ESR of paramagnetic defects in the conduction planes of Na+, K+, and H3O+ β-alumina

Paramagnetic defects created in Na+, K+, and H3O+ β-alumina single crystals by X-ray irradiation are studied by ESR. Two centers both located in the conduction planes are detected. The first one, which appears only in Na+ and K+ β-alumina, is attributed to a F+ center in a O(5) position interacting with two aluminium ions; but an alternative assignment as a AlO−Al center cannot be excluded. A comparison of Al superhyperfine linewidths between Na+ and K+ β-alumina indicates that the unpaired electron interacts also with the first neighbours conducting ions. The temperature dependence of the linewidths, which reflects the motion of Na+ ions, leads to an activation energy of 0.17 eV, a value close to those deduced from dc conductivity measurements. The second defect can be found in the three kinds of β-alumina. It is attributed to a O superoxyde ion in a non axial symmetry, the OO axis lying along the a* direction of the crystal. An analysis of the principal values of the g-tensor shows that the π antibonding level splitting of this ion, which is due to the non axial component of the crystal field, is very sensitive to the nature of the conducting ions.