RbF particles (manufactured by Kojundo Chemical Laboratory Co., Ltd.) were milled with a planetary ball mill for 6 hours. Subsequently, the crystallization temperature of the particles after the milling treatment was measured with differential scanning calorimeter (DSC). Subsequently, the particles after the milling treatment were heated in an inert gas atmosphere at a temperature 20° C. higher than the measured crystallization temperature for 1 hour. As a result, a material having a composition represented by the formula RbF was produced.

(Sample 2)

Particles of RbF and MgF₂ (both manufactured by Kojundo Chemical Laboratory Co., Ltd.) were mixed at a molar ratio RbF:MgF₂ of 90:10. Subsequently, the mixture was milled with a planetary ball mill for 6 hours. Subsequently, the crystallization temperature of the mixture after the milling treatment was measured with a DSC. Subsequently, the mixture after the milling treatment was heated in an inert gas atmosphere at a temperature 20° C. higher than the measured crystallization temperature for 1 hour. As a result, a fluoride ion conductive material having a composition represented by the formula Rb_0.9Mg_0.1F_1.1 was prepared.

(Sample 3)

A fluoride ion conductive material having a composition represented by the formula Rb_0.8Mg_0.2F_1.2 was prepared as in Sample 2 except that the mixing ratio of RbF and MgF₂ particles was RbF:MgF₂=80:20 as the molar ratio.

(Sample 4)