Amorphous thin film ionic conductors of mLiF.nAlF3

The theoretical compound LiAlF4 (=LiF.AlF3) looks promising as a thin film ionic conductor. This paper describes an experiment to determine the conduction characteristics of that compound. A mixture of LiF and AlF3 is evaporated under 1 mPa onto glass substrates kept at room temperature. The resulting 0.8 μm thick colorless transparent film is amorphous even after annealing at 600°C as judged by X-ray diffraction. AC conductivity is measured to be 1.0 × 1.0−4 S/m at room temperature in a nitrogen atmosphere. This conductivity is determined to be principally ionic in nature, from lithium ions and/or protons. Further investigation of the molar ratio, the m and n for each component molecule, reveals that the optimum molar ratio, m/n, for the highest ionic conductivity ranges between 53, corresponding to Li5Al3F14, and 11, corresponding to LiAlF4. This conductivity is 106 times larger than that of either component material.