Red photoluminescence and purple color of naturally irradiated fluorite
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ORIGINAL PAPER
Red photoluminescence and purple color of naturally irradiated fluorite M. Gaft1 · G. A. Waychunas2,3 · G. R. Rossman3 · L. Nagli1 · G. Panczer4 · D. Cheskis1 · Y. Raichlin1 Received: 30 June 2020 / Accepted: 3 October 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Natural radiation-induced red fluorescence of fluorite consists of two broad bands at 750 and 635 nm with very short decay times of 20.3 and less than 5 ns, respectively. The first one is connected to an M center compensated by Na, while the second is connected to an M + center, possibly formed as result of the M center’s destruction by UV irradiation. The optically active centers in naturally irradiated fluorite responsible for red luminescence and purple color are different from one another. The most probable reason for the purple color is colloidal calcium and not the MNa center. Keywords Fluorite · Luminescence · Natural irradiation · Purple color · M center · Na
Introduction Fluorite is calcium fluoride, CaF2, with a cubic face-centered lattice, with each fluorine ion at the center of one of the smaller cubes, obtained by dividing the unit cube into eight parts. Each Ca is coordinated by eight F ions and each F is surrounded by four Ca ions arranged at the corners of a regular tetrahedron. Fluorite was the first material where Stokes proved that the blue emission excited by UV light had a longer wavelength and was not diffuse light. For this reason, he called the blue light fluorescence (Stokes 1852; Pringsheim and Vogel 1946). Since that time, a lot of research has been devoted to fluorite emission spectra and multiple luminescence centers have been found and interpreted. Fluorite contains many of the rare earths and their luminescence, together with Mn2+ and U6+, has been carefully studied (Haberland et al. 1934; Haberland and * M. Gaft [email protected] 1
Physical Department, Ariel University, Ariel, Israel
2
Energy Geosciences Division, Lawrence Berkeley National Laboratory, MS 74‑316C, One Cyclotron Road, Berkeley, CA, USA
3
Division of Geological and Planetary Sciences, California Institute of Technology, MS 170‑25, Pasadena, CA 91125‑2500, USA
4
Institut Lumière Matière UMR 5306, CNRS, Université Lyon 1, Université de Lyon, 69622 Villeurbanne, France
Köhler 1939; Tarashchan 1978; Krasilschikova et al. 1986; Kemper et al. 2002; Gorobetz and Rogojine 2002; Sidike et al. 2003; Czaja et al. 2012; Gaft et al. 2015). The usual visible luminescence at 300 K under UV excitation is blue and is connected to E u2+ emission. Other frequent fluorescence colors are bluish white, yellowish white, pale yellow, green, violet, or violet blue. The red luminescence, accompanied by purple color, was well studied in synthetic, artificially irradiated CaF2 and ascribed to so called M and MA centers. In CaF2 crystals with cationic or anionic heterovalent impurities ( Na+, Ο2−), which isomorphically replace crystal-forming components and stimulate the formation of anion vacancies, su
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