Functionalities of a nanoporous crystal 12CaO·7Al₂O ₃ originating from the incorporation of active anions

A cage structure formed in a positively charged lattice framework of 12CaO-7Al₂O₃ (C12A7) is utilized to incorporate and release chemically unstable anions, imparting chemical and electronic activities to this material. High concentrations of oxygen radical anions, i.e. O ^- and O₂^-, are formed efficiently in these cages under a high oxygen activity condition. The O^-ion, which is the strongest oxidant among all the reactive oxygen species, can be extracted from the cages into an external vacuum by applying an electric field with thermal assistance. In contrast, hydrogen-reduction processes allow the formation of hydride (H^-) ions in the cage. H^- ion-incorporated C12A7 exhibits a persistent insulator-conductor conversion upon irradiation with ultraviolet-light or electron-beam. The irradiation-induced conversion mechanism and electronic transport characteristics are discussed on the basis of experimental evidence and first-principle theoretical calculations. Electrons released from the H^- ions are trapped by the cages and subsequent inter-cage hopping of the electrons is responsible for the electrical conduction. Furthermore, a severe reducing environment enables us to substitute electrons almost completely for anions in the cages, forming a stable inorganic electride. Finally, the formation of these active anions in this material is examined in terms of electronic structures, thermodynamics, and structural features of the cage.