ISSN 1063-7745, Crystallography Reports, 2011, Vol. 56, No. 2, pp. 339–344. © Pleiades Publishing, Inc., 2011. Original Russian Text © V.N. Shlegel, D.S. Pantsurkin, 2011, published in Kristallografiya, 2011, Vol. 56, No. 2, pp. 367–372. 339 INTRODUCTION Sillenite crystals have a number of properties important for practical applications, such as piezo- electric sensors, filters and delay lines of electromag- netic signals, electro-optical and magneto-optical field strength meters, space–time modulators, etc. It is known [1] that these devices require crystals with a low dislocation density and high optical homogeneity. Crystals of congruently melting sillenites (Bi 12 SiO 20 (BSO), Bi 12 GeO 20 (BGO), etc.) are gener- ally grown by the conventional Czochralski method [1]. In this case, one of the main problems is their opti- cal homogeneity [2–11]. The two most characteristic types of “optical” defects are selected: (i) inclusions of foreign phases and crucible material and (ii) the pres- ence of regions with enhanced optical density in the crystal bulk, which can manifest themselves as stria- tions and the so-called “growth column” [1]. Accord- ing to [7–9, 12], the formation of the growth column and the effect of selective decoration in the form of a three-bladed propeller during growth in the 〈111〉 direction are related to the growth rate anisotropy and the difference in the distribution coefficients of “pho- tochromic” impurities for polar {110} and nonpolar {100} faces present at the crystallization front. Regions with enhanced optical density may also arise due to the coexistence of two (normal and layer-by-layer) growth mechanisms at the crystallization front [13]. Steiner et al. [12] suggested that optical inhomogeneity may arise during layer-by-layer growth due to deviations in the crystal composition from stoichiometric within the homogeneity range and variations in the impurity con- centration at temperature fluctuations. According to [14], when the above-mentioned factors are mini- mized, the optical density of sillenite crystals may change because of the implementation of equilibria in these systems, leading to the formation of metastable phases, which are fairly stable in melts of oxide bis- muth-containing systems. It was shown that single crystals of stable compounds containing inclusions of metastable phases or products of their decomposition can be obtained from superheated melts at crystalliza- tion rates somewhat exceeding the optimal value. The purpose of this study was to search for the con- ditions providing reproducible layer-by-layer growth during the entire cycle of fabricating BSO crystals with a stable polyhedral front, compare the systematic fea- tures of BGO and BSO shaping during growth at a low-temperature gradient, compare the defect struc- ture in the sillenites crystals obtained by the low-ther- mal gradient Czochralski (LTG Cz) technique [15], and grow BGO and BSO crystals with a high optical homogeneity. EXPERIMENTAL BSO and BGO crystals have similar thermophysi- cal properties and structures (Table 1). The phase dia- grams of the Bi 2 O 3 –SiO 2 and Bi 2 O 3 –GeO 2 systems in the range of 6 : 1 are also similar [1]. In the initial stage of growing BSO crystals, we used our experience in the growth of BGO crystals [16]. A platinum crucible 70 mm in diameter and 150 mm high mounted in a three-zone furnace with indepen- dent temperature control circuits was used. The tem- perature gradients in this system do not exceed 1°C/cm. The charge was a stoichiometric mixture of Bi 2 O 3 (99.9999 wt %) and SiO 2 (99.9999 wt %). The Bi 2 O 3 – SiO 2 system is characterized by the formation of meta- stable phases [14, 17] and certain kinetic difficulties of SiO 2 incorporation in the melt. In view of this, we used a special procedure for preparing the melt. At a tem- perature of 800°С and higher, the heating rate did not Growth of Bi 12 GeO 20 and Bi 12 SiO 20 Crystals by the Low-Thermal Gradient Czochralski Technique V. N. Shlegel and D. S. Pantsurkin Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia e-mail: danil@niic.nsc.ru Received April 13, 2010 Abstract—Bi 12 SiO 20 crystals have been grown for the first time by the low-thermal gradient Czochralski technique in the 〈111〉 and 〈110〉 directions. The conditions for reproducible crystal growth with a high-qual- ity polyhedral faceted front are found. The systematic features of shaping Bi 12 SiO 20 and Bi 12 GeO 20 crystals, grown by the low-thermal gradient Czochralski technique, are compared. The defect formation in these crys- tals is studied and their optical homogeneity is analyzed by interferometry. DOI: 10.1134/S1063774511010226 CRYSTAL GROWTH