ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

Determination of impurities for controllable growth of high quality optical fluorite

Jordan T. Mouhovski and Albena K. Detcheva

Institute of Mineralogy and Crystallography “Acad. I. Kostov”, Acad. G. Bonchev Str., bl. 107, 1113 Sofia, Bulgaria

E-mail: jmouhovski@abv.bg

Abstract: Natural fluorite is used for growing CaF₂ boules from melt by an improved technique. Chemical treatment of the starting ore decomposes the accessory minerals, thus producing small amounts of the oxides of Si, Al, and Fe insoluble in the melt, whereas the overall content of rare earth elements (REEs) of hundreds of μg g⁻¹, remains unchanged. Analytical techniques and optical measurements provide for assessing the concentration range and trends in the distribution of residual metal impurities along the height of the boules. Solid sampling electrothermal atomic absorption spectrometry (SS-ETAAS) gives good reproducibility for impurities’ distribution within a large concentration range of 0.1–10 μg g⁻¹. The concentrations of Zn and Cu determined were found to vary within the lowest tenths of μg g⁻¹ range in the starting portions of chemically treated fluorspar and a batch of boules produced subsequently. The concentrations of both elements show a decreasing trend towards the top section within the confidential interval, the width of which confirms the definite in homogeneities in their distribution at those concentration levels. The Fe occurs in the boules below the detection limit, while the content of lead diminishes rapidly towards their upper section, probably due to a shorter path in the liquid phase before any vapour phase transition proceeds. A satisfactory correlation is found between the Pb concentration in ng g⁻¹-range and light-absorption peak intensity at 204 nm, the precise determination of which is impeded due to the overlapping bands and the light-scattering effect. Reliable determination of impurities enables optimisation of the basic purification — growing stages for the production of high grade crystals.

Keywords: natural fluorite purification – optimised crystal growth – impurity determination – spectrophotometric analyses – UV grade optical material

Full paper is available at www.springerlink.com.

DOI: 10.2478/s11696-011-0131-1

Chemical Papers 66 (3) 161–170 (2012)