Multinuclear Solid-state Nuclear Magnetic Resonance Research Articles

Cation Distribution and Anion Transport in the La3Ga5-xGe1+xO14+0.5x Langasite Structure.

Exploration of compositional disorder using conventional diffraction-based techniques is challenging for systems containing isoelectronic ions possessing similar coherent neutron scattering lengths. Here, we show that a multinuclear solid-state Nuclear Magnetic Resonance (NMR) approach provides compelling insight into the Ga3+/Ge4+ cation distribution and oxygen anion transport in a family of solid electrolytes with langasite structure and La3Ga5-xGe1+xO14+0.5x composition. Ultrahigh field 71Ga Magic Angle Spinning (MAS) NMR experiments acquired at 35.2 T offer striking resolution enhancement, thereby enabling clear detection of Ga sites in different coordination environments. Three-connected GaO4, four-connected GaO4 and GaO6 polyhedra are probed for the parent La3Ga5GeO14 structure, while one additional spectral feature corresponding to the key (Ga,Ge)2O8 structural unit which forms to accommodate the interstitial oxide ions is detected for the Ge4+-doped La3Ga3.5Ge2.5O14.75 phase. The complex spectral line shapes observed in the MAS NMR spectra are reproduced very accurately by the NMR parameters computed for a symmetry-adapted configurational ensemble that comprehensively models site disorder. This approach further reveals a Ga3+/Ge4+ distribution across all Ga/Ge sites that is controlled by a kinetically governed cation diffusion process. Variable temperature 17O MAS NMR experiments up to 700 °C importantly indicate that the presence of interstitial oxide ions triggers chemical exchange between all oxygen sites, thereby enabling atomic-scale understanding of the anion diffusion mechanism underpinning the transport properties of these materials.

Sustainable Composite Materials Based on Carnauba Wax and Montmorillonite Nanoclay for Energy Storage.

Sustainable composite materials, including carnauba wax, can store energy in the form of latent heat, and containing the wax may allow form-stable melting and crystallization cycles to be performed. Here, it is shown that carnauba wax in the molten state and the abundant nanoclay montmorillonite form stable composites with mass ratios of 50-70% (w/w). Transmission electron microscopy analysis reveals the inhomogeneous distribution of the nanoclay in the wax matrix. Analyses with infrared and multinuclear solid-state nuclear magnetic resonance (NMR) spectroscopy prove the chemical inertness of the composite materials during preparation. No new phases are formed according to studies with powder X-ray diffraction. The addition of the nanoclay increases the thermal conductivity and prevents the leakage of the phase change material, as well as reducing the time intervals of the cycle of accumulation and the return of heat. The latent heat increases in the row 69.5 ± 3.7 J/g, 95.0 ± 2.5 J/g, and 107.9 ± 1.7 J/g for the composite materials containing resp. 50%, 60% and 70% carnauba wax. Analysis of temperature-dependent 13C cross-polarization solid-state NMR spectra reveal the enhanced amorphization and altered molecular dynamics of the carnauba wax constituents in the composite materials. The amorphization also defines changes in the thermal transport mechanism in the composites compared to pure wax at elevated temperatures.

Densification of Sodium Borosilicate Glasses at Ambient Temperature: Structural Investigations by Solid-State Nuclear Magnetic Resonance and Raman Scattering

Alkali-borosilicate glasses with composition (80-x)SiO2-xB2O3-20Na2O (10 ≤ x ≤ 30) were subjected to a 25 GPa compression and decompression at room temperature, resulting in density increases between 1.4% and 1.9%. The structural changes associated with this process have been investigated and compared with uncompressed glasses having the same thermal history. Systematic trends are identified, using Raman scattering and multinuclear solid-state Nuclear Magnetic Resonance (ssNMR). Perhaps counterintuitively, pressurization tends to increase the concentration of three-coordinated boron species (B(III) units) at the expense of four-coordinated boron (B(IV) units). 23Na NMR spectra show a systematic shift toward higher frequencies in the pressurized glasses, consistent with shorter average Na-O distances. The results are consistently explained in terms of a breakage of Si-O-B4 linkages resulting in the formation of nonbridging oxygen species. Pressure effects on the spectra are reversed by annealing the glasses at their respective glass transition temperatures.

Valorizing the carbon byproduct of methane pyrolysis in batteries

While low-cost natural gas remains abundant, the energy content of this fuel can be utilized without greenhouse gas emissions through the production of molecular hydrogen and solid carbon via methane pyrolysis. In the absence of a carbon tax, methane pyrolysis is not economically competitive with current hydrogen production methods unless the carbon byproducts can be valorized. In this work, we assess the viability of the carbon byproduct produced from methane pyrolysis in molten salts as high-value-added anode or conductive additive for secondary Li-ion and Na-ion batteries. Raman characterization and electrochemical differential capacity analysis demonstrate that the use of molten salt mixtures with catalytically-active FeCl3-or MnCl2 result in more graphitic carbon co-products. These graphitic carbons exhibit the best electrochemical performance (up to 272 mAh/g of reversible capacity) when used as Li-ion anodes. For all carbon samples studied here, disordered carbon domains and retained salt species trapped and/or intercalated into the carbon structure were identified by X-ray photoelectron and multinuclear solid-state nuclear magnetic resonance spectroscopy. The latter lead to reduced electrochemical activity and reversibility, and poorer rate performance compared to commercial carbon anodes. The electronic conductivity of the pyrolyzed carbons is found to be highly dependent on their purity, with the purest carbon exhibiting an electronic conductivity nearly on par with that of commercial carbon additives. These findings suggest that more effective removal of the salt catalyst could enable applications of these carbons in secondary batteries, providing a financial incentive for the large-scale implementation of methane pyrolysis for “low-carbon” hydrogen production.

Acidic Centers on the Surface of a Crystalline α-Sn(IV) Phosphate Characterized by the Solid-State 1H, 2H, 31P, and 119Sn MAS NMR Techniques.

A layered crystalline phosphate α-Sn(HPO4)2·H2O (1), prepared and characterized in the present study by the multinuclear solid-state nuclear magnetic resonance (NMR), powder X-ray diffraction, and thermogravimetric analysis techniques, was treated with D2O and HOD imitating the reaction conditions in a water medium. The 2H solid-echo magic angle spinning NMR spectra of the products have revealed on their surface low mobile water molecules and hydronium ions, forming a structure close to the Zundel cation, [D2O···D-OD2]+. All the deuterons in the hydronium ions are tangled by hydrogen bonds with the water and the surface phosphate groups and stabilized by ionic interactions.

Solid-State Nuclear Magnetic Resonance of Triple-Cation Mixed-Halide Perovskites.

Mixed-cation lead mixed-halide perovskites are the best candidates for perovskite-based photovoltaics, thanks to their higher efficiency and stability compared to the single-cation single-halide parent compounds. TripleMix (Cs0.05MA0.14FA0.81PbI2.55Br0.45 with FA = formamidinium and MA = methylammonium) is one of the most efficient and stable mixed perovskites for single-junction solar cells. The microscopic reasons why triple-cation perovskites perform so well are still under debate. In this work, we investigated the structure and dynamics of TripleMix by exploiting multinuclear solid-state nuclear magnetic resonance (SSNMR), which can provide this information at a level of detail not accessible by other techniques. 133Cs, 13C, 1H, and 207Pb SSNMR spectra confirmed the inclusion of all ions in the perovskite, without phase segregation. Complementary measurements showed a peculiar longitudinal relaxation behavior for the 1H and 207Pb nuclei in TripleMix with respect to single-cation single-halide perovskites, suggesting slower dynamics of both organic cations and halide anions, possibly related to the high photovoltaic performances.

Pressure-induced structural evolution in boron-bearing model rhyolitic glasses under compression: Implications for boron isotope compositions and properties of deep melts in Earth’s interior

The pressure-induced structural evolutions of boron-bearing model rhyolitic melts under high pressures enable to infer the detailed geochemical processes (melting and fluid-rock-melt interactions) occurring in Earth interiors and to control the melt properties (viscosity and the boron isotope composition, δ11B) of complex magmatic melts, providing insights into the boron cycle toward the deeper part of the upper mantle (∼10 GPa). Despite the importance, the structures of multicomponent boron-bearing silicate melts above 3 GPa are currently unavailable. Here, we explore the structures, particularly, coordination transformation of constituent elements in boron-bearing nepheline and albite glasses – a model rhyolitic melts - upon compression to a depth of ∼270 km (∼9.2 GPa) in the mantle using multi-nuclear solid-state nuclear magnetic resonance (NMR) spectroscopy. The results showed that the conversion of [3]B into [4]B is prominent upon compression up to 6 GPa. In contrast, the formation of [5,6]Al is accompanied by the formation of oxygen tricluster above 6 GPa, where all the nonbridging oxygens are consumed. We quantify how the melt composition affects tendency to form highly coordinated B, Al, and Si upon compression. Particularly, the increase in the [4]B population tends to be larger for the glasses with low Si content as pressure increases to 9.2 GPa. We reveal the relationship between such structural adaptations of the compressed melts at high pressure and the melt properties, including viscosity and element partition coefficient in boron-bearing melts. The current NMR results also unravel the structural origins of 11B/10B ratios in rhyolitic melts at high pressure. Considering a preferential partitioning of 10B to [4]B, an increase in [4]B population in the melts leads to an pressure-induced enrichment of 10B. As the increase in Si/B ratio in the melts tends to decrease the pressure-induced increase in [4]B fraction, the contribution of boron coordination transformation on the 11B/10B ratios in silicate melt would be somewhat minor in deep mantle melts with increasing Si content. The detailed boron environments in rhyolitic melts at high pressure yield useful constraints for the isotope composition (11B/10B) of dense mantle melts, thereby enabling quantification of deep boron cycle.

Varying Synthesis Conditions and Comprehensive Characterization of Fluorine-Doped Hydroxyapatite Nanocrystals in a Simulated Body Fluid

Bone supports animal bodies, is the place where blood is produced, and is essential for the immune system, among other important functions. The dominant inorganic component in bone is hydroxyapatite (Hap), the structure and dynamics of which still pose many unsolved puzzles. An updated understanding of HAp is of great significance to osteology, dentistry, and the development of artificial bone and other biomaterials. In this work, HAp nanoparticles were synthesized with the wet chemical precipitation method and their structure and morphologies were controlled by varying pH and adding fluoride ions by two different routes: (1) fluoride ions were added during synthesis, and (2) fluoride ions were introduced after the samples were synthesized by soaking the samples in solutions with fluoride ions. XRD and HRTEM were employed to confirm the composition and structure, while various multinuclear (1H, 19F, 31P) solid-state nuclear magnetic resonance (NMR) methods including 1D single pulse, cross-polarization under magic-angle spinning (CPMAS), and 2D heteronuclear correlation (HETCOR) were used to characterize the structure, morphology, and dynamics, validating the general core-shell morphology in these F-HAp samples. It was found that all hydroxide ions were substituted when the fluoride ion concentration was above 0.005 M. An NMR peak corresponding to water structure emerged and the bulk water peak was shifted upfield, indicating that fluoride substitution modifies both the crystalline core and the amorphous shell of F-HAp nanoparticles. With the second route of fluoride substitution, increases in soaking time or fluoride ion concentration could increase fluoride substitution in HAp, but could not achieve complete substitution. Finally, with 1H-31P CPMAS and HETCOR, it was established that there are two types of phosphorous, one in the crystalline core (PO43−) and the other in the amorphous shell (HPO42−). These results are valuable for clarifying the fluoride substitution mechanism in HAp in biomaterials or in organisms, and provide insights for developing next generation replacement materials for bone, tooth, or coating films, drug delivery systems, etc.

Refined Structures of O-Phospho-l-serine and Its Calcium Salt by New Multinuclear Solid-State NMR Crystallography Methods.

O-phospho-l-serine (Pser) and its Ca salt, Ca[O-phospho-l-serine]·H2O (CaPser), play important roles for bone mineralization and were recently also proposed to account for the markedly improved bone-adhesive properties of Pser-doped calcium phosphate-based cements for biomedical implants. However, the hitherto few proposed structural models of Pser and CaPser were obtained by X-ray diffraction, thereby leaving the proton positions poorly defined. Herein, we refine the Pser and CaPser structures by density functional theory (DFT) calculations and contrast them with direct interatomic-distance constraints from two-dimensional (2D) nuclear magnetic resonance (NMR) correlation experimentation at fast magic-angle spinning (MAS), encompassing double-quantum–single-quantum (2Q–1Q) 1H NMR along with heteronuclear 13C{1H} and 31P{1H} correlation NMR experiments. The Pser and CaPser structures before and after refinements by DFT were validated against sets of NMR-derived effective 1H–1H, 1H–31P, and 1H–13C distances, which confirmed the improved accuracy of the refined structures. Each distance set was derived from one sole 2D NMR experiment applied to a powder without isotopic enrichment. The distances were extracted without invoking numerical spin-dynamics simulations or approximate phenomenological models. We highlight the advantages and limitations of the new distance-extraction procedure. Isotropic 1H, 13C, and 31P chemical shifts obtained by DFT calculations using the gauge including projector augmented wave (GIPAW) method agreed very well with the experimental results. We discuss the isotropic and anisotropic 13C and 31P chemical-shift parameters in relation to the previous literature, where most data on CaPser are reported herein for the first time.

Solid-state nuclear magnetic resonance of spin-9/2 nuclei 115 In and 209 Bi in functional inorganic complex oxides.

Indium and bismuth are technologically important elements, in particular as oxides for optoelectronic applications. 115 In and 209 Bi are both I = 9/2 nuclei with high natural abundances and moderately high frequencies but large nuclear electric quadrupole moments. Leveraging the quadrupolar interaction as a measure of local symmetry and polyhedral distortions for these nuclei could provide powerful insights on a range of applied materials. However, the absence of reported nuclear magnetic resonance (NMR) parameters on these nuclei, particularly in oxides, hinders their use by the broader materials community. In this contribution, solid-state 115 In and 209 Bi NMR of three recently discovered quaternary bismuth or indium oxides are reported, supported by density functional theory calculations, numerical simulations, diffraction and additional multinuclear (27 Al, 69,71 Ga, and 121 Sb) solid-state NMR measurements. The compounds LiIn2 SbO6 , BiAlTeO6 , and BiGaTeO6 are measured without special equipment at 9.4 T, demonstrating that wideline techniques such as the QCPMG pulse sequence and frequency-stepped acquisition can enable straightforward extraction of quadrupolar tensor information in I = 9/2 115 In and 209 Bi even in sites with large quadrupolar coupling constants. Relationships are described between the NMR observables and local site symmetry. These are amongst the first reports of the NMR parameters of 115 In, 121 Sb, and 209 Bi in oxides.

Bulk and Surface Chemistry of the Niobium MAX and MXene Phases from Multinuclear Solid-State NMR Spectroscopy.

MXenes, derived from layered MAX phases, are a class of two-dimensional materials with emerging applications in energy storage, electronics, catalysis, and other fields due to their high surface areas, metallic conductivity, biocompatibility, and attractive optoelectronic properties. MXene properties are heavily influenced by their surface chemistry, but a detailed understanding of the surface functionalization is still lacking. Solid-state nuclear magnetic resonance (NMR) spectroscopy is sensitive to the interfacial chemistry, the phase purity including the presence of amorphous/nanocrystalline phases, and the electronic properties of the MXene and MAX phases. In this work, we systematically study the chemistry of Nb MAX and MXene phases, Nb2AlC, Nb4AlC3, Nb2CTx, and Nb4C3Tx, with their unique electronic and mechanical properties, using solid-state NMR spectroscopy to examine a variety of nuclei (1H, 13C, 19F, 27Al, and 93Nb) with a range of one- and two-dimensional correlation, wide-line, high-sensitivity, high-resolution, and/or relaxation-filtered experiments. Hydroxide and fluoride terminations are identified, found to be intimately mixed, and their chemical shifts are compared with other MXenes. This multinuclear NMR study demonstrates that diffraction alone is insufficient to characterize the phase composition of MAX and MXene samples as numerous amorphous or nanocrystalline phases are identified including NbC, AlO6 species, aluminum nitride or oxycarbide, AlF3·nH2O, Nb metal, and unreacted MAX phase. To the best of our knowledge, this is the first study to examine the transition-metal resonances directly in MXene samples, and the first 93Nb NMR of any MAX phase. The insights from this work are employed to enable the previously elusive assignment of the complex overlapping 47/49Ti NMR spectrum of Ti3AlC2. The results and methodology presented here provide fundamental insights on MAX and MXene phases and can be used to obtain a more complete picture of MAX and MXene chemistry, to prepare realistic structure models for computational screening, and to guide the analysis of property measurements.

Transferring Lithium Ions in the Nanochannels of Flexible Metal-Organic Frameworks Featuring Superchaotropic Metallacarborane Guests: Mechanism of Ionic Conductivity at Atomic Resolution.

Metal-organic frameworks (MOFs), owing to their unique architecture, attract consistent attention in the design of high-performance Li battery materials. Here, we report a new category of ion-conducting crystalline materials for all-solid-state electrolytes based on an MIL53(Al) framework featuring a superchaotropic metallacarborane (Li+CoD-) salt and present the first quantitative data on Li+ ion sites, local dynamics, chemical exchange, and the formation of charge-transfer pathways. We used multinuclear solid-state nuclear magnetic resonance (ss-NMR) spectroscopy to examine the mechanism of ionic conductivity at atomic resolution and to elucidate order-disorder processes, framework-ion interactions, and framework breathing during the loading of Li+CoD- species and transfer of Li+ ions. In this way, the MIL53(Al)@LiCoD framework was found to adopt an open-pore conformation accompanied by a minor fraction of narrow-pore channels. The inserted Li+ ions have two states (free and bound), which both exhibit extensive motions. Both types of Li+ ions form mutually communicating chains, which are large enough to enable efficient long-range charge transfer and macroscopic conductivity. The superchaotropic anions undergo high-amplitude uniaxial rotation motions supporting the transfer of Li+ cations along them, while the fluctuations of MOF aromatic linkers support the penetration of Li+ through the channel walls. Our findings provide a detailed atomic-resolution insight into the mechanism of ionic conductivity and thus have significant implications for the design of the next generation of energy-related materials.

A novel proton conducting ionogel electrolyte based on poly(ionic liquids) and protic ionic liquid

As opposed to the traditional hydrogels and organogels, ionogels are a new class of gel family where the liquid phase, percolating through the solid phase, consists of an ionic liquid (IL). The properties inherent in the ionic liquids, including high ionic conductivity, low-volatility and wide electrochemical stability window, are imparted to the ionogels. In this work, a novel proton conducting ionogel polymer electrolyte (iGPE) based on a poly (ionic liquid) Poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide, ([PDADMA][TFSI]) and protic ionic liquid, N,N-diethylmethylammonium triflate ([DEMA][Tf]) is prepared. The influence of the protic ionic liquid content on the physical, electrochemical, and ion transport properties of the iGPE has been investigated by differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and multi-nuclear solid-state and pulse-field gradient nuclear magnetic resonance. Upon addition of 60 wt% protic IL, the conductivity of the iGPE can be enhanced to 5.8 mS/cm at 100 °C, which is signifiantly higher than that of Nafion at 120 °C (0.2 mS/cm). Solid-state and PFG NMR measurements show that although the incorporation of [PDADMA][TFSI] leads to decreased ion dynamics, it promotes proton dissociation, resulting in a lower activation energy for ion diffusion compared to the neat protic IL. Evidence of proton activity in the iGPEs was obtained by cyclic votammetry, making this material a promising candidate as a proton conducting membranes in fuel cell applications.

Tautomerism of azo dyes in the solid state studied by 15N, 14N, 13C and 1H NMR spectroscopy, X-ray diffraction and quantum-chemical calculations

The azo–hydrazo tautomerism of four model azo dyes in the solid state is investigated by multinuclear solid-state nuclear magnetic resonance (SS-NMR) spectroscopy, X-ray diffraction and quantum-chemical calculations. The measurement of 15N SS-NMR spectra is the most straightforward way to determine the tautomeric state of solid azo compounds. Unfortunately, the sensitivity of these nuclei for SS-NMR is very low and the acquisition of 15N spectra in a reasonable time may be challenging. It is demonstrated that a natural abundance of almost 100% of another nitrogen isotope, 14N, can be exploited for the investigation of the tautomerism. Indirect detection of 14N nuclei via protons is an elegant approach for the rapid and unequivocal determination of the tautomeric form. Furthermore, a comparison of experimental carbon SS-NMR chemical shifts with those obtained in solution or by quantum-chemical computations can be used for the same purpose.

A molecular fluorophore in citric acid/ethylenediamine carbon dots identified and quantified by multinuclear solid-state nuclear magnetic resonance.

The composition of fluorescent polymer nanoparticles, commonly referred to as carbon dots, synthesized by microwave-assisted reaction of citric acid and ethylenediamine was investigated by 13 C, 13 C{1 H}, 1 H─13 C, 13 C{14 N}, and 15 N solid-state nuclear magnetic resonance (NMR) experiments. 13 C NMR with spectral editing provided no evidence for significant condensed aromatic or diamondoid carbon phases. 15 N NMR showed that the nanoparticle matrix has been polymerized by amide and some imide formation. Five small, resolved 13 C NMR peaks, including an unusual ═CH signal at 84 ppm (1 H chemical shift of 5.8 ppm) and ═CN2 at 155 ppm, and two distinctive 15 N NMR resonances near 80 and 160 ppm proved the presence of 5-oxo-1,2,3,5-tetrahydroimidazo[1,2-a]pyridine-7-carboxylic acid (IPCA) or its derivatives. This molecular fluorophore with conjugated double bonds, formed by a double cyclization reaction of citric acid and ethylenediamine as first shown by Y. Song, B. Yang, and coworkers in 2015, accounts for the fluorescence of the carbon dots. Cross-peaks in a 1 H─13 C HETCOR spectrum with brief 1 H spin diffusion proved that IPCA is finely dispersed in the polyamide matrix. From quantitative 13 C and 15 N NMR spectra, a high concentration (18 ± 2 wt%) of IPCA in the carbon dots was determined. A pronounced gradient in 13 C chemical-shift perturbations and peak widths, with the broadest lines near the COO group of IPCA, indicated at least partial transformation of the carboxylic acid of IPCA by amide or ester formation.

Capping Structure of Ligand-Cysteine on CdSe Magic-Sized Clusters.

Ligand molecules capping on clusters largely affect the formation and stabilization mechanism and the property of clusters. In semiconductor CdSe clusters, cysteine is used as one of the ligands and allows the formation of ultrastable (CdSe)34 magic-sized clusters. Cysteine has sulfhydryl, amine, and carboxylate groups, all of which have coordination ability to the CdSe surface, and the bonding states of the three functional groups of ligand–cysteine on the CdSe core have not been determined. In this work, the capping structure of ligand–cysteine is examined by performing Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and multinuclear solid-state nuclear magnetic resonance (NMR) spectroscopy. FT-IR, XPS, and 1H, 13C, and 23Na magic-angle spinning NMR show that the sulfhydryl group of ligand–cysteine forms a sulfur–cadmium bond with a cadmium atom at the CdSe surface, while the carboxylate group does not contribute to the protection of the CdSe core and binds to a sodium ion contained as a counterion. 15N–{77Se} through-bond J-single quantum filtered NMR experiment reveals that the amine group of ligand–cysteine has no coordination to selenium atoms. By considering the N–Cd bond forming ratio (∼43%) revealed in our previous work, which is confirmed in this work by analyzing 13Cα signal intensity (∼42%), we concluded that cysteine capping on (CdSe)34 occurs in two ways: one involves both the sulfur–cadmium and nitrogen–cadmium bonds, and the other bears only the sulfur–cadmium bond.

Local Structures and Li Ion Dynamics in a Li10SnP2S12-Based Composite Observed by Multinuclear Solid-State NMR Spectroscopy

Multinuclear solid-state nuclear magnetic resonance spectroscopy was used in combination with Mossbauer spectroscopy and synchrotron diffraction to investigate the local and long-range structure as well as the Li-ion dynamics in a Li10SnP2S12-based composite. Although two additional phases could be detected (Li7PS6 and Li4SnS4), the Li ion dynamics turn out to be very fast with a Li diffusion coefficient of 1.6 × 10–12 m2/s, a Li+ ion conductivity of ∼2 mS/cm (both at 303 K), and a small activation barrier of 0.13 eV for single Li+ ion jumps.

Solid-state NMR and computational insights into the crystal structure of silicocarnotite-based bioceramic materials synthesized mechanochemically

In this work, we report the results of a detailed structural study of a promising bioceramic material silicocarnotite Ca5(PO4)2SiO4 (SC) synthesized from mechanochemically treated nanosized silicon-substituted hydroxyapatite by annealing at 1000°C. This novel synthetic approach represents an attractive and efficient route towards large-scale manufacturing of the silicocarnotite-based bioceramics. A combination of solid-state nuclear magnetic resonance (NMR), powder X-ray crystallography and density function theory (DFT) calculations has been implemented to characterize the phase composition of the prepared composite materials and to gain insight into the crystal structure of silicocarnotite. The phase composition analysis based on the multinuclear solid-state NMR has been found in agreement with X-ray powder diffraction indicating the minority phases of CaO (5–6wt%) and residual silicon-apatite (7–8wt%), while the rest of the material being a fairly crystalline silicocarnotite phase (86–88wt%). A combination of computational (CASTEP) and experimental methods was used to address the anionic site disorder in the silicocarnotite crystal structure. Distorted [OPO3] pyramids have appeared as an important structural motif in the SC crystal structure. The ratio between regular [PO4] and distorted [OPO3] tetrahedra is found between 2:1 and 3:1 based on XRD experiments and CASTEP calculations. The natural abundance 43Ca magic angle spinning NMR spectra of silicocarnotite are reported for the first time.

Local Structure of Rare-Earth Fluorides in Bulk and Core/Shell Nanocrystalline Materials

NaREF4 (rare-earth, RE = Y, Lu) nanocrystals (NCs) containing Ln3+-dopants are of great interest due to their unique ability to downshift, downconvert, and upconvert light. While the luminescent properties and microscale structures of such NCs are well understood, relatively little is known about their molecular-level structures, the nature of the NC cores and shells, and the interactions of the stabilizing ligands at the NC surface. To address these issues, we present a comprehensive characterization of bulk β-NaYF4, one of the most commonly used hosts for Ln3+-dopants in luminescent NCs. Using a combination of powder X-ray diffraction (pXRD) and multinuclear (89Y, 23Na, 19F) solid-state nuclear magnetic resonance (SSNMR) spectroscopy, we provide conclusive evidence that this compound crystallizes in a hexagonal P63/m structure, resolving a longstanding debate. The structure of this bulk form is related to the structure of NaYF4/NaLuF4 core/shell NCs. From the similarities between the 19F and 23Na SSNMR spectra of the bulk and NC materials, it is concluded that the NCs have the same β-NaYF4 and β-NaLuF4 phases as the bulk compounds. A series of cross-polarization NMR experiments confirm the presence of oleates on the surface of the particle via their proximity to surface Na+ sites.

Dynamic DMF Binding in MOF-5 Enables the Formation of Metastable Cobalt-Substituted MOF-5 Analogues.

Multinuclear solid-state nuclear magnetic resonance, mass spectrometry, first-principles molecular dynamics simulations, and other complementary evidence reveal that the coordination environment around the Zn2+ ions in MOF-5, one of the most iconic materials among metal–organic frameworks (MOFs), is not rigid. The Zn2+ ions bind solvent molecules, thereby increasing their coordination number, and dynamically dissociate from the framework itself. On average, one ion in each cluster has at least one coordinated N,N-dimethylformamide (DMF) molecule, such that the formula of as-synthesized MOF-5 is defined as Zn4O(BDC)3(DMF)x (x = 1–2). Understanding the dynamic behavior of MOF-5 leads to a rational low-temperature cation exchange approach for the synthesis of metastable Zn4–xCoxO(terephthalate)3 (x > 1) materials, which have not been accessible through typical high-temperature solvothermal routes thus far.