MECHANICAL RELAXATION OF TWO TYPES OF DEFECT IN MAGNESIUM OXIDE CRYSTALS.

Two point defects in magnesium oxide single crystals which give rise to internal friction peaks at 200C and at 850C are studied. The first, which has a reorientation activation energy 0.64 eV, is now shown to have 100 symmetry. The annealing behavior has been studied in some detail, and this, together with the high pre-exponential factor in the relaxation time, is consistent with the defect being a trapped charge carrier. A theory is being developed in which the relaxation rate of the Jahn-Teller distortion due to an electron held in a Coulomb potential is calculated. Measurements have been made on the 850C internal friction peak, which is believed due to vacancy: impurity chromium pairs, for specimens in 100 and 111, of commercial purity, and also specially chromium doped. The peak is composed of several relaxational modes, and the temperature of these does not shift very much with orientation. The behavior deviates strongly from that predicted by a model of closely bound vacancies, and an alternative model is being developed in which the vacancies are more remotely bound so that the relaxation shows a weaker orientation dependence. (Author)