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Iron-bearing silicate glasses at ambient conditions
Journal of Non-Crystalline Solids 275 (2000) 175±188
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Iron-bearing silicate glasses at ambient conditions Dorothee J.M. Burkhard * Scienti®c Centre for Materials Research and Institute for Mineralogy, University of Marburg, Hans Meerweinstrasse, D-35032 Marburg, Germany Received 27 January 2000; received in revised form 17 March 2000
Abstract The eect of silica and iron content (Fe2 O3 of 2, 5 and 10 mol%) on the change of both M ossbauer spectra and elastic properties, is evaluated for glasses in the systems SFC: SiO2 ±Fe2 O3 ±CaO, SFN: SiO2 ±Fe2 O3 ±Na2 O and SFL: SiO2 ± ossbauer spectroscopy glasses are paramagnetic down to 7 K. Fe2 O3 ±Li2 O. To the degree of resolution obtained by M For a given cation (Ca2 , Na or Li ) spectra of glasses are largely independent of the silica and iron content, suggesting that the local environment of iron is not aected by the degree of polymerization. However, elastic properties, available for SFL and SFN glasses, signi®cantly depend on composition. Whereas an increase in the network modi®er cation Na lowers shear moduli of SFN, all elastic moduli increase with increasing amount of Li and Fe in SFL glasses. This strong dependence of elastic properties on chemical composition on one hand, and the chemically independent Fe± O bond in SFN and SFL glasses on the other hand, suggests that in the range of compositions considered, single bond properties may not determine physical bulk properties. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Iron as the main transition element and occurring with two valence states is of great importance for understanding the structures of both synthetic and natural glasses. The structural role of iron in relation to properties of silicate glasses, such as elasticity or crystallization behavior, is important for their technological applications and for studies in earth sciences. Numerous studies have demonstrated that this structural role is complex because of site-to-site variations of bond length and coordination in an essentially unknown distribution; nevertheless, it is generally agreed with some
*
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consensus that (1) in silicate glasses with Fe3 /RFe higher than 0.5, Fe3 occurs in four-fold coordination, assumed as network former; (2) for a given cation with a cationic ®eld strength, Z/r2 and at constant redox conditions, the ratio Fe3 /Fe2 is proportional to the degree of depolymerization often expressed as the number of non-bridging oxygens per tetrahedron, NBO/T; (3) the ratio Fe3 /Fe2 decreases with Z/r2 of the network modi®er and decreases with increasing amount of total iron present in the glass [1±6]. Although in principal terms, the model of network modi®er and network former, and subsequent calculations of NBO/T has been related to bulk properties such as viscosity [7,8], physical properties are often investigated mainly with a macroscopic rather than microscopic or atomistic perspective. Here, we attempt to evaluate the relation between microscopic and macroscopic properties
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 2 - 0
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D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
based on 57 Fe M ossbauer spectroscopy and elastic properties [9], as a function of iron, alkali and alkaline earth content, in three systems, SFC: SiO2 ± Fe2 O3 ±CaO, SFN: SiO2 ±Fe2 O3 ±Na2 O and SFL: SiO2 ±Fe2 O3 ±Li2 O. We ®nd that for a given ionic charge density, Z/r2 , spectra are very similar except for intensities and that the average local environment of Fe, in the range of chemical compositions considered, is largely independent of the silica and iron contents. In contrast, elastic properties signi®cantly depend on chemical composition. These results suggest that bulk properties are controlled, at best indirectly, by properties such as the Fe±O bond or the local Fe environment. Bulk properties are more likely to be determined by properties of larger structural units. 2. Experimental Glass series, in part available from earlier studies [9,10], were considered in the systems: SFC, SFN and SFL with compositions given in Table 1. Each series consists of three sub-series with 2, 5 and 10 mol% Fe2 O3 at various CaO, Na2 O and Li2 O contents. In a single case, 15 mol% Fe2 O3 was used in an SFL disilicate composition. Glasses were prepared from reagent grade SiO2 , Fe2 O3 , and CaCO3; Na2 CO3 or Li2 CO3 . SFL and SFN samples were melted at about 1380°C, and most SFC samples at 1650°C for approximately 1 h. After quenching on a metal plate, samples were stored in a desiccator until further analyses. While compositions examined with the electron microprobe are homogeneous within 2% of the nominal composition for SFN and SFL glasses, except for a minor loss of alkalis (Li had to be estimated by dierence), SFC glasses are comparatively inhomogeneous with analytical variations of up to 6%. One reason is apparently that melting temperatures, limited to 1650°C because of the use of Pt±Au (5%) crucibles, were often not much higher than the liquidus temperature, whereas for SFL and SFN glass synthesis, melting was carried out about 100±200°C above the liquidus, enabling better homogenization. A phase separation of SFC glasses is readily seen with back scattered electrons in silica-rich
samples (Fig. 1(a) and (b)), showing SiO2 -rich blobs in a Ca±Fe richer matrix. The proximity to the immiscibility gap is likely to be another reason for this inhomogeneity. Inspection with X-ray diffraction and with transmitted light does not show crystalline fractions, with the exception of some SFL glasses with a high Li2 O content (Table 1). The M ossbauer eect was studied at the 14.4 keV emission of a 57 Co source embedded in Rh, using a triangular voltage, a conventional spectrometer and a multi-channel analyzer with 1024 channels. The optimal absorber thickness calculated after [11] varied for the dierent systems and was highest for SFC glasses where an optimal thickness could be applied only for the most Ferich samples. All spectra, calibrated to a-Fe, were ®rst collected on a large velocity scale, between ca. )12 and +12 mm/s. Since none of the spectra showed any indication of magnetic interaction, spectra were subsequently re-collected on a smaller velocity scale, between )4 and +4 mm/s, to improve resolution. In some cases spectra were taken at 7 K in the large velocity scale, to investigate the possible presence of Fe oxide clusters or nano particles. Spectra were ®tted with three symmetric doublets of Lorentzian shape, one for Fe2 and two for the most abundant Fe3 . Although this approach does not take into account the site-tosite distribution of iron in the glass evident from the characteristic line broadening (e.g., Ref. [12]), discrete line ®tting should serve the main purpose of a comparison of spectra [13]. Ultrasonic velocities of SFL and SFN, available from Ref. [9], were cross-checked in some cases by Brillouin spectroscopy, using co-planar platelets of ca. 50 lm thickness in 90° and in back-re¯ecting geometries. Because both compressional and transversal acoustic waves (Vp and Vs , respectively) depend on the refractive index n, n could be extracted and compared against the conventional usage of refractive index liquids. This cross-check of n, as well as the comparison of Vp and Vs available from ultrasonic measurements, agrees within 1±2%, in spite of dierent sample sizes: platelets of about 50 lm thickness and 1±2 mm in diameter used for Brillouin versus, blocks of several mm thickness and 0.5±1 cm in diameter used for ultrasonic measurements.
a
SFC SFC SFC SFC SFC SFC SFC SFC
SFC SFC SFC SFC SFC SFC SFC SFC
SFC SFC SFC SFC SFC SFC SFC SFC
10 mol%
5 mol%
2 mol%
11 12 13 14 15 16 17 18
1 2 3 5 6 7 8 4
21 22 23 24 25 26 27 28
67.0 64.0 61.0 58.0 55.0 52.0 49.0 46.0
67.0 64.0 61.0 55.0 52.0 49.0 46.0 58.0
67.0 64.0 61.0 58.0 55.0 52.0 49.0 46.0
SiO2 (mol%)
31.0 34.0 37.0 40.0 43.0 46.0 49.0 52.0
28.0 31.0 34.0 40.0 43.0 46.0 49.0 37.0
23.0 26.0 29.0 32.0 35.0 38.0 41.0 44.0
CaO (mol%)
1650 1650 1650 1650 1650 1650 1650 1650
1650 1650 1650 1650 1650 1650 1650 1650
1660 1660 1660 1660 1660 1550 1650 1650
Temp. (°C) (melt. prep.)
SFL SFL SFL SFL SFL
SFL SFL SFL SFL SFL
SFL SFL SFL SFL SFL
SFL
10 11 12 14 15c
0 1 2 4 5c
20 21 22 24c 25c
75.5 71.2 66.0 60.9 55.8
75.0 69.0 64.0 59.0 54.0
71.5 65.3 60.6 55.9 51.2
SiO2 (mol%)
22.5 26.8 32.0 37.1 42.2
20.0 26.0 31.0 36.0 41.0
18.5 24.7 29.4 34.1 38.8
Li2 O (mol%)
1380 1380 1380 1380 1380
1380 1380 1380 1380 1380
1380 1380 1380 1380 1380
Temp. (°C) (melt. prep.)
`Temp.' refers to the temperature of synthesis; c: partial crystallization observed with the optical microscope.
SFC
Fe2 O3
Table 1 Composition of glasses (nominal) in the systems SFC, SFL and SFNa
SFN SFN SFN SFN SFN
SFN SFN SFN SFN SFN
SFN SFN SFN SFN SFN
SFN
10 11 12 14 15
0 1 2 4 5
20 21 22 24 25
75.5 71.2 66.0 60.9 55.8
75.0 69.0 64.0 59.0 54.0
71.5 65.3 60.6 55.9 51.2
SiO2 (mol%)
22.5 26.8 32.0 37.1 42.2
20.0 26.0 31.0 36.0 41.0
18.5 24.7 29.4 34.1 38.8
Na2 O (mol%)
1380 1380 1380 1380 1380
1380 1380 1380 1380 1380
1380 1380 1380 1380 1380
Temp. (°C) (melt. prep.)
D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188 177
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D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
Fig. 1. Back-scatterd electron pictures of SFC 2 glass show a phase separation of SiO2 rich blobs (black) in a Ca±Fe-richer matrix (light-grey), (a) an overview; note the coarser separation to the left, (b) detail.
3. Results M ossbauer spectra demonstrate that SFL, SFN and SFC glasses are paramagnetic, even to temperatures as low as 7 K. The single SFL sample with 15 mol% Fe2 O3 is paramagnetic at least at room temperature. Representative spectra are displayed in Fig. 2. Spectra are characterized by a main doublet with an isomer shift (IS) in the range of Fe3 and a shoulder of Fe2 on the high velocity side. This shoulder is best pronounced for SFC glasses, followed by SFL but it is almost absent for SFN glasses. The two low velocity shoulders are always asymmetric, especially for SFC, and less so for SFL glasses, because of an overlap of the low velocity shoulders of the Fe3 and Fe2 doublets. Asymmetry should also increase with Fe2 because of its asymmetric charge distribution in the d6 shell compared to the symmetric d5 shell of Fe3 . Spectra of glasses with 5 and 10 mol% Fe2 O3 have a relatively ¯at baseline, while for glasses with 2 mol% Fe2 O3; the baseline is bent. According to visual inspection, the envelope of SFL and SFN spectra at 7 K and RT appears to be rather T-independent, whereas signi®cant changes in intensity and baseline are seen for SFC spectra (see SFC 21 and SFC 27, Fig. 2). Within each system, SFL, SFN or SFC, visual inspection
does not show signi®cant variations with changes in composition. Spectra were all ®tted with three doublets; however, there are usually several possibilities; for example, the two Fe3 doublets may be arranged either with similar IS or with similar quadrupole splitting, DEq . An interpretation of glass spectra may be referenced to spectra of crystals with comparable compositions which are summarized from Ref. [14] in Table 2. Typical values for Fe3 in four-fold coordination are for IS and DEq , 0.15± 0.3 and 0.4±1.3 mm/s, respectively, and in six-fold coordination, IS up to 0.5 and DEq around 0.4± 0.75 mm/s [15]. Further constraints, for example for the arrangement of the Fe2 doublets are provided by the need to obtain reasonable values for IS and DEq , that is, between 0.8 and 1 mm/s and around 1 and 2.5 mm/s, respectively [14,15]. SFL glasses: Only a few M ossbauer studies are known for SFL glasses [10,16,17], yet, one of these previous investigations [10] compared IS values of acmite crystals, Li- and Na-acmite, with IS of SFL and SFN glasses. The study considers spectra collected in a broad velocity range that were ®tted with two symmetric doublets for Fe3 and Fe2 . In the crystals LiFe3 Si2 O6 and NaFe3 Si2 O6 , ferric iron is in octahedral coordination with an IS around 0.39 and 0.40 mm/s, respectively. An IS for
D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
179
Fig. 2. Examples of some 57 Fe M ossbauer spectra of glasses considered in this study, SFL 24 SFC 21 and SFC 27 at RT and at 7 K. Crosses mark the original spectrum and the solid thin line the ®t results. SFC 21 and SFC 27 at 7 K are not ®tted; for discussion, see text.
Table 2 Crystalline phases with a chemistry of, or in the systems SFL, SFN and SFC, compiled from [14], and `*' are from [48] Compound
Oxidation state
Coordination
IS (RT)
Fe±X distance
Fe2 SiO4 Fe2 Si2 O6 LiFeSi2 O6 * LiFeO2 Li5 FeO4 (a,b) NaFeSi2 O6 * a NaFeO2 b NaFeO2 Na5 FeO4 Ca3 Fe2 Si3 O12 Ca2 Fe2 O5 CaFe2 O4 CaFeO3
Fe2 Fe2 Fe3 Fe3 Fe3 Fe3 Fe3 Fe3 Fe3 Fe3 Fe3 Fe3 Fe4
Octahedron Octahedron Octahedron Octahedron Tetrahedron Octahedron Octahedron Tetrahedron Tetrahedron Octahedron Octahedron Octahedron Octahedron
1.16 1.17 0.39 0.37 0.13 0.39 0.37 0.18 0.11 0.41 0.38 0.37 0.06
2.09±2.29 2.09±2.20 ) 2.08 ) ) 1.96 1.85±1.9 ) 2.02 1.89±2.15 1.96±2.12 )
Compound known, but no M ossbauer data found: Fe±Li±O: LiFe5 O8 ; Li2 Fe3 O4 ; Li5 Fe5 O8 ; Li2 Fe3 O5 ; SFN: Na2 Fe4 2 Fe2 3 Si6 O20 ; Na5 Fe(SiO3 )4 ; Fe±Ca±O: CaFeO2 ; CaFe3 O5 ; CaFe4 O7 ; Ca2 Fe7 O11 ; Ca2 Fe9 O13 ; Ca3 Fe15 o25 ; Ca2 Fe22 O33 ; Ca4 Fe2 O7 Ca4 Fe9 O17 ; Ca4 Fe14 O25 ; SFC: Ca0:54 Fe1:46 SiO4 ; (ca, Fe)2 Si2 O6 ; CaFeSiO4 .
180
D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
Fe3 above 0.3 mm/s, as found for SFL, compares better with the IS of the acmite than a value much below 0.3 mm/s, found for SFN glasses. Based on this comparison, it was suggested that in SFL glasses, Fe3 occurs in an average coordination that is higher than four-fold, either as four-fold and six-fold coordination, or as a distribution of coordination, which might include ®ve-fold coordinated Fe3 [10]. Based on this, the ®t model applied now considers two doublets for Fe3 with dierent IS, resulting in values typical for four-fold and six-fold coordinated Fe3 . SFN glasses: Previous investigations suggested that in SFN glasses with a compositional range comparable to that studied here, Fe3 is in fourfold coordination. These investigations include optical absorption and luminescence spectroscopy [18], X-ray scattering [19], X-ray, K-edge absorption spectra [20,21], besides M ossbauer spectroscopy [10,22±24] and comparative work between M ossbauer and electron spin resonance [17,25]. Applying two doublets for Fe3 , the ®t converges to two doublets with dierent DEq but similar IS values, both around 0.25±0.29 mm/s, typical for four-fold coordination, in agreement with the literature. SFC glasses: For glasses in the system SFC, both arrangements of Fe3 doublets, similar IS and similar DEq , have been applied in the past [4,26±28]. For glasses with a rather large amount of nominal Fe2 O3 , a model with dierent IS values was given preference, based on a comparison to spectra obtained under reducing conditions [28]. Spectra of glasses with less iron were ®tted with dierent DEq , suggesting tetrahedral coordination only [29,30]. For SFC glasses studied here, both arrangements were applied. They provide either one doublet with IS > 0.4 mm/s, and one with a rather low IS of < 0:2 mm/s which may be unlikely in comparison to crystalline compounds (see Table 2). Alternatively, one obtains two doublets with IS values both around 0.4 mm/s. With reference to [14,15] and data from Table 2 this would suggest six-fold coordination. It could also indicate a transitional state between four- and six-fold coordination, if the coordination change from octahedral to tetrahedral is assumed to occur for Fe3 / P Fe ratio greater than 0.5 mm/s (e.g., Ref. [31])
which is readily the case for the SFC glasses considered here. Examples of ®t results are shown in Fig. 2. A plot of IS and DEq values (Fig. 3) illustrate that within each system SFL, SFN or SFC, these parameters do not change with silica, and not with iron content, if above 5% Fe2 O3 . Note, that one sample (SFC 26) prepared at lower temperatures plots aside. Variations of DEq for Fe3 in SFC glasses with 2% Fe2 O3 which are parallel for both doublets, might be due to some inhomogeneities of the glass. The averaged parameters with standard deviation are given in Table 3. 4. Discussion 4.1. The paramagnetic nature of the glasses M ossbauer spectra demonstrate a paramagnetic nature of all glasses which means that all Fe is dissolved in the glass structure and signi®cant clustering does not occur to the extent of resolution by this technique. Such clustering, depending on the particle size, would result in a hyper®ne magnetic splitting. For example, the presence of ferrites was detected in SFL glasses with iron concentrations in the order of 40 wt% Fe2 O3 [32]. The sensitivity of M ossbauer to particle size and density distribution is a matter of discussion. A particle size in the order of 13.5 nm may be a limit of detection [33]. Hence, the presence of small Fe2 O3 centers in the glasses is well possible, although not detectable because any indication of magnetic interactions in the 7 K spectrum would be hidden by noise. For all glasses with 2 mol% nominal Fe2 O3 , but also for silica rich SFC glasses with 10 mol% Fe2 O3 , (e.g., SFC 21) the baseline is no longer horizontal. Such phenomena may indicate either spin-spin interaction resulting from super-paramagnetism, or spin-lattice interaction due to relaxation eects, e.g., [34,35]. The latter is typical for diluted systems and well-known to occur if iron-oxide concentrations are 1 mol% or less. We therefore feel that spectral features of glasses with 2 mol% may indicate the beginning of spinlattice relaxation. Since spectra with 5 mol% Fe2 O3 all have an almost horizontal base line, it is likely
D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
181
Fig. 3. IS and DEq as a function of CaO for SFC, of Li2 O for SFL and of Na2 O for SFN glasses, each with either 10, 5 and 2 mol% nominal Fe2 O3 .
0.17 0.009
0.15 0.027
0.21 0.037
0.21 0.018
0.19 0.007
0.26 0.010
0.26 0.007
0.29 0.010
0.29 0.008
SFC 2X Average S.D.
SFC X Average S.D.
SFC 1X Average S.D.
SFL 2X Average S.D.
SFL X Average S.D.
SFL 1X Average S.D.
SFN 2X Average S.D.
SFN X Average S.D.
SFN 1X Average S.D.
) )
0.26 0.006
0.25 0.004
0.40 0.012
0.42 0.008
0.38 0.026
0.52 0.065
0.45 0.037
0.48 0.035
0.94 0.032
1.07 0.027
1.01 0.061
0.83 0.067
0.89 0.030
0.95 0.131
0.99 0.011
0.96 0.060
0.93 0.038
0.80 0.043
1.11 0.061
1.12 0.022
0.77 0.033
0.93 0.028
1.01 0.031
1.11 0.051
1.19 0.049
1.19 0.022
DEq (mm/s) Fe2
Fe3 (1)
Fe3 (2)
IS (mm/s)
Fe3 (1)
) )
0.69 0.054
0.71 0.017
0.93 0.010
0.90 0.026
0.98 0.040
1.04 0.049
1.04 0.091
1.00 0.067
Fe3 (2)
2.14 0.042
1.88 0.069
2.12 0.190
2.38 0.044
2.34 0.056
2.33 0.191
2.00 0.022
2.19 0.205
2.19 0.076
Fe2
0.82 0.144
0.41 0.038
0.37 0.029
0.34 0.026
0.25 0.004
0.24 0.014
0.62 0.089
0.47 0.096
0.42 0.013
Fe3 (1)
C (mm/s)
) )
0.48 0.019
0.41 0.005
0.37 0.008
0.26 0.007
0.27 0.016
0.64 0.064
0.64 0.038
0.63 0.053
Fe3 (2)
0.64 0.165
0.45 0.100
0.29 0.154
0.48 0.046
0.27 0.025
0.29 0.075
0.67 0.014
0.67 0.045
0.66 0.034
Fe2
83.80 3.426
30.29 10.910
34.15 4.768
34.76 0.945
44.23 0.830
45.79 0.741
29.17 11.410
18.32 7.149
19.47 3.973
Fe3 (1)
Area
± ±
64.23 9.066
63.02 5.430
40.66 0.997
46.99 0.970
48.45 0.538
25.88 12.681
49.43 6.505
48.00 3.758
Fe3 (2)
16.20 3.426
5.48 2.592
2.85 1.578
24.58 1.937
8.78 1.674
5.77 1.038
44.94 5.209
32.27 3.151
32.55 6.834
Fe2
Table 3 Average and standard deviation (S.D.) of ®t parameters, isomer shift (IS), quadrapole splitting (DEq ) and width at half medium (C) for SFC, SFL and SFN glasses
182 D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
that the 7 K spectra of SFL 27 and speci®cally of SFC 21 are due to some weak super-paramagnetism as a result of higher iron concentrations and the presence nanoparticles smaller than 13 nm. 4.2. Redox state The redox state Fe3 /Fe2 may be estimated from the area ratio under the Fe3 and Fe2 doublets which is independent on the arrangement of doublets, e.g., [36]. A plot of log (Fe3 /Fe2 ) as a function of composition for all three systems (Fig. 4) shows a rather large scatter of data for SFL and SFN glasses, as to be expected for such small amounts of Fe2 and their concomitant poor resolution and large error. However, ferric iron increases with an increasing amount of Ca in SFC glasses in agreement with the literature [39]. The scatter is likely to be caused by inhomogenieties such as zonation (see Fig. 1(b)); Chaskar et al. and McCammon et al. [37,38], investigating cross-sections through iron-rich quenched SFC slags with a M ossbauer spatial resolution of about 500 lm observed a gradient in Fe3 concentration, whereby the portion of Fe3 on the surface increased with increasing CaO of the slag. These authors assigned this phenomenon to surface tension and surface activity of both iron and silica in relation to quench rates. Therefore, when selecting glass aliquots for the present study, care was taken to use comparable glass fractions with respect to their quench rates, that is portions with high quench rates from outer regions. However, in
183
several cases this was not possible if quenching caused complete breakage of the sample. Glass SFC 26, marked with an open triangle, has an exceedingly higher Fe3 content re¯ecting melting temperatures of about 100°C less compared to other SFC glasses. The much higher preparation temperatures of SFC glasses, in comparison to SFL and SFN glasses, may be one reason for redox dierences of these glasses. However, the redox state is also a function of Z/r2 and might therefore be dierent for these glasses also if prepared at the same temperature [1,4,39±42]. Fig. 4 also illustrates, in agreement with the literature [43,44], that the highest percentage of ferrous iron is observed for the smallest total iron content, that is here, for 2 mol% nominal Fe2 O3 . One way to explain this is to relate the change from Fe2 to Fe3 to too negative charges on electron donating O2ÿ [43]; because of the higher basicity of Fe2 an increasing amount of iron increases the basicity and ®nally stabilizes iron as the amphoteric Fe3 . 4.3. Line width and bond properties While spectra of crystals may have a line width C around 0.2 mm/s, the typical line broadening of glass spectra e.g., [45] is evidence for a distribution of the electronic environment of Fe (bond properties) such as bond length and coordination polyhedra, and has been modeled with various approaches e.g., [12,13,46,47]. In principle, the degree of line broadening should give a hint to the
Fig. 4. Log (Fe3 /Fe2 ) as a function of composition (triangle 10, circle 5 and square 2 mol% nominal Fe2 O3 ); despite the scatter (for explanation, see text), all glasses show highest Fe2 with the least nominal Fe2 O3 .
184
D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
degree of distribution: C is smallest for SFL glasses and is for Fe3 doublets around 0.3 mm/s. This is the range known for Li acmite spectra which have a C of 0.28 mm/s [48]. Somewhat broader lines are found for SFN glasses, but by far the largest broadening occurs for SFC glasses with C for Fe3 between 0.43 and 0.63 mm/s and upto 0.7 mm/s for Fe2 . This is approximately twice as large as for SFL glasses suggesting that there the distribution is broadest. Bond properties, such as valence orbital populations are re¯ected in the IS due to changes of the s-electron density at the nucleus. DEq re¯ects the electric ®eld gradient around iron, and decreases for Fe3 with increasing symmetry of the oxygen polyhedra sphere. For iron, because of its negative volume change upon excitation, IS increases with increasing electronegativity (increasing Z/r2 or cationic ®eld potential Z/r) of coordinated ligands, e.g. [49,50]. The expected trends of an increase in IS with Z/r2 may readily be seen from Table 3 showing SFN < SFL < SFC. For SFL, the similarity between the IS for Fe3 in six-fold coordination in both glass and crystal (LiFeSi2 O6 ) is striking (Table 2) and the comparable and narrow line width of the glass spectra suggests that the local environment of Fe3 is relatively well de®ned. However, the IS of four-fold coordinated Fe3 in the glass does not compare well with the small IS of 0.13 for Li5 FeO4 and the presence of such an environment is rather unlikely; in as much as crystal spectra are available (note that underneath Table 2, several compounds are listed of which no M ossbauer data were found), four-fold coordinated Fe3 should be part of the silicate structure in an environment that is not matched by a crystal, or the environment of Fe3 gradually changes between the 6- and four-fold coordination, and a ®t with two doublets is inappropriate. For SFN glasses, none of the known IS of crystals may directly be compared to the IS of glasses. Supported by the broader spectrum, this may suggest a broader distribution of environments and not only a distribution of distortion of polyhedra. For SFC compounds, again, IS of Fe3 in six-fold coordination compares well with that of three dierent crystals. The very broad spectra of SFC glasses suggests a broad distribution of environments.
Hence, in principle, this comparison of glass and crystal might suggest that respective crystal units or comparable environments occur in the glass structure but it may also simply re¯ect the relation between IS and bond length which is related to the chemistry rather than to the structure. As to the relation between chemistry and coordination, it has been pointed out that Fe3 occurs in four-fold coordination if the ratio Fe3 /R (or Fe3 /0.5 R2 ) < 1, whereas Fe3 should become six-fold coordinated if a charge balance of FeOÿ 4 tetrahedron is no longer given [22,29]. This relation is not necessarily evident for SFL glasses where Fe3 occurs in a coordination higher than four-fold, although in terms of stoichiometry, enough Li is available to charge-balance Fe3 . One may relate this to the very small size of the Li ion and its concomitant four-fold coordination. In an alternative view, coordination of Fe3 is controlled by the acidity and basicity of the competing network forming and modifying cations. In SFL glasses, with Li being less basic than Na, Fe3 occurs in four- and six-fold coordination but in SFN glasses with the more basic Na, Fe3 is only four-fold coordinated. This is in line with the observation that with an increasing basicity of glasses, Co2 , for example, changes the coordination from octahedral to tetrahedral, e.g., [51]. Conversely, in Fe2 O3 ±P2 O5 ±Na2 O glasses, with P being more acid than Si, Fe3 was found in tetrahedral and octahedral coordination [15,52]. IS of Fe2 suggests an assignment to six-fold coordination. However, recent studies of reduced Na±Ca silicate glasses (no ferric iron), utilizing Xray absorption (see review [53]) suggest a four-fold to ®ve-fold coordination of Fe2 in glasses and four-fold coordination in their melts, whereby the amount of ®ve-fold in the glass depends on the glass structural relaxation occurring during cooling. In later studies, Fe2 in ®ve-fold to four-fold coordination was suggested only for reduced natural materials, tektites [12], whereas in reduced synthetic SFC glasses a continuous change between distorted tetrahedral to triangular bipyramid type environment is derived from EXAFS and molecular dynamics [54]. To the authorÕs knowledge comparable studies are not yet available for less reduced glasses.
D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
185
4.4. Structural implications; M ossbauer spectroscopy vs elastic moduli The above comparison of M ossbauer spectra of various silicate glasses shows a dependence on the cation R (R Li, Na, Ca), however, within each system SFL, SFN or SFC, spectral variations are rather insigni®cant, except for the redox state effect at low iron content. Similar conclusions were derived from magnetic susceptibility measurements which, if normalized to the basicity, appeared to be independent of the coordination of Fe3 and the degree of polymerization [10]. These observations suggest relatively well de®ned local iron environments, or groups of environments, whereby a change in silica aects the concentration of iron environments but does not aect the iron environment. Comparable interpretations in Ref. [55] on binary silicate glasses suggested the coexistence of amorphous phases, for example in SiO2 ± Li2 O glasses, structural units of SiO2 , and Li2 Si2 O5 sheets, similar, but not identical to that of the disilicate crystal. Much in contrast to the results from M ossbauer spectroscopy, elastic properties show a strong dependence on chemical composition (Fig. 5); in SFL glasses, bulk moduli increase with both Fe and Li, whereby the strengthening eect of Li, known from binary glasses, is enhanced in combination with Fe [9]. In SFN glasses, Na and Fe contribute little on moduli but shear moduli decrease with Na. The more pronounced eect of Li2 O on elastic moduli compared to Na2 O is similar to the eect of Li2 O and Na2 O in di-silicate and meta-silicate melts, as apparent from bulk moduli and Vp data [56]. In binary silicate glasses, in general, alkalis may have a rigorous in¯uence; moduli decrease with decreasing SiO2 content and with decreasing cationic ®eld strength of the network modi®er cation [9,57]. If the network modi®er is bi-valent, such as Ca2 or Mg2 , the glass/melt structure is strengthened [58,59]; also very recent studies con®rm that elastic moduli are enhanced with CaO, making the structure stronger and stier [60]. Iron is assumed to have a strengthening eect [61], but the eect on moduli may depend on whether Fe3 is a network former or modi®er [62]. As discussed above, Fe3 occurs in four-fold to six-fold coor-
Fig. 5. Bulk and shear moduli of SFL and SFN glasses plotted as a function of composition; triangles, circles and squares refer to 10, 5 and 2 mol% nominal Fe2 O3 , respectively, open triangle is a partially crystallized sample; data from [9]. Dashes show moduli of the binary systems (data from [63]), the dashed line is the interpolation to SiO2 .
dination in SFL glasses; if four-fold coordinated Fe3 means network former, it should strengthen the structure, and if six-fold coordinated Fe3 means network modi®er, it should weaken the structure. Hence, for SFL, one should expect a partial net cancellation which is not really evident, questioning the relevance of the terms modi®er and former to some degree. In the discussion of coordination and elastic properties one may compare
186
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Fe3 with Al3 which has the same charge and a similar ionic radius. Fig. 6 shows that bulk and shear moduli of SiO2 ±Al2 O3 ±Li2 O (SAL) and SiO2 ±Al2 O3 ±Na2 O (SAN) glasses increase with Al2 O3 [63]. In SAL glasses, at an Al2 O3 content of 20% some irregularities occur which are probably related to a change in coordination since here Al3 > Li . In this case, Al3 is assumed to become
Fig. 6. Bulk and shear moduli of SAL and SAN glasses (data are from [63]). For SAL: circle, triangle, diamond and square refer to 20, 14, 7±9, and 4±6 mol% nominal Al2 O3 , respectively; for SAN; triangle and square to 15 and 5 mol% Al2 O3 , respectively. SAN glasses are less homogeneous than SAL glasses, comparable to the behavior of SFN versus SFL.
octahedrally coordinated, acting as network modi®er, whereas Al3 is tetrahedrally coordinated if Al < R, a condition which allows charge compensation by the R cation (e.g., discussion in Ref. [64]). Speci®cally for SAL, compositional lines are almost parallel to each other, and hence dierent to the increasing slope observed for SFL glasses (Fig. 5). In agreement with the interpretation of Al3 as network former, the activation energy for viscous ¯ow increases with Al2 O3 , but the activation energy signi®cantly decreases if Al is replaced by Fe [65], suggesting that the structural role of Fe3 is dierent from that of Al3 . Another atomistic parameter that relates to elastic properties is the bond strength which may be calculated as a summation over the bond strength of oxides, using atomization enthalpies [66,67]. Application of this method to SFL and SFN glasses provides a signi®cantly higher bond strength for SFL glasses, in agreement with experimental data [9,68]. However, a direct comparison with experimental data of elastic moduli and of microhardness illustrates that the eective bond strength of SFL glasses is signi®cantly higher than the one calculated [9,68], meaning that the speci®c interaction of compounds is more ecient in the glass than predicted from oxides. Such compounds might be structural units in the glass. So far, all evidence from M ossbauer spectroscopy, elastic properties and bond strength estimates point to the implication that physical properties may not be related in a straight forward sense to single bond properties such as the Fe±O bond. This means that the glass structures are not necessarily adequately described as a random network that may be `modi®ed' or `formed'. It appears more that the interaction of `bond groups' containing the Fe±O bond and forming some kind of structural units should be considered such that bulk properties are the sum of contributions of such units. Such units might have some similarity with crystalline structures, as discussed above. Suggestions in line with these conclusions are known from other systems. For example, sound velocity data and elastic properties of lithium borate glasses, obtained as a function of Li2 O content suggested that bulk elasticity results from the sum of contribution of units, which were speci®ed as
D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
three units, containing Li BObr Oÿ , Li BOÿ br4 , and BObr [69]. Conversely, in phosphorus-containing systems, P2 O5 ±Al2 O3 ±Na2 SiO3 , compressibility increases with P2 O5 , in spite of the incompressibility of P2 O5 tetrahedra [70]. While elastic properties and estimates of bond strength of silicate glasses may be extrapolated to liquids at least for binary systems, e.g., [71], it should be mentioned that even the order may change in multi-component liquids. Thermochemical investigations which relate the bond strength to mixing enthalpies, demonstrate that mixing becomes increasingly exothermic with the basicity of alkali and earth alkaline oxides added to silicate melts [72]. The interaction of alkalies with trivalent ions suggests that the bond strength of M1=n AlO2 in such alumino-silicates is higher for M Na than for M Li, and that in NaX3 O2 , with X either Al or Fe, the strength is higher for NaAl3 O2 than for NaFe3 O2 [73]. These observations correlate with viscosity data, in that the shear viscosity of iron-bearing silicate melts is smaller for Li- than for Na-containing melts, even to the extent that the shear viscosity of LiFeSi3 Ox is anomalously low, compared to many other liquids [74]. Moreover, independent from the presence of iron, lithium containing liquids have a lower viscosity than corresponding sodium containing silicate liquids [74,75]. 5. Conclusions Iron-bearing silicate glasses with Li2 O, Na2 O or CaO are paramagentic at least up to about 10 mol% nominal Fe2 O3 . For a given Z/r2 , the local electron environment of Fe is largely independent of the degree of polymerization and total amount of iron. Hence, an increase in iron increases the number of such equivalent Fe sites or site distributions while the addition of alkalis or alkaline earth oxides increases the degree of depolymerization, but does not aect the iron environment, except for a slight change of the redox ratio. In contrast to the local environment of iron, elastic properties of the glasses are rather sensitive to the glass chemistry of both alkali and iron content. The attempt to correlate results from M ossbauer
187
spectra with bulk properties of elastic moduli demonstrates that such bulk physical properties cannot be related directly to Fe±O bond properties. Hence, one has to infer substructural units and that the sum of their contribution determines bulk properties observed. Acknowledgements These investigations were carried out as part of a fellowship of the Heisenberg Program of the DFG which is herein acknowledged. The author thanks Dirk Sprenger, Schott and University of Mainz for stimulating discussions and for taking some M ossbauer spectra (SFL glasses). Gerti Steinbach helped preparing some of the absorbers. Access to the electron microprobe and assistance by J urgen Koepke at the Mineralogical Institute of the University of Hannover is much acknowledged. References [1] A. Paul, R.W. Douglas, Phys. Chem. Glasses 6 (1965) 207. [2] D. Virgo, B.O. Mysen, F. Seifert, Carnegie Inst. Washington Year Book 80 (1981) 308. [3] D.S. Goldman, J. Am. Ceram. Soc. 66 (1983) 205. [4] B.O. Mysen, D. Virgo, F.A. Seifert, Am. Mineral. 69 (1984) 834. [5] B.O. Mysen, Structure, Properties of Silicate Melts, Elsevier, Amsterdam, 1988. [6] B.O. Mysen, in: L.L. Perchuk, I. Kushiro (Eds.), Physical Chemistry of Magmas, Springer, New York, 1991, p. 41. [7] M.H. Manghnani, H. Sato, C.R. Rai, J. Geophys. Res. 91 (1986) 9333. [8] R.A. Secco, M.H. Manghnani, T.-C. Liu, Geophys. Res. Lett. 18 (1991) 1397. [9] D.J.M. Burkhard, Solid State Commun. 101 (1997) 903. [10] D.J.M. Burkhard, Phys. Chem. Glasses 38 (1997) 317. [11] G.J. Long, T.E. Cranshaw, G. Longworth, M ossbauer Eect Data J. 6 (1983) 42. [12] S. Rossano, E. Balan, G. Morin, J.-R. Bauer, G. Calas, C. Brouder, Phys. Chem. Mineral. 26 (1999) 530. [13] R.A. Dunlap, Hyper®ne Interact. 110 (1997) 217. [14] F. Menil, J. Phys. Chem. Solids 46 (1985) 763. [15] M.D. Dyar, Am. Mineral. 70 (1985) 304. [16] A.A. Belyustin, Y.M. Ostanevich, A.M. Pisarevski, S.B. Tomilov, U. Bai-shi, L. Cher, Sov. Phys. Solids State 7 (1965) 1163. [17] C.R. Kurkjian, E.A. Sigety, Phys. Chem. Glasses 9 (1968) 73. [18] K.E. Fox, T. Furukawa, W.B. White, Phys. Chem. Glasses 23 (1969) 169.
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[19] G.S. Henderson, M.S. Fleet, G.M. Bancroft, J. Non-Cryst. Solids 68 (1984) 333. [20] G. Calas, J. Petieu, Solid State Commun. (1983) 62. [21] G. Calas, J. Petieu, Bull. Mineral. 106 (1983) 33. [22] B.O. Mysen, D. Virgo, C. Scarfe, Am. Mineral. 70 (1980) 487. [23] M.E. Fleet, C.T. Herzberg, G.S. Henderson, E.D. Croizier, M.D. Osborne, C.M. Scarfe, Geochim. Cosmochim. Acta 48 (1984) 1455. [24] D.B. Dingwell, D. Virgo, Geochim. Cosmochim. Acta 51 (1987) 195. [25] N.A. Eissa, W.M. El-Meliegy, S.M. El-Minyawi, H.A. Sallam, Phys. Chem. Glasses 34 (1993) 31. [26] L. Pagamin, C.H.P. Lupis, P.A. Flinn, Metall. Trans. 3 (1972) 2093. [27] M.P. O'Horo, R.A. Cevy, J. Appl. Phys. 49 (1978) 1635. [28] D.B. Dingwell, M. Brearley, Geochim. Cosmochim. Acta 52 (1988) 2815. [29] G.H. Frischat, G. Tomandl, Glastechn. Ber. 42 (1969) 182. [30] G.H. Frischat, G. Tomandl, Glastechn. Ber. 44 (1971) 173. [31] D. Virgo, B.O. Mysen, Phys. Chem. Mineral. 12 (1985) 65. [32] G.R. Mather Jr., in: H.O. Hooper, A.M. de Graaf (Eds.), Amorphous Magnetism, Plenum, New York, 1973, p. 87. [33] W. K undig, H. B ommel, Phys. Rev. 142 (1966) 327. [34] S.C. Bhargave, J.E. Knudsen, S. Morup, J. Phys. Chem. Solids 40 (1979) 45. [35] E. Murad, Phys. Chem. Minerals 23 (1996) 24. Helgason, S. Steinthosson, Hyper®ne Interact. 70 (1992) [36] O. 985. [37] V. Chaskar, G.G. Richards, C.A. McCammon, Metall. Trans. B 24B (1993) 101. [38] C.A. McCammon, V. Chaskar, C.G. Richards, Nucl. Instrum. and Meth. B 76 (1993) 428. [39] A. Paul, J. Non-Cryst. Solids 123 (1990) 354. [40] R.O. Sack, I.S.E. Carmichael, M. Rivers, M.S. Ghiorso, Contrib. Mineral. Petrol. 75 (1980) 369. [41] M.P. Dickenson, P.C. Hess, Contrib. Mineral. Petrol. 78 (1983) 352. [42] I.S.E. Carmichael, M.S. Ghiorso, Earth Planet. Sci. Lett. 78 (1986) 200. [43] J.A. Duy, G.D. Chryssikos, E.I. Kamitsos, Phys. Chem. Glasses 36 (1995) 53. [44] H.D. Schreiber, J. Non-Cryst. Solids 84 (1986) 129. [45] K. Hirao, T. Komatsu, N. Soga, J. Non-Cryst. Solids 40 (1980) 315. [46] R.E. Vandenberghe, E. Grave, P.M.A. Bakker, Hyper®ne Interact. 83 (1994) 29. [47] H.V. Alberto, J.L. Pinto da Cunha, B.O. Mysen, J.M. Gil, N. Ayres-de Campos, J. Non-Cryst. Solids 194 (1996) 48. [48] E. Baum, W. Treutmann, Z. Kristallogr. 183 (1988) 273. [49] G.M. Bancroft, M ossbauer Spectroscopy. Introduction for Inorganic Chemists and Geochemists, Wiley, New York, 1973.
[50] P. G utlich, in: U. Gonser (Ed.), M ossbauer Spectroscopy, Topics in Applied Physics, Springer, Berlin, 1975, p. 53. [51] J.A. Blair, J.A. Duy, Phys. Chem. Glasses 36 (1995) 12. [52] A. Mogus-Milankoviic, D.E. Long, G.K. Marasingh, Phys. Chem. Glasses 37 (1996) 57. [53] G.E. Brown Jr., F. Farges, G. Calas, in: J.F. Stebbins, P.F. McMillan, D.B. Dingwell (Eds.), Structure, Dynamics and Properties of Silicate Melts. Rev. Minerals, vol. 32, 1995, p. 317. [54] S. Rossano et al., J. European Phys. 49 (2000) 597. [55] D. Sprenger, PhD thesis, University of Mainz, 1996. [56] J.D. Bass, in: T.J. Ahrens (Ed.), Mineral Physics and Crystallography, A Handbook of Physical Constants, AGU Reference shelf 2, Washington, 1995, p. 45. [57] L.G. Baikova, Yuk. Fedorov, V.P. Pukh, Sov. J. Glass Phys. Chem. 19 (1993) 380. [58] Y. Vaills, Y. Luspin, G. Hauret, B. Cote, F. Gervais, Solid State Commun. 82 (1992) 221. [59] Y. Vaills, Y. Luspin, G. Hauret, B. Cote, Solid State Commun. 87 (1993) 1097. [60] H. Bornh oft, R. Br uckner, Glastech. Ber. Glass Sci. Technol. 72 (1999) 315. [61] G. Gehlho, M. Thomas, in: G.W. Morey, The Properties of Glass, 2nd Ed., Am. Chem. Soc. Monograph 77, Reinhold, New York, 1954. [62] V.Y. Livshits, M.G. Potalutsyn, R.A. Nakhapeyan, Sov. J. Glass Phys. Chem. 18 (1992) 159. [63] N.P. Bansal, R.H. Doremus, Handbook of Glass Properties, Academic Press, Orlando, FL, 1986. [64] B.O. Mysen, European J. Mineral. 7 (1995) 745. [65] B.O. Mysen, D. Virgo, C. Scarfe, D.J. Cronin, Am. Mineral. 70 (1985) 487. [66] A. Makishima, J.D. Mckenzie, J. Non-Cryst. Solids 12 (1973) 35. [67] M. Yamane, J.D. Mackenzie, J. Non-Cryst. Solids 15 (1974) 153. [68] D.J.M. Burkhard, Phys. Chem. Glasses 39 (1998) 1. [69] M. Kodama, K. Yamakawa, T. Motsushita, Proc. Sym. Glass Sci. Technol. (1993) 303. [70] S. Webb, P. Courtial, Phys. Chem. Minerals 23 (1996) 205. [71] C.T. Herzberg, in: B.O. Mysen (Ed.), Magmatic Processes, Physicochemical Principles, vol. 1, The Geochem. Soc. Spec. Publ. 1986, p. 25. [72] A. Navrotsky, in: C.M. Scarfe Toronto (Ed.), Silicate Melts, Short Course Handbook, vol. 12, Mineralogical Association, Canada, 1986, p. 130. [73] A. Navrotsky, Physics and Chemistry of Earth Materials, Cambridge Topics in Mineral Physics and Chemistry 6, Cambridge University, Cambridge, 1994. [74] D.B. Dingwell, Am. Mineral. 74 (1989) 1038. [75] G. Urbain, Y. Bottinga, P. Richet, Geochim. Cosmochim. Acta 46 (1982) 1061.
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Iron-bearing silicate glasses at ambient conditions Dorothee J.M. Burkhard * Scienti®c Centre for Materials Research and Institute for Mineralogy, University of Marburg, Hans Meerweinstrasse, D-35032 Marburg, Germany Received 27 January 2000; received in revised form 17 March 2000
Abstract The eect of silica and iron content (Fe2 O3 of 2, 5 and 10 mol%) on the change of both M ossbauer spectra and elastic properties, is evaluated for glasses in the systems SFC: SiO2 ±Fe2 O3 ±CaO, SFN: SiO2 ±Fe2 O3 ±Na2 O and SFL: SiO2 ± ossbauer spectroscopy glasses are paramagnetic down to 7 K. Fe2 O3 ±Li2 O. To the degree of resolution obtained by M For a given cation (Ca2 , Na or Li ) spectra of glasses are largely independent of the silica and iron content, suggesting that the local environment of iron is not aected by the degree of polymerization. However, elastic properties, available for SFL and SFN glasses, signi®cantly depend on composition. Whereas an increase in the network modi®er cation Na lowers shear moduli of SFN, all elastic moduli increase with increasing amount of Li and Fe in SFL glasses. This strong dependence of elastic properties on chemical composition on one hand, and the chemically independent Fe± O bond in SFN and SFL glasses on the other hand, suggests that in the range of compositions considered, single bond properties may not determine physical bulk properties. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Iron as the main transition element and occurring with two valence states is of great importance for understanding the structures of both synthetic and natural glasses. The structural role of iron in relation to properties of silicate glasses, such as elasticity or crystallization behavior, is important for their technological applications and for studies in earth sciences. Numerous studies have demonstrated that this structural role is complex because of site-to-site variations of bond length and coordination in an essentially unknown distribution; nevertheless, it is generally agreed with some
*
Tel.: +49-6421 182 7026; fax: +49-6421 282 8919. E-mail address: [email protected] (D.J.M. Burkhard).
consensus that (1) in silicate glasses with Fe3 /RFe higher than 0.5, Fe3 occurs in four-fold coordination, assumed as network former; (2) for a given cation with a cationic ®eld strength, Z/r2 and at constant redox conditions, the ratio Fe3 /Fe2 is proportional to the degree of depolymerization often expressed as the number of non-bridging oxygens per tetrahedron, NBO/T; (3) the ratio Fe3 /Fe2 decreases with Z/r2 of the network modi®er and decreases with increasing amount of total iron present in the glass [1±6]. Although in principal terms, the model of network modi®er and network former, and subsequent calculations of NBO/T has been related to bulk properties such as viscosity [7,8], physical properties are often investigated mainly with a macroscopic rather than microscopic or atomistic perspective. Here, we attempt to evaluate the relation between microscopic and macroscopic properties
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 2 - 0
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based on 57 Fe M ossbauer spectroscopy and elastic properties [9], as a function of iron, alkali and alkaline earth content, in three systems, SFC: SiO2 ± Fe2 O3 ±CaO, SFN: SiO2 ±Fe2 O3 ±Na2 O and SFL: SiO2 ±Fe2 O3 ±Li2 O. We ®nd that for a given ionic charge density, Z/r2 , spectra are very similar except for intensities and that the average local environment of Fe, in the range of chemical compositions considered, is largely independent of the silica and iron contents. In contrast, elastic properties signi®cantly depend on chemical composition. These results suggest that bulk properties are controlled, at best indirectly, by properties such as the Fe±O bond or the local Fe environment. Bulk properties are more likely to be determined by properties of larger structural units. 2. Experimental Glass series, in part available from earlier studies [9,10], were considered in the systems: SFC, SFN and SFL with compositions given in Table 1. Each series consists of three sub-series with 2, 5 and 10 mol% Fe2 O3 at various CaO, Na2 O and Li2 O contents. In a single case, 15 mol% Fe2 O3 was used in an SFL disilicate composition. Glasses were prepared from reagent grade SiO2 , Fe2 O3 , and CaCO3; Na2 CO3 or Li2 CO3 . SFL and SFN samples were melted at about 1380°C, and most SFC samples at 1650°C for approximately 1 h. After quenching on a metal plate, samples were stored in a desiccator until further analyses. While compositions examined with the electron microprobe are homogeneous within 2% of the nominal composition for SFN and SFL glasses, except for a minor loss of alkalis (Li had to be estimated by dierence), SFC glasses are comparatively inhomogeneous with analytical variations of up to 6%. One reason is apparently that melting temperatures, limited to 1650°C because of the use of Pt±Au (5%) crucibles, were often not much higher than the liquidus temperature, whereas for SFL and SFN glass synthesis, melting was carried out about 100±200°C above the liquidus, enabling better homogenization. A phase separation of SFC glasses is readily seen with back scattered electrons in silica-rich
samples (Fig. 1(a) and (b)), showing SiO2 -rich blobs in a Ca±Fe richer matrix. The proximity to the immiscibility gap is likely to be another reason for this inhomogeneity. Inspection with X-ray diffraction and with transmitted light does not show crystalline fractions, with the exception of some SFL glasses with a high Li2 O content (Table 1). The M ossbauer eect was studied at the 14.4 keV emission of a 57 Co source embedded in Rh, using a triangular voltage, a conventional spectrometer and a multi-channel analyzer with 1024 channels. The optimal absorber thickness calculated after [11] varied for the dierent systems and was highest for SFC glasses where an optimal thickness could be applied only for the most Ferich samples. All spectra, calibrated to a-Fe, were ®rst collected on a large velocity scale, between ca. )12 and +12 mm/s. Since none of the spectra showed any indication of magnetic interaction, spectra were subsequently re-collected on a smaller velocity scale, between )4 and +4 mm/s, to improve resolution. In some cases spectra were taken at 7 K in the large velocity scale, to investigate the possible presence of Fe oxide clusters or nano particles. Spectra were ®tted with three symmetric doublets of Lorentzian shape, one for Fe2 and two for the most abundant Fe3 . Although this approach does not take into account the site-tosite distribution of iron in the glass evident from the characteristic line broadening (e.g., Ref. [12]), discrete line ®tting should serve the main purpose of a comparison of spectra [13]. Ultrasonic velocities of SFL and SFN, available from Ref. [9], were cross-checked in some cases by Brillouin spectroscopy, using co-planar platelets of ca. 50 lm thickness in 90° and in back-re¯ecting geometries. Because both compressional and transversal acoustic waves (Vp and Vs , respectively) depend on the refractive index n, n could be extracted and compared against the conventional usage of refractive index liquids. This cross-check of n, as well as the comparison of Vp and Vs available from ultrasonic measurements, agrees within 1±2%, in spite of dierent sample sizes: platelets of about 50 lm thickness and 1±2 mm in diameter used for Brillouin versus, blocks of several mm thickness and 0.5±1 cm in diameter used for ultrasonic measurements.
a
SFC SFC SFC SFC SFC SFC SFC SFC
SFC SFC SFC SFC SFC SFC SFC SFC
SFC SFC SFC SFC SFC SFC SFC SFC
10 mol%
5 mol%
2 mol%
11 12 13 14 15 16 17 18
1 2 3 5 6 7 8 4
21 22 23 24 25 26 27 28
67.0 64.0 61.0 58.0 55.0 52.0 49.0 46.0
67.0 64.0 61.0 55.0 52.0 49.0 46.0 58.0
67.0 64.0 61.0 58.0 55.0 52.0 49.0 46.0
SiO2 (mol%)
31.0 34.0 37.0 40.0 43.0 46.0 49.0 52.0
28.0 31.0 34.0 40.0 43.0 46.0 49.0 37.0
23.0 26.0 29.0 32.0 35.0 38.0 41.0 44.0
CaO (mol%)
1650 1650 1650 1650 1650 1650 1650 1650
1650 1650 1650 1650 1650 1650 1650 1650
1660 1660 1660 1660 1660 1550 1650 1650
Temp. (°C) (melt. prep.)
SFL SFL SFL SFL SFL
SFL SFL SFL SFL SFL
SFL SFL SFL SFL SFL
SFL
10 11 12 14 15c
0 1 2 4 5c
20 21 22 24c 25c
75.5 71.2 66.0 60.9 55.8
75.0 69.0 64.0 59.0 54.0
71.5 65.3 60.6 55.9 51.2
SiO2 (mol%)
22.5 26.8 32.0 37.1 42.2
20.0 26.0 31.0 36.0 41.0
18.5 24.7 29.4 34.1 38.8
Li2 O (mol%)
1380 1380 1380 1380 1380
1380 1380 1380 1380 1380
1380 1380 1380 1380 1380
Temp. (°C) (melt. prep.)
`Temp.' refers to the temperature of synthesis; c: partial crystallization observed with the optical microscope.
SFC
Fe2 O3
Table 1 Composition of glasses (nominal) in the systems SFC, SFL and SFNa
SFN SFN SFN SFN SFN
SFN SFN SFN SFN SFN
SFN SFN SFN SFN SFN
SFN
10 11 12 14 15
0 1 2 4 5
20 21 22 24 25
75.5 71.2 66.0 60.9 55.8
75.0 69.0 64.0 59.0 54.0
71.5 65.3 60.6 55.9 51.2
SiO2 (mol%)
22.5 26.8 32.0 37.1 42.2
20.0 26.0 31.0 36.0 41.0
18.5 24.7 29.4 34.1 38.8
Na2 O (mol%)
1380 1380 1380 1380 1380
1380 1380 1380 1380 1380
1380 1380 1380 1380 1380
Temp. (°C) (melt. prep.)
D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188 177
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D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
Fig. 1. Back-scatterd electron pictures of SFC 2 glass show a phase separation of SiO2 rich blobs (black) in a Ca±Fe-richer matrix (light-grey), (a) an overview; note the coarser separation to the left, (b) detail.
3. Results M ossbauer spectra demonstrate that SFL, SFN and SFC glasses are paramagnetic, even to temperatures as low as 7 K. The single SFL sample with 15 mol% Fe2 O3 is paramagnetic at least at room temperature. Representative spectra are displayed in Fig. 2. Spectra are characterized by a main doublet with an isomer shift (IS) in the range of Fe3 and a shoulder of Fe2 on the high velocity side. This shoulder is best pronounced for SFC glasses, followed by SFL but it is almost absent for SFN glasses. The two low velocity shoulders are always asymmetric, especially for SFC, and less so for SFL glasses, because of an overlap of the low velocity shoulders of the Fe3 and Fe2 doublets. Asymmetry should also increase with Fe2 because of its asymmetric charge distribution in the d6 shell compared to the symmetric d5 shell of Fe3 . Spectra of glasses with 5 and 10 mol% Fe2 O3 have a relatively ¯at baseline, while for glasses with 2 mol% Fe2 O3; the baseline is bent. According to visual inspection, the envelope of SFL and SFN spectra at 7 K and RT appears to be rather T-independent, whereas signi®cant changes in intensity and baseline are seen for SFC spectra (see SFC 21 and SFC 27, Fig. 2). Within each system, SFL, SFN or SFC, visual inspection
does not show signi®cant variations with changes in composition. Spectra were all ®tted with three doublets; however, there are usually several possibilities; for example, the two Fe3 doublets may be arranged either with similar IS or with similar quadrupole splitting, DEq . An interpretation of glass spectra may be referenced to spectra of crystals with comparable compositions which are summarized from Ref. [14] in Table 2. Typical values for Fe3 in four-fold coordination are for IS and DEq , 0.15± 0.3 and 0.4±1.3 mm/s, respectively, and in six-fold coordination, IS up to 0.5 and DEq around 0.4± 0.75 mm/s [15]. Further constraints, for example for the arrangement of the Fe2 doublets are provided by the need to obtain reasonable values for IS and DEq , that is, between 0.8 and 1 mm/s and around 1 and 2.5 mm/s, respectively [14,15]. SFL glasses: Only a few M ossbauer studies are known for SFL glasses [10,16,17], yet, one of these previous investigations [10] compared IS values of acmite crystals, Li- and Na-acmite, with IS of SFL and SFN glasses. The study considers spectra collected in a broad velocity range that were ®tted with two symmetric doublets for Fe3 and Fe2 . In the crystals LiFe3 Si2 O6 and NaFe3 Si2 O6 , ferric iron is in octahedral coordination with an IS around 0.39 and 0.40 mm/s, respectively. An IS for
D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
179
Fig. 2. Examples of some 57 Fe M ossbauer spectra of glasses considered in this study, SFL 24 SFC 21 and SFC 27 at RT and at 7 K. Crosses mark the original spectrum and the solid thin line the ®t results. SFC 21 and SFC 27 at 7 K are not ®tted; for discussion, see text.
Table 2 Crystalline phases with a chemistry of, or in the systems SFL, SFN and SFC, compiled from [14], and `*' are from [48] Compound
Oxidation state
Coordination
IS (RT)
Fe±X distance
Fe2 SiO4 Fe2 Si2 O6 LiFeSi2 O6 * LiFeO2 Li5 FeO4 (a,b) NaFeSi2 O6 * a NaFeO2 b NaFeO2 Na5 FeO4 Ca3 Fe2 Si3 O12 Ca2 Fe2 O5 CaFe2 O4 CaFeO3
Fe2 Fe2 Fe3 Fe3 Fe3 Fe3 Fe3 Fe3 Fe3 Fe3 Fe3 Fe3 Fe4
Octahedron Octahedron Octahedron Octahedron Tetrahedron Octahedron Octahedron Tetrahedron Tetrahedron Octahedron Octahedron Octahedron Octahedron
1.16 1.17 0.39 0.37 0.13 0.39 0.37 0.18 0.11 0.41 0.38 0.37 0.06
2.09±2.29 2.09±2.20 ) 2.08 ) ) 1.96 1.85±1.9 ) 2.02 1.89±2.15 1.96±2.12 )
Compound known, but no M ossbauer data found: Fe±Li±O: LiFe5 O8 ; Li2 Fe3 O4 ; Li5 Fe5 O8 ; Li2 Fe3 O5 ; SFN: Na2 Fe4 2 Fe2 3 Si6 O20 ; Na5 Fe(SiO3 )4 ; Fe±Ca±O: CaFeO2 ; CaFe3 O5 ; CaFe4 O7 ; Ca2 Fe7 O11 ; Ca2 Fe9 O13 ; Ca3 Fe15 o25 ; Ca2 Fe22 O33 ; Ca4 Fe2 O7 Ca4 Fe9 O17 ; Ca4 Fe14 O25 ; SFC: Ca0:54 Fe1:46 SiO4 ; (ca, Fe)2 Si2 O6 ; CaFeSiO4 .
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Fe3 above 0.3 mm/s, as found for SFL, compares better with the IS of the acmite than a value much below 0.3 mm/s, found for SFN glasses. Based on this comparison, it was suggested that in SFL glasses, Fe3 occurs in an average coordination that is higher than four-fold, either as four-fold and six-fold coordination, or as a distribution of coordination, which might include ®ve-fold coordinated Fe3 [10]. Based on this, the ®t model applied now considers two doublets for Fe3 with dierent IS, resulting in values typical for four-fold and six-fold coordinated Fe3 . SFN glasses: Previous investigations suggested that in SFN glasses with a compositional range comparable to that studied here, Fe3 is in fourfold coordination. These investigations include optical absorption and luminescence spectroscopy [18], X-ray scattering [19], X-ray, K-edge absorption spectra [20,21], besides M ossbauer spectroscopy [10,22±24] and comparative work between M ossbauer and electron spin resonance [17,25]. Applying two doublets for Fe3 , the ®t converges to two doublets with dierent DEq but similar IS values, both around 0.25±0.29 mm/s, typical for four-fold coordination, in agreement with the literature. SFC glasses: For glasses in the system SFC, both arrangements of Fe3 doublets, similar IS and similar DEq , have been applied in the past [4,26±28]. For glasses with a rather large amount of nominal Fe2 O3 , a model with dierent IS values was given preference, based on a comparison to spectra obtained under reducing conditions [28]. Spectra of glasses with less iron were ®tted with dierent DEq , suggesting tetrahedral coordination only [29,30]. For SFC glasses studied here, both arrangements were applied. They provide either one doublet with IS > 0.4 mm/s, and one with a rather low IS of < 0:2 mm/s which may be unlikely in comparison to crystalline compounds (see Table 2). Alternatively, one obtains two doublets with IS values both around 0.4 mm/s. With reference to [14,15] and data from Table 2 this would suggest six-fold coordination. It could also indicate a transitional state between four- and six-fold coordination, if the coordination change from octahedral to tetrahedral is assumed to occur for Fe3 / P Fe ratio greater than 0.5 mm/s (e.g., Ref. [31])
which is readily the case for the SFC glasses considered here. Examples of ®t results are shown in Fig. 2. A plot of IS and DEq values (Fig. 3) illustrate that within each system SFL, SFN or SFC, these parameters do not change with silica, and not with iron content, if above 5% Fe2 O3 . Note, that one sample (SFC 26) prepared at lower temperatures plots aside. Variations of DEq for Fe3 in SFC glasses with 2% Fe2 O3 which are parallel for both doublets, might be due to some inhomogeneities of the glass. The averaged parameters with standard deviation are given in Table 3. 4. Discussion 4.1. The paramagnetic nature of the glasses M ossbauer spectra demonstrate a paramagnetic nature of all glasses which means that all Fe is dissolved in the glass structure and signi®cant clustering does not occur to the extent of resolution by this technique. Such clustering, depending on the particle size, would result in a hyper®ne magnetic splitting. For example, the presence of ferrites was detected in SFL glasses with iron concentrations in the order of 40 wt% Fe2 O3 [32]. The sensitivity of M ossbauer to particle size and density distribution is a matter of discussion. A particle size in the order of 13.5 nm may be a limit of detection [33]. Hence, the presence of small Fe2 O3 centers in the glasses is well possible, although not detectable because any indication of magnetic interactions in the 7 K spectrum would be hidden by noise. For all glasses with 2 mol% nominal Fe2 O3 , but also for silica rich SFC glasses with 10 mol% Fe2 O3 , (e.g., SFC 21) the baseline is no longer horizontal. Such phenomena may indicate either spin-spin interaction resulting from super-paramagnetism, or spin-lattice interaction due to relaxation eects, e.g., [34,35]. The latter is typical for diluted systems and well-known to occur if iron-oxide concentrations are 1 mol% or less. We therefore feel that spectral features of glasses with 2 mol% may indicate the beginning of spinlattice relaxation. Since spectra with 5 mol% Fe2 O3 all have an almost horizontal base line, it is likely
D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
181
Fig. 3. IS and DEq as a function of CaO for SFC, of Li2 O for SFL and of Na2 O for SFN glasses, each with either 10, 5 and 2 mol% nominal Fe2 O3 .
0.17 0.009
0.15 0.027
0.21 0.037
0.21 0.018
0.19 0.007
0.26 0.010
0.26 0.007
0.29 0.010
0.29 0.008
SFC 2X Average S.D.
SFC X Average S.D.
SFC 1X Average S.D.
SFL 2X Average S.D.
SFL X Average S.D.
SFL 1X Average S.D.
SFN 2X Average S.D.
SFN X Average S.D.
SFN 1X Average S.D.
) )
0.26 0.006
0.25 0.004
0.40 0.012
0.42 0.008
0.38 0.026
0.52 0.065
0.45 0.037
0.48 0.035
0.94 0.032
1.07 0.027
1.01 0.061
0.83 0.067
0.89 0.030
0.95 0.131
0.99 0.011
0.96 0.060
0.93 0.038
0.80 0.043
1.11 0.061
1.12 0.022
0.77 0.033
0.93 0.028
1.01 0.031
1.11 0.051
1.19 0.049
1.19 0.022
DEq (mm/s) Fe2
Fe3 (1)
Fe3 (2)
IS (mm/s)
Fe3 (1)
) )
0.69 0.054
0.71 0.017
0.93 0.010
0.90 0.026
0.98 0.040
1.04 0.049
1.04 0.091
1.00 0.067
Fe3 (2)
2.14 0.042
1.88 0.069
2.12 0.190
2.38 0.044
2.34 0.056
2.33 0.191
2.00 0.022
2.19 0.205
2.19 0.076
Fe2
0.82 0.144
0.41 0.038
0.37 0.029
0.34 0.026
0.25 0.004
0.24 0.014
0.62 0.089
0.47 0.096
0.42 0.013
Fe3 (1)
C (mm/s)
) )
0.48 0.019
0.41 0.005
0.37 0.008
0.26 0.007
0.27 0.016
0.64 0.064
0.64 0.038
0.63 0.053
Fe3 (2)
0.64 0.165
0.45 0.100
0.29 0.154
0.48 0.046
0.27 0.025
0.29 0.075
0.67 0.014
0.67 0.045
0.66 0.034
Fe2
83.80 3.426
30.29 10.910
34.15 4.768
34.76 0.945
44.23 0.830
45.79 0.741
29.17 11.410
18.32 7.149
19.47 3.973
Fe3 (1)
Area
± ±
64.23 9.066
63.02 5.430
40.66 0.997
46.99 0.970
48.45 0.538
25.88 12.681
49.43 6.505
48.00 3.758
Fe3 (2)
16.20 3.426
5.48 2.592
2.85 1.578
24.58 1.937
8.78 1.674
5.77 1.038
44.94 5.209
32.27 3.151
32.55 6.834
Fe2
Table 3 Average and standard deviation (S.D.) of ®t parameters, isomer shift (IS), quadrapole splitting (DEq ) and width at half medium (C) for SFC, SFL and SFN glasses
182 D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
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that the 7 K spectra of SFL 27 and speci®cally of SFC 21 are due to some weak super-paramagnetism as a result of higher iron concentrations and the presence nanoparticles smaller than 13 nm. 4.2. Redox state The redox state Fe3 /Fe2 may be estimated from the area ratio under the Fe3 and Fe2 doublets which is independent on the arrangement of doublets, e.g., [36]. A plot of log (Fe3 /Fe2 ) as a function of composition for all three systems (Fig. 4) shows a rather large scatter of data for SFL and SFN glasses, as to be expected for such small amounts of Fe2 and their concomitant poor resolution and large error. However, ferric iron increases with an increasing amount of Ca in SFC glasses in agreement with the literature [39]. The scatter is likely to be caused by inhomogenieties such as zonation (see Fig. 1(b)); Chaskar et al. and McCammon et al. [37,38], investigating cross-sections through iron-rich quenched SFC slags with a M ossbauer spatial resolution of about 500 lm observed a gradient in Fe3 concentration, whereby the portion of Fe3 on the surface increased with increasing CaO of the slag. These authors assigned this phenomenon to surface tension and surface activity of both iron and silica in relation to quench rates. Therefore, when selecting glass aliquots for the present study, care was taken to use comparable glass fractions with respect to their quench rates, that is portions with high quench rates from outer regions. However, in
183
several cases this was not possible if quenching caused complete breakage of the sample. Glass SFC 26, marked with an open triangle, has an exceedingly higher Fe3 content re¯ecting melting temperatures of about 100°C less compared to other SFC glasses. The much higher preparation temperatures of SFC glasses, in comparison to SFL and SFN glasses, may be one reason for redox dierences of these glasses. However, the redox state is also a function of Z/r2 and might therefore be dierent for these glasses also if prepared at the same temperature [1,4,39±42]. Fig. 4 also illustrates, in agreement with the literature [43,44], that the highest percentage of ferrous iron is observed for the smallest total iron content, that is here, for 2 mol% nominal Fe2 O3 . One way to explain this is to relate the change from Fe2 to Fe3 to too negative charges on electron donating O2ÿ [43]; because of the higher basicity of Fe2 an increasing amount of iron increases the basicity and ®nally stabilizes iron as the amphoteric Fe3 . 4.3. Line width and bond properties While spectra of crystals may have a line width C around 0.2 mm/s, the typical line broadening of glass spectra e.g., [45] is evidence for a distribution of the electronic environment of Fe (bond properties) such as bond length and coordination polyhedra, and has been modeled with various approaches e.g., [12,13,46,47]. In principle, the degree of line broadening should give a hint to the
Fig. 4. Log (Fe3 /Fe2 ) as a function of composition (triangle 10, circle 5 and square 2 mol% nominal Fe2 O3 ); despite the scatter (for explanation, see text), all glasses show highest Fe2 with the least nominal Fe2 O3 .
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degree of distribution: C is smallest for SFL glasses and is for Fe3 doublets around 0.3 mm/s. This is the range known for Li acmite spectra which have a C of 0.28 mm/s [48]. Somewhat broader lines are found for SFN glasses, but by far the largest broadening occurs for SFC glasses with C for Fe3 between 0.43 and 0.63 mm/s and upto 0.7 mm/s for Fe2 . This is approximately twice as large as for SFL glasses suggesting that there the distribution is broadest. Bond properties, such as valence orbital populations are re¯ected in the IS due to changes of the s-electron density at the nucleus. DEq re¯ects the electric ®eld gradient around iron, and decreases for Fe3 with increasing symmetry of the oxygen polyhedra sphere. For iron, because of its negative volume change upon excitation, IS increases with increasing electronegativity (increasing Z/r2 or cationic ®eld potential Z/r) of coordinated ligands, e.g. [49,50]. The expected trends of an increase in IS with Z/r2 may readily be seen from Table 3 showing SFN < SFL < SFC. For SFL, the similarity between the IS for Fe3 in six-fold coordination in both glass and crystal (LiFeSi2 O6 ) is striking (Table 2) and the comparable and narrow line width of the glass spectra suggests that the local environment of Fe3 is relatively well de®ned. However, the IS of four-fold coordinated Fe3 in the glass does not compare well with the small IS of 0.13 for Li5 FeO4 and the presence of such an environment is rather unlikely; in as much as crystal spectra are available (note that underneath Table 2, several compounds are listed of which no M ossbauer data were found), four-fold coordinated Fe3 should be part of the silicate structure in an environment that is not matched by a crystal, or the environment of Fe3 gradually changes between the 6- and four-fold coordination, and a ®t with two doublets is inappropriate. For SFN glasses, none of the known IS of crystals may directly be compared to the IS of glasses. Supported by the broader spectrum, this may suggest a broader distribution of environments and not only a distribution of distortion of polyhedra. For SFC compounds, again, IS of Fe3 in six-fold coordination compares well with that of three dierent crystals. The very broad spectra of SFC glasses suggests a broad distribution of environments.
Hence, in principle, this comparison of glass and crystal might suggest that respective crystal units or comparable environments occur in the glass structure but it may also simply re¯ect the relation between IS and bond length which is related to the chemistry rather than to the structure. As to the relation between chemistry and coordination, it has been pointed out that Fe3 occurs in four-fold coordination if the ratio Fe3 /R (or Fe3 /0.5 R2 ) < 1, whereas Fe3 should become six-fold coordinated if a charge balance of FeOÿ 4 tetrahedron is no longer given [22,29]. This relation is not necessarily evident for SFL glasses where Fe3 occurs in a coordination higher than four-fold, although in terms of stoichiometry, enough Li is available to charge-balance Fe3 . One may relate this to the very small size of the Li ion and its concomitant four-fold coordination. In an alternative view, coordination of Fe3 is controlled by the acidity and basicity of the competing network forming and modifying cations. In SFL glasses, with Li being less basic than Na, Fe3 occurs in four- and six-fold coordination but in SFN glasses with the more basic Na, Fe3 is only four-fold coordinated. This is in line with the observation that with an increasing basicity of glasses, Co2 , for example, changes the coordination from octahedral to tetrahedral, e.g., [51]. Conversely, in Fe2 O3 ±P2 O5 ±Na2 O glasses, with P being more acid than Si, Fe3 was found in tetrahedral and octahedral coordination [15,52]. IS of Fe2 suggests an assignment to six-fold coordination. However, recent studies of reduced Na±Ca silicate glasses (no ferric iron), utilizing Xray absorption (see review [53]) suggest a four-fold to ®ve-fold coordination of Fe2 in glasses and four-fold coordination in their melts, whereby the amount of ®ve-fold in the glass depends on the glass structural relaxation occurring during cooling. In later studies, Fe2 in ®ve-fold to four-fold coordination was suggested only for reduced natural materials, tektites [12], whereas in reduced synthetic SFC glasses a continuous change between distorted tetrahedral to triangular bipyramid type environment is derived from EXAFS and molecular dynamics [54]. To the authorÕs knowledge comparable studies are not yet available for less reduced glasses.
D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
185
4.4. Structural implications; M ossbauer spectroscopy vs elastic moduli The above comparison of M ossbauer spectra of various silicate glasses shows a dependence on the cation R (R Li, Na, Ca), however, within each system SFL, SFN or SFC, spectral variations are rather insigni®cant, except for the redox state effect at low iron content. Similar conclusions were derived from magnetic susceptibility measurements which, if normalized to the basicity, appeared to be independent of the coordination of Fe3 and the degree of polymerization [10]. These observations suggest relatively well de®ned local iron environments, or groups of environments, whereby a change in silica aects the concentration of iron environments but does not aect the iron environment. Comparable interpretations in Ref. [55] on binary silicate glasses suggested the coexistence of amorphous phases, for example in SiO2 ± Li2 O glasses, structural units of SiO2 , and Li2 Si2 O5 sheets, similar, but not identical to that of the disilicate crystal. Much in contrast to the results from M ossbauer spectroscopy, elastic properties show a strong dependence on chemical composition (Fig. 5); in SFL glasses, bulk moduli increase with both Fe and Li, whereby the strengthening eect of Li, known from binary glasses, is enhanced in combination with Fe [9]. In SFN glasses, Na and Fe contribute little on moduli but shear moduli decrease with Na. The more pronounced eect of Li2 O on elastic moduli compared to Na2 O is similar to the eect of Li2 O and Na2 O in di-silicate and meta-silicate melts, as apparent from bulk moduli and Vp data [56]. In binary silicate glasses, in general, alkalis may have a rigorous in¯uence; moduli decrease with decreasing SiO2 content and with decreasing cationic ®eld strength of the network modi®er cation [9,57]. If the network modi®er is bi-valent, such as Ca2 or Mg2 , the glass/melt structure is strengthened [58,59]; also very recent studies con®rm that elastic moduli are enhanced with CaO, making the structure stronger and stier [60]. Iron is assumed to have a strengthening eect [61], but the eect on moduli may depend on whether Fe3 is a network former or modi®er [62]. As discussed above, Fe3 occurs in four-fold to six-fold coor-
Fig. 5. Bulk and shear moduli of SFL and SFN glasses plotted as a function of composition; triangles, circles and squares refer to 10, 5 and 2 mol% nominal Fe2 O3 , respectively, open triangle is a partially crystallized sample; data from [9]. Dashes show moduli of the binary systems (data from [63]), the dashed line is the interpolation to SiO2 .
dination in SFL glasses; if four-fold coordinated Fe3 means network former, it should strengthen the structure, and if six-fold coordinated Fe3 means network modi®er, it should weaken the structure. Hence, for SFL, one should expect a partial net cancellation which is not really evident, questioning the relevance of the terms modi®er and former to some degree. In the discussion of coordination and elastic properties one may compare
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D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
Fe3 with Al3 which has the same charge and a similar ionic radius. Fig. 6 shows that bulk and shear moduli of SiO2 ±Al2 O3 ±Li2 O (SAL) and SiO2 ±Al2 O3 ±Na2 O (SAN) glasses increase with Al2 O3 [63]. In SAL glasses, at an Al2 O3 content of 20% some irregularities occur which are probably related to a change in coordination since here Al3 > Li . In this case, Al3 is assumed to become
Fig. 6. Bulk and shear moduli of SAL and SAN glasses (data are from [63]). For SAL: circle, triangle, diamond and square refer to 20, 14, 7±9, and 4±6 mol% nominal Al2 O3 , respectively; for SAN; triangle and square to 15 and 5 mol% Al2 O3 , respectively. SAN glasses are less homogeneous than SAL glasses, comparable to the behavior of SFN versus SFL.
octahedrally coordinated, acting as network modi®er, whereas Al3 is tetrahedrally coordinated if Al < R, a condition which allows charge compensation by the R cation (e.g., discussion in Ref. [64]). Speci®cally for SAL, compositional lines are almost parallel to each other, and hence dierent to the increasing slope observed for SFL glasses (Fig. 5). In agreement with the interpretation of Al3 as network former, the activation energy for viscous ¯ow increases with Al2 O3 , but the activation energy signi®cantly decreases if Al is replaced by Fe [65], suggesting that the structural role of Fe3 is dierent from that of Al3 . Another atomistic parameter that relates to elastic properties is the bond strength which may be calculated as a summation over the bond strength of oxides, using atomization enthalpies [66,67]. Application of this method to SFL and SFN glasses provides a signi®cantly higher bond strength for SFL glasses, in agreement with experimental data [9,68]. However, a direct comparison with experimental data of elastic moduli and of microhardness illustrates that the eective bond strength of SFL glasses is signi®cantly higher than the one calculated [9,68], meaning that the speci®c interaction of compounds is more ecient in the glass than predicted from oxides. Such compounds might be structural units in the glass. So far, all evidence from M ossbauer spectroscopy, elastic properties and bond strength estimates point to the implication that physical properties may not be related in a straight forward sense to single bond properties such as the Fe±O bond. This means that the glass structures are not necessarily adequately described as a random network that may be `modi®ed' or `formed'. It appears more that the interaction of `bond groups' containing the Fe±O bond and forming some kind of structural units should be considered such that bulk properties are the sum of contributions of such units. Such units might have some similarity with crystalline structures, as discussed above. Suggestions in line with these conclusions are known from other systems. For example, sound velocity data and elastic properties of lithium borate glasses, obtained as a function of Li2 O content suggested that bulk elasticity results from the sum of contribution of units, which were speci®ed as
D.J.M. Burkhard / Journal of Non-Crystalline Solids 275 (2000) 175±188
three units, containing Li BObr Oÿ , Li BOÿ br4 , and BObr [69]. Conversely, in phosphorus-containing systems, P2 O5 ±Al2 O3 ±Na2 SiO3 , compressibility increases with P2 O5 , in spite of the incompressibility of P2 O5 tetrahedra [70]. While elastic properties and estimates of bond strength of silicate glasses may be extrapolated to liquids at least for binary systems, e.g., [71], it should be mentioned that even the order may change in multi-component liquids. Thermochemical investigations which relate the bond strength to mixing enthalpies, demonstrate that mixing becomes increasingly exothermic with the basicity of alkali and earth alkaline oxides added to silicate melts [72]. The interaction of alkalies with trivalent ions suggests that the bond strength of M1=n AlO2 in such alumino-silicates is higher for M Na than for M Li, and that in NaX3 O2 , with X either Al or Fe, the strength is higher for NaAl3 O2 than for NaFe3 O2 [73]. These observations correlate with viscosity data, in that the shear viscosity of iron-bearing silicate melts is smaller for Li- than for Na-containing melts, even to the extent that the shear viscosity of LiFeSi3 Ox is anomalously low, compared to many other liquids [74]. Moreover, independent from the presence of iron, lithium containing liquids have a lower viscosity than corresponding sodium containing silicate liquids [74,75]. 5. Conclusions Iron-bearing silicate glasses with Li2 O, Na2 O or CaO are paramagentic at least up to about 10 mol% nominal Fe2 O3 . For a given Z/r2 , the local electron environment of Fe is largely independent of the degree of polymerization and total amount of iron. Hence, an increase in iron increases the number of such equivalent Fe sites or site distributions while the addition of alkalis or alkaline earth oxides increases the degree of depolymerization, but does not aect the iron environment, except for a slight change of the redox ratio. In contrast to the local environment of iron, elastic properties of the glasses are rather sensitive to the glass chemistry of both alkali and iron content. The attempt to correlate results from M ossbauer
187
spectra with bulk properties of elastic moduli demonstrates that such bulk physical properties cannot be related directly to Fe±O bond properties. Hence, one has to infer substructural units and that the sum of their contribution determines bulk properties observed. Acknowledgements These investigations were carried out as part of a fellowship of the Heisenberg Program of the DFG which is herein acknowledged. The author thanks Dirk Sprenger, Schott and University of Mainz for stimulating discussions and for taking some M ossbauer spectra (SFL glasses). Gerti Steinbach helped preparing some of the absorbers. Access to the electron microprobe and assistance by J urgen Koepke at the Mineralogical Institute of the University of Hannover is much acknowledged. References [1] A. Paul, R.W. Douglas, Phys. Chem. Glasses 6 (1965) 207. [2] D. Virgo, B.O. Mysen, F. Seifert, Carnegie Inst. Washington Year Book 80 (1981) 308. [3] D.S. Goldman, J. Am. Ceram. Soc. 66 (1983) 205. [4] B.O. Mysen, D. Virgo, F.A. Seifert, Am. Mineral. 69 (1984) 834. [5] B.O. Mysen, Structure, Properties of Silicate Melts, Elsevier, Amsterdam, 1988. [6] B.O. Mysen, in: L.L. Perchuk, I. Kushiro (Eds.), Physical Chemistry of Magmas, Springer, New York, 1991, p. 41. [7] M.H. Manghnani, H. Sato, C.R. Rai, J. Geophys. Res. 91 (1986) 9333. [8] R.A. Secco, M.H. Manghnani, T.-C. Liu, Geophys. Res. Lett. 18 (1991) 1397. [9] D.J.M. Burkhard, Solid State Commun. 101 (1997) 903. [10] D.J.M. Burkhard, Phys. Chem. Glasses 38 (1997) 317. [11] G.J. Long, T.E. Cranshaw, G. Longworth, M ossbauer Eect Data J. 6 (1983) 42. [12] S. Rossano, E. Balan, G. Morin, J.-R. Bauer, G. Calas, C. Brouder, Phys. Chem. Mineral. 26 (1999) 530. [13] R.A. Dunlap, Hyper®ne Interact. 110 (1997) 217. [14] F. Menil, J. Phys. Chem. Solids 46 (1985) 763. [15] M.D. Dyar, Am. Mineral. 70 (1985) 304. [16] A.A. Belyustin, Y.M. Ostanevich, A.M. Pisarevski, S.B. Tomilov, U. Bai-shi, L. Cher, Sov. Phys. Solids State 7 (1965) 1163. [17] C.R. Kurkjian, E.A. Sigety, Phys. Chem. Glasses 9 (1968) 73. [18] K.E. Fox, T. Furukawa, W.B. White, Phys. Chem. Glasses 23 (1969) 169.
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