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Raman spectroscopic investigation on high refractive index glasses prepared from local quartz sand
Spectrochimica Acta Part A 73 (2009) 440–442
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Raman spectroscopic investigation on high refractive index glasses prepared from local quartz sand P. Dararutana a,b,e,∗ , S. Pongkrapan c,d , N. Sirikulrat e,f , M. Thawornmongkolkij g , P. Wathanakul c,d,g a
Royal Thai Army Chemical Department, Phaholyothin Road, Chatuchak, Bangkok 10900, Thailand Graduate School of Chiang Mai University, Chiang Mai, Thailand c Gemmology and Mineral Sciences Special Research Unit, Faculty of Science, Kasetsart University, Bangkok, Thailand d Department of Earth Sciences, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand e Glass and Glass Products Research and Development Laboratory, Institute for Science and Technology Research and Development, Chiang Mai University, Chiang Mai 50200, Thailand f Department of Physics, Faculty of Sciences, Chiang Mai University, Chiang Mai, Thailand g Gem and Jewelry Institute of Thailand, Bangkok 10330, Thailand b
a r t i c l e
i n f o
Article history: Received 29 June 2008 Received in revised form 15 October 2008 Accepted 23 October 2008 Keywords: High refractive index glasses Local quartz sand Physical and optical properties Raman spectroscopy
a b s t r a c t High refractive index (RI) glasses prepared from local quartz sand and compounds of heavy elements, such as, barium carbonate, lead oxide, and bismuth oxide as major ingredients were investigated using Raman spectroscopy. The results showed changes in glass structures of different doping elements, namely, Ba, Pb, and Bi. Refractive indices, densities, and UV–vis–NIR spectra of the glass samples were also measured. The Raman spectroscopy can be used to investigate and/or identify heavy glasses, local ancient glasses as well as glass jewelry. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
1. Introduction Most high refractive index (RI) glasses are generally produced from lead oxide and silica [1]. This type of glasses is highly transparent in the visible and near infrared regions and exhibits very good formation over a large compositional region. Moreover, the glass containing PbO [2–6] and B2 O3 has a radiation shielding property as the naturally occurring, stable boron isotope is a good absorber of thermal neutrons, and lead is known as a shielding material of gamma radiation [5,7]. Lead oxide is best known for its unique influence on glass structure and is widely used in glass production to prevent devitrification, to improve chemical durability, and to lower the melting temperature. With varying chemical compositions, multi-component glasses, typically consisting of network formers and modifiers, offer a wide range of desired properties for new applications, for instance, the use of heavy metal oxide glasses in IR transmitting devices, nonlinear optics, glass ceramics, optoelectronic devices, and radiation shielding purposes, etc. [8–10].
∗ Corresponding author at: Royal Thai Army Chemical Department, Phaholyothin Road, Chatuchak, Bangkok 10900, Thailand. Tel.: +66 8 15531191; fax: +66 25192866. E-mail address: [email protected] (P. Dararutana).
However, because of the toxicity on both humans and the environment, lead-free glasses were fabricated using other heavy elements, such as, barium and bismuth. It has been reported previously that local quartz sand from Thailand was used in the glass fabrication and yielded large satisfactory results [1]. Considerable numbers of studies have employed Raman spectroscopy to investigate the structures of silicate glasses and melts [1] while many have applied such technique to understanding physical properties in geological processes [2,12–17]. In this study, the technique of Raman spectroscopy was used to study the effect of barium carbonate and bismuth oxide doped in (Na2 O, K2 O)–CaO–MgO–B2 O3 –SiO2 glass system, compared with lead oxide doped glass.
2. Experiment 2.1. Preparation of glass samples The samples were prepared by a conventional melting technique [4] in the laboratory scale. The glass samples with molar composition of (Na2 O, K2 O)–BaO–CaO–MgO–B2 O3 –SiO2 , (Na2 O, K2 O)–PbO2 –CaO–MgO–B2 O3 –SiO2 and (Na2 O, K2 O)–Bi2 O3 – CaO–MgO–B2 O3 –SiO2 were prepared by melting about 150 g batch
1386-1425/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.10.036
P. Dararutana et al. / Spectrochimica Acta Part A 73 (2009) 440–442
441
compositions. The mixture was of 30 mol% domestic quartz sand, 40 mol% mixtures of sodium- and potassium-carbonates, oxides of calcium and magnesium, and boric acid. BaCO3 , PbO2 , and Bi2 O3 each with 30 mol% concentration were also added into the primary glass mixture. The homogeneous mixture in ceramic crucible was melted in an electric furnace with temperature from 1000 to 1250 ◦ C for 4 h. The melted glass was poured onto a preheated metal cylindrical mold and annealed to room temperature. 2.2. Density and refractive index measurements The values of refractive index were measured by means of the prism coupling technique, using a Rayner Duplex II refractometer at wavelength of 589 nm. The densities of the glass samples were determined by the Archimedes’ principle, using a Mettler Toledo AG 104 analytical balance with distilled water as an immersion liquid.
Fig. 1. Raman spectra of glass samples doped with different heavy elements, Ba, Pb, and Bi.
2.3. Raman spectroscopy The Raman spectra were measured using a Renishaw inVia Raman Microscope running from 240 to 2000 cm−1 . Argon laser beam of 514 nm, 2400 l mm−1 (vis) grating, 10 exposure times, 1 accumulation and 1% power (cross-polar) was used for excitation. Each glass sample was measured at least in two areas. 2.4. UV–vis absorption spectroscopy Absorption spectra were recorded at room temperature using a Perkin-Elmer Lambda 900 UV–vis–NIR spectrophotometer for wavelengths ranging between 200 and 1300 nm with ±0.8 nm accuracy in the visible range.
3.1. Preparation of glass samples It was found that the melting temperatures of the prepared glass samples containing Ba, Pb, and Bi were different. At the same concentrations, the Ba-, Pb-, and Bi-based glasses were melted at approximately 1250, 1150, and 1000 ◦ C, respectively. It was also shown by observations that the resulted colorless glass samples appeared homogeneous in colors and free of bubbles. The samples were cut and polished into thin slabs of about 1 cm × 1 cm × 0.3 cm in size. 3.2. Density and refractive index The values of both density and refractive index of Ba-, Pb-, and Bi-based glass samples were 2.91, 3.07, 3.32 g cm−3 , and 1.600, 1.690, 1.725, respectively (Table 1). Both of these values were increased with increasing atomic numbers of the heavy element being doped.
Table 1 The density and the refractive index of the prepared glass samples.
Ba Pb Bi
Barium
Lead
Bismuth
30 – –
– 30 –
– – 30
Simple oxides such as SiO2 and B2 O3 are classified as glass forming oxides, whereas Bi2 O3 and BaO are conditional glass formers, and PbO acts as both glass former and modifier [4,6]. The Raman spectra of the prepared glass samples consisted of broad peaks in the range of 240–2000 cm−1 , as shown in Fig. 1. The vibrational spectra of each investigated glass samples were dominated by bonds associated with the structural units of the heaviest cations, Ba2+ , Pb2+ , and Bi3+ . The Raman bands due to the heavy metal oxides, such as, BaO, PbO and Bi2 O3 , can be categorized into four regions [18]: (1) (2) (3) (4)
3. Results and discussions
Glass composition (mol%)
3.3. Raman spectra
Density (g cm−3 )
2.91 3.07 3.32
RI
1.600 1.690 1.725
low wave number Raman modes (<100 cm−1 ); heavy ion vibration between 70 and 160 cm−1 ; bridged anion modes in the intermediate 300–600 cm−1 range; non-bridging anion modes at higher wave numbers.
In this study, Raman spectroscopy has been measured in the bridged and non-bridging regions of wave number between 200 and 2000 cm−1 . Raman spectra of the different glasses are shown in Fig. 1 and Table 2. Raman spectra of SiO2 observed at ∼800 to ∼1200 cm−1 showed the sharp peak at ∼1100 cm−1 corresponded to Si–O–Si antisymmetric vibrations for Pb-based glass and showed the bands of both the Ba- and Bi-based glasses [5,19–22]. The observed Raman spectrum of Ba-based glass sample at ∼520 cm−1 indicated the symmetric stretching [23,24]. The recorded Raman spectrum of Pb-based glass sample at ∼580 cm−1 showed the symmetric stretching of Pb–O–Pb [19,25,26]. The Raman spectra of Bi-based glass sample at ∼510 cm−1 corresponded to the Bi–O–Bi symmetric stretching. These IR peaks were shifted [11,21,27,28].
Table 2 Raman spectra and assignments of the prepared glass samples. Peak (cm−1 )
Assignment
∼520 ∼510 ∼580
Ba–O–Ba symmetric stretching Bi–O–Bi symmetric stretching Pb–O–Pb symmetric stretching
442
P. Dararutana et al. / Spectrochimica Acta Part A 73 (2009) 440–442
Fig. 2. UV–vis–NIR spectra of Ba-, Pb- and Bi-based glass samples.
3.4. UV–vis–NIR spectra The UV–vis–NIR absorption spectra of thin specimens of the Ba-, Pb-, and Bi-based glass samples are shown in Fig. 2. The absorption began to rise slowly at 460 and showed sharp peak at 1188 nm for Ba-based glass, and 481, and 478 nm for Pb- and Bi-based glass, respectively. These absorption peaks were shifted [29–31]. The absorption edges reflected better dissolutions of Ba and Pb (both at about 300 nm) compared with Bi (∼380 nm) in glass mixtures. 4. Conclusion By means of Raman spectroscopy, the local structural peculiarities of a new glass systems; (Na2 O, K2 O)– (Na2 O, K2 O)–PbO2 –CaO–MgO– BaO–CaO–MgO–B2 O3 –SiO2 , B2 O3 –SiO2 and (Na2 O, K2 O)–Bi2 O3 –CaO–MgO–B2 O3 –SiO2 were analyzed. It can be concluded that both barium and bismuth have been used in place of lead in the lead-free, environmental-friendly glass fabrication to be used for glass jewelries and radiation shielding. Acknowledgments Part of the research has been funded by the Graduate School of Chiang Mai University. The Glass and Glass Products Research and Development Laboratory, Institute for Science and Technology of Chiang Mai University jointly supported the glass fabrication experiments. Department of Earth Sciences, the Faculty of Science, Kasetsart University, and the Gem Testing Laboratory at Gem and Jewelry Institute of Thailand (GIT) provided Raman microscope.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
P. Dararutana, et al., Advanced Materials Research 39–40 (2008) 257. C. Dayamand, et al., Materials Letters 23 (1995) 309. R.C. Lucacel, et al., Journal of Non-Crystalline Solids 353 (2007) 2020. R. Ciceo, et al., Journal of Optoelectronics and Advanced Materials 9 (2007) 747. R.C. Lucacel, et al., Solid State Sciences 9 (2007) 850. I. Ardelean, V. Timar, Journal of Optoelectronics and Advanced Materials 10 (2008) 246. P. Dararutana, et al., Abstracts of 11th International Conference on Radiation Shielding, Georgia, April 13–18, 2008, PSTR 16. S. Sindu, et al., Materials Chemistry and Physics 90 (2005) 83. I. Ardelean, D. Rusu, Journal of Optoelectronics and Advanced Materials 10 (2008) 66. H. Singh, et al., Nuclear Science and Engineering 143 (2002) 342. S. Bale, et al., Solid State Sciences 10 (2008) 326. P. McMillan, American Mineralogist 69 (1984) 622; P. McMillan, American Mineralogist 69 (1984) 645. A.M. Zahra, et al., Journal of Non-Crystalline Solids 155 (1993) 45. A.M. Efimov, Journal of Non-Crystalline Solids 253 (1999) 95. P. Colomban, Applied Physics A 79 (2004) 167. D. Maniu, et al., Romanian Reports 56 (2004) 419. A. Bertoluzza, Journal of Raman Spectroscopy 26 (2005) 751. M.E. Lines, et al., Journal of Non-Crystalline Solid 89 (1987) 163. B. Boizot, et al., Journal of Non-Crystalline Solids 325 (2003) 22. H. Jia, et al., Optical Materials 29 (2008) 445. P. Colomban, et al., Journal of Raman Spectroscopy 37 (2006) 614. R. Alzagi, et al., Journal of Non-Crystalline Solids 293–295 (2001) 471. V. Marinova, et al., Journal of Physics Condensed Matter 18 (2006) 385. M. Radovic, et al., Science of Sintering 39 (2007) 281. R.C. Lucacel, I. Ardelean, Journal of Non-Crystalline Solids 353 (2007) 2020. H. Ticha, et al., Journal of Optoelectronics and Advanced Materials 10 (2008) 2117. H. Jia, et al., Optical Materials 29 (2006) 445. D. Rusu, I. Ardelean, Materials Research Bulletin 43 (2008) 1724. X.G. Meng, et al., Optical Express 13 (2005) 1635. F. Sayilkan, et al., Journal of Materials Science 40 (2005) 5779. T. Suzuki, Y. Ohishi, Applied Physics Letters 88 (2006) 191912.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Raman spectroscopic investigation on high refractive index glasses prepared from local quartz sand P. Dararutana a,b,e,∗ , S. Pongkrapan c,d , N. Sirikulrat e,f , M. Thawornmongkolkij g , P. Wathanakul c,d,g a
Royal Thai Army Chemical Department, Phaholyothin Road, Chatuchak, Bangkok 10900, Thailand Graduate School of Chiang Mai University, Chiang Mai, Thailand c Gemmology and Mineral Sciences Special Research Unit, Faculty of Science, Kasetsart University, Bangkok, Thailand d Department of Earth Sciences, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand e Glass and Glass Products Research and Development Laboratory, Institute for Science and Technology Research and Development, Chiang Mai University, Chiang Mai 50200, Thailand f Department of Physics, Faculty of Sciences, Chiang Mai University, Chiang Mai, Thailand g Gem and Jewelry Institute of Thailand, Bangkok 10330, Thailand b
a r t i c l e
i n f o
Article history: Received 29 June 2008 Received in revised form 15 October 2008 Accepted 23 October 2008 Keywords: High refractive index glasses Local quartz sand Physical and optical properties Raman spectroscopy
a b s t r a c t High refractive index (RI) glasses prepared from local quartz sand and compounds of heavy elements, such as, barium carbonate, lead oxide, and bismuth oxide as major ingredients were investigated using Raman spectroscopy. The results showed changes in glass structures of different doping elements, namely, Ba, Pb, and Bi. Refractive indices, densities, and UV–vis–NIR spectra of the glass samples were also measured. The Raman spectroscopy can be used to investigate and/or identify heavy glasses, local ancient glasses as well as glass jewelry. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.
1. Introduction Most high refractive index (RI) glasses are generally produced from lead oxide and silica [1]. This type of glasses is highly transparent in the visible and near infrared regions and exhibits very good formation over a large compositional region. Moreover, the glass containing PbO [2–6] and B2 O3 has a radiation shielding property as the naturally occurring, stable boron isotope is a good absorber of thermal neutrons, and lead is known as a shielding material of gamma radiation [5,7]. Lead oxide is best known for its unique influence on glass structure and is widely used in glass production to prevent devitrification, to improve chemical durability, and to lower the melting temperature. With varying chemical compositions, multi-component glasses, typically consisting of network formers and modifiers, offer a wide range of desired properties for new applications, for instance, the use of heavy metal oxide glasses in IR transmitting devices, nonlinear optics, glass ceramics, optoelectronic devices, and radiation shielding purposes, etc. [8–10].
∗ Corresponding author at: Royal Thai Army Chemical Department, Phaholyothin Road, Chatuchak, Bangkok 10900, Thailand. Tel.: +66 8 15531191; fax: +66 25192866. E-mail address: [email protected] (P. Dararutana).
However, because of the toxicity on both humans and the environment, lead-free glasses were fabricated using other heavy elements, such as, barium and bismuth. It has been reported previously that local quartz sand from Thailand was used in the glass fabrication and yielded large satisfactory results [1]. Considerable numbers of studies have employed Raman spectroscopy to investigate the structures of silicate glasses and melts [1] while many have applied such technique to understanding physical properties in geological processes [2,12–17]. In this study, the technique of Raman spectroscopy was used to study the effect of barium carbonate and bismuth oxide doped in (Na2 O, K2 O)–CaO–MgO–B2 O3 –SiO2 glass system, compared with lead oxide doped glass.
2. Experiment 2.1. Preparation of glass samples The samples were prepared by a conventional melting technique [4] in the laboratory scale. The glass samples with molar composition of (Na2 O, K2 O)–BaO–CaO–MgO–B2 O3 –SiO2 , (Na2 O, K2 O)–PbO2 –CaO–MgO–B2 O3 –SiO2 and (Na2 O, K2 O)–Bi2 O3 – CaO–MgO–B2 O3 –SiO2 were prepared by melting about 150 g batch
1386-1425/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.10.036
P. Dararutana et al. / Spectrochimica Acta Part A 73 (2009) 440–442
441
compositions. The mixture was of 30 mol% domestic quartz sand, 40 mol% mixtures of sodium- and potassium-carbonates, oxides of calcium and magnesium, and boric acid. BaCO3 , PbO2 , and Bi2 O3 each with 30 mol% concentration were also added into the primary glass mixture. The homogeneous mixture in ceramic crucible was melted in an electric furnace with temperature from 1000 to 1250 ◦ C for 4 h. The melted glass was poured onto a preheated metal cylindrical mold and annealed to room temperature. 2.2. Density and refractive index measurements The values of refractive index were measured by means of the prism coupling technique, using a Rayner Duplex II refractometer at wavelength of 589 nm. The densities of the glass samples were determined by the Archimedes’ principle, using a Mettler Toledo AG 104 analytical balance with distilled water as an immersion liquid.
Fig. 1. Raman spectra of glass samples doped with different heavy elements, Ba, Pb, and Bi.
2.3. Raman spectroscopy The Raman spectra were measured using a Renishaw inVia Raman Microscope running from 240 to 2000 cm−1 . Argon laser beam of 514 nm, 2400 l mm−1 (vis) grating, 10 exposure times, 1 accumulation and 1% power (cross-polar) was used for excitation. Each glass sample was measured at least in two areas. 2.4. UV–vis absorption spectroscopy Absorption spectra were recorded at room temperature using a Perkin-Elmer Lambda 900 UV–vis–NIR spectrophotometer for wavelengths ranging between 200 and 1300 nm with ±0.8 nm accuracy in the visible range.
3.1. Preparation of glass samples It was found that the melting temperatures of the prepared glass samples containing Ba, Pb, and Bi were different. At the same concentrations, the Ba-, Pb-, and Bi-based glasses were melted at approximately 1250, 1150, and 1000 ◦ C, respectively. It was also shown by observations that the resulted colorless glass samples appeared homogeneous in colors and free of bubbles. The samples were cut and polished into thin slabs of about 1 cm × 1 cm × 0.3 cm in size. 3.2. Density and refractive index The values of both density and refractive index of Ba-, Pb-, and Bi-based glass samples were 2.91, 3.07, 3.32 g cm−3 , and 1.600, 1.690, 1.725, respectively (Table 1). Both of these values were increased with increasing atomic numbers of the heavy element being doped.
Table 1 The density and the refractive index of the prepared glass samples.
Ba Pb Bi
Barium
Lead
Bismuth
30 – –
– 30 –
– – 30
Simple oxides such as SiO2 and B2 O3 are classified as glass forming oxides, whereas Bi2 O3 and BaO are conditional glass formers, and PbO acts as both glass former and modifier [4,6]. The Raman spectra of the prepared glass samples consisted of broad peaks in the range of 240–2000 cm−1 , as shown in Fig. 1. The vibrational spectra of each investigated glass samples were dominated by bonds associated with the structural units of the heaviest cations, Ba2+ , Pb2+ , and Bi3+ . The Raman bands due to the heavy metal oxides, such as, BaO, PbO and Bi2 O3 , can be categorized into four regions [18]: (1) (2) (3) (4)
3. Results and discussions
Glass composition (mol%)
3.3. Raman spectra
Density (g cm−3 )
2.91 3.07 3.32
RI
1.600 1.690 1.725
low wave number Raman modes (<100 cm−1 ); heavy ion vibration between 70 and 160 cm−1 ; bridged anion modes in the intermediate 300–600 cm−1 range; non-bridging anion modes at higher wave numbers.
In this study, Raman spectroscopy has been measured in the bridged and non-bridging regions of wave number between 200 and 2000 cm−1 . Raman spectra of the different glasses are shown in Fig. 1 and Table 2. Raman spectra of SiO2 observed at ∼800 to ∼1200 cm−1 showed the sharp peak at ∼1100 cm−1 corresponded to Si–O–Si antisymmetric vibrations for Pb-based glass and showed the bands of both the Ba- and Bi-based glasses [5,19–22]. The observed Raman spectrum of Ba-based glass sample at ∼520 cm−1 indicated the symmetric stretching [23,24]. The recorded Raman spectrum of Pb-based glass sample at ∼580 cm−1 showed the symmetric stretching of Pb–O–Pb [19,25,26]. The Raman spectra of Bi-based glass sample at ∼510 cm−1 corresponded to the Bi–O–Bi symmetric stretching. These IR peaks were shifted [11,21,27,28].
Table 2 Raman spectra and assignments of the prepared glass samples. Peak (cm−1 )
Assignment
∼520 ∼510 ∼580
Ba–O–Ba symmetric stretching Bi–O–Bi symmetric stretching Pb–O–Pb symmetric stretching
442
P. Dararutana et al. / Spectrochimica Acta Part A 73 (2009) 440–442
Fig. 2. UV–vis–NIR spectra of Ba-, Pb- and Bi-based glass samples.
3.4. UV–vis–NIR spectra The UV–vis–NIR absorption spectra of thin specimens of the Ba-, Pb-, and Bi-based glass samples are shown in Fig. 2. The absorption began to rise slowly at 460 and showed sharp peak at 1188 nm for Ba-based glass, and 481, and 478 nm for Pb- and Bi-based glass, respectively. These absorption peaks were shifted [29–31]. The absorption edges reflected better dissolutions of Ba and Pb (both at about 300 nm) compared with Bi (∼380 nm) in glass mixtures. 4. Conclusion By means of Raman spectroscopy, the local structural peculiarities of a new glass systems; (Na2 O, K2 O)– (Na2 O, K2 O)–PbO2 –CaO–MgO– BaO–CaO–MgO–B2 O3 –SiO2 , B2 O3 –SiO2 and (Na2 O, K2 O)–Bi2 O3 –CaO–MgO–B2 O3 –SiO2 were analyzed. It can be concluded that both barium and bismuth have been used in place of lead in the lead-free, environmental-friendly glass fabrication to be used for glass jewelries and radiation shielding. Acknowledgments Part of the research has been funded by the Graduate School of Chiang Mai University. The Glass and Glass Products Research and Development Laboratory, Institute for Science and Technology of Chiang Mai University jointly supported the glass fabrication experiments. Department of Earth Sciences, the Faculty of Science, Kasetsart University, and the Gem Testing Laboratory at Gem and Jewelry Institute of Thailand (GIT) provided Raman microscope.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
P. Dararutana, et al., Advanced Materials Research 39–40 (2008) 257. C. Dayamand, et al., Materials Letters 23 (1995) 309. R.C. Lucacel, et al., Journal of Non-Crystalline Solids 353 (2007) 2020. R. Ciceo, et al., Journal of Optoelectronics and Advanced Materials 9 (2007) 747. R.C. Lucacel, et al., Solid State Sciences 9 (2007) 850. I. Ardelean, V. Timar, Journal of Optoelectronics and Advanced Materials 10 (2008) 246. P. Dararutana, et al., Abstracts of 11th International Conference on Radiation Shielding, Georgia, April 13–18, 2008, PSTR 16. S. Sindu, et al., Materials Chemistry and Physics 90 (2005) 83. I. Ardelean, D. Rusu, Journal of Optoelectronics and Advanced Materials 10 (2008) 66. H. Singh, et al., Nuclear Science and Engineering 143 (2002) 342. S. Bale, et al., Solid State Sciences 10 (2008) 326. P. McMillan, American Mineralogist 69 (1984) 622; P. McMillan, American Mineralogist 69 (1984) 645. A.M. Zahra, et al., Journal of Non-Crystalline Solids 155 (1993) 45. A.M. Efimov, Journal of Non-Crystalline Solids 253 (1999) 95. P. Colomban, Applied Physics A 79 (2004) 167. D. Maniu, et al., Romanian Reports 56 (2004) 419. A. Bertoluzza, Journal of Raman Spectroscopy 26 (2005) 751. M.E. Lines, et al., Journal of Non-Crystalline Solid 89 (1987) 163. B. Boizot, et al., Journal of Non-Crystalline Solids 325 (2003) 22. H. Jia, et al., Optical Materials 29 (2008) 445. P. Colomban, et al., Journal of Raman Spectroscopy 37 (2006) 614. R. Alzagi, et al., Journal of Non-Crystalline Solids 293–295 (2001) 471. V. Marinova, et al., Journal of Physics Condensed Matter 18 (2006) 385. M. Radovic, et al., Science of Sintering 39 (2007) 281. R.C. Lucacel, I. Ardelean, Journal of Non-Crystalline Solids 353 (2007) 2020. H. Ticha, et al., Journal of Optoelectronics and Advanced Materials 10 (2008) 2117. H. Jia, et al., Optical Materials 29 (2006) 445. D. Rusu, I. Ardelean, Materials Research Bulletin 43 (2008) 1724. X.G. Meng, et al., Optical Express 13 (2005) 1635. F. Sayilkan, et al., Journal of Materials Science 40 (2005) 5779. T. Suzuki, Y. Ohishi, Applied Physics Letters 88 (2006) 191912.