Formation of organic ion cocrystals, phase transition and ion conduction

Organic ionic plastic crystals (OIPCs) have been increasingly drawing attention recently owing to their many practical applications. In this study, seven organic salts [Cat₁][Cat₂][Ni(mnt)₂] were prepared, where mnt²⁻ = maleonitriledithiolate, and Cat₁⁺ and Cat₂⁺ represent globular quaternary ammonium cations (Me₄N⁺, Et₄N⁺, Pr₄N⁺ and Bu₄N⁺). Each salt is labeled, according to its cations, as Me, Et, Pr, Bu, MeEt, MeBu and EtBu, respectively. These salts were characterized by microanalysis, IR spectroscopy, thermogravimetry (TG), differential scanning calorimetry (DSC), and single crystal and powder X-ray diffraction techniques. Although the seven salts show analogous packing structures, consisting of alternating anion layers and cation layers, only Bu and MeBu are OIPCs, and, moreover, have multiple plastic crystal phases. Another striking feature in this family is that only cocrystals (MeEt, MeBu and EtBu) were obtained, while MePr, EtPr and PrBu were not achievable. The calculation of lattice formation energy per formula unit (E_Latt/f.u.) revealed that the cocrystal is metastable relative to its two parent salts, but achievable if the E_Latt/f.u. of the cocrystal is comparable to the average E_Latt/f.u. of its two parent salts. Besides multiple phase transitions, the critical temperature of the initial phase transition and melting points in the cocrystal shift towards low temperature compared to those of its two parent salts, and the reason was discussed. Both Bu and MeBu in plastic crystal phases display high ionic conduction (σ > 10⁻³ S cm⁻¹) and the other salts show ionic conductivities in the scope of 10⁻¹²-10⁻⁵ S cm⁻¹ at 338-473 K.