2018 年 86 巻 2 号 p. 52-56
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+.