Thermal, Structural, and Transport Properties of Solid Solutions Based on Organic Ionic Plastic Crystals (C ₃ H ₇ ) ₄ NBF ₄ and (C ₄ H ₉ ) ₄ NBF ₄

Organic ionic plastic crystals (OIPCs) are promising candidates for solid electrolytes due to their favorable properties, including incombustibility, plasticity, and high electrochemical stability. However, the ionic conductivity of many OIPCs remains insufficient for practical applications. Isovalent substitution represents a potential strategy for enhancing the transport properties. This study examines the effects of isovalent substitution with larger cations on the thermal, structural, and transport properties of a binary system based on tetrapropylammonium tetrafluoroborate (Pr4NBF4) and tetrabutylammonium tetrafluoroborate (Bu4NBF4). In the Pr4NBF4 – Bu4NBF4 system, the temperature of the polymorphic phase transition (II → I) decreased with an increasing dopant concentration. The melting temperature also decreased monotonically, from 247 to 163 °C, as the fraction of Bu4NBF4 increased. Above the II → I transition temperature, all compositions adopted a cubic phase (space group P4̅3n), isomorphic to the high-temperature phase of the pure salts, with a linear expansion of the cell parameter as the Bu4NBF4 content increased. Thermal and X–ray diffraction analyses confirmed the formation of a continuous solid solution across the entire composition range. Replacement of Pr4N+ with the large Bu4N+ cation increased ionic conductivity from 1.5 × 10–9 S × cm–1 (pure Pr4NBF4) to 6.3 × 10–7 S × cm–1 (pure Bu4NBF4) at 140 °C, which is explained by a decrease in the enthalpy of defect formation. Notably, two conductivity maxima were observed near 10 and 95 mol % Bu4NBF4, deviating from the overall trend. These enhancements are likely due to uncompensated microdeformations around impurity ions, which reduce the migration enthalpy. The study demonstrates the possibility of controlling both the thermal and transport properties of organic ionic plastic crystals through isovalent doping.