Electron paramagnetic resonance ͑EPR͒ and electron-nuclear double resonance ͑ENDOR͒ are used to identify and characterize electrons trapped by oxygen vacancies and holes trapped by lithium vacancies in lithium tetraborate ͑Li 2 B 4 O 7 ͒ crystals. Our study includes a crystal with the natural abundances of 10 B and 11 B and a crystal highly enriched with 10 B. 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 10 B and 11 B 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 is localized primarily on only one of the two neighboring boron ions while the hole stabilized by a lithium vacancy is localized on a neighboring oxygen ion with nearly equal interactions with the two boron ions adjacent to the oxygen ion.

We have compared the photovoltaic charging of the (1 0 0) surface termination for Cu doped and undoped Li 2 B 4 O 7. While the surface charging at the (1 0 0) surface of Li 2 B 4 O 7 is significantly greater than observed at (1 1 0) surface, the Cu doping plays a role in reducing the surface photovoltage effects. With Cu doping of Li 2 B 4 O 7 , the surface photovoltaic charging is much diminished at the (1 0 0) surface. The density of states observed with combined photoemission and inverse photoemission remains similar to that observed for the undoped material, except in the vicinity of the conduction band edge.