Borate compounds with the noncentrosymmetric crystal structure are attractive materials for nonlinear optical (NLO) applications, especially in the UV region [1]. Orthorhombic α-SrB₄O₇ (α-SBO) crystals have been established to possess high NLO coefficients and a very short absorption edge but they have not sufficient birefringence for the existence of the second harmonic generation (SHG) phase matching conditions [2, 3, 4]. Recently, strontium tetraborate crystals have been obtained with another structure modification and their structure has been established. The structure of this β-SrB₄O₇ (β-SBO) with the trigonal symmetry belongs to the noncentrosymmetric P3 space group, the unit cell parameters being: a = 17.145 (1) Å, c = 4.2527 (5) Å [5]. The dispersion dependence of the refraction indexes β-SBO has been investigated and SHG phase matching conditions have been calculated [6]. According to [6] β-SBO crystals can be used in Type I SHG process down to 672 nm of the fundamental wavelength. Though the NLO coefficients of β-SBO haven't been measured yet, some attempts to extend investigations on the solid solution β-SBO:Pb have already been made [7].

Small single crystals of the solid solutions β-SBO:Ba and β-SBO:Pb have been obtained and main optical birefringence of these crystals has been measured.

β-SBO itself is a metastable crystal modification of this compound [8], so β-SBO crystals cannot be obtained with a temperature higher than ~ 780°C (the stable α-SBO melting temperature is 1000°C). Solid solution crystals β-SBO:Ba have been grown from a supercooled melt of the compositions (1-x)SrB₄O₇ × xBaB₄O₇ with x = 0.05; 0.1; 0.3 at about 700 – 720°C. β-SBO:Pb has been grown at 680°C from the melt with 15 mol % of PbB₄O₇.

Single crystal X-ray diffraction data on the obtained solid solution crystals were collected using diffractometer SMART APEX II (Bruker) with CCD detector.

The birefringence of the crystals has been measured by the compensation method with a Karl ZEISS AxioScop 40 polarized light microscope and a Berek compensator. The birefringence temperature dependence has been measured with a Lincam LTS 350 temperature controlled stage in the range from – 150 to +150°C.

The results are presented in Table 1.

One can see the solid solution birefringence increase with the increase of barium concentration. The effect of lead in the solid solution on the β-SBO birefringence is much higher, so the β-SBO:Pb birefringence is slightly higher than that for β-SBO:Ba with the dopant concentration being twice higher.

The birefringence temperature dependence has a sufficiently weak linear character for all the crystals with the temperature coefficient in range 1.5-3×10^-6 grad^-1.

Barium substitutes strontium in the β-SBO structure only in three from five nonequivalent positions, though lead is distributed between all the positions.

Table 1.

Acknowledgements:

This study is supported by SB RAS Project 103, by Grant of the President of the Russian Federation for the support of leading scientific schools SS-4828.2012.2 and by RFBR Grant 11-02-00596-а.

References

1. Chuangtian Chen, Takatomo Sasaki et al. Nonlinear Optical Borate Crystals // WILEY-VCH, 2012, 388 P.

2. Oseledchik Yu.S., Prosvirnin A.L., Pisarevskiy A.I., et al. New nonlinear optic crystals: strontium and lead tetraborates // Optical Materials. – 1995. – Vol. 4. – pp. 669-674.

3. Zaitsev A.I., Aleksandrovskii A.S., Zamkov A.V., Sysoev A.M. Nonlinear optical, piezoelectric, and acoustic properties of SrB₄O₇ // Inorganic Materials. – 2006. – V. 42, No. 12. – pp. 1360-1362.

4. Petrov V., Noack F., Shen D., Pan F., Shen G., Wang X., Komatsu R., Alex V. Application of the nonlinear crystal SrB₄O₇ for ultrafast diagnostics converting to wavelengths as short as 125 nm // Optical Letters. – 2003. –V. 29, No. 4. – pp. 373-375.

5. Vasiliev A.D., Cherepakhin A.V., Zaitsev A.I. Trigonal crystal phase of strontium tetraborate, β-SrB₄O₇ // Acta Cryst. E. – 2010. – V. 66. – pp. 48-65.

6. Cherepakhin A.V. Obtainig, optical and nonllinear-optical properties of the α-, β- SrB₄O₇ and δ- BiB₃O₆. //  DSc. Thesis, L.V. Kirensky Institute of Physics. -  Krasnoyarsk. - 2011. -   URL: http://www.kirensky.ru/zdoc/chav.pdf (in Russian).

7.  Oseledchik Yu.S., Prosvirnin A.L., Lutsenko V.Yu. Nonlinear-optical phases in strontium tetraborate-lead tetraborate system // Functional materials. - V.19, No.1. -  2012. - pp 116-120.

8. Zaitsev A.I.,  Zamkov A.V.,  Koroleva N.S., Molokeev  M.S., Cherepakhin A.V. Phase formation upon crystallization of SrO·2B₂O₃ glasses // Crystallography Reports. - 2011. - V. 56, No. 1. -  pp. 44–51.

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/29740 must be provided.

1. Morphology of a polar twin structure in Czochralski grown α-SrB₄O₇ crystals
2. Growth, optical and microstructural properties of PbB₄O₇ plate crystals