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A combined experimental and computational approach to (Na2O)1−x · CaO · (ZnO)x · 2SiO2 glasses characterization
Journal of Non-Crystalline Solids 345&346 (2004) 710–714 www.elsevier.com/locate/jnoncrysol
A combined experimental and computational approach to (Na2O)1x Æ CaO Æ (ZnO)x Æ 2SiO2 glasses characterization G. Lusvardi a, G. Malavasi a,*, L. Menabue a, M.C.Menziani a, U. Segre a, M.M. Carnasciali b, A. Ubaldini b a
Department of Chemistry and SCS Center, University of Modena and Reggio Emilia, Via G. Campi 183, 41100 Modena, Italy Department of Chemistry – Industrial Chemistry and INFM, University of Genoa, Via Dodecaneso 31, 16146 Genova, Italy
b
Available online 7 October 2004
Abstract Insight into the Zn structural role in a series of glasses of composition (Na2O)1x Æ CaO Æ (ZnO)x Æ 2SiO2 (x = 0, 0.2, 0.6 and 1) has been obtained by density measurements, analysis of the crystals separated from the glasses, micro-Raman spectra and molecular dynamics simulations. We found that Zn acts as a weak tetrahedral network former independent of the glass Na content. 2004 Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 61.43.Bn; 78.30.j; 61.66.f
1. Introduction The addition of ZnO to alkali silicate glass is known to improve the glass quality by increasing mechanical properties and enhancing chemical durability [1]. Recent results obtained by means of X-ray absorption spectroscopy studies on multi-component glasses [2,3] explain the observed structure-reinforcing effect by invoking a network-forming role for the tetrahedral Zn in these glasses. These findings were confirmed/corroborated by a recently reported experimental and computational study on the quaternary glass system Na2O Æ CaO Æ 2SiO2 Æ xZnO, (where x varied in the range 0.00–0.68; x = 0.68 is the maximum of ZnO addition which still had enabled to obtain a glass) [4]. Zn was found to play the role of weak tetrahedral network former in the full range of Zn concentrations analyzed. An alternative traditional view, based on IR [5] and Raman [6] spectroscopy data and strength measurements, [7] assumes that in sodium zinc silicate glasses zinc *
Corresponding author. Tel.: +39 059 2055041; fax: +39 059 373543. E-mail address: [email protected] (G. Malavasi). 0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.08.153
can exist either in a tetrahedral (network former) and octahedral (network modifier) coordination, depending on the concentration of sodium, the 6-fold coordination being preferred at low alkali concentration. In order to gain deeper insight into the Zn local environment we prepared a series of soda-lime glass compositions of formula (Na2O)1x Æ CaO Æ (ZnO)x Æ 2SiO2 (x = 0, 0.2, 0.6 and 1), where ZnO substitutes for Na2O and the complete replacement of Na2O with ZnO has been obtained. The information derived by density measurements, analysis of the crystals separated from the glasses, micro-Raman spectra and molecular dynamics simulations have been synergistically exploited to understand the evolution of the glass structure as a function of composition. 2. Experimental 2.1. Preparation of glasses The batch compositions ((Na2O)1x Æ CaO Æ(ZnO)x Æ 2SiO2 (x = 0, 0.2, 0.6 and 1)) of the examined glasses prepared as reported in Ref. [4], are listed in Table 1.
G. Lusvardi et al. / Journal of Non-Crystalline Solids 345&346 (2004) 710–714
711
2.2. Glass characterization
3.2. XRD powder diffraction
SEM observation and EDS analysis were done on the as-quenched glasses and after the thermal treatment. Density was determined with a picnometer at room temperature with accuracy of 0.002 g/cm3. Each value is an average of three independent measurements. XRD diffraction analysis was performed on the asquenched glasses and after their thermal treatment (2 h 30 0 or 5 h) at crystallization temperature, TC (660 C for x = 0, 680 C for x = 0.2, 812 C for x = 0.6 and 880 C for x = 1); data collections were the same as reported in Ref. [4]. Micro-Raman spectra were collected on bulks and powders using a Renishaw SYSTEM 2000 (He–Ne laser, 633 nm) in the range 1500–100 cm1, making 25 accumulations of 30 s each one, using the 100% of power with an objective of 50·. Each sample has been analyzed in several different points.
The chemical formula of the unique crystal phase identified upon NC2S crystallization corresponds to the glass composition (Na2O Æ CaO Æ 2SiO2) e.g. Na2CaSi2O6, a cyclosilicate formed by [Si6O18]12 anionic rings, Na+ and Ca2+ cations connected by ionic interactions [4]. After substitution of 5 mol% of ZnO for Na2O the glass crystallization gives rise to a Zn-containing phase Na2Zn2Si2O7, in addition to Na2CaSi2O6. The crystallization of the glass with 15 mol% of ZnO (x = 0.6) leads to identify a 1-D polymeric silicate: CaSiO3 (wollastonite), and two discrete disilicates: Ca2ZnSi2O7 (hardystonite) and Na2Zn2Si2O7, listed according to the decreasing relative intensity of their main signal. The relative intensity order between CaSiO3 and Ca2ZnSi2O7 is reversed after a thermal treatment of 5 h. For x = 1 the glass composition is CaO Æ 2SiO2 Æ ZnO and the crystal phases identified and listed according to the decreasing relative intensity of the main signal are Ca2ZnSi2O7, Zn2SiO4, quartz. The main geometric features of these silicates are reported in Table 2.
2.3. Computational procedure MD simulations were performed with the DL_POLY program [8] using Cerius2 [9] as a graphical interface. The input structures for the glass compositions experimentally characterized in this work were obtained by adding randomly the appropriate number and type of ˚ edgeatoms into the simulation box of around 25 A length (Table 1). The computational protocol and potential parameters used are described in detail in Ref. [4].
3. Results The ZnO maximum concentration in the glass system (Na2O)1x Æ CaO Æ (ZnO)x Æ 2SiO2 (x = 0, 0.2, 0.6 and 1) corresponds to the complete substitution of Na2O (x = 1); the SEM micrograph and EDX analysis of all as-quenched glass reveal a homogenous distribution of the elements in all glasses. 3.1. Density The experimental density of the glasses (Table 1) increases linearly with the zinc concentration. The correlation coefficient (r) is 0.998.
3.3. Raman spectroscopy Raman spectra for the glass compositions studied are shown in Fig. 1. The spectra depend on the composition but bulk or powder samples display similar line shapes. In general the peaks are rather broad and not particularly marked. In the lowest Zn composition, at least four signals are evident at about 1030, 970, 860 and 625 cm1, respectively, that may be easily referred to the modes of the crystallized samples with the same composition. Increasing the Zn content, the peak intensity decreases and they enlarge, leading to a partial overlapping. Moreover, a large bump at 300–500 cm1 appears with the introduction of the zinc and disturbs the region of the spectrum associated with the presence of bridging oxygens (BO) or Si–O–Si linkages [12]. In particular, the high frequency portion of the Raman spectrum of the parent glass (NC2S) shows two partially distinct bands at 974 and 1023 cm1 attributable to Si–NBO (non-bridging oxygens) stretching vibrations in Q2 and Q3 silicon tetrahedral groups, respectively [12]. As Zn replaces Na, the two bands gradually merge.
Table 1 Batch composition (mol%), density and total number of atoms in the simulation box for the glasses studied Glasses
NC2S x = 0 NZ5 x = 0.2 NZ15 x = 0.6 NZ25 x = 1
mol% SiO2
ZnO
50 50 50 50
5 15 25
Na2O
CaO
25 20 10
25 25 25 25
Density (g/cm3) ± 0.002
Total number of atoms in the simulation box
2.712 2.794 2.964 3.137
1188 1166 1113 1080
712
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Table 2 Structural parameters of crystal phases obtained by glass crystallization xa
Crystal phases
NC2S
0
Na2CaSi2O6b
Cyclosilicate, stacked [Si6O18] rings connected by Na and Ca
Si = 4; Ca = 5.0; Na = 6.5
NZ5
0.2
Na2CaSi2O6 Na2Zn2Si2O7c
Polymeric arrangements of [Si2O7] units linked to [ZnO4] units
Si = 4; Na = 4.5; Zn = 4
1-D polymer with [SiO3] units Polymeric arrangements of [Si2O7] units linked to [ZnO4] units
Si = 4; Ca = 6.3 Si = 4; Ca = 8; Zn = 4
3-D polymer of alternate [SiO4] and [ZnO4] tetrahedral repeating along c axis 3-D polymeric [SiO4] units
Si = 4; Zn = 4 Si = 4
Glass
c
NZ15
0.6
CaSiO3 Ca2ZnSi2O7c Na2Zn2Si2O7
NZ25
1
Ca2ZnSi2O7 Zn2SiO4d SiO2-quartz
c
Counts [a.u.]
d
2+
x = ZnO moles. Ref. [4] and references therein. Ref. [10]. Ref. [11].
for Na, the average CN being 1.34 in NC2S, 1.48 in NZ5, 1.72 in NZ15, and 1.93 in NZ25. A parallel way to highlight this phenomenon is shown in Table 3, where the contribution of BO, NBO and three bridging oxygens (TBO) to the coordination of Si and Zn are listed. Na depletion causes a dramatic rearrangement of the Si environment, which becomes mainly constituted by 2-fold oxygens. It is worth noting that Zn manifests a clear preference to be coordinated to BO species in the full range of concentrations studied. Moreover, the percentage of triclusters (TBO) associated with Zn, already significant in NZ15, levels off to 40% in NZ25. The modifications induced by zinc substitution for Na on the network connectivity are summarized in Figs. 2 and 3, where the Qn species distribution and ring size distribution (RSD) are plotted. The number of BO surrounding the network former Si atom (SiQn) increases progressively in the series of glasses studied, the average SiQn reaching a 50% of SiQ4 species in the Na-free glass. On the contrary, although the Q4 site is always preferred by the zinc, the population of the Q3 site reaches a significant percentage in NZ25. The zinc-free glass shows an almost symmetric distribution of the various-size rings constituting the silicate network, with a marked maximum for 12-membered rings. Zinc addition promotes the formation of smaller (formed by 4–5 tetrahedral sites) or larger (8, 10 tetrahedral sites) rings, with a general decrease of the average ring size of one unit.
861
628
NZ25
100 300 500 700
974
b
Mean CN +
NZ15 NZ5 NC2S
1023
a
Structural features
900 1100 1300 1500
Raman Shift [cm-1]
Fig. 1. Raman spectra for the glass compositions studied.
3.4. Molecular dynamics simulations A complete analysis of the local structure and network connectivity of the glasses studied has been carried out. Here we report only on the most striking features. The average coordination number (CN) around silicon is four for the entire series of glasses. The zinc remains four-coordinated at low and intermediate concentrations (NZ5 and NZ15), while a significant percentage of 3-fold Zn appears in the NZ25 glass, yielding an average CN of 3.6. The CN of Na and Ca are compositional dependent: they move from 5.58 to 4.85 and from 5.24 to 4.77, respectively, passing from the zincfree to the sodium-free glass. Finally, a progressive increasing of the oxygen coordination is observed as a function of Zn substitution
Table 3 Percentage contributions (%) of NBO, BO and TBO to the Si and Zn coordination Glasses
NC2S NZ5 NZ15 NZ25
Si
Zn
NBO
BO
TBO
NBO
BO
TBO
51.3 40.3 26.4 14.4
48.7 59.3 69.1 76.7
– 0.4 4.5 8.9
– 5.8 1.6 1.3
– 87.8 72.5 59.7
– 7.4 25.9 39.0
G. Lusvardi et al. / Journal of Non-Crystalline Solids 345&346 (2004) 710–714
713
60
NC2S NZ5 NZ15 NZ25
50
Si
n
Q %
40
30
20
10
Fig. 3. Ring size distribution (RSD) for the glass compositions studied, n is the number of atoms in the rings. 0 0
1
2
3
4
5
6
n 80
NZ5 NZ15 NZ25
Zn
40
n
Q %
60
20
0 0
1
2
3
4
5
6
n Fig. 2. Qn distribution for the Si species and for the Zn species.
This bi-modal distribution evolves towards a leveling of the ring percentage formed by 4–9 Si–Zn sites in the Nafree glass.
4. Discussion The complete substitution of ZnO for Na2O in a soda-lime–silicate glass of starting composition Na2O Æ CaO Æ 2SiO2 has been obtained. The composition change is accompanied by a linear increase of density values, mainly due to the heavier zinc atomic mass and the low zinc coordination number with respect to sodium ones. This trend suggests that the structural role played by the zinc is maintained in the series of glasses studied. In fact, the results obtained by molecular dynamics simulations show a network former role of the zinc ion in the full range of concentrations studied [13,14]. The zinc is found co-polymerized with the silicate network in tetrahedral sites progressively more distorted as Na depletion takes place. The presence of triclusters (TBO) contributes to create disorder in the structures.
TBO species appear in the NZ15 glass and reaches a significant percentage in the Na-free glass. They are mainly associated with zinc and are characteristic of willemite, the second phase separated upon crystallization of the NZ25 glass. The increase in the network polymerization as a function of zinc concentration is further highlighted by the Raman spectra, which detect the structural changing occurring around the silicon site and are usually discussed in terms of Qn species. In particular, the number of structural units containing Si–Q2 (Na) or Si–Q2 (Ca) species diminishes to favour the formation of Si–O–Zn bridges. In fact, an increasing number of various-size rings constituted by Si–Zn tetrahedral sites, detected by the RSD analysis, reduces the amount of the 6-member silicate rings found in NC2S and reminiscent of the Na2CaSi2O6 crystal phase structure. More details can be obtained from the Qn distribution derived by the molecular simulations study, which shows a significant decrease of the Si–Q2 species and a parallel increase of the Si–Q4 species. Interesting is the Zn–Qn behavior: at low and intermediate zinc concentration the Zn–Q4 species is predominant; in the Na-free glass 40% of Zn–Q3 species is formed, as a consequence the average CN diminishes. Distorted sites, where only three oxygen atoms are found in the zinc first atomic shell were reported in an EXAFS study on magnesium aluminosilicate glass containing Zn and Ti [15]. The final structure of the NZ5 and NZ15 glasses obtained by the MD simulations show the presence of silicon-rich regions where Ca ions are preferentially found. Compensation for the positive charge deficiency of the Zn tetrahedrals is preferentially provided by Na ions. In the Na-depleted glass the balancing of the charge of the oxygen atoms linked to zinc is achieved in several way such as competition with Si to attract Ca as charge-compensating cation, formation of triclusters, and, in the absence of Na, distortion of the tetrahedral geometry up to a tri-coordinated pyramidal geometry. This picture of the glass structure is in full agreement with the crystal phases identified after glass crystallization, which shows that the zinc atoms are embedded in
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calcium-rich regions only when there is shortage of Na ions or in the Na-free glass which crystallizes according to the reaction: CaO + ZnO + 2SiO2 = 0.5Ca2ZnSi2O7 + 0.25Zn2SiO4 + 0.75SiO2. References [1] G. Della Mea, A. Gasparotto, M. Bettinelli, A. Montenero, R. Scaglioni, J. Non-Cryst. Solids 81 (1986) 201. [2] D.A. McKeown, I.S. Muller, A.C. Buechele, I.L. Pegg, J. NonCryst. Solids 261 (2000) 155. [3] M. Le Grand, A.Y. Ramos, G. Calas, L. Galoisy, D. Ghaleb, F. Pacaud, J. Mater. Res. 15 (2000) 2015. [4] G. Lusvardi, G. Malavasi, L. Menabue, M.C. Menziani, J. Phys. Chem. B 106 (2002) 9753.
[5] J.C. Hurt, C.J. Phillips, J. Am. Ceram. Soc. 53 (1971) 269. [6] T. Furukawa, W.B. White, J. Non-Cryst. Solids 38&39 (1980) 87. [7] T.L. Pesina, V.A. Zakreuska, O.P. Puken, J. Am. Ceram. Soc. 67 (1984) 47. [8] W. Smith, T.R. Forester, J. Mol. Graph. 14 (1996) 136. [9] Cerius2, version 4.2, Accelrys, San Diego, 2000. [10] Mincryst crystallographic database for minerals. Available from:. [11] K.H. Klaska, J.C. Eck, D. Pohl, Acta Crystallogr. Sect. B 34 (1978) 3324. [12] P.P. McMillan, Am. Mineral. 69 (1984) 622. [13] G. Calas, L. Cormier, L. Galoisy, P. Jolhvet, C.R. Chem. 5 (2002) 831. [14] A.B. Rosenthal, S.H. Garofalini, J. Am. Ceram. Soc. 70 (11) (1987) 821. [15] T. Dumas, J. Petiau, J. Non-Cryst. Solids 81 (1986) 201.
A combined experimental and computational approach to (Na2O)1x Æ CaO Æ (ZnO)x Æ 2SiO2 glasses characterization G. Lusvardi a, G. Malavasi a,*, L. Menabue a, M.C.Menziani a, U. Segre a, M.M. Carnasciali b, A. Ubaldini b a
Department of Chemistry and SCS Center, University of Modena and Reggio Emilia, Via G. Campi 183, 41100 Modena, Italy Department of Chemistry – Industrial Chemistry and INFM, University of Genoa, Via Dodecaneso 31, 16146 Genova, Italy
b
Available online 7 October 2004
Abstract Insight into the Zn structural role in a series of glasses of composition (Na2O)1x Æ CaO Æ (ZnO)x Æ 2SiO2 (x = 0, 0.2, 0.6 and 1) has been obtained by density measurements, analysis of the crystals separated from the glasses, micro-Raman spectra and molecular dynamics simulations. We found that Zn acts as a weak tetrahedral network former independent of the glass Na content. 2004 Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 61.43.Bn; 78.30.j; 61.66.f
1. Introduction The addition of ZnO to alkali silicate glass is known to improve the glass quality by increasing mechanical properties and enhancing chemical durability [1]. Recent results obtained by means of X-ray absorption spectroscopy studies on multi-component glasses [2,3] explain the observed structure-reinforcing effect by invoking a network-forming role for the tetrahedral Zn in these glasses. These findings were confirmed/corroborated by a recently reported experimental and computational study on the quaternary glass system Na2O Æ CaO Æ 2SiO2 Æ xZnO, (where x varied in the range 0.00–0.68; x = 0.68 is the maximum of ZnO addition which still had enabled to obtain a glass) [4]. Zn was found to play the role of weak tetrahedral network former in the full range of Zn concentrations analyzed. An alternative traditional view, based on IR [5] and Raman [6] spectroscopy data and strength measurements, [7] assumes that in sodium zinc silicate glasses zinc *
Corresponding author. Tel.: +39 059 2055041; fax: +39 059 373543. E-mail address: [email protected] (G. Malavasi). 0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.08.153
can exist either in a tetrahedral (network former) and octahedral (network modifier) coordination, depending on the concentration of sodium, the 6-fold coordination being preferred at low alkali concentration. In order to gain deeper insight into the Zn local environment we prepared a series of soda-lime glass compositions of formula (Na2O)1x Æ CaO Æ (ZnO)x Æ 2SiO2 (x = 0, 0.2, 0.6 and 1), where ZnO substitutes for Na2O and the complete replacement of Na2O with ZnO has been obtained. The information derived by density measurements, analysis of the crystals separated from the glasses, micro-Raman spectra and molecular dynamics simulations have been synergistically exploited to understand the evolution of the glass structure as a function of composition. 2. Experimental 2.1. Preparation of glasses The batch compositions ((Na2O)1x Æ CaO Æ(ZnO)x Æ 2SiO2 (x = 0, 0.2, 0.6 and 1)) of the examined glasses prepared as reported in Ref. [4], are listed in Table 1.
G. Lusvardi et al. / Journal of Non-Crystalline Solids 345&346 (2004) 710–714
711
2.2. Glass characterization
3.2. XRD powder diffraction
SEM observation and EDS analysis were done on the as-quenched glasses and after the thermal treatment. Density was determined with a picnometer at room temperature with accuracy of 0.002 g/cm3. Each value is an average of three independent measurements. XRD diffraction analysis was performed on the asquenched glasses and after their thermal treatment (2 h 30 0 or 5 h) at crystallization temperature, TC (660 C for x = 0, 680 C for x = 0.2, 812 C for x = 0.6 and 880 C for x = 1); data collections were the same as reported in Ref. [4]. Micro-Raman spectra were collected on bulks and powders using a Renishaw SYSTEM 2000 (He–Ne laser, 633 nm) in the range 1500–100 cm1, making 25 accumulations of 30 s each one, using the 100% of power with an objective of 50·. Each sample has been analyzed in several different points.
The chemical formula of the unique crystal phase identified upon NC2S crystallization corresponds to the glass composition (Na2O Æ CaO Æ 2SiO2) e.g. Na2CaSi2O6, a cyclosilicate formed by [Si6O18]12 anionic rings, Na+ and Ca2+ cations connected by ionic interactions [4]. After substitution of 5 mol% of ZnO for Na2O the glass crystallization gives rise to a Zn-containing phase Na2Zn2Si2O7, in addition to Na2CaSi2O6. The crystallization of the glass with 15 mol% of ZnO (x = 0.6) leads to identify a 1-D polymeric silicate: CaSiO3 (wollastonite), and two discrete disilicates: Ca2ZnSi2O7 (hardystonite) and Na2Zn2Si2O7, listed according to the decreasing relative intensity of their main signal. The relative intensity order between CaSiO3 and Ca2ZnSi2O7 is reversed after a thermal treatment of 5 h. For x = 1 the glass composition is CaO Æ 2SiO2 Æ ZnO and the crystal phases identified and listed according to the decreasing relative intensity of the main signal are Ca2ZnSi2O7, Zn2SiO4, quartz. The main geometric features of these silicates are reported in Table 2.
2.3. Computational procedure MD simulations were performed with the DL_POLY program [8] using Cerius2 [9] as a graphical interface. The input structures for the glass compositions experimentally characterized in this work were obtained by adding randomly the appropriate number and type of ˚ edgeatoms into the simulation box of around 25 A length (Table 1). The computational protocol and potential parameters used are described in detail in Ref. [4].
3. Results The ZnO maximum concentration in the glass system (Na2O)1x Æ CaO Æ (ZnO)x Æ 2SiO2 (x = 0, 0.2, 0.6 and 1) corresponds to the complete substitution of Na2O (x = 1); the SEM micrograph and EDX analysis of all as-quenched glass reveal a homogenous distribution of the elements in all glasses. 3.1. Density The experimental density of the glasses (Table 1) increases linearly with the zinc concentration. The correlation coefficient (r) is 0.998.
3.3. Raman spectroscopy Raman spectra for the glass compositions studied are shown in Fig. 1. The spectra depend on the composition but bulk or powder samples display similar line shapes. In general the peaks are rather broad and not particularly marked. In the lowest Zn composition, at least four signals are evident at about 1030, 970, 860 and 625 cm1, respectively, that may be easily referred to the modes of the crystallized samples with the same composition. Increasing the Zn content, the peak intensity decreases and they enlarge, leading to a partial overlapping. Moreover, a large bump at 300–500 cm1 appears with the introduction of the zinc and disturbs the region of the spectrum associated with the presence of bridging oxygens (BO) or Si–O–Si linkages [12]. In particular, the high frequency portion of the Raman spectrum of the parent glass (NC2S) shows two partially distinct bands at 974 and 1023 cm1 attributable to Si–NBO (non-bridging oxygens) stretching vibrations in Q2 and Q3 silicon tetrahedral groups, respectively [12]. As Zn replaces Na, the two bands gradually merge.
Table 1 Batch composition (mol%), density and total number of atoms in the simulation box for the glasses studied Glasses
NC2S x = 0 NZ5 x = 0.2 NZ15 x = 0.6 NZ25 x = 1
mol% SiO2
ZnO
50 50 50 50
5 15 25
Na2O
CaO
25 20 10
25 25 25 25
Density (g/cm3) ± 0.002
Total number of atoms in the simulation box
2.712 2.794 2.964 3.137
1188 1166 1113 1080
712
G. Lusvardi et al. / Journal of Non-Crystalline Solids 345&346 (2004) 710–714
Table 2 Structural parameters of crystal phases obtained by glass crystallization xa
Crystal phases
NC2S
0
Na2CaSi2O6b
Cyclosilicate, stacked [Si6O18] rings connected by Na and Ca
Si = 4; Ca = 5.0; Na = 6.5
NZ5
0.2
Na2CaSi2O6 Na2Zn2Si2O7c
Polymeric arrangements of [Si2O7] units linked to [ZnO4] units
Si = 4; Na = 4.5; Zn = 4
1-D polymer with [SiO3] units Polymeric arrangements of [Si2O7] units linked to [ZnO4] units
Si = 4; Ca = 6.3 Si = 4; Ca = 8; Zn = 4
3-D polymer of alternate [SiO4] and [ZnO4] tetrahedral repeating along c axis 3-D polymeric [SiO4] units
Si = 4; Zn = 4 Si = 4
Glass
c
NZ15
0.6
CaSiO3 Ca2ZnSi2O7c Na2Zn2Si2O7
NZ25
1
Ca2ZnSi2O7 Zn2SiO4d SiO2-quartz
c
Counts [a.u.]
d
2+
x = ZnO moles. Ref. [4] and references therein. Ref. [10]. Ref. [11].
for Na, the average CN being 1.34 in NC2S, 1.48 in NZ5, 1.72 in NZ15, and 1.93 in NZ25. A parallel way to highlight this phenomenon is shown in Table 3, where the contribution of BO, NBO and three bridging oxygens (TBO) to the coordination of Si and Zn are listed. Na depletion causes a dramatic rearrangement of the Si environment, which becomes mainly constituted by 2-fold oxygens. It is worth noting that Zn manifests a clear preference to be coordinated to BO species in the full range of concentrations studied. Moreover, the percentage of triclusters (TBO) associated with Zn, already significant in NZ15, levels off to 40% in NZ25. The modifications induced by zinc substitution for Na on the network connectivity are summarized in Figs. 2 and 3, where the Qn species distribution and ring size distribution (RSD) are plotted. The number of BO surrounding the network former Si atom (SiQn) increases progressively in the series of glasses studied, the average SiQn reaching a 50% of SiQ4 species in the Na-free glass. On the contrary, although the Q4 site is always preferred by the zinc, the population of the Q3 site reaches a significant percentage in NZ25. The zinc-free glass shows an almost symmetric distribution of the various-size rings constituting the silicate network, with a marked maximum for 12-membered rings. Zinc addition promotes the formation of smaller (formed by 4–5 tetrahedral sites) or larger (8, 10 tetrahedral sites) rings, with a general decrease of the average ring size of one unit.
861
628
NZ25
100 300 500 700
974
b
Mean CN +
NZ15 NZ5 NC2S
1023
a
Structural features
900 1100 1300 1500
Raman Shift [cm-1]
Fig. 1. Raman spectra for the glass compositions studied.
3.4. Molecular dynamics simulations A complete analysis of the local structure and network connectivity of the glasses studied has been carried out. Here we report only on the most striking features. The average coordination number (CN) around silicon is four for the entire series of glasses. The zinc remains four-coordinated at low and intermediate concentrations (NZ5 and NZ15), while a significant percentage of 3-fold Zn appears in the NZ25 glass, yielding an average CN of 3.6. The CN of Na and Ca are compositional dependent: they move from 5.58 to 4.85 and from 5.24 to 4.77, respectively, passing from the zincfree to the sodium-free glass. Finally, a progressive increasing of the oxygen coordination is observed as a function of Zn substitution
Table 3 Percentage contributions (%) of NBO, BO and TBO to the Si and Zn coordination Glasses
NC2S NZ5 NZ15 NZ25
Si
Zn
NBO
BO
TBO
NBO
BO
TBO
51.3 40.3 26.4 14.4
48.7 59.3 69.1 76.7
– 0.4 4.5 8.9
– 5.8 1.6 1.3
– 87.8 72.5 59.7
– 7.4 25.9 39.0
G. Lusvardi et al. / Journal of Non-Crystalline Solids 345&346 (2004) 710–714
713
60
NC2S NZ5 NZ15 NZ25
50
Si
n
Q %
40
30
20
10
Fig. 3. Ring size distribution (RSD) for the glass compositions studied, n is the number of atoms in the rings. 0 0
1
2
3
4
5
6
n 80
NZ5 NZ15 NZ25
Zn
40
n
Q %
60
20
0 0
1
2
3
4
5
6
n Fig. 2. Qn distribution for the Si species and for the Zn species.
This bi-modal distribution evolves towards a leveling of the ring percentage formed by 4–9 Si–Zn sites in the Nafree glass.
4. Discussion The complete substitution of ZnO for Na2O in a soda-lime–silicate glass of starting composition Na2O Æ CaO Æ 2SiO2 has been obtained. The composition change is accompanied by a linear increase of density values, mainly due to the heavier zinc atomic mass and the low zinc coordination number with respect to sodium ones. This trend suggests that the structural role played by the zinc is maintained in the series of glasses studied. In fact, the results obtained by molecular dynamics simulations show a network former role of the zinc ion in the full range of concentrations studied [13,14]. The zinc is found co-polymerized with the silicate network in tetrahedral sites progressively more distorted as Na depletion takes place. The presence of triclusters (TBO) contributes to create disorder in the structures.
TBO species appear in the NZ15 glass and reaches a significant percentage in the Na-free glass. They are mainly associated with zinc and are characteristic of willemite, the second phase separated upon crystallization of the NZ25 glass. The increase in the network polymerization as a function of zinc concentration is further highlighted by the Raman spectra, which detect the structural changing occurring around the silicon site and are usually discussed in terms of Qn species. In particular, the number of structural units containing Si–Q2 (Na) or Si–Q2 (Ca) species diminishes to favour the formation of Si–O–Zn bridges. In fact, an increasing number of various-size rings constituted by Si–Zn tetrahedral sites, detected by the RSD analysis, reduces the amount of the 6-member silicate rings found in NC2S and reminiscent of the Na2CaSi2O6 crystal phase structure. More details can be obtained from the Qn distribution derived by the molecular simulations study, which shows a significant decrease of the Si–Q2 species and a parallel increase of the Si–Q4 species. Interesting is the Zn–Qn behavior: at low and intermediate zinc concentration the Zn–Q4 species is predominant; in the Na-free glass 40% of Zn–Q3 species is formed, as a consequence the average CN diminishes. Distorted sites, where only three oxygen atoms are found in the zinc first atomic shell were reported in an EXAFS study on magnesium aluminosilicate glass containing Zn and Ti [15]. The final structure of the NZ5 and NZ15 glasses obtained by the MD simulations show the presence of silicon-rich regions where Ca ions are preferentially found. Compensation for the positive charge deficiency of the Zn tetrahedrals is preferentially provided by Na ions. In the Na-depleted glass the balancing of the charge of the oxygen atoms linked to zinc is achieved in several way such as competition with Si to attract Ca as charge-compensating cation, formation of triclusters, and, in the absence of Na, distortion of the tetrahedral geometry up to a tri-coordinated pyramidal geometry. This picture of the glass structure is in full agreement with the crystal phases identified after glass crystallization, which shows that the zinc atoms are embedded in
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calcium-rich regions only when there is shortage of Na ions or in the Na-free glass which crystallizes according to the reaction: CaO + ZnO + 2SiO2 = 0.5Ca2ZnSi2O7 + 0.25Zn2SiO4 + 0.75SiO2. References [1] G. Della Mea, A. Gasparotto, M. Bettinelli, A. Montenero, R. Scaglioni, J. Non-Cryst. Solids 81 (1986) 201. [2] D.A. McKeown, I.S. Muller, A.C. Buechele, I.L. Pegg, J. NonCryst. Solids 261 (2000) 155. [3] M. Le Grand, A.Y. Ramos, G. Calas, L. Galoisy, D. Ghaleb, F. Pacaud, J. Mater. Res. 15 (2000) 2015. [4] G. Lusvardi, G. Malavasi, L. Menabue, M.C. Menziani, J. Phys. Chem. B 106 (2002) 9753.
[5] J.C. Hurt, C.J. Phillips, J. Am. Ceram. Soc. 53 (1971) 269. [6] T. Furukawa, W.B. White, J. Non-Cryst. Solids 38&39 (1980) 87. [7] T.L. Pesina, V.A. Zakreuska, O.P. Puken, J. Am. Ceram. Soc. 67 (1984) 47. [8] W. Smith, T.R. Forester, J. Mol. Graph. 14 (1996) 136. [9] Cerius2, version 4.2, Accelrys, San Diego, 2000. [10] Mincryst crystallographic database for minerals. Available from: