Electrolytic Coloration And Spectral Properties Of Natural Fluorite Crystals

Using a technique of electrolytic coloration, natural fluorite crystals containing oxygen impurities are colored electrolytically for the first time, and a lot of color centers are produced in the colored crystals. Spectral measurement and analysis are performed systematically to the colored crystals, and the production and transformation mechanisms of the color centers are presented. By using measured current-time curves, the production and transformation mechanisms of the color centers are further explained.Before electrolytic coloration, there exists a large number of O^2- -V_a⁺,O^2- -V_a⁺ aggregate color centers and Yb²⁺,Sm²⁺,Ce³⁺ impurities ions in natural fluorite crystals containing oxygen impurities. After electrolytic coloration using a pointed cathode, these color centers and impurities does not disappear, and neutral fluorine atoms in interstitials, electrons trapped by Ca²⁺ interstitials, holes trapped in Ca²⁺ vacancies and F, F2 color centers are produced. In the absorption spectra of the fluorite crystals before and after the electrolytic coloration, there are intense absorption bands of the O^2- -V_a⁺ and O^2- -V_a⁺ aggregate color centers. These spectral results show that more O^2- -V_a⁺ and O^2- -V_a⁺ aggregate color centers in the fluorite crystals have not been neutralized by the containing Yb²⁺, Ce³⁺ and Sm²⁺ impurities. These O^2- -V_a⁺ and O^2- -V_a⁺ aggregate color centers can prevent the formation of the secondary alkaline earth cathode and result in no direct production of the F color centers in the fluorite crystals.Our experimental apparatus has a unique anode matrix composed of the graphite grains. It is the anode matrix makes that V color centers are produced firstly in crystal region near the anode matrix. Some V color centers are converted into F color centers under light illumination, and then, some F color centers are aggregated into F2 color centers. Data of current-time were recorded in the electrolysis, with which current-time curves are plotted. The current components of each current zone all consist of the ionic motion and electron exchange between halogen ions and anode matrix. The formation mechanism of the current zones in the current-time curves and the close relationship between the color center production and the formation of the current zones are presented. After varied experiments, in conclusion, a best effective coloration condition at temperature 813 K, voltage 1200 V and electrolysis time 75 min is found.Similarly, the O^2- -V_a⁺, O^2- -V_a⁺ aggregate color centers and Yb²⁺, Sm²⁺, Ce³⁺ impurities in natural fluorite crystals containing oxygen impurities does not disappear after the electrolytic coloration with a pointed anode, and electrons trapped by Ca²⁺ interstitials, neutral fluorine atoms in interstitials and F, F₂ color centers are produced. According to absorption spectra of the colored crystals and characteristics of the current-time curve, the formation mechanism of current zones in herein current-time curves is basically similar to that in above current-time curves using a pointed cathode.