Hydrothermal Growth Of Colorless BSO Crystals

Photorefractive BSO (Bi₁₂SiO₂₀) crystal has occupied a unique place in the sillenites family owing to its excellent optical-electrical properties. Hydrothermal technique is used to grow BSO crystals so as to lower growth temperature and minimize large thermal gradient and stress gradient during growth. Compared with flux grown crystals, the removing of yellow coloration in hydrothermal crystals also suggests the reduction in defect amount, so hydrothermal crystals may be suitable for advanced applications. In this dissertation, hydrothermal growth of BSO crystals was reported. Growth process and characterization results were showed. Different preparation methods of source material for hydrothermal growth were also discussed.1. Hydrothermal growthA gold closed liner was used to prevent impurity incorporation from the autoclave inner surface and solution corrosion to the autoclave. An aqueous solution of 5mol/L sodium hydroxide with filling grade about 60-70% was used as mineralizer, and de-ionized water was filled into the external void between the autoclave and the gold liner for keeping pressure balance. The whole system was heated to growth temperature at about 380℃for crystallization, and a typical growth period was 20-30 days.2. Source material(1) Using solidified mass prepared in a platinum crucible as source material, by two-step hydrothermal growth, colorless BSO crystals with developed (001) and (110) facets were obtained. The deep brown coloration of source material might be caused by oxidation effect, when Bi+3 turns into Bi+5 by Pt catalysis in high temperature. During hydrothermal growth, deep coloration is bleached in crystal boules due to the low segregation coefficient for Bi+5 in BSO. By two-step growth, Bi+5 content is greatly reduced, and thus corresponding color change takes place. It is indicated that impurities are excluded and intrinsic defects are greatly reduced during multistep growth.(2) The solidified mass prepared in a corundum crucible and the Bi₁₂SiO₂₀ powder prepared by co-precipitation method, could also be used as source material for hydrothermal growth. Though colorless crystals can be obtained just by one-step growth, crystal quality still needs to improve.