6533b7ddfe1ef96bd1274782

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Mollwo–Ivey relations for optical absorption bands of the atomic and F′ centres in alkali halides

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Evidence indicates that two classes of the transient IR-absorption bands: (a) with maxima at 0.27-0.36 eV in NaCI, KCI, KBr, KI and RbCl and due to shallow electron traps or bound polarons according to Jacobs (Phys. Stat. Sol. B 129 (1985) 755) and Korovkin and Lebedkina (Fiz. Tverd. Tela (Russian) 35 (1993) 642), and (b) with maxima at 0.15-0.36 eV in NaI, NaBr, NaCl : I, KCl : I I, RbCl: I and RbBr : I, due to on-centre STE localised at iodine-dimer according to Hirota et al. (J. Phys. Soc. Japan 63 (1994) 2774, Phys. Rev. B 52 (1995) 7779) and Edamatsu and Hirai (Mater. Sci. Forum 239-241 (1997) 525), are caused by the same defect. We propose that the defect is an atomic alkali impurity centre [M + ] 0 c e - , i.e. an electron e - trapped by a smaller size substitutional alkali cation impurity [M + ] 0 c . The Mollwo-Ivey plots for the transient IR-absorption bands of the zero-phonon line energy E 0 for NaCI, KCl, KBr, RbCl and NaBr, KCl:I I and/or the low-energy edge values E 0 for NaI, RbCl: I and RbBr: I versus anion-cation distance, d, are obtained for the first time. These data suggest that two types of the [M + ] 0 c e - centres are predominant: (i) [Na + ] 0 c e - in KX and RbX host crystals with the relation E 0 6.15/d 2.74 and (ii) [Li + ] 0 c e - in NaX host crystals with E 0 29.4/d 4.72 . The Mollwo-Ivey relation E 0 18.36/d 2.70 is fulfilled as well for the F' band in NaCI, KCl, KBr, KI, RbCl, RbI if we use the F' centre optical binding energy values for E 0 .