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Structure and magnetic properties of Bi2O3–GeO2–Gd2O3 glasses
PERGAMON
Solid State Communications 116 (2000) 83–86 www.elsevier.com/locate/ssc
Structure and magnetic properties of Bi2O3 –GeO2 –Gd2O3 glasses S. Simon a,*, I. Ardelean a, S. Filip b, I. Bratu c, I. Cosma d a
Faculty of Physics, Babes-Bolyai University, M. Kogalniceanu no 1, 3400 Cluj-Napoca, Romania b Department of Physics, University of Oradea, 3700 Oradea, Romania c Institute of Isotopic and Molecular Technology, 3400 Cluj-Napoca, Romania d Technical University, 3400 Cluj-Napoca, Romania
Received 29 July 1999; received in revised form 29 May 2000; accepted 29 June 2000 by P. Burlet
Abstract In this paper we present a characterisation of the local structure and magnetic properties of glasses in the xGd2O3·(100 ⫺ x) [Bi2O3·GeO2] system by using infra-red (IR), electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. The IR spectra indicate that the bismuth and germanium environments in the Bi2O3·GeO2 base glass are not strongly perturbed by gadolinium incorporation up to 30 mol% Gd2O3. The EPR spectra of the samples investigated reveal resonance sites with an unexpectedly high crystalline field in addition to the “U” spectrum, typical for Gd 3⫹ ions in disordered systems. Magnetic susceptibility measurements indicate a Curie type behaviour for x ⱕ 3 mol% and above this concentration a Curie– Weiss type behaviour. A model for the gadolinium distribution in bismuth–germanate glasses is suggested, which is consistent with these results. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Disordered system; E. Electron paramagnetic resonance; D. Superexchange interactions PACS: 61.18.Fs; 61.43.Fs; 75.20.-g; 76.30.Kg
1. Introduction Bismuth–germanate glasses are important candidates for low-loss optical fibres, infrared (IR) transmitting windows and as active media for Raman optical fibres amplifiers [1,2]. The structure of glasses in the Bi2O3 –GeO2 system has been investigated by IR, Raman, and EXAFS spectroscopies [3–5]. Bi2O3 is not traditionally known to form glass, but in some oxide systems it may have this property, because Bi 3⫹ ions are highly polarisable and the asymmetry of their oxygen coordination polyhedra can inhibit the crystallisation of melts in which they are incorporated. There are at least two reasons to introduce paramagnetic ions into diamagnetic glass matrices, such as bismuth– germanate. First, the glass formed may acquire new magnetic properties and, second, they may be studied by electron paramagnetic resonance (EPR) spectroscopy, one of the most powerful techniques for local order investigation [6]. Consequently we decided to study bismuth–germanate * Corresponding author. Tel.: ⫹40-64-194315; fax: ⫹40-64191906. E-mail address: [email protected] (S. Simon).
glasses containing gadolinium ions as magnetic resonance centres. IR, EPR and magnetic susceptibility measurements are carried out in this work in order to investigate the structure and magnetic properties of xGd2O3·(100 ⫺ x) [Bi2O3·GeO2] glass system.
2. Experimental procedures The samples were prepared by mixing the component oxides of reagent purity grade in suitable proportions to the desired compositions. The mechanically homogenised admixtures were melted in an electric furnace with supercanthal bars, in sintered corundum crucibles, directly introduced at 1250⬚C and kept for 30 min at this temperature. The glass samples were obtained by quick quenching at room temperature. The X-ray diffraction analysis of samples did not reveal any crystalline phases up to 30 mol% Gd2O3. The IR spectra were recorded at room temperature with UR20 Carl Zeiss equipment. The EPR spectra were obtained at room temperature, from powdered samples, with a JEOL type spectrometer, in X band, with a field modulation of 100 kHz. The magnetic susceptibility measurements were
0038-1098/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(00)00287-8
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S. Simon et al. / Solid State Communications 116 (2000) 83–86
Fig. 2. EPR spectra of xGd2O3·(100 ⫺ x)[Bi2O3·GeO2] glasses.
Fig. 1. IR absorption spectra of xGd2O3·(100 ⫺ x)[Bi2O3·GeO2] glasses.
carried out in the temperature range 80–300 K, using a Faraday type balance.
3. Results and discussion The IR absorption spectra of xGd2O3·(100 ⫺ x) [Bi2O3·GeO2] samples with different Gd2O3 content are shown in Fig. 1. No major difference between IR spectra up to 30 mol% Gd2O3 are observed. The most important features of the IR spectra are observed in the wave number range between 500 and 1100 cm ⫺1 where the Ge–O–Ge and O–Ge–O stretching and deformation modes are active. Within this region there is a well-defined peak around 750 cm ⫺1 assigned to GeO6 octahedra and a shoulder at 900 cm ⫺1 (indicated by an arrow in Fig. 1). This shoulder appears at about the same position as the second peak identified for the other bismuth–germanate glasses [4] and is associated with GeO4 tetrahedra. The poor resolution of
IR spectra could be an indication of a higher local disorder in vitreous samples. The EPR spectra, illustrated in Fig. 2, show for the low gadolinium concentration region x ⬍ 5 mol% well-known features with g 艑 6:0; 2.8 and 2.0 of “U” spectrum [6,7], typical for Gd 3⫹ ion in disordered polycrystalline matrices. They are associated with Gd 3⫹ ions disposed in tetrahedral, octahedral or cubic sites with moderate distortion. In these sites the gadolinium ions experience a relatively weak crystalline field and they are characterised by a coordination number higher than six [7]. The most important difference between Gd 3⫹ EPR spectra of bismuth–germanate glasses and the “U” spectrum is the occurrence of an asymmetric line with g 艑 4:8: This line indicates a relatively strong crystal field with an orthorhombic symmetry and is associated with gadolinium ions with a coordination number lower than six. The tendency of earthions to have a high coordination number is known [8,9] but in these bismuth–germanate glasses, a part of Gd 3⫹ ions replaces bismuth or germanium ions, that are predominantly four coordinated with oxygen ions. The bismuth–germanate matrices impose an unusual environment for gadolinium ions due to their hardness. This hardness is strongly
S. Simon et al. / Solid State Communications 116 (2000) 83–86
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Fig. 4. The gadolinium concentration dependence of the paramagnetic Curie temperature.
Fig. 3. The temperature dependence of the reciprocal magnetic susceptibility for samples with x 0:5 and 1 mol% (a); x 3; 5, 10 and 30 mol% (b).
confirmed by IR results that show almost the same type of spectra up to x 30 mol% Gd2O3. At higher content of gadolinium oxide, the EPR spectra become less resolved as a result of dipolar interaction between the Gd 3⫹ ions. The suggested behaviour is also supported by magnetic susceptibility measurements data. For high gadolinium concentrations x ⬎ 5 mol% the EPR spectra become less resolved and they are dominated by a large line.
The different temperature dependencies (Fig. 3) of the reciprocal magnetic susceptibility, a Curie-type for x ⱕ 3 mol% and a Curie–Weiss type for x ⬎ 3 mol% samples, prove that above that concentration, a fraction of gadolinium ions interact by a superexchange type mechanism. The negative values of the paramagnetic Curie temperatures (Fig. 4) and their increase in absolute magnitude with Gd2O3 content, for x ⬎ 3 mol%; reveal that these exchange interactions are of antiferromagnetic type. The magnetic moment for the gadolinium ions mGd3⫹ 7:9 ^p0:05 mB ; calculated with the relation mGd3⫹ 2; 827 CMx =x where CMx represents the molar Curie constant, is in good agreement with the theoretical value of the magnetic moment of Gd 3⫹ ions in free ion state [8]. Taking into account the radius of the involved ions, it is are better accommoexpected that Gd 3⫹ ions r ⬇ 1:20 A than in Ge 4⫹ ions sites dated in Bi 3⫹ ions sites r ⬇ 1:20 A in bismuth–germanate matrices. It is well r ⬇ 0:53 A known that Gd 3⫹ ions tend to impose their preferential environment with a high coordination number [9], that strongly differs from the lower coordination at Bi 3⫹ or Ge 4⫹ ions sites. In this competition, a part of the Gd 3⫹ ions will be subjected to an unusual environment in glasses, with a strong crystalline field that will give rise to the line with g 艑 4:8 in the EPR spectra, while another part of the Gd 3⫹ ions will be disposed in sites of relatively weak crystalline field, like in polycrystalline systems, giving rise to the “U” spectrum.
4. Conclusions The IR spectra indicate that the bismuth–germanate matrices are relatively rigid and the Bi 3⫹ and Ge 4⫹
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S. Simon et al. / Solid State Communications 116 (2000) 83–86
environments are less affected by the gadolinium presence up to 30 mol% Gd2O3 in the base glass. The EPR spectra of the samples investigated reveal, in addition to the “U” pattern, typical of Gd 3⫹ ions in disordered systems, a line with g 艑 4:8 assigned to low coordinated Gd 3⫹ ions disposed in sites with unexpected high crystalline field. Another part of the Gd 3⫹ ions will impose a preferential environment, with relatively a weak crystalline field like in other vitreous oxide matrices and will give rise to the “U” spectrum. The first type of Gd 3⫹ ions characterised by high crystalline field will act as vitreous network former and the second type of Gd 3⫹ ions characterised by weak crystalline field act as network modifier. The magnetic measurements evidence that the Gd 3⫹ ions in Bi2O3 –GeO2 glass matrix are randomly distributed in the whole composition range and they are subjected to dipolar interactions up to 3 mol % Gd 3⫹ and to superexchange interactions of antiferromagnetic type above this concentration.
References [1] K. Nassau, D.L. Wood, D.L. Chadwick, Appl. Optics 21 (1982) 4276. [2] C. Lin, J. Opt. Commun. 4 (1983) 2. [3] J.E. Canale, R.A. Condiate Jr., K. Nassau, B.C. Cornilsen, J. Can. Ceram. Soc. 55 (1986) 50. [4] R. Capelleti, E. Colombi, G. Gnappi, P.P. Lottici, A. Montenero, in: L.D. Pye, et al. (Eds.), The Physics of Non-Crystalline Solids, Taylor and Francis, London, 1992, p. 112. [5] P.P. Lottici, A. Antonioli, C. Rozzetti, A. Montenero, in: L.D. Pye, et al. (Eds.), The Physics of Non-Crystalline Solids, Taylor and Francis, London, 1992, p. 101. [6] D.L. Griscom, J. Non-Cryst. Solids 40 (1980) 211. [7] I.E. Iton, C.M. Brodbeck, S.L. Suib, G.D. Stucky, J. Chem. Phys. 79 (1983) 1185. [8] I. Ardelean, E. Burzo, D. Mitulescu-Ungur, S. Simon, J. NonCryst. Solids 146 (1992) 256. [9] C.M. Brodbeck, I.E. Iton, J. Chem. Phys. 83 (1985) 4285.
Solid State Communications 116 (2000) 83–86 www.elsevier.com/locate/ssc
Structure and magnetic properties of Bi2O3 –GeO2 –Gd2O3 glasses S. Simon a,*, I. Ardelean a, S. Filip b, I. Bratu c, I. Cosma d a
Faculty of Physics, Babes-Bolyai University, M. Kogalniceanu no 1, 3400 Cluj-Napoca, Romania b Department of Physics, University of Oradea, 3700 Oradea, Romania c Institute of Isotopic and Molecular Technology, 3400 Cluj-Napoca, Romania d Technical University, 3400 Cluj-Napoca, Romania
Received 29 July 1999; received in revised form 29 May 2000; accepted 29 June 2000 by P. Burlet
Abstract In this paper we present a characterisation of the local structure and magnetic properties of glasses in the xGd2O3·(100 ⫺ x) [Bi2O3·GeO2] system by using infra-red (IR), electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. The IR spectra indicate that the bismuth and germanium environments in the Bi2O3·GeO2 base glass are not strongly perturbed by gadolinium incorporation up to 30 mol% Gd2O3. The EPR spectra of the samples investigated reveal resonance sites with an unexpectedly high crystalline field in addition to the “U” spectrum, typical for Gd 3⫹ ions in disordered systems. Magnetic susceptibility measurements indicate a Curie type behaviour for x ⱕ 3 mol% and above this concentration a Curie– Weiss type behaviour. A model for the gadolinium distribution in bismuth–germanate glasses is suggested, which is consistent with these results. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Disordered system; E. Electron paramagnetic resonance; D. Superexchange interactions PACS: 61.18.Fs; 61.43.Fs; 75.20.-g; 76.30.Kg
1. Introduction Bismuth–germanate glasses are important candidates for low-loss optical fibres, infrared (IR) transmitting windows and as active media for Raman optical fibres amplifiers [1,2]. The structure of glasses in the Bi2O3 –GeO2 system has been investigated by IR, Raman, and EXAFS spectroscopies [3–5]. Bi2O3 is not traditionally known to form glass, but in some oxide systems it may have this property, because Bi 3⫹ ions are highly polarisable and the asymmetry of their oxygen coordination polyhedra can inhibit the crystallisation of melts in which they are incorporated. There are at least two reasons to introduce paramagnetic ions into diamagnetic glass matrices, such as bismuth– germanate. First, the glass formed may acquire new magnetic properties and, second, they may be studied by electron paramagnetic resonance (EPR) spectroscopy, one of the most powerful techniques for local order investigation [6]. Consequently we decided to study bismuth–germanate * Corresponding author. Tel.: ⫹40-64-194315; fax: ⫹40-64191906. E-mail address: [email protected] (S. Simon).
glasses containing gadolinium ions as magnetic resonance centres. IR, EPR and magnetic susceptibility measurements are carried out in this work in order to investigate the structure and magnetic properties of xGd2O3·(100 ⫺ x) [Bi2O3·GeO2] glass system.
2. Experimental procedures The samples were prepared by mixing the component oxides of reagent purity grade in suitable proportions to the desired compositions. The mechanically homogenised admixtures were melted in an electric furnace with supercanthal bars, in sintered corundum crucibles, directly introduced at 1250⬚C and kept for 30 min at this temperature. The glass samples were obtained by quick quenching at room temperature. The X-ray diffraction analysis of samples did not reveal any crystalline phases up to 30 mol% Gd2O3. The IR spectra were recorded at room temperature with UR20 Carl Zeiss equipment. The EPR spectra were obtained at room temperature, from powdered samples, with a JEOL type spectrometer, in X band, with a field modulation of 100 kHz. The magnetic susceptibility measurements were
0038-1098/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(00)00287-8
84
S. Simon et al. / Solid State Communications 116 (2000) 83–86
Fig. 2. EPR spectra of xGd2O3·(100 ⫺ x)[Bi2O3·GeO2] glasses.
Fig. 1. IR absorption spectra of xGd2O3·(100 ⫺ x)[Bi2O3·GeO2] glasses.
carried out in the temperature range 80–300 K, using a Faraday type balance.
3. Results and discussion The IR absorption spectra of xGd2O3·(100 ⫺ x) [Bi2O3·GeO2] samples with different Gd2O3 content are shown in Fig. 1. No major difference between IR spectra up to 30 mol% Gd2O3 are observed. The most important features of the IR spectra are observed in the wave number range between 500 and 1100 cm ⫺1 where the Ge–O–Ge and O–Ge–O stretching and deformation modes are active. Within this region there is a well-defined peak around 750 cm ⫺1 assigned to GeO6 octahedra and a shoulder at 900 cm ⫺1 (indicated by an arrow in Fig. 1). This shoulder appears at about the same position as the second peak identified for the other bismuth–germanate glasses [4] and is associated with GeO4 tetrahedra. The poor resolution of
IR spectra could be an indication of a higher local disorder in vitreous samples. The EPR spectra, illustrated in Fig. 2, show for the low gadolinium concentration region x ⬍ 5 mol% well-known features with g 艑 6:0; 2.8 and 2.0 of “U” spectrum [6,7], typical for Gd 3⫹ ion in disordered polycrystalline matrices. They are associated with Gd 3⫹ ions disposed in tetrahedral, octahedral or cubic sites with moderate distortion. In these sites the gadolinium ions experience a relatively weak crystalline field and they are characterised by a coordination number higher than six [7]. The most important difference between Gd 3⫹ EPR spectra of bismuth–germanate glasses and the “U” spectrum is the occurrence of an asymmetric line with g 艑 4:8: This line indicates a relatively strong crystal field with an orthorhombic symmetry and is associated with gadolinium ions with a coordination number lower than six. The tendency of earthions to have a high coordination number is known [8,9] but in these bismuth–germanate glasses, a part of Gd 3⫹ ions replaces bismuth or germanium ions, that are predominantly four coordinated with oxygen ions. The bismuth–germanate matrices impose an unusual environment for gadolinium ions due to their hardness. This hardness is strongly
S. Simon et al. / Solid State Communications 116 (2000) 83–86
85
Fig. 4. The gadolinium concentration dependence of the paramagnetic Curie temperature.
Fig. 3. The temperature dependence of the reciprocal magnetic susceptibility for samples with x 0:5 and 1 mol% (a); x 3; 5, 10 and 30 mol% (b).
confirmed by IR results that show almost the same type of spectra up to x 30 mol% Gd2O3. At higher content of gadolinium oxide, the EPR spectra become less resolved as a result of dipolar interaction between the Gd 3⫹ ions. The suggested behaviour is also supported by magnetic susceptibility measurements data. For high gadolinium concentrations x ⬎ 5 mol% the EPR spectra become less resolved and they are dominated by a large line.
The different temperature dependencies (Fig. 3) of the reciprocal magnetic susceptibility, a Curie-type for x ⱕ 3 mol% and a Curie–Weiss type for x ⬎ 3 mol% samples, prove that above that concentration, a fraction of gadolinium ions interact by a superexchange type mechanism. The negative values of the paramagnetic Curie temperatures (Fig. 4) and their increase in absolute magnitude with Gd2O3 content, for x ⬎ 3 mol%; reveal that these exchange interactions are of antiferromagnetic type. The magnetic moment for the gadolinium ions mGd3⫹ 7:9 ^p0:05 mB ; calculated with the relation mGd3⫹ 2; 827 CMx =x where CMx represents the molar Curie constant, is in good agreement with the theoretical value of the magnetic moment of Gd 3⫹ ions in free ion state [8]. Taking into account the radius of the involved ions, it is are better accommoexpected that Gd 3⫹ ions r ⬇ 1:20 A than in Ge 4⫹ ions sites dated in Bi 3⫹ ions sites r ⬇ 1:20 A in bismuth–germanate matrices. It is well r ⬇ 0:53 A known that Gd 3⫹ ions tend to impose their preferential environment with a high coordination number [9], that strongly differs from the lower coordination at Bi 3⫹ or Ge 4⫹ ions sites. In this competition, a part of the Gd 3⫹ ions will be subjected to an unusual environment in glasses, with a strong crystalline field that will give rise to the line with g 艑 4:8 in the EPR spectra, while another part of the Gd 3⫹ ions will be disposed in sites of relatively weak crystalline field, like in polycrystalline systems, giving rise to the “U” spectrum.
4. Conclusions The IR spectra indicate that the bismuth–germanate matrices are relatively rigid and the Bi 3⫹ and Ge 4⫹
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S. Simon et al. / Solid State Communications 116 (2000) 83–86
environments are less affected by the gadolinium presence up to 30 mol% Gd2O3 in the base glass. The EPR spectra of the samples investigated reveal, in addition to the “U” pattern, typical of Gd 3⫹ ions in disordered systems, a line with g 艑 4:8 assigned to low coordinated Gd 3⫹ ions disposed in sites with unexpected high crystalline field. Another part of the Gd 3⫹ ions will impose a preferential environment, with relatively a weak crystalline field like in other vitreous oxide matrices and will give rise to the “U” spectrum. The first type of Gd 3⫹ ions characterised by high crystalline field will act as vitreous network former and the second type of Gd 3⫹ ions characterised by weak crystalline field act as network modifier. The magnetic measurements evidence that the Gd 3⫹ ions in Bi2O3 –GeO2 glass matrix are randomly distributed in the whole composition range and they are subjected to dipolar interactions up to 3 mol % Gd 3⫹ and to superexchange interactions of antiferromagnetic type above this concentration.
References [1] K. Nassau, D.L. Wood, D.L. Chadwick, Appl. Optics 21 (1982) 4276. [2] C. Lin, J. Opt. Commun. 4 (1983) 2. [3] J.E. Canale, R.A. Condiate Jr., K. Nassau, B.C. Cornilsen, J. Can. Ceram. Soc. 55 (1986) 50. [4] R. Capelleti, E. Colombi, G. Gnappi, P.P. Lottici, A. Montenero, in: L.D. Pye, et al. (Eds.), The Physics of Non-Crystalline Solids, Taylor and Francis, London, 1992, p. 112. [5] P.P. Lottici, A. Antonioli, C. Rozzetti, A. Montenero, in: L.D. Pye, et al. (Eds.), The Physics of Non-Crystalline Solids, Taylor and Francis, London, 1992, p. 101. [6] D.L. Griscom, J. Non-Cryst. Solids 40 (1980) 211. [7] I.E. Iton, C.M. Brodbeck, S.L. Suib, G.D. Stucky, J. Chem. Phys. 79 (1983) 1185. [8] I. Ardelean, E. Burzo, D. Mitulescu-Ungur, S. Simon, J. NonCryst. Solids 146 (1992) 256. [9] C.M. Brodbeck, I.E. Iton, J. Chem. Phys. 83 (1985) 4285.