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Upconversion fluorescence in Er3+-doped Na2O–GeO2 glasses
Journal of Non-Crystalline Solids 259 (1999) 57±62
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Upconversion ¯uorescence in Er3 -doped Na2O±GeO2 glasses Hideo Yamada 1, Kazuo Kojima * Department of Chemistry, Faculty of Science and Engineering, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan
Abstract Electron spin resonance (ESR) and upconversion ¯uorescence spectra have been investigated for Er3 -doped Na2 O± GeO2 glasses with molar compositions of xNa2 O á (100 ÿ x)GeO2 á 0.7Er2 O3 prepared by the melt quenching method. The g-value for Er3 derived from ESR spectrum and both the peak position and the integrated intensity of upconversion ¯uorescence spectra changed in the so-called germanate anomaly region of the glass. The upconversion ¯uorescence intensity correlated to the electron±phonon coupling strength. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction In oxide glasses when compared to ¯uoride glasses, it is generally dicult to generate upconversion ¯uorescence due to the doped rare-earth ions, since the phonon energies of oxide glass networks are larger than in ¯uoride glasses in which upconversion is observed [1±3]. However, upconversion ¯uorescence has been observed in some oxide glasses such as germanate, tellurite and gallate glasses that have relatively smaller phonon energies [4±10]. We have reported upconversion ¯uorescence of Er3 -doped GeO2 -based glasses prepared by the sol±gel method [11±13]. In this paper we describe electron spin resonance (ESR) and upconversion ¯uorescence * Corresponding author. Tel.: +81-77 561 2780; fax: +81-77 561 2659. E-mail address: [email protected] (K. Kojima) 1 Present address: New Glass Research Division, Yamamura Glass Co., Ltd., 2-1-18, Naruohama, Nishinomiya, Hyogo 6638142, Japan.
spectra of Er3 -containing Na2 O±GeO2 glasses prepared by the melt quenching method. The results are discussed in connection with the so-called germanate anomaly which means that the density or refractive index reached a maximum around 15 mol% Na2 O [14±16], and in connection with phonon energies and electron±phonon coupling strengths. 2. Experimental procedures Germanate glasses with molar compositions of xNa2 O á (100 ÿ x)GeO2 á 0.7Er2 O3 and similar glasses containing 5 mol% of Er2 O3 or Eu2 O3 were prepared from Na2 CO3 , GeO2 , Er(NO3 )3 á 6H2 O and Eu2 O3 . The mixed powders were melted in a platinum crucible at 1400°C for 30 min. The liquid was poured on a stainless steel plate and pressed by another stainless steel plate from the top. Two surfaces of a sample which are at right angles to each other were polished on a paper coated with 3000 mesh corundum prior to use for upconversion ¯uorescence measurements.
0022-3093/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 4 9 5 - 0
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The insertion in Fig. 1 shows ESR spectra, taken at 4.2 K, of xNa2 O á (100 ÿ x)GeO2 á 0.7Er2 O3 samples. A broad signal at about 80 mT is due to Er3 ions in the glass [18]. The corresponding signal for the sample with x 3 splits into some components, probably indicating that Er3 ions are inhomogeneously introduced in this glass with a smaller Na2 O content. The g-values evaluated
from the spectra, where g hm/b Hr, h the PlanckÕs constant, m the microwave frequency, b the Bohr magneton, and Hr is the magnetic ®eld at which the maximum amplitude of a component is observed, are given in Fig. 1 as a function of Na2 O content. The g-value decreases with increasing Na2 O content; the rate of decrease with increasing x changes around x 15 where the so-called germanate anomaly occurs. Fig. 2 gives upconversion ¯uorescence pro®le changes in the visible green region when a sample, 14Na2 O á 86GeO2 á 0.7Er2 O3 , was excited at positions located various distances from a glass surface upon which the upconversion ¯uorescence was observed. The spectral pro®le becomes ill-resolved as the distance increases because of the self-absorption of Er3 ions. Therefore, the excitation beam should be incident just upon the surface where
Fig. 1. Na2 O content dependence of g-values for Er3 evaluated from ESR spectra shown in the insertion, taken at 4.2 K, of xNa2 O á (100 ÿ x)GeO2 á 0.7Er2 O3 glasses. The line is drawn as a guide for the eye.
Fig. 2. Dependence of upconversion spectra on the distance (L) between the surface of the sample where upconversion ¯uorescence was detected and the position where an excitation laser beam was focused as shown in the insertion.
Upconversion ¯uorescence was collected perpendicular to the direction of the excitation beam from an 805 nm laser diode at room temperature. ESR spectrum was recorded at 4.2 K. Methods for optical and ESR measurements have been described elsewhere [12,17]. 3. Results
H. Yamada, K. Kojima / Journal of Non-Crystalline Solids 259 (1999) 57±62
59
upconversion ¯uorescence was measured. Upconversion ¯uorescence spectra were measuredin this manner for xNa2 O á (100 ÿ x)GeO2 á 0.7Er2 O3 glasses (Fig. 3). Green ¯uorescence bands around 525 and 550 nm are assigned to the transitions of 2 H11=2 ® 4 I15=2 and 4 S3=2 ® 4 I15=2 , respectively. A red ¯uorescence band due to the 4 F9=2 ® 4 I15=2 transition was also observed. Fig. 4 shows absorption spectra of a glass 30Na2 O á 70GeO2 á 5Er2 O3 measured at room temperature and 77 K. Four peaks were observed at 523, 534, 547 and 557 nm in the spectrum recorded at room temperature. However, two of them, 534 and 557 nm, were quenched at 77 K. Phonon side bands (PSB) accompanied by the electronic transition of 7 F0 ® 5 D2 for Eu3 (about 465 nm) in the glasses xNa2 O á (100 ÿ x) GeO2 á 5Eu2 O3 are shown in Fig. 5. PSB was measured to
Fig. 4. Absorption spectra of a glass 30Na2 O á 70GeO2 á 5Er2 O3 measured at room temperature and 77 K. The inset shows the assignments.
discuss the Na2 O content dependence of the intensity of the upconversion ¯uorescence. 4. Discussion
Fig. 3. Upconversion spectra of xNa2 O á (100 ÿ x)GeO2 á 0.7 Er2 O3 glasses.
Assuming a cubic crystal ®eld of six-fold coordination for Er3 , the ground state of the Er3 ion, 4 I15=2 , is split into two Kramers doublets (C6 and C7 ) and three quadruplets (C8 ). When the C6 and C7 is the lowest energy level, the g-values of the C6 and C7 states have been predicted theoretically to be 6.0 and 6.8, respectively [18±21]. Energies of the C6 and C7 states in single crystal of ErBa2 Cu3 O6 have been calculated to be nearly degenerate, assuming a cubic crystal ®eld [21]. Therefore, the C6 and C7 states may be energetically interchangeable. We suggest that the result shown in Fig. 1 may
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H. Yamada, K. Kojima / Journal of Non-Crystalline Solids 259 (1999) 57±62
Er3 in a single crystal [21]. From these results, the peaks of 523 and 534 nm are assigned to the transitions from the common lowest state of 2 H11=2 to the lowest state and the next lowest state of 4 I15=2 , respectively. In the same way, the peaks of 547 and 557 nm can be attributed to the transitions from the common lowest state of 4 S3=2 to the two same states of 4 I15=2 . These assignments can be also supported by the upconversion pro®le change shown in Fig. 2; the eect of self-absorption at the peaks of 523 and 547 nm is due to true ground state absorption transitions. The green upconversion spectrum illustrated in Fig. 3 was deconvoluted into four Gaussian component bands discussed above. Figs. 6 and 7 show peak positions and integrated intensities, respectively, of these component bands as a function of
Fig. 5. Absorption spectra of xNa2 O á (100 ÿ x)GeO2 á 5Eu2 O3 glasses showed zero phonon lines (ZPL) and phonon side bands (PSB).
indicate that Er3 ions in glasses with smaller and larger Na2 O contents have a lowest level of C6 and C7 , respectively, and that the interchange of C6 and C7 occurs in the germanate anomaly region. Each of the green upconversion ¯uorescence bands shown in Fig. 3 splits into two peaks for glasses with more than 14 mol% of Na2 O; for example, in the glass with x 30 there are four peaks at 523, 534, 547 and 557 nm. These four peaks were also observed in the absorption spectrum recorded at room temperature as seen in Fig. 4. At 77 K the two peaks of 534 and 557 nm were quenched. The energy dierence of about 390 cmÿ1 between the 523 and 557 nm peaks nearly corresponds with that of about 330 cmÿ1 between the 547 and 557 nm peaks. Actually, an energy difference of about 390 cmÿ1 has been calculated between the C8 and the degenerate C7 , C6 states for
Fig. 6. Na2 O content dependence of peak positions of upconversion ¯uorescence four component bands of xNa2 O á (100 ÿ x)GeO2 á 0.7Er2 O3 glasses. Lines are drawn as guides for the eye.
H. Yamada, K. Kojima / Journal of Non-Crystalline Solids 259 (1999) 57±62
Fig. 7. Na2 O content dependence of integrated intensities of upconversion ¯uorescence four component bands of xNa2 O á (100 ÿ x)GeO2 á 0.7Er2 O3 glasses. The symbols that indicate the four component bands correspond to those in Fig. 5. Lines are drawn as guides for the eye.
Na2 O content. The peak position did not change in a monotonic fashion in the germanate anomaly region and the splitting of each of the 2 H11=2 and 4 S3=2 bands increased with Na2 O content (Fig. 6). This increase is probably responsible for the enlargement of Stark splitting resulting from the increase of a local electric ®eld around the Er3 ions. The local ®eld increases as eective negative charges of oxygen atoms increase with the addition of Na2 O. While conversely, the integrated intensities gave maxima for all the component bands (Fig. 7). PSB spectra at about 800 cmÿ1 shown in Fig. 5, which agree with those of previous results [22,23], are ascribed to Ge±O stretching vibrations of the GeO4 tetrahedral structural unit involving the non-bridging oxygens. The zero phonon line (ZPL) and the PSB are deconvoluted into Gaussian bands and then both phonon energy and electron±phonon coupling strength are estimated (Fig. 8), the latter being evaluated as the ratio of the integrated intensity of the PSB to that of the ZPL. It is known that the greater the phonon energy and the electron±phonon coupling strength, the weaker the upconversion ¯uorescence, because of the increase in the non-radiative decay [24,25]. Fig. 8 shows that near the germanate anomaly region the electron±phonon coupling strength has a minimum, while the phonon energy is a maximum. Therefore, the maximum intensity of upconversion ¯uorescence shown in Fig. 7 may relate
61
Fig. 8. Phonon energy (h) and electron±phonon coupling strength (s) as a function of Na2 O content for xNa2 O á (100 ÿ x)GeO2 á 5Eu2 O3 glasses. Lines are drawn as guides for the eye.
not to the phonon energy, but to the electron± phonon coupling strength. 5. Conclusions The g-value for Er3 derived from ESR spectrum taken at 4.2 K changed near the so-called germanate anomaly region. By deconvoluting the upconversion ¯uorescence spectra into four Gaussian bands, the peak positions and the integrated intensities were obtained. In the germanate anomaly region the former gave somewhat irregular shifts and the latter had a maximum. The maximum related to the electron±phonon coupling strength that was evaluated from the phonon side bands accompanied by the electronic transition of Eu3 ions doped in the same host glasses. References [1] D.C. Yeh, W.A. Sibley, M. Suscavage, M.G. Drexhage, J. Appl. Phys. 62 (1987) 266. [2] K. Okada, K. Miura, I. Masuda, T. Yamashita, Mater. Sci. Forum 32&33 (1988) 523. [3] P. Le Gall, E.W.J.L. Oomen, A.M.A. van Dongen, Riv. Staz. Sper. Vetro (1990) 57. [4] S. Tanabe, K. Hirao, N. Soga, J. Non-Cryst. Solids 122 (1990) 79. [5] K. Hirao, S. Todoroki, S. Tanabe, N. Soga, in: Proceedings of the Seventh International Conference On Phys. Non-Cryst. Solids, Cambridge, 1991, p. 611.
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[6] C. Zhu, X. Lu, Z. Zhang, J. Non-Cryst. Solids 144 (1992) 89. [7] Z. Pan, S.H. Morgan, A. Loper, V. King, B.H. Long, W.E. Collins, J. Appl. Phys. 77 (1995) 4688. [8] Z. Pan, S.H. Morgan, K. Dyer, A. Ueda, H. Liu, J. Appl. Phys. 79 (1996) 8906. [9] Z. Pan, S.H. Morgan, J. Lumin. 75 (1997) 301. [10] H. Nii, K. Ozaki, M. Herren, M. Morita, J. Lumin. 76&77 (1998) 116. [11] M. Yamazaki, K. Kojima, in: Proceedings of the XVIIth International Congress on Glass, vol. 4, Beijing, 1995, p. 264. [12] M. Yamazaki, K. Kojima, J. Mater. Sci. Lett. 14 (1995) 813. [13] K. Kojima, T. Fukuda, M. Yamazaki, Chem. Lett. (1997) 931. [14] A.O. Ivanov, K.S. Evstropiev, Dokl. Akad. Nauk. SSSR 145 (1962) 797. [15] M.K. Murthy, J. Ip, Nature 201 (1964) 285.
[16] G.S. Henderson, M.E. Fleet, J. Non-Cryst. Solids 134 (1991) 259. [17] T. Tsuneoka, K. Kojima, S. Bojja, J. Non-Cryst. Solids 202 (1996) 297. [18] L.D. Bogomolova, T.F. Dolgolenko, Fiz. Khim. Stekl. 1 (1975) 300. [19] M. Abraham, R.A. Weeks, G.W. Clark, C.B. Finch, Phys. Rev. 137 (1965) A139. [20] M.M. Abraham, C.B. Finch, J.L. Kolopus, J.T. Lewis, Phys. Rev. B 3 (1971) 2855. [21] M.X. Huang, J. Barak, S.M. Bhagat, J.L. Peng, J. Magn. Magn. Mater. 117 (1992) 195. [22] S. Todoroki, K. Hirao, N. Soga, J. Alloys Comp. 193 (1993) 207. [23] M. Wachtler, A. Speghini, S. Pigorini, R. Rolli, M. Bettinelli, J. Non-Cryst. Solids 217 (1997) 111. [24] T. Miyakawa, D.L. Dexter, Phys. Rev. B 1 (1970) 2691. [25] S. Tanabe, S. Yoshii, K. Hirao, N. Soga, Phys. Rev. B 45 (1992) 4620.
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Upconversion ¯uorescence in Er3 -doped Na2O±GeO2 glasses Hideo Yamada 1, Kazuo Kojima * Department of Chemistry, Faculty of Science and Engineering, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan
Abstract Electron spin resonance (ESR) and upconversion ¯uorescence spectra have been investigated for Er3 -doped Na2 O± GeO2 glasses with molar compositions of xNa2 O á (100 ÿ x)GeO2 á 0.7Er2 O3 prepared by the melt quenching method. The g-value for Er3 derived from ESR spectrum and both the peak position and the integrated intensity of upconversion ¯uorescence spectra changed in the so-called germanate anomaly region of the glass. The upconversion ¯uorescence intensity correlated to the electron±phonon coupling strength. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction In oxide glasses when compared to ¯uoride glasses, it is generally dicult to generate upconversion ¯uorescence due to the doped rare-earth ions, since the phonon energies of oxide glass networks are larger than in ¯uoride glasses in which upconversion is observed [1±3]. However, upconversion ¯uorescence has been observed in some oxide glasses such as germanate, tellurite and gallate glasses that have relatively smaller phonon energies [4±10]. We have reported upconversion ¯uorescence of Er3 -doped GeO2 -based glasses prepared by the sol±gel method [11±13]. In this paper we describe electron spin resonance (ESR) and upconversion ¯uorescence * Corresponding author. Tel.: +81-77 561 2780; fax: +81-77 561 2659. E-mail address: [email protected] (K. Kojima) 1 Present address: New Glass Research Division, Yamamura Glass Co., Ltd., 2-1-18, Naruohama, Nishinomiya, Hyogo 6638142, Japan.
spectra of Er3 -containing Na2 O±GeO2 glasses prepared by the melt quenching method. The results are discussed in connection with the so-called germanate anomaly which means that the density or refractive index reached a maximum around 15 mol% Na2 O [14±16], and in connection with phonon energies and electron±phonon coupling strengths. 2. Experimental procedures Germanate glasses with molar compositions of xNa2 O á (100 ÿ x)GeO2 á 0.7Er2 O3 and similar glasses containing 5 mol% of Er2 O3 or Eu2 O3 were prepared from Na2 CO3 , GeO2 , Er(NO3 )3 á 6H2 O and Eu2 O3 . The mixed powders were melted in a platinum crucible at 1400°C for 30 min. The liquid was poured on a stainless steel plate and pressed by another stainless steel plate from the top. Two surfaces of a sample which are at right angles to each other were polished on a paper coated with 3000 mesh corundum prior to use for upconversion ¯uorescence measurements.
0022-3093/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 4 9 5 - 0
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The insertion in Fig. 1 shows ESR spectra, taken at 4.2 K, of xNa2 O á (100 ÿ x)GeO2 á 0.7Er2 O3 samples. A broad signal at about 80 mT is due to Er3 ions in the glass [18]. The corresponding signal for the sample with x 3 splits into some components, probably indicating that Er3 ions are inhomogeneously introduced in this glass with a smaller Na2 O content. The g-values evaluated
from the spectra, where g hm/b Hr, h the PlanckÕs constant, m the microwave frequency, b the Bohr magneton, and Hr is the magnetic ®eld at which the maximum amplitude of a component is observed, are given in Fig. 1 as a function of Na2 O content. The g-value decreases with increasing Na2 O content; the rate of decrease with increasing x changes around x 15 where the so-called germanate anomaly occurs. Fig. 2 gives upconversion ¯uorescence pro®le changes in the visible green region when a sample, 14Na2 O á 86GeO2 á 0.7Er2 O3 , was excited at positions located various distances from a glass surface upon which the upconversion ¯uorescence was observed. The spectral pro®le becomes ill-resolved as the distance increases because of the self-absorption of Er3 ions. Therefore, the excitation beam should be incident just upon the surface where
Fig. 1. Na2 O content dependence of g-values for Er3 evaluated from ESR spectra shown in the insertion, taken at 4.2 K, of xNa2 O á (100 ÿ x)GeO2 á 0.7Er2 O3 glasses. The line is drawn as a guide for the eye.
Fig. 2. Dependence of upconversion spectra on the distance (L) between the surface of the sample where upconversion ¯uorescence was detected and the position where an excitation laser beam was focused as shown in the insertion.
Upconversion ¯uorescence was collected perpendicular to the direction of the excitation beam from an 805 nm laser diode at room temperature. ESR spectrum was recorded at 4.2 K. Methods for optical and ESR measurements have been described elsewhere [12,17]. 3. Results
H. Yamada, K. Kojima / Journal of Non-Crystalline Solids 259 (1999) 57±62
59
upconversion ¯uorescence was measured. Upconversion ¯uorescence spectra were measuredin this manner for xNa2 O á (100 ÿ x)GeO2 á 0.7Er2 O3 glasses (Fig. 3). Green ¯uorescence bands around 525 and 550 nm are assigned to the transitions of 2 H11=2 ® 4 I15=2 and 4 S3=2 ® 4 I15=2 , respectively. A red ¯uorescence band due to the 4 F9=2 ® 4 I15=2 transition was also observed. Fig. 4 shows absorption spectra of a glass 30Na2 O á 70GeO2 á 5Er2 O3 measured at room temperature and 77 K. Four peaks were observed at 523, 534, 547 and 557 nm in the spectrum recorded at room temperature. However, two of them, 534 and 557 nm, were quenched at 77 K. Phonon side bands (PSB) accompanied by the electronic transition of 7 F0 ® 5 D2 for Eu3 (about 465 nm) in the glasses xNa2 O á (100 ÿ x) GeO2 á 5Eu2 O3 are shown in Fig. 5. PSB was measured to
Fig. 4. Absorption spectra of a glass 30Na2 O á 70GeO2 á 5Er2 O3 measured at room temperature and 77 K. The inset shows the assignments.
discuss the Na2 O content dependence of the intensity of the upconversion ¯uorescence. 4. Discussion
Fig. 3. Upconversion spectra of xNa2 O á (100 ÿ x)GeO2 á 0.7 Er2 O3 glasses.
Assuming a cubic crystal ®eld of six-fold coordination for Er3 , the ground state of the Er3 ion, 4 I15=2 , is split into two Kramers doublets (C6 and C7 ) and three quadruplets (C8 ). When the C6 and C7 is the lowest energy level, the g-values of the C6 and C7 states have been predicted theoretically to be 6.0 and 6.8, respectively [18±21]. Energies of the C6 and C7 states in single crystal of ErBa2 Cu3 O6 have been calculated to be nearly degenerate, assuming a cubic crystal ®eld [21]. Therefore, the C6 and C7 states may be energetically interchangeable. We suggest that the result shown in Fig. 1 may
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H. Yamada, K. Kojima / Journal of Non-Crystalline Solids 259 (1999) 57±62
Er3 in a single crystal [21]. From these results, the peaks of 523 and 534 nm are assigned to the transitions from the common lowest state of 2 H11=2 to the lowest state and the next lowest state of 4 I15=2 , respectively. In the same way, the peaks of 547 and 557 nm can be attributed to the transitions from the common lowest state of 4 S3=2 to the two same states of 4 I15=2 . These assignments can be also supported by the upconversion pro®le change shown in Fig. 2; the eect of self-absorption at the peaks of 523 and 547 nm is due to true ground state absorption transitions. The green upconversion spectrum illustrated in Fig. 3 was deconvoluted into four Gaussian component bands discussed above. Figs. 6 and 7 show peak positions and integrated intensities, respectively, of these component bands as a function of
Fig. 5. Absorption spectra of xNa2 O á (100 ÿ x)GeO2 á 5Eu2 O3 glasses showed zero phonon lines (ZPL) and phonon side bands (PSB).
indicate that Er3 ions in glasses with smaller and larger Na2 O contents have a lowest level of C6 and C7 , respectively, and that the interchange of C6 and C7 occurs in the germanate anomaly region. Each of the green upconversion ¯uorescence bands shown in Fig. 3 splits into two peaks for glasses with more than 14 mol% of Na2 O; for example, in the glass with x 30 there are four peaks at 523, 534, 547 and 557 nm. These four peaks were also observed in the absorption spectrum recorded at room temperature as seen in Fig. 4. At 77 K the two peaks of 534 and 557 nm were quenched. The energy dierence of about 390 cmÿ1 between the 523 and 557 nm peaks nearly corresponds with that of about 330 cmÿ1 between the 547 and 557 nm peaks. Actually, an energy difference of about 390 cmÿ1 has been calculated between the C8 and the degenerate C7 , C6 states for
Fig. 6. Na2 O content dependence of peak positions of upconversion ¯uorescence four component bands of xNa2 O á (100 ÿ x)GeO2 á 0.7Er2 O3 glasses. Lines are drawn as guides for the eye.
H. Yamada, K. Kojima / Journal of Non-Crystalline Solids 259 (1999) 57±62
Fig. 7. Na2 O content dependence of integrated intensities of upconversion ¯uorescence four component bands of xNa2 O á (100 ÿ x)GeO2 á 0.7Er2 O3 glasses. The symbols that indicate the four component bands correspond to those in Fig. 5. Lines are drawn as guides for the eye.
Na2 O content. The peak position did not change in a monotonic fashion in the germanate anomaly region and the splitting of each of the 2 H11=2 and 4 S3=2 bands increased with Na2 O content (Fig. 6). This increase is probably responsible for the enlargement of Stark splitting resulting from the increase of a local electric ®eld around the Er3 ions. The local ®eld increases as eective negative charges of oxygen atoms increase with the addition of Na2 O. While conversely, the integrated intensities gave maxima for all the component bands (Fig. 7). PSB spectra at about 800 cmÿ1 shown in Fig. 5, which agree with those of previous results [22,23], are ascribed to Ge±O stretching vibrations of the GeO4 tetrahedral structural unit involving the non-bridging oxygens. The zero phonon line (ZPL) and the PSB are deconvoluted into Gaussian bands and then both phonon energy and electron±phonon coupling strength are estimated (Fig. 8), the latter being evaluated as the ratio of the integrated intensity of the PSB to that of the ZPL. It is known that the greater the phonon energy and the electron±phonon coupling strength, the weaker the upconversion ¯uorescence, because of the increase in the non-radiative decay [24,25]. Fig. 8 shows that near the germanate anomaly region the electron±phonon coupling strength has a minimum, while the phonon energy is a maximum. Therefore, the maximum intensity of upconversion ¯uorescence shown in Fig. 7 may relate
61
Fig. 8. Phonon energy (h) and electron±phonon coupling strength (s) as a function of Na2 O content for xNa2 O á (100 ÿ x)GeO2 á 5Eu2 O3 glasses. Lines are drawn as guides for the eye.
not to the phonon energy, but to the electron± phonon coupling strength. 5. Conclusions The g-value for Er3 derived from ESR spectrum taken at 4.2 K changed near the so-called germanate anomaly region. By deconvoluting the upconversion ¯uorescence spectra into four Gaussian bands, the peak positions and the integrated intensities were obtained. In the germanate anomaly region the former gave somewhat irregular shifts and the latter had a maximum. The maximum related to the electron±phonon coupling strength that was evaluated from the phonon side bands accompanied by the electronic transition of Eu3 ions doped in the same host glasses. References [1] D.C. Yeh, W.A. Sibley, M. Suscavage, M.G. Drexhage, J. Appl. Phys. 62 (1987) 266. [2] K. Okada, K. Miura, I. Masuda, T. Yamashita, Mater. Sci. Forum 32&33 (1988) 523. [3] P. Le Gall, E.W.J.L. Oomen, A.M.A. van Dongen, Riv. Staz. Sper. Vetro (1990) 57. [4] S. Tanabe, K. Hirao, N. Soga, J. Non-Cryst. Solids 122 (1990) 79. [5] K. Hirao, S. Todoroki, S. Tanabe, N. Soga, in: Proceedings of the Seventh International Conference On Phys. Non-Cryst. Solids, Cambridge, 1991, p. 611.
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[6] C. Zhu, X. Lu, Z. Zhang, J. Non-Cryst. Solids 144 (1992) 89. [7] Z. Pan, S.H. Morgan, A. Loper, V. King, B.H. Long, W.E. Collins, J. Appl. Phys. 77 (1995) 4688. [8] Z. Pan, S.H. Morgan, K. Dyer, A. Ueda, H. Liu, J. Appl. Phys. 79 (1996) 8906. [9] Z. Pan, S.H. Morgan, J. Lumin. 75 (1997) 301. [10] H. Nii, K. Ozaki, M. Herren, M. Morita, J. Lumin. 76&77 (1998) 116. [11] M. Yamazaki, K. Kojima, in: Proceedings of the XVIIth International Congress on Glass, vol. 4, Beijing, 1995, p. 264. [12] M. Yamazaki, K. Kojima, J. Mater. Sci. Lett. 14 (1995) 813. [13] K. Kojima, T. Fukuda, M. Yamazaki, Chem. Lett. (1997) 931. [14] A.O. Ivanov, K.S. Evstropiev, Dokl. Akad. Nauk. SSSR 145 (1962) 797. [15] M.K. Murthy, J. Ip, Nature 201 (1964) 285.
[16] G.S. Henderson, M.E. Fleet, J. Non-Cryst. Solids 134 (1991) 259. [17] T. Tsuneoka, K. Kojima, S. Bojja, J. Non-Cryst. Solids 202 (1996) 297. [18] L.D. Bogomolova, T.F. Dolgolenko, Fiz. Khim. Stekl. 1 (1975) 300. [19] M. Abraham, R.A. Weeks, G.W. Clark, C.B. Finch, Phys. Rev. 137 (1965) A139. [20] M.M. Abraham, C.B. Finch, J.L. Kolopus, J.T. Lewis, Phys. Rev. B 3 (1971) 2855. [21] M.X. Huang, J. Barak, S.M. Bhagat, J.L. Peng, J. Magn. Magn. Mater. 117 (1992) 195. [22] S. Todoroki, K. Hirao, N. Soga, J. Alloys Comp. 193 (1993) 207. [23] M. Wachtler, A. Speghini, S. Pigorini, R. Rolli, M. Bettinelli, J. Non-Cryst. Solids 217 (1997) 111. [24] T. Miyakawa, D.L. Dexter, Phys. Rev. B 1 (1970) 2691. [25] S. Tanabe, S. Yoshii, K. Hirao, N. Soga, Phys. Rev. B 45 (1992) 4620.