Research of the Central Core in Bi₁₂SiO₂₀Crystals

Abstract

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 μm 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.

X-ray diffraction topography observations of the core in Bi12SiO20 crystals doped with Mn

“…Some authors [5] explain the central core as a consequence of the so-called "facet growth" when facets are formed during the crystal growth process that preferentially absorb photochromic impurities, thus forming the so-called "growth bands". Such problems usually arise due to changes in parameters that are important for the growth process like distortion of the thermal field, and hence fluctuations in the melt composition, undercooling, metastable phases, or changes in the impurity distribution coefficients [6][7][8][9][10][11]. Our Raman spectroscopy studies on undoped BSO crystals showed no increased impurity concentration and the differences in the measured spatially resolved Raman spectra can be explained with only slight local misorienations in the crystal lattice [12].…”