Electron spin resonance studies of defects in ionic crystals

Abstract

Defects in single crystals of silver halides and calcium oxide have been investigated principally by electron spin resonance spectroscopy. Studies of the properties of iron dissolved in silver chloride and bromide have revealed that threshold temperatures exist below which Fe2+ cannot trap a hole during illumination. In the case of silver chloride, a trigonal Fe3+ spectrum is formed after irradiation at about 170°K. Harming the crystal to 200°K converts this spectrum to a cubic spectrum, which is interpreted as arising from the migration of an Ag+ ion away from the complex. In silver bromide no trigonal spectrum from Fe 3+ has been detected. Instead illumination at about 160°K produces the cubic Fe3+ spectrum, at characteristically lower temperatures than that for the equivalent complex in silver chloride. Attempts to observe the ions V2+ and Mn4+ in silver chloride, and Cu2+ in silver bromide have been unsuccessful.
The use of relatively unstrained crystals of calcium oxide has made possible resolution of the hyperfine structure of the isotopes 155 and 157 of gadolinium in cubic symmetry, and also the transferred hyperfine stucture of the (single electron) F+ centre from ligand Ca43 (0.13% abundant) and 017 (0.037% abundant). Investigation of electron and hole trapping processes during ultraviolet irradiation has revealed a new spectrum in chemically reduced crystals, which has been attributed to Ti+ ions on substitutional octahedral sites. Oxidizing annealing and quenching studies indicate that hole centres may be created by heat treatment alone, and a new centre containing two holes, not previously observed in calcium oxide is reported. The strain induced by quenching enables double quantum transitions in the Mn+ spectrum to be observed.