Spectroscopic studies of chromium-doped silica sol–gel glasses

Journal of Non-Crystalline Solids 288 (2001) 56±65

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Spectroscopic studies of chromium-doped silica sol±gel glasses W. Strez k a,*, P.J. Dere n a, E. èukowiak, J. Hanuza a, H. Drulis a, A. Bednarkiewicz a, V. Gaishun b a

Institute for Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wrocøaw, Poland b Gomel State University, Gomel, Belarus Received 1 August 2000; received in revised form 11 April 2001

Abstract The results of EPR and optical measurements of Cr-doped silica glasses obtained by means of sol±gel technology are presented. The e€ect of Cr concentration on optical behavior was studied. Thermoluminescence was observed after heating previously cooled and UV irradiated samples. The EPR measurements were performed for UV irradiated samples and a strong temperature dependence of the signal was noticed. The nature of optically active Cr centers was discussed. It is concluded that the EPR signal is associated with the Cr5‡ ions at tetrahedral sites whereas the emission is attributed to the ligand±metal charge transfer transitions of Cr6‡ ions coupled to Cr5‡ . No evidence was found for the presence of Cr3‡ ions in silica gel glass. Ó 2001 Elsevier Science B.V. All rights reserved.

1. Introduction In recent years a great interest has been observed in the study of the optical properties of SiO2 glasses obtained by sol±gel technology and containing optically active metal ions [1]. This technology makes it possible to obtain good optical quality monolithic `quartz' glasses with high concentration of active elements. It was found that during the gelation process metal ions can enter in di€erent valencies. The hydrolysis reaction creates the extreme chemical conditions leading to the changes of metal ion charges. For instance, different valencies have been observed for lantha-

* Corresponding author. Tel.: +48-71 343 50 21; fax: +48-71 344 10 29. E-mail address: [email protected] (W. Strez k).

nides like Ce [2] and Pr [3,4] ions in silica sol±gel glasses. Recently, we have reported the preliminary studies of luminescence properties of Cr-doped silica sol±gel glasses [5]. We have noted that the absorption spectra resemble those reported for tetrahedral Cr4‡ sites by Hommerich et al. [6]. However, no emission characteristic for Cr4‡ was measured and tentatively the observed visible emission was ascribed to the Cr3‡ ions. Unfortunately, a more careful analysis of the emission properties of Cr-doped silica gel glasses [7] has excluded a signi®cant contribution of Cr3‡ ions. Quite recently Ramanan and Ganguli [8] have studied the e€ect of pH and temperature on the absorption and EPR spectra of Cr-doped silica gels obtained from hexavalent chromium prepared via sols. They have shown that the chromium ions were incorporated as Cr3‡ ; Cr5‡ and Cr6‡ and have discussed the EPR signal in terms of Cr5‡ ion.

0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 6 1 0 - X

W. Strez k et al. / Journal of Non-Crystalline Solids 288 (2001) 56±65

The emission spectra of Cr-doped silica gel glasses were recently studied by Herren et al. [9]. In their preliminary report the authors discussed the luminescence behavior of sol±gel glass in terms of Cr5‡ . In a later report [10] they changed the interpretation following the earlier work of Hazenkamp and Blasse [11] on Cr-doped SiO2 as being due to the ligand±metal charge transfer transitions of Cr6‡ ions. In the present paper we report the studies of spectroscopic properties of Cr-doped silica gel glass to solve the problem of its optical behavior. In particular the e€ect of chromium concentration on optical spectra was investigated and the nature of Cr species co-existing in the silica sol±gel glass is discussed. 2. Experimental The samples of silica gel glasses were prepared from tetraethyl-orthosilicate Si…OC2 H5 †4 ± TEOS (10 wt%), distillated water, hydrochloric acid as catalytic agent and aerosil (®nely dispersed powdered SiO2 ). The preparation of monolithic xerogels was conducted by adding the aerosil into the sol which was neutralized with NH4 OH solution. The obtained xerogels were then impregnated by water solution of CrF3 and dried at 60°C. After this they were heated at the temperature range up to 1473 K in oxygen atmosphere. For further experiments we have used samples with nominal concentrations of Cr ions 0.01, 0.02 and 0.04 mol%. The color of the lowest concentration sample was white±yellow whereas for high concentration it changed to green. The obtained samples were transparent with the transmittance T ˆ 89% and T ˆ 78% for white±yellow and green sample, respectively. Transmittance was measured at 550 nm for 2.33 mm thick sample. In course of our experiments we have prepared also the silica glass impregnated with Cr…NO3 †3 salts. We did not observe signi®cant changes of absorption and emission spectra. The color of the samples was dependent only on the concentration of Cr ions. The obtained samples contained a signi®cant amount of hydroxyl groups as determined from IR absorption spectra to be 2000 ppm.

57

The absorption measurements were performed on a Cary Varian 2300 spectrophotometer at 300 K. The emission spectra were measured with a Jobin Yvon spectrophotometer THR 1000 using an argon laser ILA 120 (Zeiss Jena) as an excitation source. The excitation spectra were measured with an SLM Aminco SPF-500 spectro¯uorometer. The emission spectra in the range below 1 lm were recorded by means of the Bruker IFS 88 spectrometer equipped with Raman module FRA 106. Thermoluminescence was measured using the Ocean Optics spectrophotometer. The EPR measurements were made with the X-band spectrometer in the temperature range 4±300 K. The microwave frequency has been measured with a standard accuracy of 10 6 GHz. The magnetic ®eld resolution is higher than 10 2 mT. It allows to calculate the gfactor value with an accuracy of 10 4 . The standard errors of the linewidth and intensity measurements are shown in an appropriate ®gures.

3. Experimental results 3.1. Absorption spectra The absorption spectra of Cr-doped silica sol± gel glasses heated at 1300 K are illustrated in Fig. 1. They consist of two broadbands peaking at around 670 and 460 nm. The ®rst one is very di€use and extends from 550 to nearly 1300 nm.

Fig. 1. Absorption spectra of Cr-doped silica glasses obtained by sol±gel process for three concentrations of chromium ions heated at 1300 K.

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W. Strez k et al. / Journal of Non-Crystalline Solids 288 (2001) 56±65

The absorption spectra were measured for different concentrations of active ions up to 2000 nm. As can be observed the di€use band in red range extending from 550 to 900 nm has increased signi®cantly in intensity with the Cr content compared to the bands in violet and UV range. For heavydoped glass the red tail has extended up to 1200 nm. Analyzing the observed absorption band as a set of Lorentzial (see inset in Fig. 1) we have distinguished the four di€erent maxima as indicated in the inset. The most intense were found at 628, 701 and 808 nm. The less intense were located at 937 nm. We have noted also that the di€use band and the band at 460 nm changed in a similar manner with concentration in contrast to the band at 250 nm assigned to the Cr6‡ which did not increase so strongly. 3.2. Emission spectra The emission spectra of the Cr-doped silica sol± gel glasses were measured for di€erent concentrations of active ions (see Fig. 2). They were characterized by a broad, structureless band at 655 nm slightly dependent on concentration. Using as excitation line the 1.05 lm line of Nd:YAG laser the broadband emission with the maximum at 1.49 lm was observed. It is shown in Fig. 3. The possible origin of this emission band will be discussed later. For comparative purposes we have measured the emission spectra of the matrix glass obtained

Fig. 2. Emission spectra of Cr-doped silica sol±gel glasses measured for two di€erent concentrations (0.01 and 0.04 mol%) of chromium ions, kexc ˆ 308 nm, T ˆ 77 K.

under similar preparation conditions. No emission in infrared region was observed for undoped glass. The emission lifetimes of Cr-doped silica-gel glasses were measured for di€erent concentrations of active ions. The emission decay curves were earlier reported by us [7] for green, heavy-doped glass sample. We did not ®nd signi®cant changes of decay times with concentration. It means that there is no ecient energy transfer between emitting species leading to concentration quenching. 3.3. Thermoluminescence behavior In course of luminescence studies we have found that the Cr-doped silica gel glasses have demonstrated the thermoluminescence behavior after heating previously irradiated samples with an excimer laser (308 nm). In the beginning the sample did not exhibit any luminescence but with increasing temperature, approximately at 250 K it started to emit a red luminescence at 612 nm, the same shape as that one observed after laser excitation. The observed thermoluminescence is shown in Fig. 4. It was observed only after UV pumping with excimer laser 308 nm. 3.4. Excitation spectra To get insight into the nature of emission centers we have measured the excitation spectra recorded for di€erent concentrations of active ions.

Fig. 3. Emission spectra of Cr-doped silica sol±gel glasses measured for two di€erent concentrations of chromium ions excited at 1.06 lm.

W. Strez k et al. / Journal of Non-Crystalline Solids 288 (2001) 56±65

59

Fig. 4. Thermoluminescence behavior of Cr-doped silica sol± gel glasses.

The examples of excitation spectra measured at room temperature are illustrated in Fig. 5. The spectra consisted apart from the UV band also two characteristic intense and inhomogeneously broadened bands centered around 500 and 400 nm, and a smaller one at 280 nm which was observed distinctly only for smaller concentration of Cr ions. We have found that with increasing concentration the maxima of excitation bands have been shifted into the red. Moreover the maxima of red components increased with concentration while the blue decreased. One can also note that with increasing concentration the contribution from UV region also decreases what is manifested by a shift of the absorption edge associated with the MLCT bands of Cr6‡ into the blue. The excitation spectra as can be seen are inhomogeneously broadened which means that di€erent species may contribute to the observed visible emission. The nature of these species (will be shown later) is associated with the pair of Cr5‡ and Cr6‡ giving rise to the MLCT transition. 3.5. EPR spectra EPR experiments have been performed for a pure silica-gel glass sample (without Cr) and for two doped samples with 0.04 and 0.01 mol% of Cr content. In an undoped virgin sample the small, rather tiny EPR signal with g-values: gII ˆ 2:0000 and g? ˆ 1:9975 has been observed, respectively

Fig. 5. Excitation spectra of Cr-doped silica sol±gel glasses measured for di€erent concentrations of chromium ions.

Fig. 6. EPR spectrum of Cr-doped silica sol±gel glass.

(see the inset of Fig. 6). Its integral intensity increases a little bit after laser irradiation. The Cr-doped silica sol±gel glass samples showed the same EPR signal. However, after the low temperature UV irradiation of samples the additional, very intensive EPR signal appeared. In

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W. Strez k et al. / Journal of Non-Crystalline Solids 288 (2001) 56±65

Fig. 7. Temperature dependence of EPR signal linewidth of Crdoped silica sol±gel glass.

Fig. 8. E€ect of temperature on EPR signal intensity of Crdoped silica sol±gel glass.

Fig. 6 the EPR spectra registered at 70 K are shown. This radiation-induced signal is relatively broad with the peak-to-peak width of about 2:5  0:15 mT. The resonance line has a characteristic shape of EPR spectra measured for paramagnetic ions in crystalline ®elds with axial symmetry and with random orientations of their nearest polyhedra relative to external magnetic ®eld (powder like). The lineshape indicates that g? > gII where g? ˆ 1:968  0:001 and gII ˆ 1:883  0:001. The experimentally measured linewidth is slightly temperature dependent (Fig. 7). The integral line intensity is proportional to the Cr concentration and changes with temperature as shown in Fig. 8. At low temperatures the line intensity increases with increasing temperature and then it starts to decay, so at about 140 K the amplitude of the observed signal was comparable with a noise niveau of spectrometer.

Strong UV absorption bands at 320 and 250 nm may be attributed to the charge transfer transitions of Cr6‡ ion. The bands in visible and infrared range are di€erent than those assigned to the Cr3‡ doped glasses and resemble those observed for Cr-doped glass obtained under weak oxidizing conditions [6]. There the broad di€use band which peaked at around 670 nm has been assigned to Cr4‡ ion. A similar pattern of absorption spectrum was observed for Cr-doped Li2 MgSiO4 crystal by Anino et al. [12]. The authors have observed three di€erent Cr valence states of chromium …Cr4‡ ; Cr5‡ ; Cr6‡ † appearing simultaneously in this crystal. The bands centered at 460 and 670 nm have been assigned to the Cr4‡ ion. The di€use broadband located at the edge of the broadband with the maximum at 850 nm have been ascribed to the Cr5‡ ion. The peaks found at 628, 701 and 808 nm may be attributed to the tetrahedral Cr4‡ or octahedral Cr5‡ ions, The less intense were located at 937 nm and were assigned to the tetrahedral Cr5‡ sites. Changes with concentration of the di€use band and the band at 460 nm mean that the relative content of Cr6‡ ions compared to the Cr5‡ and Cr4‡ ions decreases with concentration.

4. Discussion of results 4.1. Absorption The shape of the absorption bands does not resemble the typical spectra of Cr3‡ -doped glasses, which are characterized by the two well-resolved maxima in the visible range combined with the spin-allowed 4 A2 ! 4 T2 and 4 A2 ! 4 T1 transitions.

4.2. Emission and excitation spectra The emission observed is reabsorbed by broad di€use absorption band (see Fig. 1). We did not

W. Strez k et al. / Journal of Non-Crystalline Solids 288 (2001) 56±65

observe however, any signi®cant changes of the emission band shape with concentration. It was found that the spectra excited by argon laser lines of 488 and 514 nm were almost identical to those measured by the 308 nm excimer laser line. The emission in the infrared region below 1000 nm up 2000 nm was assigned to the Cr4‡ or Cr5‡ centers. No thermoluminescence was observed after pumping with the argon laser 488 and 514 nm lines. It means that the thermoluminescence originates from the self-trapped electrons. The stored energy is then after heating recombined to the metal ligand charge transfer (MLCT) state. The thermoluminescence was relatively long lived. It was observed even a few seconds after switching o€ the laser illumination due to progressive heating of the sample. Such a long-lasting luminescence is most probably linked with the metastable center formed by capture of the UV generated electron at the oxygen vacancy (later discussed). Such a trapped electron resembles an F‡ -like center. Since no UV-induced thermoluminescence was observed for a pure silica sol±gel glass we assume that the mechanism of creation of a F‡ like center is associated with the Cr ion Cr ion ‡ UV quantum ± …Cr ion†‡ ‡ e

…1†

e ‡ oxygen vacancy ± F‡ -like center:

…2†

In course of measurements we have found that the strongest thermoluminescence has taken place for a small concentration of Cr ions. A decrease of thermoluminescence with increasing concentration is most probably associated with reabsorption process which increases with concentration. Regarding the excitation spectra it is important to note that there are no corresponding bands in the absorption spectra. It means that the emission species are associated, apart from Cr6‡ ions, with a small content of Cr ions of di€erent valencies. They are also not directly combined with the Cr species responsible for the absorption in the range of infrared di€use bands. The e€ect of concentration on optical spectra was observed to be most distinctive in the case of the excitation ones.

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4.3. EPR spectra This type of EPR spectra observed is connected with the so-called E0 -type centers which are detected in natural quartz and also in neutron- and c-irradiated samples. It is the simplest intrinsic point defect in quartz. The model for the E01 center in am- and crystalline SiO2 is an unpaired electron in a dangling sp3 hybrid orbital of pyramidal SiO33 where an oxygen ion, O is extracted from a SiO4 tetrahedron. Assignment of EPR spectra measured in glasses is sometimes rather complex. The EPR lines in glasses doped with Cr ions may be interpreted as resulting from Cr ions with di€erent valencies as well as resulting from possible di€erent resonance centers, for example the vacancy centers accompanying the doped ions replacing non-equivalent atoms in crystal. The EPR measurements were reported for many Cr-doped systems [13±17]. There appears usually the question of Cr ions in di€erent valencies with the glass under investigation responsible for the optical, electronic and EPR transitions. We did not observe the characteristic resonance associated with the single Cr3‡ ions expected at g  5 [18]. The resonance line at g  2 is often ascribed to the exchange of antiferromagnetic coupled pairs Cr3‡ ±Cr3‡ aggregating together if there is observed the concentration dependence of the g-factor. Another possibility that could explain a strong signal at g  2 is the d1 con®guration (Cr5‡ ) in tetrahedral site symmetry observed in the other glasses prepared under oxidizing conditions [14,15]. We accept this possibility which is in line with our optical measurements. The charge compensation mechanism and the temperature instability of the Cr5‡ state are discussed in the next section. 5. Nature of Cr valence states in silica gel glass The oxidation state of the chromium in sol±gel glasses as well as its co-ordination number is controversial. Dependencies on the heat treatment temperature and the ®nal pH value the chromium ions could be incorporated as Cr3‡ ; Cr5‡ and Cr6‡

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in gel matrix [8]. With increasing temperature the conversion Cr3‡ ! C6‡ is promoted. On the other hand the increase of the pH value causes an increase of the Cr5‡ content. Other studies have postulated an existence of the chromium in the sol±gel glass at 4+ oxidation state [2] because of the similarity of the absorption spectra to those reported by Hommerich et al. [6] for Cr4‡ -doped glass. The experimental conditions applied in our synthesis should lead to the formation of the chromium ions at highest oxidation states in spite of the fact that the starting substances contain the Cr2 O3 oxide. The obtained Cr-doped silica gel exhibits the absorption spectrum consisting of a structureless di€usive contour in the range 500± 900 nm with a maximum near 650 nm and a resolved band at 450 nm. The shape of this spectrum does not resemble that typical for Cr3‡ -doped glass. In our opinion the crucial role in the doped silica gel glass is played by the charge compensation e€ect on the rule of the radius ratios qe ˆ rH =r0 . The e€ective ionic radii extracted from Shannon's original work [18] for chromium ions at di€erent oxidation states and co-ordination numbers are listed in Table 1. The e€ective ionic radius re of Cr6‡ in fourfold  being the same as for Si4‡ co-ordination is 0.26 A Table 1 The dependence of e€ective ionic radii (re ) and radius ratios …he ) of Cr and Si ions on co-ordination number (CN) after Shannon [18] Ion

CN

re

he

Cr2‡

6(ls) 6(hs) 6 4 6 4 6 4 6 4 6 2 3 4 6 8

0.73 0.80 0.615 0.41 0.55 0.345 0.49 0.26 0.44 0.26 0.40 1.35 1.36 1.38 1.40 1.42

0.54 0.59 0.46 0.30 0.41 0.26 0.36 0.19 0.33

Cr3‡ Cr4‡ Cr5‡ Cr6‡ Si4‡ O2

 Therefore the substitution of the Si4‡ in (0.26 A). tetrahedral site by Cr6‡ is highly favorable given the high covalency of Cr(VI)±O bonds. In such a case the charge compensation of the CrO24 situated at SiO44 site is realized intrinsically by the formation of Si vacancies. One Si vacancy can compensate two Cr6‡ ions due to its fourfold valency. Therefore half of the Cr6‡ centers is compensated by the adjacent vacancy and the other half is compensated non-locally, giving the remaining electrons also to the Si vacancy complex and to the Cr6‡ ±Cr5‡ center. The re …Cr5‡ † in fourfold co-ordination is 0.345  A and it is much larger than that one corre A similar situation takes sponding to Si4‡ (0.26 A). place when the e€ective ionic radii for these ions in six-co-ordination polyhedrons are compared:  and re …Si4‡ †  re …Cr5‡ †CNˆ6 ˆ 0:49 A CNˆ6 ˆ 0:40 A. 5‡ Therefore both these substitutions of the Cr into tetrahedral and octahedral gaps are equally probable. The substitution of the Si(IV) in tetrahe Fe (0.49 A),  dral sites by Al(III) (re ˆ 0:39 A),   Ge(IV) (0.39 A) and Cr(IV) (0.41A) is well known in silicates. The relative stability of Cr5‡ at the tetrahedral site, as compared to those at the octahedral ones could be due to the higher covalency of the Cr(V)±O bonds in tetrahedral co-ordination. The prediction based on the radius ratio rule [19] also indicates that the most probable is substitution of the Cr5‡ ion into tetrahedral hole. The rH =r0 value for the CN ˆ 4 equals to 0.25 and for CN ˆ 6 to 0.35, i.e. both vary in the range 0:225 < qe < 0:414 characteristic for the tetrahedral co-ordination [19]. The charge compensation mechanism described in the present paper is based on the co-operation between the two valence states of chromium ions, Cr6‡ and Cr5‡ . The adjacent and non-local charge compensations of the doped ions make the equilibrium Cr6‡ ‡ e ! Cr5‡ very labile and the amounts of both centers can vary in dependence on preparation conditions. Our studies exhibit also that the photons can in¯uence the relative content of both ions in silica sol±gel glass. Several types of charge compensation appear in the Cr-doped silica-gel matrix. As it was mentioned above one Si vacancy compensates two Cr6‡ -doped ions. Another possibility is when Cr5‡

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63

Fig. 9. Compensational schemes of Cr ions in silica sol±gel matrix: (a) two doped Cr6‡ ions are compensated by one cationic vacancy, (b) four doped Cr5‡ ions are compensated by one cationic vacancy, (c) one vacancy compensates two Cr5‡ and one Cr6‡ ions.

ions are included. Then one vacancy compensates four Cr5‡ ions or two Cr5‡ ions and one Cr6‡ ion. 1 Fig. 9 illustrates the compensational schemes in which, apart of the Si4‡ ; Cr5‡ ; Cr6‡ and O2 , the OH anions should be taken into account, particularly for the fresh samples. All these ions participate in the oxidation±reduction process in the matrix. Schematically it can be described in the following way: ‰O3 CrV OHŠ

2

2.

3

‡ ‰O3 SiIV OHŠ 2

) ‰O3 CrVIˆOt Š

‡ ‰O3 SiIII Š

3

‡ H2 O

…3†

where Ot denotes the terminal double-bonded oxygen and  the oxygen vacancy. An increase of temperature strongly in¯uences the Cr5‡ ! Cr6‡ oxidation. The redox process given in such a manner explains the several observations: 1. With increasing temperature up to 1000°C two features were noted: an increase of the UV absorption with the shift of the absorption edge towards longer wavelengths (350 nm), a pro1 The vacancies are manifested in IR and Raman spectroscopy. The stretching and bending vibrations of the SiO4 and SiOSi units are strongly bonded due to the oxygen vacancies of the silica framework. Moreover, the longwavelength shift of these bands in SiO2 glasses is observed in comparison to the SiO2 crystals [20].

3. 4.

5.

gressive disappearance of the band at 600 nm and an increase in the red transmission. All these e€ects indicate the generation of the higher oxidation states. With increasing heat treatment temperature the concentration of the Cr6‡ ions was found to increase with simultaneous decrease in the amount of Cr5‡ ions. This is seen from the behavior of the intensity of EPR signal at g  2 associated with Cr5‡ ion which decreases with increase of temperature. The intensity of the m(OH) infrared band decreases depending on the time of calcination and temperature. The high temperature heated sol±gel samples are highly amorphous indicating that the chromium ions are incorporated in the SiO2 network giving a high-defect structure. Cr6‡ has a considerable size and mass mismatch with Si4‡ . This substitution causes a charge imbalance that is not compensated by any additional charge-compensating dopant. The structure accommodates such a charge imbalance by forming defects in the structure, leading to structure distortion and in consequence to progressive broadening of the absorption bands up to amorphous type. The fresh silica-gel and the samples dried up to 80°C contain some amount of octahedrally co-ordinated Cr3‡ [8]. Heat-treated samples

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obtained under oxidative condition do not exhibit the presence of the Cr3‡ ions in absorption as well as EPR spectra. The presence of the Cr3‡ ions in fresh samples is allowed because of the OH ions present in the silica gel. Therefore it may be postulated that under synthesis of glassy materials the following process runs: CrN Ox OHm: ‡ SiIV O3 OH3 ) O ‡ SiIII O3 3 ‡ H2 O; ˆ CrN ‡1 Om: x

…4†

where x ˆ 3 or 5; N means the oxidation states 3+, 4+, 5+ and  is the oxygen vacancy. The heating at higher temperature shifts the reaction towards higher valency states and a progressive disappearance of the Cr3‡ and small amounts of Cr5‡ ions. The ®nal Cr±SiO2 amorphous system may contain only the highest states, i.e. Cr6‡ and probably trace amounts of Cr5‡ ions.

lanthanum zirconate titanate) ceramics. They have found an emission at 9000 cm 1 …1:1 lm† at 10 K and assigned it to be due to the octahedral Cr5‡ sites. We did not observe an emission in this range. The emission observed by us was observed at 1.48 lm and should be assigned to the tetrahedral Cr5‡ sites. The Cr5‡ …d1 † has two energy 2 T2 and 2 E terms. If we assume that the origin of two bands observed in the excitation spectra corresponds to the Cr5‡ at the tetrahedral symmetry we may simply assign them to these terms. The distance between them may be calculated from the peak maxima being something about 8500±7000 cm 1 . Such an energy transition was observed in absorption spectra therefore we can assign the emission band in infrared corresponding to the 1.48 lm to the 2 T2 ! 2 E emission transition of Cr5‡ at tetrahedral sites. The schematic con®guration co-ordinate diagram for the relaxation processes involving Cr6‡

6. Mechanism of optical relaxations Following the results of emission spectra we can conclude that the orange emission was observed under excitation of Cr-doped sol±gel silica glass in a broad excitation range. The spectra were identical for UV excitation as well as for 488 and 514 nm. Quite di€erent conclusions can be drawn from thermoluminescence data. We have noted that it occurred only after irradiating with the excimer laser. No thermoluminescence was observed for 488 and 514 nm argon laser excitations. Therefore we can conclude that there are some electron traps created at low temperature which after heating relax to the state giving rise to the orange emission. Following the previous discussion of the nature of Cr centers in silica gel glass and the results of absorption measurements we see that the dominant contribution to the optical transitions in the UV region arises from the CT transition of Cr6‡ ions. The weak absorption bands which were resolved in red and infrared region could be ascribed to the small amounts of the Cr5‡ centers. Such centers are present in the silica gel glass independent of the UV illumination. Recently Murakami et al. [21] have reported the near-infrared luminescence spectrum in chromium-doped sol±gel PLZT (lead

Fig. 10. Schematic energy level diagrams for Cr-doped silica sol±gel glass.

W. Strez k et al. / Journal of Non-Crystalline Solids 288 (2001) 56±65

and Cr5‡ ions is shown in Fig. 10. We deduce the following picture of relaxation processes in our system: …Cr6‡ †0 ‡ hmUV ) …Cr6‡ †CT ‡

…Cr6‡ †CT ) …Cr5‡ † ‡ e e ‡ oxygen vacancy ) F‡ -like center F‡ -like center () …Cr6‡ †CT thermal equilibrium ‡

…Cr5‡ † ) IR…1:48 lm† emission ‡ …Cr5‡ †0 …Cr5‡ †0 () …Cr6‡ †CT thermal equilibrium …Cr6‡ †CT () LMCT orange emission ‡

whereby …Cr5‡ † we denoted the photo-oxidized Cr5‡ to discriminate it from the Cr6‡ center. The ground level 2 E of Cr5‡ ion is in thermal equilibrium with the LMCT state of Cr6‡ ion so after relaxation to the ground state the excitation energy is transferred to the Cr6‡ ion. In conclusion our data conclusively demonstrate that the pair of coupled Cr6‡ and Cr5‡ ions give a dominant contribution to the visible emission observed in silica gel glass. References [1] R. Reisfeld, C.K. Joergensen, Struct. Bonding 77 (1992) 207. [2] A.A. Boiko, J. Legendziewicz, J. Sokolnicki, E. èukowiak, W. Strez k, E.N. Poddenezhny, J. Appl. Spectrosc. 62 (4) (1995) 18.

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[3] A.A. Boiko, E. èukowiak, J. Sokolnicki, J. Legendziewicz, W. Strez k, E.N. Poddenezhny, J. Appl. Spectrosc. 62 (4) (1995) 22. [4] G.E. Malashkevich, E.N. Poddenezhny, I.M. Melnichenko, A.A. Boiko, V.E. Gaishun, W. Strez k, J. Appl. Spectrosc. 62 (5) (1995) 54. [5] P. Dere n, E. èukowiak, M. Suszy nska, W. Strez k, J. Appl. Spectrosc. 62 (4) (1995) 53. [6] U. Hommerich, H. Eilers, W.M. Yen, J.S. Hayden, M.K. Aston, J. Lumin. 60&61 (1994) 119. [7] W. Strez k, E. èukowiak, P.J. Dere n, I. Trabjerg, Cz. Koepke, G.E. Malashkevich, V.I. Gaishun, Proc. SPIE, Tunable Solid State Lasers, vol. 3176, 1996, p. 249. [8] S.R. Ramanan, D. Ganguli, J. Non-Cryst. Solids 212 (1997) 299. [9] M. Herren, K. Yamanaka, N. Miyazaki, M. Morita, J. Lumin. 72±74 (1997) 417. [10] M. Morita, N. Miyazaki, S. Murakami, M. Herren, D. Rau, J. Lumin. 76±77 (1998) 238. [11] M.F. Hazenkamp, G. Blasse, J. Phys. Chem. 96 (1992) 3442. [12] C. Anino, J. Thery, D. Vivien, Proc. SPIE, Tunable Solid State Lasers, vol. 3176, 1996, p. 38. [13] R.H. Clarke, L.J. Andrews, H.A. Frank, Chem. Phys. Lett. 85 (1982) 161. [14] F. Durville, B. Champagnon, E. Duval, G. Boulon, F. Gaume, A.F. Wright, A.N. Fitch, Phys. Chem. Glasses 25 (1984) 126. [15] E. Moya, C. Zaldo, B. Briat, V. Topa, F.J. Lopez, J. Phys. Chem. Solids 54 (1993) 890. [16] F.J. Lopez, E. Moya, C. Zaldo, Solid State Commun. 76 (1990) 1169. [17] G. Fuxi, Optical and Spectroscopic Properties of Glass, Springer, Shanghai, 1992. [18] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. [19] F. Scordari, in: C. Giacovazzo (Ed.), Fundamentals of Crystallography, Ionic Crystals, International Union of Crystallography, Oxford University, Oxford, 1995, p. 403. [20] G. Lucovsky, W.B. Pollard, in: J.D. Joannopoulos, G. Lucovsky (Eds.),Vibrational Properties of Amorphous Alloys in Topics in Applied Physics, vol. 56, Springer, Berlin. [21] S. Murakami, M. Herren, D. Rau, T. Sakurai, M. Morita, J. Lumin. 83&84 (1999) 215.