Is lead part of the glass network in lead glasses Is lead part of the glass network in lead glasses? Recent insights and practical consequences L.Montagne and G.Palavit Ecole Nationale Superieure de Chimie de Lille -France XVI ICF Technical Exchange Conference Karlovy Vary 9-12 th october 2004

Pb environment in PbO PbO litharge massicot a b c Pb+2 O-2

Low field strength (z/ri2) Large size Specificities of Lead free 6s2 electron pair Low field strength (z/ri2) Large size PbO3 /PbO4 environments are prefered in crystals

Main techniques for the examination of the local environment of lead in glasses 207Pb Solid-state NMR X-Rays and neutrons diffraction Pb-LIIIedge XAFS X-ray photoelectron spectroscopy (XPS): binding energies from Pb 4f7/2, Pb 4f5/2.. Vibrational spectroscopies

Outline NMR, basics and scales Lead in binary glasses PbO-SiO2 Comparative studies: PbO-SiO2, PbO-P2O5 Lead in complex glasses: PbO-B2O3-Al2O3-SiO2 glasses PbO-P2O5-SiO2 glasses PbO-P2O5-Na2O-CaO-Al2O3-SiO2 glasses

Nuclear Magnetic Resonance ? b Bo > 0 Bo = 0 DE = h n Access to chemical informations: bonding, geometry,... 29Si, 207Pb, 31P, ...

NMR of glasses: the scales Phase separation: Demixion, crystallisation SiO4 Na+ Nucleus 207Pb, 29Si,... 1st-2nd neighbours: Coordinence, Former/modifier Connectivity : SiOSi, SiOPb,BOSi,... PbO4

Outline NMR, basics and scales Lead in binary glasses PbO-SiO2 Comparative studies: PbO-SiO2, PbO-P2O5 Lead in complex glasses: PbO-B2O3-Al2O3-SiO2 glasses PbO-P2O5-SiO2 glasses PbO-P2O5-Na2O-CaO-Al2O3-SiO2 glasses

207Pb NMR spectra of PbO-SiO2 glasses PbO w.% 90 72 61 4000 0 -4000 ppm Covalent (former) SiOPb ionic (modifier) SiO...Pb

lead environment in PbO-SiO2 glasses Pb 2+ appears to be more donating than cationic potential predicts  large involvement of the inert pair of electrons Larger coordinence in glasses than in crystals Current model proposes a disproportionation : 2 Si-O-Pb  Si-O-Si + Pb-O-Pb I.A.Gee, 2001

Outline NMR, basics and scales Lead in binary glasses PbO-SiO2 Comparative studies: PbO-SiO2, PbO-P2O5 Lead in complex glasses: PbO-B2O3-Al2O3-SiO2 glasses PbO-P2O5-SiO2 glasses PbO-P2O5-Na2O-CaO-Al2O3-SiO2 glasses

Structure of lead phosphate glasses Chemical shift / ppm -4500 -3000 -1500 1500 3000 60.2 42.2 34.4 32.1 27.6 51.8 mol% P2O5 207Pb NMR ionic (modifier) Covalent (former) Chemical shift / ppm 20 10 -20 -30 -40 -50 -10 21.4 27.6 32.1 34.4 42.2 51.8 60.2 mol% P2O5 31P NMR Depolymerisation We have also described the local environment of lead cations in these lead and cadmium phosphate glasses using NMR. We can observe that the chemical shift of the resonance shifts towards more positive ones as the amount of modifying oxides increases. We associate this chemical shift evolution to an increase of the lead-oxygen bond covalency. The structural modifications of the lead and cadmium phosphate glasses observed by phosphorus NMR and lead NMR can be correlated to the increase of the glass temperature as P2O5 content decreases . Then we can conclude that the excess of lead chloride and cadmium chloride in the batch to be converted induces the reticulation of the depolymerised phosphate network by P-O-Pb bonds. Increasing lead in PbO-P2O5 glasses : 31P indicates a depolymerisation of the phosphate network 207Pb indicates a transition from network modifier to former S. Donze, L. Montagne, G. Palavit, Phys. Chem. Glasses (2001)

Geometries of oxygen environments of Pb 2+ cations in PbO-SiO2 and PbO-P2O5 glasses Neutron diffraction Distance and number of Pb-O bonds PbO-SiO2 PbO-P2O5 Shortest bonds 3 x 0.230 nm 5 x 0.247 nm Additional oxygen neighbours 1 x 0.275 nm 3 x 0.28nm total 4 8 Do not depend on the PbO content, but on the nature of the network-forming oxide (U. Hoppe, J. Non-cryst. solids 328, 1-3 (2003)146)

Outline NMR, basics and scales Lead in binary glasses PbO-SiO2 Comparative studies: PbO-SiO2, PbO-P2O5 Lead in complex glasses: PbO-B2O3-Al2O3-SiO2 glasses PbO-P2O5-SiO2 glasses PbO-P2O5-Na2O-CaO-Al2O3-SiO2 glasses

Pb environment in complex oxide glasses? What is the influence of another network former, like P2O5, B2O3 or Al2O3 in lead silicate glasses? (1) PbO-B2O3-Al2O3-SiO2 glasses - addition of B2O3 to lead silicate glasses - substitution of B2O3 by Al2O3 (2) PbO-P2O5-SiO2 glasses - Effect of P2O5 doping in lead silicate and aluminosilicate glasses

(1) PbO-B2O3-Al2O3-SiO2 glasses - Addition of B2O3 to lead silicate glasses: The covalent character of Pb-O increases with the amount of PbO (similar to PbO-SiO2) stabilisation of BO3 units at the expense of BO4. - Substitution of B2O3 by Al2O3 : PbO[(B2O3)1-z(Al2O3)Z] SiO2 glasses promotion of BO3 species at the expense of BO4. BO3 induces a decrease of surface tension s. Fujino, J. Am. Ceram. Soc. 87(2004)10 Sawvel, S.Chinn, W.Bourcier, R.Maxwell,Chem. Mater., in press (2004)

(2) Effect of phosphate doping in lead silicate glasses Why P2O5 ?  P2O5 increases the chemical durability of lead silicate glasses Effect of phosphate addition on lead release (24h exposure, 4% acetic acid) Base glass Phosphate doped Na2O-CaO-PbO-SiO2 Glasses+1mol%P2O5 R.L. Lehman, J. Am. Ceram. Soc., 65 (1982) 410 and 75 (1992) 2194.

(2) Effect of phosphate doping in lead silicate glasses Is the reduction of lead release due to: - Surface crystal formation (5PbO-P2O5-SiO2) ? Complexing of Pb2+ by the phosphate into the glass structure ? R.L. Lehman (1992)  Need for a structural approach

(2) Effect of phosphate doping in lead silicate glasses Strategy of the structural approach : P2O5 in Na-silicate glasses PbO-P2O5 in Na-Ca-Al silicate glasses

 Isolated phosphate anions P2O5 in Na-silicate glasses 31P MAS-NMR :  No P-O-Si bonds,  Isolated phosphate anions sodium orthophosphate Na3PO4 sodium pyrophosphate Na4P2O7 67SiO2 - 33Na2O + 1,3 mol.% (P2O5 ; Na2O) L. Montagne, G.Palavit et al., Glastech. Ber. Glass Sci. Technol. 71C (1998) 204

P2O5 in Na-silicate glasses The influence of P2O5 on the silicate network depends on how P2O5 is introduced : silicate network polymerisation increased P2O5 + 4Si-O-Na Na4P2O7 + 2Si-O-Si P2O5 + 6Si-O-Na 2Na3PO4 + 3Si-O-Si 29Si NMR + P2O5 silicate network polymerisation unchanged + Na3PO4 Dissolution of Na3PO4 L. Montagne, G.Palavit et al., J. Non-Cryst. Solids, 263&264 (2000) 312-317

P2O5 in Na-silicate glasses Viscosity at 1250°C silicate network polymerisation increased + P2O5 1 1,4 1,8 2,2 2,6 5,7 6,1 6,5 6,9 10 4 .1/T(K -1 ) log h (P) silicate network polymerisation unchanged + Na3PO4 1 1,4 1,8 2,2 2,6 5,7 6,1 6,5 6,9 10 4 .1/T(K -1 ) log h (P) L. Montagne, G. Palavit et al., Glastech. Ber. Glass Sci. Technol. 73 (12) (2000) 380-384

P2O5 in Na-silicate glasses Liquidus temperature silicate network polymerisation increased + P2O5 T L (°C) Phase Si60Ca20Na20 1181 Silica Si60Ca20Na20+1,3(M + ;PO 4 x- ) 1161 Si60Ca27Na13 1264 a - CS Si60Ca27Na13+1,3(M 1230 CS Si60Ca34Na6 1380 Si60Ca34Na6+1,3(M 1360 silicate network polymerisation unchanged + Na3PO4

PbO-P2O5 in Na-Ca-Al silicate glasses (ppm) -120 -110 -100 -90 -80 -70 -60 -50 Base glass +10% +5% +20% 29Si NMR Polymerization of the silicate glass network, which depends on the proportion of PbO-P2O5 L. Montagne, G. Palavit et al., in preparation

PbO-P2O5 in Na-Ca-Al silicate glasses (ppm) -60 -50 -40 -30 -20 -10 10 20 30 PbO-P2O5 31P NMR Depolymerization of the phosphate chains Dissociation of lead phosphate into Ca,Na monomers (Ca,Na)PO4

PbO-P2O5 in Na-Ca-Al silicate 1) Na2O-CaO-Al2O3-SiO2 glass

PbO-P2O5 in Na-Ca-Al silicate 2) Addition of P2O5 to Na2O-CaO-Al2O3-SiO2 glass

PbO-P2O5 in Na-Ca-Al silicate 3) Addition of PbO-P2O5 in Na2O-CaO-Al2O3-SiO2 glass

(2) Effect of phosphate doping in lead silicate glasses Is the reduction of lead measured in solution due to: - Surface crystal formation (5PbO-P2O5-SiO2) ? Complexing of Pb2+ by the phosphate into the glass structure ? R.L. Lehman (1992)

Conclusions The local structure around lead depends : 1) on its quantity in the glasses: High: covalent bonding (network former), low: ionic bonding (network modifier). 2) on the nature of the network former(s) Structural informations from other nuclei (29Si, 31P) are as important as that from lead (207Pb) The consequence on properties depends on indirect effect of lead on the other network formers (Al2O3, B2O3, P2O5).