Kyropoulos growth and characterization of monoclinic α-Bi2B8O15 single crystal with a noncentrosymmetric structure

Abstract

Monoclinic bismuth-based borate crystal α-Bi₂B₈O₁₅ was grown by the Kyropoulos method. High-resolution X-ray diffraction analysis showed that the full width at half maximum of the Y plate was about 0.04° for the α-Bi₂B₈O₁₅ crystal. The ideal morphology of the α-Bi₂B₈O₁₅ crystal was analyzed based on the refined structure obtained by the Rietveld method and it was found that the α-Bi₂B₈O₁₅ crystal had a layered structure along the b axis. X-Ray photoelectron spectroscopy and transmission spectroscropy were performed together with analysis of the density, mechanical, and thermal properties and the electrical resistivity of the α-Bi₂B₈O₁₅ crystal. Furthermore, the opto-electric properties were evaluated from the perspective of structural distortion. The results supported that the α-Bi₂B₈O₁₅ crystal possessed good chemical and thermal stability. The specific heat (0.61 J g⁻¹ K⁻¹) was larger than that of the α-BiB₃O₆ crystal (0.48 J g⁻¹ K⁻¹) at room temperature. In addition, the α-Bi₂B₈O₁₅ crystal had a relatively large band gap (4.41 eV) and a wide transmission range (260 nm–4.43 μm). The infrared cut-off edge was 4.43 μm, larger than other borate crystals, such as β-BaB₂O₄, CsB₃O₅, and LiB₃O₅ crystals. Besides, a high electrical resistivity was observed for the α-Bi₂B₈O₁₅ crystal, reaching as high as 1.5 × 10¹⁰ Ω cm at 300 °C along the Y axis, nearly two orders of magnitude higher than that of the La₃Ga₅SiO₁₄ crystal (∼8 × 10⁸ Ω cm). The multifunctional α-Bi₂B₈O₁₅ crystal may have potential for opto-electric applications.