Impact of Mineralizing Agents on Aluminum Distribution and Acidity of ZSM-5 Zeolites.

Intensive research on improving the catalytic properties of zeolites is focused on modulating their acidity and the distribution of associated Al sites. Herein, by studying a series of ZSM-5 zeolites over a broad range of Al content, we demonstrate how the nature of the mineralizing agent (F- or OH- ) used in hydrothermal syntheses directly impacts Al sites distribution. The proportions of Al sites, probed by 27 Al NMR, depend on the Si/Al ratio for F- , but remain identical for OH- (from Si/Al=30 to 760). This leads to contrasting variations in weak and strong acidities. Such opposite effect of mineralizers is explained by the spatial location of negative charges and the resulting balance between short- and long-range electrostatic interactions. This understanding paves the way for additional and simple opportunities to control zeolites' acidity.

Stabilization of the Trigonal Langasite Structure in Ca3Ga2-2xZnxGe4+xO14 (0 ≤ x ≤ 1) with Partial Ordering of Three Isoelectronic Cations Characterized by a Multitechnique Approach.

Crystallization of oxide glasses rich in Zn2+, Ga3+, and Ge4+ is of interest for the synthesis of new transparent ceramics. In this context, we report the identification and detailed structural characterization of a new solid solution Ca3Ga2-2xZnxGe4+xO14 (0 ≤ x ≤ 1). These compounds adopt the trigonal langasite structure type, offering three possible crystallographic sites for the coordination of isoelectronic Zn2+, Ga3+, and Ge4+. We used neutron diffraction to determine distributions of Ga3+/Ge4+ and Zn2+/Ge4+ in the simpler end members Ca3Ga2Ge4O14 and Ca3ZnGe5O14, while for the complex intermediate member Ca3GaZn0.5Ge4.5O14, we used an original approach combining quantitative 2D analysis of atomic-resolution STEM-EDS maps with neutron diffraction. This revealed that, across the solid solution, the tetrahedral D sites remain fully occupied by Ge4+, while Zn2+, Ga3+, and the remaining Ge4+ are shared between octahedral B- and tetrahedral C sites in proportions that depend upon their relative ionic radii. The adoption of the trigonal langasite structure by glass-crystallized Ca3ZnGe5O14, a compound that was previously observed only in a distorted monoclinic langasite polymorph, is attributed to substantial disorder between Zn2+ and Ge4+ over the B and C sites. The quantitative 2D refinement of atomic-resolution STEM-EDS maps is applicable to a wide range of materials where multiple cations with poor scattering contrast are distributed over different crystallographic sites in a crystal structure.

Sodium Site Exchange and Migration in a Polar Stuffed-Cristobalite Framework Structure.

The study of ionic dynamics in solids is essential to understanding and developing modern energy technologies. Here we study the ionic dynamics of orthorhombic Na2MgSiO4, an interesting case of a polar stuffed-cristobalite-type structure that contains two inequivalent Na sites within the channels of the magnesium silicate tetrahedral framework. Its preparation by a solid-state reaction method favors the presence of ∼2% of Na vacancies, converting it into a pure Na ionic conductor with an optimized ionic conductivity of ∼10-5 S cm-1 at 200 °C. The macroscopic migration has been characterized through impedance spectroscopy and molecular dynamics simulation, which proves the pure Na ionic character of the compound through hopping between Na1 and Na2 sites, forming three-dimensional migration zigzag-shaped paths. High-resolution solid-state 23Na magic-angle-spinning (MAS) NMR spectroscopy is employed to characterize the local structure and microscopic dynamics of Na-ion transport in Na2MgSiO4. Remarkably, variable-temperature 23Na MAS NMR and two-dimensional exchange spectroscopy evidence for the first time a Na site exchange phenomenon at room temperature, which further triggers Na ionic conduction at elevated temperatures.

Iterative baseline correction algorithm for dead time truncated one-dimensional solid-state MAS NMR spectra.

We present an algorithm suitable for automatically correcting rolling baseline coming from time-domain truncation induced by the dead time in pulse-acquire one-dimensional MAS NMR spectra. It relies on an iterative estimation of the baseline restricted in the time-domain by the dead time duration combined with a histogram filter allowing adaptive selection of the baseline points. This method does not make any assumption regarding the NMR resonances line shapes or widths and does not modify the acquired free induction decay points. This makes it suitable for accurate deconvolution and quantification of single-pulse MAS NMR spectra. The baseline correction accuracy is evaluated on synthetic solid-state spectra of 19F, 71Ga, and 23Na by comparing the fitted baseline to the theoretical one. The versatility of the algorithm is also exemplified on three additional solid-state spectra of 23Na and 71Ga. The algorithm is made available to the community through a user-friendly standalone Matlab® application.

Structure and dynamics of high-temperature strontium aluminosilicate melts.

We report the study of high-temperature melts (1600-2300 °C) and related glasses in the SrO-Al2O3-SiO2 phase diagram considering three series: (i) depolymerized ([SrO]/[Al2O3] = 3); (ii) fully polymerized ([SrO]/[Al2O3] = 1); and (iii) per-aluminous ([SrO]/[Al2O3] < 1). By considering the results from high-temperature 27Al NMR and high-temperature neutron diffraction, we demonstrate that the structure of the polymerized melts is controlled by a close-to-random distribution of Al and Si in the tetrahedral sites, while the depolymerized melts show smaller rings with a possible loss of non-bridging oxygens on AlO4 units during cooling for high-silica compositions. A few five-fold coordinated VAl sites are present in all compositions, except per-aluminous ones where high amounts of high-coordinated aluminium are found in glasses and melts with complex temperature dependence. In high-temperature melts, strontium has a coordination number of 8 or less, i.e. less than in the corresponding glasses. The dynamics of high-temperature melts were studied from 27Al NMR relaxation and compared to macroscopic shear viscosity data. These methods provide correlation times in close agreement. At very high temperatures, the NMR correlation times can be related to the oxygen self-diffusion coefficient, and we show a decrease of the latter with increasing Si/(Al + Si) ratios for polymerized melts with no compositional dependence for depolymerized ones. The dominant parameter controlling the temperature dependence of the aluminum environment of all melts is the distribution of Al-(OSi)p(OAl)(4-p) units.

Local environments of boron heteroatoms in non-crystalline layered borosilicates.

Boron heteroatom distributions are shown to be significantly different in two closely related layered borosilicates synthesized with subtly different alkylammonium surfactant species. The complicated order and disorder near framework boron sites in both borosilicates were characterized at the molecular level by using a combination of multi-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy techniques and first-principles calculations. Specifically, two-dimensional (2D) solid-state J-mediated (through-bond) (11)B{(29)Si} NMR analyses provide direct and local information on framework boron sites that are covalently bonded to silicon sites through bridging oxygen atoms. The resolution and identification of correlated signals from distinct (11)B-O-(29)Si site pairs reveal distinct distributions of boron heteroatoms in layered borosilicate frameworks synthesized with the different C16H33N(+)Me3 and C16H33N(+)Me2Et structure-directing surfactant species. The analyses establish that boron atoms are distributed non-selectively among different types of silicon sites in the layered C16H33N(+)Me3-directed borosilicate framework, whereas boron atoms are preferentially incorporated into incompletely condensed Q(3)-type sites in the C16H33N(+)Me2Et-directed borosilicate material. Interestingly, framework boron species appear to induce framework condensation of their next-nearest-neighbor silicon sites in the C16H33N(+)Me3-directed borosilicate. By comparison, the incorporation of boron atoms is found to preserve the topology of the C16H33N(+)Me2Et-directed borosilicate frameworks. The differences in boron site distributions and local boron-induced structural transformations for the two surfactant-directed borosilicates appear to be due to different extents of cross-linking of the siliceous frameworks. The molecular-level insights are supported by density functional theory (DFT) calculations, which show the distinct influences of boron atoms on the C16H33N(+)Me3- and C16H33N(+)Me2Et-directed borosilicate frameworks, consistent with the experimental observations.

Exploring electrolyte organization in supercapacitor electrodes with solid-state NMR.

Supercapacitors are electrochemical energy-storage devices that exploit the electrostatic interaction between high-surface-area nanoporous electrodes and electrolyte ions. Insight into the molecular mechanisms at work inside supercapacitor carbon electrodes is obtained with (13)C and (11)B ex situ magic-angle spinning nuclear magnetic resonance (MAS-NMR). In activated carbons soaked with an electrolyte solution, two distinct adsorption sites are detected by NMR, both undergoing chemical exchange with the free electrolyte molecules. On charging, anions are substituted by cations in the negative carbon electrode and cations by anions in the positive electrode, and their proportions in each electrode are quantified by NMR. Moreover, acetonitrile molecules are expelled from the adsorption sites at the negative electrode alone. Two nanoporous carbon materials were tested, with different nanotexture orders (using Raman and (13)C MAS-NMR spectroscopies), and the more disordered carbon shows a better capacitance and a better tolerance to high voltages.

Topological, geometric, and chemical order in materials: insights from solid-state NMR.

Unlike the long-range order of ideal crystalline structures, local order is an intrinsic characteristic of real materials and often serves as the key to the tuning of their properties and their final applications. Although researchers can easily assess local ordering using two-dimensional imaging techniques with resolution that approaches the atomic level, the diagnosis, description, and qualification of local order in three dimensions is much more challenging. Solid-state nuclear magnetic resonance (NMR) and its panel of continually developing instruments and methods enable the local, atom-selective characterization of structures and assemblies ranging from the atomic to the nanometer length scales. By making use of the indirect J-coupling that distinguishes chemical bonds, researchers can use solid-state NMR to characterize a variety of materials, ranging from crystalline compounds to amorphous or glassy materials. In crystalline compounds showing some disorder, we describe and distinguish the contributions of topology, geometry, and local chemistry in ways that are consistent with X-ray diffraction and computational approaches. We give examples of materials featuring either chemical disorder in a topological order or topological disorder with local chemical order. For glasses, we show that we can separate geometric and chemical contributions to the local order by identifying structural motifs with a viewpoint that extends from the atomic scale up to the nanoscale. As identified by solid state NMR, the local structure of amorphous materials or glasses consists of well-identified structural entities up to at least the nanometer scale. Instead of speaking of disorder, we propose a new description for these structures as a continuous assembly of locally defined structures, an idea that draws on the concept of locally favored structures (LFS) introduced by Tanaka and coworkers. This idea provides a comprehensive picture of amorphous structures based on fluctuations of chemical composition and structure over different length scales. We hope that these local or molecular insights will allow researchers to consider key questions related to nucleation and crystallization, as well as chemically (spinodal decomposition) or density-driven (polyamorphism) phase separation, which could lead to future applications in a variety of materials.

La10W2O21: an anion-deficient fluorite-related superstructure with oxide ion conduction.

The crystal structure of La10W2O21, which has to be reformulated (La5.667W0.333)LaWO14□2, is best described, on average, by a 2 × 2 × 2 anion-deficient fluorite-related superstructure cubic cell, with space group F4 3m, Z = 4, and a = 11.17932(6) Å, similar to Y7ReO14--δ. The 32 cations are distributed with lanthanum on the 4a-site, tungsten on the 4b-site, and a partial occupancy of the 24g-site by La (94%) and W. The 56 oxygen atoms occupy four 16e-sites, three of them fully and with an occupancy of 1/2 for the fourth one. Others M10W2O21 (M = Er, Y) adopt a 3 × 2 × 2 fluorite superstructure with W in octahedral sites, whereas W is mainly in tetrahedral sites in La10W2O21. Several powerful techniques such as crystal image furnace synthesis, (139)La nuclear magnetic resonance (NMR) and convergent beam electron diffraction (CBED) were used to achieve our results. Transmission electron microscopy (microdiffraction, CBED, and Tanaka patterns) brought us the real symmetry, showing that indeed classical cubic twinning along the 3-fold axis does take place. The surprising La/W mixed site is nicely confirmed by (139)La NMR. This compound exhibits interesting fast oxide ion conducting properties, comparable with LAMOX (Lacorre et al. Nature 2000, 404, 856-858) at low temperature. As opposed to many ionic conductors, no temperature structural transition is observed. Its conductivity is about 6.4 × 10(-4) S·cm(-1) at 700 °C.

NMR parameters in column 13 metal fluoride compounds (AlF3, GaF3, InF3 and TlF) from first principle calculations.

The relationship between the experimental (19)F isotropic chemical shift and the (19)F isotropic shielding calculated using the gauge including projector augmented-wave (GIPAW) method with PBE functional is investigated in the case of GaF3, InF3, TlF and several AlF3 polymorphs. It is shown that the linear correlation between experimental and DFT-PBE calculated values previously established on alkali, alkaline earth and rare earth of column 3 basic fluorides (Sadoc et al., Phys. Chem. Chem. Phys. 13 (2011) 18539-18550) remains valid in the case of column 13 metal fluorides, indicating that it allows predicting (19)F solid state NMR spectra of a broad range of crystalline fluorides with a relatively good accuracy. For the isostructural α-AlF3, GaF3 and InF3 phases, PBE-DFT geometry optimization leads to noticeably overbended M-F-M bond angles and underestimated (27)Al, (71)Ga and (115)In calculated quadrupolar coupling constants. For the studied compounds, whose structures are built of corner shared MF6 octahedra, it is shown that the electric field gradient (EFG) tensor at the cationic sites is not related to distortions of the octahedral units, in contrast to what previously observed for isolated AlF6 octahedra in fluoroaluminates.

Synthesis and structure determination of CaSi(1/3)B(2/3)O(8/3): a new calcium borosilicate.

This article reports on the identification, synthesis, and in-situ structure determination of a new crystalline calcium borosilicate compound of composition CaSi(1/3)B(2/3)O(8/3). Synthesis was carried out by complete crystallization on annealing from a corresponding glassy composition in the widely studied CaO-SiO2-B2O3 ternary system. The crystallographic structure was determined ab initio using electron diffraction information and the charge flipping algorithm performed on synchrotron and neutron powder diffraction data collected in situ at high temperature. CaSi(1/3)B(2/3)O(8/3) is found to crystallize in the Pna2(1) (no. 33) orthorhombic space group, with a = 12.1025(4) Å, b = 5.2676(1) Å, c = 3.7132(1) Å, and V = 236.71(1) Å(3) at 650 °C. Solid-state (29)Si and (11)B NMR experiments have confirmed the existence of finite chains along the c axis, formed by corner-sharing SiO4 tetrahedra and BO3 units. Silicon and boron species share a crystallographic site, and the Si/B distribution induces different possible arrangements of the chains which are discussed in light of DFT calculations. At room temperature, the existence of a superstructure, resulting from the ordering within nanoscale domains, was explored by transmission electron microscopy.

Nature and structure of aluminum surface sites grafted on silica from a combination of high-field aluminum-27 solid-state NMR spectroscopy and first-principles calculations.

The determination of the nature and structure of surface sites after chemical modification of large surface area oxides such as silica is a key point for many applications and challenging from a spectroscopic point of view. This has been, for instance, a long-standing problem for silica reacted with alkylaluminum compounds, a system typically studied as a model for a supported methylaluminoxane and aluminum cocatalyst. While (27)Al solid-state NMR spectroscopy would be a method of choice, it has been difficult to apply this technique because of large quadrupolar broadenings. Here, from a combined use of the highest stable field NMR instruments (17.6, 20.0, and 23.5 T) and ultrafast magic angle spinning (>60 kHz), high-quality spectra were obtained, allowing isotropic chemical shifts, quadrupolar couplings, and asymmetric parameters to be extracted. Combined with first-principles calculations, these NMR signatures were then assigned to actual structures of surface aluminum sites. For silica (here SBA-15) reacted with triethylaluminum, the surface sites are in fact mainly dinuclear Al species, grafted on the silica surface via either two terminal or two bridging siloxy ligands. Tetrahedral sites, resulting from the incorporation of Al inside the silica matrix, are also seen as minor species. No evidence for putative tri-coordinated Al atoms has been found.

Synthesis and structure resolution of RbLaF4.

The synthesis and structure resolution of RbLaF(4) are described. RbLaF(4) is synthesized by solid-state reaction between RbF and LaF(3) at 425 °C under a nonoxidizing atmosphere. Its crystal structure has been resolved by combining neutron and synchrotron powder diffraction data refinements (Pnma,a = 6.46281(2) Å, b = 3.86498(1) Å, c = 16.17629(4) Å, Z = 4). One-dimensional (87)Rb, (139)La, and (19)F MAS NMR spectra have been recorded and are in agreement with the proposed structural model. Assignment of the (19)F resonances is performed on the basis of both (19)F-(139)La J-coupling multiplet patterns observed in a heteronuclear DQ-filtered J-resolved spectrum and (19)F-(87)Rb HMQC MAS experiments. DFT calculations of both the (19)F isotropic chemical shieldings and the (87)Rb, (139)La electric field gradient tensors using the GIPAW and PAW methods implemented in the CASTEP code are in good agreement with the experimental values and support the proposed structural model. Finally, the conductivity of RbLaF(4) and luminescence properties of Eu-doped LaRbF(4) are investigated.

Lithium diffusion in lithium nitride by pulsed-field gradient NMR.

Lithium self-diffusion coefficients are measured for the first time using (7)Li Pulsed-Field Gradient Nuclear Magnetic Resonance (PFG-NMR) in a crystalline inorganic powder of α-Li(3)N between 534 K and 774 K. The diffusion of lithium cations is anisotropic, and the activation energy for the diffusion within the Li(2)N layers was found to be 0.150 ± 0.009 eV.

Effects of the orientation of the 23Na-29Si dipolar vector on the dipolar mediated heteronuclear solid state NMR correlation spectrum of crystalline sodium silicates.

Dipolar-Heteronuclear Multiple Quantum Correlation (D-HMQC) experiment based on SR4(2)(1) recoupling was shown as a very efficient probe of spatial proximities in ordered or disordered materials. As crystalline sodium silicates have been extensively studied using 1D and 2D MAS NMR experiments and DFT calculations, they have been used as candidate model systems to perform this D-HMQC experiment. In this work, we demonstrate that the combination of (29)Si and (23)Na NMR at high magnetic field and DFT calculations makes it possible to revisit the assignment of the NMR signature of the δ-Na(2)Si(2)O(5) polymorph. A D-HMQC experiment performed on this crystalline sample reveals lineshape distortions on the (23)Na powder patterns extracted from the 2D correlation. Numerical simulations showed that these distortions result from an effect of the relative orientation between the (23)Na quadrupolar tensor and the (23)Na-(29)Si dipolar vector at the origin of the magnetization transfer.

A solid-state NMR study of C(70): a model molecule for amorphous carbons.

We show that natural abundance, solid-state MAS-NMR (13)C INADEQUATE spectra can be recorded for crystallized C(70), using the through-bond J-coupling for the magnetization transfer. The effect of strong J-coupling can be lessened at high magnetic fields, allowing the observation of cross-peaks between close resonances. DFT calculations of the chemical shifts show an excellent agreement with the experimental values. A correlation is observed between the average CCC bond angles and the (13)C chemical shift, offering a way to understand the dispersion of (13)C chemical shifts in nanoporous activated carbons in terms of local deviations from planarity.

Structure resolution of Ba5Al3F19 and investigation of fluorine ion dynamics by synchrotron powder diffraction, variable-temperature solid-state NMR, and quantum computations.

The room temperature structure of Ba(5)Al(3)F(19) has been solved using electron microscopy and synchrotron powder diffraction data. One-dimensional (1D) (27)Al and ultrafast magic-angle-spinning (MAS) (19)F NMR spectra have been recorded and are in agreement with the proposed structural model for Ba(5)Al(3)F(19). The (19)F isotropic chemical shift and (27)Al quadrupolar parameters have been calculated using the CASTEP code from the experimental and density functional theory geometry-optimized structures. After optimization, the calculated NMR parameters of both the (19)F and (27)Al nuclei show improved consistency with the experimental values, demonstrating that the geometry optimization step is necessary to obtain more accurate and reliable structural data. This also enables a complete and unambiguous assignment of the (19)F MAS NMR spectrum of Ba(5)Al(3)F(19). Variable-temperature 1D MAS (19)F NMR experiments have been carried out, showing the occurrence of fluorine ion mobility. Complementary insights were obtained from both two-dimensional (2D) exchange and 2D double-quantum dipolar recoupling NMR experiments, and a detailed analysis of the anionic motion in Ba(5)Al(3)F(19) is proposed, including the distinction between reorientational processes and chemical exchange involving bond breaking and re-formation.

Characterization and properties of novel gallium-doped calcium phosphate ceramics.

Addition of a gallium (Ga) precursor in the typical reaction protocols used for the preparation of ß-tricalcium phosphate (ß-TCP) led to novel Ga-doped ß-TCP ceramics with rhombohedral structures (R3c space group). From the refinement of their X-ray diffraction patterns, it was found that the incorporation of Ga in the ß-TCP network occurs by substitution of one of the five calcium (Ca) sites, while occupation of another Ca site decreases in inverse proportion to the Ga content in the structure. The Ga local environment and the modification of the phosphorus environments due to the Ga/Ca substitution in Ga-doped ß-TCP compounds are probed using (31)P and (71)Ga magic-angle spinning NMR. A decrease of the unit cell volume is observed with increasing Ga content, together with improved mechanical properties. Indeed, the compressive strength of these new bioceramics is enhanced in direct proportion of the Ga content, up to a 2.6-fold increase as compared to pure ß-TCP.

1H and 19F ultra-fast MAS double-quantum single-quantum NMR correlation experiments using three-spin terms of the dipolar homonuclear Hamiltonian.

Measuring internuclear distances through dipolar interaction is a major challenge for solid-state nuclear magnetic resonance (NMR) spectroscopy. Obtaining reliable interatomic distances provides an access to the local structure in ordered or disordered solids. We show that at magic angle spinning (MAS) frequencies larger than ca. 50 kHz, some of the three-spin terms of the homogeneous homonuclear dipolar Hamiltonian can be used to promote the creation of double-quantum coherences between neighbouring (1)H or (19)F spins without using dipolar recoupling pulse sequences in the Dipolar Homonuclear Homogeneous Hamiltonian (DH(3)) double-quantum/single-quantum correlation experiment. This makes it possible to probe inter-nuclear spatial proximity with limited risk of probe or sample damage from radio-frequency (RF) irradiation, and is fully appropriate for fast repetition rate offering sensitivity gains in favourable cases. Experimental demonstrations are supported by multi-spin numerical simulations, which points to new possibilities for the characterization of spin-system geometries.

Broadband inversion for MAS NMR with single-sideband-selective adiabatic pulses.

We explain how and under which conditions it is possible to obtain an efficient inversion of an entire sideband family of several hundred kHz using low-power, sideband-selective adiabatic pulses, and we illustrate with some experimental results how this framework opens new avenues in solid-state NMR for manipulating spin systems with wide spinning-sideband (SSB) manifolds. This is achieved through the definition of the criteria of phase and amplitude modulation for designing an adiabatic inversion pulse for rotating solids. In turn, this is based on a framework for representing the Hamiltonian of the spin system in an NMR experiment under magic angle spinning (MAS). Following earlier ideas from Caravatti et al. [J. Magn. Reson. 55, 88 (1983)], the so-called "jolting frame" is used, which is the interaction frame of the anisotropic interaction giving rise to the SSB manifold. In the jolting frame, the shift modulation affecting the nuclear spin is removed, while the Hamiltonian corresponding to the RF field is frequency modulated and acquires a spinning-sideband pattern, specific for each crystallite orientation.