Coloration of Pure and Doped Calcium Fluoride Crystals at 20°C and -190°C

Coloration of pure Ca${\mathrm{F}}_2$ crystals with 2.5-Mev electrons at room-temperature results in bands at 580, 400, 335, and 225 mμ. In Y${\mathrm{F}}_3$-doped crystals the bands are in the same spectral positions as in pure crystals. They are, however, enhanced and their intensity ratios are significantly changed, the 400-mμ band being predominant. NaF-doped crystals show an even more drastic change: Coloration at room temperature produces bands at 605, 385, and 330 mμ and is deeper than in either pure or Y${\mathrm{F}}_3$-doped crystals. Since the 400-mμ band is strongly enhanced by Y${\mathrm{F}}_3$ addition which forms ${\mathrm{F}}^{-}$ interstitials, it is correlated to neutral fluorine atoms in interstitial positions. Similarly, the 605-mμ band in crystals doped with NaF, which creates ${\mathrm{F}}^{-}$ vacancies, is correlated to electrons trapped in ${\mathrm{F}}^{-}$ ion vacancies ($F$ centers). The enhancement of the 385-mμ band by NaF is explained by the reduction of the activation energy required for formation of interstitials and vacancies. Of several possible correlations the 330-mμ band is probably connected with a hole trapped in a ${\mathrm{Ca}}^{2+}$ vacancy and that at 225 mμ with an electron trapped by a ${\mathrm{Ca}}^{2+}$ interstitial. The bands at 440 and 200 mμ which appear in NaF-doped crystals only must be connected with Na ions. Coloration at -190°C produces strong changes in the absorption spectrum. In pure and Y${\mathrm{F}}_3$-doped crystals bands appear at 550, 320, and 270 mμ while in NaF-doped crystals bands are formed at 440, 390, 315, and 200 mμ in addition to the 550-mμ band. The primary process of coloration at low temperature cannot differ from that at room temperature but the secondary processes are strongly influenced by temperature.