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*Ir4+ ion‐free 12CaO·7Al2O3 single crystal grown by the CZ method

Ir4+ ion‐free 12CaO•7Al2O3 (C12A7) single crystals that were colorless and transparent have been grown by the CZ method under 0.5% oxygen‐containing nitrogen atmospheres for the first time. Powder X‐ray diffraction patterns and X‐ray single‐crystal diffraction were used to investigate the microstructure and quality of the C12A7 single crystal. The results show that the crystal was a C12A7 single crystal with tetragonal symmetry. The content of Ir4+ in the C12A7 single crystal was tested by ICP along with visible‐light transmittance. There was no Ir4+ in the C12A7 crystal grown in a 0.5% oxygen‐containing nitrogen atmosphere. Contents of Ir4+ in C12A7 crystal grown under 1% and 2% oxygen‐containing nitrogen atmospheres were 500 ppm and 400 ppm, respectively. The microstructure of C12A7 single crystal was determined by the content of oxygen in the growth process: the occurrence of bubbles and color increase when the content of oxygen is increased. Bubbles and color decrease when the oxygen content is lowered, bubbles disappeared and the color of C12A7 turns into transparent when the content of oxygen is 0.5%, the C12A7 phase decomposed when the content of oxygen was lower than 0.5%. So, the content of oxygen was responsible for the bubbles, color and cracking, and these also can be limited by controlling the content of oxygen.