Single crystals of low temperature modification of barium borate β–BaB₂O₄ (BBO) are an important material for nonlinear optical application. Due to the phase transition at 925°C, β–BaB₂O₄ crystals are commonly grown from high temperature solution.

In present work phase equilibria in Li, Ba // BO₂, F ternary reciprocal system have been studied. LiF is considered as the promising solvent for β–BaB₂O₄ crystal growth due to the essential viscosity reduction. Also, lithium ions should not enter isomorphically into the structure of barium borate because of crystallochemical characteristics. In mentioned reciprocal system only boundary sections BaB₂O₄–LiBO₂ [1], BaB₂O₄–BaF₂ [2], BaF₂–LiF [3], LiBO₂–LiF [4] have been investigated yet.

We investigated phase equilibria in Li, Ba // BO₂, F ternary reciprocal system using the modified visual  polythermal analysis (VPA), differential thermal analysis (DTA), methods of spontaneous crystallization on a platinum loop,  solid–state synthesis and X–ray powder diffraction analysis.

It has been found that BaB₂O₄–LiF system is not quasi–binary. In the concentration interval of 32.5–40 mol % LiF and 40–60 mol % LiF the area of primary crystallization of β–BaB₂O₄ and Li₂Ba₄B₁₀O₂₀, respectively, were found.

The β–BaB₂O₄ crystal was grown by top–seeded solution growth (TSSG) technique in a precise furnace with a heat field of 3–fold axis symmetry L3 [5]. A high–temperature solution of 1 kg 65 mol % BaB₂O₄ – 35 mol. % LiF composition was melted in a platinum crucible (90 mm in diameter and 95 mm in height). A crystal was grown on a 5 × 5 mm² seed oriented along the optical axis. The cooling rate was 0.2–0.3 °C/day. Total temperature decrease was only 21.5 °C due to the very narrow crystallization area, but  the yield coefficient was as high as 7.44 g/(kg·°C). The crystal was 60 mm in diameter and 25 mm in height, its weight was 160 g.

There are the four invariant points in Li, Ba // BO₂, F ternary reciprocal system (except boundary sections). The BaB₂O₄–LiBaF₃, BaB₂O₄–(0.6 BaF₂–0.4 LiF), BaB₂O₄–(0.7 BaF₂–0.3 LiF) systems has been studied in detail and much broader crystallization area of BaB₂O₄ was found in Li, Ba // BO₂, F system. We suppose that the use of various compositions in this system is of great interest for the growth of large β–BaB₂O₄ crystals.

1. Shilie Pan, Jared P. Smit, Courtney H. Lanier, Michael R. Marvel, Laurence D. Marks, and Kenneth R. Poeppelmeier. Optical floating zone growth of β–BaB₂O₄ from a LiBa₂B₅O₁₀–based solvent// Crystal growth & design. 2007. V. 7. № 8. pp. 1561–1564.

2. Bekker T. B., Fedorov P. P., Kokh A. E. Phase formation in the BaB₂O₄–BaF₂ system // Crystallography Reports. 2012. V. 57. № 4. pp. 574–578.

3. Bychkova N.A., Bergman A.G. – ZHOKH, 1956, V.26, P.639 (in Russian).

4. Petit M.G., Jaeger M. – «C. r. Acad. Sci.», 1957, V. 244, № 13, P. 1734.

5. Kokh A.E., Kononova N.G., Mokruchnikov P.W., J. Crystal Growth 216 (2000) 369. 

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