High-temperature Nuclear Magnetic Resonance Research Articles

High-temperature in situ11B NMR study of network dynamics in boron-containing glass-forming liquids

In situ high-temperature nuclear magnetic resonance (NMR) spectroscopy can be very useful for probing changes in structure and dynamics in glass-forming liquids, and is a unique method for observing chemical exchange among structural species ( e.g. BO 3–BO 4, Q n–Q n+1, and NBO–BO) at the seconds to microseconds time scales. High-temperature 11B MAS NMR line shape measurements were made at about 100 K above the glass transitions on (Na 2O) 0.3(B 2O 3) 0.7 and (Na 2O) 0.2(B 2O 3) 0.21(Al 2O 3) 0.08(SiO 2) 0.51 glass-forming liquids. BO 3 and BO 4 groups are well resolved in 11B MAS NMR spectra at 14.1 T with sample spinning at 5000 Hz. At higher temperatures, partial peak coalescence occurred due to exchange of BO 3 and BO 4. Temperature effects on borate speciation were also determined by varying the fictive temperature ( T f ) of glasses, where T f estimated from differential scanning calorimetry measurements. We combined these complementary data sets to model structural exchange in the liquid state. The time scale of BO 3–BO 4 exchange from NMR data, τ NMR, appears to be “decoupled” from that of the macroscopic shear relaxation process τ s derived from the viscosity, however, at higher temperatures, τ s approaches τ NMR. The “decoupling” at lower temperature may be related to intermediate-range compositional heterogeneities, and/or fast modifier cation diffusivities which trigger “unsuccessful” network exchange events.

Investigation of Fluoroacidity in Molten Fluorides by the Combination of High Temperature NMR and Molecular Dynamics

The acidity of a molten salt will act directly on the formation of complexes involving the cations of the melt. The determination of the speciation with varying overall composition therefore provides an indirect measure of the acidity. This opens a route for building up the acidity scale through the use of some specific in situ spectroscopic approaches. To determine the speciation in the liquid state, Nuclear Magnetic Resonance (NMR) appears as the ideal candidate, as it allows estimating the relative concentration of the various complexes formed in the melt. In parallel, Molecular Dynamics (MD) simulations are the unavoidable complement to this experimental technique. In this work we describe the structure and the dynamics in the LiF-CaF2 system.

In Situ Experimental Evidence for a Nonmonotonous Structural Evolution with Composition in the Molten LiF−ZrF4 System

We propose in this paper an original approach to study the structure of the molten LiF-ZrF(4) system up to 50 mol % ZrF(4), combining high-temperature nuclear magnetic resonance (NMR) and extended X-ray absorption fine structure (EXAFS) experiments with molecular dynamics (MD) calculations. (91)Zr high-temperature NMR experiments give an average coordination of 7 for the zirconium ion on all domains of composition. MD simulations, in agreement with EXAFS experiments at the K-edge of Zr, provide evidence for the coexistence of three different Zr-based complexes, [ZrF(6)](2-), [ZrF(7)](3-), and [ZrF(8)](4-), in the melt; the evolution of the concentration of these species upon addition of ZrF(4) is quantified. Smooth variations are observed, apart from a given composition at 35 mol % ZrF(4), for which an anomalous point is observed. Concerning the anion coordination, we observe a predominance of free fluorides at low concentrations in ZrF(4), and an increase of the number of bridging fluoride ions between complexes with addition of ZrF(4).

Bimodal phase percolation model for the structure of Ge-Se glasses and the existence of the intermediate phase

A detailed nuclear magnetic resonance and Raman study of ${\text{Ge}}_{x}{\text{Se}}_{1\ensuremath{-}x}$ glasses indicate that the glass structure is composed of intertwined microdomains of ${\text{GeSe}}_{2}$ and ${\text{Se}}_{\text{n}}$. Static nuclear magnetic resonance spectra of glasses ranging from $0\ensuremath{\le}x\ensuremath{\le}\frac{1}{3}$ reveal the absence of Ge-Se-Se fragments in the structure. High temperature nuclear magnetic resonance showing considerable line narrowing confirms this observation. More importantly, the fraction of Se-Se-Se obtained by integration of nuclear magnetic resonance lines matches closely the percentage predicted for a bimodal phase model and is not consistent with the existence of Ge-Se-Se fragments. Raman spectra collected on the same glass also confirm the existence of ${\text{GeSe}}_{2}$ domains up to high selenium concentrations. The mobility of the ${\text{Se}}_{\text{n}}$ phase observed at high temperature while the ${\text{GeSe}}_{2}$ phase remains rigid is consistent with their respective underconstrained and overconstrained structural nature. The proposed bimodal phase percolation model is consistent with the original Phillips and Thorpe theory however it is clearly at odds with the intermediate phase model which predicts large amounts of Ge-Se-Se fragments in the structure. A calorimetric study performed over a wide range of cooling/heating rates shows a narrow composition dependence centered at $⟨\text{r}⟩=2.4$ in contrast with the wide reversibility window observed by Modulated Differential Scanning Calorimetry. This suggests that the observation of the reversibility window associated with the intermediate phase in Ge-Se glasses could be an experimental artifact resulting from the use of a single modulation frequency.

High temperature NMR approach of mixtures of rare earth and alkali fluorides: An insight into the local structure

In situ high temperature nuclear magnetic resonance in molten fluoride mixtures gives some structural picture of the complexes existing in the melt, i.e. of their nature and relative proportion. Thanks to the development of a laser heating system associated with a close crucible in boron nitride, we can describe experimentally the evolution of these complexes from the anions and the cations point of view. By 19F NMR, we have shown the existence of three kinds of fluorine atoms depending on the composition: free fluorine like in pure LiF (non-bonded), bridging fluorine in melts rich in LnF 3 in addition with terminal fluorine singly bonded to one rare earth. Data obtained by NMR spectroscopy are also combined with EXAFS measurements, again thanks to a specific development of the sample holder adapted with molten fluorides and high temperature. This study is a part of our systematic investigation of the different Alk–LnF 3 systems by NMR and EXAFS spectroscopy.

Laser-Heated High-Temperature NMR: A Time-Resolution Study

The time resolution achievable in in situ high-temperature nuclear magnetic resonance experiments is investigated using laser heating of refractory materials. Three case studies using 27Al in alumina nanoparticles, 29Si in silicon carbide and 23Na in a glass-forming mixture of sodium carbonate and quartz have been conducted to distinguish the cases of (a) a fast-relaxing, high natural abundance nucleus, (b) a probe nucleus with low abundance and low spin–lattice relaxation rate, and (c) a complex and changing system of industrial relevance. The most suitable nucleus for in situ high-temperature studies is one with high abundance but slow relaxation because the differential relaxation time between hot and cold parts of the sample effectively removes the signal from the cold material. There is no "in situ penalty" from the diminishing Boltzmann polarization at high temperature since this effect is balanced by a corresponding increase of the spin–lattice relaxation rate.

Structure and dynamics of oxide melts and glasses: A view from multinuclear and high temperature NMR

From a presentation of the various nuclear magnetic resonance (NMR) experiments that allows to characterize the local structure and dynamics of oxide glasses and melts, we show that it becomes possible to evidence not only the details of the coordination state of the constituting atoms but also the nature of polyatomic molecular motifs extending over several chemical bonds.

Cs 4P 2Se 10: A new compound discovered with the application of solid-state and high temperature NMR

The new compound Cs 4P 2Se 10 was serendipitously produced in high purity during a high-temperature synthesis done in a nuclear magnetic resonance (NMR) spectrometer. 31P magic angle spinning (MAS) NMR of the products of the synthesis revealed that the dominant phosphorus-containing product had a chemical shift of −52.8 ppm that could not be assigned to any known compound. Deep reddish brown well-formed plate-like crystals were isolated from the NMR reaction ampoule and the structure was solved with X-ray diffraction. Cs 4P 2Se 10 has the triclinic space group P-1 with a=7.3587(11) Å, b=7.4546(11) Å, c=10.1420(15) Å, α=85.938(2)°, β=88.055(2)°, and γ=85.609(2)° and contains the [P 2Se 10] 4− anion. To our knowledge, this is the first compound containing this anion that is composed of two tetrahedral (PSe 4) units connected by a diselenide linkage. It was also possible to form a glass by quenching the melt in ice water, and Cs 4P 2Se 10 was recovered upon annealing. The static 31P NMR spectrum at 350 °C contained a single peak with a −35 ppm chemical shift and a ∼7 ppm peak width. This study highlights the potential of solid-state and high-temperature NMR for aiding discovery of new compounds and for probing the species that exist at high temperature.

Structural Changes above the Glass Transition and Crystallization in Aluminophosphate Glasses: An in Situ High-Temperature MAS NMR Study

We present an in situ high-temperature nuclear magnetic resonance study on the structural changes in aluminophosphate glasses occurring in the temperature range between the glass transition temperature Tg and the crystallization temperature Tc, Tg < T < Tc. Decisive changes in the network organization between Tg and Tc in potassium aluminophosphate glasses in the compositional range 50K2O-xAl2O3-(50 - x)P2O5 with 2.5 < x < 20 could be monitored for the first time employing 1D 31P- and 27Al-MAS NMR. Accompanying ex situ NMR experiments (31P-RFDR NMR and 31P-{27Al} CP-HETCOR NMR) on devitrified samples were performed at room temperature to further characterize the phases formed during the crystallization process. The structural role of boron-which is known to inhibit the crystallization process in these aluminophosphate glasses-on short and intermediate length scales was analyzed employing 11B-MQMAS, 11B-{27Al} TRAPDOR and 11B-{31P} REDOR NMR spectroscopy.

A low- and high-temperature superconducting NMR magnet: design and performance results

A nuclear magnetic resonance (NMR) magnet comprised of low- and high-temperature superconducting (LTS/HTS) coils has been built and operated at a frequency of 360 MHz. The magnet operates at 4.2 K with the LTS background coil operated in persistent mode, while the HTS insert is in driven mode. The HTS insert is an assembly of 50 double pancakes, each wound with Bi-2223/Ag tape. The paper describes design and performance results of the LTS/HTS magnet.

Low-cost high-temperature NMR probe head

The design of a simple high-temperature nuclear magnetic resonance (NMR) probe head for narrow-bore magnets is presented. It covers the temperature range from 20 to 1300°C, necessitating a heating power of below 100 W. Several probe heads of this design, manufactured for NMR solenoids with bores from 30 to 54 mm have shown good stability and long life times.

Hydrogen diffusion in titanium and zirconium hydrides

Titanium and zirconium form hydrides TiH(D) x and ZrH(D) x with hydrogen concentrations between x≈1.5 (Ti) or 1.6 (Zr) and x=2.0 (room temperature). In these hydrides, the metal atoms form a fcc (δ-phase) or a fct (ϵ-phase) lattice in which the hydrogen atoms occupy tetrahedral interstitial sites. All the tetrahedral sites are occupied at the maximum concentration x=2.0. The hydrogen atoms in titanium and zirconium represent a model system for a concentrated lattice gas. We studied hydrogen and deuterium diffusion in titanium and zirconium hydride by mechanical spectroscopy (vibrating reed technique, temperatures from 5 to 400 K, frequencies between 160 and 1300 Hz). The experiments yielded large hydrogen-induced Zener-relaxation peaks between 240 and 340 K from which the jump rates of the hydrogen interstitials were determined with the help of a theoretical model for the Zener relaxation in a concentrated lattice gas. The jump rates follow an Arrhenius relation with activation energies of 0.49±0.04 eV (H in titanium and zirconium), 0.60±0.04 eV (D in titanium) and 0.51±0.04 eV (D in zirconium). Extrapolation of the present jump rates to higher temperatures allows a comparison with diffusion data from previous high-temperature nuclear magnetic resonance and neutron-scattering measurements. The comparison yields a perfect agreement for titanium hydride, and a poor one for zirconium hydride. The poor agreement for zirconium hydride indicates differences in the microscopic diffusion mechanism between low and high temperatures, which do not exist in the case of titanium hydride.

NMR Spectroscopy of Phase Transitions in Minerals

Nuclear magnetic resonance (NMR) spectroscopy has been used extensively since the 1960’s to study phase transformations. When Lippmaa et al. (1980) presented the first significant high-resolution solid-state NMR spectroscopic study of minerals, the study of structural phase transitions by NMR was a mature field, appearing primarily in the physics literature (e.g., Blinc 1981; Rigamonti 1984). The sensitivity of NMR spectroscopy to short-range structure (first- and second-coordination spheres) and low-frequency dynamics (i.e., frequencies much lower than the thermal vibrations of atoms) make it useful for determining changes in the structure and dynamics of solids that occur near phase transitions. This combination of characteristic time- and distance-scales is not easily accessible by other techniques. Widely studied phase transformations include order-disorder and displacive transitions in compounds with perovskite, antifluorite, β-K2SO4 (A2BX4), KH2PO4 (“KDP”), and other structure types, including some mineral phases (e.g., colemanite; Theveneau and Papon 1976). These studies used primarily single-crystal techniques and were mostly limited to phases that exhibit high symmetry and possess one crystallographic site for the nucleus studied. In some cases, the NMR data can help distinguish order-disorder from purely displacive transition mechanisms. Also, careful measurement of NMR relaxation rates provides information on the spectral density at low frequencies, which reflects softening lattice vibrations or freezing-in of rotational disorder modes. Many phase transitions interesting to mineralogists became accessible to NMR spectroscopy in the late 1980’s with the commercial availability of magic-angle-spinning (MAS) NMR probe assemblies capable of operation at extended temperatures (up to about 600°C) and the development of high-temperature NMR capability at Stanford (Stebbins 1995b). These technical advances enabled many of the structural transitions that occur in minerals to be studied in situ by NMR, including those in phases for which large single crystals are not available. MAS-NMR …

Structural study of Na8[AlSiO4]6(CO3)x(HCOO)2−2x(H2O)4x, 0.2≤x≤1, synthesized in organic solvents: order and disorder of carbonate and formate anions in sodalite

Structural differences between sodalite “as-synthesized” in the system NaOCH3–2SiO2–Al2O3–Na2CO3–(CH3OH) and the calcined product were investigated. The characterization of the sodalites was carried out by X-ray powder diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy, as well as by 29Si and 23Na magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and 23Na satellite transition (SATRAS) NMR spectroscopy. Reaction products of the solvent/base were identified by {1H}13C cross-polarization MAS NMR and 1H MAS NMR. It can be shown that formate results from solvent–base reactions and is enclathrated in the β cages of the sodalite besides carbonate. Calcination at 773 K gives rise to the conversion of formate to carbonate, carbon dioxide and water, which leads to a total ordering of carbonate in the sodalite matrix. Rietveld structure refinements, thermal analysis and high-temperature 23Na and 1H MAS NMR studies confirm this behavior of the templates.

High temperature nuclear magnetic resonance studies of oxide melts

Recently, application of in situ, high temperature nuclear magnetic resonance (NMR) spectroscopy to study silicate, borate and aluminate melts has lead to a variety of findings on structural changes with temperature (particularly in cation coordination number), and on the dynamics of exchange and diffusion. New data are presented on chemical shifts and spin-lattice relaxation of 23Na in a number of silicate liquids to about 1200°C, and these results are discussed in light of other recent studies. These results clearly demonstrate the mixed alkali effect on cation motion. New high temperature magic angle spinning 11B NMR spectra for a sodium borate liquid are also presented. As in previous studies of aluminate and silicate melts, the latter data set suggests that the mechanism of viscous flow is closely tied to local exchange of cationic species.

Dynamics of a glass-forming system: 11B NMR of B2O3

The dynamics of the relaxation processes in a glass-forming system, B2O3, was investigated by means of 11B nuclear magnetic resonance (NMR). Using a homemade high temperature NMR probe, we collected NMR data over a wide temperature range from room temperature to 1200 °C. The NMR data were interpreted in terms of a Fourier transform of the Kohlrausch decay function, f(t)=exp[−(t/τc)−b], where the parameter b varied from 0 to 1. The temperature dependence of τc and b in the decay function was estimated by using both the data from a 11B NMR longitudinal relaxation and a line shape measurement at each temperature. Above 800 °C, the NMR data were well simulated by a single exponential decay of the function (i.e., b=1). Below 800 °C, stretched exponential was introduced to the simulation with the b parameters of 0.6 and 0.8. An Arrhenius plot of τc showed a bend at around 600 °C, which indicates the existence of two distinct reorientational processes crossing each other at that temperature. Below 600 °C, an almost linear dependence of the logarithm of τc vs the inverse of temperature with the activation energy of 40 kJ/mol was observed. This process persists below the glass transition temperature. Above 600 °C, the temperature dependence of τc became non-Arrhenius-like and was identical with that of the previous relaxation measurements. The isotropic chemical shift for the B2O3 melt suggests that the network structure constructed from the BO3 triangle is preserved in the whole temperature range.

High temperature NMR observation of mobile phases up to 1500°C

A new high temperature Nuclear Magnetic Resonance device using double laser heating system has been developed for NMR experiments up to 1500°C. Different systems have been investigated, both of theoritical and applied interest. We report here three examples, where the appearance and the role of mobile species are characterized by 27 Al, 23 Na and 19 F NMR signal evolution with temperature : solid - liquid transition in cryolite Na3 AlF6 , β-α alumina transformation with fluoride mineralizer, and the effects of mineralizer CaF2 on the high temperature reactions in the clinker formation.

Magnesium and Calcium Aluminate Liquids: In Situ High-Temperature 27 Al NMR Spectroscopy

The use of high-temperature nuclear magnetic resonance (NMR) spectroscopy provides a means of investigating the structure of refractory aluminate liquids at temperatures up to 2500 K. Time-averaged structural information indicates that the average aluminum coordination for magnesium aluminate (MgAl(2)O(4)) liquid is slightly greater than for calcium aluminate (CaAl(2)O(4)) liquid and that in both liquids it is close to four. Ion dynamics simulations for these liquids suggest the presence of four-, five-, and six-coordinated aluminate species, in agreement with NMR experiments on fast-quenched glasses. These species undergo rapid chemical exchange in the high-temperature liquids, which is evidenced by a single Lorentzian NMR line.

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.

A compact, high temperature nuclear magnetic resonance probe for use in a narrow-bore superconducting magnet

We report the design of a nuclear magnetic resonance (NMR) probe that can be used in narrow-bore superconducting solenoids for the observation of nuclear induction at high temperatures. The probe is compact, highly sensitive, and stable in continuous operation at temperatures up to 1050 °C. The essential feature of the probe is a water-cooled NMR coil that contains the sample-furnace; this design maximizes sensitivity and circuit stability by maintaining the probe electronics at ambient temperature. We demonstrate the design by showing high temperature 17O NMR spectra and relaxation measurements in solid barium bismuth oxide and yttria-stabilized zirconia.