The aim of the present paper is to study the birefringence properties of the Ba 2Na 3 2F and Ba 7(BO 3) 4 −yF 2+3 y crystals. Optical and magnetic properties of the Ba 7(BO 3) 4 −yF 2+3 y phase were described in, , however its birefringence and potential for nonlinear optical applications have remained unexamined. Owing to this phenomenon these crystals are promising sensor applicable to fixing X-ray images. The induced absorption can be removed by illumination with a wavelength of 300 − 400 nm. Crystals of Ba 7(BO 3) 4−yF 2+3y solid solutions display a phenomenon that has never been described for borates before – X-ray irradiation of Ba 7(BO 3) 4 −yF 2+3y crystals induces strong absorption in the visible range and, as the result, coloring of the crystals up to dark purple. With the use of solid-state synthesis and differential thermal analysis it has been found that both hypothetical end-members ‘Ba 7(BO 3) 3F 5’ and ‘Ba 7(BO 3) 4F 2’ do not exist: X-ray diffraction patterns of the sample with ‘Ba 7(BO 3) 3F 5’ stoichiometry contain – in addition to the above hexagonal phase – strong peaks of BaF 2 cubic phase, samples with ‘Ba 7(BO 3) 4F 2’ stoichiometry – strong peaks of Ba 5(BO 3) 3F phase ( Pnma, a = 7.60788 (12), b = 14.8299 (2), c = 10.28650 (16) Å).
The Ba 7(BO 3) 4 − yF 2+3y phase reveals new type of anionic isomorphic substitution according to the scheme 4– ↔ 4–. Our study of the birefringence and absorption coefficients of Ba 2Na 3 2F compound in terahertz range (0.2–2 THz) has shown that it has better combination of properties for polarizing applications than high-temperature modification of barium borate: α-BaB 2O 4 Δ n/n = 0.22/2.73 = 0.08, Ba 2Na 3 2F Δ n/n = 0.45/2.85 = 0.16. Bulk crystals of Ba 2Na 3 2F were grown by top-seeded solution growth from the composition 50 mol.% BaB 2O 4, 50 mol.% NaF. The Ba 2Na 3 2F compound was initially found in the BaB 2O 4 – NaF system and is formed due to the chemical reaction between BaB 2O 4 and NaF: 3BaB 2O 4 + 3NaF → Ba 2Na 3 2F + BaF 2. Our investigation of phase equilibria in the Na, Ba, B//O, F system led to the discovery of the new centrosymmetric Ba 2Na 3 2F (BNBF) ( P6 3/ m) and non-centrosymmetric Ba 7(BO 3) 4− yF 2+3 y ( P6 3) phases. The latters are of increasing interest owing to their technological application and materials design challenges. 1) combines main solvents for the growth of nonlinear optical low-temperature modification of barium borate β-BaB 2O 4: Na 2O, NaF, ,, NaBaBO 3, BaF 2, and is perspective for fluoride borate crystal growth, ,. Quaternary reciprocal system Na, Ba, B//O, F (Fig.
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