Fast Room-Temperature Mg2+ Conductivity in Mg(BH4)2·1.6NH3-Al2O3 Nanocomposites

Design of new functional materials with fast Mg-ion mobility is crucial for the development of competitive solid-state magnesium batteries. Herein, we present new nanocomposites, Mg(BH₄)₂·1.6NH₃-Al₂O₃, reaching a high magnesium conductivity of σ(Mg²⁺) = 2.5 × 10^-5 S cm^-1 at 22 °C assigned to favorable interfaces between amorphous state Mg(BH₄)₂·1.6NH₃; inert and insulating Al₂O₃ nanoparticles; and a minor fraction of crystalline material, mainly Mg(BH₄)₂·2NH₃. Furthermore, quasi-elastic neutron scattering reveals that the Mg²⁺-ion mobility in the solid state appears to be correlated to relatively slow motion of NH₃ molecules rather than the fast dynamics of BH₄^- complexes. The nanocomposite is compatible with a metallic Mg anode and shows stable Mg²⁺ stripping/plating in a symmetric cell and an electrochemical stability of ∼1.2 V. The nanocomposite has high mechanical stability and ductility and is a promising Mg²⁺ electrolyte for future solid-state magnesium batteries.

Pair distribution function and 71Ga NMR study of aqueous Ga3+ complexes

The atomic structures, and thereby the coordination chemistry, of metal ions in aqueous solution represent a cornerstone of chemistry, since they provide first steps in rationalizing generally observed chemical information. However, accurate structural information about metal ion solution species is often surprisingly scarce. Here, the atomic structures of Ga³⁺ ion complexes were determined directly in aqueous solutions across a wide range of pH, counter anions and concentrations by X-ray pair distribution function analysis and ⁷¹Ga NMR. At low pH (<2) octahedrally coordinated gallium dominates as either monomers with a high degree of solvent ordering or as Ga-dimers. At slightly higher pH (pH ≈ 2–3) a polyoxogallate structure is identified as either Ga₃₀ or Ga₃₂ in contradiction with the previously proposed Ga₁₃ Keggin structures. At neutral and slightly higher pH nanosized GaOOH particles form, whereas for pH > 12 tetrahedrally coordinated gallium ions surrounded by ordered solvent are observed. The effects of varying either the concentration or counter anion were minimal. The present study provides the first comprehensive structural exploration of the aqueous chemistry of Ga³⁺ ions with atomic resolution, which is relevant for both semiconductor fabrication and medical applications.

With changing pH four different structural regions in Ga³⁺ aqueous solutions are observed. In contrast the effects of different anions and concentrations are minimal.

CO2 mineralization of demolished concrete wastes into a supplementary cementitious material – a new CCU approach for the cement industry

This contribution discusses the carbon capture and utilization (CCU) approach based on CO2 mineralization of cement paste from recycled concrete as new approach to capture CO2 and significantly contribute to the reduction in CO2 emissions associated with cement production. The current literature suggests that all CO2 released from the decomposition of limestone during clinker production can be sequestered by carbonation of the end-of-life cement paste. This carbonation can be achieved in a few hours at ambient temperature and pressure and with a relatively low CO2 concentration (< 10 %) in the gas. The carbonation of cement paste produces calcite and an amorphous alumina-silica gel, the latter being a pozzolanic material that can be utilized as a supplementary cementitious material. The pozzolanic reaction of the alumina-silica gel is very rapid as a result of its high specific surface and amorphous structure. Thus, composite cements containing carbonated cement paste are characterized by a rapid strength gain. The successful implementation of this CCU approach relies also on improved concrete recycling techniques and methods currently under development to separate out the cement paste fines and such. Full concrete recycling will further improve the circular utilization of cement and concrete by using recycled aggregates instead of natural deposits of aggregates.  Although the feasibility of the process has already been demonstrated at the industrial scale, there are still several open questions related to optimum carbonation conditions and the performance of carbonated material in novel composite cements.

Probing the validity of the spinel inversion model: a combined SPXRD, PDF, EXAFS and NMR study of ZnAl2O4

Spinels are of essential interest in the solid-state sciences with numerous important materials adopting this crystal structure. One defining feature of spinel compounds is their ability to accommodate a high degree of tailorable point defects, and this significantly influences their physical properties. Standard defect models of spinels often only consider metal atom inversion between octahedral and tetrahedral sites, thereby neglecting other defects such as interstitial atoms. In addition, most studies rely on a single structural characterization technique, and this may bias the result and give uncertainty about the correct crystal structure. Here we explore the virtues of multi-technique investigations to limit method and model bias. We have used Pair Distribution Function analysis, Rietveld refinement and Maximum Entropy Method analysis of Powder X-ray Diffraction data, Zn edge Extended X-ray Absorption Fine Structure, and solid-state 27Al Nuclear Magnetic Resonance to study the structural defects in ZnAl2O4 spinel samples prepared by either microwave hydrothermal synthesis, supercritical flow synthesis, or spark plasma sintering. In addition, the samples were subjected to thermal post treatments. The study demonstrates that numerous synthesis dependent defects are present and that the different synthesis pathways allow for defect tailoring within the ZnAl2O4 structure. This suggests a pathway forward for optimization of the physical properties of spinel materials.

Optical Sensing of pH and O2 in the Evaluation of Bioactive Self-Healing Cement

Leakage from cementitious structures with a retaining function can have devastating environmental consequences. Leaks can originate from cracks within the hardened cementitious material that is supposed to seal the structure off from the surrounding environment. Bioactive self-healing concretes containing bacteria capable of microbially inducing CaCO₃ precipitation have been suggested to mitigate the healing of such cracks before leaking occurs. An important parameter determining the biocompatibility of concretes and cements is the pH environment. Therefore, a novel ratiometric pH optode imaging system based on an inexpensive single-lens reflex (SLR) camera was used to characterize the pH of porewater within cracks of submerged hydrated oil and gas well cement. This enabled the imaging of pH with a spatial distribution in high resolution (50 μm per pixel) and a gradient of 1.4 pH units per 1 mm. The effect of fly ash substitution and hydration time on the pH of the cement surface was evaluated by this approach. The results show that pH is significantly reduced from pH >11 to below 10 with increasing fly ash content as well as hydration time. The assessment of bioactivity in the cement was evaluated by introducing superabsorbent polymers with encapsulated Bacillus alkalinitrilicus endospores into the cracks. The bacterial activity was measured using oxygen optodes, which showed the highest bacterial activity with increasing amounts of fly ash substitution in the cement, correlating with the decrease in the pH. Overall, our results demonstrate that the pH of well cements can be reliably measured and modified to sustain the microbial activity.

Synthesis of ZSM-23 (MTT) zeolites with different crystal morphology and intergrowths: effects on the catalytic performance in the conversion of methanol to hydrocarbons

A series of zeolite ZSM-23 samples prepared with different organic structure-directing agents, gave rise to crystalline phases, which show a large variety of particle dimensions.

Potassium octahydridotriborate: diverse polymorphism in a potential hydrogen storage material and potassium ion conductor

Hydrogen storage properties and polymorphism in KB₃H₈. The order–disorder polymorphic transition results in disordered B₃H₈⁻ anions, facilitating cation mobility.

Hydrogenation properties of lithium and sodium hydride – closo-borate, [B10H10]2− and [B12H12]2−, composites

The hydrogen absorption properties of metal closo-borate/metal hydride composites are studied under high hydrogen pressures.

The structure-directing amine changes everything: structures and optical properties of two-dimensional thiostannates

Two different two-dimensional thiostannates (SnS) were synthesized using tris(2-aminoethyl)amine (tren) or 1-(2-aminoethyl)piperidine (1AEP) as structure-directing agents. Both structures consist of negatively charged thiostannate layers with charge stabilizing cations sandwiched in-between. The fundamental building units are Sn₃S₄ broken-cube clusters connected by double sulfur bridges to form polymeric (Sn₃S₇^2-)_n honeycomb hexagonal layers. The compounds are members of the R-SnS-1 family of structures, where R indicates the type of cation. Despite consisting of identical structural units, the band gaps of the two semiconducting compounds were found to differ substantially at 2.96 eV (violet-blue light) and 3.21 eV (UV light) for tren-SnS-1 and 1AEP-SnS-1, respectively. Aiming to explain the observed differences in optical properties, the structures of the two thiostannates were investigated in detail based on combined X-ray diffraction, solid-state ¹³C and ¹¹⁹Sn MAS NMR spectroscopy and scanning electron microscopy studies. The compound tren-SnS-1 has a hexagonal structure consisting of planar SnS layers with regular hexagonal pores and disordered cations, whereas 1AEP-SnS-1 has an orthorhombic unit cell with ordered cations, distorted hexagonal pores and non-planar SnS layers. In the formation of 1AEP-SnS-1, an intramolecular reaction of the structure-directing piperidine takes place to form an N-heterobicyclic cation through in situ C-H activation. Hirshfeld surface analysis was used to investigate the interaction between the SnS layers and cations in 1AEP-SnS-1 and revealed that the most nucleophilic part of the SnS sheets is one of the two crystallographically distinct double sulfur bridges.

Ionic liquids grafted onto graphene oxide as a new multifunctional heterogeneous catalyst and its application in the one-pot multi-component synthesis of hexahydroquinolines

The synergistic effect of ionic liquids and carboxylic acids in the reaction mechanism improved the catalytic activity of supported ILs.

Metal borohydride formation from aluminium boride and metal hydrides

Formation and quantification of metal borohydrides at high pressure, p(H₂) = 600 bar, and elevated temperature from AlB₂-MH_x (M = Li, Na, Mg, Ca) composites.

Quantification of the boron speciation in alkali borosilicate glasses by electron energy loss spectroscopy

Transmission electron microscopy and related analytical techniques have been widely used to study the microstructure of different materials. However, few research works have been performed in the field of glasses, possibly due to the electron-beam irradiation damage. In this paper, we have developed a method based on electron energy loss spectroscopy (EELS) data acquisition and analyses, which enables determination of the boron speciation in a series of ternary alkali borosilicate glasses with constant molar ratios. A script for the fast acquisition of EELS has been designed, from which the fraction of BO4 tetrahedra can be obtained by fitting the experimental data with linear combinations of the reference spectra. The BO4 fractions (N4) obtained by EELS are consistent with those from (11)B MAS NMR spectra, suggesting that EELS can be an alternative and convenient way to determine the N4 fraction in glasses. In addition, the boron speciation of a CeO2 doped potassium borosilicate glass has been analyzed by using the time-resolved EELS spectra. The results clearly demonstrate that the BO4 to BO3 transformation induced by the electron beam irradiation can be efficiently suppressed by doping CeO2 to the borosilicate glasses.

Hydrogen-fluorine exchange in NaBH4-NaBF4

Hydrogen-fluorine exchange in the NaBH4-NaBF4 system is investigated using a range of experimental methods combined with DFT calculations and a possible mechanism for the reactions is proposed. Fluorine substitution is observed using in situ synchrotron radiation powder X-ray diffraction (SR-PXD) as a new Rock salt type compound with idealized composition NaBF2H2 in the temperature range T = 200 to 215 °C. Combined use of solid-state (19)F MAS NMR, FT-IR and DFT calculations supports the formation of a BF2H2(-) complex ion, reproducing the observation of a (19)F chemical shift at -144.2 ppm, which is different from that of NaBF4 at -159.2 ppm, along with the new absorption bands observed in the IR spectra. After further heating, the fluorine substituted compound becomes X-ray amorphous and decomposes to NaF at ~310 °C. This work shows that fluorine-substituted borohydrides tend to decompose to more stable compounds, e.g. NaF and BF3 or amorphous products such as closo-boranes, e.g. Na2B12H12. The NaBH4-NaBF4 composite decomposes at lower temperatures (300 °C) compared to NaBH4 (476 °C), as observed by thermogravimetric analysis. NaBH4-NaBF4 (1:0.5) preserves 30% of the hydrogen storage capacity after three hydrogen release and uptake cycles compared to 8% for NaBH4 as measured using Sievert's method under identical conditions, but more than 50% using prolonged hydrogen absorption time. The reversible hydrogen storage capacity tends to decrease possibly due to the formation of NaF and Na2B12H12. On the other hand, the additive sodium fluoride appears to facilitate hydrogen uptake, prevent foaming, phase segregation and loss of material from the sample container for samples of NaBH4-NaF.

Nanoconfined NaAlH4: prolific effects from increased surface area and pore volume

Nanoconfinement is a promising technique to improve the properties of nanomaterials such as the kinetics for hydrogen release and uptake and the stability during cycling. Here we present a systematic study of nanoconfined NaAlH4 in nanoporous scaffolds with increasing surface area and pore volume and almost constant pore sizes in the range of 8 to 11 nm. A resorcinol formaldehyde carbon aerogel was CO2-activated under different conditions and provided aerogels with BET surface areas of 704, 1267 and 2246 m(2) g(-1) and total pore volumes of 0.91, 1.30 and 2.21 mL g(-1), respectively. Nanoconfinement of NaAlH4 was achieved by melt infiltration and (27)Al MAS NMR reveals that the respective scaffolds incorporate 68, 82 and 91 wt% NaAlH4, for the above-mentioned samples, while the remaining fraction decomposes to metallic Al indicating that increasing CO2-activation tends to facilitate the infiltration process. The frequencies for the (23)Na and (27)Al MAS NMR centerband resonances from NaAlH4 vary systematically for the infiltrated samples and are shifted towards higher frequency and become more narrow with increasing degree of CO2 activation of the scaffolds. This new effect is attributed to increasing interactions with conduction electrons from increasingly graphite-/graphene-like scaffolds. The bulk versus nanoconfined ratio of NaAlH4 was investigated using Rietveld refinement, revealing that the majority of added NaAlH4 is confined inside the nanopores. The hydrogen desorption kinetics decreased with increasing surface area and the hydrogen storage capacity is more stable and decreases less during continuous hydrogen release and uptake cycles. In fact, the available amount of hydrogen (2.7 wt% H2) was more than doubled compared to the nanoconfinement in the non-activated carbon aerogel (1.3 wt% H2). Furthermore, it was demonstrated that Ti-functionalization of the CO2-activated aerogels combines the high storage capacity with fast hydrogen release kinetics from NaAlH4 which fully decomposes into Na3AlH6 at T ≤ 100 °C.

Magic-angle spinning solid-state multinuclear NMR on low-field instrumentation

Mobile and cost-effective NMR spectroscopy exploiting low-field permanent magnets is a field of tremendous development with obvious applications for arrayed large scale analysis, field work, and industrial screening. So far such demonstrations have concentrated on relaxation measurements and lately high-resolution liquid-state NMR applications. With high-resolution solid-state NMR spectroscopy being increasingly important in a broad variety of applications, we here introduce low-field magic-angle spinning (MAS) solid-state multinuclear NMR based on a commercial ACT 0.45 T 62 mm bore Halbach magnet along with a homebuilt FPGA digital NMR console, amplifiers, and a modified standard 45 mm wide MAS probe for 7 mm rotors. To illustrate the performance of the instrument and address cases where the low magnetic field may offer complementarity to high-field NMR experiments, we demonstrate applications for (23)Na MAS NMR with enhanced second-order quadrupolar coupling effects and (31)P MAS NMR where reduced influence from chemical shift anisotropy at low field may facilitate determination of heteronuclear dipole-dipole couplings.

Hydrogen reversibility of LiBH4–MgH2–Al composites

Numerous intermediates are involved in the decomposition mechanism of LiBH₄–MgH₂–Al and the cyclic stability is evaluated.

Double cross-polarization MAS NMR in the assignment of abundant-spin resonances: ¹⁹F-{²⁹Si}-¹⁹F FBCP/MAS NMR of fluoride ions incorporated in calcium silicate hydrate (C-S-H) phases

A new version of the double cross-polarization MAS NMR experiment, which transfers polarization Forth and Back (FBCP) between high- and low-γ spin nuclei, is presented. The pulse sequence is demonstrated by ¹⁹F-{²⁹Si}-¹⁹F and ¹⁹F-{¹³C}-¹⁹F FBCP NMR spectra of a mixture of cuspidine (Ca₄Si₂O₇F₂) and Teflon (-CF₂-)(n). The experiment is useful for assignment of the high-γ spin resonances, as demonstrated by ¹⁹F-{²⁹Si}-¹⁹F FBCP NMR of a fluoride-containing calcium-silicate-hydrate (C-S-H) phase, where the ¹⁹F resonance from fluoride ions incorporated in the interlayer structure of the C-S-H phase is identified.

Improved hydrogen storage kinetics of nanoconfined NaAlH₄ catalyzed with TiCl₃ nanoparticles

Nanoparticles of NaAlH(4) have been infiltrated in nanoporous carbon aerogel with TiCl(3) nanoparticles in order to explore possible synergetic effects between nanoconfinement and a functionalized catalytic scaffold. Resorcinol formaldehyde carbon aerogels with an average pore size of 17 nm and total pore volume of 1.26 mL/g were infiltrated with TiCl(3) to obtain an aerogel doped with 3.0 wt % TiCl(3) nanoparticles. NaAlH(4) was melt-infiltrated into the functionalized carbon aerogel at 189 °C and p(H(2)) ∼ 186-199 bar. Energy-dispersive spectrometry (EDS) combined with focused ion beam (FIB) techniques revealed the presence of Na, Al, Ti, and Cl inside the aerogel scaffold material. The infiltrated NaAlH(4) was X-ray amorphous, whereas (27)Al magic-angle spinning (MAS) NMR spectroscopy confirmed the presence of nanoconfined NaAlH(4). Temperature-programmed desorption mass spectroscopy (TPD-MS) and Sieverts' measurements demonstrated significantly improved hydrogen desorption kinetics for this new nanoconfined NaAlH(4)-TiCl(3) material as compared to nanoconfined NaAlH(4) without the catalysts TiCl(3) and to bulk ball-milled samples of NaAlH(4)-TiCl(3). We find that the onset temperature for hydrogen release was close to room temperature (T(onset) = 33 °C), and the hydrogen release rate reached a maximum value at 125 °C, which demonstrates favorable synergetic effects between nanoconfinement and catalyst addition.

Natural abundance solid-state 95Mo MAS NMR of MoS2 reveals precise 95Mo anisotropic parameters from its central and satellite transitions

Precise values are reported for a quite large (95)Mo quadrupole coupling and an unusually large (95)Mo chemical shift anisotropy in MoS(2), values that have been retrieved by analysis of a well-resolved, highly complex 14.1 T (95)Mo MAS NMR spectrum displaying both the central and satellite transitions.

A strategy for acquisition and analysis of complex natural abundance (33)S solid-state NMR spectra of a disordered tetrathio transition-metal anion

A strategy, involving (i) sensitivity enhancement for the central transition (CT) by population transfer (PT) employing WURST inversion pulses to the satellite transitions (STs) in natural abundance (33)S MAS NMR for two different MAS frequencies (nu(r)=5.0 and 10.0kHz) at 14.1T and (ii) a (33)S static QCPMG experiment at 19.6T, has allowed acquisition and analysis of very complex solid-state (33)S CT NMR spectra for the disordered tetrathioperrhenate anion ReS(4)(-) in [(C(2)H(5))(4)N][ReS(4)]. This strategy of different NMR experiments combined with spectral analysis/simulations has allowed determination of precise values for two sets of quadrupole coupling parameters (C(Q) and eta(Q)) assigned to the two different S sites for the four sulfur atoms in the ReS(4)(-) anion in the ratio S1:S2=1:3. These sets of C(Q), eta(Q) values for the S1 and S2 site are quite similar and the magnitudes of the quadrupole coupling constants (C(Q)=2.2-2.5MHz) are a factor of about three larger than observed for other tetrathiometalates A(2)MS(4) (A=NH(4), Cs, Rb and M=W, Mo). In addition, the spectral analysis also leads to a determination of the chemical shift anisotropy (CSA) parameters (delta(sigma) and eta(sigma)) for the S1 and S2 site, however, with much lower precisions (about 20% error margins) compared to those for C(Q), eta(Q), because the magnitudes of the two CSAs (i.e., delta(sigma)=60-90ppm) are about a factor of six smaller than observed for the other tetrathiometalates mentioned above. This large difference in the magnitudes of the anisotropic parameters C(Q) and delta(sigma) for the ReS(4)(-) anion, compared to those for the WS(4)(2-) and MoS(4)(2-) anions determined previously under identical experimental conditions, accounts for the increased complexity of the PT-enhanced (33)S MAS spectra observed for the ReS(4)(-) anion in this study. This difference in C(Q) also contributes significantly to the intensity distortions observed in the outer wings of the CTs when employing PT from the STs under conditions of slow-speed MAS.

Improved quantification of alite and belite in anhydrous Portland cements by (29)Si MAS NMR: effects of paramagnetic ions

The applicability, reliability, and repeatability of 29Si MAS NMR for determination of the quantities of alite (Ca3SiO5) and belite (Ca2SiO4) in anhydrous Portland cement was investigated in detail for 11 commercial Portland cements and the results compared with phase quantifications based on powder X-ray diffraction combined with Rietveld analysis and with Taylor-Bogue calculations. The effects from paramagnetic ions (Fe3+) on the spinning sideband intensities, originating from dipolar couplings between 29Si and the spins of the paramagnetic electrons, were considered and analyzed in spectra recorded at four magnetic fields (4.7-14.1T) and this has led to an improved quantification of alite and belite from (29)Si MAS NMR spectra recorded at "high" spinning speeds of nu(R)=12.0-13.0kHz using 4 or 5mm rotors. Furthermore, the impact of Fe3+ ions on the spin-lattice relaxation was studied by inversion-recovery experiments and it was found that the relaxation is overwhelmingly dominated by the Fe3+ ions incorporated as guest-ions in alite and belite rather than the Fe3+ sites present in the intimately mixed ferrite phase (Ca2Al(x)Fe(2-)(x)O5).

Sensitivity enhancement in natural-abundance solid-state 33S MAS NMR spectroscopy employing adiabatic inversion pulses to the satellite transitions

The WURST (wideband uniform rate smooth truncation) and hyperbolic secant (HS) pulse elements have each been employed as pairs of inversion pulses to induce population transfer (PT) between the four energy levels in natural abundance solid-state (33)S (spin I=3/2) MAS NMR, thereby leading to a significant gain in intensity for the central transition (CT). The pair of inversion pulses are applied to the satellite transitions for a series of inorganic sulfates, the sulfate ions in the two cementitious materials ettringite and thaumasite, and the two tetrathiometallates (NH(4))(2)WS(4) and (NH(4))(2)MoS(4). These materials all exhibit (33)S quadrupole coupling constants (C(Q)) in the range 0.1-1.0 MHz, with precise C(Q) values being determined from analysis of the PT enhanced (33)S MAS NMR spectra. The enhancement factors for the WURST and HS elements are quite similar and are all in the range 1.74-2.25 for the studied samples, in excellent agreement with earlier reports on HS enhancement factors (1.6-2.4) observed for other spin I=3/2 nuclei with similar C(Q) values (0.3-1.2 MHz). Thus, a time saving in instrument time by a factor up to five has been achieved in natural abundance (33)S MAS NMR, a time saving which is extremely welcome for this important low-gamma nucleus.

Long-term stability of rotor-controlled MAS frequencies to 0.1 Hz proved by 14N MAS NMR experiments and simulations

Experimental and simulated 14N MAS NMR spectra of the NH4+ ions in the two polymorphs, mS60 and mP60, of (NH4)2MoO4 are used to illustrate that a long-term stability of rotor-controlled MAS frequencies to 0.1 Hz can be achieved using commercial instrumentation (MAS speed controller and 7.5 mm MAS probe with a single marked rotor) attached to a highly pressure-stabilized air supply. A new modification of the STARS simulation software employs a Gaussian distribution for the experimental spinning frequency around the frequency set for the MAS speed controller. A simulated spectrum is then obtained by summation of several calculated spectra for evenly spaced spinning frequencies around the set frequency with relative weight factors corresponding to the Gaussian distribution.

Single-Crystal Growth and Characterization of Disilver(I) Monofluorophosphate(V), Ag2PO3F: Crystal Structure, Thermal Behavior, Vibrational Spectroscopy, and Solid-State 19F, 31P, and 109Ag MAS NMR Spectroscopy

Single crystals of disilver(I) monofluorophosphate(V), Ag2PO3F (1), were obtained by slow evaporation of a diluted aqueous Ag2PO3F solution. Compound 1 adopts a new structure type and crystallizes in the monoclinic space group C2/c with eight formula units and lattice parameters of a = 9.2456(8) A, b = 5.5854(5) A, c = 14.7840(13) A, and beta = 90.178(2) degrees. The crystal structure of 1 [R(F2 > 2sigma(F2) = 0.0268, wR(F2 all) = 0.0665] is composed of three crystallographically independent Ag+ cations and PO3F2- anions as single building units. The oxygen environment around each of the Ag+ cations is different, with one Ag+ in distorted octahedral (d(Ag-O) = 2.553 A), one in nearly rectangular (d(Ag-O) = 2.445 A), and one in distorted tetrahedral (d(Ag-O) = 2.399 A) coordination. Additional Ag-F contacts to more remote F atoms located at distances >2.80 A augment the coordination polyhedra for the two latter Ag+ cations. The monofluorophosphate anion deviates slightly from C3v symmetry and exhibits the characteristic differences in bond lengths, with a mean of 1.510 A for the P-O bonds and one considerably longer P-F bond of 1.575(2) A. Compound 1 was further characterized by vibrational spectroscopy (Raman and IR) and solid-state 19F, 31P, and 109Ag MAS NMR spectroscopy. The value for the isotropic one-bond P-F coupling constant in 1 is 1JPF = -1045 Hz. Thermal analysis (TG, DSC) revealed a reversible phase transition at 308 degrees C, which is very close to the decomposition range of 1. Under release of POF3, Ag4P2O7 and Ag3PO4 are the thermal decomposition products at temperatures above 450 degrees C.

Advancements in natural abundance solid-state33S MAS NMR: characterization of transition-metal MS bonds in ammonium tetrathiometallates

We report the first (33)S chemical shift anisotropy (CSA) data as obtained from a combined determination of (33)S CSA and quadrupole coupling parameters utilizing the observation of both the (33)S (I = 3/2) central and satellite transitions in a natural abundance (33)S MAS NMR study aimed at characterizing the two important tetrathiometallates (NH4)(2)MoS(4) and (NH4)(2)WS(4).

Probing Crystal Structures and Transformation Reactions of Ammonium Molybdates by 14N MAS NMR Spectroscopy

The unique high-resolution feature offered by 14N magic-angle spinning (MAS) NMR spectroscopy of ammonium ions has been used to characterize the crystal structures of various ammonium molybdates by their 14N quadrupole coupling parameters, i.e., CQ, the quadrupole coupling constant, and etaQ, the asymmetry parameter. Two polymorphs of diammonium monomolybdate, (NH4)2MoO4, recently structurally characterized by single-crystal X-ray diffraction (XRD) and named mS60 and mP60, show distinct but different 14N MAS NMR spectra from each of which two sets of characteristic 14N CQ and etaQ values have been obtained. Similarly, the well-characterized ammonium polymolybdates (NH4)2Mo2O7, (NH4)6Mo7O24.4H2O, and (NH4)6Mo8O27.4H2O also give rise to distinct and characteristic 14N MAS NMR spectra. In particular, it is noted that simulation of the experimental (NH4)6Mo7O24.4H2O spectrum requires an iterative fit with six independent NH4+ sites. For the slow spinning frequencies employed (nu(r) = 1500-3000 Hz), all 14N MAS NMR spectra of the ammonium molybdates in this study are fingerprints of their identity. These different 14N MAS NMR fingerprints are shown to be an efficient tool in qualitative and quantitative assessment of the decomposition of (NH4)2MoO4 in humid air. Finally, by a combination of the 14N and 95Mo MAS NMR experiments performed here, it has become clear that a recent report of the 95Mo MAS spectra and data for the mS60 and mP60 polymorphs of (NH4)2MoO4 are erroneous because the sample examined had decomposed to (NH4)2Mo2O7.