Single-crystal rods and fibers of Bi12SiO20 were grown directly from a rod of mixed Bi2O3 and SiO2 powders in a floating zone device heated by an infrared source with a special light shutter. The source rod was pressed from the mixed powders at room temperature; to avoid possible contamination, hot pressing and melting with die or crucible were not employed. The following are the main results: (a) The length of the stable molten zone is an increasing function of the diameter of the grown crystal rod. (b) Non-transparency is a major defect in a small diameter crystal rod or fiber. The growth velocity should be less than the critical transparent velocity (the critical value below which the grown crystal is transparent throughout). For growth of a large diameter crystal rod (diameter ≥3 mm), the growth velocity should be less than both the critical transparent velocity and the critical cracking velocity (the critical value below which the grown crystal is free of cracks). In general, both the critical transparent and critical cracking velocities are decreasing functions of the diameter of the grown crystal rod. (c) Use of a single crystal with high thermal conductivity (e.g., Al2O3) as a seed can dramatically increase both the critical transparent and critical cracking velocities for growth of a single crystal of diameter larger than 3 mm. (d) The pulling down floating zone method is more suitable than the pedestal growth method because of the low surface tension and high density of the Bi12SiO20 melt. (e) The grown Bi12SiO20 crystal is a pure body-centered cubic γ phase and has a good infrared transmission spectrum.

Metastable δ-Bi12SiO20 may crystallize from the overheated Bi12SiO20 melt and transform into stable γ-Bi12SiO20 at about 569.5 °C during the subsequent slow cooling process. The transition δ-Bi12SiO20 → γ-Bi12SiO20 is irreversible and the γ-Bi12SiO20 is stable up to the melting temperature. By quenching the Bi12SiO20 melt, pure δ-Bi12SiO20 can be obtained at room temperature. The quenched δ-Bi12SiO20 crystal is nontransparent and has a space group of Fm3m (225) and a lattice constant of 55.417 Å at 20 °C. The quenched metastable δ-Bi12SiO20 can transform into pure γ-Bi12SiO20 at 382.5–386.1 °C with an exothermic heat of 31.68–32.38 J/g. The transition-produced δ-Bi12SiO20 crystal is still nontransparent and has a large lattice distortion. The transition δ-Bi12SiO20 → γ-Bi12SiO20 causes about 6% volume contraction, which may result in cracking of the grown crystal. By controlling the growth parameters, this transition can be effectively avoided.

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Reaction pathways in the synthesis of three photorefractive silicates—γ‐Bi12 SiO20 (BSO), γ‐Bi12 GeO20 (BGO), and gamma‐Bi12 TiO20 (BTO)—were systematically investigated. The main results were as follows: (i) all the reactions of the form 6Bi2O3+ MO2→> γ‐Bi12 MO20 (SR1 for M = Si, SR2 for M = Ge, SR3 for M = Ti) in the solid state seemed to be diffusion‐controlled processes and were affected by both temperature and time, where the reaction temperature increases in the order SR1 < SR2 < SR3; (ii) the metastable phases Bi2 SiO5 (tetragonal) in reaction SR1, Bi2 GeO5 (orthorhombic) in reaction SR2, Bi4 Ti3 O12 (orthorhombic) in reaction SR3 may be formed and seemed to greatly accelerate the above‐mentioned solid‐state reaction processes; and (iii) for a continuous heating process, pure γ‐Bi12 SiO20 and γ‐Bi12 GeO20 could be produced before melting, whereas pure γ‐Bi12 TiO20 could not be produced, even if all the mixed phases had melted.