There is provided a C12A7 compound capable of selectively and reversibly clathrating X.sup.- ions, such as active oxygen radicals, with improved controllability. The C12A7 compound clathrating OH.sup.- ions and On.sup.- at a concentration of 2.times.10.sup.19 cm.sup.-3 or less can be prepared by subjecting a mixed material containing Ca and Al to a solid phase reaction at a firing temperature not lower than 1200.degree. C. and lower than 1449.degree. C. in a dry oxygen atmosphere having an oxygen partial pressure of 0.1 atm or more and a water vapor partial pressure of 10.sup.-3 atm or less, keeping the resultant product at a temperature of 1200.degree. C. or higher, and cooling rapidly. The C12A7 compound is heat-treated to clathrate OH.sup.- ions at a concentration of 2.times.10.sup.19 cm.sup.-3 or less, and On.sup.- at a high concentration of more than 2.times.10.sup.19 cm.sup.-3. The compound can reversibly clathrate or release On.sup.- ion radicals through the elevation or lowering of the temperature thereof in a temperature range of 300 to 850.degree. C. The compound is useful for an oxidation catalyst, an anti-bacterial agent, an ion conducting material, an electrode for a solid-electrolyte fuel cell, and the like.