Progress has been made in the study of calcium fluoride crystal defects by the Chinese Academy of Sciences. Calcium fluoride crystals feature a wide transmission wavelength range, low dispersion effects, and strong resistance to laser damage, making them core components in advanced instruments such as ultraviolet lithography systems, high-resolution space cameras, and high-precision optical detection equipment. However, the open cubic structure of calcium fluoride leaves half of the octahedral lattice sites vacant, resulting in a dislocation density of 10⁵/cm², which significantly impairs performance. Therefore, precisely identifying, analyzing, and controlling dislocation defects within the crystal is a breakthrough point for achieving near-theoretical-limit performance of calcium fluoride crystals and meeting extreme application requirements. Due to the sensitivity of F-ions to electron beams, atomic-level characterization of dislocations in calcium fluoride crystals has posed significant challenges, and traditional characterization techniques have struggled to resolve and clarify their dislocation configurations and formation mechanisms.
In response, a research team led by Dr. Liangbi Su and Dr. Bo Zhang from the Shanghai Institute of Ceramics, Chinese Academy of Sciences, addressed the technical bottlenecks in atomic-level characterization of dislocations in calcium fluoride crystals. They developed a multi-scale integrated characterization system combining "chemical etching method-high-resolution electron backscatter diffraction (HR-EBSD)-integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM)." The study developed a high-resolution EBSD technique based on the Kikuchi line rotation compensation algorithm, overcoming the limitations of traditional EBSD precision, and combined it with chemical etching to achieve precise localization and characterization of small orientation differences for two types of dislocations in crystals: "freely distributed" and "regularly aggregated." Additionally, the study employed ultra-low-dose iDPC-STEM technology to directly resolve the F/Ca atomic arrangement at the atomic scale for the first time and clearly capture the true configuration of dislocations.
Analysis of the morphology, distribution, and orientation differences of dislocations in calcium fluoride crystals
Furthermore, by integrating crystal growth experiments, first-principles calculations, and structural characterization, the researchers elucidated the mechanism of dislocation formation in calcium fluoride crystals. The study found that excessive temperature gradients cause lattice mismatch, thereby promoting dislocation formation. Additionally, the study clarified the causal relationship between vacancy-preferred aggregation and Frank incomplete dislocation nucleation at the atomic scale. Based on these findings, the study proposed strategies to suppress/reduce dislocation defects and validated the possibility of "dislocation self-regulation" through heat treatment experiments. The dislocation density in calcium fluoride crystals was reduced from 10⁵ cm⁻² to 10³ cm⁻² level, providing a practical pathway for achieving low-defect calcium fluoride crystals.
This study addresses the long-standing scientific challenge of achieving "atomic-level resolution" of defects in calcium fluoride crystals and establishes a complete cognitive chain from the macroscopic to the mesoscopic and atomic levels of defects, providing critical scientific basis for the preparation of crystals with near-theoretical-limit performance.
