NMR investigations unveil phase composition-property correlations in Sr0.55Na0.45SiO2.775 fast ion conductor.

This paper reports results of 23Na and 29Si solid-state NMR investigations carried out on sodium strontium silicate ion conductor, Sr0.55Na0.45SiO2.775 and presents the first experimental evidence to show that different synthesis conditions induce multiple devitrified phases. Along with 1-dimensional NMR, 23Na MQMAS spectra have been used to identify the phases corresponding to polymorphs of Na2Si2O5, in addition to the crystalline SrSiO3 and the glass/amorphous Na2Si2O5 phases. The surprising observation of about an order of magnitude higher ionic conductivity achieved in devitrified samples is attributed to the growth of the crystalline δ-Na2Si2O5 phase within the amorphous Na2Si2O5 phase domains, identified using NMR. Together with XRD and conductivity measurement data, the study leads to the identification of the chemical phase composition and an understanding of the composition-property-structure correlation in this material. Present findings, while do not show any evidence of Na doping in the SrSiO3 phase confirming earlier reports, explain the large discrepancy in the conductivity reported in the literature.

Dynamic Nuclear Polarization Enhanced Multiple-Quantum Spin Counting of Molecular Assemblies in Vitrified Solutions.

Crystallization pathways are essential to various industrial, geological, and biological processes. In nonclassical nucleation theory, prenucleation clusters (PNCs) form, aggregate, and crystallize to produce higher order assemblies. Microscopy and X-ray techniques have limited utility for PNC analysis due to the small size (0.5-3 nm) and time stability constraints. We present a new approach for analyzing PNC formation based on 31P nuclear magnetic resonance (NMR) spin counting of vitrified molecular assemblies. The use of glassing agents ensures that vitrification generates amorphous aqueous samples and offers conditions for performing dynamic nuclear polarization (DNP)-amplified NMR spectroscopy. We demonstrate that molecular adenosine triphosphate along with crystalline, amorphous, and clustered calcium phosphate materials formed via a nonclassical growth pathway can be differentiated from one another by the number of dipolar coupled 31P spins. We also present an innovative approach for examining spin counting data, demonstrating that a knowledge-based fitting of integer multiples of cosine wave functions, instead of the traditional Fourier transform, provides a more physically meaningful retrieval of the existing frequencies. This is the first report of multiquantum spin counting of assemblies formed in solution as captured under vitrified DNP conditions, which can be useful for future analysis of PNCs and other aqueous molecular clusters.

Local structures and Al/Si ordering in lanthanum aluminosilicate glasses explored by advanced 27Al NMR experiments and molecular dynamics simulations.

The structures of 15 La-Al-Si-O glasses, whose compositions span 11-28 mol% La(2)O(3), 11-30 mol% Al(2)O(3), and 45-78 mol% SiO(2), are explored over both short and intermediate length-scales by using a combination of solid-state (27)Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations. MAS NMR reveals Al speciations dominated by AlO(4) groups, with minor but significant fractions of AlO(5) (5-10%) and AlO(6) (≲3%) polyhedra present in all La(2)O(3)-Al(2)O(3)-SiO(2) glasses; the amounts of Al([5]) and Al([6]) coordinations increase for decreasing molar fraction of Si. The MD simulations reproduce this compositional trend, with the fractional populations of AlO(p) groups (p = 4, 5, 6) according well with the experimental results. The modeled La speciations mainly involve LaO(6) and LaO(7) polyhedra, giving a range of average La(3+) coordination numbers between 6.0 and 6.6; the latter increases slightly for decreasing Si content of the sample. Besides the expected bridging and non-bridging O species, minor contributions of oxygen triclusters (≤9%) and free O(2-) ions (≤4%) are observed in all MD data. The glass structures exhibit a pronounced Al/Si disorder; the MD simulations reveal essentially random SiO(4)-SiO(4), SiO(4)-AlO(p) and AlO(p)-AlO(q) (p, q = 4, 5, 6) associations, including significant amounts of AlO(4)-AlO(4) contacts, regardless of the n(Al)/n(Si) molar ratio of the glass. The strong violation of Al([4])-Al([4]) avoidance is verified by 2D (27)Al NMR experimentation that correlates double-quantum and single-quantum coherences, here applied for the first time to aluminosilicate glasses, and evidencing AlO(p)-AlO(q) connectivities dominated by AlO(4)-AlO(4) and AlO(4)-AlO(5) pairs. The potential bearings from distinct fictive temperatures of the experimental and modeled glass structures are discussed.

Phase diagram of NdFeAsO(1-x)F(x): essential role of chemical composition.

In this Article, we provided the complete phase diagram of NdFeAsO(1-x)F(x) solid solution as a function of electron doping thanks to the careful determination of F- and O-content and phase content. We gave direct evidence of the source of F-depletion in the superconducting main phase, that is, the formation of oxyfluoride spurious phase. The approach reported in this work clearly showed that to give reliable results on these complex new superconducting materials, a rigorous control of the chemical composition of the considered phases has to be carried out.

Low-power broadband homonuclear dipolar recoupling in MAS NMR by two-fold symmetry pulse schemes for magnetization transfers and double-quantum excitation.

We provide an experimental, numerical, and high-order average Hamiltonian evaluation of an open-ended series of homonuclear dipolar recoupling sequences, SR [Formula: see text] with p=1,2,3,…. While operating at a very low radio-frequency (rf) power, corresponding to a nutation frequency of 1/2 of the magic-angle spinning (MAS) rate (ωnut=ωr/2), these recursively generated double-quantum (2Q) dipolar recoupling schemes offer a progressively improved compensation to resonance offsets and rf inhomogeneity for increasing pulse-sequence order p. The excellent recoupling robustness to these experimental obstacles, as well as to CSA, is demonstrated for 2Q filtering (2QF) experiments and for driving magnetization transfers in 2D NMR correlation spectroscopy, where the sequences may provide either double or zero quantum dipolar Hamiltonians during mixing. Experimental and numerical demonstrations, which mostly target conditions of "ultra-fast" MAS (≳50kHz) and high magnetic fields, are provided for recoupling of (13)C across a wide range of isotropic and anisotropic chemical shifts, as well as dipolar coupling constants, encompassing [2,3-(13)C2]alanine, [1,3-(13)C2]alanine, diammonium [1,4-(13)C2]fumarate, and [U-(13)C]tyrosine. When compared at equal power levels, a superior performance is observed for the SR [Formula: see text] sequences with p⩾3 relative to existing and well-established 2Q recoupling techniques. At ultra-fast MAS, proton decoupling is redundant during the homonuclear dipolar recoupling of dilute spins in organic solids, which renders the family of SR [Formula: see text] schemes the first efficient 2Q recoupling option for general applications, such as 2Q-1Q correlation NMR and high-order multiple-quantum excitation, under truly low-power rf conditions.

Multiple-quantum spin counting in magic-angle-spinning NMR via low-power symmetry-based dipolar recoupling.

By using a symmetry-based R2(8)(1)R2(8)(-1) double-quantum (2Q) dipolar recoupling sequence, we demonstrate high-order multiple-quantum coherence (MQC) excitation at fast magic-angle spinning (MAS) frequencies up to 34 kHz. This scheme combines several attractive features, such as a relatively high dipolar scaling factor, good compensation to rf-errors, isotropic and anisotropic chemical shifts, as well as an ultra-low radio-frequency (rf) power requirement. The latter translates into nutation frequencies below 30 kHz for MAS rates up to 60 kHz, thereby permitting rf application for very long excitation periods without risk of damaging the NMR probehead or sample, while the compensation to chemical shifts improves as the MAS rate increases. (31)P MQC spin counting is demonstrated on powders of calcium hydroxyapatite (Ca5(PO4)3OH) and anhydrous sodium diphosphate (Na4P2O7), from which all even coherence orders up to 30 and 14 were detected, respectively, over the respective MAS ranges of 15-24 kHz and 20-34 kHz. The amplitude distributions among the (31)P MQC orders depend on the precise nutation frequency during recoupling, despite that the highest detected order was relatively insensitive to this parameter. An observed gradual transition from a Gaussian to exponential functionality of the MQC amplitude-profile is discussed in relation to the prevailing approach to derive spin-cluster sizes by fitting the MQC amplitude-distribution to a Gaussian decay, where minor systematic deviations between the model and experimental data are frequently reported.

Analysis of the local structure of phosphorus-substituted LAMOX oxide ion conductors.

The effect of partial substitution of molybdenum by phosphorus on the global and local structural arrangement of the fast oxide-ion conductor La(2)Mo(2)O(9) (LAMOX) has been studied by X-ray powder diffraction as well as (139)La and (31)P solid state NMR. The diffraction patterns show that La(2)Mo(2-y)P(y)O(9-y/2) forms a solid solution at low phosphorus concentrations, and that there is a structural phase transition upon increasing phosphorus concentration. This phase transition is also reflected in (139)La and (31)P NMR spectra. The possibility to excite (31)P multiple-quantum coherences of one of the (31)P NMR signals gives evidence of an accumulation of phosphorus atoms on neighbouring Mo-type sites already before formation of three-dimensional precipitates. On the basis of our X-ray and NMR results we propose a possible structural arrangement of the compound La(2)Mo(2-y)P(y)O(9-y/2) that explains the experimental observations by crystal twinning.