Spin Resonance of Point Defects in Magnesium Oxide

Abstract

Transition metal impurity ions and intrinsic defects such as trapped electron and trapped hole centers are readily detected in magnesium oxide crystals by electron spin resonance. The valence state of impurity ions may readily be changed by heat treatment or by irradiation. For nickel and cobalt one may observe both the 2+ and 1+ states, while for iron, one additionally observes the 3+ state. The extra positive charge of trivalent ions is usually compensated by positive ion vacancies. Vacancies, either single or in clusters, alter the electric field symmetry about an impurity ion or intrinsic defect. Hence they may indicate their presence by altering the symmetry from cubic to tetragonal, rhombic or lower type. The impurities are generally substitutional, but one interstitial type has been found. Isolated positive ion vacancies can trap either one or two holes, in each case producing centers of axial electric field symmetry. Other hole centers are observed after extensive atomic displacements have been produced by neutron irradiation. Negative ion vacancies are difficult to produce except by such irradiation. Electrons trapped at negative ion vacancies are largely localized upon the vacancy, as is evident from the small hyperfine splitting of adjacent 25Mg ions. If crystals are heated in order to lead to association of vacancies, a second type of F center is formed. With protracted neutron irradiation, yet a third type of F center is produced. Optical studies are being undertaken to learn more about these paramagnetic centers.