Hayashi Katsuro, Sushko Peter V, Shluger Alexander L, Hirano Masahiro, Hosono Hideo
Frontier Collaborative Research Center and Materials and Structures Laboratory, Tokyo Institute of Technology, S2-13, 4259 Nagatsuta, Yokohama 226-8503, Japan.
J Phys Chem B. 2005 Dec 22;109(50):23836-42. doi: 10.1021/jp053990p.
The 12CaO.7Al2O3 (C12A7) crystal with a nanoporous lattice framework exhibits high electrical conductivity with an activation energy of approximately 1.5 eV when equilibrated in a hydrogen atmosphere above approximately 800 degrees C. The high conductivity is preserved in a quenched state below approximately 600 degrees C with a reduced activation energy of approximately 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 approximately 1 eV, which agrees well with an experimentally obtained enthalpy change, approximately 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.
具有纳米多孔晶格框架的12CaO·7Al2O3（C12A7）晶体，在约800℃以上的氢气氛中平衡时表现出高电导率，其激活能约为1.5eV。在约600℃以下的淬火状态下，高电导率得以保留，激活能降低至约0.8eV。这种复杂的电导率行为与晶格框架笼中氢化物离子（H-）的掺入有关。电动势测量表明，电导率的主要载流子是电子，质子（H+）贡献较小，排除了H-离子在笼间直接迁移的可能性。这些观察结果与从头算计算相结合得出结论，电子是由H-离子热解离成两个电子和一个质子而产生的，质子通过与骨架外氧化物离子（O2-）反应进一步转化为OH-离子。理论计算评估的初始（H- + O2-）和最终（2e- + OH-）状态之间的能量差小至约1eV，这与实验获得的焓变约1.4eV非常吻合。因此，骨架外氢与氧物种之间的内部平衡是载流子电子热产生的原因。还表明，含H-的C12A7经光照射也能达到相同的导电（2e- + OH-）状态。在这种情况下，H-的光电离形成一个电子和一个H0原子，然后在热辅助下形成一个OH-离子和另一个电子。光致电导率的持续性可通过室温下逆过程的缓慢动力学来解释。
