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The EPR spectra of some transition ions in lead-silicate glasses
Journal of Non-Crystalline Solids 27 (1978) 427-435 © North-Holland Publishing Company
THE EPR SPECTRA OF SOME TRANSITION IONS IN LEAD-SILICATE GLASSES
UD. BOGOMOLOVA, V.A. JACHKIN, V.N. LAZUKIN and V.A. SHMUCLER Institute of Nuclear Physics, Moscow State University, Moscow 117234, USSR
Received 29 April 1977
1. Introduction The study of optical spectra of the transition elements in lead-containing silicate glasses [1] has shown that the optical spectra and colouring of these glasses are essentially different from those of sodium-silicate glasses containing no lead. In the present work, the EPR spectra of Cu 2+, Mn 2+ and V 4+ ions in binary lead-silicate glasses are investigated and analyzed. Previously, the EPR spectra of Cu 2+ ions and those of Mn 2+ ions for a single sample of 33 PbO • 67 SiO 2 were investigated in refs. [2] and [3]. In addition, in the present work, the EPR spectra are measured for comparison in some samples of!ead-borate glasses. In order to determine the location of the colouring ion relative to lead ions in various glasses, we also discuss the EPR data for Cu 2+ and V 4+ in lead-phosphate glasses [4]. Binary glasses of the P b O - S i O 2 system were prepared from chemically pure reactants PbO and SiO 2 in the concentration range 2 2 - 8 0 mol% PbO in platinum crucibles in electrical furnaces. Samples were prepared in air atmosphere under temperature conditions depending on the glass composition. The EPR spectra were measured using an X-band radiospectrometer EPR-3 at room temperature. The optical spectra were recorded using the spectrophotometer SP-8.
Results 2.1. EPR o f V4+ ions
In ref. [1 ] it was mentioned that as V20 s is introduced in glasses of the composition Na20 - 3SiO 2 - 2PbO no optical absorption bands for the low-valence forms of vanadium are observed. It was also indicated that vanandium is present there as V s+ whose charge transfer band determines the absorption edge position for the glasses under study. In binary lead-silicate glasses containing PbO in excess of 50 mo1% no optical absorption bands of V 3+ and V 4+ ions are observed neither is the EPR signal 427
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L.D. Bogomolova et al. / E P R spectra o f transition ions
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• - 45 $102 • Fig. 1. Short-wavelength edge o f the optical transmission spectra o f vanadium-containing • 55 PbO glasses: (I) 0% V 2 0 5 , (II) 0.5 wt% V 2 0 s, (III) 5 wt% V 2 0 5 .
from V 4+ ions and V20 s to 5 wt% is added, even in melting under special reduction conditions (addition of charcoal). Here the shift of the edge of UV absorption of glasses towards the long-wavelength part of the spectrum as the V2Os content increases may be observed. Fig. 1 illustrates such a shift for glass containing 45 mol% SiO2 and 55 mol% PbO which has been melted under air atmosphere conditions. In binary lead-silicate glasses containing from 30 to 50 mol% PbO, which were prepared under normal conditions, EPR signals are observed whose intensity decreases by a factor of 3 as the PbO concentration increases from 30 to 50 mol%. The EPR spectrum for glass with 30 mol% PbO and 0.5 wt% V20 s is shown in fig. 2, together with the EPR spectrum of VO 2÷ in sodium-silicate glasses containing 30 mol% Na20. One can readily notice that in lead-silicate glasses the EPR lines are much broader than those in the sodium-silicate glasses. However, the shape of the spectrum enables one to suggest that is due to the V 4÷ ion which is present in leadsilicate glasses in the vanadyl-like form. It is very difficult exactly to determine the parameters of the spin hamiltonian from this spectrum• If one assumes that the observed shape of the spectrum is due to the V 4+ ion in a single structural position with strongly distorted immediate surroundings, the above parameters will be as follows: gll = 1.906 -+ 0.010; All = (160 + 5) 10 -4 c m - l ; g± = 1.969 + 0.003; A z = (60 + 3) 10 -4 cm -1. However, a very flat top of the hfs components enables one to suggest that V4+ ions can be found at least in two non-equivalent positions characterized by the following values of the spectral paramaters: (1) gll = 1.912 + 0.010; All = (170 + 5) 10 -4 cm-1; (II)gll = 1.891 -+ 0.010;All = (161 + 5) 10 -4 cm -1.
429
L.D. Bogomolova et al. / EPR spectra o f transition ions
I • ......
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2900
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Fig. 2. Fragments of EPR spectra of V4+ in glasses of the systems PbO-SiO 2 and Na20-SiO 2 : (I) 30 PbO - 70 SiO2; (II) 30 Na20 • 70 SiO2. In lead-borate glasses the EPR spectrum of V 4+ ions already has a weak intensity at 30 mol% PbO and vanishes at 40 mol%. In comparison with alkali-borate glasses containing 30 mol% R20, its lines are considerably broader and its parameters have the following values: gll = 1.933 -+ 0.005;A II= (168 +- 5) 10 - 4 cm -1. The EPR spectra of V 4÷ ions in lead-phosphate glasses have the same shape and spectral parameters as those of other phosphate 'glasses with modifiers o f the second group [4]. 2.2. E P R spectra o f Cu 2÷ ions
The EPR spectra o f Cu 2+ ions in lead-silicate glasses were studied in ref. [2]. Fig. 3 shows the dependences of hfs constants. Here, A U and gll values for sodium-silicate and lead-silicate glasses, as obtained from new measurements in which the systematic error that crept in the results of ref. [2] has been eliminated. The maximum of the optical absorption band of copper (760 nm) is displaced in the binary lead-silicate glass 45 SiO 2 • 55 PbO toward the shorter wavelengths, as compared with that for sodium-silicate glass (790 nm) (see ref. [1 ]).
430
L.D. Bogomolova et al. /EPR spectra o f transition ions
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Fig. 3. The dependence of the EPR parameters All (1, 2) and gll (I, II) for the Cu 2+ ion in glasses of the system P b O - S i O 2 (I and 1) and N a 2 0 - S i O 2 (II and 2).
In lead-phosphate glasses one observes the ordinary EPR spectru~ of Cu 2+ ions, which is the same as for other phosphate glasses with the second group modifiers, although its parameters are somewhat different [4]. Having the largest g-factor and the smallest hfs constant, the Cu 2+ ion is also characterized by the longest-wavelength maximum of the optical absorption band [4]. In the range 3 0 - 6 0 tool% PbO, the hfs constant Art in lead-borate glasses decreases from 1 5 8 . 1 0 -4 to 149. 10 -4 cm -1 at a practically unchanged value of the g-factor: glt = 2.338 + 2.335 (+0.003). 2.3. EPR spectra o f Mn 2+ ions In the present work, the EPR spectra of Mn 2+ ions in glasses of the system P b O SiO2 containing from 30 to 80 tool% PbO have been investigated. For comparison we have also studied the behavior of the spectra of Mn 2+ in sodium-silicate glasses at Na20 concentrations from 17 to 50 mol%. In both types of the systems the MnO concentration was 0.1 wt%. The experimental spectra for some glasses are shown in fig. 4. From this figure one can see that in both types of glasses the lines with g = 4.3 and g = 2.0 are observed, which are typical of the Mn 2+ ion, the line intensities being functions of the glass composition. In both systems these lines exhibit hyper-
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Fig. 4. EPR spectra of Mn 2+ in glasses of the systems P b O - S i O 2 and N a 2 0 - S i O 2. (I) Total spectrum o f Mn 2+ in P b O - S i O glasses: (a) 30 P b 0 • 70 SiO2; (b) 75 PbO • 25 SiO 2. (II) The EPR line of Mn 2+ w i t h g = 4.3 for some glasses: (a) 30 N a 2 0 • 70 SIO2; (b) 50 PbO • 50 SiO2; (c) 80 PbO • SiO 2. (III) The EPR line of Mn 2+ ions with g = 2.0 for (a) N a 2 0 - S i O 2 and (b) P b O - S i O 2 glasses. The central part of the lines with g = 4.3 is due
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L.D. Bogomolova et al. / EPR spectra o f transition ions
fine structure. In most of the compositions the number of hfs components exceeds six, which is expected for the isotropic hyperfine structure due to the allowed transitions in the interaction of an unpaired electron with the magnetic moment of the Mn ss nucleus. The values of the spectral parameters are given in section 3. We have experimentally established the following regularities: (1) With the increasing content of both PbO and Na20 the intensity of both lines decreases several times; this appears to be caused by the shift of equilibrium toward the higher valence forms of Mn with the increasing content of the ionic components of glass. (2) In lead-silicate glasses the intensity of the line with g = 4.3 is several times higher than that of the line with g = 2.0. in low-lead samples. As the concentration of PbO increases the line with g = 2.0 practically vanishes. The additional structure of the line with g = 4.3 also vanishes, and the usual hfs "sextet" was observed: (3) In sodium-silicate glasses the intensity of the line with g = 2.0 is about seven times as high as that of the line with g = 4.3.
3. Discussion By now, much experimental EPR data have been obtained for Cu 2+ and V4÷ ions in various oxide glasses. The main result is that the spectral parameters of Cu 2+ and V 4+ ions are highly sensitive to the glass composition. The character of the compositional dependence of the spectral parameters makes possible determination of the paramagnetic ion location in the glass network. Consider from this standpoint the change in the EPR parameters of Cu 2+ and V 4+ ions in silicate glasses. From fig. 3 one can see how strongly the Na20 content affects the parameters of Cu 2÷ in sodium-silicate glass. Table 1 presents the parameters of Cu 2+ and VO 2÷ ions (.4 II and gu) for some glasses corresponding to an approximately disilicate composition with different modifiers, which have been taken from our earlier works. It is easy to see that the EPR parameters of Cu 2+ are strongly dependent on the nature of the modifier, whereas the EPR parameters of V 4+ are practically independent of the modifier, but they vary in a narrow range as the Na20 content increases Table 1 Glass composition Na20 • 2SiO2 K20 • 2SiO2 BaO • 2SiO2 SrO • 2SiO2
Cu2+
V4+
gll
All' 104 cm-I
gl[
All"104 cm-I
2.351 2.340 2.367 2.359
150 165 140.2 138.8
1.939 1.939 1.936
169 170 168
L.D. Bogomolova et al. / EPR spectra o f transition ions
433
from 17 to 37 mol% in sodium-silicate glasses: gll from 1.932 to 1.939, AII from 172 to 168,A1 from 38 to 5 4 . 1 0 -4 cm - l andg± from 1.969 to 1.964. It has been suggested [5] that disilicate glasses have a layer structure with cationic layers separated by those of tetrahedrons, SiO 2. On the basis of the established strong dependence of the spectral parameters of Cu 2÷ on the nature of the modifier it may be assumed that Cu 2÷ ions are located in the immediate vicinity of modifiers, i.e. in cationic layers of silicate glass. In ref. [6] it has been demonstrated that complexes formed by Cu 2+ ions in oxide glasses are similar to aquacomplexes [Cu(H20)6] 2+ formed in hydrated crystals and frozen solutions. Apparently the presence of alkali and alkali-earth cations is essential for the stabilization of such complexes. The fact that in the vanadyl ion the parameters remain practically unchanged enables one to suggest that in silicate glasses the vanadyl ions are located between the oxygen-silicon tetrahedrons. Although the vanadyl complex in some oxide glasses also has features in common with the aquavanadyl complexes, it seems to have a wider range of stable positions owing to the double oxygen bond and the presence of some or other ions is not critical for its stabilization. In this aspect, the EPR data for Cu 2+ and V 4+ ions in lead-silicate glasses are readily interpreted. If one assumes that the lead ions in silicate glasses serve both as modifiers and as a network-former [7], then, being located near the lead modifier, Cu 2+ will give rise to the EPR spectrum parameters which are significantly different from those for other modifiers. Indeed, for lead-disilicates we have gtl = 2.363, A II = 135 • 10 -4 cm -1. The incorporation of lead into the glass network will manifest itself rather weakly in the EPR spectra of Cu 2+, because in the given case the effect will be indirect - this can be seen in the slow change in parameters with the varying lead content. (see fig. 3). It is different with the V 4+ ions. Being incorporated precisely in the glass network, these ions react to the presence of Pb ions in the glass network. The spectra observed in lead-silicate glasses and presented in fig. 2 are quite different from all spectra ever observed in oxide glasses, both in shape and in parameters. The spectral parameters for vanadyl are primarily determined by the type of the network-former and the fact that in the given case we have an unusual set of parameters suggests that lead does play the role of network-former. The large linewidth indicates a highly irregular surrounding of the vanadyl ion created by non-equivalent oxygen atoms. According to [7], "covalent" lead ions are present in lead-silicate glasses at all PbO concentrations, but their proportion increases with incresing lead concentration. At PbO concentrations from 30 to 50 mol%, at which we have succeeded in observing the spectrum of V 4+ ions, out of five oxygen atoms surrounding vanadium, one or two seem to be bonded with lead. The distortion of the environment and the non-equivalence of positions or vanadium are produced exactly by the inconstancy of the number of such oxygen atoms. It is also possible that at PbO concentrations of >50 mol%, there appear direct bonds with charge-transfer between V and Pb, as a consequence of which tetravalent vanadium cannot be obtained, even under the reduction conditions of melting.
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L.D. Bogomolova et al. / E P R spectra o f transition ions
At this point one should note the different role played by lead in lead-silicate and lead-phosphate glasses. In the latter, lead is present only in the divalent state and it serves only as a modifier. This appears to account for the fact that lead-phosphate glasses are not essentially different in their properties from other phosphate glasses with modifiers of the second group [8]. The EPR and optical spectra of Cu 2+ and VO 2÷ in lead-phosphate glasses are the same as those in other phosphate glasses, displaying only regular shifts of the spectral parameters. A considerable broadening of the EPR lines of VO 2÷ ions and a steep decrease of its intensity with increasing PbO content suggest that in lead-borate glasses lead may be incorporated into the glass network, and this result is also consistent with the data of ref. [7]. As follows from fig. 4, the EPR spectra of Mn 2÷, both in lead-silicate and in sodium-silicate glasses, have a complicated shape: the number of hfs components exceeds six and, in most compositions, in both lines. Such lines shapes have been observed in the earliest EPR studies [3,9] of Mn 2÷ glasses, but have not yet been discussed. An attempt to analyze the reasons for the doublet splitting of the hfs components of the line with g = 4.3 in chalcogenide glasses has been made in ref. [10], in which the appearance of two sextets of lines is attributed to the presence of two non-equivalent positions of Mn 2+. The doublet splitting of hfs in both lines observed in our experiment suggests, then, that there are four, and not two, nonequivalent positions of Mn 2÷ ions in the silicate glasses. If, however, one considers the character of the splitting and the greater probability of its appearance at low Mn concentrations (when the splitting is not masked by the line broadening) in various glasses, then, in our opinion, one should look for some general cause of the appeara.nce of additional splittings in the spectrum of Mn 2÷. Since the origin of individual components of the spectrum is obscure, we find it difficult to determine their parameters. If one assumes that they are indeed due to two centres, for the line with g = 4.3 in lead-silicate glasses one has the following values: A (I) = 86 Oe; A (II) = 88 Oe;in sodium-silicate glasses A (I) = 80 Oe;A (II) ~ 83 Oe;g (I) - g (II) = 0,137. The hfs constants for the line with g = 2.0 are in both cases somewhat smaller (Al = 77 and A2 = 80 Oe for lead-silicate glasses and about 75 Oe for sodium-silicate ones). In those lead-silicate glasses (PbO > 60 mol%) in which one sextet is observed on the line with g = 4.3 the hfs constant A is equal to 78 Oe. The obtained results on the EPR of Mn 2÷ in lead-silicate glasses do not easily lend themselves to interpretation because of the uncertain origin of the spectrum shape. One may only state that, in contrast to the assumptions made in [1 ], a fraction of Mn ions is present in lead-silicate glasses in the low-valence form Mn 2+. Most of the Mn 2÷ ions are located in a high crystalline field. The ions located in a weak field practically vasnish with the growing lead content. A more detailed interpretation may be given only after the reason for the appearance of the additional lines in the Mn hfs spectra has been found. The explanation of the additional lines may well be sought in the anisotropy effects [11 ] or in the forbidden transitions [3,12]. The only fact that ensures from our results is that the cause seems to be identical for the two lines and is subject to additional studies.
L.D. Bogomolova et al. / E P R spectra o f transition ions
435
4. Conclusion We have measured and analyzed the EPR spectra of copper, vanadium and manganese ions in lead-silicate and other lead-containing glasses. The EPR and optical properties of these ions have been shown to be determined both the location of lead in the glass network and by the position of the coloring ion relative to lead.
Acknowledgement The authors avail themselves of the opportunity to thank N.A. Belova and I.A. Sasova for preparation of glasses and T.F. Dolgolenko for assistance with optical measurements.
References [11 J.S. Stroud, J. Am. Ceram. Soc. 54 (8) (1971) 401. [2] V.N. Lazukin, Z.M. Syritskaya, N.A. Belova, L.D. Bogomolova, V.N. Ovsyannikov, V.A. Fedorova. Dokl. Akad. Nauk SSSR, 208 (2) (1973) 314. [3] E.I. Abdrashitova, N.F. Yafaev, Fiz. Tverdogo Tela 9 (11) (1967) 3172. [4] L.D. Bogomolova, V.A. Jachkin, V.N. Lazukin, T.K. Pavluskina, V.A. Shmuckler, to be published [5] W.L. Konijnendijk, J.M. Stevels, J. Non-Crystalline Solids 21 (1976) 447. [6] L.D. Bogomolova, V.A. Jachkin, V.N. Lazukin, in: Magnetic Resonance and Related Phenomena, ed. H. Brunner, K.H. Hausser, and S. Schweitzer (Groupement Ampere, Heidelberg-Geneva, 1976) p. 235. [7] K.S. Kim, P.J. Bray, J. Chem. Phys. 61 (11) (1976) 4459. [8] E. Kordes, W. Vogel, R. Feterowsky, Z. Electrochem. 57 (1953) 282. [9] S.G. Lunter, G.O. Karapetyan, N.M. Bokin, D.M. Yudin, Fiz. Tverdogo Tela 9 (10) (1967) 2874. [10] V.N. Lazukin, I.V. Chepeleva, E.A. Zhilinskaya, A.P. Chernov, Phys Stat. Sol. (b) 69 (1975) 399. [11] P.C. Taylor and P.J. Bray, J. Phys. Chem. Solids 33 (1972) 43. [12] H.W. de Wijn and R.F. van Balderen, J. Chem. Phys. 46 (1967) 1381.
THE EPR SPECTRA OF SOME TRANSITION IONS IN LEAD-SILICATE GLASSES
UD. BOGOMOLOVA, V.A. JACHKIN, V.N. LAZUKIN and V.A. SHMUCLER Institute of Nuclear Physics, Moscow State University, Moscow 117234, USSR
Received 29 April 1977
1. Introduction The study of optical spectra of the transition elements in lead-containing silicate glasses [1] has shown that the optical spectra and colouring of these glasses are essentially different from those of sodium-silicate glasses containing no lead. In the present work, the EPR spectra of Cu 2+, Mn 2+ and V 4+ ions in binary lead-silicate glasses are investigated and analyzed. Previously, the EPR spectra of Cu 2+ ions and those of Mn 2+ ions for a single sample of 33 PbO • 67 SiO 2 were investigated in refs. [2] and [3]. In addition, in the present work, the EPR spectra are measured for comparison in some samples of!ead-borate glasses. In order to determine the location of the colouring ion relative to lead ions in various glasses, we also discuss the EPR data for Cu 2+ and V 4+ in lead-phosphate glasses [4]. Binary glasses of the P b O - S i O 2 system were prepared from chemically pure reactants PbO and SiO 2 in the concentration range 2 2 - 8 0 mol% PbO in platinum crucibles in electrical furnaces. Samples were prepared in air atmosphere under temperature conditions depending on the glass composition. The EPR spectra were measured using an X-band radiospectrometer EPR-3 at room temperature. The optical spectra were recorded using the spectrophotometer SP-8.
Results 2.1. EPR o f V4+ ions
In ref. [1 ] it was mentioned that as V20 s is introduced in glasses of the composition Na20 - 3SiO 2 - 2PbO no optical absorption bands for the low-valence forms of vanadium are observed. It was also indicated that vanandium is present there as V s+ whose charge transfer band determines the absorption edge position for the glasses under study. In binary lead-silicate glasses containing PbO in excess of 50 mo1% no optical absorption bands of V 3+ and V 4+ ions are observed neither is the EPR signal 427
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L.D. Bogomolova et al. / E P R spectra o f transition ions
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E 5O'/.
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• - 45 $102 • Fig. 1. Short-wavelength edge o f the optical transmission spectra o f vanadium-containing • 55 PbO glasses: (I) 0% V 2 0 5 , (II) 0.5 wt% V 2 0 s, (III) 5 wt% V 2 0 5 .
from V 4+ ions and V20 s to 5 wt% is added, even in melting under special reduction conditions (addition of charcoal). Here the shift of the edge of UV absorption of glasses towards the long-wavelength part of the spectrum as the V2Os content increases may be observed. Fig. 1 illustrates such a shift for glass containing 45 mol% SiO2 and 55 mol% PbO which has been melted under air atmosphere conditions. In binary lead-silicate glasses containing from 30 to 50 mol% PbO, which were prepared under normal conditions, EPR signals are observed whose intensity decreases by a factor of 3 as the PbO concentration increases from 30 to 50 mol%. The EPR spectrum for glass with 30 mol% PbO and 0.5 wt% V20 s is shown in fig. 2, together with the EPR spectrum of VO 2÷ in sodium-silicate glasses containing 30 mol% Na20. One can readily notice that in lead-silicate glasses the EPR lines are much broader than those in the sodium-silicate glasses. However, the shape of the spectrum enables one to suggest that is due to the V 4÷ ion which is present in leadsilicate glasses in the vanadyl-like form. It is very difficult exactly to determine the parameters of the spin hamiltonian from this spectrum• If one assumes that the observed shape of the spectrum is due to the V 4+ ion in a single structural position with strongly distorted immediate surroundings, the above parameters will be as follows: gll = 1.906 -+ 0.010; All = (160 + 5) 10 -4 c m - l ; g± = 1.969 + 0.003; A z = (60 + 3) 10 -4 cm -1. However, a very flat top of the hfs components enables one to suggest that V4+ ions can be found at least in two non-equivalent positions characterized by the following values of the spectral paramaters: (1) gll = 1.912 + 0.010; All = (170 + 5) 10 -4 cm-1; (II)gll = 1.891 -+ 0.010;All = (161 + 5) 10 -4 cm -1.
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L.D. Bogomolova et al. / EPR spectra o f transition ions
I • ......
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Fig. 2. Fragments of EPR spectra of V4+ in glasses of the systems PbO-SiO 2 and Na20-SiO 2 : (I) 30 PbO - 70 SiO2; (II) 30 Na20 • 70 SiO2. In lead-borate glasses the EPR spectrum of V 4+ ions already has a weak intensity at 30 mol% PbO and vanishes at 40 mol%. In comparison with alkali-borate glasses containing 30 mol% R20, its lines are considerably broader and its parameters have the following values: gll = 1.933 -+ 0.005;A II= (168 +- 5) 10 - 4 cm -1. The EPR spectra of V 4÷ ions in lead-phosphate glasses have the same shape and spectral parameters as those of other phosphate 'glasses with modifiers o f the second group [4]. 2.2. E P R spectra o f Cu 2÷ ions
The EPR spectra o f Cu 2+ ions in lead-silicate glasses were studied in ref. [2]. Fig. 3 shows the dependences of hfs constants. Here, A U and gll values for sodium-silicate and lead-silicate glasses, as obtained from new measurements in which the systematic error that crept in the results of ref. [2] has been eliminated. The maximum of the optical absorption band of copper (760 nm) is displaced in the binary lead-silicate glass 45 SiO 2 • 55 PbO toward the shorter wavelengths, as compared with that for sodium-silicate glass (790 nm) (see ref. [1 ]).
430
L.D. Bogomolova et al. /EPR spectra o f transition ions
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Fig. 3. The dependence of the EPR parameters All (1, 2) and gll (I, II) for the Cu 2+ ion in glasses of the system P b O - S i O 2 (I and 1) and N a 2 0 - S i O 2 (II and 2).
In lead-phosphate glasses one observes the ordinary EPR spectru~ of Cu 2+ ions, which is the same as for other phosphate glasses with the second group modifiers, although its parameters are somewhat different [4]. Having the largest g-factor and the smallest hfs constant, the Cu 2+ ion is also characterized by the longest-wavelength maximum of the optical absorption band [4]. In the range 3 0 - 6 0 tool% PbO, the hfs constant Art in lead-borate glasses decreases from 1 5 8 . 1 0 -4 to 149. 10 -4 cm -1 at a practically unchanged value of the g-factor: glt = 2.338 + 2.335 (+0.003). 2.3. EPR spectra o f Mn 2+ ions In the present work, the EPR spectra of Mn 2+ ions in glasses of the system P b O SiO2 containing from 30 to 80 tool% PbO have been investigated. For comparison we have also studied the behavior of the spectra of Mn 2+ in sodium-silicate glasses at Na20 concentrations from 17 to 50 mol%. In both types of the systems the MnO concentration was 0.1 wt%. The experimental spectra for some glasses are shown in fig. 4. From this figure one can see that in both types of glasses the lines with g = 4.3 and g = 2.0 are observed, which are typical of the Mn 2+ ion, the line intensities being functions of the glass composition. In both systems these lines exhibit hyper-
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Fig. 4. EPR spectra of Mn 2+ in glasses of the systems P b O - S i O 2 and N a 2 0 - S i O 2. (I) Total spectrum o f Mn 2+ in P b O - S i O glasses: (a) 30 P b 0 • 70 SiO2; (b) 75 PbO • 25 SiO 2. (II) The EPR line of Mn 2+ w i t h g = 4.3 for some glasses: (a) 30 N a 2 0 • 70 SIO2; (b) 50 PbO • 50 SiO2; (c) 80 PbO • SiO 2. (III) The EPR line of Mn 2+ ions with g = 2.0 for (a) N a 2 0 - S i O 2 and (b) P b O - S i O 2 glasses. The central part of the lines with g = 4.3 is due
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L.D. Bogomolova et al. / EPR spectra o f transition ions
fine structure. In most of the compositions the number of hfs components exceeds six, which is expected for the isotropic hyperfine structure due to the allowed transitions in the interaction of an unpaired electron with the magnetic moment of the Mn ss nucleus. The values of the spectral parameters are given in section 3. We have experimentally established the following regularities: (1) With the increasing content of both PbO and Na20 the intensity of both lines decreases several times; this appears to be caused by the shift of equilibrium toward the higher valence forms of Mn with the increasing content of the ionic components of glass. (2) In lead-silicate glasses the intensity of the line with g = 4.3 is several times higher than that of the line with g = 2.0. in low-lead samples. As the concentration of PbO increases the line with g = 2.0 practically vanishes. The additional structure of the line with g = 4.3 also vanishes, and the usual hfs "sextet" was observed: (3) In sodium-silicate glasses the intensity of the line with g = 2.0 is about seven times as high as that of the line with g = 4.3.
3. Discussion By now, much experimental EPR data have been obtained for Cu 2+ and V4÷ ions in various oxide glasses. The main result is that the spectral parameters of Cu 2+ and V 4+ ions are highly sensitive to the glass composition. The character of the compositional dependence of the spectral parameters makes possible determination of the paramagnetic ion location in the glass network. Consider from this standpoint the change in the EPR parameters of Cu 2+ and V 4+ ions in silicate glasses. From fig. 3 one can see how strongly the Na20 content affects the parameters of Cu 2÷ in sodium-silicate glass. Table 1 presents the parameters of Cu 2+ and VO 2÷ ions (.4 II and gu) for some glasses corresponding to an approximately disilicate composition with different modifiers, which have been taken from our earlier works. It is easy to see that the EPR parameters of Cu 2+ are strongly dependent on the nature of the modifier, whereas the EPR parameters of V 4+ are practically independent of the modifier, but they vary in a narrow range as the Na20 content increases Table 1 Glass composition Na20 • 2SiO2 K20 • 2SiO2 BaO • 2SiO2 SrO • 2SiO2
Cu2+
V4+
gll
All' 104 cm-I
gl[
All"104 cm-I
2.351 2.340 2.367 2.359
150 165 140.2 138.8
1.939 1.939 1.936
169 170 168
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from 17 to 37 mol% in sodium-silicate glasses: gll from 1.932 to 1.939, AII from 172 to 168,A1 from 38 to 5 4 . 1 0 -4 cm - l andg± from 1.969 to 1.964. It has been suggested [5] that disilicate glasses have a layer structure with cationic layers separated by those of tetrahedrons, SiO 2. On the basis of the established strong dependence of the spectral parameters of Cu 2÷ on the nature of the modifier it may be assumed that Cu 2÷ ions are located in the immediate vicinity of modifiers, i.e. in cationic layers of silicate glass. In ref. [6] it has been demonstrated that complexes formed by Cu 2+ ions in oxide glasses are similar to aquacomplexes [Cu(H20)6] 2+ formed in hydrated crystals and frozen solutions. Apparently the presence of alkali and alkali-earth cations is essential for the stabilization of such complexes. The fact that in the vanadyl ion the parameters remain practically unchanged enables one to suggest that in silicate glasses the vanadyl ions are located between the oxygen-silicon tetrahedrons. Although the vanadyl complex in some oxide glasses also has features in common with the aquavanadyl complexes, it seems to have a wider range of stable positions owing to the double oxygen bond and the presence of some or other ions is not critical for its stabilization. In this aspect, the EPR data for Cu 2+ and V 4+ ions in lead-silicate glasses are readily interpreted. If one assumes that the lead ions in silicate glasses serve both as modifiers and as a network-former [7], then, being located near the lead modifier, Cu 2+ will give rise to the EPR spectrum parameters which are significantly different from those for other modifiers. Indeed, for lead-disilicates we have gtl = 2.363, A II = 135 • 10 -4 cm -1. The incorporation of lead into the glass network will manifest itself rather weakly in the EPR spectra of Cu 2+, because in the given case the effect will be indirect - this can be seen in the slow change in parameters with the varying lead content. (see fig. 3). It is different with the V 4+ ions. Being incorporated precisely in the glass network, these ions react to the presence of Pb ions in the glass network. The spectra observed in lead-silicate glasses and presented in fig. 2 are quite different from all spectra ever observed in oxide glasses, both in shape and in parameters. The spectral parameters for vanadyl are primarily determined by the type of the network-former and the fact that in the given case we have an unusual set of parameters suggests that lead does play the role of network-former. The large linewidth indicates a highly irregular surrounding of the vanadyl ion created by non-equivalent oxygen atoms. According to [7], "covalent" lead ions are present in lead-silicate glasses at all PbO concentrations, but their proportion increases with incresing lead concentration. At PbO concentrations from 30 to 50 mol%, at which we have succeeded in observing the spectrum of V 4+ ions, out of five oxygen atoms surrounding vanadium, one or two seem to be bonded with lead. The distortion of the environment and the non-equivalence of positions or vanadium are produced exactly by the inconstancy of the number of such oxygen atoms. It is also possible that at PbO concentrations of >50 mol%, there appear direct bonds with charge-transfer between V and Pb, as a consequence of which tetravalent vanadium cannot be obtained, even under the reduction conditions of melting.
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At this point one should note the different role played by lead in lead-silicate and lead-phosphate glasses. In the latter, lead is present only in the divalent state and it serves only as a modifier. This appears to account for the fact that lead-phosphate glasses are not essentially different in their properties from other phosphate glasses with modifiers of the second group [8]. The EPR and optical spectra of Cu 2+ and VO 2÷ in lead-phosphate glasses are the same as those in other phosphate glasses, displaying only regular shifts of the spectral parameters. A considerable broadening of the EPR lines of VO 2÷ ions and a steep decrease of its intensity with increasing PbO content suggest that in lead-borate glasses lead may be incorporated into the glass network, and this result is also consistent with the data of ref. [7]. As follows from fig. 4, the EPR spectra of Mn 2÷, both in lead-silicate and in sodium-silicate glasses, have a complicated shape: the number of hfs components exceeds six and, in most compositions, in both lines. Such lines shapes have been observed in the earliest EPR studies [3,9] of Mn 2÷ glasses, but have not yet been discussed. An attempt to analyze the reasons for the doublet splitting of the hfs components of the line with g = 4.3 in chalcogenide glasses has been made in ref. [10], in which the appearance of two sextets of lines is attributed to the presence of two non-equivalent positions of Mn 2+. The doublet splitting of hfs in both lines observed in our experiment suggests, then, that there are four, and not two, nonequivalent positions of Mn 2÷ ions in the silicate glasses. If, however, one considers the character of the splitting and the greater probability of its appearance at low Mn concentrations (when the splitting is not masked by the line broadening) in various glasses, then, in our opinion, one should look for some general cause of the appeara.nce of additional splittings in the spectrum of Mn 2÷. Since the origin of individual components of the spectrum is obscure, we find it difficult to determine their parameters. If one assumes that they are indeed due to two centres, for the line with g = 4.3 in lead-silicate glasses one has the following values: A (I) = 86 Oe; A (II) = 88 Oe;in sodium-silicate glasses A (I) = 80 Oe;A (II) ~ 83 Oe;g (I) - g (II) = 0,137. The hfs constants for the line with g = 2.0 are in both cases somewhat smaller (Al = 77 and A2 = 80 Oe for lead-silicate glasses and about 75 Oe for sodium-silicate ones). In those lead-silicate glasses (PbO > 60 mol%) in which one sextet is observed on the line with g = 4.3 the hfs constant A is equal to 78 Oe. The obtained results on the EPR of Mn 2÷ in lead-silicate glasses do not easily lend themselves to interpretation because of the uncertain origin of the spectrum shape. One may only state that, in contrast to the assumptions made in [1 ], a fraction of Mn ions is present in lead-silicate glasses in the low-valence form Mn 2+. Most of the Mn 2÷ ions are located in a high crystalline field. The ions located in a weak field practically vasnish with the growing lead content. A more detailed interpretation may be given only after the reason for the appearance of the additional lines in the Mn hfs spectra has been found. The explanation of the additional lines may well be sought in the anisotropy effects [11 ] or in the forbidden transitions [3,12]. The only fact that ensures from our results is that the cause seems to be identical for the two lines and is subject to additional studies.
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4. Conclusion We have measured and analyzed the EPR spectra of copper, vanadium and manganese ions in lead-silicate and other lead-containing glasses. The EPR and optical properties of these ions have been shown to be determined both the location of lead in the glass network and by the position of the coloring ion relative to lead.
Acknowledgement The authors avail themselves of the opportunity to thank N.A. Belova and I.A. Sasova for preparation of glasses and T.F. Dolgolenko for assistance with optical measurements.
References [11 J.S. Stroud, J. Am. Ceram. Soc. 54 (8) (1971) 401. [2] V.N. Lazukin, Z.M. Syritskaya, N.A. Belova, L.D. Bogomolova, V.N. Ovsyannikov, V.A. Fedorova. Dokl. Akad. Nauk SSSR, 208 (2) (1973) 314. [3] E.I. Abdrashitova, N.F. Yafaev, Fiz. Tverdogo Tela 9 (11) (1967) 3172. [4] L.D. Bogomolova, V.A. Jachkin, V.N. Lazukin, T.K. Pavluskina, V.A. Shmuckler, to be published [5] W.L. Konijnendijk, J.M. Stevels, J. Non-Crystalline Solids 21 (1976) 447. [6] L.D. Bogomolova, V.A. Jachkin, V.N. Lazukin, in: Magnetic Resonance and Related Phenomena, ed. H. Brunner, K.H. Hausser, and S. Schweitzer (Groupement Ampere, Heidelberg-Geneva, 1976) p. 235. [7] K.S. Kim, P.J. Bray, J. Chem. Phys. 61 (11) (1976) 4459. [8] E. Kordes, W. Vogel, R. Feterowsky, Z. Electrochem. 57 (1953) 282. [9] S.G. Lunter, G.O. Karapetyan, N.M. Bokin, D.M. Yudin, Fiz. Tverdogo Tela 9 (10) (1967) 2874. [10] V.N. Lazukin, I.V. Chepeleva, E.A. Zhilinskaya, A.P. Chernov, Phys Stat. Sol. (b) 69 (1975) 399. [11] P.C. Taylor and P.J. Bray, J. Phys. Chem. Solids 33 (1972) 43. [12] H.W. de Wijn and R.F. van Balderen, J. Chem. Phys. 46 (1967) 1381.