Combined 17O NMR and 11B-31P double resonance NMR studies of sodium borophosphate glasses

Reassessment of Electrical and Dielectric Properties in the Borophosphate Glass System: A Promising Solid Electrolyte for High-Temperature Batteries

This study investigates the conduction mechanism of ternary sodium borophosphate glass 30Na2O-(70 - x)B2O3-xP2O5 with 0 ≤ x ≤ 35 mol % from a different perspective, focusing on previously unreported high-temperature electrical and dielectric properties for potential solid electrolytes in high-temperature batteries. The glass composition with B2O3/P2O5 = 1 exhibits a conductivity of approximately 10-4 S/cm at 250 °C. Dielectric analysis supports this improved conduction, showing higher dielectric values and minimal energy dissipation during storage, indicating promising conductivity and favorable dielectric properties. This enhancement is attributed to the large-polaron (QMT) model, deduced from the power law exponent, due to the creation and spreading of lattice distortion of a long-range order with interconnected B4-O-P1 and B4-O-P2 linkages. Contrary to previous results, the glass transition temperature does not vary coherently with the conductivity and activation energy, displaying a discontinuity at 14 mol %. This discontinuity is caused by the initial extreme depolymerization of P2O5, leading to an increase in nonbridging oxygens (NBOs) within the glass network and forming B4-O-P0 linkages. Despite this, the ionic mobility of Na+ is continuously enhanced, correlated with the increase in the molar volume. This new perspective highlights the significant impact of both free volume expansion and reduced Coulombic effects on conduction improvement.

Ab initio study of stability and quadrupole coupling constants in borophosphates

The DFT method was used to predict the formation energies and quadrupole coupling constants CQ in a series of borophosphates: Li3BP2O8, Li2NaBP2O8, Na3BP2O8, Li2B3PO8, Na5B2P3O13, LiNa2B5P2O14 and Na3B6PO13 composed of different networks and different amounts of borate and phosphate units. The change in formation energies with increasing number of B atoms in this series is attributed to the multiplicity of boron sites and is explained by density of states calculations. The calculated CQ values of 7Li, 23Na and 11B are correlated with the coordination and distortion of polyhedra to elucidate the influence of local and more distant environments. As for the CQ of 11B, it should be in the ranges of 0.26-0.36, 0.48-0.84 and ∼1 MHz for boron tetrahedral distortion indices of 0.004-0.013, 0.015-0.019 and 0.033, respectively, whereas CQ ∼3.0 MHz corresponds to boron in a triangular site. The obtained numerical relationships make it possible to predict the quadrupole frequencies for these nuclei based only on their local environment, and vice versa, to propose structural models from NMR data. These results provide guidance for studying similar characteristics of other borophosphates, the structure of which varies depending on the initial reaction, composition and temperature.

Solid State NMR: A Powerful Tool for the Characterization of Borophosphate Glasses

This review will show how solid state nuclear magnetic resonance (NMR) has contributed to a better understanding of the borophosphate glass structure. Over the last fifteen years, 1D and 2D magic angle spinning (MAS)-NMR has been used to produce key information about both local and medium range organization in this type of glass. After a brief presentation on borophosphate glasses, the paper will focus on the description of the local order of phosphate and borate species obtained by 1D 31P-and 11B-MAS-NMR experiments, with a special emphasis on the improvements obtained at high magnetic fields on the borate speciation description. The last part of this review will show how correlation NMR provided new insights into the intermediate length scale order. Special attention will be paid to the quantitative data retrieved from 11B/31P REDOR-based NMR sequences and to the qualitative connectivity schemes observed on the 2D 11B/31P maps edited with the heteronuclear multiple quantum coherence (HMQC) NMR techniques.

Observation of proximities between spin-1/2 and quadrupolar nuclei in solids: Improved robustness to chemical shielding using adiabatic symmetry-based recoupling

We introduce a novel heteronuclear dipolar recoupling based on the R2₁^-1 symmetry, which uses the tanh/tan (tt) shaped pulse as a basic inversion element and is denoted R2₁^-1(tt). Using first-order average Hamiltonian theory, we show that this sequence is non-γ-encoded and that it reintroduces the |m| = 1 spatial component of the Chemical Shift Anisotropy (CSA) of the irradiated isotope and its heteronuclear dipolar interactions. Using numerical simulations and one-dimensional (1D) ²⁷Al-{³¹P} through-space D-HMQC (Dipolar Heteronuclear Multiple-Quantum Correlation) experiments on VPI-5, we compare the performances of this recoupling to those of other non-γ-encoded |m| = 1 heteronuclear recoupling schemes: REDOR (Rotational-Echo DOuble Resonance), SFAM (Simultaneous Frequency and Amplitude Modulation) and R4₂^-1(tt). Such comparison indicates that the R2₁^-1(tt) scheme is more robust to CSA, offset and radiofrequency field inhomogeneities than the other schemes. We take advantage of the high robustness of R2₁^-1(tt) to CSA and offset to demonstrate the possibility to correlate the signals of ²⁰⁷Pb isotope with those of neighboring half-integer spin quadrupolar nuclei. Such approach is demonstrated experimentally by acquiring ¹¹B-{²⁰⁷Pb} D-HMQC 2D spectra of Pb₄O(BO₃)₂ crystalline powder.

Heteronuclear correlation experiments of 23Na-27Al in rotating solids

We demonstrated that the heteronuclear correlation experiments between two quadrupolar nuclei, ²³Na and ²⁷Al, with close Larmor frequencies can be achieved via D-HMQC and D-RINEPT approaches by using a diplexer connected to a conventional probe in magic-angle-spinning solid-state NMR. Low-power heteronuclear dipolar recoupling schemes can be applied on ²³Na or ²⁷Al to establish polarization transfers between the central transitions of ²³Na and ²⁷Al for a model compound, NaAlO₂. Further, we showed a practical implementation of the two dimensional ²³Na-²⁷Al dipolar-based heteronuclear correlation experiment on a heterogeneous catalyst, Na₂CO₃/γ-Al₂O₃. This allows to determine spatial proximities between different ²³Na and ²⁷Al sites, thus the surface Na species adjacent to octahedral-coordination Al can be clearly discriminated.

P-O-B(3) linkages in borophosphate glasses evidenced by high field (11)B/(31)P correlation NMR

The long-standing debate about the presence of P-O-B(3) linkages in glasses has been solved by high-field scalar correlation NMR. Previously suggested by dipolar NMR methods, the presence of such species has been definitively demonstrated by (11)B((31)P) J-HMQC NMR techniques. The results indicate that borophosphate networks contain P-O-B(3) bonds and thus present a higher degree of atomic homogeneity than previously thought.

A review on immobilization of phosphate containing high level nuclear wastes within glass matrix--present status and future challenges

Immobilization of phosphate containing high level nuclear wastes within commonly used silicate glasses is difficult due to restricted solubility of P(2)O(5) within such melts and its tendency to promote crystallization. The situation becomes more adverse when sulfate, chromate, etc. are also present within the waste. To solve this problem waste developers have carried out significant laboratory scale research works in various phosphate based glass systems and successfully identified few formulations which apparently look very promising as they are chemically durable, thermally stable and can be processed at moderate temperatures. However, in the absence of required plant scale manufacturing experiences it is not possible to replace existing silicate based vitrification processes by the phosphate based ones. A review on phosphate glass based wasteforms is presented here.

Effect of pressure on structure of oxide glasses at high pressure: Insights from solid-state NMR of quadrupolar nuclides

Revealing the structure of oxide glasses at high pressure remains a fundamental yet difficult problem in modern physical and chemical sciences. The recent advances in solid-state NMR techniques used for quadrupolar nuclides offer a considerably improved resolution of atomic sites, unveiling previously unknown structural details of oxides glasses at high pressure. Here, we present an overview of the recent progress and insights by high-resolution multi-nuclear triple quantum magic angle spinning (3QMAS) NMR into pressure-induced changes in coordination number, connectivity, and topological disorder in oxide glasses quenched from melts at high pressure. (11)B and (27)Al 3QMAS NMR studies of oxide glasses show that the formation of highly coordinated Al (([5,6])Al) and four coordinated ([4])B are prevalent at high pressure up to 8 GPa. The formation of oxygen clusters linking these highly coordinated framework units and Si (e.g., ([5,6])Al-O-([4])Si, ([5,6])Si-O-([4])Si, and Na-O-([5,6])Si) is observed in the (17)O NMR spectra at higher pressure, leading to an overall increase in the degree of polymerization with pressure. (23)Na MAS NMR spectra of diverse oxide glasses at high pressure and high magnetic field also indicate that the Na-O bond distance may decrease with pressure. Pressure-induced changes in structurally relevant NMR parameters such as the (17)O quadrupolar coupling product (P(q)) for the Si-O-Si cluster and (27)Al P(q) for Al sites in oxide glasses indicate the occurrence of pressure-induced reductions in the Si-O-Si angle and an increase in the Al-O bond length distribution with pressure, indicating an increase in the overall topological disorder in oxide glasses with pressure. All the pressure-induced changes in structure and topology are characterized by strong composition dependence. These experimental results highlight a new opportunity to investigate the molecular structures of silicate melts at high pressure and reveal connections between the microscopic signatures of anomalous and non-linear changes in the macroscopic properties of the corresponding liquids. While many challenges still remain in the synthesis of oxide glasses with wider range of melt composition at higher pressure above 12 GPa, recent progress in enhancement of sensitivity and resolution in the solid state NMR hold strong promise for study exploring additional details of connectivity among quadrupolar nuclides and medium-range order of the more complex, multi-components glasses at high pressure.

Characterization of local environments in crystalline borophosphates using single and double resonance NMR

(11)B and (31)P magic-angle spinning as well as (11)B{(31)P} and (31)P{(11)B} rotational echo double resonance (REDOR) NMR have been applied to characterize the local environments in the crystalline borophosphates K(3)[BP(3)O(9)(OH)(3)], NH(4)[ZnBP(2)O(8)] and Rb(3)[B(2)P(3)O(11)(OH)(2)]. Dipolar second moment values extracted from the REDOR curves at short evolution times (DeltaS/S(0) < or = 0.2) are in reasonable agreement with those calculated from the internuclear distances in the corresponding crystal structures. In particular, the method is found to be useful for distinguishing between boron and phosphorus local environments with different numbers of B-O-P connectivities, making REDOR a well-suited tool for medium-range order investigations in glasses.

Site discrimination in the crystalline borophosphate Na5B2P3O13 using advanced solid-state NMR techniques

A comprehensive solid-state NMR investigation on crystalline Na(5)B(2)P(3)O(13) is presented. Triple-quantum magic angle spinning (TQMAS) and rotational echo double resonance (REDOR) studies are used for accurate determinations of the (11)B, (23)Na and (31)P interaction parameters. Based on these results and complementary quantum mechanical calculations, plausible site assignments can be derived. Generally, the results show that detailed, quantitative information about structures in borophosphate compounds can be obtained by investigating both the local site environments characterized by chemical shift and quadrupolar interaction parameters and the correlated dipolar interactions to atoms in the second coordination sphere.

93Nb and 17O NMR chemical shifts of niobiophosphate compounds

Niobiophosphate compounds with a large range of niobium and oxygen environments were studied with (93)Nb and (17)O solid-state NMR. (93)Nb isotropic chemical shift of pure niobate Nb(ONb)(6), pure phosphate Nb(OP)(6) and mixed phosphate-niobate Nb(OP)(x)(ONb)((6-x)) (1<or= x<or=5) sites were measured at a high magnetic field (18.8T). (93)Nb chemical shifts were found to be sensitive to the variation of local charge on Nb, but not to the local bond geometry (i.e. crystallographic site and edge or corner connectivity). A systematic shift to high field of the (93)Nb chemical shift is measured when x increases. Then, (17)O NMR spectra of a series of enriched samples provided the chemical shift and quadrupolar parameters for 4 types of oxygen environment (P-O-P, P-O-Na, P-O-Nb and Nb-O-Nb). Finally, Nb-O-Nb sites were characterized by a large (17)O chemical shift anisotropy.

Experimental and Theoretical 17O NMR Study of the Influence of Hydrogen-Bonding on CO and O−H Oxygens in Carboxylic Solids

A systematic solid-state 17O NMR study of a series of carboxylic compounds, maleic acid, chloromaleic acid, KH maleate, KH chloromaleate, K2 chloromaleate, and LiH phthalate.MeOH, is reported. Magic-angle spinning (MAS), triple-quantum (3Q) MAS, and double angle rotation (DOR) 17O NMR spectra were recorded at high magnetic fields (14.1 and 18.8 T). 17O MAS NMR for metal-free carboxylic acids and metal-containing carboxylic salts show featured spectra and demonstrate that this combined, where necessary, with DOR and 3QMAS, can yield site-specific information for samples containing multiple oxygen sites. In addition to 17O NMR spectroscopy, extensive quantum mechanical calculations were carried out to explore the influence of hydrogen bonding at these oxygen sites. B3LYP/6-311G++(d,p) calculations of 17O NMR parameters yielded good agreement with the experimental values. Linear correlations are observed between the calculated 17O NMR parameters and the hydrogen bond strengths, suggesting the possibility of estimating H-bonding information from 17O NMR data. The calculations also revealed intermolecular H-bond effects on the 17O NMR shielding tensors. It is found that the delta11 and delta22 components of the chemical shift tensor at O-H and C=O, respectively, are aligned nearly parallel with the strong H-bond and shift away from this direction as the H-bond interaction weakens.

A new 17O-isotopic enrichment method for the NMR characterisation of phosphate compounds

Heating phosphate compounds under (17)O-enriched water vapour is an easy and rapid method to prepare homogeneously enriched and pure samples for the acquisition of (17)O NMR spectra with a good sensitivity.