Sillenite crystal

BGO crystals with eulithine structure have a very small absorption coefficient in the wavelength band where the main part of the blackbody radiation is concentrated. In contrast to this, the absorption coefficient of sillenite crystals reaches 0.3-0.4cm in the wavelength range from 2 to 6 tm. One can expect that absorption and emission of radiation by a crystal appreciably influence the heat-transfer process. Two growth processes were simulated. The first was performed in the setup used for the growth of BGO eulithine crystals, while the second was carried out in the Laboratory of Crystal Growth of the Autonomic University of Madrid. Results of simulation are described in Refs. [38, 45], respectively. Here, we focus on the second process. 

Fig. 8.7 Temperature fields in the sillenite crystal in the case of specular reflection at the shoulder surface. (Left half) Temperature isolines for calculation domains A (left) and B (right). (Right half) Radial temperature distributions for several values of the z-coordinate. Solid symbols correspond to calculations performed for domain A and open symbols for domain B.

The temperature fields in the case of specular reflection correlate surprisingly well with the dark core observed in the center of the grown sillenite crystals. The peculiarities of the temperature field near the axis in Fig. 8.7 can be responsible for the appearance of corresponding elevated thermal stresses revealed in Ref [47]. In siUenite crystals grown by LTG technology [45] the dark core was absent and distortion of isotherms was appreciably less. 

Cover Diagram A fragment of the crystal structure of sillenite - a framework composed of comer-sharing Bi04 tetrahedra and flat BiOs pyramids. 

Finally, a class of compounds which is gaining attention because of their photorefractive properties is the sillenites, Bii2MO20 (M = Si, Ge, other metal ions). They can be prepared by a variety of methods but, for several derivatives, the most satisfactory route is a hydrothermal one. Thus, single crystals of these compounds may soon be prepared for commercial purposes [110]. 

Among the piezoelectric (and non-ferroelectric) crystals the cubic (synunetry 23) sillenite Bii2SiO20 and isomorphous Bii2GeO20 and tetragonal (symmetry Amm) bariiun germanium titanate Ba2Ge2Ti08 and isomorphous fresnoite Ba2Si2TiOg seems to be perspective crystal materials. 

Fig. 1.12 Three examples for crystal growth from solution Boron-sillenite 6(2462039 (PSG 23) [15] (top left), 6i2ZnB207 (PSG mm2) [unpublished], Bi2Ga409 (PSG mmm) [22] (bottom).

M. Burianek and M. Muehlberg, 1997, Crystal growth of boron sillenite Bi24B2039 , Cryst. Res. Technol. 32, 1023-1027. 

The method described is efficient if the optical thickness is not too large. Examples of growth simulations of bismuth germanate crystals with eulithine and sillenite structure are presented below. Properties of the materials considered are listed in Table 8.1. 

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