Oxygen vacancies in LiAlO2 crystals

Singly ionized oxygen vacancies are produced in LiAlO2 crystals by direct displacement events during a neutron irradiation. These vacancies, with one trapped electron, are referred to as VO+ centers. They are identified and characterized using electron paramagnetic resonance (EPR) and optical absorption. The EPR spectrum from the VO+ centers is best monitored near 100 K with low microwave power. When the magnetic field is along the [001] direction, this spectrum has a g value of 2.0030 and well-resolved hyperfine interactions of 310 and 240 MHz with the two Al27 nuclei that are adjacent to the oxygen vacancy. A second EPR spectrum, also showing hyperfine interactions with two Al27 nuclei, is attributed to a metastable state of the VO+ center. An optical absorption band peaking near 238 nm is assigned to VO+ centers. Bleaching light from a Hg lamp converts a portion of the VO+ centers to VO0 centers (these latter centers are oxygen vacancies with two trapped electrons). The VO0 centers have an absorption band peaking near 272 nm, a photoluminescence band peaking near 416 nm, and a photoluminescence excitation band peaking near 277 nm. Besides the oxygen-vacancy EPR spectra, a holelike spectrum with a resolved, but smaller, hyperfine interaction with one Al27 nucleus is present in LiAlO2 after the neutron irradiation. This spectrum is tentatively assigned to doubly ionized aluminum vacancies.