FLUORIDE ION CONDUCTOR CONTAINING POTASSIUM, ALKALINE EARTH METAL, AND FLUORINE, AND FLUORIDE ION SECONDARY BATTERY INCLUDING THE SAME

A fluoride ion conductor contains potassium, at least one alkaline earth metal selected from the group consisting of calcium, barium, and strontium, and fluorine. The fluoride ion conductor includes a phase of a compound containing potassium, at least one alkaline earth metal, and fluorine.