State Scientific-Research and Project Institute of Rare-Metal Industry “Giredmet”, Moscow, Russia:

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

This paper investigates the dependence of spectral transmittance of thallium halide crystals KRS-5 (КРС-5) (TlBr – TlI) on growing atmosphere. The purpose of experiments is the choice of conditions, minimizing the decomposition products formation on the final stage of crystal obtaining for laser engineering tools. Cation and anion impurities are intensively removed on the stages of salt purification before growing. Decomposition products are removed and reproduced on each stage and activate the thallium halide dissociation process. Such impurities appear in the melt KRS-5 with interaction of TlI with oxygen and water vapors. The formed thallium oxides are introduced into the growing crystal, decreasing its glassiness and causing the decomposition process, which leads to the failure of optical elements of laser system. According to the results of the earlier investigations, decomposition processes may be under the effect of the crystal-growing atmosphere. Growing in vacuum allows to obtain the crystals KRS-5 with high spectral passing. This paper shows the choice of the atmosphere, providing the obtaining of low light scattering coefficients (which do not change in time), in the visible part of spectrum and laser irradiation absorption in the range of 10 μm. It is the halogenating atmosphere, formed by iodine vapors.
This work was carried out with the financial support of the Ministry of Education and Science of Russian Federation. Federal Target Program “Investigations and developments on the priority ways of development of the scientific-technical complex of Russia for 2014–2020” within the project “Development of the technology of obtaining of new optical materials for tools and apparatuses of laser and/or infrared technics”. Agreement No. 14.576.21.0054. Unique identifier of applied scientific investigations (project) RFMEFI57614X0054.

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