High temperature (35)Cl nuclear magnetic resonance study of the LiCl-KCl system and the effect of CeCl3 dissolution.

This paper examines the dynamics of the LiCl-KCl system over a range of temperatures in order to understand the local structure surrounding chlorine, which is the common ion in these systems, during molten salt pyro-processing. Chlorine-35 nuclear magnetic resonance (NMR) is sensitive to the local environments of the resonant nuclei and their motion on a diffusive timescale. Thus, it is a good probe of the atomic scale processes controlling the viscosities, diffusivities and conductivities of these molten salts. The average isotropic chemical shifts (((35)Cl)δ) and spin-lattice relaxation times (T1) of (35)Cl in (Li,K)Cl salt mixtures have been obtained over a compositional range of 0-100 mol% KCl with an interval of 10 mol% using high temperature nuclear magnetic resonance (NMR) spectroscopy from room temperature up to 890 °C. The ((35)Cl)δ in the two end member salts are consistent with the cation-anion radius ratio as previously measured on the solid halides and the average radius ratio of cation to anion, can be used to explain the variation of ((35)Cl)δ with composition. The quadrupolar interaction is found to be responsible for the spin-lattice relaxation of the (35)Cl, and the activation energies for T1 relaxation have been obtained for all compositions. The measured T1 ((35)Cl) activation energies do not vary linearly with composition and peak at 50% KCl, which also coincides with the Chemla point for this system. They also are in good agreement with the values from equivalent conductivity measurements. To investigate the response of the system to solutes, 8 wt% of CeCl3 was added to the pure LiCl as a surrogate actinide. The shift induced was 120 ppm and the activation energy for the T1 ((35)Cl) increased by a factor of four. This is a promising preliminary result for probing the effect of actinide dissolution on the dynamics of these pyro-processing salts.

A 23Na magic angle spinning nuclear magnetic resonance, XANES, and high-temperature X-ray diffraction study of NaUO3, Na4UO5, and Na2U2O7.

The valence state of uranium has been confirmed for the three sodium uranates NaU(V)O3/[Rn](5f(1)), Na4U(VI)O5/[Rn](5f(0)), and Na2U(VI)2O7/[Rn](5f(0)), using X-ray absorption near-edge structure (XANES) spectroscopy. Solid-state (23)Na magic angle spinning nuclear magnetic resonance (MAS NMR) measurements have been performed for the first time, yielding chemical shifts at -29.1 (NaUO3), 15.1 (Na4UO5), and -14.1 and -19 ppm (Na1 8-fold coordinated and Na2 7-fold coordinated in Na2U2O7), respectively. The [Rn]5f(1) electronic structure of uranium in NaUO3 causes a paramagnetic shift in comparison to Na4UO5 and Na2U2O7, where the electronic structure is [Rn]5f(0). A (23)Na multi quantum magic angle spinning (MQMAS) study on Na2U2O7 has confirmed a monoclinic rather than rhombohedral structure with evidence for two distinct Na sites. DFT calculations of the NMR parameters on the nonmagnetic compounds Na4UO5 and Na2U2O7 have permitted the differentiation between the two Na sites of the Na2U2O7 structure. The linear thermal expansion coefficients of all three compounds have been determined using high-temperature X-ray diffraction: αa = 22.7 × 10(-6) K(-1), αb = 12.9 × 10(-6) K(-1), αc = 16.2 × 10(-6) K(-1), and αvol = 52.8 × 10(-6) K(-1) for NaUO3 in the range 298-1273 K; αa = 37.1 × 10(-6) K(-1), αc = 6.2 × 10(-6) K(-1), and αvol = 81.8 × 10(-6) K(-1) for Na4UO5 in the range 298-1073 K; αa = 6.7 × 10(-6) K(-1), αb = 14.4 × 10(-6) K(-1), αc = 26.8 × 10(-6) K(-1), αß = -7.8 × 10(-6) K(-1), and αvol = -217.6 × 10(-6) K(-1) for Na2U2O7 in the range 298-573 K. The α to ß phase transition reported for the last compound above about 600 K was not observed in the present studies, either by high-temperature X-ray diffraction or by differential scanning calorimetry.

Coupling XRD, EXAFS, and 13C NMR to study the effect of the carbon stoichiometry on the local structure of UC(1±x).

A series of uranium carbide samples, prepared by arc melting with a C/U ratio ranging from 0.96 to 1.04, has been studied by X-ray diffraction (XRD), (13)C nuclear magnetic resonance (NMR), and extended X-ray absorption fine structure (EXAFS). XRD determines phase uniqueness and the increase of the lattice parameter versus the carbon content. In contrast, (13)C NMR detects the different carbon environments in the lattice and in this study, clearly identifies the presence of discrete peaks for carbon in the octahedral lattice site in UC and an additional peak associated with excess carbon in hyperstoichiometric samples. Two peaks associated with different levels of carbon deficiency are detected for all hypostoichiometric compositions. More than one carbon environment is always detected by (13)C NMR. This exemplifies the difficulty in obtaining a perfect stoichiometric uranium monocarbide UC(1.00). The (13)C MAS spectra of uranium carbides exhibit the effects resulting from the carbon content on both the broadening of the peaks and on the Knight shift. An abrupt spectral change occurs between hypo- and hyperstoichiometric samples. The results obtained by EXAFS highlight subtle differences between the different stoichiometries, and in the hyperstoichiometric samples, the EXAFS results are consistent with the excess carbon atoms being in the tetrahedral interstitial position.

A nuclear magnetic resonance spectrometer concept for hermetically sealed magic angle spinning investigations on highly toxic, radiotoxic, or air sensitive materials.

A concept to integrate a commercial high-resolution, magic angle spinning nuclear magnetic resonance (MAS-NMR) probe capable of very rapid rotation rates (70 kHz) in a hermetically sealed enclosure for the study of highly radiotoxic materials has been developed and successfully demonstrated. The concept centres on a conventional wide bore (89 mm) solid-state NMR magnet operating with industry standard 54 mm diameter probes designed for narrow bore magnets. Rotor insertion and probe tuning take place within a hermetically enclosed glovebox, which extends into the bore of the magnet, in the space between the probe and the magnet shim system. Oxygen-17 MAS-NMR measurements demonstrate the possibility of obtaining high quality spectra from small sample masses (~10 mg) of highly radiotoxic material and the need for high spinning speeds to improve the spectral resolution when working with actinides. The large paramagnetic susceptibility arising from actinide paramagnetism in (Th(1-x)U(x))O2 solid solutions gives rise to extensive spinning sidebands and poor resolution at 15 kHz, which is dramatically improved at 55 kHz. The first (17)O MAS-NMR measurements on NpO(2+x) samples spinning at 55 kHz are also reported. The glovebox approach developed here for radiotoxic materials can be easily adapted to work with other hazardous or even air sensitive materials.

Structural transformations and anomalous viscosity in the B2O3 melt under high pressure.

Liquid B2O3 represents an archetypical oxide melt with a superhigh viscosity at the melting temperature. We present the results of the in situ x-ray diffraction study and the in situ viscosity measurements of liquid B2O3 under high pressure up to 8 GPa. Additionally, the 11B solid state NMR spectroscopy study of B2O3 glasses quenched from the melt at five different pressures has been carried out. Taken together, the results obtained provide understanding of the nature of structural transformations in liquid B2O3. The fraction of the boroxol rings in the melt structure rapidly decreases with pressure. From pressures of about 4.5 GPa, four-coordinated boron states begin to emerge sharply, reaching the fraction 40%-45% at 8 GPa. The viscosity of the B2O3 melt along the melting curve drops by 4 orders of magnitude as the pressure increases up to 5.5 GPa and remains unchanged on further pressure increase.

A time resolved 27Al NMR study of the cooling process of liquid alumina from 2450 degrees C to crystallisation.

Nuclear magnetic resonance is able to give insight into the structure and dynamics of liquids at very high temperature (T > 2000 degrees C). 27Al NMR spectra have been recorded every 25 ms during the cooling of an aerodynamically levitated liquid alumina droplet from 2450 degrees C to crystallisation in less than 3 s. The temperature is measured jointly by pyrometry and NMR, and this time resolved experiment provides a unique way of exploring the temperature dependence of both the structure (shift) and the dynamics (relaxation time of 27Al of the liquid and the supercooled liquid alumina until the crystallisation of alpha-Al2O3. The apparent Arrhenian activation energy of 130 kJ mol-1, derived from T1 measurement, is understood as the signature of the macroscopic viscosity in this high temperature liquid.

71Ga and 69Ga nuclear magnetic resonance study of beta-Ga2O3: resolution of four- and six-fold coordinated Ga sites in static conditions.

71Ga and 69Ga nuclear magnetic resonance (NMR) spectra have been obtained for beta-Ga2O3 at magnetic fields of 11.7 and 7.0 T with a combination of static and spinning samples and using echo techniques. Isotropic chemical shifts and well constrained electric field gradient (EFG) tensors were measured for the GaIV and GaVI sites in beta-Ga2O3 and are reported for the first time. Due to its high quadrupolar coupling constant the GaIV site in beta-Ga2O3 is only resolved in static conditions. Analysis of spectral discontinuities obtained for different isotopes and magnetic field strengths and a method of readily obtaining very broad spectra without point by point acquisition demonstrate an approach that should be very useful in constraining the chemical shifts and quadrupole parameters involved in solid-state gallium NMR spectra.

The nature of the glass transition in a silica-rich oxide melt.

The atomic-scale dynamics of the glass-to-liquid transition are, in general, poorly understood in inorganic materials. Here, two-dimensional magic angle spinning nuclear magnetic resonance spectra collected just above the glass transition of K(2)Si(4)O(9) at temperatures as high as 583 degrees C are presented. Rates of exchange for silicon among silicate species, which involves Si-O bond breaking, have been measured and are shown to be closely related in time scale to those defined by viscosity. Thus, even at viscosities as high as 10(10) pascal seconds, local bond breaking (in contrast to the cooperative motion of large clusters) is of major importance in the control of macroscopic flow and diffusion.

Quantification of the disorder in network-modified silicate glasses.

Local order in silicate glasses has been observed by many experimental techniques to be similar to that in crystalline materials. Details of the intermediate-range order are more elusive, but essential for understanding the lack of long-range symmetry in glasses and the effect of composition on glass structure. Two-dimensional 17O dynamic-angle-spinning nuclear magnetic resonance experiments reveal intermediate-range order in the distribution of inter-tetrahedral (Si-O-Si) bond angles and a high degree of order in the disposition of oxygen atoms around the network-modifying cations.

Effects of high temperature on silicate liquid structure: a multinuclear NMR study.

The structure of a silicate liquid changes with temperature, and this substantially affects its thermodynamic and transport properties. Models used by geochemists, geophysicists, and glass scientists need to include such effects. In situ, high-temperature nuclear magnetic resonance (NMR) spectroscopy on (23)Na, (27)A1, and (29)Si was used to help determine the time-averaged structure of a series of alkali aluminosilicate liquids at temperatures to 1320 degrees C. Isotropic chemical shifts for (29)Si increase (to higher frequencies) with increasing temperature, probably in response to intermediate-range structural changes such as the expansion of bonds between nonbridging oxygens and alkali cations. In contrast, isotropic chemical shifts for (27)Al decrease with increasing temperature, indicating that more significant short-range structural changes take place for aluminum, such as an increase in mean coordination number. The spectrum of a sodium aluminosilicate glass clearly indicates that at least a few percent of six-coordinated aluminum was present in the liquid at high temperature.

Solid-state phosphorus-31 nuclear magnetic resonance differentiation of bone mineral and synthetic apatite used to fill bone defects.

Bioabsorption of synthetic apatite compounds used to promote bone healing and remodeling has been difficult to evaluate. In this study, solid-state phosphorus-31 nuclear magnetic resonance (NMR) has been used to characterize and quantitate bone mineral and a synthetic apatite in order to establish a model for bioabsorption studies. Pulverized solid samples of cortical rabbit bone and a synthetic fluoridated apatite were examined in vitro at variable degrees of hydration. A 9.4 T superconducting spectrometer was used to obtain 31P magic angle spinning NMR spectra and T1 relaxation times. Quantitation was attempted in mixed samples using T1 recovery data. Bone mineral and synthetic apatite could be distinguished by chemical shift and T1 relaxation time in variable hydration states, and were readily differentiated in mixtures by their T1 relaxation time. NMR estimates of relative proportions of components in mixed samples were accurate within 2% of evaluations based on weight. Solid-state 31P NMR therefore provides a suitable method for monitoring the bioabsorption of synthetic apatites.

Nuclear magnetic resonance spectroscopy in the Earth sciences: structure and dynamics.

Detailed knowledge of the structure and dynamics of the materials that make up the earth is necessary for fundamental understanding of most geological processes. Nuclear magnetic resonance spectroscopy is beginning to play an important role in investigations of inorganic solid materials, as well as of liquids and organic compounds; it has already contributed substantially to our knowledge of minerals and rocks, compositionally simplified analogs of magmas, and the surfaces of silicate crystals. The technique is particularly useful for determining local structure and ordering state in crystals, glasses, and liquids, and is sensitive to atomic motion at the time scales of diffusion and viscosity in silicates. New techniques offer promise for increased resolution for quadrupolar nuclei and for extension of experiments to high temperature and pressure.