Electron paramagnetic resonance and electron-nuclear double resonance study of trapped-hole centers in LiB₃O₅ crystals

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) have been used to characterize two distinct hole centers in single crystals of LiB₃O₅ (commonly referred to as LBO). Irradiating a crystal at 77 K with x rays produces an intense four-line holelike EPR signal, with the structure arising from the hyperfine interaction with one ¹¹B nucleus. Warming the crystal to approximately 130 K destroys the first hole center and allows a second less intense four-line holelike EPR signal to be observed (also interacting with one ¹¹B nucleus). The second hole center decays between 150 and 200 K. EPR and ENDOR angular dependence data were used to determine the g matrix and the ¹¹B hyperfine and nuclear quadrupole matrices for each hole center. We suggest that the first (less thermally stable) center is a self-trapped hole. In this defect, the hole is localized primarily on an oxygen ion between a threefold bonded boron and a fourfold bonded boron, and the self-trapping occurs because of a significant relaxation of the neighboring fourfold boron away from the hole. GAUSSIAN 98 calculations, using a (B₃O₇H₄)⁰ cluster to represent the defect and the nearby lattice, support this self-trapping mechanism. A similar model is suggested for the second hole center, except in this case a neighboring lithium vacancy is included to provide the increased thermal stability. These trapped-hole centers are of interest because of their possible role in the unwanted transient optical absorption produced in LiB₃O₅ crystals at room temperature by high-power pulsed ultraviolet lasers.