Through the first-principles local-density-$\text{approximation}+\text{Hubbard}\text{\ensuremath{-}}U$ (LDA+$U$) electronic-structure calculation method, the microscopic properties of the Nb antisite $({\text{Nb}}_{\text{Li}})$ and the electron-lattice interaction are investigated. The atomic structure is found to depend on the capture of electrons at the defect level, and especially when the defect level is occupied by two electrons, the ${\text{Nb}}_{\text{Li}}$ undergoes a large-lattice-relaxation (LLR), accompanied with the formation of the deep level. The main driving force toward the LLR is suggested to be the orbital hybridization of the defect level state and the conduction-band state. As a result, the ${\text{Nb}}_{\text{Li}}$ defect exhibits a negative-$U$ property. Based on the computational results, several well-known light-induced phenomena in ${\text{LiNbO}}_{3}$ and the polaron model are discussed.

First-principles study of microscopic properties of the Nb antisite in LiNbO 3 : Comparison to phenomenological polaron theory

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to identify and characterize electrons trapped by oxygen vacancies and holes trapped by lithium vacancies in lithium tetraborate (Li2B4O7) crystals. Our study includes a crystal with the natural abundances of B10 and B11 and a crystal highly enriched with B10. The as-grown crystals contain isolated oxygen vacancies, lithium vacancies, and copper impurities, all in nonparamagnetic charge states. During an irradiation at 77 K with 60 kV x-rays, doubly ionized oxygen vacancies trap electrons while singly ionized lithium vacancies and monovalent copper impurities trap holes. The vacancies return to their preirradiation charge states when the temperature of the sample is increased to approximately 90 K. Hyperfine interactions with B10 and B11 nuclei, observed between 13 and 40 K in the radiation-induced EPR and ENDOR spectra, provide models for the two vacancy-related defects. The electron trapped by an oxygen vacancy ...

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Identification of electron and hole traps in lithium tetraborate (Li2B4O7) crystals: Oxygen vacancies and lithium vacancies

Radiation-induced spin centers in vitreous boron trioxide and alkali borate glasses were studied using pulsed electron paramagnetic resonance (EPR). It is shown that electrons and holes in these glasses are trapped on valence alternation defects, undercoordinated oxygen (holes) and overcoordinated oxygen (electrons). The local environment around these defects has major effect on spin parameters of the corresponding spin centers. The electronic and atomic structure of spin-1/2 centers and their diamagnetic precursors is analyzed using semiempirical and ab initio calculations.

Magnetic resonance studies on radiation-induced point defects in mixed oxide glasses. I. Spin centers in B2O3 and alkali borate glasses

The recombination of photoinduced free ${\mathrm{Nb}}_{\mathrm{Nb}}^{4+}$ and bound ${\mathrm{Nb}}_{\mathrm{Li}}^{4+}$ small polarons to ${\mathrm{Nb}}_{\mathrm{Li}}^{4+}\ensuremath{\mathbin:}{\mathrm{Nb}}_{\mathrm{Nb}}^{4+}$ bipolarons is investigated in nominally pure, reduced ${\mathrm{LiNbO}}_{3}$ single crystals by means of excited-state-absorption spectroscopy. We discovered a two-component decay of the light-induced absorption ${\ensuremath{\alpha}}_{\mathrm{li}}(t)$ for probe light at $\ensuremath{\lambda}=785\text{ }\text{ }\mathrm{nm}$ and moderate pump beam intensities (${I}_{\mathrm{p}}\ensuremath{\ll}670\text{ }\text{ }\mathrm{GW}/{\mathrm{m}}^{2}$). These experimental results give strong evidence for the existence of a two-path recombination of the photoinduced polarons. A corresponding model taking into account hopping charge transport and trapping is presented.

Evidence for Two-Path Recombination of Photoinduced Small Polarons in Reduced LiNbO~3

The influence of chemical reduction on the particular number densities of light-induced free small ${\mathrm{Nb}}_{\mathrm{Nb}}^{4+}$ electron polarons, bound small ${\mathrm{Nb}}_{\mathrm{Li}}^{4+}$ electron polarons, bound small ${\mathrm{Nb}}_{\mathrm{Li}}^{4+}:{\mathrm{Nb}}_{\mathrm{Nb}}^{4+}$ electron bipolarons, and bound small ${\mathrm{O}}^{\ensuremath{-}}$ hole polarons is investigated in nominally pure, congruently melting $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ by means of excited-state absorption spectroscopy Characteristic changes in the sign of the light-induced absorption in the blue-green spectral range and distinctive dependencies on the pump beam intensity reflect the increasing contribution of electron polarons generated upon single intense ns-laser pulses at $\ensuremath{\lambda}=532\phantom{\rule{03em}{0ex}}\mathrm{nm}$ with increasing degree of reduction The entire data set including its time dependence and spectral properties is consistently explained within a model taking into account the presence of all four types of electron and hole polarons, and residual ${\mathrm{Fe}}^{2+∕3+}$ impurities The model includes one- and two-photon excitation processes for polaron generation, optical gating of bipolarons, and direct and two-step polaronic recombination processes Our results indicate a mutual independence of two-photon hole polaron generation and one-photon dissociation processes of bipolarons, at least for moderate degrees of reduction

Influence of chemical reduction on the particular number densities of light-induced small electron and hole polarons in nominally pure Li Nb O 3

Electron paramagnetic resonance (EPR), electron‐nuclear double resonance, optical absorption, and thermoluminescence have been used to investigate radiation‐induced point defects in a single crystal of lithium triborate (LiB3O5) Two prominent defects are observed after irradiation near liquid‐nitrogen temperature with 60 kV x rays A four‐line EPR spectrum, with 122 G splittings, is assigned to a trapped‐hole center, and another four‐line EPR spectrum, with 120 G splittings, is assigned to a trapped‐electron center In each case, the nucleus responsible for the observed hyperfine is 11B The trapped hole is localized on an oxygen ion and has a weak hyperfine interaction with one neighboring boron nucleus, whereas the trapped electron is localized primarily on a boron ion with a correspondingly larger hyperfine interaction Both defects become thermally unstable near 125 K, and their decay (ie, recombination) correlates with an intense thermoluminescence peak at this same temperature An optical absorp

J. K. Tyminski

Papers

Point defects in lithium triborate (LiB3O5) crystals