SYMMETRY OF THE MODEL OF A CRYSTAL WITH A PERIODIC DEFECT - POINT-DEFECTS IN MGO CRYSTAL

A model of a periodic defect (supercell model) is often used for study ing the electronic and vibrational structure of point defects in cryst als. The symmetry of this model is determined by the site symmetry of the point defect embedded in a crystal and the symmetry of the host cr ystal. The procedure of the determination of all the possible space gr oups describing the symmetry of the model of a periodic defect is elab orated in the present paper. This procedure is applied to point defect s in MgO crystal. There are generated the space groups of the supercel l model of an interstitial oxygen atom in volume-, face- and edge-cent ered positions and of molecular oxygen ion O-2(2-) in anion site orien ted along the [111], [110] and [001] directions. For the case of [110] orientation the results of nonempirical Hartree-Fock LCAO calculation s are given for different supercell choices.