Phase Behavior of the [N(C₂H₅)₄][BF₄]-[N(C₃H₇)₄][BF₄] Binary System

抄録

Phase behavior of the [N₂₂₂₂][BF₄]-[N₃₃₃₃][BF₄] (N₂₂₂₂⁺ = tetraethylammonium, and N₃₃₃₃⁺ = tetrapropylammonium,) binary system has been investigated by differential scanning calorimetry and powder X-ray diffraction. Two solid-solid phase transitions are observed in differential scanning calorimetric curve in the range of x([N₃₃₃₃][BF₄]) = 0.1–0.9 (x([N₃₃₃₃][BF₄]) = the molar fraction of [N₃₃₃₃][BF₄]) reflecting the solid-solid phase transition of each single salt, and only slight shift was observed for the transition temperatures. Powder X-ray diffraction patterns confirmed phase transitions. Although the amount of the minor constituent is low in the solid solution phase based on each single salt at 313, 373, and 423 K, Phase I of [N₂₂₂₂][BF₄] at 483 K with the NaCl-type structure, which is regarded as an ionic plastic crystal (IPC) phase, can accommodate [N₃₃₃₃][BF₄] up to the level where significant change in lattice parameter is observed (5.7% volume expansion). Drop of the liquidus line was observed for the salts with the mixing ratio x([N₃₃₃₃][BF₄]) = 0.8 to 0.9 approaching to the eutectic temperature of 475 K. Ionic conductivity of Phase I increases by two orders of magnitude from x([N₃₃₃₃][BF₄]) = 0 to 0.1 owing to the lattice expansion by inclusion of N₃₃₃₃⁺.