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Influence of europium ions on structure and crystallization properties of bismuth borate glasses and glass ceramics
Journal of Non-Crystalline Solids 354 (2008) 5475–5479
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Influence of europium ions on structure and crystallization properties of bismuth borate glasses and glass ceramics Petru Pascuta a,*, Gheorghe Borodi b, Eugen Culea a a b
Physics Department, Technical University, C. Daicoviciu 15, 400020 Cluj-Napoca, Romania National Institute for R&D of Isotopic and Molecular Technology, P.O. Box 700, RO-400293 Cluj-Napoca, Romania
a r t i c l e
i n f o
Article history: Received 24 January 2008 Received in revised form 11 August 2008 Available online 20 October 2008 PACS: 81.05.Kf 81.05.Pj 61.05.cp 78.30.j Keywords: Glass ceramics X-ray diffraction FTIR measurements Borates
a b s t r a c t Glasses of the xEu2O3 (100x)[2Bi2O3 B2O3] system with 0 6 x 6 25 mol% have been characterized by X-ray diffraction and FTIR spectroscopy measurements. Melting at 1100 °C and the rapid cooling at room temperature permitted us to obtain glass samples. In order to improve the local order and to develop crystalline phases, the glass samples were kept at 625 °C for 24 h. After heat treatment two crystalline phases were put into evidence. One of the crystalline phases was observed for the host glass matrix, the x = 0 mol% sample, and belongs to the cubic system. The second one was observed for the x = 25 mol% sample and was find to be orthorhombic with two unit cell parameters very close to each other. For the samples with 0 < x < 25 mol% there is a mixture of the two mentioned phases. FTIR spectroscopy data suggest that both Bi2O3 and B2O3 play the glass network former role while the europium ions play the network modifier role in the studied glasses. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Glasses based on heavy metal oxides, such as Bi2O3, are interesting because their properties are exploited in many applications such as wave-guides in non-linear optics, radiation shielding windows, scintillation counters, optical transmission devices, thermal and mechanical sensors as well as optical devices, such as optical fibers, optical switching, optical memory etc. [1–4]. Glasses containing two glass-forming oxides, such as the bismuth borate glasses, which have superior properties, have also a wide range of practical applications. B2O3 is a typical glass former while Bi2O3 is a conditional glass former. B2O3 alone can form a glass network that consists of BO3 triangles forming three-member (boroxol) rings connected by B–O–B linkages [5]. It was reported that addition of a network modifier in borate glasses causes a progressive change of some of the triangular BO3 structural units into BO4 tetrahedra which are incorporated in more complex cyclic groups such as di-, tri-, tetra- or penta-borate groups [6]. Despite the fact that Bi2O3 is not a classical glass former, due to the high polarizability of the Bi3+ ions, in the presence of conventional glass formers such as B2O3 it may build a glass network consisting of both * Corresponding author. Tel.: +40 264 401 262; fax: +40 264 595 355. E-mail address: [email protected] (P. Pascuta). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.09.010
BiO6 octahedral and BiO3 pyramidal units [7,8]. A lot of studies were reported over the past several decades on structural, electrical and magnetic properties of bismuth based glasses such as bismuth borate glasses [2], vanadium phosphorus bismuthate glasses [9], gadolinium bismuth germanate glasses [4,10], copper bismuth borate glasses [3], vanadium bismuthate glasses [11], europium lead bismuthate glasses [12] lead bismuthate glasses [13], lithium bismuthate glasses [14] and iron bismuthate glasses [15]. On the other hand, glasses containing rare-earth ions have been investigated for their optical, electrical and magnetic properties. These properties confer to them important applications in many fields such as the laser technology, telecommunications technology (for lasers, amplifiers and optical fiber), electronics (phosphors, electro-luminescent devices, memory devices and flat-panel display), etc. [16–21]. Most of the rare earth ions incorporated in glasses present a single valence state, namely the 3+ one [20,22] while europium ions were reported to be present in both their 2+ and 3+ valence states [23,24]. Having in view that the addition of europium ions in glasses usually induces significant changes in their magnetic and optical behavior [20,22–24], opening thus opportunities in the finding of new applications, the careful structural investigations of glasses containing Eu2O3 becomes necessary. The aim of the present study was to obtain by means of FTIR spectroscopy specific data regarding the local structure of bismuth
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borate glasses doped with europium ions in order to evidence (i) changes of the Bi2O3 and B2O3 structural units and (ii) formation of new crystalline phases induced by the thermal treatment. Furthermore, our interest was to establish the structural changes induced by the addition of europium ions in the bismuth borate host glass matrices. The presence of two network forming oxides, the classical B2O3 and the conditional Bi2O3 glass former, the possible participation in the glass structure of both boron and bismuth ions with more than one stabile coordination (and thus, the presence of several structural units namely, BO3, BO4, BiO3 and BiO6), the capability of the bismuth polyhedra and of the borate structural groups to form independent interconnected networks and, finally, the presence of a high content of rare earth ions increase the interest of the present study. 2. Experimental Glasses of the xEu2O3 (100x)[2Bi2O3 B2O3] system were prepared using Bi2O3, B2O3 and Eu2O3 of high purity (99.9%) in suitable proportion. The mechanically homogenized mixtures were melted in sintered corundum crucibles at 1100 °C, in an electric furnace. The samples were put into the electric furnace at the mentioned temperature. After 10 minutes, the molten material was quenched at room temperature by pouring onto a stainless-steel plate. The glass samples were subject to partial crystallization by a heat treatment applied at 625 °C for 24 h. The X-ray diffraction measurements of the as prepared and heat treated samples were obtained by using a diffractometer with a monochromator of graphite for the Cu Ka radiation (k = 1.54 Å), at room temperature. The FTIR absorption spectra of the glasses and glass ceramics were obtained in the 360–1600 cm1 spectral range by using the KBr pellet technique. In order to obtain good quality spectra, the samples were crushed in an agate mortar to obtain particles of micrometer size. This procedure was applied every time to the fragments of bulk glass to avoid structural modifications due to ambient moisture. 3. Results 3.1. X-ray diffraction data The vitreous or/and crystalline nature of the xEu2O3 (100x)[2Bi2O3 B2O3] glass system with various contents of europium oxide (0 6 x 6 25 mol%) was tested by X-ray diffraction. Xray diffraction analysis shows that the as prepared samples were
non-crystalline (Fig. 1(a)). The diffractograms from this figure show two halos for each studied composition which is characteristic of amorphous compounds. After the heat treatment applied at 625 °C for 24 h, some structural changes were observed, namely two crystalline phases appeared in the structure of the samples (Fig. 1(b)). The degree of crystallinity is higher for the samples that contain more europium ions (Fig. 1(b)). A pure crystalline phase was found for the sample without europium ions, x = 0, and was labeled as phase 1. A second crystalline phase was found for sample containing 25 mol% Eu2O3 and was labeled as phase 2. With increasing of the Eu2O3 content on can see a gradual disappearance of the phase 1 and the appearance of phase 2. The X-ray powder diffraction patterns were indexed for both phases in order to establish the unit cell of the samples (Fig. 1(b)). In order to index the powder diffraction patterns we used the Dicvol method [25] and a grid search indexing program [26]. 3.2. FTIR data The experimental FTIR spectra of the studied untreated glasses are shown in Fig. 2(a). FTIR spectroscopy was used to obtain essential information concerning the arrangement of the structural units of these glasses. A simple inspection of the spectral features presented in Fig. 2(a) shows that they are those characteristic of the base glass matrix, 2Bi2O3 B2O3, and no dramatic changes occur with increasing the Eu2O3 content in the glasses. To get quantitative information about the structural groups present in the glass network the spectra were submitted to a deconvolution procedure. This procedure was made by using the Spectra Manager program and a Gaussian type function and allowed us a better identification of the IR absorption bands and, respectively, their better assignment. Fig. 3(a) shows as an example the deconvolution of the IR spectrum, in Gaussian bands, for the glass containing 5 mol% Eu2O3. The obtained IR bands and their assignment are presented in Table 1. In order to determine the influence of the heat treatment on the structure of the studied glasses we compared the FTIR spectra of untreated glasses (Fig. 2(a)) with those recorded for the heat treated samples (Fig. 2(b)). As an example, Fig. 3(b) shows the deconvolution in Gaussian bands for the glass ceramics containing 5 mol% Eu2O3. The obtained bands and their assignment are presented in Table 1. The FTIR spectra for the heat treated samples exhibit changes, namely those illustrated by the narrowing of the previous bands and the appearance of some new bands in the spectra. In the FTIR spectrum of the glass ceramic sample appears five new bands located at 478, 590, 615, 775 and 830 cm1.
Fig. 1. The X-ray diffraction patterns of the xEu2O3 (100x)[2Bi2O3 B2O3] samples: as prepared (a) and heat treated (b).
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Fig. 2. FTIR spectra of the xEu2O3 (100x)[2Bi2O3 B2O3] samples: as prepared (a) and heat treated (b).
Fig. 3. Deconvoluted FTIR spectra of the xEu2O3 (100x)[2Bi2O3 B2O3] samples: as prepared (a) and heat treated (b), using a Gaussian-type function for x = 5 mol%.
Table 1 Wavenumbers and assignments for the FTIR features from the spectra of xEu2O3 (100x)[2Bi2O3 B2O3] samples: as prepared and heat treated Wavenumber (cm1) Glasses
Glass ceramics
440–458
448–451 476–486 500–535 588–590 601–614 697–714 765–775 825–834 861–869 885–899
527–542
701–703
857–868
932–975 1029– 1036 1192– 1206 1298– 1319 1424– 1430
949-972 1007–1039 1175–1223 1269–1322 1363–1412
Assignments
Bi–O bonds in BiO6 units Bi–O bend in BiO3 units Bi–O bend in BiO6 units Bi–O stretch in BiO6 units B–O–B bend O3B–O–BO4 bend Bi-O stretch in BiO3 units Bi–O bonds in BiO6 units B–O stretch in BO4 units from tri-, tetra- and pentaborate groups B–O stretch in BO4 units from di-borate groups B–O stretch in BO4 units from tri-, tetra- and pentaborate groups B–Osymm stretch in BO3 units from pyro- and orthoborate groups B–O stretch in BO3 units from varied types of borate groups B–O stretch in BO2O units from varied types of borate groups
The structural changes involved by the addition of Eu2O3 were analyzed based on the changes produced by the europium ions
in the relative population of triangular and tetrahedral borate units in glasses [27,28]. To follow the evolution of the triangular and tetrahedral borate units in the studied samples we used the fraction of four-coordination boron atoms, N4, as was defined previously [27,28]:
N4 ¼
A4 ; A3 þ A4
ð1Þ
where A4 and A3 denote the areas of the BO4 and BO3 units. Fig. 4 shows the plots of the fraction of the four-coordinated boron atoms, N4, versus the Eu2O3 content of the glass and glass ceramic samples. For glass ceramics, the amount of BO4 increase as the content of Eu2O3 increase up to 15 mol% Eu2O3. A further addition of Eu2O3 over 15 mol% led to a decrease in the amount of BO4. On the other hand, for the glass samples, N4 decreases with increasing the Eu2O3 content for all the compositional range, more evidently for x > 15 mol%. 4. Discussion X-ray diffraction data of the heat treated sample without europium ions evidenced that the phase 1 crystallizes in a cubic system having the a = 10.2878 Å lattice parameter. For the phase 2 we obtained two equivalent solutions. One of the solutions shows that the compound belongs to the tetragonal system having the a = 3.7873 Å, b = 3.7873 Å and c = 14.9747 Å lattice parameters with a unit cell volume of V = 214.74 Å3. Taking
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Fig. 4. Fraction of the four-coordinated boron atoms N4 versus the Eu2O3 content for the xEu2O3 (100x)[2Bi2O3 B2O3] samples: as prepared (s) and heat treated (j). The lines are drawn as a guide for the eyes.
into account the forbidden reflexes the most probable space group is the I4. Another solution of the indexing procedure is the orthorhombic system with the a = 14.9789 Å, b = 5.3586 Å and c = 5.3523 Å lattice parameters with a unit cell volume of V = 429.61 Å3. In this case the possible space group is the F222. We notice that the difference between the b and c lattice parameters is very small. If we consider the a ffiffiffi= b = 3.7873 Å and multiply this value with pffiffiffi lattice parameters p 2 we obtain 5.35 Å, 2 3:7873 ¼ 5:356. The difference between the two cases consists in the way we chose the unit cell axes. Thus, in the first case a and b unit cell parameters were chosen as the sides of the square base of the unit cell while in the second case they were chosen as the diagonals of the same square base. The volume of the unit cell in the second case is twice of that for the first case. We consider that the second case is more probable and thus the crystalline phase present in the studied samples is orthorhombic (almost tetragonal) with a = 14.9789 Å, b = 5.3586 Å and c = 5.3523 Å as lattice parameters. The characteristic FTIR features of the binary 2Bi2O3 B2O3 glass matrix spectrum are the Bi–O bond vibrations in the BiO6 units, namely the bands located at 457 and 860 cm1 [29,30], the Bi–O bending vibration in BiO6 units located at 535 cm1 [30], the bending vibrations of B–O–B linkage in the borate network at 700 cm1 [31–33], the B–O stretching vibrations in BO4 units from di-borate groups at 916 cm1 [31–33], the B–O stretching vibrations in BO4 units from tri-, tetra-and penta-borate groups at 1017 cm1 [31–33], the B–O asymmetric stretching vibrations in BO3 units from pyro- and ortho- borate groups at 1183 cm1, the B–O stretching vibration in BO3 units from varied types of borate groups at 1298 cm1 [31–33] and the B–O stretching vibrations in BO2O units at 1437 cm1 [31–33]. No major changes were observed in the FTIR spectra of the studied glasses with the addition of europium ions. This suggests that the xEu2O3 (100x)[2Bi2O3 B2O3] glasses present a relatively high rigidity of the vitreous host glass matrix. The Bi3+ cations are incorporated in the network of these glasses as BiO6 units, while the boron atoms takes part in the glass network in the BO3 and BO4 units. For the heat treated samples the bands located at 478 and 830 cm1 can be due the Bi–O bending and stretching vibrations in BiO3 units [29,30]. This suggests the conversion of the BiO6 into BiO3 structural units with increasing the europium ions content in the glass ceramic samples. Thus, the presence of europium ions in the glass ceramics seems to influence the surrounding of the Bi3+ cations favoring the formation of the BiO3 units. The Bi–O stretch-
ing vibrations in BiO6 units give the bands from 590 and 615 cm1 [34,35]. These bands are put in evidence for x P 5 mol% and their intensity increases slowly with increasing of europium ions content. The band at 775 cm1 can be due to the O3B–O– BO4 bending vibrations [31–33] while the band at 890 cm1 was ascribed to the B–O stretching vibrations in BO4 units from tri-, tetra- and penta-borate groups [31–33]. As the Eu2O3 content increases the FTIR spectra become broader and more bands are present. Thus, the presence in the analyzed glass ceramic of a higher Eu2O3 content (x P 5 mol%) generates significant changes in the short-range order structure. Accordingly, we can conclude that the heat treatment induces an ordering process of bismuthate and borate structural groups in the samples where both BiO3 and BiO6 units are present while the B3+ cations are incorporated in the glass ceramic network in the BO3 and BO4 units in all the investigated samples. It was previously reported [31,36] that in borate glasses appears an isomerization between the three- and four-coordinated boron species, namely BO3 , BO4 (1). The increase of N4 can be explained by the reaction (1) shifted to the right in this studied composition range. This is due to the conversion of BO3 into BO4 structural units as the content of the glass modifier Eu2O3 increases, namely the BO3 units in the bismuth borate glasses prefer a coordination change from 3 to 4 rather than producing nonbridging oxygen. This conversion leads to the increase in the stability of the glass ceramics. The reverse structural change from the BO4 units to the nonbridging oxygen containing BO3 units occurred when more than 15 mol% of Eu2O3 is added and this reduces the stability of the glass and glass ceramic samples. The fourfold boron atoms are favored in the glass ceramics as compared with the threefold ones while for glasses the threefold ones are dominated. 5. Conclusions Glasses of the xEu2O3 (100x)[2Bi2O3 B2O3] system were obtained within a large concentration range, namely 0 6 x 6 25 mol%. X-ray diffraction analysis shows that the obtained samples were non-crystalline. After a heat treatment applied at 625 °C for 24 h two new crystalline phases appear. The first one was observed for the host glass matrix while the second one was observed for the sample containing 25 mol%. Between these two extreme composition samples, x = 0 mol% and x = 25 mol%, there is a mixture of the two crystalline phases. The crystalline phase corresponding to the host glass ceramic matrix belongs to cubic system, while the crystalline phase corresponding to glass ceramics containing 25 mol% Eu2O3 belongs to the orthorhombic crystallographic system with the a and b cell parameters almost equals. The FTIR study shows that both Bi2O3 and B2O3 play the network former role and the europium ions play a network modifier role in the studied samples. The glasses network consists of BiO6, BO3 and BO4 structural units while the network of the glass ceramic consists of BiO3, BiO6, BO3 and BO4 units. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Y. Takahashi, Y. Benino, T. Fujiwara, T. Komatsu, J. Appl. Phys. 95 (2004) 3503. Y. Cheng, H. Xiao, W. Guo, W. Guo, Thermochim. Acta 444 (2006) 172. F.H. ElBatal, S.Y. Marzouk, N. Nada, S.M. Desouky, Physica B 391 (2007) 88. P. Pascuta, E. Culea, Mater. Lett. 62 (2008) 4127. J. Krogh-Moe, Phys. Chem. Glasses 3 (1962) 101. R.L. Mozzi, B.E. Waren, J. Appl. Crystallogr. 3 (1970) 251. W.H. Dumbaugh, Phys. Chem. Glasses 27 (1986) 119. S. Hazra, S. Mandal, A. Ghosh, Phys. Rev. B 56 (1997) 8021. A. Ghosh, J. Chem. Phys. 102 (1995) 1385. S. Simon, I. Ardelean, S. Filip, I. Bratu, I. Cosma, Solid State Commun. 116 (2000) 83. [11] A. Ghosh, B.K. Chaudhuri, J. Mater. Sci. 22 (1987) 2369. [12] E. Culea, L. Pop, S. Simon, M. Culea, J. Magn. Magn. Mater. 290&291 (2005) 1465.
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A. Pan, A. Ghosh, J. Non-Cryst. Solids 271 (2000) 157. A. Ghosh, J. Chem. Phys. 112 (2000) 1503. A. Ghosh, J. Appl. Phys. 66 (1989) 2425. M. Nogami, T. Nagakura, T. Hayakawa, J. Lumin. 86 (2000) 117. B. Sun, H. Song, J. Wang, H. Peng, X. Zhang, S. Lu, J. Zhang, H. Xia, Chem. Phys. Lett. 368 (2003) 412. C.F. Song, P. Yang, M.K. Lu, D. Xu, D.R. Yuan, Z.Q. Liu, J. Phys. Chem. Solids 64 (2003) 491. E. Culea, L. Pop, V. Simon, M. Neumann, I. Bratu, J. Non-Cryst. Solids 337 (2004) 62. P. Pascuta, M. Bosca, M. Culea, S. Simon, E. Culea, Mod. Phys. Lett. B 22 (2008) 447. V. Venkatramu, P. Babu, C.K. Jayasankar, Spectrochim. Acta A 63 (2006) 276. E. Burzo, I. Ardelean, D. Mitulescu, J. Mater. Sci. Lett. 11 (1992) 1492. D. Matulescu, M.I. Chipara, E. Burzo, I. Ardelean, Phys. Stat. Solidi (a) 161 (1997) 451.
[24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
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T. Ristoiu, E. Culea, J. Non-Cryst. Solids 279 (2001) 93. A. Boultif, D. Louer, J. Appl. Cryst 24 (1991) 987. G. Mihailescu, G. Borodi, I. Bratu, J.M. Gavira, Roum. Rep. Phys. 51 (1999) 983. K. El-Egili, Physica B 325 (2003) 340. P. Pascuta, L. Pop, S. Rada, M. Bosca, E. Culea, J. Mater. Sci: Mater. El. 19 (2008) 424. V. Dimitrov, Y. Dimitriev, A. Montenero, J. Non-Cryst. Solids 180 (1994) 51. Y. Hu, N.H. Liu, U.L. Lin, J. Mater. Sci. 33 (1998) 229. Y.D. Yiannopoulos, G.D. Chyssikos, E.I. Kamitsos, Phys. Chem. Glasses 42 (2001) 164. M. Abo-Naf, F.H. ElBatal, M.A. Azooz, Mater. Chem. Phys. 77 (2002) 846. A. Kumar, S.B. Rai, D.K. Rai, Mater. Res. Bull. 38 (2003) 333. I. Ardelean, S. Cora, R. Ciceo Lucacel, O. Hulpus, Solid State Sci. 7 (2005) 1438. P. Pascuta, L. Pop, S. Rada, M. Bosca, E. Culea, Vibrat. Spectrosc. (2008), doi:10.1016/j.vibspec.2008.01.011. R. Ciceo Lucacel, C. Marcus, V. Timar, I. Ardelean, Solid State Sci. 9 (2007) 850.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Influence of europium ions on structure and crystallization properties of bismuth borate glasses and glass ceramics Petru Pascuta a,*, Gheorghe Borodi b, Eugen Culea a a b
Physics Department, Technical University, C. Daicoviciu 15, 400020 Cluj-Napoca, Romania National Institute for R&D of Isotopic and Molecular Technology, P.O. Box 700, RO-400293 Cluj-Napoca, Romania
a r t i c l e
i n f o
Article history: Received 24 January 2008 Received in revised form 11 August 2008 Available online 20 October 2008 PACS: 81.05.Kf 81.05.Pj 61.05.cp 78.30.j Keywords: Glass ceramics X-ray diffraction FTIR measurements Borates
a b s t r a c t Glasses of the xEu2O3 (100x)[2Bi2O3 B2O3] system with 0 6 x 6 25 mol% have been characterized by X-ray diffraction and FTIR spectroscopy measurements. Melting at 1100 °C and the rapid cooling at room temperature permitted us to obtain glass samples. In order to improve the local order and to develop crystalline phases, the glass samples were kept at 625 °C for 24 h. After heat treatment two crystalline phases were put into evidence. One of the crystalline phases was observed for the host glass matrix, the x = 0 mol% sample, and belongs to the cubic system. The second one was observed for the x = 25 mol% sample and was find to be orthorhombic with two unit cell parameters very close to each other. For the samples with 0 < x < 25 mol% there is a mixture of the two mentioned phases. FTIR spectroscopy data suggest that both Bi2O3 and B2O3 play the glass network former role while the europium ions play the network modifier role in the studied glasses. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Glasses based on heavy metal oxides, such as Bi2O3, are interesting because their properties are exploited in many applications such as wave-guides in non-linear optics, radiation shielding windows, scintillation counters, optical transmission devices, thermal and mechanical sensors as well as optical devices, such as optical fibers, optical switching, optical memory etc. [1–4]. Glasses containing two glass-forming oxides, such as the bismuth borate glasses, which have superior properties, have also a wide range of practical applications. B2O3 is a typical glass former while Bi2O3 is a conditional glass former. B2O3 alone can form a glass network that consists of BO3 triangles forming three-member (boroxol) rings connected by B–O–B linkages [5]. It was reported that addition of a network modifier in borate glasses causes a progressive change of some of the triangular BO3 structural units into BO4 tetrahedra which are incorporated in more complex cyclic groups such as di-, tri-, tetra- or penta-borate groups [6]. Despite the fact that Bi2O3 is not a classical glass former, due to the high polarizability of the Bi3+ ions, in the presence of conventional glass formers such as B2O3 it may build a glass network consisting of both * Corresponding author. Tel.: +40 264 401 262; fax: +40 264 595 355. E-mail address: [email protected] (P. Pascuta). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.09.010
BiO6 octahedral and BiO3 pyramidal units [7,8]. A lot of studies were reported over the past several decades on structural, electrical and magnetic properties of bismuth based glasses such as bismuth borate glasses [2], vanadium phosphorus bismuthate glasses [9], gadolinium bismuth germanate glasses [4,10], copper bismuth borate glasses [3], vanadium bismuthate glasses [11], europium lead bismuthate glasses [12] lead bismuthate glasses [13], lithium bismuthate glasses [14] and iron bismuthate glasses [15]. On the other hand, glasses containing rare-earth ions have been investigated for their optical, electrical and magnetic properties. These properties confer to them important applications in many fields such as the laser technology, telecommunications technology (for lasers, amplifiers and optical fiber), electronics (phosphors, electro-luminescent devices, memory devices and flat-panel display), etc. [16–21]. Most of the rare earth ions incorporated in glasses present a single valence state, namely the 3+ one [20,22] while europium ions were reported to be present in both their 2+ and 3+ valence states [23,24]. Having in view that the addition of europium ions in glasses usually induces significant changes in their magnetic and optical behavior [20,22–24], opening thus opportunities in the finding of new applications, the careful structural investigations of glasses containing Eu2O3 becomes necessary. The aim of the present study was to obtain by means of FTIR spectroscopy specific data regarding the local structure of bismuth
5476
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borate glasses doped with europium ions in order to evidence (i) changes of the Bi2O3 and B2O3 structural units and (ii) formation of new crystalline phases induced by the thermal treatment. Furthermore, our interest was to establish the structural changes induced by the addition of europium ions in the bismuth borate host glass matrices. The presence of two network forming oxides, the classical B2O3 and the conditional Bi2O3 glass former, the possible participation in the glass structure of both boron and bismuth ions with more than one stabile coordination (and thus, the presence of several structural units namely, BO3, BO4, BiO3 and BiO6), the capability of the bismuth polyhedra and of the borate structural groups to form independent interconnected networks and, finally, the presence of a high content of rare earth ions increase the interest of the present study. 2. Experimental Glasses of the xEu2O3 (100x)[2Bi2O3 B2O3] system were prepared using Bi2O3, B2O3 and Eu2O3 of high purity (99.9%) in suitable proportion. The mechanically homogenized mixtures were melted in sintered corundum crucibles at 1100 °C, in an electric furnace. The samples were put into the electric furnace at the mentioned temperature. After 10 minutes, the molten material was quenched at room temperature by pouring onto a stainless-steel plate. The glass samples were subject to partial crystallization by a heat treatment applied at 625 °C for 24 h. The X-ray diffraction measurements of the as prepared and heat treated samples were obtained by using a diffractometer with a monochromator of graphite for the Cu Ka radiation (k = 1.54 Å), at room temperature. The FTIR absorption spectra of the glasses and glass ceramics were obtained in the 360–1600 cm1 spectral range by using the KBr pellet technique. In order to obtain good quality spectra, the samples were crushed in an agate mortar to obtain particles of micrometer size. This procedure was applied every time to the fragments of bulk glass to avoid structural modifications due to ambient moisture. 3. Results 3.1. X-ray diffraction data The vitreous or/and crystalline nature of the xEu2O3 (100x)[2Bi2O3 B2O3] glass system with various contents of europium oxide (0 6 x 6 25 mol%) was tested by X-ray diffraction. Xray diffraction analysis shows that the as prepared samples were
non-crystalline (Fig. 1(a)). The diffractograms from this figure show two halos for each studied composition which is characteristic of amorphous compounds. After the heat treatment applied at 625 °C for 24 h, some structural changes were observed, namely two crystalline phases appeared in the structure of the samples (Fig. 1(b)). The degree of crystallinity is higher for the samples that contain more europium ions (Fig. 1(b)). A pure crystalline phase was found for the sample without europium ions, x = 0, and was labeled as phase 1. A second crystalline phase was found for sample containing 25 mol% Eu2O3 and was labeled as phase 2. With increasing of the Eu2O3 content on can see a gradual disappearance of the phase 1 and the appearance of phase 2. The X-ray powder diffraction patterns were indexed for both phases in order to establish the unit cell of the samples (Fig. 1(b)). In order to index the powder diffraction patterns we used the Dicvol method [25] and a grid search indexing program [26]. 3.2. FTIR data The experimental FTIR spectra of the studied untreated glasses are shown in Fig. 2(a). FTIR spectroscopy was used to obtain essential information concerning the arrangement of the structural units of these glasses. A simple inspection of the spectral features presented in Fig. 2(a) shows that they are those characteristic of the base glass matrix, 2Bi2O3 B2O3, and no dramatic changes occur with increasing the Eu2O3 content in the glasses. To get quantitative information about the structural groups present in the glass network the spectra were submitted to a deconvolution procedure. This procedure was made by using the Spectra Manager program and a Gaussian type function and allowed us a better identification of the IR absorption bands and, respectively, their better assignment. Fig. 3(a) shows as an example the deconvolution of the IR spectrum, in Gaussian bands, for the glass containing 5 mol% Eu2O3. The obtained IR bands and their assignment are presented in Table 1. In order to determine the influence of the heat treatment on the structure of the studied glasses we compared the FTIR spectra of untreated glasses (Fig. 2(a)) with those recorded for the heat treated samples (Fig. 2(b)). As an example, Fig. 3(b) shows the deconvolution in Gaussian bands for the glass ceramics containing 5 mol% Eu2O3. The obtained bands and their assignment are presented in Table 1. The FTIR spectra for the heat treated samples exhibit changes, namely those illustrated by the narrowing of the previous bands and the appearance of some new bands in the spectra. In the FTIR spectrum of the glass ceramic sample appears five new bands located at 478, 590, 615, 775 and 830 cm1.
Fig. 1. The X-ray diffraction patterns of the xEu2O3 (100x)[2Bi2O3 B2O3] samples: as prepared (a) and heat treated (b).
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Fig. 2. FTIR spectra of the xEu2O3 (100x)[2Bi2O3 B2O3] samples: as prepared (a) and heat treated (b).
Fig. 3. Deconvoluted FTIR spectra of the xEu2O3 (100x)[2Bi2O3 B2O3] samples: as prepared (a) and heat treated (b), using a Gaussian-type function for x = 5 mol%.
Table 1 Wavenumbers and assignments for the FTIR features from the spectra of xEu2O3 (100x)[2Bi2O3 B2O3] samples: as prepared and heat treated Wavenumber (cm1) Glasses
Glass ceramics
440–458
448–451 476–486 500–535 588–590 601–614 697–714 765–775 825–834 861–869 885–899
527–542
701–703
857–868
932–975 1029– 1036 1192– 1206 1298– 1319 1424– 1430
949-972 1007–1039 1175–1223 1269–1322 1363–1412
Assignments
Bi–O bonds in BiO6 units Bi–O bend in BiO3 units Bi–O bend in BiO6 units Bi–O stretch in BiO6 units B–O–B bend O3B–O–BO4 bend Bi-O stretch in BiO3 units Bi–O bonds in BiO6 units B–O stretch in BO4 units from tri-, tetra- and pentaborate groups B–O stretch in BO4 units from di-borate groups B–O stretch in BO4 units from tri-, tetra- and pentaborate groups B–Osymm stretch in BO3 units from pyro- and orthoborate groups B–O stretch in BO3 units from varied types of borate groups B–O stretch in BO2O units from varied types of borate groups
The structural changes involved by the addition of Eu2O3 were analyzed based on the changes produced by the europium ions
in the relative population of triangular and tetrahedral borate units in glasses [27,28]. To follow the evolution of the triangular and tetrahedral borate units in the studied samples we used the fraction of four-coordination boron atoms, N4, as was defined previously [27,28]:
N4 ¼
A4 ; A3 þ A4
ð1Þ
where A4 and A3 denote the areas of the BO4 and BO3 units. Fig. 4 shows the plots of the fraction of the four-coordinated boron atoms, N4, versus the Eu2O3 content of the glass and glass ceramic samples. For glass ceramics, the amount of BO4 increase as the content of Eu2O3 increase up to 15 mol% Eu2O3. A further addition of Eu2O3 over 15 mol% led to a decrease in the amount of BO4. On the other hand, for the glass samples, N4 decreases with increasing the Eu2O3 content for all the compositional range, more evidently for x > 15 mol%. 4. Discussion X-ray diffraction data of the heat treated sample without europium ions evidenced that the phase 1 crystallizes in a cubic system having the a = 10.2878 Å lattice parameter. For the phase 2 we obtained two equivalent solutions. One of the solutions shows that the compound belongs to the tetragonal system having the a = 3.7873 Å, b = 3.7873 Å and c = 14.9747 Å lattice parameters with a unit cell volume of V = 214.74 Å3. Taking
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Fig. 4. Fraction of the four-coordinated boron atoms N4 versus the Eu2O3 content for the xEu2O3 (100x)[2Bi2O3 B2O3] samples: as prepared (s) and heat treated (j). The lines are drawn as a guide for the eyes.
into account the forbidden reflexes the most probable space group is the I4. Another solution of the indexing procedure is the orthorhombic system with the a = 14.9789 Å, b = 5.3586 Å and c = 5.3523 Å lattice parameters with a unit cell volume of V = 429.61 Å3. In this case the possible space group is the F222. We notice that the difference between the b and c lattice parameters is very small. If we consider the a ffiffiffi= b = 3.7873 Å and multiply this value with pffiffiffi lattice parameters p 2 we obtain 5.35 Å, 2 3:7873 ¼ 5:356. The difference between the two cases consists in the way we chose the unit cell axes. Thus, in the first case a and b unit cell parameters were chosen as the sides of the square base of the unit cell while in the second case they were chosen as the diagonals of the same square base. The volume of the unit cell in the second case is twice of that for the first case. We consider that the second case is more probable and thus the crystalline phase present in the studied samples is orthorhombic (almost tetragonal) with a = 14.9789 Å, b = 5.3586 Å and c = 5.3523 Å as lattice parameters. The characteristic FTIR features of the binary 2Bi2O3 B2O3 glass matrix spectrum are the Bi–O bond vibrations in the BiO6 units, namely the bands located at 457 and 860 cm1 [29,30], the Bi–O bending vibration in BiO6 units located at 535 cm1 [30], the bending vibrations of B–O–B linkage in the borate network at 700 cm1 [31–33], the B–O stretching vibrations in BO4 units from di-borate groups at 916 cm1 [31–33], the B–O stretching vibrations in BO4 units from tri-, tetra-and penta-borate groups at 1017 cm1 [31–33], the B–O asymmetric stretching vibrations in BO3 units from pyro- and ortho- borate groups at 1183 cm1, the B–O stretching vibration in BO3 units from varied types of borate groups at 1298 cm1 [31–33] and the B–O stretching vibrations in BO2O units at 1437 cm1 [31–33]. No major changes were observed in the FTIR spectra of the studied glasses with the addition of europium ions. This suggests that the xEu2O3 (100x)[2Bi2O3 B2O3] glasses present a relatively high rigidity of the vitreous host glass matrix. The Bi3+ cations are incorporated in the network of these glasses as BiO6 units, while the boron atoms takes part in the glass network in the BO3 and BO4 units. For the heat treated samples the bands located at 478 and 830 cm1 can be due the Bi–O bending and stretching vibrations in BiO3 units [29,30]. This suggests the conversion of the BiO6 into BiO3 structural units with increasing the europium ions content in the glass ceramic samples. Thus, the presence of europium ions in the glass ceramics seems to influence the surrounding of the Bi3+ cations favoring the formation of the BiO3 units. The Bi–O stretch-
ing vibrations in BiO6 units give the bands from 590 and 615 cm1 [34,35]. These bands are put in evidence for x P 5 mol% and their intensity increases slowly with increasing of europium ions content. The band at 775 cm1 can be due to the O3B–O– BO4 bending vibrations [31–33] while the band at 890 cm1 was ascribed to the B–O stretching vibrations in BO4 units from tri-, tetra- and penta-borate groups [31–33]. As the Eu2O3 content increases the FTIR spectra become broader and more bands are present. Thus, the presence in the analyzed glass ceramic of a higher Eu2O3 content (x P 5 mol%) generates significant changes in the short-range order structure. Accordingly, we can conclude that the heat treatment induces an ordering process of bismuthate and borate structural groups in the samples where both BiO3 and BiO6 units are present while the B3+ cations are incorporated in the glass ceramic network in the BO3 and BO4 units in all the investigated samples. It was previously reported [31,36] that in borate glasses appears an isomerization between the three- and four-coordinated boron species, namely BO3 , BO4 (1). The increase of N4 can be explained by the reaction (1) shifted to the right in this studied composition range. This is due to the conversion of BO3 into BO4 structural units as the content of the glass modifier Eu2O3 increases, namely the BO3 units in the bismuth borate glasses prefer a coordination change from 3 to 4 rather than producing nonbridging oxygen. This conversion leads to the increase in the stability of the glass ceramics. The reverse structural change from the BO4 units to the nonbridging oxygen containing BO3 units occurred when more than 15 mol% of Eu2O3 is added and this reduces the stability of the glass and glass ceramic samples. The fourfold boron atoms are favored in the glass ceramics as compared with the threefold ones while for glasses the threefold ones are dominated. 5. Conclusions Glasses of the xEu2O3 (100x)[2Bi2O3 B2O3] system were obtained within a large concentration range, namely 0 6 x 6 25 mol%. X-ray diffraction analysis shows that the obtained samples were non-crystalline. After a heat treatment applied at 625 °C for 24 h two new crystalline phases appear. The first one was observed for the host glass matrix while the second one was observed for the sample containing 25 mol%. Between these two extreme composition samples, x = 0 mol% and x = 25 mol%, there is a mixture of the two crystalline phases. The crystalline phase corresponding to the host glass ceramic matrix belongs to cubic system, while the crystalline phase corresponding to glass ceramics containing 25 mol% Eu2O3 belongs to the orthorhombic crystallographic system with the a and b cell parameters almost equals. The FTIR study shows that both Bi2O3 and B2O3 play the network former role and the europium ions play a network modifier role in the studied samples. The glasses network consists of BiO6, BO3 and BO4 structural units while the network of the glass ceramic consists of BiO3, BiO6, BO3 and BO4 units. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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