Bubble Formation and Evolution in Calcium Fluoride Crystals: the Role of Melt/Crystal Interface Behavior

doi:10.15541/jim20260091

Abstract

Abstract: Calcium fluoride (CaF₂) single crystals are essential materials in precision optics due to their excellent optical properties. However, bubble defects formed during melt growth significantly affect crystal quality and yield, making them a primary concern in crystal preparation. This study aims to investigate the formation and evolution mechanisms of bubble defects to support the development of large‑size, high‑quality optical crystals. The morphology, distribution, and influence of bubbles on crystal properties were examined using microscopic characterization, spectral analysis, and statistical methods. In addition, the melt/crystal interface during growth was monitored using a visualization system based on the vertical Bridgman method, allowing the reconstruction of interface evolution in the bubble formation region. The results reveal four types of bubbles: spherical, spindle‑shaped, elongated, and tadpole‑like. Bubbles first appear at the center of the crystal at a height of 95  mm. In the early stage, individual bubbles are relatively large, and the tails of tadpole‑like bubbles point along the normal direction of the melt/crystal interface. As the interface transitions from concave to convex, the area ratio of bubbles to the crystal cross‑section increases continuously until the interface becomes fully convex at 156  mm, where bubbles occupy the entire cross‑section. Additionally, during the initial stage of bubble formation, the ratio of crystal growth rate to crucible descent rate changes from negative to positive and increases to a maximum of 2.5. These findings demonstrate that bubble formation is closely linked to the morphology and kinetic behavior of the melt/crystal interface, providing a theoretical foundation for suppressing bubble defects through temperature field optimization and interface growth rate control, and offering new insights for defect management in other melt‑grown crystals.

Key words: calcium fluoride single crystal, vertical Bridgman method, melt/crystal interface, bubble defect, X-ray imaging

ZHANG Xiaohui, ZHANG Zhonghan, LIU Yang, JIANG Dapeng, KOU Huamin, WU Anhua, SU Liangbi. Bubble Formation and Evolution in Calcium Fluoride Crystals: the Role of Melt/Crystal Interface Behavior[J]. Journal of Inorganic Materials, DOI: 10.15541/jim20260091.

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