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Electron paramagnetic resonance and optical absorption studies of Cu2+ spin probe in MgO–Na2O–B2O3 ternary glasses
Journal of Non-Crystalline Solids 278 (2000) 205±212
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Electron paramagnetic resonance and optical absorption studies of Cu2 spin probe in MgO±Na2O±B2O3 ternary glasses G. Ramadevudu, Md. Shareefuddin, N. Sunitha Bai, M. Lakshmipathi Rao, M. Narasimha Chary * Department of Physics, Osmania University, Hyderabad 500 007, India Received 13 July 1999; received in revised form 3 November 1999
Abstract Electron paramagnetic resonance and optical absorption studies of MgO±Na2 O±B2 O3 glasses were made by introducing Cu2 as a spin probe. The electron paramagnetic resonance spectra of all the glass samples recorded at X-band frequencies have similar spectral features. It is observed that the spin-Hamiltonian parameters calculated from the EPR spectra are in¯uenced by the glass composition. The Cu2 ions are in well-de®ned axial sites but subjected to small distortion leading to the broadening of the spectra. The spin-Hamiltonian parameter values indicate that the ground state of Cu2 is dx2 ÿy 2 orbital (2 B1g ) and the site symmetry around Cu2 ions is tetragonally distorted octahedral. The optical absorption spectra exhibited a broad band corresponding to the d±d transition bands of Cu2 ion. The bond parameters indicate a slight covalency for the r and in-plane p-bonds, compared to the out-of-plane p-bonds. The theoretical optical basicity parameter values were evaluated and it was observed that sp which is a measure of Lewis basicity of non-bridging oxide ions in the glasses varies in a non-linear manner with increasing optical basicity. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Electron paramagnetic resonance (EPR) spectroscopy has been proved to be a powerful experimental tool for determining the coordination and environment of paramagnetic ions in glass [1± 3]. EPR investigation of oxide glasses doped with transition metal (TM) ions have been extensively studied to obtain information on the glassy network and to identify the site symmetry around the
* Corresponding author. Tel.: +91-40 701 8951; fax: +91-40 701 9020. E-mail address: mnchary_phy@redimail.com (M. Narasimha Chary).
TM ions [4±7]. Copper (II) (Cu2 ) is the most amenable ion for EPR studies. The main advantage of using Cu2 as the spin probe is that its EPR spectra can easily be recorded at room temperature, the spectrum is simple and the spread of the spectrum is large enough to detect minute changes in the coordination sphere [8]. Glasses containing TM ions exhibit memory and photoconducting properties [9]. Information on the local structure may be obtained by using optical spectroscopy methods. The position of the optical absorption peak is sensitive to the composition of the glass [10]. The local environment of TM ions incorporated in a glass may change due to the ligand ®eld variations, which will be re¯ected in the EPR and
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optical spectra. By correlating EPR and optical data the information on metal ligand bond nature can be obtained [3]. The properties of a glass can often be altered by the addition of a network modi®er to the basic constituent, which is termed as the network former (e.g. B2 O3 ). The commonly used network modi®ers are the alkali and alkaline earth oxides [11]. It was observed that the properties of an alkali oxide glass show a non-linear behaviour when one kind of alkali is gradually replaced by another [12]. This departure from linearity is called the mixed alkali eect. Similar observations were made in the case of mixed alkali±alkaline earth oxide glasses [13]. This phenomenon is called mixed oxide eect [13]. In this paper, we report EPR and optical absorption studies of Cu2 spin probe in the ternary glass system MgO±Na2 O±B2 O3 . The in¯uence of varying the concentrations of alkali (Na2 O) and alkaline earth (MgO) oxides, which act as network modi®ers on the spin-Hamiltonian parameters, is discussed. The main application of MgO±Na2 O±B2 O3 glasses is that they can be used as solid-state electrolytes in the fabrication of solid-state batteries. Magnesium (Mg) can be used as one of the electrodes since it is non-hygroscopic and less reactive, compared to sodium.
Table 1 Composition of glasses (mol%) S. no 1. 2. 3. 4. 5.
Composition
Tg (°C)
MgO
Na2 O
B2 O3
CuO
5 10 12 15 17
25 20 18 15 13
69 69 69 69 69
1 1 1 1 1
470 485 497 515 534
con®rm the glassy nature of the prepared samples. Table 1 gives the composition in mol% of the glasses studied in the present investigation, along with the glass transition temperatures (Tg ). EPR spectra of the crushed glasses were recorded at room temperature (310 K) using an ESR spectrometer (model JEOL-JES-FE3X) at X-band frequencies with 100 kHz ®eld modulation. The magnetic ®eld was scanned between 0.23 and 0.43 T. The organic radical a,a-diphenyl-b-picrylhydrazyl (DPPH) with a g value of 2.0036 was used as the standard g marker. Optical absorption spectra were recorded at 310 K using UV±VIS spectrophotometer in the wavelength region 300±1000 nm. The peak positions were obtained by using the peak-pick programme provided with the spectrometer. The uncertainty in the observed wavelength is about 1 nm.
2. Experimental
3. Results
The starting materials used in the present study were analytical grade magnesium oxide (MgO), sodium carbonate (Na2 CO3 ), and boric acid (H3 BO3 ). These materials were weighed to get the required composition and ground in a mortar with a pestle for 1 h to obtain homogeneous mixtures. The base glass composition was xMgO± (30)x)Na2 O±69B2 O3 to which 1 mol% of Cu2 was added in the form of CuO as the spin probe. Each batch was melted in porcelain crucible in an electric furnace at 1273 K for about 30 min. The homogeneous melt was rapidly quenched on to a stainless steel plate maintained at a temperature of 373 K. The glasses were annealed for 24 h also at 373 K to relieve the mechanical stresses. An X-ray diractometer and a thermal analyser were used to
3.1. EPR spectra No EPR signal was observed for undoped (without Cu2 ) glasses indicating the absence of TM ions. An EPR signal was observed for all the glasses containing Cu2 ions. Fig. 1 shows the EPR spectrum of Cu2 in 15MgO±15Na2 O± 69B2 O3 glass. The EPR spectra of Cu2 ions in the remaining glasses exhibited the same spectral features. Similar EPR line shapes were observed earlier in oxide glasses doped with Cu2 ions [14± 17]. The Cu2 ion (S 1/2) has a nuclear spin I 3/2 for both Cu63 and Cu65 (69.09% and 30.91% isotopic abundance, respectively). Therefore 2I 1, i.e. four parallel and four perpendicular hyper®ne lines could be observed. In most
G. Ramadevudu et al. / Journal of Non-Crystalline Solids 278 (2000) 205±212
207
Hamiltonian can be employed in the analysis of the EPR spectra [20±22] which is given below: H bgk Hz Sz g? Hx Sx Hy Sy Ak Iz Sz A? Ix Sx Iy Sy ;
Fig. 1. EPR spectrum of Cu2 in 15 MgO±15Na2 O±69B2 O3 .
1
where z is the symmetry axis, b the Bohr magneton, S and I are the electron and nuclear spin operators, Hx ; Hy ; Hz are the static magnetic ®eld components, gk and g? are the parallel and perpendicular components of the g tensor and Ak and A? are parallel and perpendicular components of the hyper®ne tensor A. The nuclear quadrupole contribution is neglected [23]. The solution to the spin-Hamiltonian gives the expressions for the peak position related to the principal values of g and A tensors as [24] hm gk bH mAk 15=4 ÿ m2
A2? 2gk bH
2
and Fig. 2. Computer-simulated EPR spectrum of Cu2 in 15MgO± 15Na2 O±69B2 O3 .
of the cases, however, the isotopic splittings are not resolved owing to the nearly identical nuclear moments and the large line widths resulting from the random orientation of magnetic complexes in the glasses. In all the EPR spectra recorded in the present investigation three weak parallel components were observed in the low ®eld region. However, the perpendicular components are not resolved leading to an intense line in the high ®eld region. It can be observed that the high ®eld side of the spectrum is more intense than the low ®eld side. The EPR spectra were also computer simulated [18,19]. Fig. 2 presents the computer simulated EPR spectrum of Cu2 in 15MgO±15Na2 O± 69B2 O3 glass. Spectroscopic splitting (g) and hyper®ne (A) tensors with axial symmetry have been assumed in the analysis of EPR spectra of oxide glasses [20]. The Jahn±Teller eect causes predominantly an elongated octahedral coordination with four short in-plane bond lengths. Therefore an axial spin-
hm g? bH mA? 15=4 ÿ m2 A2k
A2? ; 4g? bH 3
for the parallel and perpendicular hyper®ne peaks, respectively. Here m is the microwave frequency. The calculated spin-Hamiltonian parameters (SHP) are given in Table 2.
3.2. Optical absorption spectra The optical absorption spectra of all the glasses studied reveal only a broad absorption band. Fig. 3 presents the optical absorption spectrum of Cu2 in 15MgO±15Na2 O±69B2 O3 glass. The optical absorption values are given in Table 3. The variation of optical absorption maximum (k) with MgO content is shown in Fig. 4. With increasing MgO content the optical absorption maximum shifts towards longer wavelengths. The theoretical optical basicity K values were evaluated using the basicity moderating parameters which constitute the glass [25,26] and are given in Table 3.
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Table 2 Spin-Hamiltonian parameters of Cu2 ion in xMgO±(30)x)Na2 O±69B2 O3 glasses S. no.
gk
g?
Ak 104 (cmÿ1 )
Ak 104 (cmÿ1 )
1. 2. 3. 4. 5.
2.352 0.002 2.348 0.002 2.345 0.002 2.338 0.002 2.336 0.002
2.098 0.002 2.097 0.002 2.095 0.002 2.095 0.002 2.093 0.002
153 2 155 2 158 2 162 2 164 2
19.52 19.4 2 19.3 2 19.6 2 19.5 2
Fig. 3. Optical absorption spectrum of Cu2 in 15MgO± 15Na2 O±69B2 O3 . Table 3 Optical absorption bands and K values for Cu2 ions in xMgO± (30)x)Na2 O±69B2 O3 glasses S. no.
K
DE (cmÿ1 )
1. 2. 3. 4. 5.
0.5095 0.5065 0.5057 0.5052 0.5051
14085 13927 13736 13513 13333
4. Discussion 4.1. EPR spectra The observed g values are characteristic of Cu2 coordinated by six ligands, which form an octahedron elongated along the z-axis. The general nature of the ligand coordination can be obtained [27] from the fact that gk > g? > ge ge g factor of free electron 2:0023: Only an environment elongated along one of the cube axes can yield this result. In the present investigation it is
Fig. 4. Variation of optical absorption maxima (k) with MgO content.
observed that gk > g? > ge . Therefore from the g values and shape of the EPR spectra, it can be concluded that the ground state of the Cu2 is dx2 ÿy 2 orbital (2 B1g state),the Cu2 ions being located in tetragonally distorted octahedral sites [20,27]. Detailed calculations have shown that the elongated structures are usually more favourable than the compressed structures. Various experimental data also con®rmed that the Cu2 ion generally exists in solutions, crystals and glasses in octahedral symmetry with a tetragonal distortion [28±31]. The high gk values indicate the presence of a CuO6 chromophore [32,33]. The gk and Ak values are found to be dependent on the glass composition while g? and A? (Table 2)
G. Ramadevudu et al. / Journal of Non-Crystalline Solids 278 (2000) 205±212
values are essentially constant. The variation of gk and Ak with increasing MgO content is non-linear (Fig. 5). As the MgO content increases gk decreases and Ak increases indicating the change in the tetragonal distortion of Cu2 [34]. Variation in gk and Ak values may be associated with the change in the environment of Cu2 , i.e. in the ligand ®eld at the site of Cu2 which may be attributed to the structural changes in the glass. In the B2 O3 glasses, the addition of network modi®ers (MgO and Na2 O) leads to an increase in the coordination number of a certain portion of the boron atoms from 3 to 4. It is assumed that the resulting glass is composed of both triangular and tetrahedral units which form a relatively open network with holes between the oxygen atoms of sucient size to accommodate the Na and Mg ions [11]. As a doubly charged cation, Mg2 is suciently strong to split the network. Thus, sucient non-bridging oxygens are available for good coordination in the broken network. The alkali oxide Na2 O makes available additional weakly bonded O2ÿ for each Mg2 , i.e. Mg2 captures the O2ÿ from Na2 O and this happens at the expense of Na2 O coordination. Na should remain in the neighbourhood of the next stronger Mg2 than that it should be incorporated separately into the rigid network. The con®guration Na±O±Mg is energetically favoured [13]. The solubility of the Cu2 increases with the addition of the network modi®ers presumably due to the
Fig. 5. Variation of gk and Ak with MgO content.
209
coordination of the metal ion by the extra oxygen ions. Thus, incorporation of MgO in the glass will in¯uence the ®eld at the site of Cu2 , which in turn will re¯ect in the non-linear variation of the spinHamiltonian parameters as observed in the present case. The line width of the parallel hyper®ne components was found to increase with increasing values of the nuclear spin quantum number mI (Fig. 1), which may be due to ¯uctuations in both the ligand ®elds and bond covalencies from one copper (II) complex to the next, giving rise to a narrow distribution in g [27]. The mobility of the paramagnetic species in the glass medium may also aect the EPR line shape. 4.2. Optical absorption spectra The optical absorption spectrum is in¯uenced by the host structure into which the TM ions are incorporated. In oxide glasses, the TM ions mostly form coordination complexes with doubly charged oxygen as the ligands. In a regular octahedral complex one optical absorption peak is expected corresponding to the 2 Eg ® 2 T2g transition [10]. But Cu2 , being a d9 ion, experiences a strong Jahn±Teller distortion, which leads to the splitting of energy levels [35] and causes predominantly an elongated octahedral coordination with four short in-plane bond lengths and two longer axial bond lengths (D4h site symmetry). Accordingly three transitions, viz. 2 B1g ® 2 A1g , 2 B1g ® 2 B2g and 2 B1g ® 2 Eg are expected. But only a single optical absorption maximum was observed in most of the cases [36,37]. This single optical band was interpreted [38] as the overlap of all the three transitions. In most of the cases, however, it was found that a unique decomposition into three bands was not possible. Various authors [8,22,39±41] have placed the 2 B1g ®2 B2g and 2 B1g ® 2 A1g transitions under the observed band while 2 B1g ® 2 Eg transition is considered to be hidden under the intense charge transfer absorption in the UV region. It was assumed [8,40] as a reasonable approximation that, of the two transitions under the observed band, the one at shorter wavelength has the same maximum position, as does the envelope itself. Hence most of the authors [4,7,8,39,42±45]
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assigned the observed optical peak to the 2 B1g ®2 B2g transition (DExy ) and have used this value in the evaluation of the bond parameters. In the present case also the optical absorption was assigned to 2 B1g ® 2 B2g (DExy ) transition to evaluate the bond parameters. With increasing MgO content the optical absorption maximum shifts towards longer wavelength which is attributed to the reduction of ligand ®eld around Cu2 ion, as incorporation of MgO may modify the boron±oxygen network by converting some of the BO3 units into BO4 units [2,11].
Table 4 Bonding coecients for Cu2 ions in xMgO±(30)x)Na2 O± 69B2 O3 glasses S. no.
a2
b21
b2
sp (%)
sr (%)
1. 2. 3. 4. 5.
0.860 0.861 0.865 0.869 0.871
0.865 0.844 0.821 0.787 0.770
0.895 0.894 0.890 0.890 0.883
27.08 31.26 35.78 42.52 45.92
30.74 30.41 29.53 28.62 28.16
DExz;yz
2
4.3. Cu ±ligand bond nature Using the antibonding molecular orbitals of appropriate symmetry, the EPR and optical data can be related to the bonding coecients of Cu2 [22,27,39]. The bonding between the Cu2 ion and its ligands can be described in terms of the covalency parameters a2 , b2 and b21 . a2 describes the inplane r-bonding with the copper dx2 ÿy 2 orbital, b2 describes the out-of-plane p-bonding with the dxz and dyz orbital and the b21 parameter is a measure of in-plane p-bonding with the dxy orbital. a2 lies between 0.5 and 1.0, the limits of pure covalent and pure ionic bonding, respectively. The terms, b21 and b2 can be interpreted similarly. The bonding parameters were evaluated using the equations given below [8,18,27], a2 ÿ Ak =P gk ÿ 2 3=7 g? ÿ 2 0:04;
1656K 2 ; g? ÿ 2:0023
7
where K 2 is the orbital reduction factor ( 0.77). An error of 20% in this optical transition assignment (DExz;yz ) may introduce a 5% error in the calculated bonding coecient b2 [7,22]. Using the Eqs. (4)±(7) a2 , b2 and b21 were calculated and are tabulated in Table 4. The errors associated with a2 , b21 and b2 are 1.5%, 3% and 5%, respectively. The values of a2 and b21 indicate slight covalency for the r-and in-plane pbonds compared to the out-of-plane p-bonds (b2 values). The normalized covalency of Cu2 ±O inplane bonding of r and p symmetries (sr and sp , respectively) were calculated using the equation [4] sr
200 1 ÿ S 1 ÿ a2 % 1 ÿ 2S
8
4 DExz;yz ; 828a2
5
DExy ; 3312a2
6
b2 g? =ge ÿ 1 b2 gk =ge ÿ 1
where P is the dipolar hyper®ne coupling parameter ( 0.036 cmÿ1 ), DExy and DExz;yz are the heights of the dxy and dxz;yz molecular orbital levels above the ground state dx2 ÿy 2 , respectively [8,22]. In order to evaluate the bond parameters the optical data is required. As already discussed, the position of the observed absorption maximum of Cu2 indicates the values of DExy [4,8,22,34,42]. The corresponding value of DExz;yz was calculated using the approximation [7]
Fig. 6. Variation of sp and sr with optical basicity (K).
G. Ramadevudu et al. / Journal of Non-Crystalline Solids 278 (2000) 205±212
References
and sp 200 1 ÿ b21 %;
211
9
where S is the overlap integral Soxygen 0:076. Table 4 gives the values of sr and sp . The variation of sr and sp with optical basicity is shown in Fig. 6. sp , which is a measure of Lewis basicity of nonbridging oxide ions in glasses, varies in a non-linear manner with increasing K. The non-linear variation of sp may be attributed to the mixed oxide effect [13,45]. There is little variation of sr with K values. 5. Conclusions 1. From EPR and optical measurements, it is clear that Cu2 is present in all the glasses investigated and they exist in tetragonally distorted octahedral sites with dx2 y 2 (2 B1g ) ground state. 2. The spin-Hamiltonian parameters are in¯uenced by the composition of glasses, which may be attributed to the change of ligand ®eld strength around Cu2 . 3. With increasing MgO content, the ligand ®eld strength is reduced around the Cu2 ion, due to the modi®cation of the boron±oxygen network. 4. The bond parameter values indicate slight covalency for the r and in-plane p-bonds compared to the out-of-plane p-bonds. 5. sp which is a measure of Lewis basicity of nonbridging oxide ion varies in a non-linear manner with increasing K values, which may attributed to the mixed oxide eect. Acknowledgements The authors thank Professor B.A. Sastry, Head, Department of Physics, Osmania University, Hyderabad (India) for providing laboratory facilities. Md. Shareefuddin thank the Council for Scienti®c and Industrial Research (CSIR), New Delhi (India) for the award of Senior Research Associateship (SRA) while N. Sunitha Bai thank the CSIR for awarding Research Associateship (RA).
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Electron paramagnetic resonance and optical absorption studies of Cu2 spin probe in MgO±Na2O±B2O3 ternary glasses G. Ramadevudu, Md. Shareefuddin, N. Sunitha Bai, M. Lakshmipathi Rao, M. Narasimha Chary * Department of Physics, Osmania University, Hyderabad 500 007, India Received 13 July 1999; received in revised form 3 November 1999
Abstract Electron paramagnetic resonance and optical absorption studies of MgO±Na2 O±B2 O3 glasses were made by introducing Cu2 as a spin probe. The electron paramagnetic resonance spectra of all the glass samples recorded at X-band frequencies have similar spectral features. It is observed that the spin-Hamiltonian parameters calculated from the EPR spectra are in¯uenced by the glass composition. The Cu2 ions are in well-de®ned axial sites but subjected to small distortion leading to the broadening of the spectra. The spin-Hamiltonian parameter values indicate that the ground state of Cu2 is dx2 ÿy 2 orbital (2 B1g ) and the site symmetry around Cu2 ions is tetragonally distorted octahedral. The optical absorption spectra exhibited a broad band corresponding to the d±d transition bands of Cu2 ion. The bond parameters indicate a slight covalency for the r and in-plane p-bonds, compared to the out-of-plane p-bonds. The theoretical optical basicity parameter values were evaluated and it was observed that sp which is a measure of Lewis basicity of non-bridging oxide ions in the glasses varies in a non-linear manner with increasing optical basicity. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Electron paramagnetic resonance (EPR) spectroscopy has been proved to be a powerful experimental tool for determining the coordination and environment of paramagnetic ions in glass [1± 3]. EPR investigation of oxide glasses doped with transition metal (TM) ions have been extensively studied to obtain information on the glassy network and to identify the site symmetry around the
* Corresponding author. Tel.: +91-40 701 8951; fax: +91-40 701 9020. E-mail address: mnchary_phy@redimail.com (M. Narasimha Chary).
TM ions [4±7]. Copper (II) (Cu2 ) is the most amenable ion for EPR studies. The main advantage of using Cu2 as the spin probe is that its EPR spectra can easily be recorded at room temperature, the spectrum is simple and the spread of the spectrum is large enough to detect minute changes in the coordination sphere [8]. Glasses containing TM ions exhibit memory and photoconducting properties [9]. Information on the local structure may be obtained by using optical spectroscopy methods. The position of the optical absorption peak is sensitive to the composition of the glass [10]. The local environment of TM ions incorporated in a glass may change due to the ligand ®eld variations, which will be re¯ected in the EPR and
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 2 5 5 - 6
206
G. Ramadevudu et al. / Journal of Non-Crystalline Solids 278 (2000) 205±212
optical spectra. By correlating EPR and optical data the information on metal ligand bond nature can be obtained [3]. The properties of a glass can often be altered by the addition of a network modi®er to the basic constituent, which is termed as the network former (e.g. B2 O3 ). The commonly used network modi®ers are the alkali and alkaline earth oxides [11]. It was observed that the properties of an alkali oxide glass show a non-linear behaviour when one kind of alkali is gradually replaced by another [12]. This departure from linearity is called the mixed alkali eect. Similar observations were made in the case of mixed alkali±alkaline earth oxide glasses [13]. This phenomenon is called mixed oxide eect [13]. In this paper, we report EPR and optical absorption studies of Cu2 spin probe in the ternary glass system MgO±Na2 O±B2 O3 . The in¯uence of varying the concentrations of alkali (Na2 O) and alkaline earth (MgO) oxides, which act as network modi®ers on the spin-Hamiltonian parameters, is discussed. The main application of MgO±Na2 O±B2 O3 glasses is that they can be used as solid-state electrolytes in the fabrication of solid-state batteries. Magnesium (Mg) can be used as one of the electrodes since it is non-hygroscopic and less reactive, compared to sodium.
Table 1 Composition of glasses (mol%) S. no 1. 2. 3. 4. 5.
Composition
Tg (°C)
MgO
Na2 O
B2 O3
CuO
5 10 12 15 17
25 20 18 15 13
69 69 69 69 69
1 1 1 1 1
470 485 497 515 534
con®rm the glassy nature of the prepared samples. Table 1 gives the composition in mol% of the glasses studied in the present investigation, along with the glass transition temperatures (Tg ). EPR spectra of the crushed glasses were recorded at room temperature (310 K) using an ESR spectrometer (model JEOL-JES-FE3X) at X-band frequencies with 100 kHz ®eld modulation. The magnetic ®eld was scanned between 0.23 and 0.43 T. The organic radical a,a-diphenyl-b-picrylhydrazyl (DPPH) with a g value of 2.0036 was used as the standard g marker. Optical absorption spectra were recorded at 310 K using UV±VIS spectrophotometer in the wavelength region 300±1000 nm. The peak positions were obtained by using the peak-pick programme provided with the spectrometer. The uncertainty in the observed wavelength is about 1 nm.
2. Experimental
3. Results
The starting materials used in the present study were analytical grade magnesium oxide (MgO), sodium carbonate (Na2 CO3 ), and boric acid (H3 BO3 ). These materials were weighed to get the required composition and ground in a mortar with a pestle for 1 h to obtain homogeneous mixtures. The base glass composition was xMgO± (30)x)Na2 O±69B2 O3 to which 1 mol% of Cu2 was added in the form of CuO as the spin probe. Each batch was melted in porcelain crucible in an electric furnace at 1273 K for about 30 min. The homogeneous melt was rapidly quenched on to a stainless steel plate maintained at a temperature of 373 K. The glasses were annealed for 24 h also at 373 K to relieve the mechanical stresses. An X-ray diractometer and a thermal analyser were used to
3.1. EPR spectra No EPR signal was observed for undoped (without Cu2 ) glasses indicating the absence of TM ions. An EPR signal was observed for all the glasses containing Cu2 ions. Fig. 1 shows the EPR spectrum of Cu2 in 15MgO±15Na2 O± 69B2 O3 glass. The EPR spectra of Cu2 ions in the remaining glasses exhibited the same spectral features. Similar EPR line shapes were observed earlier in oxide glasses doped with Cu2 ions [14± 17]. The Cu2 ion (S 1/2) has a nuclear spin I 3/2 for both Cu63 and Cu65 (69.09% and 30.91% isotopic abundance, respectively). Therefore 2I 1, i.e. four parallel and four perpendicular hyper®ne lines could be observed. In most
G. Ramadevudu et al. / Journal of Non-Crystalline Solids 278 (2000) 205±212
207
Hamiltonian can be employed in the analysis of the EPR spectra [20±22] which is given below: H bgk Hz Sz g? Hx Sx Hy Sy Ak Iz Sz A? Ix Sx Iy Sy ;
Fig. 1. EPR spectrum of Cu2 in 15 MgO±15Na2 O±69B2 O3 .
1
where z is the symmetry axis, b the Bohr magneton, S and I are the electron and nuclear spin operators, Hx ; Hy ; Hz are the static magnetic ®eld components, gk and g? are the parallel and perpendicular components of the g tensor and Ak and A? are parallel and perpendicular components of the hyper®ne tensor A. The nuclear quadrupole contribution is neglected [23]. The solution to the spin-Hamiltonian gives the expressions for the peak position related to the principal values of g and A tensors as [24] hm gk bH mAk 15=4 ÿ m2
A2? 2gk bH
2
and Fig. 2. Computer-simulated EPR spectrum of Cu2 in 15MgO± 15Na2 O±69B2 O3 .
of the cases, however, the isotopic splittings are not resolved owing to the nearly identical nuclear moments and the large line widths resulting from the random orientation of magnetic complexes in the glasses. In all the EPR spectra recorded in the present investigation three weak parallel components were observed in the low ®eld region. However, the perpendicular components are not resolved leading to an intense line in the high ®eld region. It can be observed that the high ®eld side of the spectrum is more intense than the low ®eld side. The EPR spectra were also computer simulated [18,19]. Fig. 2 presents the computer simulated EPR spectrum of Cu2 in 15MgO±15Na2 O± 69B2 O3 glass. Spectroscopic splitting (g) and hyper®ne (A) tensors with axial symmetry have been assumed in the analysis of EPR spectra of oxide glasses [20]. The Jahn±Teller eect causes predominantly an elongated octahedral coordination with four short in-plane bond lengths. Therefore an axial spin-
hm g? bH mA? 15=4 ÿ m2 A2k
A2? ; 4g? bH 3
for the parallel and perpendicular hyper®ne peaks, respectively. Here m is the microwave frequency. The calculated spin-Hamiltonian parameters (SHP) are given in Table 2.
3.2. Optical absorption spectra The optical absorption spectra of all the glasses studied reveal only a broad absorption band. Fig. 3 presents the optical absorption spectrum of Cu2 in 15MgO±15Na2 O±69B2 O3 glass. The optical absorption values are given in Table 3. The variation of optical absorption maximum (k) with MgO content is shown in Fig. 4. With increasing MgO content the optical absorption maximum shifts towards longer wavelengths. The theoretical optical basicity K values were evaluated using the basicity moderating parameters which constitute the glass [25,26] and are given in Table 3.
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Table 2 Spin-Hamiltonian parameters of Cu2 ion in xMgO±(30)x)Na2 O±69B2 O3 glasses S. no.
gk
g?
Ak 104 (cmÿ1 )
Ak 104 (cmÿ1 )
1. 2. 3. 4. 5.
2.352 0.002 2.348 0.002 2.345 0.002 2.338 0.002 2.336 0.002
2.098 0.002 2.097 0.002 2.095 0.002 2.095 0.002 2.093 0.002
153 2 155 2 158 2 162 2 164 2
19.52 19.4 2 19.3 2 19.6 2 19.5 2
Fig. 3. Optical absorption spectrum of Cu2 in 15MgO± 15Na2 O±69B2 O3 . Table 3 Optical absorption bands and K values for Cu2 ions in xMgO± (30)x)Na2 O±69B2 O3 glasses S. no.
K
DE (cmÿ1 )
1. 2. 3. 4. 5.
0.5095 0.5065 0.5057 0.5052 0.5051
14085 13927 13736 13513 13333
4. Discussion 4.1. EPR spectra The observed g values are characteristic of Cu2 coordinated by six ligands, which form an octahedron elongated along the z-axis. The general nature of the ligand coordination can be obtained [27] from the fact that gk > g? > ge ge g factor of free electron 2:0023: Only an environment elongated along one of the cube axes can yield this result. In the present investigation it is
Fig. 4. Variation of optical absorption maxima (k) with MgO content.
observed that gk > g? > ge . Therefore from the g values and shape of the EPR spectra, it can be concluded that the ground state of the Cu2 is dx2 ÿy 2 orbital (2 B1g state),the Cu2 ions being located in tetragonally distorted octahedral sites [20,27]. Detailed calculations have shown that the elongated structures are usually more favourable than the compressed structures. Various experimental data also con®rmed that the Cu2 ion generally exists in solutions, crystals and glasses in octahedral symmetry with a tetragonal distortion [28±31]. The high gk values indicate the presence of a CuO6 chromophore [32,33]. The gk and Ak values are found to be dependent on the glass composition while g? and A? (Table 2)
G. Ramadevudu et al. / Journal of Non-Crystalline Solids 278 (2000) 205±212
values are essentially constant. The variation of gk and Ak with increasing MgO content is non-linear (Fig. 5). As the MgO content increases gk decreases and Ak increases indicating the change in the tetragonal distortion of Cu2 [34]. Variation in gk and Ak values may be associated with the change in the environment of Cu2 , i.e. in the ligand ®eld at the site of Cu2 which may be attributed to the structural changes in the glass. In the B2 O3 glasses, the addition of network modi®ers (MgO and Na2 O) leads to an increase in the coordination number of a certain portion of the boron atoms from 3 to 4. It is assumed that the resulting glass is composed of both triangular and tetrahedral units which form a relatively open network with holes between the oxygen atoms of sucient size to accommodate the Na and Mg ions [11]. As a doubly charged cation, Mg2 is suciently strong to split the network. Thus, sucient non-bridging oxygens are available for good coordination in the broken network. The alkali oxide Na2 O makes available additional weakly bonded O2ÿ for each Mg2 , i.e. Mg2 captures the O2ÿ from Na2 O and this happens at the expense of Na2 O coordination. Na should remain in the neighbourhood of the next stronger Mg2 than that it should be incorporated separately into the rigid network. The con®guration Na±O±Mg is energetically favoured [13]. The solubility of the Cu2 increases with the addition of the network modi®ers presumably due to the
Fig. 5. Variation of gk and Ak with MgO content.
209
coordination of the metal ion by the extra oxygen ions. Thus, incorporation of MgO in the glass will in¯uence the ®eld at the site of Cu2 , which in turn will re¯ect in the non-linear variation of the spinHamiltonian parameters as observed in the present case. The line width of the parallel hyper®ne components was found to increase with increasing values of the nuclear spin quantum number mI (Fig. 1), which may be due to ¯uctuations in both the ligand ®elds and bond covalencies from one copper (II) complex to the next, giving rise to a narrow distribution in g [27]. The mobility of the paramagnetic species in the glass medium may also aect the EPR line shape. 4.2. Optical absorption spectra The optical absorption spectrum is in¯uenced by the host structure into which the TM ions are incorporated. In oxide glasses, the TM ions mostly form coordination complexes with doubly charged oxygen as the ligands. In a regular octahedral complex one optical absorption peak is expected corresponding to the 2 Eg ® 2 T2g transition [10]. But Cu2 , being a d9 ion, experiences a strong Jahn±Teller distortion, which leads to the splitting of energy levels [35] and causes predominantly an elongated octahedral coordination with four short in-plane bond lengths and two longer axial bond lengths (D4h site symmetry). Accordingly three transitions, viz. 2 B1g ® 2 A1g , 2 B1g ® 2 B2g and 2 B1g ® 2 Eg are expected. But only a single optical absorption maximum was observed in most of the cases [36,37]. This single optical band was interpreted [38] as the overlap of all the three transitions. In most of the cases, however, it was found that a unique decomposition into three bands was not possible. Various authors [8,22,39±41] have placed the 2 B1g ®2 B2g and 2 B1g ® 2 A1g transitions under the observed band while 2 B1g ® 2 Eg transition is considered to be hidden under the intense charge transfer absorption in the UV region. It was assumed [8,40] as a reasonable approximation that, of the two transitions under the observed band, the one at shorter wavelength has the same maximum position, as does the envelope itself. Hence most of the authors [4,7,8,39,42±45]
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assigned the observed optical peak to the 2 B1g ®2 B2g transition (DExy ) and have used this value in the evaluation of the bond parameters. In the present case also the optical absorption was assigned to 2 B1g ® 2 B2g (DExy ) transition to evaluate the bond parameters. With increasing MgO content the optical absorption maximum shifts towards longer wavelength which is attributed to the reduction of ligand ®eld around Cu2 ion, as incorporation of MgO may modify the boron±oxygen network by converting some of the BO3 units into BO4 units [2,11].
Table 4 Bonding coecients for Cu2 ions in xMgO±(30)x)Na2 O± 69B2 O3 glasses S. no.
a2
b21
b2
sp (%)
sr (%)
1. 2. 3. 4. 5.
0.860 0.861 0.865 0.869 0.871
0.865 0.844 0.821 0.787 0.770
0.895 0.894 0.890 0.890 0.883
27.08 31.26 35.78 42.52 45.92
30.74 30.41 29.53 28.62 28.16
DExz;yz
2
4.3. Cu ±ligand bond nature Using the antibonding molecular orbitals of appropriate symmetry, the EPR and optical data can be related to the bonding coecients of Cu2 [22,27,39]. The bonding between the Cu2 ion and its ligands can be described in terms of the covalency parameters a2 , b2 and b21 . a2 describes the inplane r-bonding with the copper dx2 ÿy 2 orbital, b2 describes the out-of-plane p-bonding with the dxz and dyz orbital and the b21 parameter is a measure of in-plane p-bonding with the dxy orbital. a2 lies between 0.5 and 1.0, the limits of pure covalent and pure ionic bonding, respectively. The terms, b21 and b2 can be interpreted similarly. The bonding parameters were evaluated using the equations given below [8,18,27], a2 ÿ Ak =P gk ÿ 2 3=7 g? ÿ 2 0:04;
1656K 2 ; g? ÿ 2:0023
7
where K 2 is the orbital reduction factor ( 0.77). An error of 20% in this optical transition assignment (DExz;yz ) may introduce a 5% error in the calculated bonding coecient b2 [7,22]. Using the Eqs. (4)±(7) a2 , b2 and b21 were calculated and are tabulated in Table 4. The errors associated with a2 , b21 and b2 are 1.5%, 3% and 5%, respectively. The values of a2 and b21 indicate slight covalency for the r-and in-plane pbonds compared to the out-of-plane p-bonds (b2 values). The normalized covalency of Cu2 ±O inplane bonding of r and p symmetries (sr and sp , respectively) were calculated using the equation [4] sr
200 1 ÿ S 1 ÿ a2 % 1 ÿ 2S
8
4 DExz;yz ; 828a2
5
DExy ; 3312a2
6
b2 g? =ge ÿ 1 b2 gk =ge ÿ 1
where P is the dipolar hyper®ne coupling parameter ( 0.036 cmÿ1 ), DExy and DExz;yz are the heights of the dxy and dxz;yz molecular orbital levels above the ground state dx2 ÿy 2 , respectively [8,22]. In order to evaluate the bond parameters the optical data is required. As already discussed, the position of the observed absorption maximum of Cu2 indicates the values of DExy [4,8,22,34,42]. The corresponding value of DExz;yz was calculated using the approximation [7]
Fig. 6. Variation of sp and sr with optical basicity (K).
G. Ramadevudu et al. / Journal of Non-Crystalline Solids 278 (2000) 205±212
References
and sp 200 1 ÿ b21 %;
211
9
where S is the overlap integral Soxygen 0:076. Table 4 gives the values of sr and sp . The variation of sr and sp with optical basicity is shown in Fig. 6. sp , which is a measure of Lewis basicity of nonbridging oxide ions in glasses, varies in a non-linear manner with increasing K. The non-linear variation of sp may be attributed to the mixed oxide effect [13,45]. There is little variation of sr with K values. 5. Conclusions 1. From EPR and optical measurements, it is clear that Cu2 is present in all the glasses investigated and they exist in tetragonally distorted octahedral sites with dx2 y 2 (2 B1g ) ground state. 2. The spin-Hamiltonian parameters are in¯uenced by the composition of glasses, which may be attributed to the change of ligand ®eld strength around Cu2 . 3. With increasing MgO content, the ligand ®eld strength is reduced around the Cu2 ion, due to the modi®cation of the boron±oxygen network. 4. The bond parameter values indicate slight covalency for the r and in-plane p-bonds compared to the out-of-plane p-bonds. 5. sp which is a measure of Lewis basicity of nonbridging oxide ion varies in a non-linear manner with increasing K values, which may attributed to the mixed oxide eect. Acknowledgements The authors thank Professor B.A. Sastry, Head, Department of Physics, Osmania University, Hyderabad (India) for providing laboratory facilities. Md. Shareefuddin thank the Council for Scienti®c and Industrial Research (CSIR), New Delhi (India) for the award of Senior Research Associateship (SRA) while N. Sunitha Bai thank the CSIR for awarding Research Associateship (RA).
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