AND UDC 535.34The effect of cation impurities on structure-sensitive properties of alkali-halide crystals has been known for a long time [1]. Since a lithium fluoride crystal is an optical material, transparent down to 105 rim, it is interesting to consider the effect of cation impurities on its optical absorption in the vacuum ultraviolet region.From the theory of electronic processes in ionic compounds [2], we know that additional absorption can occur by adding cation impurities to alkali-halide crystals. This absorption can be due to electron-vibration transitions in the impurity ion itself as well as electron-vibration transitions in the halide ion of the impurity cation, or absorption by a cation exciton.The effect of magnesium impurities on the optical properties of LiF crystals is especially important since magnesium salts are similar to lithium salts in their chemical properties [3] which leads to an appreciable magnesium content in commercial LiF crystals [4].The effect of magnesium on the optical absorption spectrum of LiF crystals was investigated in [5][6][7][8]. According to [8], the presence of the Mg 2+ ion in LiF crystals gives rise to additional bands at 230 and < 210 nm in the optical absorption spectrum. Here, the observed absorption is considered to be associated with the presence of magnesium ions at the crystal lattice sites. In [5] it is indicated that the presence of magnesium in quantities of 2 • 10 -4 wt.% and larger in LiF crystals does not produce any absorption bands in the spectral range of 105-600 nm. However, decreasing the magnesium concentration by a factor of ten produces a noticeable absorption in the wavelength region below 130 nm.Thus, the effect of magnesium on the optical absorption spectrum of LiF crystals has not been studied adequately, up to the present time. In this connection we have investigated the optical absorption spectra of LiF crystals containing magnesium, introduced both as MgF 2 and MgO. The crystals were grown from high-purity starting materials in a vacuum by the Stockbarger method. For doping we used high-purity MgF 2 (crystal (1)), magnesium fluoride recrystallized from the melt (crystal (2)), and single-crystal magnesium oxide (crystal (3)).Crystals (1) and (2) were grown from a melt containing 3.6.10-2 wt. % magnesium. In growing crystals (3), magnesium was added to the melts in the amounts of 3.6.10-2 and 3.6 • 10-3 wt.~c. The magnesium content (wt. %) in the single crystals was determined by emission spectroscopy in which the relative error did not exceed 20%. The optical absorption spectra were studied in the 105-700 nm region using an SP-68 vacuum monochromator and an SF-4 spectrophotometer. The wavelengths were recorded with a precision of 1 nm. The absorption coefficients were determined by a differential method [9] and the relative error for the measurements did not exceed 10~c.Absorption spectra of samples cleaved from the lower portions of crystals (1), (2), and (3) are shown in Fig. 1 from which we see that in ~he additional absorption spectr...
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