Lionel Montagne

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.

Structural characterisation of phosphate materials: new insights into the spatial proximities between phosphorus and quadrupolar nuclei using the D-HMQC MAS NMR technique

We show in this article how the spatial proximity between phosphorus and quadrupolar nuclei can be efficiently and easily investigated with the D-HMQC (Dipolar Hetero-nuclear Multiple-Quantum Coherences) NMR technique. Compared to the commonly used CP-HETCOR (Cross-Polarisation HETero-nuclear CORrelation) sequence, the D-HMQC pulse scheme exhibits a higher sensitivity and a better robustness with respect to spinning frequency, electronic shielding and quadrupole interaction, and thus does not require time-consuming and complicated optimisation procedures. The advantages of the D-HMQC are demonstrated in this article through the acquisition of (31)P/S through-space two-dimensional correlation NMR spectra providing unreported structural information on (i) a sodium alumino-silicate glass doped with only 3% of P(2)O(5), (ii) a potassium boro-phosphate glass containing BO(3) and BO(4) groups and (iii) a crystalline zirconium vanado-phosphate. All these systems, representative of the most important mixed phosphate network materials, cannot be correctly investigated with the conventional CP-HETCOR NMR technique.

X-ray photoelectron spectroscopy and nuclear magnetic resonance structural study of phosphorus oxynitride glasses, ‘LiNaPON’

Abstract In this study, the structure of mixed alkali nitrided phosphate glasses ‘LiNaPON’, of composition Li 0.5 Na 0.5 PO 3−3x/2 N x (0 , was investigated using X-ray photoelectron spectroscopy (XPS), 31P magic angle spinning (MAS) and double quantum (DQ) nuclear magnetic resonance (NMR) spectroscopies. XPS results show that nitrogen N3− exists as two-coordinated –N atoms and three-coordinated –N atoms, respectively bonded to two and three phosphorus atoms, thus forming mixed PO3N and PO2N2 tetrahedra. Nitrogen atoms substitute for bridging –O– as well as non-bridging O oxygen atoms and a correlation was observed between the O1s and N1s spectra. 31P MAS NMR results show that PO4, PO3N and PO2N2 tetrahedra coexist within the glass network. From our 31P MAS and double quantum NMR results, we propose a model of nitrogen/oxygen substitution in which nitrogen atoms preferentially substitute for oxygen atoms shared by a PO4 and a PO3N tetrahedron. Consequently, oxynitride micro-domains grow at the expense of oxygenated areas in such a way that connectivity between PO4 tetrahedra is retained, even for the most nitrogen-rich glass compositions. Our data demonstrate that the nitrogen substitution for oxygen is not random.

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.

Observation of proximities between spin-1/2 and quadrupolar nuclei: which heteronuclear dipolar recoupling method is preferable?

We have recently shown that the dipolar-mediated heteronuclear multiple-quantum coherence (D-HMQC) method allows observing through-space proximities between spin-1/2 ((1)H, (13)C, (31)P...) and quadrupolar ((23)Na, (27)Al...) nuclei. However, the D-HMQC effectiveness depends on the choice of the heteronuclear dipolar recoupling sequence. Here, we compare the efficiency and the robustness of four rotor-synchronized sequences: the symmetry-based ones, R4(1)(2)R4(1)(-2) and its super-cycled version, SR4(1)(2), and two schemes based on simultaneous amplitude and frequency modulations, denoted SFAM-1 and SFAM-2. For the SFAM methods, we point out efficient recoupling conditions that facilitate their experimental optimization and we introduce analytical expressions for the buildup of D-HMQC signal in the case of an isolated spin pair. We show that the main differences between these four sequences lie in the number of adjustable parameters and in their robustness with respect to chemical shift and homonuclear dipolar interactions. The relative performances of these four recoupling sequences are analyzed using average Hamiltonian theory, numerical simulations, and (27)Al-{(31)P} D-HMQC experiments on crystalline aluminophosphate.

SPAM-MQ-HETCOR: an improved method for heteronuclear correlation spectroscopy between quadrupolar and spin-1/2 nuclei in solid-state NMR

The recently introduced concept of soft pulse added mixing (SPAM) is used in two-dimensional heteronuclear correlation (HETCOR) NMR experiments between half-integer quadrupolar and spin-1/2 nuclei. The experiments employ multiple quantum magic angle spinning (MQMAS) to remove the second order quadrupolar broadening and cross polarization (CP) or refocused INEPT for magnetization transfer. By using previously unexploited coherence pathways, the efficiency of SPAM-MQ-HETCOR NMR is increased by a factor of almost two without additional optimization. The sensitivity gain is demonstrated on a test sample, AlPO(4)-14, using CP and INEPT to correlate (27)Al and (31)P nuclei. SPAM-3Q-HETCOR is then applied to generate (27)Al-(31)P spectra of the devitrified 41Na(2)O-20.5Al(2)O(3)-38.5P(2)O(5) glass and the silicoaluminophosphate ECR-40. Finally, the method allowed the acquisition of the first high resolution solid-state correlation spectra between (27)Al and (29)Si.

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.