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Mollwo–Ivey relations for optical absorption bands of the atomic and F′ centres in alkali halides
Radiation Measurements 33 (2001) 779–783
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Mollwo–Ivey relations for optical absorption bands of the atomic and F centres in alkali halides V. Ziraps ∗ Institute of Solid State Physics, University of Latvia, 8 Kengaraga St., LV-1063 Riga, Latvia Received 20 August 2000; received in revised form 22 January 2001; accepted 23 March 2001
Abstract Evidence indicates that two classes of the transient IR-absorption bands: (a) with maxima at 0.27–0:36 eV in NaCl, KCl, 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, 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+ ]0c e− , i.e. an electron e− trapped by a smaller size substitutional alkali cation impurity [M+ ]0c . The Mollwo–Ivey plots for the transient IR-absorption bands of the zero-phonon line energy E0 for NaCl, KCl, KBr, RbCl and NaBr, KCl : I and=or the low-energy edge values E0 for NaI, RbCl : I and RbBr : I versus anion–cation distance, d, are obtained for the Brst time. These data suggest that two types of the [M+ ]0c e− centres are predominant: (i) [Na+ ]0c e− in KX and RbX host crystals with the relation E0 ≈ 6:15=d2:74 and (ii) [Li+ ]0c e− in NaX host crystals with E0 ≈ 29:4=d4:72 . The Mollwo–Ivey relation E0 ≈ 18:36=d2:70 is fulBlled as well for the F c 2001 Elsevier Science band in NaCl, KCl, KBr, KI, RbCl, RbI if we use the F centre optical binding energy values for E0 . Ltd. All rights reserved. Keywords: Alkali halides; F centres; Self-trapped excitons; Transient IR-absorption bands
1. Introduction Shallow electron traps (electrically neutral defects Dn ) in alkali halide crystals play a very important role in many low-temperature (from LHeT to RT) thermally stimulated relaxation phenomena. Under ionising irradiation, electrons (e− ) are eEectively trapped by the neutral traps Dn forming the negatively charged centres Dn e− . A large number of these centres are formed in close association with Vk -type centres resulting in dipolar correlated pairs {Dn e− : : : Vk }, {Dn e− : : : VkA } and {Dn e− : : : Vk (XY− )}, where A and X are the alkali and halogen atoms in alkali halide crystal AX, respectively, and Y the impurity halogen atom in AX crystal.
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Fax: +371-711-2583. E-mail address: [email protected] (V. Ziraps).
If the alkali impurity cations (M+ ) of smaller size substitute for the larger host lattice cations (A+ ) in an alkali halide (AX) crystal, then shallow electron traps, i.e., neutral centres which capture electrons, like [M+ ]0c , can be formed. During ionising irradiation, electrons are trapped by such neutral centres, thus forming the negatively charged centres like [M+ ]0c e− . The centres [M+ ]0c e− are alkali impurity atoms placed in the cation sites and therefore, could be called as atomic alkali impurity centres. Consequently, the main neutral traps for electrons in alkali halides are F centres and atomic alkali impurity centres [M+ ]0c . Under ionising irradiation, the formation and accumulation of the isolated charged colour centres F and [M+ ]0c e− , as well as their dipolar associations with Vk -type centres, such as {F : : : Vk }, {F : : : VkA }, {F : : : Vk (XY− )}, {[M+ ]0c e− : : : Vk }, {[M+ ]0c e− : : : VkA }
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V. Ziraps / Radiation Measurements 33 (2001) 779–783
and {[M+ ]0c e− : : : Vk (XY− )}, are expected with a very high yield. In this paper, we report our results of a correlative analysis of data published since 1966 by several research groups concerning the transient IR-absorption bands with maxima at 0.27–0:36 eV in the F or M band photoexcited additive or subtractive coloured crystals NaCl, KCl, KBr, KI and RbCl (A-class IR bands) and in the UV-laser excited crystals NaCl : I, KCl : I, RbCl : I and RbBr : I (B-class IR bands). We show that the same Mollwo–Ivey plots are fulBlled for both classes of the IR bands, if we plot not the IR band peak energy values but the more distinct and deBnite values of the IR band zero-phonon line energy E0 and=or the IR band low-energy edge energy E0 . These Mollwo–Ivey power law relations for the IR bands conBrm our previous suggestions and thesis (Ziraps, 1996) that both classes of the above transient IR-absorption bands are caused by the same type defects and are rather due to transitions of the shallow trapped electron in the atomic alkali impurity centre [M+ ]0c e− . It is also shown that the Mollwo–Ivey relation is approximately fulBlled for the F bands in alkali halides. 2. Experimental results and discussion Two classes (marked as A and B) of the photoinduced transient IR-absorption bands at LHeT or LNT with maxima in the range from 0.27 to 0:36 eV in NaCl, KCl, KBr, KI and RbCl (the A-class IR bands) and in the iodine-doped crystals NaCl : I, NaBr : I, KCl : I, KBr : I, RbCl : I and RbBr : I, as well as in NaI and NaBr (the B-class IR bands) have been detected and investigated in several laboratories. The A-class IR bands with maxima at 0.27–0:36 eV in additively or subtractively coloured NaCl, KCl, KBr, KI and RbCl crystals were detected during a pulsed photoexcitation of the F or M centres and were related with the shallow trapped electrons (Park and Faust, 1966; Schneider, 1978; Jacobs, 1985) or bound polarons (Korovkin and Lebedkina, 1993). The B-class IR bands with maxima at 0.15 –0:36 eV were detected during pulsed UV-light excitation by an excimer laser and were related to the on-centre STE at the iodine-dimer in heavily iodine-doped (about 1 mol%) crystals NaCl : I, NaBr : I, KCl : I, KBr : I, RbCl : I and RbBr : I (Hirota et al., 1994, 1995; Edamatsu and Hirai, 1997; Shirai and Kan’no, 1996) and in NaI and NaBr—related to the on-centre STE (Edamatsu and Hirai, 1997). The published data relating to the A-class and B-class transient IR-absorption bands have been summarised, compiled and carefully analysed (Ziraps, 1996, 2000). From this analysis, Ziraps showed that there is a very close correlation between the A-class and B-class IR bands. The IR bands are similar in band location, shape, vibration (phonon) structure and other features. For KCl : I, the IR bands exactly coincide at 10 or 80 K and overlap with a small shift for RbCl : I, KBr : I and NaCl : I at 10 K. The corresponding transient IR-band is at 0.27–0:36 eV at 11–15 K in KCl, RbCl, KBr
and NaCl and are claimed to be due to shallow trapped electrons (Park and Faust, 1966; Jacobs, 1985; Schneider, 1978) or bound polarons (Korovkin and Lebedkina, 1993). In addition, the vibration structure of the transient IR-band in KCl : I at 10 K (Hirota et al., 1995) is very similar to the vibration structure of the transient IR band due to the shallow trapped electrons in KCl at 4.2–21 K (Jacobs, 1985; Korovkin and Lebedkina, 1993). Schneider (1978) and Ziraps (1996, 2000) showed that these shallow traps in the alkali halide AX crystal are caused by substitutional alkali impurity cations [M+ ]0c of smaller size than the host crystal cations A+ . Because of the free atom ionisation potential diEerence NU = U (M0 ) − U (A0 ) ¿ 0, the neutral isoelectronic impurity centre [M+ ]0c can trap an electron and form a charged point defect [M+ ]0c e− , the so-called “atomic alkali impurity” centre. We propose that these data indicate that the B-class IR bands (with maxima at 0.15 –0:36 eV) proposed as electron transitions in the on-centre STE localised at the iodine dimer in ACl : I and ABr : I (Hirota et al., 1994, 1995; Edamatsu and Hirai, 1997; Shirai and Kan’no, 1996) or the on-centre STE in NaBr and NaI (Edamatsu and Hirai, 1997) crystals are rather due to electrons trapped by shallow traps, i.e., during the UV-laser excitation of the iodine-dimers [I2− ]+ aa and subsequent electron transfer to neighbouring shallow traps [M+ ]0c : {[M+ ]0c : : : [I− ; I− ]0aa } + h (6:42 eV) → {[M+ ]0c : : : [(I2= )0aa ]∗ } → {[M+ ]0c e− : : : [I2− ]+ aa }:
(1)
Thus donor-type centres [M+ ]0c e− and=or their associations with the genetic hole centres [I2− ]+ aa , i.e., donor–acceptor pairs {[M+ ]0c e− : : : [I2− ]+ aa } can be formed with a high yield. We have found, and in this paper demonstrate, that even for these very wide and asymmetric absorption (transient IR and F ) bands in alkali halides, the Mollwo–Ivey relation E0 ≈ a=dn (where d is the nn anion–cation distance, a the constant, n the exponent) is valid. The data (E0 values) for the Mollwo–Ivey plots (Figs. 1 and 2) are taken from the published literature (references are given in Bgure captions). If we take not the band peak energy values but the more deBnite values of the zero-phonon line energy E0 values (for the IR bands) and=or the band low-energy edge energy E0 values for the IR bands and the F bands, then the Mollwo– Ivey relation is valid for the transient IR-absorption bands (Fig. 1) and F bands (Fig. 2). The Mollwo–Ivey plot curves (Fig. 1, curves 1, 2) of the IR band zero-phonon line energy E0 values—for NaCl, KCl, KBr, RbCl (Jacobs, 1985; Korovkin and Lebedkina, 1993) and NaBr, KCl : I (Hirota et al., 1995; Edamatsu and Hirai, 1997)—and=or the IR band edge energy E0 (±0:03 eV) values—for NaI, RbCl : I, RbBr : I (Hirota et al., 1994; Edamatsu and Hirai, 1997; Shirai and Kan’no, 1996)— versus anion–cation nn-distance d evidence that two types of the atomic alkali impurity centres [M+ ]0c e− in the AX
V. Ziraps / Radiation Measurements 33 (2001) 779–783
Fig. 1. The Mollwo–Ivey plots of the zero-phonon line and=or the low-energy edge energy E0 values of the transient IR-bands (with maxima at 0.15 –0:36 eV) versus the anion–cation nn distance d (in 0:1 nm): (1) for NaX host crystals: NaCl data from Jacobs (1985), Schneider (1978) and Korovkin and Lebedkina (1993) and NaCl : I, NaBr and NaI data from Hirota et al. (1994) and Edamatsu and Hirai (1997). (2) for KX and RbX host crystals: KCl, KBr and RbCl data from Jacobs (1985); KCl data from Schneider (1978); KCl and KBr data from Korovkin and Lebedkina (1993); KI data from Park and Faust (1966) and RbCl : I and RbBr : I data from Hirota et al. (1994, 1995), Edamatsu and Hirai (1997) and Shirai and Kan’no (1996).
Fig. 2. The Mollwo–Ivey plots of the F centre optical binding (the F absorption band low-energy edge) energy E0 (±0:2 eV) values versus the anion–cation nn distance d (in 0:1 nm) for alkali halide crystals: KCl, KBr, KI, RbCl and RbI data from Lynch and Robinson (1968); NaCl and KCl data from Zhang et al. (1994) and KCl data from HRartel and LRuty (1964) and Ishii and Endo (1968). The Mollwo–Ivey power law E0 ≈ 18:36=d2:70 is approximately valid.
host crystals are predominant (Fig. 1): (i) [Li+ ]0c e− centres in NaX host crystals with the relation E0 ≈ 29:4=d4:72 (curve 1); (ii) [Na+ ]0c e− centres in KX and RbX host crystals with the relation E0 ≈ 6:15=d2:74 (curve 2).
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We demonstrate (Fig. 2) that the Mollwo–Ivey relation is fulBlled as well for the F band low-energy edge energy (the optical binding energy) E0 (±0:2 eV) values for the NaCl, KCl, KBr, KI, RbCl, RbI crystals (the data are taken from Lynch and Robinson, 1968; Zhang et al., 1994; HRartel and LRuty, 1964). A relation E0 ≈ 18:36=d2:70 is approximately valid for the F bands. The exponent n values in the above relations give information about the electron localisation depth in these neutral traps and the electron wave function spatial distribution (the diEuseness and extension of the electronic states) around the point defect. In alkali halide crystals for electrons trapped in the anion site defects (with the defect eEective charge q = +1), the exponent n is below 2.0, e.g., for the F centres, U centres, [Cu− ]0a , [Ag− ]0a and [Au− ]0a the exponent n values are in the range n ≈ 0:4–2.0. This means that there is a relatively deep electron localisation. In the case of electrons trapped by neutral traps—for the atomic alkali impurity centres [M+ ]0c e− the n values are higher: from n ≈ 2:74 (for the [Na+ ]0K e− and [Na+ ]0Rb e− centres in the KX and RbX hosts) to n ≈ 4:72 (for [Li+ ]0Na e− in the NaX hosts). The higher n values for the [Li+ ]0Na e− centres relative to the [Na+ ]0K e− and [Na+ ]0Rb e− centres indicate that the trap [Li+ ]0Na is shallower (and the electron wave function is more extended and diEuse) than the traps [Na+ ]0K and [Na+ ]0Rb . In alkali halide (AX) crystals, the electron localisation depth (and exponent n values) in the [M+ ]0c e− centres is determined by the free alkali impurity atom (M0 ) and free host alkali metal atom (A0 ) ionisation potential differences NU (M0 ; A0 ): viz., NU (Na0 ; K 0 ) ≈ 0:80 eV, NU (Na0 ; Rb0 ) ≈ 0:96 eV and NU (Li0 ; Na0 ) ≈ 0:25 eV. Such a correlation between the NU and n values in the case of the NaX host crystals (NU (Li0 ; Na0 ) ≈ 0:25 eV and n ≈ 4:72) and in the case of the KX and RbX hosts (NU (Na0 ; K 0 ) ≈ 0:80 eV, NU (Na0 ; Rb0 ) ≈ 0:96 eV and n ≈ 2:74) is indeed observed. It must be stressed that the optical absorption bands for both classes of shallow trapped electrons (the F centres and the [M+ ]0c e− centres) have some distinctive and similar features. Both bands (the F band and the transient IR-band) are distinctively asymmetric and have a large relative half-width k = W1=2 =Emax , where W1=2 is the half-width and Emax the peak energy of the band. For the F band, the parameter k decreases from the values k ≈ 0:9–1.3 (RbX, KX) to k ≈ 0:4 (NaCl) in the series RbBr, RbCl, KBr, CsF, KCl, KCl : Na and NaCl in which the F centre electron binding energy E0 increases. In the case of the transient IR bands for the atomic alkali impurity centres the parameter k ≈ 0:32– 0.65 (NaX, KX, RbCl, RbCl : I, RbBr : I). Let us consider how we can explain the coincidence of the B-class IR band location, shape and structure in the iodine-doped alkali halide (AX) crystals ACl : I and ABr : I, UV-laser excited at LHeT or LNT, with the corresponding A-class IR band location, shape and structure in the additive coloured crystals ACl and ABr, photoexcited in F or M
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bands at LHeT or LNT. Moreover, how we can explain the correlated decay kinetics ( 6 1:0 s) of the transient IR-absorption band and the UV-luminescence (observed by Hirota et al., 1994, 1995; Edamatsu and Hirai, 1997) of the so-called on-centre STE localised at iodine dimer [(I22− )0aa ]∗ is as follows: It must be taken into account that because of the volume compensation eEect during the growth of the heavily iodine-doped (about 1 mol%) crystals, and=or subsequent thermal treatment, the accidental (trace) alkali impurity cations [M+ ]0c of smaller size (like Li+ in NaCl : I; Li+ and Na+ in KCl : I, KBr : I, RbCl : I and RbBr : I) are expected to be predominantly associated with the larger iodine dimers [I− ; I− ]0aa , thus reducing the lattice distortions, internal stresses and forming the neutral impurity-related defect pairs {[M+ ]0c : : : [I− ; I− ]0aa }. Then we can realise a formation in ACl : I and ABr : I crystals (during laser UV-photoexcitation of the iodine-dimers [I− ; I− ]0aa ) the donor–acceptor type dipolar pairs {[M+ ]0c e− : : : [I2− ]+ aa } with a very small lifetime (see reaction (1)). It results in the B-class transient IR-bands with maxima in the range of 0.23–0:36 eV, as observed by Hirota et al. (1994, 1995) and Edamatsu and Hirai (1997). Subsequent electron thermoactivated or tunnel transitions and recombination within such donor–acceptor type defect pairs {[M+ ]0c e− : : : [I2− ]+ aa } can explain the correlated decay kinetics ( 6 1:0 s) of the transient IR-absorption band and UV-luminescence quanta h emission (observed by Hirota et al., 1994, 1995; Edamatsu and Hirai, 1997) of the so-called on-centre STE localised at iodine dimer [(I22− )0aa ]∗ : + 0 2− 0 ∗ {[M+ ]0c e− : : : [I2− ]+ aa } → {[M ]c : : : [(I2 )aa ] } + 0 − − 0 → {[M ]c : : : [I ; I ]aa } + h :
(2)
In summary, the optical absorption and thermoactivation spectroscopy data evidence that the A-class and the B-class transient IR-absorption bands in the range of 0.27–0:36 eV are caused by the same defect—probably, the atomic alkali impurity centre [M+ ]0c e− . Both classes of the IR-absorption bands have a very similar shape, vibration (phonon) structure and coincide in energy scale, as well as fulBl the same Mollwo–Ivey power law.
3. Conclusions Evidence states that the alkali impurity ions M+ of smaller size can substitute the larger A+ cations in alkali halide AX crystals thus forming a shallow electron traps, i.e., the alkali impurity atomic centres [M+ ]0c e− . We propose that the same centre, i.e., a shallow electron trap—the atomic alkali impurity centre [M+ ]0c e− , responsible for the A-class transient IR-absorption bands with maxima at 0.27–0:36 eV in NaCl, KCl, KBr, KI and RbCl, is responsible for the B-class transient IR-absorption bands with maxima at 0.15 –0:36 eV in NaI, NaBr, NaCl : I, NaBr : I, KCl : I, KBr : I, RbCl : I and
RbBr : I. Both classes of the transient IR-absorption bands have the same location in energy scale, a very similar shape, half-width and vibration (phonon) structure. Because of the volume compensation eEect (reduction of the internal stresses) in the crystal the alkali impurity cations [M+ ]0c of smaller size in heavily iodine-doped alkali halide crystals AX : I (1 mol%) are expected to be mainly associated with the larger iodine dimers [I− ; I− ]aa in close pairs {[M+ ]0c : : : [I− ; I− ]0aa }. In the UV-laser photoexcitation experiments for the AX : I crystals, performed by Hirota et al., (1994, 1995), Edamatsu and Hirai (1997) and Shirai and Kan’no (1996), it is expected that the electron transfer within the pairs {[M+ ]0c : : : [I− ; I− ]0aa } takes place with a high yield, and as a result the transient donor–acceptor pairs {[M+ ]0c e− : : : [I2− ]+ aa } are formed (reaction (1)). Subsequent electron–hole recombination within such pairs can explain the correlated decay kinetics of the IR bands (the B-class IR bands) and luminescence (reaction (2)). The Mollwo–Ivey plot curves of the IR-absorption band zero-phonon line energy E0 values (for NaCl, KCl, KBr, RbCl, as well as for NaBr, KCl : I) and=or the IR-band low-energy edge energy E0 (±0:03 eV) values (for NaI, KBr : I, RbCl : I, RbBr : I) versus the anion–cation nndistance d are demonstrated for the Brst time. Two types of the atomic alkali impurity centres in AX hosts are predominant: (i) the [Na+ ]0c e− centres in the KX and RbX host crystals with the Mollwo–Ivey relation E0 ≈ 6:15=d2:74 and (ii) the [Li+ ]0c e− centres in the NaX host crystals with the Mollwo–Ivey relation E0 ≈ 29:4=d4:72 . The higher exponent n values in the case of the [Li+ ]0Na e− centre relative to the [Na+ ]0K e− and [Na+ ]0Rb e− centres evidence that the electron trap [Li+ ]0Na is shallower (and the electron wave function is more extended and diEuse) than the traps [Na+ ]0K and [Na+ ]0Rb . The electron localisation depth and the exponent n in the above alkali impurity traps is highly determined by the free alkali impurity atom (M0 ) and free host alkali metal atom (A0 ) ionisation potential diEerences NU (M0 ; A0 ). Such a correlation between the NU and n values in the case of the NaX host crystals (NU (Li0 ; Na0 ) ≈ 0:25 eV and n ≈ 4:72) and the KX and RbX host crystals (NU (Na0 ; K 0 ) ≈ 0:80 eV, NU (Na0 ; Rb0 ) ≈ 0:96 eV and n ≈ 2:74) is indeed observed. The Mollwo–Ivey plot—the F centre optical binding energy E0 (±0:2 eV) in alkali halides (the published data) versus anion–cation nn distance d gives the relation E0 ≈ 16:8=d2:64 . References Edamatsu, K., Hirai, M., 1997. Infrared studies of self-trapped excitons. Mater. Sci. Forum 239 –241, 525–530. HRartel, H., LRuty, F., 1964. Zur bildungskinetik der F-folgezentren in KCl. Z. Phys. 177, 369–384. Hirota, S., Edamatsu, K., Kondo, Y., Hirai, M., 1994. The transient absorption due to self-trapped excitons localized at
V. Ziraps / Radiation Measurements 33 (2001) 779–783 iodine dimers in alkali halide crystals. J. Phys. Soc. Japan 63, 2774–2779. Hirota, S., Edamatsu, K., Kondo, Y., Itoh, T., Hirai, M., 1995. Infrared transient absorption and electronic state of localized self-trapped excitons in KCl : I. Phys. Rev. B 52, 7779–7782. Ishii, T., Endo, T., 1968. Photochemical reaction and photocoductivity of additively colored KCl crystals at low temperature. J. Phys. Soc. Japan 24, 524–533. Jacobs, G., 1985. Transient Ie centres in alkali halides. Phys. Stat. Sol. B 129, 755–762. Korovkin, E.V., Lebedkina, T.A., 1993. Infrared optical absorption of polarons in gamma-irradiated NaCl, KCl, KBr crystals. Fiz. Tverd. Tela (Russian) 35, 642–647. Lynch, D.W., Robinson, D.A., 1968. Study of the F center in several alkali halides. Phys. Rev. 174, 1050–1059. Park, K., Faust, W.L., 1966. Infrared absorption spectrum of the ∗ F center in KI. Phys. Rev. Lett. 17, 137–138. Schneider, I., 1978. Electron trapping by alkali-ion impurities in alkali halides. Solid State Commun. 25, 1027–1029.
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Shirai, M., Kan’no, K., 1996. Transient absorption due to the triplet relaxed excitons of one-center type in RbCl : I. In: Schreiber, M. (Ed.), Proceedings of the Second International Conference on Excitonic Processes in Condensed Matter. Dresden University Press, Dresden, pp. 215–218. Zhang, C.G., Leung, C.H., Song, K.S., 1994. Two-electron defect system in ionic crystals: application to F centres in alkali halides. J. Phys.: Condens. Matter 6, 7715–7723. Ziraps, V., 1996. On the nature of the transient IR-absorption bands at 0.2–0:5 eV in alkali halide crystal. The 13th International Conference on Defects in Insulating Materials. Program and Abstracts. Wake Forest University, Winston–Salem, NC, USA, p. 268. Ziraps, V., 2000. Shallow electron traps in alkali halide crystals. The 14th International Conference on Defects in Insulating Materials. Programme and Abstracts. Eskom Conference Centre, Johannesburg–Midrand, South Africa, p. 65.
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Mollwo–Ivey relations for optical absorption bands of the atomic and F centres in alkali halides V. Ziraps ∗ Institute of Solid State Physics, University of Latvia, 8 Kengaraga St., LV-1063 Riga, Latvia Received 20 August 2000; received in revised form 22 January 2001; accepted 23 March 2001
Abstract Evidence indicates that two classes of the transient IR-absorption bands: (a) with maxima at 0.27–0:36 eV in NaCl, KCl, 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, 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+ ]0c e− , i.e. an electron e− trapped by a smaller size substitutional alkali cation impurity [M+ ]0c . The Mollwo–Ivey plots for the transient IR-absorption bands of the zero-phonon line energy E0 for NaCl, KCl, KBr, RbCl and NaBr, KCl : I and=or the low-energy edge values E0 for NaI, RbCl : I and RbBr : I versus anion–cation distance, d, are obtained for the Brst time. These data suggest that two types of the [M+ ]0c e− centres are predominant: (i) [Na+ ]0c e− in KX and RbX host crystals with the relation E0 ≈ 6:15=d2:74 and (ii) [Li+ ]0c e− in NaX host crystals with E0 ≈ 29:4=d4:72 . The Mollwo–Ivey relation E0 ≈ 18:36=d2:70 is fulBlled as well for the F c 2001 Elsevier Science band in NaCl, KCl, KBr, KI, RbCl, RbI if we use the F centre optical binding energy values for E0 . Ltd. All rights reserved. Keywords: Alkali halides; F centres; Self-trapped excitons; Transient IR-absorption bands
1. Introduction Shallow electron traps (electrically neutral defects Dn ) in alkali halide crystals play a very important role in many low-temperature (from LHeT to RT) thermally stimulated relaxation phenomena. Under ionising irradiation, electrons (e− ) are eEectively trapped by the neutral traps Dn forming the negatively charged centres Dn e− . A large number of these centres are formed in close association with Vk -type centres resulting in dipolar correlated pairs {Dn e− : : : Vk }, {Dn e− : : : VkA } and {Dn e− : : : Vk (XY− )}, where A and X are the alkali and halogen atoms in alkali halide crystal AX, respectively, and Y the impurity halogen atom in AX crystal.
∗
Fax: +371-711-2583. E-mail address: [email protected] (V. Ziraps).
If the alkali impurity cations (M+ ) of smaller size substitute for the larger host lattice cations (A+ ) in an alkali halide (AX) crystal, then shallow electron traps, i.e., neutral centres which capture electrons, like [M+ ]0c , can be formed. During ionising irradiation, electrons are trapped by such neutral centres, thus forming the negatively charged centres like [M+ ]0c e− . The centres [M+ ]0c e− are alkali impurity atoms placed in the cation sites and therefore, could be called as atomic alkali impurity centres. Consequently, the main neutral traps for electrons in alkali halides are F centres and atomic alkali impurity centres [M+ ]0c . Under ionising irradiation, the formation and accumulation of the isolated charged colour centres F and [M+ ]0c e− , as well as their dipolar associations with Vk -type centres, such as {F : : : Vk }, {F : : : VkA }, {F : : : Vk (XY− )}, {[M+ ]0c e− : : : Vk }, {[M+ ]0c e− : : : VkA }
c 2001 Elsevier Science Ltd. All rights reserved. 1350-4487/01/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 1 ) 0 0 1 0 6 - 8
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V. Ziraps / Radiation Measurements 33 (2001) 779–783
and {[M+ ]0c e− : : : Vk (XY− )}, are expected with a very high yield. In this paper, we report our results of a correlative analysis of data published since 1966 by several research groups concerning the transient IR-absorption bands with maxima at 0.27–0:36 eV in the F or M band photoexcited additive or subtractive coloured crystals NaCl, KCl, KBr, KI and RbCl (A-class IR bands) and in the UV-laser excited crystals NaCl : I, KCl : I, RbCl : I and RbBr : I (B-class IR bands). We show that the same Mollwo–Ivey plots are fulBlled for both classes of the IR bands, if we plot not the IR band peak energy values but the more distinct and deBnite values of the IR band zero-phonon line energy E0 and=or the IR band low-energy edge energy E0 . These Mollwo–Ivey power law relations for the IR bands conBrm our previous suggestions and thesis (Ziraps, 1996) that both classes of the above transient IR-absorption bands are caused by the same type defects and are rather due to transitions of the shallow trapped electron in the atomic alkali impurity centre [M+ ]0c e− . It is also shown that the Mollwo–Ivey relation is approximately fulBlled for the F bands in alkali halides. 2. Experimental results and discussion Two classes (marked as A and B) of the photoinduced transient IR-absorption bands at LHeT or LNT with maxima in the range from 0.27 to 0:36 eV in NaCl, KCl, KBr, KI and RbCl (the A-class IR bands) and in the iodine-doped crystals NaCl : I, NaBr : I, KCl : I, KBr : I, RbCl : I and RbBr : I, as well as in NaI and NaBr (the B-class IR bands) have been detected and investigated in several laboratories. The A-class IR bands with maxima at 0.27–0:36 eV in additively or subtractively coloured NaCl, KCl, KBr, KI and RbCl crystals were detected during a pulsed photoexcitation of the F or M centres and were related with the shallow trapped electrons (Park and Faust, 1966; Schneider, 1978; Jacobs, 1985) or bound polarons (Korovkin and Lebedkina, 1993). The B-class IR bands with maxima at 0.15 –0:36 eV were detected during pulsed UV-light excitation by an excimer laser and were related to the on-centre STE at the iodine-dimer in heavily iodine-doped (about 1 mol%) crystals NaCl : I, NaBr : I, KCl : I, KBr : I, RbCl : I and RbBr : I (Hirota et al., 1994, 1995; Edamatsu and Hirai, 1997; Shirai and Kan’no, 1996) and in NaI and NaBr—related to the on-centre STE (Edamatsu and Hirai, 1997). The published data relating to the A-class and B-class transient IR-absorption bands have been summarised, compiled and carefully analysed (Ziraps, 1996, 2000). From this analysis, Ziraps showed that there is a very close correlation between the A-class and B-class IR bands. The IR bands are similar in band location, shape, vibration (phonon) structure and other features. For KCl : I, the IR bands exactly coincide at 10 or 80 K and overlap with a small shift for RbCl : I, KBr : I and NaCl : I at 10 K. The corresponding transient IR-band is at 0.27–0:36 eV at 11–15 K in KCl, RbCl, KBr
and NaCl and are claimed to be due to shallow trapped electrons (Park and Faust, 1966; Jacobs, 1985; Schneider, 1978) or bound polarons (Korovkin and Lebedkina, 1993). In addition, the vibration structure of the transient IR-band in KCl : I at 10 K (Hirota et al., 1995) is very similar to the vibration structure of the transient IR band due to the shallow trapped electrons in KCl at 4.2–21 K (Jacobs, 1985; Korovkin and Lebedkina, 1993). Schneider (1978) and Ziraps (1996, 2000) showed that these shallow traps in the alkali halide AX crystal are caused by substitutional alkali impurity cations [M+ ]0c of smaller size than the host crystal cations A+ . Because of the free atom ionisation potential diEerence NU = U (M0 ) − U (A0 ) ¿ 0, the neutral isoelectronic impurity centre [M+ ]0c can trap an electron and form a charged point defect [M+ ]0c e− , the so-called “atomic alkali impurity” centre. We propose that these data indicate that the B-class IR bands (with maxima at 0.15 –0:36 eV) proposed as electron transitions in the on-centre STE localised at the iodine dimer in ACl : I and ABr : I (Hirota et al., 1994, 1995; Edamatsu and Hirai, 1997; Shirai and Kan’no, 1996) or the on-centre STE in NaBr and NaI (Edamatsu and Hirai, 1997) crystals are rather due to electrons trapped by shallow traps, i.e., during the UV-laser excitation of the iodine-dimers [I2− ]+ aa and subsequent electron transfer to neighbouring shallow traps [M+ ]0c : {[M+ ]0c : : : [I− ; I− ]0aa } + h (6:42 eV) → {[M+ ]0c : : : [(I2= )0aa ]∗ } → {[M+ ]0c e− : : : [I2− ]+ aa }:
(1)
Thus donor-type centres [M+ ]0c e− and=or their associations with the genetic hole centres [I2− ]+ aa , i.e., donor–acceptor pairs {[M+ ]0c e− : : : [I2− ]+ aa } can be formed with a high yield. We have found, and in this paper demonstrate, that even for these very wide and asymmetric absorption (transient IR and F ) bands in alkali halides, the Mollwo–Ivey relation E0 ≈ a=dn (where d is the nn anion–cation distance, a the constant, n the exponent) is valid. The data (E0 values) for the Mollwo–Ivey plots (Figs. 1 and 2) are taken from the published literature (references are given in Bgure captions). If we take not the band peak energy values but the more deBnite values of the zero-phonon line energy E0 values (for the IR bands) and=or the band low-energy edge energy E0 values for the IR bands and the F bands, then the Mollwo– Ivey relation is valid for the transient IR-absorption bands (Fig. 1) and F bands (Fig. 2). The Mollwo–Ivey plot curves (Fig. 1, curves 1, 2) of the IR band zero-phonon line energy E0 values—for NaCl, KCl, KBr, RbCl (Jacobs, 1985; Korovkin and Lebedkina, 1993) and NaBr, KCl : I (Hirota et al., 1995; Edamatsu and Hirai, 1997)—and=or the IR band edge energy E0 (±0:03 eV) values—for NaI, RbCl : I, RbBr : I (Hirota et al., 1994; Edamatsu and Hirai, 1997; Shirai and Kan’no, 1996)— versus anion–cation nn-distance d evidence that two types of the atomic alkali impurity centres [M+ ]0c e− in the AX
V. Ziraps / Radiation Measurements 33 (2001) 779–783
Fig. 1. The Mollwo–Ivey plots of the zero-phonon line and=or the low-energy edge energy E0 values of the transient IR-bands (with maxima at 0.15 –0:36 eV) versus the anion–cation nn distance d (in 0:1 nm): (1) for NaX host crystals: NaCl data from Jacobs (1985), Schneider (1978) and Korovkin and Lebedkina (1993) and NaCl : I, NaBr and NaI data from Hirota et al. (1994) and Edamatsu and Hirai (1997). (2) for KX and RbX host crystals: KCl, KBr and RbCl data from Jacobs (1985); KCl data from Schneider (1978); KCl and KBr data from Korovkin and Lebedkina (1993); KI data from Park and Faust (1966) and RbCl : I and RbBr : I data from Hirota et al. (1994, 1995), Edamatsu and Hirai (1997) and Shirai and Kan’no (1996).
Fig. 2. The Mollwo–Ivey plots of the F centre optical binding (the F absorption band low-energy edge) energy E0 (±0:2 eV) values versus the anion–cation nn distance d (in 0:1 nm) for alkali halide crystals: KCl, KBr, KI, RbCl and RbI data from Lynch and Robinson (1968); NaCl and KCl data from Zhang et al. (1994) and KCl data from HRartel and LRuty (1964) and Ishii and Endo (1968). The Mollwo–Ivey power law E0 ≈ 18:36=d2:70 is approximately valid.
host crystals are predominant (Fig. 1): (i) [Li+ ]0c e− centres in NaX host crystals with the relation E0 ≈ 29:4=d4:72 (curve 1); (ii) [Na+ ]0c e− centres in KX and RbX host crystals with the relation E0 ≈ 6:15=d2:74 (curve 2).
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We demonstrate (Fig. 2) that the Mollwo–Ivey relation is fulBlled as well for the F band low-energy edge energy (the optical binding energy) E0 (±0:2 eV) values for the NaCl, KCl, KBr, KI, RbCl, RbI crystals (the data are taken from Lynch and Robinson, 1968; Zhang et al., 1994; HRartel and LRuty, 1964). A relation E0 ≈ 18:36=d2:70 is approximately valid for the F bands. The exponent n values in the above relations give information about the electron localisation depth in these neutral traps and the electron wave function spatial distribution (the diEuseness and extension of the electronic states) around the point defect. In alkali halide crystals for electrons trapped in the anion site defects (with the defect eEective charge q = +1), the exponent n is below 2.0, e.g., for the F centres, U centres, [Cu− ]0a , [Ag− ]0a and [Au− ]0a the exponent n values are in the range n ≈ 0:4–2.0. This means that there is a relatively deep electron localisation. In the case of electrons trapped by neutral traps—for the atomic alkali impurity centres [M+ ]0c e− the n values are higher: from n ≈ 2:74 (for the [Na+ ]0K e− and [Na+ ]0Rb e− centres in the KX and RbX hosts) to n ≈ 4:72 (for [Li+ ]0Na e− in the NaX hosts). The higher n values for the [Li+ ]0Na e− centres relative to the [Na+ ]0K e− and [Na+ ]0Rb e− centres indicate that the trap [Li+ ]0Na is shallower (and the electron wave function is more extended and diEuse) than the traps [Na+ ]0K and [Na+ ]0Rb . In alkali halide (AX) crystals, the electron localisation depth (and exponent n values) in the [M+ ]0c e− centres is determined by the free alkali impurity atom (M0 ) and free host alkali metal atom (A0 ) ionisation potential differences NU (M0 ; A0 ): viz., NU (Na0 ; K 0 ) ≈ 0:80 eV, NU (Na0 ; Rb0 ) ≈ 0:96 eV and NU (Li0 ; Na0 ) ≈ 0:25 eV. Such a correlation between the NU and n values in the case of the NaX host crystals (NU (Li0 ; Na0 ) ≈ 0:25 eV and n ≈ 4:72) and in the case of the KX and RbX hosts (NU (Na0 ; K 0 ) ≈ 0:80 eV, NU (Na0 ; Rb0 ) ≈ 0:96 eV and n ≈ 2:74) is indeed observed. It must be stressed that the optical absorption bands for both classes of shallow trapped electrons (the F centres and the [M+ ]0c e− centres) have some distinctive and similar features. Both bands (the F band and the transient IR-band) are distinctively asymmetric and have a large relative half-width k = W1=2 =Emax , where W1=2 is the half-width and Emax the peak energy of the band. For the F band, the parameter k decreases from the values k ≈ 0:9–1.3 (RbX, KX) to k ≈ 0:4 (NaCl) in the series RbBr, RbCl, KBr, CsF, KCl, KCl : Na and NaCl in which the F centre electron binding energy E0 increases. In the case of the transient IR bands for the atomic alkali impurity centres the parameter k ≈ 0:32– 0.65 (NaX, KX, RbCl, RbCl : I, RbBr : I). Let us consider how we can explain the coincidence of the B-class IR band location, shape and structure in the iodine-doped alkali halide (AX) crystals ACl : I and ABr : I, UV-laser excited at LHeT or LNT, with the corresponding A-class IR band location, shape and structure in the additive coloured crystals ACl and ABr, photoexcited in F or M
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bands at LHeT or LNT. Moreover, how we can explain the correlated decay kinetics ( 6 1:0 s) of the transient IR-absorption band and the UV-luminescence (observed by Hirota et al., 1994, 1995; Edamatsu and Hirai, 1997) of the so-called on-centre STE localised at iodine dimer [(I22− )0aa ]∗ is as follows: It must be taken into account that because of the volume compensation eEect during the growth of the heavily iodine-doped (about 1 mol%) crystals, and=or subsequent thermal treatment, the accidental (trace) alkali impurity cations [M+ ]0c of smaller size (like Li+ in NaCl : I; Li+ and Na+ in KCl : I, KBr : I, RbCl : I and RbBr : I) are expected to be predominantly associated with the larger iodine dimers [I− ; I− ]0aa , thus reducing the lattice distortions, internal stresses and forming the neutral impurity-related defect pairs {[M+ ]0c : : : [I− ; I− ]0aa }. Then we can realise a formation in ACl : I and ABr : I crystals (during laser UV-photoexcitation of the iodine-dimers [I− ; I− ]0aa ) the donor–acceptor type dipolar pairs {[M+ ]0c e− : : : [I2− ]+ aa } with a very small lifetime (see reaction (1)). It results in the B-class transient IR-bands with maxima in the range of 0.23–0:36 eV, as observed by Hirota et al. (1994, 1995) and Edamatsu and Hirai (1997). Subsequent electron thermoactivated or tunnel transitions and recombination within such donor–acceptor type defect pairs {[M+ ]0c e− : : : [I2− ]+ aa } can explain the correlated decay kinetics ( 6 1:0 s) of the transient IR-absorption band and UV-luminescence quanta h emission (observed by Hirota et al., 1994, 1995; Edamatsu and Hirai, 1997) of the so-called on-centre STE localised at iodine dimer [(I22− )0aa ]∗ : + 0 2− 0 ∗ {[M+ ]0c e− : : : [I2− ]+ aa } → {[M ]c : : : [(I2 )aa ] } + 0 − − 0 → {[M ]c : : : [I ; I ]aa } + h :
(2)
In summary, the optical absorption and thermoactivation spectroscopy data evidence that the A-class and the B-class transient IR-absorption bands in the range of 0.27–0:36 eV are caused by the same defect—probably, the atomic alkali impurity centre [M+ ]0c e− . Both classes of the IR-absorption bands have a very similar shape, vibration (phonon) structure and coincide in energy scale, as well as fulBl the same Mollwo–Ivey power law.
3. Conclusions Evidence states that the alkali impurity ions M+ of smaller size can substitute the larger A+ cations in alkali halide AX crystals thus forming a shallow electron traps, i.e., the alkali impurity atomic centres [M+ ]0c e− . We propose that the same centre, i.e., a shallow electron trap—the atomic alkali impurity centre [M+ ]0c e− , responsible for the A-class transient IR-absorption bands with maxima at 0.27–0:36 eV in NaCl, KCl, KBr, KI and RbCl, is responsible for the B-class transient IR-absorption bands with maxima at 0.15 –0:36 eV in NaI, NaBr, NaCl : I, NaBr : I, KCl : I, KBr : I, RbCl : I and
RbBr : I. Both classes of the transient IR-absorption bands have the same location in energy scale, a very similar shape, half-width and vibration (phonon) structure. Because of the volume compensation eEect (reduction of the internal stresses) in the crystal the alkali impurity cations [M+ ]0c of smaller size in heavily iodine-doped alkali halide crystals AX : I (1 mol%) are expected to be mainly associated with the larger iodine dimers [I− ; I− ]aa in close pairs {[M+ ]0c : : : [I− ; I− ]0aa }. In the UV-laser photoexcitation experiments for the AX : I crystals, performed by Hirota et al., (1994, 1995), Edamatsu and Hirai (1997) and Shirai and Kan’no (1996), it is expected that the electron transfer within the pairs {[M+ ]0c : : : [I− ; I− ]0aa } takes place with a high yield, and as a result the transient donor–acceptor pairs {[M+ ]0c e− : : : [I2− ]+ aa } are formed (reaction (1)). Subsequent electron–hole recombination within such pairs can explain the correlated decay kinetics of the IR bands (the B-class IR bands) and luminescence (reaction (2)). The Mollwo–Ivey plot curves of the IR-absorption band zero-phonon line energy E0 values (for NaCl, KCl, KBr, RbCl, as well as for NaBr, KCl : I) and=or the IR-band low-energy edge energy E0 (±0:03 eV) values (for NaI, KBr : I, RbCl : I, RbBr : I) versus the anion–cation nndistance d are demonstrated for the Brst time. Two types of the atomic alkali impurity centres in AX hosts are predominant: (i) the [Na+ ]0c e− centres in the KX and RbX host crystals with the Mollwo–Ivey relation E0 ≈ 6:15=d2:74 and (ii) the [Li+ ]0c e− centres in the NaX host crystals with the Mollwo–Ivey relation E0 ≈ 29:4=d4:72 . The higher exponent n values in the case of the [Li+ ]0Na e− centre relative to the [Na+ ]0K e− and [Na+ ]0Rb e− centres evidence that the electron trap [Li+ ]0Na is shallower (and the electron wave function is more extended and diEuse) than the traps [Na+ ]0K and [Na+ ]0Rb . The electron localisation depth and the exponent n in the above alkali impurity traps is highly determined by the free alkali impurity atom (M0 ) and free host alkali metal atom (A0 ) ionisation potential diEerences NU (M0 ; A0 ). Such a correlation between the NU and n values in the case of the NaX host crystals (NU (Li0 ; Na0 ) ≈ 0:25 eV and n ≈ 4:72) and the KX and RbX host crystals (NU (Na0 ; K 0 ) ≈ 0:80 eV, NU (Na0 ; Rb0 ) ≈ 0:96 eV and n ≈ 2:74) is indeed observed. The Mollwo–Ivey plot—the F centre optical binding energy E0 (±0:2 eV) in alkali halides (the published data) versus anion–cation nn distance d gives the relation E0 ≈ 16:8=d2:64 . References Edamatsu, K., Hirai, M., 1997. Infrared studies of self-trapped excitons. Mater. Sci. Forum 239 –241, 525–530. HRartel, H., LRuty, F., 1964. Zur bildungskinetik der F-folgezentren in KCl. Z. Phys. 177, 369–384. Hirota, S., Edamatsu, K., Kondo, Y., Hirai, M., 1994. The transient absorption due to self-trapped excitons localized at
V. Ziraps / Radiation Measurements 33 (2001) 779–783 iodine dimers in alkali halide crystals. J. Phys. Soc. Japan 63, 2774–2779. Hirota, S., Edamatsu, K., Kondo, Y., Itoh, T., Hirai, M., 1995. Infrared transient absorption and electronic state of localized self-trapped excitons in KCl : I. Phys. Rev. B 52, 7779–7782. Ishii, T., Endo, T., 1968. Photochemical reaction and photocoductivity of additively colored KCl crystals at low temperature. J. Phys. Soc. Japan 24, 524–533. Jacobs, G., 1985. Transient Ie centres in alkali halides. Phys. Stat. Sol. B 129, 755–762. Korovkin, E.V., Lebedkina, T.A., 1993. Infrared optical absorption of polarons in gamma-irradiated NaCl, KCl, KBr crystals. Fiz. Tverd. Tela (Russian) 35, 642–647. Lynch, D.W., Robinson, D.A., 1968. Study of the F center in several alkali halides. Phys. Rev. 174, 1050–1059. Park, K., Faust, W.L., 1966. Infrared absorption spectrum of the ∗ F center in KI. Phys. Rev. Lett. 17, 137–138. Schneider, I., 1978. Electron trapping by alkali-ion impurities in alkali halides. Solid State Commun. 25, 1027–1029.
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Shirai, M., Kan’no, K., 1996. Transient absorption due to the triplet relaxed excitons of one-center type in RbCl : I. In: Schreiber, M. (Ed.), Proceedings of the Second International Conference on Excitonic Processes in Condensed Matter. Dresden University Press, Dresden, pp. 215–218. Zhang, C.G., Leung, C.H., Song, K.S., 1994. Two-electron defect system in ionic crystals: application to F centres in alkali halides. J. Phys.: Condens. Matter 6, 7715–7723. Ziraps, V., 1996. On the nature of the transient IR-absorption bands at 0.2–0:5 eV in alkali halide crystal. The 13th International Conference on Defects in Insulating Materials. Program and Abstracts. Wake Forest University, Winston–Salem, NC, USA, p. 268. Ziraps, V., 2000. Shallow electron traps in alkali halide crystals. The 14th International Conference on Defects in Insulating Materials. Programme and Abstracts. Eskom Conference Centre, Johannesburg–Midrand, South Africa, p. 65.