| 摘要 |
PROBLEM TO BE SOLVED: To produce a single crystal useful for high-accurcy optical systems, having high transmissivity, low distorted and having no turbidness in its inner portion by removing adsorbate or accretion on the surface of a fluoride single crystal to clean the surface and then by heat-treating the single crystal through retaining it in a heated state and gradually cooling it. SOLUTION: The method of this invention is to purify the surface of a fluoride single crystal before heat treatment, in consideration of the fact that the cause of turbidness in its inner portion of a fluoride single crystal due to heat treatment is based on impurities, stain, moisture or oxygen-containing ingredients respectively attached or adsorbed on the surface of a treating object. And the above method is to conduct heat treatment by retaining the fluoride single crystal in a heated state and gradually cooling it, followed by removing an alteration layer formed on its surface due to the heat treatment because even a fluoride single crystal the surface of which is cleaned before heat treatment is etched on its surface due to the existence of heat or a fluorinating agent during the heat treatment, which leads to the formation of alteration layer causing the quality degradation of the fluoride single crystal. |
