ESR investigation of the[Al]0center in tetragonal GeO2

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Abstract
γ-ray, x-ray, or uv irradiation of an Al-doped tetragonal GeO2 single crystal at 77 K generates holelike centers in four inequivalent sites. The ESR spectrum for one of the sites consists of a six-line hyperfine pattern corresponding to the interaction of a single unpaired spin with an Al27 (100% abundant, I=52) nucleus. Symmetry considerations and the spin-Hamiltonian parameters show that the hole is localized in the 2pz orbital of one of four (out of six) nearest-neighbor oxygen anions. Owing to the low point symmetry (Cs) at the trapped-hole lattice site, the g, A, and Q tensors have only one common principal axis, which lies along a [110] direction (the z→ axis). Spin-Hamiltonian parameters are: gz′=2.0040±0.0002, gy′=2.0143±0.0002, gx′=2.0202±0.0002, Az′′=4.51±0.05, Ay′′=3.57±0.05, and Ax′′=4.50±0.05, where the A components are in units of 10−4 cm−1. The axes y→′ and y→′′ make angles of 11.6° ± 0.5° and 36.6° ± 1.0° relative to the [001] direction, respectively, and z→′=z→′′=z→. A point-ion crystal-field calculation provides support for the defect model. Characteristics of the optical absorption spectra for the center are predicted assuming transitions between