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Study on Tb3+ containing high silica and low silica calcium aluminate glasses: Impact of optical basicity
Spectrochimica Acta Part A 94 (2012) 180–185
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Study on Tb3+ containing high silica and low silica calcium aluminate glasses: Impact of optical basicity Atul D. Sontakke, K. Annapurna ∗ Glass Science and Technology Section, CSIR-Central Glass and Ceramic Research Institute, 196 Raja S. C. Mullick Road, Kolkata 700032, India
a r t i c l e
i n f o
Article history: Received 21 November 2011 Received in revised form 14 February 2012 Accepted 22 March 2012 Keywords: Glass Infrared spectroscopy Optical spectroscopy Optical basicity Luminescent materials
a b s t r a c t Two series of glasses based on high silica (CAS) and low silica calcium aluminates (LSCA) have been investigated for their structural, optical and Tb3+ luminescence properties. The compositional modification reduces host phonon energy in LSCA glasses. Still, LSCA glasses exhibit Tb3+ green luminescence quenching, whereas no quenching observed in CAS glasses. Material property influence on this behaviour has been discussed with an insight into the redox state of active ions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The extensive ability of property tailoring by compositional modifications makes glasses most preferable engineered material for numerous applications. Its revolutionary role in optical communication and solid-state lasers attracted tremendous attention in the last five decades resulting in significant exploration of glass science to achieve desired properties [1,2]. Host phonon energy is one of such intrinsic property of glass which plays key role in favouring the luminescence performance of active ions by limiting or allowing the multi-phonon relaxation of excited levels [3,4]. Among several glasses, silicates in general and high silica calciumalumino-silicate (CAS) in particular are of special interest with great industrial and scientific importance [5–7]. These glasses exhibit maximum phonon energy of about 1000–1100 cm−1 , which limits their suitability for infra-red as well as up-conversion luminescence applications. It is possible to reduce the phonon energy of these glasses by partially substituting silica contents with alumina. Thus the maximum phonon energy of such low silica calcium aluminate (LSCA) glasses comes down to 750–800 cm−1 [8]. Further, the comparatively high mechanical strength, better chemical durability and non-toxicity of LSCA glasses over heavy metal oxides and other low phonon energy host glasses makes them favourable for infrared luminescence and visible up-conversion applications.
∗ Corresponding author. Tel.: +91 33 2473 3469; fax: +91 33 2473 0957. E-mail addresses: [email protected], [email protected] (K. Annapurna). 1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.03.052
Nevertheless, such alterations in chemical composition of glasses may develop adverse effects on other constructive properties. Usually the compositional changes bring about structural rearrangements and modify the optical basicity of glass [9]. Basicity defines the metal–ligand interactions in terms of electron density carried by the ligand and affects the oxidation state of dopant metal ions, which reflects on their optical as well as spectroscopic properties [10]. Such consequences of optical basicity can be well studied experimentally by using multi-valence ions as probe, since their redox equilibrium is highly sensitive to optical basicity of glass. In the present work, CAS and LSCA glasses doped with varied concentration of Tb2 O3 have been prepared and studied for the effect of compositional variations on their structural, optical and luminescence properties. Terbium has been selected as an active ion owing to its ability to exhibit multi-valence (Tb3+ and Tb4+ ) states. In addition to this, the blue emission of Tb3+ from 5 D3 excited level is sensitive to host phonon energy; whereas, that of green emission from 5 D4 level has negligible dependence making it an ideal ion to study the host effect. 2. Experimental Two series of glasses having chemical compositions (Series 1, CAS: 23CaO–5Al2 O3 –58SiO2 –4MgO–10NaF and Series 2, LSCA: 57.3CaO–28.7Al2 O3 –10SiO2 –4MgO in mol%) with varied Tb2 O3 concentration of 0.25, 0.5, 1, 2, 4, 8, 16, 24, 32 and 40 in excess wt% were prepared by conventional melt quenching technique. For both series, glass melting was carried out in a pure platinum crucible in 1450–1600 ◦ C temperature range for one to one and half hours followed by annealing at 590–750 ◦ C depending upon their
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and 560–400 cm−1 are attributed to the vibrational transitions of Al O Si and Si O− stretchings, symmetric stretching of Al O in [AlO6 ] octahedral unit and bending modes of Al O Al and Si O Al respectively [7,11]. The comparison of overall spectral appearance in both LSCA and CAS glasses suggests that the majority of Al3+ ions are tetrahedrally coordinated in LSCA glasses, whereas they have octahedral coordination in CAS glasses. That is, in CAS glass network, [AlO6 ] unit represents the role of Al3+ ion as network modifier; while at high Al2 O3 content, Al3+ ions forms [AlO4 ] tetrahedral units with the aid of Ca2+ ions and act as network formers as in the present LSCA glasses which contain only 10 mol% of SiO2 [12]. Such replacement of [SiO4 ] structural units by [AlO4 ]− in LSCA glasses results in the reduction of effective glass phonon energy from 1040 cm−1 in CAS glasses to 790 cm−1 in LSCA glasses. 3.2. Physical properties and glass optical basicity
Fig. 1. FTIR reflectance spectra of CAS and LSCA glasses.
composition. The obtained glasses were labelled on the basis of dopant ion concentrations. The infrared reflectance spectra were recorded in 400–1500 cm−1 range to study the structural properties of glasses using Fourier Transform Infrared (FTIR) spectrometer (Perkin-Elmer, Model: 1615 series). The density of well annealed glasses was measured by Archimedes’ buoyancy principle using water as immersion liquid on Mettler-Tollado digital balance fitted with a density measurement kit. Refractive indices were recorded with an accuracy of 10−5 on a Prism Coupler refractometer (Model Metricon M-2010, Pennington, NJ) built-in with five different lasers as illuminating sources. The optical absorption spectral measurements were carried out on a UV–Vis–NIR spectrophotometer (Perkin-Elmer, Model: Lambda-20) in the 200–1100 nm wavelength range. The luminescence spectra of Tb3+ ions were recorded on a fluorescence spectrophotometer (Model: Quantum Masterenhanced NIR, from Photon Technologies International, USA). 3. Results and discussion 3.1. Glass structure Fig. 1 presents the recorded FTIR reflectance spectra of CASTb0.25 and LSCA-Tb0.25 glasses, which demonstrate significant structural differences as evidenced from the deconvoluted components for individual glasses. For CAS-Tb0.25 glass, the most intense band is observed at around 1040 cm−1 due to asymmetric stretching vibration of Si O Si bond of [SiO4 ] tetrahedral units, with a shoulder at around 940 cm−1 due to non bridging Si O− vibrations [7]. In addition to this, a broad hump has been observed at around 550–780 cm−1 due to the vibrations of Al O bond in [AlO6 ] octahedral unit, which coexists with two inter-tetrahedral Si O Si vibrations in the region of 600–800 cm−1 . The Si O Si bending vibration is found at around 430–470 cm−1 region. At 1130 cm−1 , a weak band is observed due to stretching vibrations of [SiO4 ]poly units indicating the presence of polymerized silicate network in CAS glasses [7]. For LSCA glasses, four major absorption bands have been observed with most intense band centred at around 790 cm−1 , which has been identified as due to the symmetric stretching vibrational transitions of Al O Al bond in [AlO4 ] tetrahedra [11]. Similarly, the bands at around 950 cm−1 , 650 cm−1
The complete change of glass network from CAS to LSCA glasses as discussed in earlier section has significantly affected their physical and optical characteristics. The measured density and refractive indices of both series of glasses have been listed in Table 1. It is observed that the LSCA glasses exhibit higher values of density and refractive index over CAS series of glasses. In general, the Al O ˚ is higher than Si O bond length (∼1.6 A) ˚ in bond length (∼1.75 A) tetrahedral network [13,14] indicating more volume fraction for [AlO4 ] tetrahedron over [SiO4 ] tetrahedron. However, the formation of [AlO4 ]− tetrahedral units reduces the non-bridging oxygen (NBO) in LSCA glass matrix to compensate extra charge on aluminium ion giving rise to a denser network. Table 1 also enlists the optical basicity of present glasses along with their molecular electronic polarizability. The optical basicity has been calculated from the chemical composition of glasses using the relation [15], glass =
X (O) i
(1)
i
i
where Xi (O) is the fraction of oxygen atoms carried by the individual Mx Oy oxide to the total number of oxygen atoms in glass and i is the basicity moderating parameter. The basicity moderating parameter, i can be evaluated from the Pauling’s electronegativity of cation (Mi ) using the relation i = 1.36(Mi − 0.26). The LSCA glasses are found to exhibit higher values of basicity than that of CAS glasses. In case of oxide materials, the basicity shows direct dependence on polarization state of oxygen ions as well as metal–oxygen bond ionicity [10,15]. Thus the bonding type between metal ion and anion and in turn the corresponding bond strength is vital in defining the basicity of glasses. As the basicity increases, the ionic characteristics of the glass also increase as can be seen from the glass ionicity (Iglass ) values of present glasses listed in Table 1, which have been calculated using the relation [15,16], Iglass =
i
1 1 − exp − (o − Mi )2 4
(2)
where o and Mi are Pauling’s electronegativity for oxygen and metal ions respectively. This enhanced ionicity results in the reduced bond strength, which thereby reduces the effective phonon energy as observed in present study. The Al3+ ions owing to their lower field strength over Si4+ , form less covalent bonding with oxygen leading to the higher ionicity of Al O bond than that of Si O bond. Further, the optical basicity is increasing continuously with the increase of Tb2 O3 concentration in both series of glasses. As the rare earth oxides (as network modifiers) are comparatively basic in nature, they contribute to increase the overall optical basicity of glasses.
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Fig. 2. Absorption spectra of CAS and LSCA glasses with different Tb2 O3 contents. (Inset: Base glass corrected absorption spectra. The sharp cuts at around 420 nm, 380 nm and 327 nm are due to intrinsic noise of instrument.)
3.3. Optical absorption spectra Fig. 2 represents the optical absorption spectra of CAS and LSCA glasses with different Tb2 O3 contents. For CAS glasses, the spectra exhibit weak absorption peaks centred at 485 nm, 376 nm, 369 nm, 352 nm, 338 nm and 317 nm. These peaks have been assigned to the intra-band f–f transitions from ground state 7 F6 to higher excited levels 5 D4 , (5 G6 , 5 D3 ), 5 L10 , (5 L8 , 5 G4 ), (5 G2 , 5 L ) and (5 H , 5 D 3+ 4f8 electronic config6 6 1, 2 ) respectively, of Tb uration. These absorption bands could not be detected in LSCA series of glasses. Further, the UV absorption edge is found to be shifting towards higher wavelength with increase in Tb2 O3
concentration in both series of glasses. This has resulted in the reduction of glass optical band gap energies as can be seen from Table 1. This red-shift is comparatively less in CAS glasses where it is attributed to the increased absorption due to 7 Dj and 9 Dj levels of 4f7 5d1 configuration of Tb3+ ions as has been depicted in the base glass corrected absorption spectra of CAS glasses in the inset of Fig. 2 [7]. In case of LSCA glasses, this red shift of UV band edge is significantly strong extending to visible region at higher dopant concentrations. This shift may be due to the formation of Tb4+ ions. Since the LSCA glasses exhibit higher optical basicity (∼0.7), the dopant ions to exist in higher oxidation state (Tb4+ ) become
Table 1 Density (d), average molecular weight (Mavg ), refractive index (ne ), optical basicity (), molecular electronic polarizability (˛e ), glass ionicity (Iglass ) and UV band gap energy for direct (Egd ) and indirect (Egi ) transitions. Glass
d (g/cm3 )
Mavg (g/mol)
ne
˛e (A˚ 3 )
Series 1 CAS-Tb0.25 CAS-Tb0.5 CAS-Tb1 CAS-Tb2 CAS-Tb4 CAS-Tb8 CAS-Tb16 CAS-Tb24 CAS-Tb32 CAS-Tb40
2.694 2.696 2.708 2.725 2.776 2.834 2.964 3.161 3.270 3.403
59.77 59.88 60.15 60.62 61.60 63.52 67.28 70.96 74.55 78.05
1.559 1.555 1.559 1.562 1.565 1.569 1.581 1.600 1.609 1.619
0.5240 0.5242 0.5247 0.5258 0.5278 0.5318 0.5394 0.5469 0.5539 0.5607
2.839 2.826 2.843 2.859 2.865 2.911 2.998 3.044 3.129 3.189
Series 2 LSCA-Tb0.25 LSCA-Tb0.5 LSCA-Tb1 LSCA-Tb2 LSCA-Tb4 LSCA-Tb8 LSCA-Tb16 LSCA-Tb24 LSCA-Tb32 LSCA-Tb40
2.934 2.938 2.946 2.972 3.010 3.088 3.227 3.369 3.487 3.590
69.14 69.28 69.56 70.15 71.22 73.41 77.69 81.85 85.89 89.90
1.665 1.662 1.665 1.668 1.671 1.677 1.686 1.697 1.705 1.713
0.6909 0.6910 0.6913 0.6919 0.6932 0.6956 0.7002 0.7046 0.7087 0.7127
3.469 3.459 3.476 3.487 3.507 3.549 3.631 3.710 3.794 3.891
Iglass
Band gap (eV) Egd
Egi
0.6073 0.6075 0.6077 0.6081 0.6091 0.6108 0.6143 0.6176 0.6208 0.6238
5.03 4.94 4.86 4.81 4.77 4.71 4.65 4.54 4.48 4.45
4.64 4.54 4.50 4.48 4.41 4.35 4.29 4.17 4.10 4.05
0.6640 0.6641 0.6642 0.6645 0.6650 0.6660 0.6679 0.6697 0.6715 0.6732
3.93 3.86 3.70 3.42 3.01 2.65 2.56 2.51 2.45 2.40
3.23 3.20 3.01 2.14 2.09 2.08 2.59 2.04 2.02 1.97
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Fig. 3. Images of CAS and LSCA series of glasses with varied Tb2 O3 concentration. (For interpretation of the references to colour in the text, the reader is referred to the web version of the article.)
more probable [17]. This may be possibly because of the fact that, the network modifying nature of terbium ions, which similar to Ca2+ , expected to help Al3+ ions in charge compensation of [AlO4 ]− tetrahedral units in the network and thereby attain the higher oxidation state (Tb4+ ). To further examine the existence of Tb4+ ions, base glass corrected absorption spectra of LSCA glasses have been recorded and are shown in the inset. The spectrum of LSCA-Tb0.25 glass shows a broad absorption band covering 230–500 nm wavelength region. This band exhibits two distinct absorptions peaking at around 282 nm and 335 nm. The former is due to the f → d transition of Tb3+ ions while the later can be attributed to Tb4+ absorption, which is becoming more and more significant with the increase in Tb2 O3 concentration causing red shift in UV band edge. This strong absorption of Tb4+ has resulted in light yellow to strong amber coloured tint in LSCA glasses as can be seen from the glass images shown in Fig. 3.
3.4. Luminescence spectra and quenching behaviour The emission and excitation spectra of Tb3+ ions in CAS and LSCA glasses are shown in Fig. 4a and b respectively. The emission spectra of CAS glasses on excitation into f → d band of Tb3+ reveal several emission transitions from its 5 D3 and 5 D4 excited levels to ground state multiplets. The emission bands centred at 378 nm, 418 nm, 438 nm, 457 nm and 472 nm are due to 5D → 7F 3 6, 5, 4, 3 and 2 , whereas, those at 482 nm, 542 nm, 586 nm and 625 nm are attributed to 5 D4 → 7 F6, 5, 4 and 3 respectively [7]. Similar emission transitions have been detected for LSCA glasses. The variation of normalized emission intensity from 5 D3 level (blue) and 5 D4 level (green) is pictorially represented in Fig. 5. The blue emission intensity increases first with the increase in dopant concentration and then quenches for further increase in concentration. This quenching is slower in LSCA series of glasses compared to
Fig. 4. (a) Emission and excitation spectra of Tb3+ ions in CAS series of glasses. (b) Emission and excitation spectra of Tb3+ ions in LSCA series of glasses.
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Fig. 6. Normalized spectral overlap of Tb4+ absorption with Tb3+ absorption and emission spectrum in LSCA glasses. Inset shows the simplified representation of Tb3+ → Tb4+ energy transfer mechanism.
Fig. 5. Plot of normalized intensities for blue and green emission with Tb2 O3 contents in CAS and LSCA glasses. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
CAS glasses. For green emission, completely different results have observed. In CAS glasses, the emission intensity increases continuously for the concentration range studied and no quenching has found. However, the LSCA glasses exhibit a decrease in emission intensity for glasses having Tb2 O3 content above 8 wt%. The quenching of blue emission occurs predominantly due to the resonant energy transfer assisted cross-relaxation (RETCR) mechanism among neighbouring Tb3+ ions following the path 5 D3 → 5 D4 :7 F6 → 7 F0 and it becomes more prominent with increasing dopant concentration owing to the reduced Tb3+ –Tb3+ inter-ionic separation [7,18]. The multi-phonon relaxation can also depopulate the 5 D3 level. However, it is comparatively weak owing to a large energy difference of about 6000 cm−1 requiring more than five phonons in CAS glasses and seven in LSCA glasses. But, its influence can still be experienced from the observed slowed-down quenching rate in LSCA glasses, which may have resulted from the reduced multi-phonon relaxation due to lower phonon energy of LSCA glasses. For Tb3+ green emission, concentration quenching is a rare phenomenon, which generally occurs at higher dopant concentration under strong excitation due to the energy migration among Tb3+ ions leading to cooperative up-conversion [7,18]. In the present investigation, CAS glasses do not show any quenching in green luminescence but it is detected in LSCA glasses at considerably lower concentration. It suggests that the observed quenching of green luminescence in LSCA glasses might have been influenced by the presence of Tb4+ ions. It is because, the strong absorption of Tb4+ in LSCA glasses covers both absorption and emission bands of Tb3+ ions which can directly mask the excitation of Tb3+ as well as facilitate the energy transfer from Tb3+ to Tb4+ as can be seen from Fig. 6. The Tb3+ → Tb4+ energy transfer predominantly occurs from 5 D3 and 5 D4 excitation levels of Tb3+ ions, since upper excitation levels are short lived and relaxes non-radiatively through multi-phonon and cross-relaxation processes. Along with the energy transfer from
Tb3+ to Tb4+ , masking of excitation radiation by strong absorption band of Tb4+ results in an inefficient pumping of Tb3+ ions in LSCA glasses and leads to the luminescence quenching. As shown in Fig. 6, the selective decrease in Tb3+ excitation intensity in 300–450 nm wavelength range in LSCA glasses which exactly overlapped by Tb4+ absorption further strengthens above assumption of masking effect by Tb4+ ions. Thus, from the observation of both CAS and LSCA series of glasses, the effect of lower phonon energy of LSCA glasses has mainly reflected on the blue emission arising from 5 D3 excitation level of Tb3+ ions, but the consequent rise in optical basicity influenced the redox state of terbium ions leading to the formation of higher oxidation state (Tb4+ ), which has acted as quencher for Tb3+ green emission. 4. Conclusions In summary, the compositional modification of CAS to LSCA glasses reduced the effective host phonon energy from about 1040 cm−1 to 790 cm−1 resulting in a weaker multi-phonon relaxation of excitation population of dopant ions in LSCA glasses. But, such alteration in chemical composition from CAS to LSCA glasses significantly enhanced the glass optical basicity and electronic polarizability. This enhanced optical basicity of LSCA glasses partially shifted the redox equilibrium of terbium ions towards the formation of higher valence state (Tb4+ ). These non-luminescent Tb4+ ions have acted as quenching centres for Tb3+ luminescence owing to their strong absorption in UV–visible region causing green luminescence quenching in LSCA glasses. Acknowledgements Authors would like to thank Prof. I. Manna, Director, CGCRI for his kind encouragement and permission to publish this work that was carried out in an In-house project (OLP-0288). Authors also thank Dr. Ranjan Sen for his continued support. Mr. A.D.S. is thankful to the CSIR, New Delhi for the award of Senior Research Fellowship to him. References [1] M. Yamane, Y. Asahara, Glasses for Photonics, Cambridge University Press, 2000. [2] G. Fuxi, Laser Materials, World Scientific Publications Co. Ltd., Singapore, 1995.
A.D. Sontakke, K. Annapurna / Spectrochimica Acta Part A 94 (2012) 180–185 [3] B. Cole, J. Sanghera, B. Shaw, B. Harbison, I.D. Aggarwal, Low phonon energy glass and fibers doped with rare earth ions, USPTO 6128429, 2000. [4] U. Unrau, Proceedings of the 2nd International Conference on Transparent ´ Optical Networks, ICTON, Gdansk, Poland, June 5–8, 2000. [5] J.E. Shelby, J. Am. Ceram. Soc. 68 (1985) 155–158. [6] D.R. Neuville, L. Cormier, D. Massiot, Chem. Geol. 229 (2006) 173–185. [7] A.D. Sontakke, K. Biswas, K. Annapurna, J. Lumin. 129 (2009) 1347–1355. [8] S.L. Oliveira, S.M. Lima, T. Catunda, L.A.O. Nunes, J.H. Rohling, A.C. Bento, M.L. Baesso, Appl. Phys. Lett. 84 (2004) 359–361. [9] V. Dimitrov, T. Komatsu, J. Non-Cryst. Solids 249 (1999) 160–179. [10] J.A. Duffy, M.D. Ingram, S. Fong, Phys. Chem. Chem. Phys. 2 (2000) 1829–1833.
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[11] K.J. Rao, Structural Chemistry of Glasses, Springer Publication, 2002. [12] J.J. Videau, J. Etourneau, J. Rocherulle, P. Verdier, Y. Laurent, J. Eur. Ceram. Soc. 17 (1997) 1955–1961. [13] A.C. Hannon, J.M. Parker, J. Non-Cryst. Solids 274 (2000) 102–109. [14] D.R. Neuville, G.S. Henderson, L. Cormier, D. Massiot, Am. Mineral. 95 (2010) 1580–1589. [15] P. Balt˘a, J. Optoelectron. Adv. Mater. 13 (2011) 1377–1384. [16] R. Moretti, Ann. Geophys. 48 (2005) 583–608. [17] S. Liu, G. Zhao, W. Ruan, Z. Yao, T. Xie, J. Jin, H. Ying, J. Wang, G. Han, J. Am. Ceram. Soc. 91 (2008) 2740–2742. [18] Y.Y. Choi, K.S. Sohn, H.D. Park, S.Y. Choi, J. Mater. Res. 16 (2001) 881–889.
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Study on Tb3+ containing high silica and low silica calcium aluminate glasses: Impact of optical basicity Atul D. Sontakke, K. Annapurna ∗ Glass Science and Technology Section, CSIR-Central Glass and Ceramic Research Institute, 196 Raja S. C. Mullick Road, Kolkata 700032, India
a r t i c l e
i n f o
Article history: Received 21 November 2011 Received in revised form 14 February 2012 Accepted 22 March 2012 Keywords: Glass Infrared spectroscopy Optical spectroscopy Optical basicity Luminescent materials
a b s t r a c t Two series of glasses based on high silica (CAS) and low silica calcium aluminates (LSCA) have been investigated for their structural, optical and Tb3+ luminescence properties. The compositional modification reduces host phonon energy in LSCA glasses. Still, LSCA glasses exhibit Tb3+ green luminescence quenching, whereas no quenching observed in CAS glasses. Material property influence on this behaviour has been discussed with an insight into the redox state of active ions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The extensive ability of property tailoring by compositional modifications makes glasses most preferable engineered material for numerous applications. Its revolutionary role in optical communication and solid-state lasers attracted tremendous attention in the last five decades resulting in significant exploration of glass science to achieve desired properties [1,2]. Host phonon energy is one of such intrinsic property of glass which plays key role in favouring the luminescence performance of active ions by limiting or allowing the multi-phonon relaxation of excited levels [3,4]. Among several glasses, silicates in general and high silica calciumalumino-silicate (CAS) in particular are of special interest with great industrial and scientific importance [5–7]. These glasses exhibit maximum phonon energy of about 1000–1100 cm−1 , which limits their suitability for infra-red as well as up-conversion luminescence applications. It is possible to reduce the phonon energy of these glasses by partially substituting silica contents with alumina. Thus the maximum phonon energy of such low silica calcium aluminate (LSCA) glasses comes down to 750–800 cm−1 [8]. Further, the comparatively high mechanical strength, better chemical durability and non-toxicity of LSCA glasses over heavy metal oxides and other low phonon energy host glasses makes them favourable for infrared luminescence and visible up-conversion applications.
∗ Corresponding author. Tel.: +91 33 2473 3469; fax: +91 33 2473 0957. E-mail addresses: [email protected], [email protected] (K. Annapurna). 1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.03.052
Nevertheless, such alterations in chemical composition of glasses may develop adverse effects on other constructive properties. Usually the compositional changes bring about structural rearrangements and modify the optical basicity of glass [9]. Basicity defines the metal–ligand interactions in terms of electron density carried by the ligand and affects the oxidation state of dopant metal ions, which reflects on their optical as well as spectroscopic properties [10]. Such consequences of optical basicity can be well studied experimentally by using multi-valence ions as probe, since their redox equilibrium is highly sensitive to optical basicity of glass. In the present work, CAS and LSCA glasses doped with varied concentration of Tb2 O3 have been prepared and studied for the effect of compositional variations on their structural, optical and luminescence properties. Terbium has been selected as an active ion owing to its ability to exhibit multi-valence (Tb3+ and Tb4+ ) states. In addition to this, the blue emission of Tb3+ from 5 D3 excited level is sensitive to host phonon energy; whereas, that of green emission from 5 D4 level has negligible dependence making it an ideal ion to study the host effect. 2. Experimental Two series of glasses having chemical compositions (Series 1, CAS: 23CaO–5Al2 O3 –58SiO2 –4MgO–10NaF and Series 2, LSCA: 57.3CaO–28.7Al2 O3 –10SiO2 –4MgO in mol%) with varied Tb2 O3 concentration of 0.25, 0.5, 1, 2, 4, 8, 16, 24, 32 and 40 in excess wt% were prepared by conventional melt quenching technique. For both series, glass melting was carried out in a pure platinum crucible in 1450–1600 ◦ C temperature range for one to one and half hours followed by annealing at 590–750 ◦ C depending upon their
A.D. Sontakke, K. Annapurna / Spectrochimica Acta Part A 94 (2012) 180–185
181
and 560–400 cm−1 are attributed to the vibrational transitions of Al O Si and Si O− stretchings, symmetric stretching of Al O in [AlO6 ] octahedral unit and bending modes of Al O Al and Si O Al respectively [7,11]. The comparison of overall spectral appearance in both LSCA and CAS glasses suggests that the majority of Al3+ ions are tetrahedrally coordinated in LSCA glasses, whereas they have octahedral coordination in CAS glasses. That is, in CAS glass network, [AlO6 ] unit represents the role of Al3+ ion as network modifier; while at high Al2 O3 content, Al3+ ions forms [AlO4 ] tetrahedral units with the aid of Ca2+ ions and act as network formers as in the present LSCA glasses which contain only 10 mol% of SiO2 [12]. Such replacement of [SiO4 ] structural units by [AlO4 ]− in LSCA glasses results in the reduction of effective glass phonon energy from 1040 cm−1 in CAS glasses to 790 cm−1 in LSCA glasses. 3.2. Physical properties and glass optical basicity
Fig. 1. FTIR reflectance spectra of CAS and LSCA glasses.
composition. The obtained glasses were labelled on the basis of dopant ion concentrations. The infrared reflectance spectra were recorded in 400–1500 cm−1 range to study the structural properties of glasses using Fourier Transform Infrared (FTIR) spectrometer (Perkin-Elmer, Model: 1615 series). The density of well annealed glasses was measured by Archimedes’ buoyancy principle using water as immersion liquid on Mettler-Tollado digital balance fitted with a density measurement kit. Refractive indices were recorded with an accuracy of 10−5 on a Prism Coupler refractometer (Model Metricon M-2010, Pennington, NJ) built-in with five different lasers as illuminating sources. The optical absorption spectral measurements were carried out on a UV–Vis–NIR spectrophotometer (Perkin-Elmer, Model: Lambda-20) in the 200–1100 nm wavelength range. The luminescence spectra of Tb3+ ions were recorded on a fluorescence spectrophotometer (Model: Quantum Masterenhanced NIR, from Photon Technologies International, USA). 3. Results and discussion 3.1. Glass structure Fig. 1 presents the recorded FTIR reflectance spectra of CASTb0.25 and LSCA-Tb0.25 glasses, which demonstrate significant structural differences as evidenced from the deconvoluted components for individual glasses. For CAS-Tb0.25 glass, the most intense band is observed at around 1040 cm−1 due to asymmetric stretching vibration of Si O Si bond of [SiO4 ] tetrahedral units, with a shoulder at around 940 cm−1 due to non bridging Si O− vibrations [7]. In addition to this, a broad hump has been observed at around 550–780 cm−1 due to the vibrations of Al O bond in [AlO6 ] octahedral unit, which coexists with two inter-tetrahedral Si O Si vibrations in the region of 600–800 cm−1 . The Si O Si bending vibration is found at around 430–470 cm−1 region. At 1130 cm−1 , a weak band is observed due to stretching vibrations of [SiO4 ]poly units indicating the presence of polymerized silicate network in CAS glasses [7]. For LSCA glasses, four major absorption bands have been observed with most intense band centred at around 790 cm−1 , which has been identified as due to the symmetric stretching vibrational transitions of Al O Al bond in [AlO4 ] tetrahedra [11]. Similarly, the bands at around 950 cm−1 , 650 cm−1
The complete change of glass network from CAS to LSCA glasses as discussed in earlier section has significantly affected their physical and optical characteristics. The measured density and refractive indices of both series of glasses have been listed in Table 1. It is observed that the LSCA glasses exhibit higher values of density and refractive index over CAS series of glasses. In general, the Al O ˚ is higher than Si O bond length (∼1.6 A) ˚ in bond length (∼1.75 A) tetrahedral network [13,14] indicating more volume fraction for [AlO4 ] tetrahedron over [SiO4 ] tetrahedron. However, the formation of [AlO4 ]− tetrahedral units reduces the non-bridging oxygen (NBO) in LSCA glass matrix to compensate extra charge on aluminium ion giving rise to a denser network. Table 1 also enlists the optical basicity of present glasses along with their molecular electronic polarizability. The optical basicity has been calculated from the chemical composition of glasses using the relation [15], glass =
X (O) i
(1)
i
i
where Xi (O) is the fraction of oxygen atoms carried by the individual Mx Oy oxide to the total number of oxygen atoms in glass and i is the basicity moderating parameter. The basicity moderating parameter, i can be evaluated from the Pauling’s electronegativity of cation (Mi ) using the relation i = 1.36(Mi − 0.26). The LSCA glasses are found to exhibit higher values of basicity than that of CAS glasses. In case of oxide materials, the basicity shows direct dependence on polarization state of oxygen ions as well as metal–oxygen bond ionicity [10,15]. Thus the bonding type between metal ion and anion and in turn the corresponding bond strength is vital in defining the basicity of glasses. As the basicity increases, the ionic characteristics of the glass also increase as can be seen from the glass ionicity (Iglass ) values of present glasses listed in Table 1, which have been calculated using the relation [15,16], Iglass =
i
1 1 − exp − (o − Mi )2 4
(2)
where o and Mi are Pauling’s electronegativity for oxygen and metal ions respectively. This enhanced ionicity results in the reduced bond strength, which thereby reduces the effective phonon energy as observed in present study. The Al3+ ions owing to their lower field strength over Si4+ , form less covalent bonding with oxygen leading to the higher ionicity of Al O bond than that of Si O bond. Further, the optical basicity is increasing continuously with the increase of Tb2 O3 concentration in both series of glasses. As the rare earth oxides (as network modifiers) are comparatively basic in nature, they contribute to increase the overall optical basicity of glasses.
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Fig. 2. Absorption spectra of CAS and LSCA glasses with different Tb2 O3 contents. (Inset: Base glass corrected absorption spectra. The sharp cuts at around 420 nm, 380 nm and 327 nm are due to intrinsic noise of instrument.)
3.3. Optical absorption spectra Fig. 2 represents the optical absorption spectra of CAS and LSCA glasses with different Tb2 O3 contents. For CAS glasses, the spectra exhibit weak absorption peaks centred at 485 nm, 376 nm, 369 nm, 352 nm, 338 nm and 317 nm. These peaks have been assigned to the intra-band f–f transitions from ground state 7 F6 to higher excited levels 5 D4 , (5 G6 , 5 D3 ), 5 L10 , (5 L8 , 5 G4 ), (5 G2 , 5 L ) and (5 H , 5 D 3+ 4f8 electronic config6 6 1, 2 ) respectively, of Tb uration. These absorption bands could not be detected in LSCA series of glasses. Further, the UV absorption edge is found to be shifting towards higher wavelength with increase in Tb2 O3
concentration in both series of glasses. This has resulted in the reduction of glass optical band gap energies as can be seen from Table 1. This red-shift is comparatively less in CAS glasses where it is attributed to the increased absorption due to 7 Dj and 9 Dj levels of 4f7 5d1 configuration of Tb3+ ions as has been depicted in the base glass corrected absorption spectra of CAS glasses in the inset of Fig. 2 [7]. In case of LSCA glasses, this red shift of UV band edge is significantly strong extending to visible region at higher dopant concentrations. This shift may be due to the formation of Tb4+ ions. Since the LSCA glasses exhibit higher optical basicity (∼0.7), the dopant ions to exist in higher oxidation state (Tb4+ ) become
Table 1 Density (d), average molecular weight (Mavg ), refractive index (ne ), optical basicity (), molecular electronic polarizability (˛e ), glass ionicity (Iglass ) and UV band gap energy for direct (Egd ) and indirect (Egi ) transitions. Glass
d (g/cm3 )
Mavg (g/mol)
ne
˛e (A˚ 3 )
Series 1 CAS-Tb0.25 CAS-Tb0.5 CAS-Tb1 CAS-Tb2 CAS-Tb4 CAS-Tb8 CAS-Tb16 CAS-Tb24 CAS-Tb32 CAS-Tb40
2.694 2.696 2.708 2.725 2.776 2.834 2.964 3.161 3.270 3.403
59.77 59.88 60.15 60.62 61.60 63.52 67.28 70.96 74.55 78.05
1.559 1.555 1.559 1.562 1.565 1.569 1.581 1.600 1.609 1.619
0.5240 0.5242 0.5247 0.5258 0.5278 0.5318 0.5394 0.5469 0.5539 0.5607
2.839 2.826 2.843 2.859 2.865 2.911 2.998 3.044 3.129 3.189
Series 2 LSCA-Tb0.25 LSCA-Tb0.5 LSCA-Tb1 LSCA-Tb2 LSCA-Tb4 LSCA-Tb8 LSCA-Tb16 LSCA-Tb24 LSCA-Tb32 LSCA-Tb40
2.934 2.938 2.946 2.972 3.010 3.088 3.227 3.369 3.487 3.590
69.14 69.28 69.56 70.15 71.22 73.41 77.69 81.85 85.89 89.90
1.665 1.662 1.665 1.668 1.671 1.677 1.686 1.697 1.705 1.713
0.6909 0.6910 0.6913 0.6919 0.6932 0.6956 0.7002 0.7046 0.7087 0.7127
3.469 3.459 3.476 3.487 3.507 3.549 3.631 3.710 3.794 3.891
Iglass
Band gap (eV) Egd
Egi
0.6073 0.6075 0.6077 0.6081 0.6091 0.6108 0.6143 0.6176 0.6208 0.6238
5.03 4.94 4.86 4.81 4.77 4.71 4.65 4.54 4.48 4.45
4.64 4.54 4.50 4.48 4.41 4.35 4.29 4.17 4.10 4.05
0.6640 0.6641 0.6642 0.6645 0.6650 0.6660 0.6679 0.6697 0.6715 0.6732
3.93 3.86 3.70 3.42 3.01 2.65 2.56 2.51 2.45 2.40
3.23 3.20 3.01 2.14 2.09 2.08 2.59 2.04 2.02 1.97
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Fig. 3. Images of CAS and LSCA series of glasses with varied Tb2 O3 concentration. (For interpretation of the references to colour in the text, the reader is referred to the web version of the article.)
more probable [17]. This may be possibly because of the fact that, the network modifying nature of terbium ions, which similar to Ca2+ , expected to help Al3+ ions in charge compensation of [AlO4 ]− tetrahedral units in the network and thereby attain the higher oxidation state (Tb4+ ). To further examine the existence of Tb4+ ions, base glass corrected absorption spectra of LSCA glasses have been recorded and are shown in the inset. The spectrum of LSCA-Tb0.25 glass shows a broad absorption band covering 230–500 nm wavelength region. This band exhibits two distinct absorptions peaking at around 282 nm and 335 nm. The former is due to the f → d transition of Tb3+ ions while the later can be attributed to Tb4+ absorption, which is becoming more and more significant with the increase in Tb2 O3 concentration causing red shift in UV band edge. This strong absorption of Tb4+ has resulted in light yellow to strong amber coloured tint in LSCA glasses as can be seen from the glass images shown in Fig. 3.
3.4. Luminescence spectra and quenching behaviour The emission and excitation spectra of Tb3+ ions in CAS and LSCA glasses are shown in Fig. 4a and b respectively. The emission spectra of CAS glasses on excitation into f → d band of Tb3+ reveal several emission transitions from its 5 D3 and 5 D4 excited levels to ground state multiplets. The emission bands centred at 378 nm, 418 nm, 438 nm, 457 nm and 472 nm are due to 5D → 7F 3 6, 5, 4, 3 and 2 , whereas, those at 482 nm, 542 nm, 586 nm and 625 nm are attributed to 5 D4 → 7 F6, 5, 4 and 3 respectively [7]. Similar emission transitions have been detected for LSCA glasses. The variation of normalized emission intensity from 5 D3 level (blue) and 5 D4 level (green) is pictorially represented in Fig. 5. The blue emission intensity increases first with the increase in dopant concentration and then quenches for further increase in concentration. This quenching is slower in LSCA series of glasses compared to
Fig. 4. (a) Emission and excitation spectra of Tb3+ ions in CAS series of glasses. (b) Emission and excitation spectra of Tb3+ ions in LSCA series of glasses.
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Fig. 6. Normalized spectral overlap of Tb4+ absorption with Tb3+ absorption and emission spectrum in LSCA glasses. Inset shows the simplified representation of Tb3+ → Tb4+ energy transfer mechanism.
Fig. 5. Plot of normalized intensities for blue and green emission with Tb2 O3 contents in CAS and LSCA glasses. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
CAS glasses. For green emission, completely different results have observed. In CAS glasses, the emission intensity increases continuously for the concentration range studied and no quenching has found. However, the LSCA glasses exhibit a decrease in emission intensity for glasses having Tb2 O3 content above 8 wt%. The quenching of blue emission occurs predominantly due to the resonant energy transfer assisted cross-relaxation (RETCR) mechanism among neighbouring Tb3+ ions following the path 5 D3 → 5 D4 :7 F6 → 7 F0 and it becomes more prominent with increasing dopant concentration owing to the reduced Tb3+ –Tb3+ inter-ionic separation [7,18]. The multi-phonon relaxation can also depopulate the 5 D3 level. However, it is comparatively weak owing to a large energy difference of about 6000 cm−1 requiring more than five phonons in CAS glasses and seven in LSCA glasses. But, its influence can still be experienced from the observed slowed-down quenching rate in LSCA glasses, which may have resulted from the reduced multi-phonon relaxation due to lower phonon energy of LSCA glasses. For Tb3+ green emission, concentration quenching is a rare phenomenon, which generally occurs at higher dopant concentration under strong excitation due to the energy migration among Tb3+ ions leading to cooperative up-conversion [7,18]. In the present investigation, CAS glasses do not show any quenching in green luminescence but it is detected in LSCA glasses at considerably lower concentration. It suggests that the observed quenching of green luminescence in LSCA glasses might have been influenced by the presence of Tb4+ ions. It is because, the strong absorption of Tb4+ in LSCA glasses covers both absorption and emission bands of Tb3+ ions which can directly mask the excitation of Tb3+ as well as facilitate the energy transfer from Tb3+ to Tb4+ as can be seen from Fig. 6. The Tb3+ → Tb4+ energy transfer predominantly occurs from 5 D3 and 5 D4 excitation levels of Tb3+ ions, since upper excitation levels are short lived and relaxes non-radiatively through multi-phonon and cross-relaxation processes. Along with the energy transfer from
Tb3+ to Tb4+ , masking of excitation radiation by strong absorption band of Tb4+ results in an inefficient pumping of Tb3+ ions in LSCA glasses and leads to the luminescence quenching. As shown in Fig. 6, the selective decrease in Tb3+ excitation intensity in 300–450 nm wavelength range in LSCA glasses which exactly overlapped by Tb4+ absorption further strengthens above assumption of masking effect by Tb4+ ions. Thus, from the observation of both CAS and LSCA series of glasses, the effect of lower phonon energy of LSCA glasses has mainly reflected on the blue emission arising from 5 D3 excitation level of Tb3+ ions, but the consequent rise in optical basicity influenced the redox state of terbium ions leading to the formation of higher oxidation state (Tb4+ ), which has acted as quencher for Tb3+ green emission. 4. Conclusions In summary, the compositional modification of CAS to LSCA glasses reduced the effective host phonon energy from about 1040 cm−1 to 790 cm−1 resulting in a weaker multi-phonon relaxation of excitation population of dopant ions in LSCA glasses. But, such alteration in chemical composition from CAS to LSCA glasses significantly enhanced the glass optical basicity and electronic polarizability. This enhanced optical basicity of LSCA glasses partially shifted the redox equilibrium of terbium ions towards the formation of higher valence state (Tb4+ ). These non-luminescent Tb4+ ions have acted as quenching centres for Tb3+ luminescence owing to their strong absorption in UV–visible region causing green luminescence quenching in LSCA glasses. Acknowledgements Authors would like to thank Prof. I. Manna, Director, CGCRI for his kind encouragement and permission to publish this work that was carried out in an In-house project (OLP-0288). Authors also thank Dr. Ranjan Sen for his continued support. Mr. A.D.S. is thankful to the CSIR, New Delhi for the award of Senior Research Fellowship to him. References [1] M. Yamane, Y. Asahara, Glasses for Photonics, Cambridge University Press, 2000. [2] G. Fuxi, Laser Materials, World Scientific Publications Co. Ltd., Singapore, 1995.
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