JSC “Giredmet”, Moscow, Russia:

I. S. Lisitskiy, Head of Laboratory of High-Purity Halide Materials for Optics
G. V. Polyakova, Senior Researcher, e-mail: gradan@mail.ru
V. F. Golovanov, Senior Researcher
M. S. Kuznetsov, Senior Researcher

Dependence of structural perfection of thallium halide crystals on their growing conditions was investigated. The crystals were grown using Bridgman-Stockbarger technique. Stable temperatures on the heaters of top and bottom furnace parts during the whole process change the work space temperatures because of the heat exchange alteration between the parts during the diaphragm covering by lowering ampoule and heat removal throw the growing crystal, moving into the lowering temperature area. At the same time, position and shape of the crystallization interface are changed, which leads to the change of growing crystal structure. The blocks, which disordered orientation may reach several degrees, are formed, which leads to reduction of mechanical strength and degradation of optical characteristics of obtained material. X-ray investigations of the structure of crystals, grown in various temperature modes, have shown that mono-block homogeneously polygonized structure may be obtained only during the stabilization of the position of relief crystallization interface. X-ray investigations defined the presence of two parameters – two grids with various cell parameters in the crystals of solid TlBr – TlI solutions. These two parameters are shown independently on the growing conditions and are saved in the crystals during the heating to the temperature of 180 ^oC, which allows to suppose the connection of this phenomena with modifications of lattice cell of TlI, crystallized from the melt with cubical grid. At the same time, during the cooling to the temperature below 180 ^оС, TlI has the rhombic modification. TlBr should be a stabilizer of the cubical cell of TlI. However TlI may form the rhombic phase “islands”, fixed as pseudo-cubic.
This work was carried out with the financial support of the Ministry of Education and Science of Russian Federation; the Federal Target Program “Investigations and developments of the priority ways of development of scientific-technical complex of Russia for 2014–2020”, within the project “Development of technology of obtaining of new optical materials for the infrared equipment”; Agreement No. 15.576.21.0054. Unique identifier of the applied scientific investigations (project) is RFMEFI57614X0054.

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