Research of the central core in Bi12SiO20 crystals

The structure of the so-called ''central core'' within Czochralski grown Bi12SiO20 Crystals is investigated by the laser projection. A finer structure with a number of optical inhomogeneities with irregular forms and sizes about 2-3 mu m is observed within the core. The conditions for facet growth and absorption of impurities on the facets are described. Positron lifetime measurements are carried out within and outside the core. The results show a higher concentration of defects in regions within the core. The optical rotatory power in regions within and outside of the core was measured, too. An evaluation of the distribution of the different structural units and the dopants is made. Some suppositions about the kind and the role of the defective center are given.