Hydride Ion as a Two-Electron Donor in a Nanoporous Crystalline Semiconductor 12CaO·7Al2O3

• Published in: The journal of physical chemistry. B Vol. 109; no. 50; pp. 23836 - 23842
• Main Authors: Hayashi, Katsuro, Sushko, Peter V, Shluger, Alexander L, Hirano, Masahiro, Hosono, Hideo
• Format: Journal Article
• Language: English
• Published: American Chemical Society 22.12.2005
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp053990p

The 12CaO·7Al2O3 (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 (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 ∼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 Ho 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.