Triple-quantum correlation NMR experiments in solids using J-couplings

We show that triple-quantum–single-quantum (TQ–SQ) correlation spectra of crystalline and disordered solids can be obtained under MAS using pulse sequences based on through-bond J-couplings. The feasibility of the experiments in coupled spin-1/2 systems is demonstrated for fully 13C-labelled L-alanine and Pb3P4O13 crystalline compounds, considered as model three-spin and four-spin systems, respectively. In the case of phosphate glasses, we show that the obtained TQ–SQ correlation spectra provide an improved description of the glass forming network connectivities and of the chain length distribution in the disordered network

Structural characterization of water-bearing silicate and aluminosilicate glasses by high-resolution solid-state NMR

Various one- and two-dimensional high-resolution solid-state NMR techniques have been applied to hydrous silicate and aluminosilicate glasses: simple acquisition, cross-polarization (CP-MAS), heteronuclear correlation (HETCOR), dipolar dephasing, spin counting, double-quantum correlation, and rotational echo double resonance (REDOR). The comparison of the results obtained for sodium tetrasilicate and phonolite glasses suggests that the water incorporation mechanisms are qualitatively similar for these two compositions. From proton NMR experiments, we observe no evidence of proton clustering and a wide range of chemical shifts ranging from 0 to 16 ppm, even for the aluminosilicate phonolite glass, identifying at least three types of hydroxyl (OH) protons in addition to molecular H2O. This variety of OH groups can be discussed in terms of hydrogen bonding strength. For both compositions, the results indicate some depolymerization of the tetrahedral network, but the picture cannot be so simple as to completely exclude any of the different previously proposed models for water incorporation in silicate glasses

New Insights into the Molecular Structures, Compositions, and Cation Distributions in Synthetic and Natural Montmorillonite Clays

International audienceWe present a detailed investigation of the molecular structure of montmorillonite, an aluminosilicate clay with important applications in materials sciences, such as for catalysis, drug delivery, or as a waste barrier. Solid-state 29Si, 27Al, 25Mg, and 1H nuclear magnetic resonance (NMR) measurements combined with density functional theory (DFT) calculations provide a comprehensive picture of the local structure and composition of a synthetic clay and its naturally-occurring analogue. A revised composition is proposed based on NMR results that allow the identification and quantification of the signatures of otherwise undetectable non-crystalline impurities, thus largely complementing the traditional elemental analyses. Solid-state 1H NMR at fast magic-angle spinning (MAS) and high magnetic field provide quantitative information on intra- and inter-layer local environments that are crucial for the determination of the amount of Mg/Al substitution within the octahedral layer. In combination with DFT calculations of energies, it suggests that pairs of adjacent Mg atoms are unfavorable, leading to a non-random cationic distribution within the layers

Mechanism of Calcium Incorporation Inside Sol–Gel Silicate Bioactive Glass and the Advantage of Using Ca(OH)2 over Other Calcium Sources

Calcium is an essential component of osteogenesis and is often required for imparting significant bioactivity to synthetic bone substitutes and, in particular, silicate-based materials. However, the mechanism of calcium incorporation inside sol–gel silicates is poorly understood. In this work, we shed light on the determinant parameters for incorporation of calcium into acid–base-catalyzed sol–gel silicates at ambient temperature: increasing the pH above the isoelectric point of silicic acid and the nature of the calcium counterion in the calcium precursor are found to be the key. Based on our proposed reaction sequence, we were able to compare calcium precursors and select an ideal candidate compound for the synthesis of bioactive glasses (BG) and organic–inorganic hybrids at ambient temperature. Reproducible syntheses and gel times of SiO2–CaO BG were obtained using calcium hydroxide (CH), and we demonstrate its usability in the synthesis of promising BG–polycaprolactone hybrid scaffolds. BG and hybrids prepared with CH were able to form nanocrystalline nonstoichiometric apatite in simulated body fluid. The increased reliability of low-temperature syntheses associated with the use of a stable and inexpensive alkaline-earth precursor are major steps toward the translation of calcium silicate hybrids or other alkaline-earth silicates from bench to clinic

Atomic Insights into Aluminium-Ion Insertion in Defective Anatase for Batteries

International audienceAluminium batteries constitute a safe and sustainable high‐energy‐density electrochemical energy‐storage solution. Viable Al‐ion batteries require suitable electrode materials that can readily intercalate high‐charge Al3+ ions. Here, we investigate the Al3+ intercalation chemistry of anatase TiO2 and how chemical modifications influence the accommodation of Al3+ ions. We use fluoride‐ and hydroxide‐doping to generate high concentrations of titanium vacancies. The coexistence of these hetero‐anions and titanium vacancies leads to a complex insertion mechanism, attributed to three distinct types of host sites: native interstitial sites, single vacancy sites, and paired vacancy sites. We demonstrate that Al3+ induces a strong local distortion within the modified TiO2 structure, which affects the insertion properties of the neighbouring host sites. Overall, specific structural features induced by the intercalation of highly polarising Al3+ ions should be considered when designing new electrode materials for polyvalent batteries

A computationally-guided non-equilibrium synthesis approach to materials discovery in the SrO-Al₂O₃-SiO₂ phase field

Glass-crystallisation synthesis is coupled to probe structure prediction for the guided discovery of new metastable oxides in the SrO-Al2O3-SiO2 phase field, yielding a new ternary ribbon-silicate, Sr2Si3O8. In principle, this methodology can be applied to a wide range of oxide chemistries by selecting an appropriate non-equilibrium synthesis route

Highly Nonstoichiometric YAG Ceramics with Modified Luminescence Properties

Y3Al5O12 (YAG) is a widely used phosphor host. Its optical properties are controlled by chemical substitution at its YO8 or AlO6/AlO4 sublattices, with emission wavelengths defined by rare-earth and transition-metal dopants that have been explored extensively. Nonstoichiometric compositions Y3+xAl5-xO12 (x ≠ 0) may offer a route to new emission wavelengths by distributing dopants over two or more sublattices simultaneously, producing new local coordination environments for the activator ions. However, YAG typically behaves as a line phase, and such compositions are therefore challenging to synthesize. Here, a series of highly nonstoichiometric Y3+xAl5-xO12 with 0 ≤ x ≤ 0.40 is reported, corresponding to ≤20% of the AlO6 sublattice substituted by Y3+, synthesized by advanced melt-quenching techniques. This impacts the up-conversion luminescence of Yb3+/Er3+-doped systems, whose yellow-green emission differs from the red-orange emission of their stoichiometric counterparts. In contrast, the YAG:Ce3+ system has a different structural response to nonstoichiometry and its down-conversion emission is only weakly affected. Analogous highly nonstoichiometric systems should be obtainable for a range of garnet materials, demonstrated here by the synthesis of Gd3.2Al4.8O12 and Gd3.2Ga4.8O12. This opens pathways to property tuning by control of host stoichiometry, and the prospect of improved performance or new applications for garnet-type materials.Financial support was provided by the ANR-18-CE08-0012 PERSIST and ANR-20-CE08-0007 CAPRE projects of the French National Research Agency (ANR) and the CNRS, the I+D+I Grants PID2021-122328OB-100 funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”. PhD studentships for WC and XF were financed by the Chinese Scholarship Council (project numbers 201808450100 and 202008450026). Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. EXAFS beamtime was provided by the SOLEIL synchrotron (Gif-sur-Yvette, France) under project 99210047. The project benefitted from the microscopy facilities of the Platform MACLE-CVL which was co-funded by the European Union and Centre-Val de Loire Region (FEDER).Peer reviewe

Novel phosphate–phosphonate hybrid nanomaterials applied to biology

International audienceA new process for preparing oligonucleotide arrays is described that uses surface grafting chemistry which is fundamentally different from the electrostatic adsorption and organic covalent binding methods normally employed. Solid supports are modified with a mixed organic/inorganic zirconium phosphonate monolayer film providing a stable, well-defined interface. Oligonucleotide probes terminated with phosphate are spotted directly to the zirconated surface forming a covalent linkage. Specific binding of terminal phosphate groups with minimal binding of the internal phosphate diesters has been demonstrated. On the other hand, the reaction of a bisphosphonate bone resorption inhibitor (Zoledronate) with calcium deficient apatites (CDAs) was studied as a potential route to local drug delivery systems active against bone resorption disorders. A simple mathematical model of the Zoledronate/CDA interaction was designed that correctly described the adsorption of Zoledronate onto CDAs. The resulting Zoledronate-loaded materials were found to release the drug in different phosphate-containing media, with a satisfactory agreement between experimental data and the values predicted from the model

NMR parameters in alkali, alkaline earth and rare earth fluorides from first principle calculations

19F isotropic chemical shifts for alkali, alkaline earth and rare earth of column 3 basic fluorides are measured and the corresponding isotropic chemical shieldings are calculated using the GIPAW method. When using PBE exchange correlation functional for the treatment of the cationic localized empty orbitals of Ca2+, Sc3+ (3d) and La3+ (4f), a correction is needed to accurately calculate 19F chemical shieldings. We show that the correlation between experimental isotropic chemical shifts and calculated isotropic chemical shieldings established for the studied compounds allows to predict 19F NMR spectra of crystalline compounds with a relatively good accuracy. In addition, we experimentally determine the quadrupolar parameters of 25Mg in MgF2 and calculate the electric field gradient of 25Mg in MgF2 and 139La in LaF3 using both PAW and LAPW methods. The orientation of the EFG components in the crystallographic frame, provided by DFT calculations, is analysed in term of electron densities. It is shown that consideration of the quadrupolar charge deformation is essential for the analysis of slightly distorted environments or highly irregular polyhedra.Comment: 18 pages, 8 figures, 4 tables and ES