice experiences the strongest attraction, such as hydrogen bonding, reflected in the shortest CH⋅⋅⋅O distance of 2.293 Å, which is believed to hinder the rotation and translation of ions, thus decreases the plasticity of OIPC, and also results in the preservation of the long-range crystalline order. The single OIPC phase transition here is due to the growth in the rotational motions of the cations and anions. These observations are different from the previously reported phosphonium salt, suggesting that the interionic force and chemical structures significantly affect the physical, thermodynamic and phase behavior of OIPCs.