EPR and optical absorption studies of radiation-produced defects in sodium beta -alumina

The EPR and optical absorption spectra of X-ray irradiated single crystal of Na beta -alumina have been examined. The most prominent absorptions are tentatively ascribed to an F⁺ centre at the O(5) oxygen ion site in the conduction plane. To this defect the authors assign an axially symmetric EPR spectrum which shows a resolved hyperfine structure due to interaction of the single unpaired spin with two equivalent ²⁷Al(I=⁵/ ₂) nuclei. The measured spin-Hamiltonian parameters are g/sub ///=2.0076(2), g_{perpendicular to}=2.0032(2), A/sub ///=1.665(2) mT, A_{perpendicular to}=1.617(2) mT, the defect symmetry axis coinciding with the crystal c axis. From the EPR linewidth it is inferred that the hyperfine interaction with three equivalent ²³Na nuclei is of order 0.2 mT. This defect is also optically active having a strongly polarised s to p_z transition with mean energy E_F=4.15(5) eV. The corresponding s to p_x,y optical transitions are not observed in the range of photon energies from 4.2-6.8 eV.