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

The 12CaO center dot 7Al(2)O(3) (C12A7) crystal with a nanoporous lattice framework exhibits high electrical conductivity with an activation energy of similar to 1.5 eV when equilibrated in a hydrogen atmosphere above similar to 800 degrees C. The high conductivity is preserved in a quenched state below similar to 600 degrees C with a reduced activation energy of similar to 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 ion. A combination of these observations with the ab initio calculations leads to the migration of the H 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 (O2-). The energy difference between the initial (H- + O2-) and the final (2e(-) + OH-) states as evaluated by the theoretical calculation is as small as similar to 1 eV, which agrees well with an experimentally obtained enthalpy change, similar to 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.