Transport of hydrogenic species in crystalline oxides:: radiation and electric-field-enhanced diffusion

Thermal diffusion of protons, deuterons and tritons forming OH-, OD- and OT- radicals respectively was monitored by infrared absorption measurements in MgO, Al2O3, LiNbO3 and TiO2 single crystals. The electric charge and/or ionic radius is shown to be more important than mass in affecting the diffusion behaviour in these oxides. The influence of selected impurities and crystal orientation on the diffusion parameters was also investigated. Thermal diffusion of protons occupying substitutional anion vacancies (hydride ions) or [H-](+) centres was studied in thermochemically reduced MgO crystals. Simulations using an ab initio Hartree-Fock cluster approach indicate that the mobile defect is more compatible with the H- ion than with the proton. Application of even a moderate electric field is very effective in enhancing the H+ <----> D+ exchange in crystals containing hydroxyl ions. In addition, deuterons can be effectively incorporated in crystals with undetectable hydrogen concentrations by applying moderate electric fields at elevated temperatures. The incorporation of deuterons occurs without proton replacement, which indicates the possibility of D+ (H+) doping. Under electron irradiation, otherwise stable hydrogenic species become mobile at temperatures as low as 85 K. Ionizing radiation breaks the O-H bond with exceedingly large cross-sections (108 barns at room temperature), which is a strong function of the irradiating temperature. The displacement cross-section of protons is twice that of deuterons. Radiation induced displacement of protons from hydride ions at room temperature is also discussed. Out-diffusion of hydrogen isotopes can be induced in TiO2 crystals near room temperature by breaking the hydroxyl bond by electron irradiation and subsequently sweeping out hydrogenic species along the c-axis by application of an electric field.