Growth characteristics of single-crystal rods and fibers of Bi12SiO20 by the floating zone method

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.