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

Abstract

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.

Effect of H and He irradiation on cavity formation and blistering in ceramics

“…The cavity microstructure for 1 MeV H ion irradiated MgO was not examined in detail, but the cavities appeared to occur as platelets coinciding with the loops as opposed to a collection of small spherical cavities. Since hydrogen diffusion in MgO has been reported to be considerably enhanced by the ionizing radiation associated with proton irradiation [88,89], it is possible that some of the implanted hydrogen may have diffused away from the bombarded region during the irradiation.…”

Hydrogen-isotope transport induced by an electric field inα−Al2O3single crystals

“…Hydrogen exists in most as-grown oxide crystals as a ubiquitous impurity that enters the lattice during crystal growth and profoundly influences the electronic, mechanical, and optical characteristics of these materials, especially for device applications. [1][2][3][4][5][6][7][8][9][10][11][12][13] Hydrogen ions normally occupy stable configurations in oxide crystals and do not diffuse until several hundred degrees above room temperature. [13][14][15] Hydrogen and its isotopes-deuterium and tritium-diffuse much more slowly in insulators than in metals.…”

Hydrogen and deuterium diffusion in non-stoichiometric spinel

“…Pieces of this material were prepared as 1 × 1 × 2 mm polished rectangular prisms, cut using a diamond wire saw and polished gently using up to 1 µm diamond polishing suspensions. Composition of the material, as determined by electron microprobe (EMP) (details in Section 2.2) is as follows: Mg: 6.62(5)%, Al: 47.43(14)%, O: 47.52 (15), where values are averaged element weight percent, and values in parentheses are standard deviations on the last figure for averaged compositions. No B was detected in the sample during analysis (detection limit 150-200 ppmw), and the fact that it is colourless implies no contamination due to the presence of even trace amounts of transition metals such as Fe and Cr.…”

“…In particular, non-stoichiometric spinel contains high concentrations of M site vacancies, so additionally, protonation of vacancies would appear to be highly favourable due to the strong net negative charge associated with cation vacancies (i.e. associated H i -V /// Al defects involving single or multiple protons) [10,[14][15][16]. However, in all IR spectra here relative areas of all O-H contributions remain unchanged across diffusion profiles, during either H + loss or H + gain.…”

“…hydroxyl) in the deep Earth [3,[11][12][13]. In addition to the insight which it provides on H + incorporation in major mantle NAMs, non-stoichiometric spinel has several potential technological applications, most notably as a proton conductor and H + sensor, and has been the subject of numerous investigations of electrochemical properties [14][15][16]. Spinel sensu stricto and ringwoodite are both minerals within the spinel mineral class.…”