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Growth and characterization of Li₂B₄O₇ single crystals pure and doped with Co ions

Danuta Piwowarska ¹, Sławomir Kaczmarek ¹, Marek Berkowski ², Ireneusz Stefaniuk ³

1. Szczecin University of Technology, Institute of Physics, Al. Piastów 48/49, Szczecin 70-311, Poland
2. Polish Academy of Sciences, Institute of Physics, al. Lotników 32/46, Warszawa 02-668, Poland
3. University of Rzeszow, Rejtana 16 A, Rzeszów 35-310, Poland

Abstract

The paper reports on the growth conditions of Li₂B₄O₇ single crystals in both pure and Co doped ions. The crystals were grown by the Czochralski method at a growth rate of 0.6 mm/h and rotation rate 5.0 rpm in the Institute of Physics, Szczecin University of Technology, Poland. Technological factors affecting the quality of the crystals, both pure and doped, have been discussed. As raw materials boric oxide (B₂O₃), lithium bicarbonate (Li₂CO₃) and dopant ions (Co) in the form of oxides of purity 4N and 5N were used. The growth processes were carried out without protective atmosphere (in the presence of oxygen), with the use of platinum crucible, 50 mm in diameter and height. The optimum initial composition of melt has been defined, which is dependent on the speed of boric oxide evaporation reaching 90 mg/h. A schematic phase diagram of this solution is proposed. Starting concentrations of Co in the melt were: 0.5, 0.85 and 1 mol. % with respect to Li₂CO₃. Absorption and EPR spectra were analyzed with the aim of define of oxidation state and site of cobalt ions. Optical measurements show that Co-doped Li₂B₄O₇ exhibit distinct absorption bands near 500, 1100 and 1500 nm associated with electronic transitions characteristic of Co²⁺ ions at octahedral Li⁺ site positions. Low-temperature EPR measurements show that in the Li₂B₄O₇:Co crystal two types of Co²⁺ complexes can be distinguished. Obtained single crystals Li₂B₄O₇ has been used in the processing system on the second harmonic generation of laser YAG:Nd.

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