Pulsed cathodoluminescence of two-alkali sodium potassium silicate ...

ISSN 1087-6596, Glass Physics and Chemistry, 2006, Vol. 32, No. 1, pp. 28–32. © Pleiades Publishing, Inc., 2006. Original Russian Text © A.F. Zatsepin, D.A. Zatsepin, V.I. Solomonov, V.B. Guseva, S.O. Cholakh, 2006, published in Fizika i Khimiya Stekla.

Pulsed Cathodoluminescence of Two-Alkali Sodium Potassium Silicate Glasses A. F. Zatsepin*, D. A. Zatsepin**, V. I. Solomonov***, V. B. Guseva*, and S. O. Cholakh* * Ural State Technical University, ul. Mira 19, Yekaterinburg, 620002 Russia ** Institute of Metal Physics, Ural Division, Russian Academy of Sciences, ul. S. Kovalevskoi 18, Yekaterinburg, 620219 Russia *** Institute of Electrophysics, Ural Division, Russian Academy of Sciences, ul. Komsomol’skaya 34, Yekaterinburg, 620216 Russia Received February 15, 2005

Abstract—Alkali silicate glasses of variable composition 22xNa2O · 22(1 – x)K2O · 3CaO · 75SiO2 with equimolecular replacement of sodium ions by potassium ions are investigated using pulsed cathodoluminescence. It is revealed that localized electronic states interact with vibrations of two types, namely, polarization vibrations of the silicon–oxygen network with the frequency ν0 = 820 cm–1 and bending vibrations of the modifier sublattice. At low concentrations of one of the alkali components (x < 0.1), bending vibrations are observed at two frequencies. These frequencies coincide with those of the corresponding vibrations in one-alkali systems containing Na (530 cm–1) and K (520 cm–1). At higher concentrations (x in the range ~0.14–0.86), there occur bending vibrations of the cationic sublattice with a frequency of 420 cm–1. This can be interpreted as a luminescence analog of two-alkali (mixed-alkali) effect. DOI: 10.1134/S1087659606010020

cence centers (O≡Si–O–…Me+ quasi-molecular complexes) associated with the spatially inhomogeneous distribution of alkali modifiers in the glass matrix. Apart from the band of L1 centers dominating in disordered alkali silicate matrices, the pulsed cathodoluminescence spectra contain the luminescence band attributed to L2 centers whose nearest environment has a more ordered structure [10–12]. The appearance of a fine structure in the pulsed cathodoluminescence spectra corresponding to L1 centers was interpreted as a manifestation of electron–vibration interactions of two types, namely, vibronic interactions (ν0 = 820 cm–1) and electron–phonon interactions with frequencies ν1 = 640, 530, and 520 cm–1 for lithium, sodium, and potassium glasses, respectively [10, 11]. The revealed dependence of the frequency of phonon modes contributing to the vibrational structure of the luminescence spectra of one-alkali glasses on the cation nature allows us to assume that the luminescence properties of L centers (localized electronic states) in mixed-alkali systems can turn out to be rather sensitive to the presence of modifiers of several types and their concentration.

INTRODUCTION A wide use of vitreous and amorphous materials in high-tech fields (such as industrial optics, microelectronics, laser and fiber-optics engineering, and radiation technologies), requires detailed knowledge of the properties of the electronic subsystem, because they determine a set of electronic optical characteristics of the material. In turn, the electronic spectrum depends substantially on the atomic structure of the material due to the static effects associated with the degree of disordering or the presence of microsized and nanosized heterogeneous regions and dynamic processes caused by the interaction of the electronic subsystem with atomic vibrations of the glass network [1, 2]. It is known that the mixed-alkali effect, which manifests itself in a nonadditive behavior of composition– property dependences, can be observed in complex silicate glasses containing several types of modifier ions in the silicon–oxygen network. This effect is revealed for a number of macroscopic properties (predominantly associated with the transport phenomena), such as diffusion, ionic electrical conduction, gas permeability, etc. [3–9]. In this respect, it is of interest to investigate the possible manifestation of similar nonadditive effects for electronic and optical properties of glasses. In our earlier works [10, 11], a number of alkali silicate glasses containing alkali cations of one type were studied using pulsed cathodoluminescence. It was found that the pulsed cathodoluminescence spectra exhibit bands corresponding to two types of L lumines-

In the present work, the pulsed cathodoluminescence spectra of silicate glasses containing modifier cations of two types were measured and their vibrational structure was studied in order to reveal a manifestation of the two-alkali effect in the luminescence characteristics. 28

PULSED CATHODOLUMINESCENCE

SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE Model silicate glasses of variable composition 22xNa2O · 22(1 – x)K2O · 3CaO · 75SiO2 (x = 0.00, 0.05, 0.09, 0.14, 0.20, 0.27, 0.41, 0.50, 0.59, 0.73, 0.80, 0.86, 0.91, 0.95, 1.00) with equimolecular replacement of sodium ions by potassium ions served as the objects of our investigation. Samples in the form of optically transparent plates 2–4 mm thick were prepared by melting the corresponding reactants in corundum crucibles at a temperature of 1450°C, followed by quenching and annealing at 400°C for 2 h. The buffer additive CaO (3 mol %) introduced for increasing the chemical durability of the glasses did not affect their luminescence properties [13, 14]. Iron oxide (≈10–2–10–4 wt %) was the majority impurity. The cathodoluminescence spectra were excited with a pulsed electron beam (electron energy, 180 keV; current density, 700 A/cm2; pulse duration, 2 ns) generated by a RADAN-220 compact accelerator [15, 16]. The luminescence spectrum with a width of 150 nm at the exit of the diffraction polychromator was recorded on a 512-element charge-coupled device interfaced with a computer. The absolute error in the measurement of the wavelengths was equal to 0.4 nm. The width of the instrumental function in the spectral range 25000– 28000 cm–1 was equal to 60 cm–1. The luminescence spectra were recorded in the wavelength range 350– 650 nm. We measured the luminescence spectra integrated over the time, t ph

I(λ) =

∫ I ( λ, t )dt,

(1)

0

where the lower and upper limits of integration corresponded to the initial instant of exposure of the sample to the electron beam and the time of recording by a photoreceiver tph, respectively. The spectra were recorded in two modes: at tph = 200 ms (I) and tph = 1–20000 µs (II). In the first mode, the integrated spectrum of the luminescence pulse was recorded and spectral information was averaged over 16 luminescence pulses. In the second mode, we measured the spectrum of the nearafterglow caused by single-pulse irradiation. In both cases, the accelerator start-up was synchronized with a pulse of the photoreceiver start. The electron beam transmitted through air induced a line emission spectrum of atmospheric gases predominantly in the range 400–550 nm. The intensity of the most intense lines was more than two orders of magnitude lower than that of the luminescence of glasses. Before measurements, the emission spectrum of atmospheric gases was recorded as a background and then was automatically subtracted from the measured luminescence spectrum of glasses. Upon exposure to one electron beam pulse, the temperature increased by 0.05 K in the irradiated sample GLASS PHYSICS AND CHEMISTRY

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Intensity, rel. units L1

L2

L1

(a)

L2 (b)

400

600 Wavelength, nm

Fig. 1. Pulsed cathodoluminescence spectra recorded in the pulse repetition mode for sodium potassium glasses at alkali cation contents x = (a) 0.41 and (b) 0.59. Fine structures of the pulsed cathodoluminescence spectra recorded in the single-pulse mode are shown for comparison.

and by no more than 1–10 K directly in the irradiated region. Therefore, the irradiation did not lead to evaporation of atoms (ions) from the sample. This was confirmed by the fact that no lines associated with the atoms and ions in the glass composition were revealed in the emission spectrum. RESULTS Figure 1 shows the pulsed cathodoluminescence spectra of the model alkali silicate glasses of variable composition with equimolecular replacement of sodium ions by potassium ions. As in [1, 2], the spectra measured in mode I (the pulsed cathodoluminescence spectra integrated over the recording time) contain two broad overlapping bands with maxima at wavelengths of 345–357 nm (3.5 eV) and 402 nm (3.1 eV). The energy positions of these bands correspond to luminescence ranges of L centers, and this luminescence is due to the electronic transitions in intrinsic localized states of the glass. The band at 3.5 eV is attributed to the L1 centers observed in disordered alkali silicate matrices [2]. The spectral characteristics of the band at 3.1 eV coincide with the photoluminescence parameters of the L2 centers, which are considered a variety of L centers with a more ordered structure of the nearest environment [2, 12]. The nature of the third band at 500 nm (2.5 eV) is not known and requires separate discussion. The large width of the luminescence bands can be associated with the local inhomogeneity of the glass struc2006

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ZATSEPIN et al. Intensity, arb. units

Intensity, rel. units

ν0 = 820 cm–1 (a) 4

ν2 = 420 cm–1

3

ν0 = 820 cm–1 2

(b)

ν2 = 420 cm–1

1 27000

28500 Wave number, cm–1

26000

Fig. 2. Evolution of the fine structure of the pulsed cathodoluminescence spectra of 22xNa2O · 22(1 – x)K2O · 3CaO · 75SiO2 sodium potassium glasses (x = 0.2) after completing the electron pulse. The time step is 1 µs. Single-pulse irradiation mode. Numbers near curves indicate the signal relaxation times in microseconds.

28000 Wave number, cm–1

Fig. 3. Fine structure of the pulsed cathodoluminescence spectra recorded in the single-pulse mode for 22xNa2O · 22(1 – x)K2O · 3CaO · 75SiO2 sodium potassium glasses at alkali cation contents x = (a) 0.2 and (b) 0.8. The time after completing the electron pulse is 2 µs.

ture and the corresponding differences in the luminescence characteristics. The luminescence spectra measured in mode II for the two-alkali glasses are characterized by a number of regularities similar to those observed for one-alkali glasses. Specifically, the fine structure (as in the spectra of one-alkali glasses) manifests itself in the range 340– 400 nm (Fig. 1). A rapid decrease in the intensities of the lines and the intensity redistribution in the fine structure of the spectra are most likely explained by the relaxation interaction of the electronic subsystem with lattice vibrations. The analysis of the spectra with due regard for the data on the time relaxation of the signals (Fig. 2) enabled us to separate the known group of lines with ν0 = 820 cm–1 (Fig. 3) in the spectra of the mixedalkali glasses. However, there are specific features in the spectra of the mixed-alkali glasses. In particular, the groups of lines at frequencies ν1 characteristic of onealkali glasses are observed in the spectra of the samples under investigation only at low degrees of substitution of one of the alkali modifiers Na2O or K2O (x = 0.05– 0.09) (Fig. 4). At higher degrees of substitution (x in the range from 0.14 to 0.86), these groups of lines virtually disappear, but there arises a new series of lines with ν2 = 420 cm–1 (Figs. 3, 4). The transformation of vibrational modes and the change in the vibrational frequencies are most pronounced in the vicinity of x = 0.50, i.e., at the degree of substitution at which the dependences

of the macroscopic properties of alkali silicate glasses exhibit a maximum associated with the two-alkali effect [3–9, 17]. DISCUSSION As in the case of one-alkali glasses, the results obtained in the study of the pulsed cathodoluminescence spectra of the two-alkali glasses indicate that their structure contains L centers of two types with different degrees of local ordering [12]. Upon excitation with a single electron pulse, the fine structure associated with the L1 centers in the near-afterglow spectrum, by analogy with [10], can be attributed to the interaction of the radiative electronic transition with localized vibrations of nonbridging oxygen atoms (ν0 = 820 cm–1). The frequency separations ν1 = 520 cm–1 (ν1 = 530 cm–1) and ν2 = 420 cm–1 should be interpreted as a manifestation of the electron–phonon interaction with the participation of bending vibrations of the modifier sublattice [11]. At low concentrations of one of the alkali oxides (x = 0.05–0.09), the observation of both vibrational frequencies, ν1 = 530 cm–1 and ν1 = 520 cm–1 (Fig. 4), which coincide with the frequencies of the phonon modes of L1 centers in the one-alkali glasses, revealed the coexistence of two types of isolated luminescence

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centers in the two-alkali systems: L1(Na) and L1(K) centers that are not substantially affected by dissimilar cations. In the framework of this approach, the corresponding band in the integrated spectrum is a superposition of the luminescence bands associated with the dissimilar discrete one-alkali centers L1(Me+) (Me = Na, K) and the difference between the vibrational frequencies of the lines in the fine structure of the pulsed cathodoluminescence spectra is explained by the specific features of vibronic interactions of particular onealkali centers. It should be noted that the above effect of small additives can manifest itself not only in the fine structure of the pulsed cathodoluminescence spectra but also in the concentration dependences of the lifetime of excited states of L centers [18]. At high degrees of substitution x = 0.14–0.86, owing to the high concentrations of the dissimilar alkali oxides in the amorphous matrix, cations can enter into a strong ion–ion (Na+…K+) interaction, which results in a nonadditive change in a number of physical properties of the glass, including the vibrational and electronic spectra. Actually, at comparable concentrations of different alkali cations, the fine structure of the luminescence spectrum involves not the lines of vibrations at the frequencies ν1(Na) and ν1(K) but the lines of vibrations at the new frequency ν2(Na, K) (Figs. 3, 4). This frequency ν2 differs from the frequencies ν1 of the one-alkali phonon modes and is not equal to the sum of these frequencies. The analysis demonstrates that, in this case, the laws of conservation of energy and momentum are violated and, hence, we cannot speak about the simultaneous interaction of two normal modes corresponding to vibrations of cations of a specific type. Previously, the appearance of the fine structure associated with the L1 centers in one-alkali glasses was explained by the interaction of lattice phonons with the electronic transition between s orbitals of alkali ions and 2p states of nonbridging oxygen atoms [10, 11]. According to the traditional model of an L1 center with the participation of one alkali ion (O≡Si–O–…Me+), it is most reasonable to expect that, at considerable concentrations of the second oxide, the spectrum characteristic of the “small additive effect” (Fig. 4) should be retained. The observed disappearance of two isolated modes and the appearance of the new mixed mode (Figs. 3, 4) in the fine structure can be explained within the extended model of an L center [12]. According to Trukhin [12], L centers are cluster groupings that consist of several alkali modifier cations located in the vicinity of O≡Si–O–…Me+ groups; i.e., the corresponding structural fragment can be represented in the form O≡Si–O–…nMe+, where n > 1. In two-alkali glasses at x = 0.14–0.86, cations of different types should be located, with a high probability, in the nearest environment of a nonbridging oxygen atom. In this case, the formula of the L center can be written as GLASS PHYSICS AND CHEMISTRY

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ν, cm–1 ν0 = 820 cm–1 800

600 ν1(Na) = 530 cm–1 ν1(K) = 520 cm–1

ν2(Na, K) = 420 cm–1

400 0 K

0.5 x

1.0 Na

Fig. 4. Dependences of the frequency separations ν0, ν1, and ν2 observed in the fine structure of the pulsed cathodoluminescence spectra on the alkali oxide content x for sodium potassium glasses.

+

+

O≡Si–O–…(n Me 1 /m Me 2 ). The lines corresponding to these centers in the fine structure of the pulsed cathodoluminescence spectra should be associated with the electronic transitions between 2p states of nonbridging oxygen atoms and electronic states formed by mixing s orbitals of dissimilar alkali cations. The presence of a single phonon mode (whose frequency differs from the vibrational frequencies in onealkali systems) in the fine structure of the luminescence spectra of the glasses with x = 0.14–0.86 indicates that the interaction of the radiative electronic transition with the initial one-alkali modes and the mixing of the initial modes in proportion to the degree of substitution of alkali cations are absent in two-alkali systems. The nonadditive interaction between the electronic transitions of L1 centers and vibrations of modifier cations in the glasses with x = 0.14–0.86 can be treated as a manifestation of the effect similar to the traditional twoalkali effect due to the nonadditive changes in the macroscopic properties and structure of glasses containing considerable amounts of the second alkali oxide [3–9, 17]. Therefore, the extended model of mixed-alkali centers L1 allows us to explain consistently the appearance of mixed vibrational modes in the pulsed cathodoluminescence spectra and their manifestation in vibronic 2006

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interactions involving the electronic states of tails of the energy bands. CONCLUSIONS Thus, the above investigations into the vibrational structure of the pulsed cathodoluminescence spectra of alkali silicate glasses of variable composition 22xNa2O · 22(1 – x)K2O · 3CaO · 75SiO2 revealed a luminescence analog of the two-alkali effect. The revealed effect is observed at high degrees of substitution (x in the range ~0.14–0.86) and can be attributed to the formation of mixed vibrational modes. The main regularities of the given effect can be explained within the modified model of L centers represented in the form O≡Si–O–… + + (n Me 1 /m Me 2 ). This model takes into account the interaction of nonbridging oxygen atoms with several modifier cations of the nearest environment. The small additive effect, which consists in retaining the individual vibrational modes characteristic of one-alkali glasses, is observed at low concentrations of the second alkali oxide. The results obtained for L luminescence centers demonstrated that the composition and atomic structure of alkali silicate glasses substantially affect the properties of the electronic subsystem. The data obtained give grounds to believe that the fine structure of the pulsed cathodoluminescence spectra can be used for evaluating the quality of optical materials when designing precision vitreous media. ACKNOWLEDGMENTS This work was supported by the Ministry of Education and Science of the Russian Federation, the US Civilian Research and Development Foundation for the New Independent States of the Former Soviet Union (Annex BF4M05, EK-005-X2 [REC-005], BRHE 2004 post-doctoral fellowship award Y2-EP-05-11), and the Russian Foundation for Basic Research (project no. 0502-16448). REFERENCES 1. Mott, N.F. and Davis, E.A., Electronic Processes in NonCrystalline Materials, Oxford: Clarendon, 1979. 2. Vainshtein, I.A., Zatsepin, A.F., and Kortov, V.S., Specific Features of the Urbach Rule Manifestation in Vitreous Materials, Fiz. Khim. Stekla, 1999, vol. 25, no. 1, pp. 85–95 [Glass Phys. Chem. (Engl. transl.), 1999, vol. 25, no. 1, pp. 67–74].

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