Growth and spectrum of a novel birefringent α-BaB2O4 crystal

doi:10.1016/S0022-0248(98)00162-6

Journal of Crystal Growth

Volume 191, Issue 3, 15 July 1998, Pages 517-519

https://doi.org/10.1016/S0022-0248(98)00162-6 Get rights and content

Abstract

A novel birefringent α-BaB₂O₄ crystal, 40 mm in diameter, 35 mm in height has been grown successfully in our laboratory by Czochralski method. The proportion of B₂O₃ and BaO of starting material, crystal cracking and the transmission spectrum of α-BaB₂O₄ crystal have been briefly analyzed.

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 n_o=1.655 and n_e=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.

Section snippets

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 R $3 ̄$ c, 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)...
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