Localised Electronic States of Defect Aggregates in Neutron Irradiated Magnesium Oxide

The alkaline earth oxides have the same crystal structure as the alkali halides and consequently defects with electronic states and symmetry properties analogous with the M-and R-centers are observed after neutron irradiation. The divalent nature of the host lattice also offers additional potential for electron trapping : in this respect the R-like centres have been especially interesting. The application of uniaxial stress techniques has confirmed that optical zero-phonon lines at 5 248 A, 6 419 A and 6 490 A are associated with such centres. The line at 5 248 A is strongly dichroic under stress at 4 OK and the results are indicative of E + A tansitions. The lack of such dichroic behaviour in the 6 419 A and 6 490 A lines confirm that these lines are associated wiht A -t E transitions. Calculation of the electronic states of theses trigonal defects suggest that the 5 248 A, 6 419 A and 6 490 A lines are due to clusters of three anion vacancies lying in a (111) plane and containing 5, 6 and 7 electrons respectively. In addition to defects which are readily recognised on account of their Mand R-like characteristics, there are defects that are not analogous with any defects so far identified in the alkali halides. Two such defects have spin triplet states (S = 1) at 300 O K but have spin singlet states at 4 OK. Models for these defects involve exchange coupled pairs of F-centres in next nearest neighbour association along the [l001 axes (tetragonal symmetry) and [l101 axes (orthorhombic symmetry). A third defect, designated the F2-centre by Wertz, has optical properties in accord with those expected for a single electron bound to an anion-cation vacancy pair. More recent paramagnetic resonance measurements at Q-band strongly support the proposed model for this defect.