It was pointed out that the identification of a large number of paramagnetic p-type impurity centers by means of electron spin resonance, following X-irradiation, -irradiation or electrolytic/additive coloring of crystals which contained cations from group-IIIA or group-IVA, was due mainly to the high sensitivity of the resultant spectra to the intensity and symmetry of the local crystal field. This property made the p-type centers valuable microscopic probes for studying variations in the local crystal field that were induced by external factors or which were associated with structural phase transitions. The tendency to sp-hybridization and the formation of sp-sp molecular bonds, which led to the formation of impurity molecular centers (Tl2+, Pb23+) and to the stabilization of electron self-trapped centers in PbCl2, were other important properties of these centers.

S.V.Nistor: Materials Science Forum, 1997, 239-241, 543-8