Hydride ion as a two-electron donor in a nanoporous crystalline semiconductor 12CaO·7Al₂O₃

The 12CaO·7Al₂0₃ (C12A7) crystal with a nanoporous lattice framework exhibits high electrical conductivity with an activation energy of ∼ 1.5 eV when equilibrated in a hydrogen atmosphere above ∼800°C. The high conductivity is preserved in a quenched state below ∼600°C with a reduced activation energy of ∼0.8 eV, Such complex behavior in electrical conductivity is associated with incorporation of hydride ions (H^-) in cages of the lattice framework. Electromotive force measurements reveal that the major carrier for the conductivity is electron with a small contribution by proton (H⁺), ruling out the possibility of direct intercage migration of the H^- ion. A combination of these observations with the ab initio calculations leads to the conclusion that the electrons are thermally generated from the H^- ion by the dissociation into two electrons and an proton, which is further converted to an OH^- ion via reaction with an extraframework oxide ion (0^2-). The energy difference between the initial (H^- + O^2-) and the final (2e^- + OH^-) states as evaluated by the theoretical calculation is as small as ∼1 eV, which agrees well with an experimentally obtained enthalpy change, ∼1.4 eV. Thus, internal equilibration between the extraframework hydrogen and the oxygen species is responsible for the thermal generation of the carrier electron. It is also suggested that the same conductive (2e^- + OH^-) state is reached by the photoirradiation of H^--containing C12A7. In this case the photoionization of H^- forms an electron and an H ^o atom, which then forms an OH^- ion and another electron with thermal assistance. The persistence of photoinduced conductivity is explained by the slow kinetics of the reverse process at room temperature.