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

There was investigated the dependence of spectral transmittance of crystals of thallium halides KRS-5 (КРС-5) (TlBr – TlI) and KRS-6 (КРС-6) (TlCl – TlBr) on growing atmosphere. The aim of the experiments is the choice of the conditions, allowing to reduce the thallium halide decomposition to minimum on the final stage of optical materials obtaining for infrared technique devices. Decomposition products are impurities, activating the thallium halide dissociation process. They are removed and recovered again during the each process of salt purification and crystal growing. Such impurities in KRS-5 melt appear during the interaction of thallium iodide with oxygen, where Tl₂O and Tl₂O₃ are formed together with iodine molecules. At the same time, these impurities appear in KRS-6 melt during the dissociation of thallium chloride molecules with thallium formation and chlorine release into the atmosphere. The formed impurities are introduced into the growing crystals, reduce their transparency, and cause the halide decomposition process. The atmosphere, including the crystallization process, can make an influence on decomposition during the growing process. Experiments on the crystals KRS-5 and KRS-6 growing (with 30 mm diameter and 120 mm length) were carried out in different atmospheres: air, vacuum and argon. The carried out research made a conclusion that the crystals KRS-5 must be grown in vacuum, while the crystals KRS-6 must be grown in air.
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 the 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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