Electron spin resonance and optical studies of the double-hole (V 0 ) centre in MgO

Isolated cation vacancies are created by neutron irradiation of MgO crystals maintained at approximately 32 degrees C; these vacancies trap one positive hole to form stable V - centres (Mg O + (=)O Mg). Subsequent X-irradiation at 77K causes some V - centres to trap an additional hole to form V 0 centres (Mg O + (=)O + Mg). In these representations, only derivations from normal site charge are shown. Independently, some unstable V M centres (Mg O + (=)O M + ) are formed. Here M + represents a substitutional trivalent ion. ESR spectra of V - and V M centres are indistinguishable at 77K; however, at room temperature, the V - centres give an isotropic line which permits the unambiguous determination of their number. Decay of V 0 and V M centres at room temperature regenerates the number of V - centres originally present before X-irradiation. Analysis of the optical absorption of V 0 centres shows that at 77K their absorption band in MgO is centred at 2.37 eV and has a half-width of 0.95 eV. Measurements of the oscillator strength of the V 0 centre give f=0.24, approximately 1.7 times that of the V - centre.