Investigation of the Physico-Chemical Properties of Plastic Electrolytes in a Binary System (<i>n</i>-C₄H₉)₄NBF₄–(<i>n</i>-C₄H₉)₃CH₃NBF₄

A comprehensive study of the influence of (n-C4H9)3CH3NBF4 on the transport properties of (n-C4H9)4NBF4 ─representatives of the class of organic ionic-plastic crystals ─ has been carried out. In pure salts there are several solid–solid phase transitions in the temperature range 25 °C─Tmelting: in (n-C4H9)4NBF4 ─ at 67 °C and Tmelting = 162 °C, and in (n-C4H9)3CH3NBF4 ─ at 56 and 105 °C and Tmelting = 158 °C. According to XRD and DSC data, the introduction of the second salt leads to the stabilization of high-temperature plastic cubic phases (Pm-3n or P-43n) of the salts in the solid solution series at room temperature. Vickers microhardness measurements showed that the high-temperature phases of the salts are characterized by lower Hv values than the low-temperature phases: Hv ((n-C4H9)3CH3NBF4) = 1.7 ± 0.1, which is 5.5 times lower than that of the low-temperature phase. The microhardness of the solid solutions has close values (Hv = 1.4–1.8), which indicates the presence of plasticity of the materials. The conductivity of the solid solutions has a nontrivial character: the increase in conductivity by 0.5–0.7 orders of magnitude is caused by the substitution of large cations (n-C4H9)4N+ by smaller (n-C4H9)3CH3N+, and vice versa by the substitution of small cations by large ones at low dopant concentrations (10 mol %), but high dopant concentrations (more than 10 mol %) led to a decrease in conductivity and the appearance of a minimum on the conductivity isotherms. The obtained concentration dependence of the conductivity can be explained by the opposite influence of two factors:the occurrence of mechanical stresses in the lattice, leading to an increase in the ion mobility, and an increase in the enthalpy of defect formation, leading to a decrease in the conductivity, with the first factor predominating.