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Structural studies of potassium silicate glasses with fixed iron content and their relation to similar alkali silicates
Journal of Non-Crystalline Solids 518 (2019) 85–91
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Structural studies of potassium silicate glasses with fixed iron content and their relation to similar alkali silicates
T
Manjunath T. Nayaka, J.A. Erwin Desaa, , V. Raghvendra Reddyb, C. Nayakc, D. Bhattacharyyac, S.N. Jhac ⁎
a
Department of Physics, Goa University, Taleigao Plateau, Goa 403206, India UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India c Atomic and Molecular Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India b
ARTICLE INFO
ABSTRACT
Keywords: Potassium Silicate Glass Fe3O4 Structure
Silicate glasses containing potassium oxide of molar percentages 12%, 16% and 20% each with 8% iron oxide have been prepared and examined by Raman, Extended X-ray absorption fine structure (EXAFS) and Mössbauer spectroscopy. For the 12% and 16% of iron potassium silicate glasses, analysis of the Raman data indicated a maximum of Q2 linkages of the silica tetrahedra while for the 20% iron potassium silicate sample, the Q3 and Q4 linkages were dominant. EXAFS data found the Fe ion to be in tetrahedral coordination with oxygen and a FeeO distance of ~1.85 Å. The isomeric shifts in the Mössbauer spectra found the Fe3+ to Fe2+ ratios to vary with potassium content from 2:1 for the 12% to 8:1 for the 20% potassium glass. Spectral areas showed iron ions to be tetrahedrally coordinated to oxygen with quadrupole splitting showing distortion of these units. From the distribution of Qn linkages it is found that at 16 mol% of the alkali, the lithium iron silicates have more open structures than the sodium and potassium iron silicate glasses. The more open structure of sodium silicate glass when iron is added could have important implications for its use as an electrolytic material.
1. Introduction The addition of alkali oxides to pure silica are known to improve the range of applications of this versatile tetrahedral glassy network [1–4]. In addition to the alkali elements, if a transition metal such as iron is a constituent of the glass, the resulting system is one in which the alkali and iron affect the structures and physical properties in different ways. The present authors have earlier reported the structural effects of lithium [5] and sodium [6] additions to iron containing silicate glasses. We consider here an extension of the latter studies by incorporation of potassium in place of the lithium or sodium ions. Potassium oxide is used in the production of colored glazes and for improving clarity and intensity of colored glasses. Viscosity of the melt is also lowered with the addition of potassium [7]. The glass transition temperature range is increased when potassium is added to the glass. Similarly, some interesting magnetic properties result when potassium is added to the iron silicate glass - as reported here. Study of iron in glass is complex due to the mixed redox states of Fe and the distortions at these sites. Each of the Fe3+ and Fe2+ sites are known to have regular or distorted 4/5/6 coordinated sites which may segregate to form clusters of Fe rich ions [8] making these systems more ⁎
challenging to study. There have been many studies of iron alkali silicate glasses in which the iron content (as Fe2O3) varies [4,5,9–11]. All the studies give lot of insight into the changes of the iron environment in the glass matrix. However, it remains unclear if these changes are due to the changes in concentration of iron content. Moreover, in most of these works the authors have used Fe2O3 as a starting constituent in order to include only ferric ions (Fe3+). However, in almost all of the glasses prepared under oxidising conditions it is found that some of the Fe3+ gets reduced to Fe2+ in the ratio equivalent to 2:1. It has been reported that the polarisation of the lattice by the unpaired electrons of Fe2+ to Fe3+ causes electrical conduction in such glasses [12–15]. Iron ions may also play the role of network modifier or former depending on its concentration in the glassy matrix [16]. Kurkjian and Sigety [17] studied the role of Fe [II] in silicate glass and have concluded that the iron sites are tetrahedrally occupied, whereas Bishay and Kinawi [18] with similar studies have reported Fe [II] to be in both tetrahedral and octahedral sites. A few authors have used Raman spectroscopy to probe the structural changes occurring in such vitreous systems through the study of their vibrational bands [19–20]. For example, the spectral feature attributed to the Q2 band of anti-
Corresponding author. E-mail address: [email protected] (J.A.E. Desa).
https://doi.org/10.1016/j.jnoncrysol.2019.04.025 Received 14 February 2019; Received in revised form 12 April 2019; Accepted 18 April 2019 Available online 21 May 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.
Journal of Non-Crystalline Solids 518 (2019) 85–91
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composition using Energy-dispersive X-ray spectrometer (EDX). Samples were mounted on a sample holder made of aluminium with the help of a carbon-conductive adhesive tape. Sputter coater (Auto fine coater Leica EM ACE 200) was used for coating the samples with gold‑palladium prior to viewing by a field-emission scanning electron microscope (Quanta FEG 250, FEI, Netherlands) fitted with EDAX (Smart Insight, USA) for analyses. Every sample was examined by point analyses at three different locations on the sample. For each sample, the average of the three data sets is tabulated in Table 2. A Raman spectrometer (AIRIX Corp. Tokyo, Japan) STR 500 confocal micro spectrometer imaging spectroscope with focal length of 500 mm was used for collecting the Raman spectra. The excitation wavelength was achieved by 12.5 mW, 532 nm solid state laser at 50× magnification with integration time for each sample being 50 s. The samples were measured from 100 cm−1 to 3000 cm−1 (three scans for each sample) at room temperature. Fig. 1(a) shows the averaged spectra of the samples. Baseline subtractions as explained by Ph Coloumban [22] were performed on the Raman data to retain only the ‘molecular’ SieO Raman signature (600–1200 cm−1) and eliminate the so-called ‘Boson peak’. The pronounced but broad Raman maxima were deconvoluted by fitting Gaussian peaks (Fig. 1 b) at the positions known to correspond to the different Qn vibrations [21,23–30]. The areas of each of the Gaussian fits were then found and placed on a relative scale of peak areas (Table 3). The redox state and environment of iron were analysed by element specific techniques (iron in this case) such as Mössbauer and EXAFS. EXAFS data were collected at the Fe K-edge (7112 eV) using the scanning EXAFS beam line (BL-09) at the Indus-2 Synchrotron Source, RRCAT, Indore [31–32]. A standard sample in the form of a Fe foil was used in the energy calibration, of which the detailed procedure of energy calibration can be found in G. Bunker [33]. Beam size at the sample was 1 mm × 1 mm with an energy step of 0.5 eV. For the EXAFS, the samples (~ 30 mg) were mixed with cellulose and pressed to form pellets. All the samples were scanned three times in transmission mode at room temperature. The final data presented (Fig. 3 a) are accordingly averaged data from the three scans. Details of the EXAFS technique and measurement used by the present authors have been given elsewhere [6]. A 57Co radioactive source was used in the Mössbauer experiment. The transmission mode of measurement with constant acceleration in the velocity range ± 11 mm/s relative to α-Fe was used. Calibration was with a natural iron absorber at ambient temperature. Data analysis used the NORMOS DIST/SITE software. Ratios of Fe3+ to Fe2+ were obtained through the usual methods of isomer shift, quadrupole splitting and line width analysis.
Table 1 Compositions in mol% of the chemicals in the glass samples (Thomas Baker, Purity ≥99%)-Potassium-silicate‑iron oxide glasses. Sample code
SiO2%
K2O %
Fe3O4%
K1 K2 K3 K4
80 80 76 72
20 12 16 20
0 8 8 8
symmetric coupled modes of Fe3+O4-SiO4 shows the presence and variation of Fe3+ in tetrahedral configuration [21]. In the present work, the glassy system chosen for study has been the one in which the potassium oxide molar content varies from 12% to 20% while Fe3O4 was kept at a fixed molar value of 8%, with SiO2 varying accordingly. The iron percentage was kept at 8 mol% as this was found to be the maximum value for a stable and fully vitreous system at the lowest content of potassium oxide (12 mol%). 2. Experimental 2.1. Materials The chemical compositional formula of the glass samples was; (100x-y)SiO2.xK2O.yFe3O4 with y = 8% in 3 samples and x = 12%,16%, 20% for the others (Table 1). Appropriate molar percentages of the constituents were weighed out for 15 g of the mixture. This was finely ground and homogenised using an agate pestle and mortar. The mixture was held in a platinum crucible and heated in a Carbolite 1600 furnace. Initially the samples were heated to 900C for 1 h for the potassium carbonate (K2CO3) to decompose to potassium oxide (K2O). Further heating to 1400C and maintenance at this temperature for 15 mins completed the heat treatment. The melt was then quenched in air to room temperature within the crucible. The glass samples were obtained in broken fragments during cooling which were then annealed by transferring them to a preheated oven at 350C for two hours before being left to cool slowly overnight in the furnace. X-ray diffraction (with MoKα X-rays) data on all the samples confirmed them to be vitreous by the absence of Bragg reflection peaks on the broad diffraction maxima. 2.2. Instrumentation and data analysis All the glass samples were examined for the changes in the Table 2 Comparison of the atomic % of samples before and after syntheses. K1 Atoms
Si
K
Fe
O
Initial Comp. Final Comp. (EDX)
0.267 0.219 ± 0.035
0.133 0.081 ± 0.028
0.000 0.000
0.600 0.700 ± 0.090
K2 Atoms Initial Comp. Final Comp. (EDX)
Si 0.241 0.283 ± 0.044
K 0.072 0.086 ± 0.038
Fe 0.073 0.086 ± 0.034
O 0.614 0.545 ± 0.095
Si 0.229 0.191 ± 0.043
K 0.096 0.067 ± 0.033
Fe 0.073 0.051 ± 0.035
O 0.602 0.689 ± 0.087
Si 0.217 0.251 ± 0.047
K 0.120 0.154 ± 0.030
Fe 0.073 0.107 ± 0.033
O 0.590 0.489 ± 0.103
K3 Atoms Initial Comp. Final Comp. (EDX) K4 Atoms Initial Comp. Final Comp. (EDX)
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Fig. 1. (a): Raman spectra of potassium silicate glass with and without iron (the blue bar and yellow bar show the shifts in the main peaks due to the inclusion of iron and then due to the variation in potassium content). (b): De-convoluted Raman spectra of potassium silicate glasses with and without iron ions.
3. Results and discussions
tetrahedra. However, for the maximum potassium content (20%) in this iron containing samples there is an abrupt decrease in the scattering due to Q2 with substantial increase in the feature associated with Q3 and Q4 units (Fig. 1(b)). This suggests that the large potassium content in these iron silicates enhances the network connectivity of the silica ions. The Q2 feature at ~980 cm−1 which is attributed to the symmetric vibrations of the two-corner shared iron and silica tetrahedra [21] is seen to occur at the same wavenumber for both the 12% and 16% Na containing samples. Also, it may be noted that the feature attributed to only iron (~650 cm−1–710 cm−1) is severely diminished in the latter
Fig. 1(a) shows the measured and the normalized room temperature Raman spectra (300 cm−1 to 1200 cm−1) for the potassium silicate glasses with and without iron. The spectra clearly show changes not only after the introduction of iron, but also with variation of potassium content in the iron containing samples. The absorption maxima (~1049 cm−1) for samples without iron is due to the Q3 units of the silica tetrahedra. With introduction of iron the maxima shift to a lower wavenumber (~960 cm −1) associated with Q2 units of the silica
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samples and also for a drop in Q2 (known for Fe3+O4-SiO4 corner connected tetrahedra) in the Raman spectra. The results from EXAFS and Mössbauer spectroscopy suggests that the majority of the glass samples have iron in its tetrahedral form although the presence of octahedral sites cannot be ruled out in the present systems. Finally, the major contribution to the spectra of these glasses are from Fe3+ and hence the glass samples are highly oxidized in nature.
Table 3 Peak (band) positions and relative fractional areas ( ± 0.5%) of the de-convoluted Raman spectra of potassium silicate glasses with and without iron. Peak
Q0
Q1
Q2
Q3
Q4
K1
Position Area
730 0.0438
880 0.0219
930 0.0219
1049 0.8759
1149 0.0366
K2
Position Area
769 0.0197
910 0.0329
965 0.6842
1054 0.1185
1114 0.1447
K3
Position Area
767 0.0347
910 0.1458
960 0.5347
1042 0.1667
1102 0.1181
K4
Position Area
850 0.0209
893 0.0069
999 0.1189
1046 0.4616
1132 0.3917
3.1. Qn linkages in the alkali iron silicate glasses Table 3 shows the positions (in wavenumbers) and areas of the main features of the spectra of Fig. 1(b). The relative intensities of the constituent Qn features of Fig. 1(b) (K1 to K4) were calculated by finding the relative fractional area pertaining to each Qn on a normalized scale. These values are represented in the bar graphs of Fig. 2 (a). In the latter, the relative heights of the Q0 to Q4 are depicted for each K1 to K4 and the collated data displayed as shown. It may be observed that for the highest percentage of potassium i.e. K4 which also contains iron, there is an abrupt and substantial increase in Q3 and Q4 components as compared to the values of these two Qn components in the K1, K2 and K3 glasses. Thus, the effect of adding 20 mol% K2O to the iron silicate network is to promote strong inter-tetrahedral corner sharing and a continuous random network in which the Fe ion has both network forming (the Fe3+ ions) and modifying roles (Fe2+ ions). For the K2 and K3 glasses, the Q2 linkages are the dominant ones, while for the K1 (no iron content) it is the Q3 linkages that are important. Attempts by the present authors to characterize the lithium and sodium iron silicate glasses have been reported earlier [5–6]. Fig. 2(b) shows a comparative summary of the main Qn linkages for the lithium, sodium and potassium alkali silicate glasses with and without iron. It may be noted that when no iron is included, the lithium glass has a maximum at Q2 while the other two show a maximum at Q3 indicative of a more open network for the lithium glass. For the glasses with 8 mol% iron, the following observations may be made: In these three glass systems, iron has the effect of lowering the Qn when it is added to the alkali silicate. However, at higher alkali content the Qn increases for the lithium and potassium glasses. In the case of the sodium silicate glass, the Qn changes from Q3 and remains at Q2 for all the concentrations of sodium. The variation of Qn with alkali content in these iron silicates is likely to be related to the relative sizes of each alkali ion type. The smaller lithium ion may be accommodated/bonded to the network such that on average, there is only one bridging oxygen site (i.e. Q [1]) at 16 mol% lithium. For the other two iron alkali silicates at 16 mol% and 20 mol% of the alkali, the two and three bridging oxygen sites may be more energetically favoured due to the bonding/accommodation to the network of these larger radii ions. In summary, for both the lithium and sodium iron-containing glasses the maximum linkages are those of the Q2 while for the potassium iron silicate, Q2 linkages are important at lower alkali concentrations while for the higher alkali content of 20 mol%, the Q3 and Q4 units predominate. This is also supported by the glass transition
two glasses. A summary of the effects of sodium and lithium content on the features pertaining to different linkages is discussed separately in Section 3.2 below. The role of iron can further be understood in more detail by using the data obtained from specific techniques such as EXAFS and Mössbauer spectroscopy. While EXAFS is sensitive to small changes in the 1st and even the 2nd neighbouring shells of the iron atoms, it is worth mentioning that it is less sensitive to the wide variation of the inter-atomic distances of disordered structures [34]. In oxide glasses like the one under study every iron ion will have varying co-ordination number and bond lengths. The EXAFS spectra are shown in Fig. 3(a), with an inset of pre-edge appearing due to high excitation energy of FeeK edge (7112 eV). The χ v/s r plots extracted from the Fourier transformed data of the K – space (shown in the inset) assuming the values of the FeeO shells is shown in Fig. 3 (b/c/d). The final results of the fits are tabulated in Table 4. The bond lengths are found to be ~ 1.85 Å which is lower than the expected bond length of 1.86 Å to 1.90 Å. This suggests that the oxygen ions are in much closer neighbourhood of iron. This could be due to the potassium which seen to promote the network forming ability of iron ions. The low bond length and co-ordination number show that the iron favours the tetrahedral array of oxygen's in the present glass systems. The Mössbauer spectra shown in Fig. 4 are as expected for iron in a silicate glass matrix. The doublets in the fitted hyperfine parameters confirms the presence of Fe3+ along with Fe2+. The parameters obtained from the fit are tabulated in Table 5. The isomer shift (IS) values of both Fe3+ and Fe2+ are characteristic of the four co-ordinated iron ions, indicating that the iron ions are mostly in tetrahedral sites [35]. Furthermore, the IS values of Fe3+ and Fe2+ (~ 0.24 mm/s and ~0.87 mm/s respectively) are consistent even with the variation of potassium content indicating that the iron sites remain stable even with the variation of potassium content. The Quadrupole Splitting (QS) values are seen to be increasing with increase in potassium content, which suggests that there are slight distortions in the tetrahedral structure with increase in potassium content. This could also be the reason for low FeeO bond lengths in the present Table 4 Best fit parameter values of the EXAFS analysis of the glass samples with iron.
K2 K3 K4
N
Relative N
S02
σ2
e0
delr
Reff
R ( ± 0.003)
4.000 4.000 4.000
3.995 4.320 4.437
0.749 0.810 0.832
0.00782 0.00435 0.00587
−1.76 1.84 −2.61
−0.001 −0.008 −0.004
1.860 1.860 1.860
1.859 1.852 1.856
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Table 5 Mössbauer spectroscopic results of potassium silicate glasses with iron. Samples
3+
K2 K3 K4
Fe3+
FWHM(mm/s) 2+
Fe
Fe
0.62 ± 0.02 0.55 ± 0.01 0.54 ± 0.02
0.62 ± 0.02 0.55 ± 0.02 0.54 ± 0.02
Fe2+
Area%
IS (mm/s)
QS (mm/s)
IS (mm/s)
QS (mm/s)
Fe3+
Fe2+
0.26 ± 0.01 0.24 ± 0.01 0.23 ± 0.01
0.67 ± 0.02 0.72 ± 0.01 0.76 ± 0.01
0.87 ± 0.03 0.87 ± 0.03 0.88 ± 0.05
2.09 ± 0.05 2.24 ± 0.06 2.32 ± 0.09
67.8 85.6 88.3
32.2 14.3 11.7
Fig. 2. (a): Fractional Qn area as obtained from Table 3. (b): Comparisons of corner connectives of silica tetrahedra in: (i) alkali silicate glasses without iron; (ii),(iii), (iv) alkali silicate glasses with and without iron.
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Journal of Non-Crystalline Solids 518 (2019) 85–91
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Fig. 3. (a) Experimental EXAFS spectra of the glass samples measured at the Fe K-edge (inset shows the pre-edge of iron). Fig. 3 (b,c,d): Experimental χ(r) versus r plots of the glass samples at the Fe K edge extracted using the Fourier Transform of k (insets).
the Mössbauer spectra give evidence for the four-connected Fe2+ and Fe3+ ions and their relative ratios, while the quadrupole splitting shows distortion of the four-connected Fe ions with increasing potassium ion concentration with possibility of octahedral coordination of the Fe as well. The lithium and sodium containing iron silicate glasses have been studied earlier by the present authors and together with the study reported here on potassium iron silicate glass, have led to the following overall view of the role of the alkali as a function of its concentration in the glass: The lithium iron silicates have both Q1 and Q2 as the dominant linkages in this range of lithium doping. The sodium iron silicate glasses on the other hand display a dominance of Q2 for the iron-containing samples and Q3 for the sodium glass without iron. In the case of the potassium containing glasses, Q2 linkages are the most common for the lower potassium containing samples while Q3 and Q4 are the main connectivities for the higher percentages of potassium glass. The finding that the sodium iron silicate glass has a more open structure, may be of use in its application as a solid alkali-based electrolyte. Likewise, for the lithium and potassium iron silicate glasses more open structures result when the alkali content is lowered to 16 mol%.
temperature of each sample – as measured by differential thermal analyses – appeared to be higher for the well-connected structure (higher Qn) and lower for the more open structure (lower Qn). 4. Conclusions Silicate glasses having a fixed 8 M % of Fe and varying potassium molar contents from 12% to 20% have been studied by Raman scattering, EXAFS at the Fe edge and by Mössbauer spectroscopy. Raman spectra were analysed to show that for the 12% and 16% potassium containing samples, the dominant linkage of the silica tetrahedra was the Q2 while for the 20 mol% glass with iron, the distribution shifted to Q3 and Q4 indicating – for this glass – substantial growth of a well-connected tetrahedral network (46% Q3 and 39% Q4). The role of the iron ion in these glasses was understood by the EXAFS and Mössbauer spectra. The EXAFS data beyond the Fe edge unambiguously showed a tetrahedral coordination of Fe to oxygen with the FeeO bond length of about 1.85 Å. The participation of Fe in the network connectivity is thus strongly supported by these data. Some FeO4 tetrahedra are thus likely to link to SiO4 units to form the continuous random network of this silicate glass. The isomeric shift data of
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Fig. 4. Mössbauer spectra of the potassium silicate glasses with iron. Experimental data are represented by black dots. The best fit to the data are represented by solid lines with D1 for Fe3+ and D2 for Fe2+ components.
Acknowledgments The authors gratefully acknowledge funding for this work by the UGC-DAE CSR Project CRS-M-209. Thanks are also due to Materials Research Centre (MNIT, Jaipur) for collecting the Raman spectroscopic data and to BITS Pilani-Goa for help with the EDX data. Manjunath T. Nayak gratefully acknowledges support of a UGC BSR Fellowship as a Senior Research Fellow. References [1] K.J. Rao, Structural Chemistry of Glasses, Elsevier Science, 2002, pp. 23–35. [2] J.E. Shelby, Introduction to glass science and technology, R. Soc. Chem. (2005) 81–82.
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Structural studies of potassium silicate glasses with fixed iron content and their relation to similar alkali silicates
T
Manjunath T. Nayaka, J.A. Erwin Desaa, , V. Raghvendra Reddyb, C. Nayakc, D. Bhattacharyyac, S.N. Jhac ⁎
a
Department of Physics, Goa University, Taleigao Plateau, Goa 403206, India UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India c Atomic and Molecular Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India b
ARTICLE INFO
ABSTRACT
Keywords: Potassium Silicate Glass Fe3O4 Structure
Silicate glasses containing potassium oxide of molar percentages 12%, 16% and 20% each with 8% iron oxide have been prepared and examined by Raman, Extended X-ray absorption fine structure (EXAFS) and Mössbauer spectroscopy. For the 12% and 16% of iron potassium silicate glasses, analysis of the Raman data indicated a maximum of Q2 linkages of the silica tetrahedra while for the 20% iron potassium silicate sample, the Q3 and Q4 linkages were dominant. EXAFS data found the Fe ion to be in tetrahedral coordination with oxygen and a FeeO distance of ~1.85 Å. The isomeric shifts in the Mössbauer spectra found the Fe3+ to Fe2+ ratios to vary with potassium content from 2:1 for the 12% to 8:1 for the 20% potassium glass. Spectral areas showed iron ions to be tetrahedrally coordinated to oxygen with quadrupole splitting showing distortion of these units. From the distribution of Qn linkages it is found that at 16 mol% of the alkali, the lithium iron silicates have more open structures than the sodium and potassium iron silicate glasses. The more open structure of sodium silicate glass when iron is added could have important implications for its use as an electrolytic material.
1. Introduction The addition of alkali oxides to pure silica are known to improve the range of applications of this versatile tetrahedral glassy network [1–4]. In addition to the alkali elements, if a transition metal such as iron is a constituent of the glass, the resulting system is one in which the alkali and iron affect the structures and physical properties in different ways. The present authors have earlier reported the structural effects of lithium [5] and sodium [6] additions to iron containing silicate glasses. We consider here an extension of the latter studies by incorporation of potassium in place of the lithium or sodium ions. Potassium oxide is used in the production of colored glazes and for improving clarity and intensity of colored glasses. Viscosity of the melt is also lowered with the addition of potassium [7]. The glass transition temperature range is increased when potassium is added to the glass. Similarly, some interesting magnetic properties result when potassium is added to the iron silicate glass - as reported here. Study of iron in glass is complex due to the mixed redox states of Fe and the distortions at these sites. Each of the Fe3+ and Fe2+ sites are known to have regular or distorted 4/5/6 coordinated sites which may segregate to form clusters of Fe rich ions [8] making these systems more ⁎
challenging to study. There have been many studies of iron alkali silicate glasses in which the iron content (as Fe2O3) varies [4,5,9–11]. All the studies give lot of insight into the changes of the iron environment in the glass matrix. However, it remains unclear if these changes are due to the changes in concentration of iron content. Moreover, in most of these works the authors have used Fe2O3 as a starting constituent in order to include only ferric ions (Fe3+). However, in almost all of the glasses prepared under oxidising conditions it is found that some of the Fe3+ gets reduced to Fe2+ in the ratio equivalent to 2:1. It has been reported that the polarisation of the lattice by the unpaired electrons of Fe2+ to Fe3+ causes electrical conduction in such glasses [12–15]. Iron ions may also play the role of network modifier or former depending on its concentration in the glassy matrix [16]. Kurkjian and Sigety [17] studied the role of Fe [II] in silicate glass and have concluded that the iron sites are tetrahedrally occupied, whereas Bishay and Kinawi [18] with similar studies have reported Fe [II] to be in both tetrahedral and octahedral sites. A few authors have used Raman spectroscopy to probe the structural changes occurring in such vitreous systems through the study of their vibrational bands [19–20]. For example, the spectral feature attributed to the Q2 band of anti-
Corresponding author. E-mail address: [email protected] (J.A.E. Desa).
https://doi.org/10.1016/j.jnoncrysol.2019.04.025 Received 14 February 2019; Received in revised form 12 April 2019; Accepted 18 April 2019 Available online 21 May 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.
Journal of Non-Crystalline Solids 518 (2019) 85–91
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composition using Energy-dispersive X-ray spectrometer (EDX). Samples were mounted on a sample holder made of aluminium with the help of a carbon-conductive adhesive tape. Sputter coater (Auto fine coater Leica EM ACE 200) was used for coating the samples with gold‑palladium prior to viewing by a field-emission scanning electron microscope (Quanta FEG 250, FEI, Netherlands) fitted with EDAX (Smart Insight, USA) for analyses. Every sample was examined by point analyses at three different locations on the sample. For each sample, the average of the three data sets is tabulated in Table 2. A Raman spectrometer (AIRIX Corp. Tokyo, Japan) STR 500 confocal micro spectrometer imaging spectroscope with focal length of 500 mm was used for collecting the Raman spectra. The excitation wavelength was achieved by 12.5 mW, 532 nm solid state laser at 50× magnification with integration time for each sample being 50 s. The samples were measured from 100 cm−1 to 3000 cm−1 (three scans for each sample) at room temperature. Fig. 1(a) shows the averaged spectra of the samples. Baseline subtractions as explained by Ph Coloumban [22] were performed on the Raman data to retain only the ‘molecular’ SieO Raman signature (600–1200 cm−1) and eliminate the so-called ‘Boson peak’. The pronounced but broad Raman maxima were deconvoluted by fitting Gaussian peaks (Fig. 1 b) at the positions known to correspond to the different Qn vibrations [21,23–30]. The areas of each of the Gaussian fits were then found and placed on a relative scale of peak areas (Table 3). The redox state and environment of iron were analysed by element specific techniques (iron in this case) such as Mössbauer and EXAFS. EXAFS data were collected at the Fe K-edge (7112 eV) using the scanning EXAFS beam line (BL-09) at the Indus-2 Synchrotron Source, RRCAT, Indore [31–32]. A standard sample in the form of a Fe foil was used in the energy calibration, of which the detailed procedure of energy calibration can be found in G. Bunker [33]. Beam size at the sample was 1 mm × 1 mm with an energy step of 0.5 eV. For the EXAFS, the samples (~ 30 mg) were mixed with cellulose and pressed to form pellets. All the samples were scanned three times in transmission mode at room temperature. The final data presented (Fig. 3 a) are accordingly averaged data from the three scans. Details of the EXAFS technique and measurement used by the present authors have been given elsewhere [6]. A 57Co radioactive source was used in the Mössbauer experiment. The transmission mode of measurement with constant acceleration in the velocity range ± 11 mm/s relative to α-Fe was used. Calibration was with a natural iron absorber at ambient temperature. Data analysis used the NORMOS DIST/SITE software. Ratios of Fe3+ to Fe2+ were obtained through the usual methods of isomer shift, quadrupole splitting and line width analysis.
Table 1 Compositions in mol% of the chemicals in the glass samples (Thomas Baker, Purity ≥99%)-Potassium-silicate‑iron oxide glasses. Sample code
SiO2%
K2O %
Fe3O4%
K1 K2 K3 K4
80 80 76 72
20 12 16 20
0 8 8 8
symmetric coupled modes of Fe3+O4-SiO4 shows the presence and variation of Fe3+ in tetrahedral configuration [21]. In the present work, the glassy system chosen for study has been the one in which the potassium oxide molar content varies from 12% to 20% while Fe3O4 was kept at a fixed molar value of 8%, with SiO2 varying accordingly. The iron percentage was kept at 8 mol% as this was found to be the maximum value for a stable and fully vitreous system at the lowest content of potassium oxide (12 mol%). 2. Experimental 2.1. Materials The chemical compositional formula of the glass samples was; (100x-y)SiO2.xK2O.yFe3O4 with y = 8% in 3 samples and x = 12%,16%, 20% for the others (Table 1). Appropriate molar percentages of the constituents were weighed out for 15 g of the mixture. This was finely ground and homogenised using an agate pestle and mortar. The mixture was held in a platinum crucible and heated in a Carbolite 1600 furnace. Initially the samples were heated to 900C for 1 h for the potassium carbonate (K2CO3) to decompose to potassium oxide (K2O). Further heating to 1400C and maintenance at this temperature for 15 mins completed the heat treatment. The melt was then quenched in air to room temperature within the crucible. The glass samples were obtained in broken fragments during cooling which were then annealed by transferring them to a preheated oven at 350C for two hours before being left to cool slowly overnight in the furnace. X-ray diffraction (with MoKα X-rays) data on all the samples confirmed them to be vitreous by the absence of Bragg reflection peaks on the broad diffraction maxima. 2.2. Instrumentation and data analysis All the glass samples were examined for the changes in the Table 2 Comparison of the atomic % of samples before and after syntheses. K1 Atoms
Si
K
Fe
O
Initial Comp. Final Comp. (EDX)
0.267 0.219 ± 0.035
0.133 0.081 ± 0.028
0.000 0.000
0.600 0.700 ± 0.090
K2 Atoms Initial Comp. Final Comp. (EDX)
Si 0.241 0.283 ± 0.044
K 0.072 0.086 ± 0.038
Fe 0.073 0.086 ± 0.034
O 0.614 0.545 ± 0.095
Si 0.229 0.191 ± 0.043
K 0.096 0.067 ± 0.033
Fe 0.073 0.051 ± 0.035
O 0.602 0.689 ± 0.087
Si 0.217 0.251 ± 0.047
K 0.120 0.154 ± 0.030
Fe 0.073 0.107 ± 0.033
O 0.590 0.489 ± 0.103
K3 Atoms Initial Comp. Final Comp. (EDX) K4 Atoms Initial Comp. Final Comp. (EDX)
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Fig. 1. (a): Raman spectra of potassium silicate glass with and without iron (the blue bar and yellow bar show the shifts in the main peaks due to the inclusion of iron and then due to the variation in potassium content). (b): De-convoluted Raman spectra of potassium silicate glasses with and without iron ions.
3. Results and discussions
tetrahedra. However, for the maximum potassium content (20%) in this iron containing samples there is an abrupt decrease in the scattering due to Q2 with substantial increase in the feature associated with Q3 and Q4 units (Fig. 1(b)). This suggests that the large potassium content in these iron silicates enhances the network connectivity of the silica ions. The Q2 feature at ~980 cm−1 which is attributed to the symmetric vibrations of the two-corner shared iron and silica tetrahedra [21] is seen to occur at the same wavenumber for both the 12% and 16% Na containing samples. Also, it may be noted that the feature attributed to only iron (~650 cm−1–710 cm−1) is severely diminished in the latter
Fig. 1(a) shows the measured and the normalized room temperature Raman spectra (300 cm−1 to 1200 cm−1) for the potassium silicate glasses with and without iron. The spectra clearly show changes not only after the introduction of iron, but also with variation of potassium content in the iron containing samples. The absorption maxima (~1049 cm−1) for samples without iron is due to the Q3 units of the silica tetrahedra. With introduction of iron the maxima shift to a lower wavenumber (~960 cm −1) associated with Q2 units of the silica
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samples and also for a drop in Q2 (known for Fe3+O4-SiO4 corner connected tetrahedra) in the Raman spectra. The results from EXAFS and Mössbauer spectroscopy suggests that the majority of the glass samples have iron in its tetrahedral form although the presence of octahedral sites cannot be ruled out in the present systems. Finally, the major contribution to the spectra of these glasses are from Fe3+ and hence the glass samples are highly oxidized in nature.
Table 3 Peak (band) positions and relative fractional areas ( ± 0.5%) of the de-convoluted Raman spectra of potassium silicate glasses with and without iron. Peak
Q0
Q1
Q2
Q3
Q4
K1
Position Area
730 0.0438
880 0.0219
930 0.0219
1049 0.8759
1149 0.0366
K2
Position Area
769 0.0197
910 0.0329
965 0.6842
1054 0.1185
1114 0.1447
K3
Position Area
767 0.0347
910 0.1458
960 0.5347
1042 0.1667
1102 0.1181
K4
Position Area
850 0.0209
893 0.0069
999 0.1189
1046 0.4616
1132 0.3917
3.1. Qn linkages in the alkali iron silicate glasses Table 3 shows the positions (in wavenumbers) and areas of the main features of the spectra of Fig. 1(b). The relative intensities of the constituent Qn features of Fig. 1(b) (K1 to K4) were calculated by finding the relative fractional area pertaining to each Qn on a normalized scale. These values are represented in the bar graphs of Fig. 2 (a). In the latter, the relative heights of the Q0 to Q4 are depicted for each K1 to K4 and the collated data displayed as shown. It may be observed that for the highest percentage of potassium i.e. K4 which also contains iron, there is an abrupt and substantial increase in Q3 and Q4 components as compared to the values of these two Qn components in the K1, K2 and K3 glasses. Thus, the effect of adding 20 mol% K2O to the iron silicate network is to promote strong inter-tetrahedral corner sharing and a continuous random network in which the Fe ion has both network forming (the Fe3+ ions) and modifying roles (Fe2+ ions). For the K2 and K3 glasses, the Q2 linkages are the dominant ones, while for the K1 (no iron content) it is the Q3 linkages that are important. Attempts by the present authors to characterize the lithium and sodium iron silicate glasses have been reported earlier [5–6]. Fig. 2(b) shows a comparative summary of the main Qn linkages for the lithium, sodium and potassium alkali silicate glasses with and without iron. It may be noted that when no iron is included, the lithium glass has a maximum at Q2 while the other two show a maximum at Q3 indicative of a more open network for the lithium glass. For the glasses with 8 mol% iron, the following observations may be made: In these three glass systems, iron has the effect of lowering the Qn when it is added to the alkali silicate. However, at higher alkali content the Qn increases for the lithium and potassium glasses. In the case of the sodium silicate glass, the Qn changes from Q3 and remains at Q2 for all the concentrations of sodium. The variation of Qn with alkali content in these iron silicates is likely to be related to the relative sizes of each alkali ion type. The smaller lithium ion may be accommodated/bonded to the network such that on average, there is only one bridging oxygen site (i.e. Q [1]) at 16 mol% lithium. For the other two iron alkali silicates at 16 mol% and 20 mol% of the alkali, the two and three bridging oxygen sites may be more energetically favoured due to the bonding/accommodation to the network of these larger radii ions. In summary, for both the lithium and sodium iron-containing glasses the maximum linkages are those of the Q2 while for the potassium iron silicate, Q2 linkages are important at lower alkali concentrations while for the higher alkali content of 20 mol%, the Q3 and Q4 units predominate. This is also supported by the glass transition
two glasses. A summary of the effects of sodium and lithium content on the features pertaining to different linkages is discussed separately in Section 3.2 below. The role of iron can further be understood in more detail by using the data obtained from specific techniques such as EXAFS and Mössbauer spectroscopy. While EXAFS is sensitive to small changes in the 1st and even the 2nd neighbouring shells of the iron atoms, it is worth mentioning that it is less sensitive to the wide variation of the inter-atomic distances of disordered structures [34]. In oxide glasses like the one under study every iron ion will have varying co-ordination number and bond lengths. The EXAFS spectra are shown in Fig. 3(a), with an inset of pre-edge appearing due to high excitation energy of FeeK edge (7112 eV). The χ v/s r plots extracted from the Fourier transformed data of the K – space (shown in the inset) assuming the values of the FeeO shells is shown in Fig. 3 (b/c/d). The final results of the fits are tabulated in Table 4. The bond lengths are found to be ~ 1.85 Å which is lower than the expected bond length of 1.86 Å to 1.90 Å. This suggests that the oxygen ions are in much closer neighbourhood of iron. This could be due to the potassium which seen to promote the network forming ability of iron ions. The low bond length and co-ordination number show that the iron favours the tetrahedral array of oxygen's in the present glass systems. The Mössbauer spectra shown in Fig. 4 are as expected for iron in a silicate glass matrix. The doublets in the fitted hyperfine parameters confirms the presence of Fe3+ along with Fe2+. The parameters obtained from the fit are tabulated in Table 5. The isomer shift (IS) values of both Fe3+ and Fe2+ are characteristic of the four co-ordinated iron ions, indicating that the iron ions are mostly in tetrahedral sites [35]. Furthermore, the IS values of Fe3+ and Fe2+ (~ 0.24 mm/s and ~0.87 mm/s respectively) are consistent even with the variation of potassium content indicating that the iron sites remain stable even with the variation of potassium content. The Quadrupole Splitting (QS) values are seen to be increasing with increase in potassium content, which suggests that there are slight distortions in the tetrahedral structure with increase in potassium content. This could also be the reason for low FeeO bond lengths in the present Table 4 Best fit parameter values of the EXAFS analysis of the glass samples with iron.
K2 K3 K4
N
Relative N
S02
σ2
e0
delr
Reff
R ( ± 0.003)
4.000 4.000 4.000
3.995 4.320 4.437
0.749 0.810 0.832
0.00782 0.00435 0.00587
−1.76 1.84 −2.61
−0.001 −0.008 −0.004
1.860 1.860 1.860
1.859 1.852 1.856
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Table 5 Mössbauer spectroscopic results of potassium silicate glasses with iron. Samples
3+
K2 K3 K4
Fe3+
FWHM(mm/s) 2+
Fe
Fe
0.62 ± 0.02 0.55 ± 0.01 0.54 ± 0.02
0.62 ± 0.02 0.55 ± 0.02 0.54 ± 0.02
Fe2+
Area%
IS (mm/s)
QS (mm/s)
IS (mm/s)
QS (mm/s)
Fe3+
Fe2+
0.26 ± 0.01 0.24 ± 0.01 0.23 ± 0.01
0.67 ± 0.02 0.72 ± 0.01 0.76 ± 0.01
0.87 ± 0.03 0.87 ± 0.03 0.88 ± 0.05
2.09 ± 0.05 2.24 ± 0.06 2.32 ± 0.09
67.8 85.6 88.3
32.2 14.3 11.7
Fig. 2. (a): Fractional Qn area as obtained from Table 3. (b): Comparisons of corner connectives of silica tetrahedra in: (i) alkali silicate glasses without iron; (ii),(iii), (iv) alkali silicate glasses with and without iron.
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Fig. 3. (a) Experimental EXAFS spectra of the glass samples measured at the Fe K-edge (inset shows the pre-edge of iron). Fig. 3 (b,c,d): Experimental χ(r) versus r plots of the glass samples at the Fe K edge extracted using the Fourier Transform of k (insets).
the Mössbauer spectra give evidence for the four-connected Fe2+ and Fe3+ ions and their relative ratios, while the quadrupole splitting shows distortion of the four-connected Fe ions with increasing potassium ion concentration with possibility of octahedral coordination of the Fe as well. The lithium and sodium containing iron silicate glasses have been studied earlier by the present authors and together with the study reported here on potassium iron silicate glass, have led to the following overall view of the role of the alkali as a function of its concentration in the glass: The lithium iron silicates have both Q1 and Q2 as the dominant linkages in this range of lithium doping. The sodium iron silicate glasses on the other hand display a dominance of Q2 for the iron-containing samples and Q3 for the sodium glass without iron. In the case of the potassium containing glasses, Q2 linkages are the most common for the lower potassium containing samples while Q3 and Q4 are the main connectivities for the higher percentages of potassium glass. The finding that the sodium iron silicate glass has a more open structure, may be of use in its application as a solid alkali-based electrolyte. Likewise, for the lithium and potassium iron silicate glasses more open structures result when the alkali content is lowered to 16 mol%.
temperature of each sample – as measured by differential thermal analyses – appeared to be higher for the well-connected structure (higher Qn) and lower for the more open structure (lower Qn). 4. Conclusions Silicate glasses having a fixed 8 M % of Fe and varying potassium molar contents from 12% to 20% have been studied by Raman scattering, EXAFS at the Fe edge and by Mössbauer spectroscopy. Raman spectra were analysed to show that for the 12% and 16% potassium containing samples, the dominant linkage of the silica tetrahedra was the Q2 while for the 20 mol% glass with iron, the distribution shifted to Q3 and Q4 indicating – for this glass – substantial growth of a well-connected tetrahedral network (46% Q3 and 39% Q4). The role of the iron ion in these glasses was understood by the EXAFS and Mössbauer spectra. The EXAFS data beyond the Fe edge unambiguously showed a tetrahedral coordination of Fe to oxygen with the FeeO bond length of about 1.85 Å. The participation of Fe in the network connectivity is thus strongly supported by these data. Some FeO4 tetrahedra are thus likely to link to SiO4 units to form the continuous random network of this silicate glass. The isomeric shift data of
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Fig. 4. Mössbauer spectra of the potassium silicate glasses with iron. Experimental data are represented by black dots. The best fit to the data are represented by solid lines with D1 for Fe3+ and D2 for Fe2+ components.
Acknowledgments The authors gratefully acknowledge funding for this work by the UGC-DAE CSR Project CRS-M-209. Thanks are also due to Materials Research Centre (MNIT, Jaipur) for collecting the Raman spectroscopic data and to BITS Pilani-Goa for help with the EDX data. Manjunath T. Nayak gratefully acknowledges support of a UGC BSR Fellowship as a Senior Research Fellow. References [1] K.J. Rao, Structural Chemistry of Glasses, Elsevier Science, 2002, pp. 23–35. [2] J.E. Shelby, Introduction to glass science and technology, R. Soc. Chem. (2005) 81–82.
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