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Ultraviolet and visible transmission spectra of heavy germanate glasses containing Sn2+ and Ce3+
Journal of Non-Crystalline Solids 326&327 (2003) 343–347 www.elsevier.com/locate/jnoncrysol
Ultraviolet and visible transmission spectra of heavy germanate glasses containing Sn2þ and Ce3þ Guorong Chen a,b,*, S. Baccaro b, A. Cecilia b, Yongjuan Du a, M. Montecchi b, Jiaxiang Nie a, Shan Wang b, Yonghui Zhang a a
Department of Inorganic Materials, East China University of Science and Technology, Box 306, 130 Meilong Road, Shanghai 200237, PeopleÕs Republic of China b ENEA-FIS/ION, Via Anguillarese 301, 00060 S. Maria di Galeria, Rome, Italy
Abstract Six heavy germanate glasses containing GeO2 , Gd2 O3 , BaO, SnO2 and La2 O3 are prepared. Ultraviolet and visible transmission spectra are measured and compared. Special attention is paid to the UV absorption edges of these glasses which prove moderate and suitable for hosting scintillating Ce3þ activators. Red shifts of UV absorption bands assigned to the solute Sn2þ and Ce3þ in these glasses are observed. Reasonable explanations are given from the viewpoint of different mechanisms of UV absorption referring to the different types of metal ions existing in the glass network. The relationship between such red shift and compositions of glass matrices is fully discussed in terms of the so-called Ôoptical basicityÕ of glasses. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.25.Gz; 73.61.Jc
1. Introduction In the past decades many investigations on glasses for special applications at the short wavelengths have been reported. Particularly the possibility of using glasses as the ultraviolet and visible transmitting matrices for hosting different functional ions have been well studied. While the
* Corresponding author. Tel.: +86-21 6425 2647; fax: +86-21 6424 6381. E-mail addresses: [email protected], [email protected] (G. Chen).
majority of work in this respect has focused on borate, phosphate, silicate and fluoride glasses [1,2], only a little attention has been given to heavy-metal oxide (HMO) glasses. However, demands for HMO glasses in different fields have increased in recent years because of their unique propertiess, for instances, switching capability, non-linear optical behavior, higher radiation hardness, and so on [3]. In this paper a set of heavy germanate glasses are investigated as hosts for scintillating Ce3þ ions, with the emphasis on their UV transmitting properties. The dependence of their UV absorption band positions on glass compositions is discussed in terms of the so-called Ôoptical basicityÕ of glass systems.
0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-3093(03)00430-7
G. Chen et al. / Journal of Non-Crystalline Solids 326&327 (2003) 343–347
2. Experimental Germanate glasses were prepared for the present work with the glass compositions reported in Table 1. As can be seen, glasses # 1, # 2 and # 3 are matrices containing GeO2 , Gd2 O3 , BaO and SnO2 or La2 O3 , while samples # 4, # 5 and # 6 are doped with Ce3þ ions. Besides the heaver glass network formers GeO2 , all other components are heavy metal oxides. Such compositions are considered for the purpose of producing scintillating glasses with the density higher than 5 g/cm3 so as to be suitable for the application in high energy physics experiments. The aim of adding tin dioxide is to improve both the glass forming ability and the chemical durability [4,5]. Gadolinium oxide was introduced because of its possible involvement in glass forming system at the comparatively higher concentrations, as well as its possible mechanism of enhancing energy transfer towards to Ce3þ emission center which was already demonstrated in the Gd-containing phosphate and silicate glasses [1]. In some compositions La2 O3 was substituted for a same amount of Gd2 O3 in order to investigate any possible effect, if any, induced by different rare earth involvement on transmitting properties of glasses. Starting materials used to prepare glass samples include germanium, tin, gadolinium and lanthanum oxides, as well as barium carbonate (BaCO3 ) and cerium nitrate (Ce(NO3 )3 ) for introducing BaO and Ce2 O3 , respectively. The purity of all raw materials are 3 N without inclusion of transition metals. Batches were mixed and then melted in the fused quartz crucibles at 1500 °C under the controlled reducing atmosphere (N2 plus graphite). Glass melts were put into the steel moulds for quenching in air and annealed in an electric oven
at 600 °C for 3 h, after which the annealing oven was turned off until the samples were finally cooled to the room temperature. Glasses such prepared are optically transparent. The glass blocks taken from the moulds were ground to rectangular size (approximately 40 mm 10 mm 10 mm) and then polished for transmission property measurements. Transmission spectra were measured in the ultraviolet and visible region by a spectrophotometer equipped with an integrating sphere.
3. Results Transmission spectra of glass matrices # 1, # 2 and # 3 are shown in Fig. 1. From this figure it can be seen that the UV absorption band of the glass # 2 (with SnO2 ) is located at the longer wavelength
0.8
Transmittance
344
0.6 #1 #2 #3
0.4
0.2
0 300
400
500
600
700
800
Wavelength (nm)
Fig. 1. Transmission spectra of glass matrices (thickness 40 mm).
Table 1 Glass compositions (mol%) Glass No.
GeO2
Gd2 O3
BaO
SnO2
La2 O3
Ce2 O3 (wt%)
Bandgap (eV 0.01)
1 2 3 4 5 6
60 55 60 60 55 60
5 10 5 5 10 5
35 30 30 35 30 30
– 5 – – 5 –
– – 5 – – 5
– – – 0.5 0.5 0.5
3.35 3.06 3.40 2.88 3.02 3.06
G. Chen et al. / Journal of Non-Crystalline Solids 326&327 (2003) 343–347
Transmittance
0.8
0.6 #2 #5
0.4
0.2
0 400
500
600
700
800
Wavelength (nm)
Fig. 3. Transmission spectra of glasses containing SnO2 and doped with Ce3þ (thickness 40 mm).
0.8
Transmittance
by comparison with those of glasses # 1 and # 3. This absorption band is known to be due to the lowest 5s–5p transition of Sn2þ ions which were reduced from Sn4þ ions during melting under the controlled reducing atmosphere. It is worth noting here that the Sn2þ 5s–5p transition bands in the present germanate glass matrix suffers the obvious red shift with respect to those in the borosilicate, phosphate and fluoride glass matrices, as reported by other investigators. In their observations, the similar red shift of the UV absorption bands induced by Sn2þ also occurred which was found to be in the order of fluoride ! phosphate ! borosilicate glasses [6]. In Figs. 2–4 comparisons of transmitting spectra between glass matrices and their relevant Ce3þ doped counterparts are reported. The longer UV cut-off positions are observed for the Ce3þ -doped glasses, and they are all determined by the lowest 4f–5d transition of Ce3þ ions. In the present germanate glass matrices these bands experience the further red shift when compared with our previous work on the Ce3þ -doped phosphate and silicate scintillating glasses, where the Ce3þ 4f–5d transition bands shifted from phosphate to silicate glasses [1]. Some other investigators witnessed the same red shift for the Ce3þ -doped fluoride and phosphate glasses which was in the order from fluoride to phosphates [7].
345
0.6 #3 #6
0.4
0.2
0 300
0.8
400
500
600
700
800
Transmittance
Wavelength (nm)
Fig. 4. Transmission spectra of glasses containing La2 O3 and doped with Ce3þ (thickness 40 mm).
0.6 #1 #4
0.4
0.2
0 300
400
500
600
700
800
Wavelength (nm)
Fig. 2. Transmission spectra of glasses doped with Ce3þ (thickness 40 mm).
To further manifest the influence of glass matrix composition on the position of 4f–5d transition bands induced by Ce3þ ions, the transmitting spectra of all three Ce3þ -doped glasses are compared together, as illustrated in Fig. 5. It is evidenced from this figure that the UV cut-off positions of these glasses doped with the same concentration of Ce3þ (0.5 wt%) ions are glass matrix composition dependent. The glass # 4, which contains the highest amount of alkali earth
346
G. Chen et al. / Journal of Non-Crystalline Solids 326&327 (2003) 343–347
Transmittance
0.8
0.6 #4 #5 #6
0.4
0.2
0 400
500
600
700
800
Wavelength (nm)
Fig. 5. Transmission spectra of glasses doped with Ce3þ (thickness 40 mm).
oxide (BaO) is found to have the lowest Ce3þ 4f–5d transition energy. In other words, the biggest red shift occurs for the glass matrix having the highest basicity, which is identical to the work of [8], where the pronounced Ce3þ 4f–5d band showed the remarkable red shift with the increasing alkali oxide (Na2 O) content in the borate glasses [8].
4. Discussion Since the scintillating glasses developed for high energy particle experiments are required to possess both high density and high transparency over the ultraviolet and visible region, special attention has to be paid to their UV absorption edges which could be influenced by variations of glass compositions corresponding to the increased density of glasses. Some works in this respect disclosed the negative effect induced by some heavier metal oxides on the UV cut-off positions of glasses which shifted to the wavelength longer than that related to the doped Ce3þ emission centers [9]. The germanate glass matrices in the present work show the moderate UV cut-off edges which are located at the wavelength longer than those of phosphate and silicate scintillating glasses [1], but shorter than the absorption bands induced by the doped Ce3þ cations in these glass matrices, inferring
their suitability as the hosts for scintillating Ce3þ activators. Observation on the red shift of UV absorption edge (Figs. # 1, # 2, # 3 and # 4) can be discussed from viewpoint of different type of metal ions existing in glasses. Generally speaking, the p-block metal ions having oxidation numbers two units less than their group number, such as Sn2þ with the outer 5s2 configuration, and some rare earth ions, such as Ce3þ with the outer 4f1 configuration, tend to influence the UV absorption edge of glasses directly and sensitively to the variation of glass compositions. This is because the mechanism of UV absorption operating for these cations involves the electronic transitions between orbitals located essentially on the ion itself [10]. As the energy levels of these orbitals are governed by the attraction of the screened positive nucleus, the degree of their covalent bonding to surrounding ligands plays an important and straightforward role in affecting this attractive force. Since the ability of oxygen atoms to donate negative charge to cations is affected by their state of polarization which depends simultaneously on the presence of other cationic species in the glass network, such as the network former Ge4þ and the modifiers Ba2þ , Gd3þ , La3þ in the present work, it follows that this electron donor power is most likely correlated to the chemical composition of glass matrices. The electron donor power of oxygen atoms can be expressed in terms of the optical basicity as proposed by Duffy from the 6s–6p transition of Pb2þ in glasses [10]. The optical basicity of oxygen atom approaches a maximum when it is uninfluenced by surrounding cations, such as alkali or earth alkaline cations which are almost nonpolarizing, whereas the polarized oxygen is less able to donate charge to a solute metal ion in the glasses, thus showing the decreasing optical basicity with the increasing covalent bonding of oxygen atoms with other cations, especially the glass network formers. Taking into account all these considerations, the red shift phenomena occurring in the present work can be well explained. Observations on the longer Sn2þ and Ce3þ absorption bands for the present heavy germanate glasses than those previously investigated for the fluoride, phosphate
G. Chen et al. / Journal of Non-Crystalline Solids 326&327 (2003) 343–347
and silicate glass systems [6,7] are consistent to the decreasing covalent bonding of oxygen atoms with the glass network former cations in the order of phosphorous pentoxide ! silica ! germanium oxide, indicating that germanium oxide has the largest optical basicity among these glass network formers. Fluoride is an exception in that fluorine ions have the lowest electron donor power due to the higher electronegativity compared with oxygen ions so as to show the shortest Sn2þ and Ce3þ absorption bands among the glass matrices involved [6,7]. The biggest red shift of UV absorption edge occurring to the Ce3þ -doped glass (# 4) is the consequence of the effect coming from the higher concentration of alkali earth oxide (BaO) which awards the glass matrix (# 1) with the highest optical basicity among other two so as to decrease most the Ce3þ 4f–5d transition in the glass. In Fig. 3 the shift of the UV edge due to Ce3þ doping seems much smaller compared with the other two as shown in Figs. 2 and 4. This is due to the fact that the absorption band induced by Sn2þ in the matrix approaches to that by Ce3þ , while other two matrices were Sn2þ free.
5. Conclusion The present heavy germanate glasses containing GeO2 , Gd2 O3 , BaO and SnO2 or La2 O3 have the UV absorption edges located at a wavelength shorter than that of the typical Ce3þ emission center and therefore suitable for hosting scintillating Ce3þ ions. The UV absorption bands assigned to electron transitions of the solute Sn2þ and Ce3þ ions in the present glasses experience the obvious red shift by comparison with those in the fluoride, phosphate
347
and silicate glasses. The higher optical basicity of germanium oxide compared with other glass network formers explains the reason. The UV absorption band position of Ce3þ ions is glass matrix composition dependent, and the concentration of alkali earth oxide plays an important role.
Acknowledgements The present work is supported by the fund of National Natural Science Foundation of China (NSFC No. 50242017) and the fund of Development Project of Shanghai Priority Academic Discipline.
References [1] S. Baccaro, R. DallÕIgna, P. Fabeni, M. Martini, J.A. Mares, F. Meinardi, M. Nikl, K. Nitsch, G.P. Pazzi, P. Polato, C. Susini, A. Vedda, G. Zanella, R. Zannoni, J. Lumin. 87–89 (2000) 673. [2] P.R. Hobson, Proc. Crystal 2000 Int. Workshop, Heavy Scintillators for Scientific and Industrial Applications, Editions Frontieres, France 1993, p. 343. [3] W.H. Dumbaugh, J.C. Lapp, J. Am. Ceram. Soc. 75 (9) (1992) 2315. [4] M.M. Karim, D. Holland, Phys. Chem. Glasses 36 (5) (1995) 206. [5] A.E. Marino, S.R. Arrasmith, L.L. Gregg, S.D. Jacobs, Guorong Chen, Yongjuan Du, J. Non-Cryst. Solids 289 (1–3) (2001) 37. [6] D. Ehrt, P. Ebeling, U. Natura, Proc. Int. Congr. Glass, Edinburgh, vol. 1, 2001, p. 84. [7] H. Ebendorff-Heidepriem, D. Ehrt, Opt. Mater. 15 (2000) 7. [8] J.A. Duffy, Phys. Chem. Glasses 37 (1996) 45. [9] X. Liu, D.B. Hollis, J. McDougall, Phys. Chem. Glasses 37 (4) (1996) 160. [10] J.A. Duffy, M.D. Ingram, J. Non-Cryst. Solids 21 (1976) 373.
Ultraviolet and visible transmission spectra of heavy germanate glasses containing Sn2þ and Ce3þ Guorong Chen a,b,*, S. Baccaro b, A. Cecilia b, Yongjuan Du a, M. Montecchi b, Jiaxiang Nie a, Shan Wang b, Yonghui Zhang a a
Department of Inorganic Materials, East China University of Science and Technology, Box 306, 130 Meilong Road, Shanghai 200237, PeopleÕs Republic of China b ENEA-FIS/ION, Via Anguillarese 301, 00060 S. Maria di Galeria, Rome, Italy
Abstract Six heavy germanate glasses containing GeO2 , Gd2 O3 , BaO, SnO2 and La2 O3 are prepared. Ultraviolet and visible transmission spectra are measured and compared. Special attention is paid to the UV absorption edges of these glasses which prove moderate and suitable for hosting scintillating Ce3þ activators. Red shifts of UV absorption bands assigned to the solute Sn2þ and Ce3þ in these glasses are observed. Reasonable explanations are given from the viewpoint of different mechanisms of UV absorption referring to the different types of metal ions existing in the glass network. The relationship between such red shift and compositions of glass matrices is fully discussed in terms of the so-called Ôoptical basicityÕ of glasses. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.25.Gz; 73.61.Jc
1. Introduction In the past decades many investigations on glasses for special applications at the short wavelengths have been reported. Particularly the possibility of using glasses as the ultraviolet and visible transmitting matrices for hosting different functional ions have been well studied. While the
* Corresponding author. Tel.: +86-21 6425 2647; fax: +86-21 6424 6381. E-mail addresses: [email protected], [email protected] (G. Chen).
majority of work in this respect has focused on borate, phosphate, silicate and fluoride glasses [1,2], only a little attention has been given to heavy-metal oxide (HMO) glasses. However, demands for HMO glasses in different fields have increased in recent years because of their unique propertiess, for instances, switching capability, non-linear optical behavior, higher radiation hardness, and so on [3]. In this paper a set of heavy germanate glasses are investigated as hosts for scintillating Ce3þ ions, with the emphasis on their UV transmitting properties. The dependence of their UV absorption band positions on glass compositions is discussed in terms of the so-called Ôoptical basicityÕ of glass systems.
0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-3093(03)00430-7
G. Chen et al. / Journal of Non-Crystalline Solids 326&327 (2003) 343–347
2. Experimental Germanate glasses were prepared for the present work with the glass compositions reported in Table 1. As can be seen, glasses # 1, # 2 and # 3 are matrices containing GeO2 , Gd2 O3 , BaO and SnO2 or La2 O3 , while samples # 4, # 5 and # 6 are doped with Ce3þ ions. Besides the heaver glass network formers GeO2 , all other components are heavy metal oxides. Such compositions are considered for the purpose of producing scintillating glasses with the density higher than 5 g/cm3 so as to be suitable for the application in high energy physics experiments. The aim of adding tin dioxide is to improve both the glass forming ability and the chemical durability [4,5]. Gadolinium oxide was introduced because of its possible involvement in glass forming system at the comparatively higher concentrations, as well as its possible mechanism of enhancing energy transfer towards to Ce3þ emission center which was already demonstrated in the Gd-containing phosphate and silicate glasses [1]. In some compositions La2 O3 was substituted for a same amount of Gd2 O3 in order to investigate any possible effect, if any, induced by different rare earth involvement on transmitting properties of glasses. Starting materials used to prepare glass samples include germanium, tin, gadolinium and lanthanum oxides, as well as barium carbonate (BaCO3 ) and cerium nitrate (Ce(NO3 )3 ) for introducing BaO and Ce2 O3 , respectively. The purity of all raw materials are 3 N without inclusion of transition metals. Batches were mixed and then melted in the fused quartz crucibles at 1500 °C under the controlled reducing atmosphere (N2 plus graphite). Glass melts were put into the steel moulds for quenching in air and annealed in an electric oven
at 600 °C for 3 h, after which the annealing oven was turned off until the samples were finally cooled to the room temperature. Glasses such prepared are optically transparent. The glass blocks taken from the moulds were ground to rectangular size (approximately 40 mm 10 mm 10 mm) and then polished for transmission property measurements. Transmission spectra were measured in the ultraviolet and visible region by a spectrophotometer equipped with an integrating sphere.
3. Results Transmission spectra of glass matrices # 1, # 2 and # 3 are shown in Fig. 1. From this figure it can be seen that the UV absorption band of the glass # 2 (with SnO2 ) is located at the longer wavelength
0.8
Transmittance
344
0.6 #1 #2 #3
0.4
0.2
0 300
400
500
600
700
800
Wavelength (nm)
Fig. 1. Transmission spectra of glass matrices (thickness 40 mm).
Table 1 Glass compositions (mol%) Glass No.
GeO2
Gd2 O3
BaO
SnO2
La2 O3
Ce2 O3 (wt%)
Bandgap (eV 0.01)
1 2 3 4 5 6
60 55 60 60 55 60
5 10 5 5 10 5
35 30 30 35 30 30
– 5 – – 5 –
– – 5 – – 5
– – – 0.5 0.5 0.5
3.35 3.06 3.40 2.88 3.02 3.06
G. Chen et al. / Journal of Non-Crystalline Solids 326&327 (2003) 343–347
Transmittance
0.8
0.6 #2 #5
0.4
0.2
0 400
500
600
700
800
Wavelength (nm)
Fig. 3. Transmission spectra of glasses containing SnO2 and doped with Ce3þ (thickness 40 mm).
0.8
Transmittance
by comparison with those of glasses # 1 and # 3. This absorption band is known to be due to the lowest 5s–5p transition of Sn2þ ions which were reduced from Sn4þ ions during melting under the controlled reducing atmosphere. It is worth noting here that the Sn2þ 5s–5p transition bands in the present germanate glass matrix suffers the obvious red shift with respect to those in the borosilicate, phosphate and fluoride glass matrices, as reported by other investigators. In their observations, the similar red shift of the UV absorption bands induced by Sn2þ also occurred which was found to be in the order of fluoride ! phosphate ! borosilicate glasses [6]. In Figs. 2–4 comparisons of transmitting spectra between glass matrices and their relevant Ce3þ doped counterparts are reported. The longer UV cut-off positions are observed for the Ce3þ -doped glasses, and they are all determined by the lowest 4f–5d transition of Ce3þ ions. In the present germanate glass matrices these bands experience the further red shift when compared with our previous work on the Ce3þ -doped phosphate and silicate scintillating glasses, where the Ce3þ 4f–5d transition bands shifted from phosphate to silicate glasses [1]. Some other investigators witnessed the same red shift for the Ce3þ -doped fluoride and phosphate glasses which was in the order from fluoride to phosphates [7].
345
0.6 #3 #6
0.4
0.2
0 300
0.8
400
500
600
700
800
Transmittance
Wavelength (nm)
Fig. 4. Transmission spectra of glasses containing La2 O3 and doped with Ce3þ (thickness 40 mm).
0.6 #1 #4
0.4
0.2
0 300
400
500
600
700
800
Wavelength (nm)
Fig. 2. Transmission spectra of glasses doped with Ce3þ (thickness 40 mm).
To further manifest the influence of glass matrix composition on the position of 4f–5d transition bands induced by Ce3þ ions, the transmitting spectra of all three Ce3þ -doped glasses are compared together, as illustrated in Fig. 5. It is evidenced from this figure that the UV cut-off positions of these glasses doped with the same concentration of Ce3þ (0.5 wt%) ions are glass matrix composition dependent. The glass # 4, which contains the highest amount of alkali earth
346
G. Chen et al. / Journal of Non-Crystalline Solids 326&327 (2003) 343–347
Transmittance
0.8
0.6 #4 #5 #6
0.4
0.2
0 400
500
600
700
800
Wavelength (nm)
Fig. 5. Transmission spectra of glasses doped with Ce3þ (thickness 40 mm).
oxide (BaO) is found to have the lowest Ce3þ 4f–5d transition energy. In other words, the biggest red shift occurs for the glass matrix having the highest basicity, which is identical to the work of [8], where the pronounced Ce3þ 4f–5d band showed the remarkable red shift with the increasing alkali oxide (Na2 O) content in the borate glasses [8].
4. Discussion Since the scintillating glasses developed for high energy particle experiments are required to possess both high density and high transparency over the ultraviolet and visible region, special attention has to be paid to their UV absorption edges which could be influenced by variations of glass compositions corresponding to the increased density of glasses. Some works in this respect disclosed the negative effect induced by some heavier metal oxides on the UV cut-off positions of glasses which shifted to the wavelength longer than that related to the doped Ce3þ emission centers [9]. The germanate glass matrices in the present work show the moderate UV cut-off edges which are located at the wavelength longer than those of phosphate and silicate scintillating glasses [1], but shorter than the absorption bands induced by the doped Ce3þ cations in these glass matrices, inferring
their suitability as the hosts for scintillating Ce3þ activators. Observation on the red shift of UV absorption edge (Figs. # 1, # 2, # 3 and # 4) can be discussed from viewpoint of different type of metal ions existing in glasses. Generally speaking, the p-block metal ions having oxidation numbers two units less than their group number, such as Sn2þ with the outer 5s2 configuration, and some rare earth ions, such as Ce3þ with the outer 4f1 configuration, tend to influence the UV absorption edge of glasses directly and sensitively to the variation of glass compositions. This is because the mechanism of UV absorption operating for these cations involves the electronic transitions between orbitals located essentially on the ion itself [10]. As the energy levels of these orbitals are governed by the attraction of the screened positive nucleus, the degree of their covalent bonding to surrounding ligands plays an important and straightforward role in affecting this attractive force. Since the ability of oxygen atoms to donate negative charge to cations is affected by their state of polarization which depends simultaneously on the presence of other cationic species in the glass network, such as the network former Ge4þ and the modifiers Ba2þ , Gd3þ , La3þ in the present work, it follows that this electron donor power is most likely correlated to the chemical composition of glass matrices. The electron donor power of oxygen atoms can be expressed in terms of the optical basicity as proposed by Duffy from the 6s–6p transition of Pb2þ in glasses [10]. The optical basicity of oxygen atom approaches a maximum when it is uninfluenced by surrounding cations, such as alkali or earth alkaline cations which are almost nonpolarizing, whereas the polarized oxygen is less able to donate charge to a solute metal ion in the glasses, thus showing the decreasing optical basicity with the increasing covalent bonding of oxygen atoms with other cations, especially the glass network formers. Taking into account all these considerations, the red shift phenomena occurring in the present work can be well explained. Observations on the longer Sn2þ and Ce3þ absorption bands for the present heavy germanate glasses than those previously investigated for the fluoride, phosphate
G. Chen et al. / Journal of Non-Crystalline Solids 326&327 (2003) 343–347
and silicate glass systems [6,7] are consistent to the decreasing covalent bonding of oxygen atoms with the glass network former cations in the order of phosphorous pentoxide ! silica ! germanium oxide, indicating that germanium oxide has the largest optical basicity among these glass network formers. Fluoride is an exception in that fluorine ions have the lowest electron donor power due to the higher electronegativity compared with oxygen ions so as to show the shortest Sn2þ and Ce3þ absorption bands among the glass matrices involved [6,7]. The biggest red shift of UV absorption edge occurring to the Ce3þ -doped glass (# 4) is the consequence of the effect coming from the higher concentration of alkali earth oxide (BaO) which awards the glass matrix (# 1) with the highest optical basicity among other two so as to decrease most the Ce3þ 4f–5d transition in the glass. In Fig. 3 the shift of the UV edge due to Ce3þ doping seems much smaller compared with the other two as shown in Figs. 2 and 4. This is due to the fact that the absorption band induced by Sn2þ in the matrix approaches to that by Ce3þ , while other two matrices were Sn2þ free.
5. Conclusion The present heavy germanate glasses containing GeO2 , Gd2 O3 , BaO and SnO2 or La2 O3 have the UV absorption edges located at a wavelength shorter than that of the typical Ce3þ emission center and therefore suitable for hosting scintillating Ce3þ ions. The UV absorption bands assigned to electron transitions of the solute Sn2þ and Ce3þ ions in the present glasses experience the obvious red shift by comparison with those in the fluoride, phosphate
347
and silicate glasses. The higher optical basicity of germanium oxide compared with other glass network formers explains the reason. The UV absorption band position of Ce3þ ions is glass matrix composition dependent, and the concentration of alkali earth oxide plays an important role.
Acknowledgements The present work is supported by the fund of National Natural Science Foundation of China (NSFC No. 50242017) and the fund of Development Project of Shanghai Priority Academic Discipline.
References [1] S. Baccaro, R. DallÕIgna, P. Fabeni, M. Martini, J.A. Mares, F. Meinardi, M. Nikl, K. Nitsch, G.P. Pazzi, P. Polato, C. Susini, A. Vedda, G. Zanella, R. Zannoni, J. Lumin. 87–89 (2000) 673. [2] P.R. Hobson, Proc. Crystal 2000 Int. Workshop, Heavy Scintillators for Scientific and Industrial Applications, Editions Frontieres, France 1993, p. 343. [3] W.H. Dumbaugh, J.C. Lapp, J. Am. Ceram. Soc. 75 (9) (1992) 2315. [4] M.M. Karim, D. Holland, Phys. Chem. Glasses 36 (5) (1995) 206. [5] A.E. Marino, S.R. Arrasmith, L.L. Gregg, S.D. Jacobs, Guorong Chen, Yongjuan Du, J. Non-Cryst. Solids 289 (1–3) (2001) 37. [6] D. Ehrt, P. Ebeling, U. Natura, Proc. Int. Congr. Glass, Edinburgh, vol. 1, 2001, p. 84. [7] H. Ebendorff-Heidepriem, D. Ehrt, Opt. Mater. 15 (2000) 7. [8] J.A. Duffy, Phys. Chem. Glasses 37 (1996) 45. [9] X. Liu, D.B. Hollis, J. McDougall, Phys. Chem. Glasses 37 (4) (1996) 160. [10] J.A. Duffy, M.D. Ingram, J. Non-Cryst. Solids 21 (1976) 373.