Growth of Single Crystals of Incongruently Melting Yttrium Iron Garnet by Flame Fusion Process

Single crystals of yttrium iron garnet (Y₃Fe₅O₁₂) have been grown using the flame fusion process, even though the compound is reported to melt incongruently. The growth of these single crystals involves a mechanism different from that which has been proposed for the growth of single crystals of incongruently melting mullite. Crystal boules were grown at varying linear growth rates and analyzed with chemical, X-ray, and metallographic techniques. With high linear growth rates, the samples are uniformly polycrystalline and three-phase, containing Fe₂O₃, YFeO₃, and Y₃Fe₅O₁₂. When slow linear growth rates are used, single-crystal Y₃Fe₅O₁₂ can be grown. The mechanism is as follows: At the beginning of growth the first phase to precipitate is YFeO₃, and during this stage in growth the molten cap becomes enriched in Fe₂O₃, compared with the Y₃Fe₅O₁₂ composition. The liquid cap composition thus changes to the limit of the peritectic on the Fe₂O₃-rich side, and Y₃Fe₆O₁₂ then crystallizes from the bottom of the melt as Y₃Fe₅O₁₂ powder is added to the top of the molten cap. The central sections of these boules are single-crystal yttrium iron garnet.