Growth and spectrum of a novel birefringent α-BaB2O4 crystal
Introduction
Barium metaborate (BaB2O4) is known to crystallize in two modifications, the high-temperature α-phase and low-temperature β-phase. The α–β phase transformation temperature is 925°C under a pressure of 1 atm [1]. So far, the low-temperature phase BaB2O4 (β-BaB2O4) crystal has been well-known as an excellent nonlinear optical crystal [2]. However, the high-temperature phase BaB2O4 (α-BaB2O4) crystal is still a completely novel birefringent optical crystal. Currently, the birefringent crystal widely used in the world is the natural calcite crystal. But it is seriously confined to optical homogeneity, small size and difficulty to process precisely due to its cleavability. Therefore, for a long time, many efforts have been made to seek for a new birefringent crystal having high quality, large size, good chemical stability and mechanical work ability. The high-temperature phase α-BaB2O4 crystal is a highly birefringent crystal characterized by a wide transmission range from 189 to 3500 nm. As a negative uniaxial crystal, the refractive indices of this crystal are about no=1.655 and ne=1.539 at 1064 nm. So it makes α-BaB2O4 crystal to be a potential substitute for the natural CaCO3 crystal and YVO4 crystal especially in the ultraviolet wave band.
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Structure of α-BaB2O4 crystal
Both α-BaB2O4 and β-BaB2O4 crystals have a trigonal structure, but the point group and space lattice parameters of β-BaB2O4 crystal are 3R, and a=b=1.2532 nm, c=1.2712 nm, α=β=90°, γ=120°, respectively [3]. In contrast to the β-phase, the point group and space lattice parameters of α-BaB2O4 crystal are Rc, and a=b=0.7235 nm, c=3.9192 nm, α=β=90°, γ=120°, respectively. Therefore, α-BaB2O4 crystal is centrosymmetric and is not a nonlinear optical material.
Preparation of starting material
From the phase equilibrium diagram of BaO–B2O3 system [4], the high-temperature phase of BaB2O4 occurs as the primary phase in mixtures containing approximately 67.5% and 31.5% B2O3 (wt%), which melts congruently at 1095±5°C and 1 atm.
Unfortunately, under this strict stoichiometric composition, an integrated α-BaB2O4 crystal having a large size and high optical quality could not have been obtained using the Czochralski method because of the strong volatility of the B2O3 component. Three
Acknowledgements
This work was supported initially by Casix Co. Ltd. The authors are indebted to Mrs. Qiao Jingwen and Mr. Wang Haobin for helpful comments on the orientation and quality examination of α-BaB2O4 crystal.
References (4)
- K.H. Hubner, Neues Jahrb. Mineral. Monatsh. (1969)...
- Chuangtian Chen, Yicheng Wu, Rukang Li, J. Crystal growth 99 (1990)...
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