Gerard Palavit

Phosphate speciation in Na2O–CaO–P2O5–SiO2 and Na2O–TiO2–P2O5–SiO2 glasses

P2O5 was added in amounts of 0.4–2.5 mol% to (5.1–41.9)Na2O–(5.3–35.1)CaO–(57.0–66.9)SiO2 and (35.4–38)Na2O–(4.7–9.8)TiO2–(50–58.7)SiO2 glasses. Sodium and calcium ions were also added to maintain the charge balance on the phosphate complexes, without modification of the silicate condensation. 31P magic-angle-spinning nuclear magnetic resonance spectra of Na2O–CaO–P2O5–SiO2 glasses show that the phosphate complexes are monophosphate and diphosphate. The linear dependance of 31P chemical shifts with CaO/(CaO+Na2O) is consistent with a random distribution of sodium and calcium around phosphate complexes, except for diphosphates at CaO/(CaO+Na2O) less than 0.3. The relative proportion of mono- and diphosphate is analyzed in terms of an acid–base reaction involving exchange of O2−. We suggest that the amount of monophosphate larger than predicted by basicity analysis is consistent with calcium monophosphate complexes forming, and shifting the acid–base equilibria. An additional discrete 31P resonance in the Na2O–TiO2–P2O5–SiO2 glass spectra is attributed to titanium bearing phosphate complexes. They reveal the formation of Ti–O–P covalent bonds, owing to the electrostatic field strength of Ti4+ ions.

Acid dissolution of sodium–calcium metaphosphate glasses

Abstract The kinetics and mechanism of dissolution of (50-x)Na2O–xCaO–50P2O5 metaphosphate glasses in aqueous solution at pH=3 are reported. Kinetic data, the presence of a hydrated layer and etch pits lead to the conclusion that the dissolution at pH=3 is limited by surface reactions. Solution analysis and surface characterization show that dissolution is congruent. Local structure and composition in the hydrated layer is identical to that in the pristine glass. After a linear period, the dissolution rate decreases, because the increase in ionic strength of the leaching solution results in a modification of the hydrated layer.

Effect of ZnO on the properties of (100 - x)(NaPO3)-xZnO glasses

Abstract Glasses of the (100 - x)(NaPO3)-xZnO system with 0 ≤ x ≤ 33.3 mol% have been prepared with a conventional melting procedure. The density and index of refraction of (100 - x)(NaPO3)-xZnO glasses increases linearly with x, in accordance with the network modifying behavior of ZnO. For x less than 15%, the glass transition temperature, the activation energy for viscous flow and the aqueous durability are affected by the decrease in phosphate chain length, owing to the very weak zinc-oxygen bonding energy. For larger x (15 ≤ x ≤ 33.3), these properties increase because the covalent character of zinc-oxygen bonds enables the formation of a mixed zinc-phosphate network.

17O nuclear magnetic resonance study of Na2O–P2O5 glasses

Abstract The preparation and structural investigation of 17O-enriched xNa2O–(100−x)P2O5 glasses (46.5⩽x⩽62.8) by nuclear magnetic resonance (NMR) is described. Enriched phosphoric acid was prepared by hydrolysis of PCl5 with 17O-enriched water and neutralized with sodium carbonate. The sodium metaphosphate was then melted at 800 °C for 15 h and quenched. Polyphosphate and ultraphosphate glass compositions were prepared by remelting the metaphosphate with sodium carbonate and phosphorus pentoxide, respectively. 31P magic angle sample spinning (MAS) NMR was used to determine the Na2O/P2O5 content in the glasses. 17O NMR spectra (quadrupole echo for non-rotating samples and multiple-quantum excitation for rotating samples (MQMAS)) show two oxygen sites in the samples with large quadrupolar coupling constants (4.7 and 7.7 MHz), in accordance with the high phosphorus electronegativity. According to the correlation of 17O quadrupolar constants with bond ionicity, these two components are attributed to bridging P–O–P and non-bridging P–O⋯Na oxygens. The average P–O–P bond angle is estimated with the quadrupolar asymmetry derived from the fit of the static echo spectra. The MQMAS spectrum shows a distribution of non-bridging oxygen chemical shifts, attributed to a variation of bond length and angle.

Structure and ionic conductivity of sodium titanophosphate glasses

Abstract 50Na 2 O– x TiO 2 –(50− x )P 2 O 5 (with x =0, 5, 10, 15) glasses have been characterized by infrared spectroscopy, 31 P magic angle spinning nuclear magnetic resonance (MAS–NMR) and 31 P double-quantum (DQ) MAS–NMR. MAS–NMR spectra show that phosphate network depolymerization occurs when x increases. Infrared spectra indicate that titanium is incorporated as TiO 6/2 units. DQ MAS–NMR enables us to characterize the Q n , ij phosphate units. Their chemical shifts, measured on DQ MAS–NMR spectra, were used as constraints for the deconvolution of one-dimensional MAS–NMR spectra. It indicates the formation of diphosphate units bonded to sodium and titanium Q 1,1 (Na,Ti) , even at the lowest x value. A structural unit formed by the sequence –Q 2,22 (Na) –Q 2,21 (Na) –Q 1,2 (Na,Ti) –TiO 6/2 –Q 1,1 (Na,Ti) –Q 1,1 (Na,Ti) –TiO 6/2 can be proposed. The increase of conductivity and glass transition temperature with x is discussed from this structural model.

Local structure of zinc ultraphosphate glasses containing large amount of hydroxyl groups: 31P and 1H solid state nuclear magnetic resonance investigation

Abstract Zinc ultraphosphate glasses containing variable amounts of hydroxyl groups were prepared by melting phosphoric acid and zinc oxide at 900°C for different times. The 1 H- 31 P cross polarization (CP) combined with magic angle spinning (MAS) nuclear magnetic resonance (NMR) at variable contact time show clearly the presence of Q 2 sites including those bonded to H + , Q 2 (H), and those bonded to Zn 2+ , Q 2 (Zn), and Q 3 sites. Moreover, the detailed examination of the line widths reveals that the Q n sites are distributed in the glass matrix, particularly the Q 2 (H) sites. A quantification of the measurements indicates an evolution of the relative fractions of the sites versus the melting time. 1 H MAS NMR reveals the existence of at least three kind of protons characterized by different isotropic chemical shifts: 17 ± 0.2, 13 ± 0.2 and 8.6 ± 0.2 ppm. These protons are identified as those involved, respectively, in Q 2 (H)…Q 2 (Zn) groups, in Q 2 (H)…Q 2 (H) groups and those of water molecules adsorbed on the glass surface. The measurements of spin lattice and spin-spin relaxation times indicate that the water molecules are strongly adsorbed and that the protons involved in Q 2 (H)…Q 2 (Zn) and in Q 2 (H)…Q 2 (H)…Q 2 (H) are homogeneously distributed in the glass matrix, in good agreement with the analysis of 31 P NMR data.

Surface alteration of zinc ultraphosphate glass in humid air at 140°C

A mechanism for the alteration of zinc ultraphosphate glass by humid air at 140°C is presented. The glass was prepared by melting a H 3 PO 4 -ZnO batch at 900°C for one hour, its composition is 54.6(±1.5)P 2 O 5 -34.2(±0.5)ZnO-11.2(±0.4)H 2 O. 1 H and 31 P nuclear magnetic resonance shows that water adsorption occurs and simultaneaously leads to the formation of monophosphoric (H 3 PO 4 ) and diphosphoric (H 4 P 2 O 7 ) acids. The surface conductivity data are consistent with the formation on the glass surface of an acidic solution of phosphates. The thermogravimetric data shows that this solution reaches equilibrium with the atmospheric water, that decreases the adsorption rate of water.

207Pb and 113Cd NMR and XPS characterization of PbO-PbCl2-CdCl2 glasses

Abstract A structural characterization of PbO–PbCl2–CdCl2 oxychloride glasses was achieved using 207 Pb and 113 Cd nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS). 207 Pb static NMR spectra exhibit two broad components, whose intensity change with oxygen–chlorine substitution in the glass composition. 207 Pb phase-adjusted spinning sidebands (PASS) NMR shows that the lead first co-ordination sphere in PbO–PbCl2–CdCl2 glasses contains both oxygen and chlorine atoms and that the Pb bonding environment varies from an ionic environment to a more covalent bonding state. From the study of crystalline Pb3O2Cl2, the correlation between 207 Pb isotropic and anisotropic chemical shift is extended to oxychloride compounds. Oxygen 1s photoelectron spectra contain two components that are consistent with a glass formation through Pb–O–Pb bonds. 113 Cd NMR spectra show that Cd is mainly ionically bonded to chlorine and acts as network modifier for all glass compositions.

Structural analysis of the thermal conversion of metal chlorides into phosphate glasses

Abstract The conversion reaction of metal chlorides (NaCl, KCl, PbCl 2 , CdCl 2 ) into phosphate glasses is studied via a structural approach. The glass compositions are: Na: 8.5–15.4%; K: 7.4–13.2% ; Pb: 1.6–2.6%; Cd: 0.4–0.7%; Cl: 0.2–20.1%; P: 11.6–20.4%; O: 36.5–60.8% (at. mol%). 31 P magic-angle-spinning nuclear magnetic resonance and 31 P double quantum analyses are employed for determining the structural evolution of the phosphate network as function of the batch composition and the melting temperatures (700°C and 900°C). The Q 1 /Q 2 ratios, measured by 31 P spectrum deconvolution, are in accordance with the elemental analyses of the samples prepared at 700°C and 900°C. 23 Na magic-angle-spinning nuclear magnetic resonance shows the presence of sodium in the amorphous phases and in NaCl crystals in devitrified samples. Transfer of populations in double-resonance experiments (TRAPDOR) are also consistent with the hypothesis that there is no specific bonding of sodium to a particular Q n unit in the phosphate network.

Thermal evolution of phosphorodiamidic acid as a model for nitrogen stability in phosphate glasses

The thermal evolution at a heating rate of 3°C min−1 of phosphorodiamidic acid, HPO2(NH2)2, was studied up to 600°C. Thermogravimetric analysis revealed three stages at 120, 320 and 600°C. Nuclear magnetic resonance and Fourier transform-infrared analysis have been used to characterize the thermal products. At 120°C, phosphorodiamidic acid condenses without any weight loss into an ammonium salt of P,P′-diamidoimidodiphosphoric acid. It is transformed at 320°C into a more condensed product containing 17.7 wt % nitrogen and showing P-NH-P and P-O-P linkages. At 600°C, the product still contains 10 wt% nitrogen. Phosphorus nuclear magnetic resonance shows that it is composed of nitrogen-containing Q3 groups and ultraphosphate Q3 groups. It is concluded that nitrogen cannot be held in the phosphate network if it contains hydroxyl groups, and that incorporation of nitrogen requires both reducing and nitriding conditions.