High-resolution 29Si Research Articles

Surface and Bulk Stabilization of Silicon Anodes Via in-Situ Formation of Li-M-Si Zintl Phases Using Multivalent Cation Additives

Silicon is drawing attention as the upcoming anode material for the next generation of lithium-ion batteries due to its higher capacity compared to commercial graphite. However, silicon anions formed during lithiation are highly reactive with binder and electrolyte components creating an unstable SEI layer and limiting the calendar life of silicon anodes. In this study, the reactivity of lithium silicide and the formation of unstable SEI layer is mitigated by utilizing the use of a mixture of Ca and Mg multivalent cations as an electrolyte additive for Si anodes to improve their calendar life. Ca and Mg ions in the electrolyte diffuse into silicon particles forming relatively thermodynamically stable quaternary Li-Ca-Mg-Si Zintl phases with less chemical reactivity, in an in-situ fashion, stabilizing the bulk. Ca and Mg cations also react with the F anions in the electrolyte, forming a layer of nanocrystalline CaF2 and MgF2 that is closely coated around the silicon particles. The CaF2-enabled new SEI is strong and dense, which effectively protects the silicon core from side reactions, leading to lower capacity decay after calendar aging at high voltage. To understand the mechanism of ternary/ quaternary Zintl phase formation and its dynamics upon lithiation/delithiation, high-resolution solid-state 7Li and 29Si nuclear magnetic resonance (NMR) are utilized to directly probe the local Li and Si environments on Si electrodes harvested from coin and pouch cells at various states of (de)lithiation. The effect of multivalent cation additives on bulk and surface of aged silicon anodes was studied by multiple structural characterization techniques such as Electron Microscopy, HRXRD, ex situ and operando solid-state NMR. The results show that the electrodes that are gone through cycling and calendar aging with multivalent cations tended to have less cracking on the electrode surface, less lithium trapping in the bulk and the presence of mixed cations enhances cation migration and formation of quaternary Zintl phases stabilizing bulk and forming a more stable SEI in comparison to baseline electrolytes.

Development of Green Conversion Sol-Gel Coatings for Corrosion Protection of AZ61 Magnesium Alloy

The urgent need to address the challenges posed by climate change has led to increased research and development efforts aimed at creating lightweight materials to reduce greenhouse gas emissions. This study focuses on corrosion protection of the commercial magnesium alloy AZ61, valued for its wide-ranging applications in engineering and industry, including automotive, aeronautics, and biomedical engineering. Despite its interesting mechanical properties, the AZ61 alloy exhibits poor corrosion resistance in saline aqueous environments, prompting significant research into coatings to enhance its durability.This research aims to design and prepare organic-inorganic hybrid sol-gel coatings for corrosion protection of the AZ61 alloy. The inorganic phase, mainly based on tetramethyl orthosilicate (TMOS), serves as a cross-linking agent, while four organofunctionalized silanes containing hydrolysable methoxy groups and functional organic groups into the same molecule are used as precursors for the organic component of these hybrid coatings. Specifically, the organofunctionalized silanes selected are γ-methacryloxypropyltrimethoxysilane (MAPTMS), γ -glycidoxypropyltrimethoxysilane (GPTMS), γ -aminopropyltrimethoxysilane (AMPTMS), and γ -mercaptopropyltrimethoxysilane (MPTMS). These silanes are chosen for their ability to create organopolysiloxane coatings with sufficient chemical compatibility and flexibility to accommodate other species without phase segregation or cracking. These properties are essential to create a suitable structural environment to encapsulate the selected ecofriendly corrosion inhibitors that are added in later stages of the study, thereby yielding a processable system that serves as a functional phase of active corrosion protection coatings for magnesium alloys.Physicochemical characterization techniques are employed to understand the chemical environment within the hybrid networks and confirm the successful formation of organopolysiloxane networks. Thermogravimetry and differential thermal analysis (TG/DTA), Fourier transform infrared spectroscopy (FTIR), and high-resolution solid-state 13C and 29Si nuclear magnetic resonance spectroscopies are used for these purposes.The performance of coated AZ61 samples is analysed using the aforementioned organic-inorganic hybrid gels. These sol-gel coatings are applied using immersion techniques (dip-coating) on unpolished (as-received) and polished AZ61 samples, to determine the influence of surface conditions on their behaviour during immersion tests in 0.6, 0.06 and 0.006 M NaCl aqueous solutions. Open circuit potential (OCP) measurements and global and localized electrochemical impedance spectroscopies (EIS, LEIS) are applied with this purpose. The microstructure and texture of the coatings, both before and after the corrosion tests, are observed by optical microscopy (OM) and scanning electron microscopy (SEM). Moreover, Energy dispersive X-ray (EDX) microanalyses are performed on several areas of the coated samples after the corrosion tests.In the last phase of the study, modifications to the sol-gel formulations are explored by incorporating environmentally friendly corrosion inhibitors such as cysteine (L-Cys) and benzotriazole (BTA), as well as cross-linking agents like hexamethoxymethylmelamine (HMMM) in presence of an acid catalysts, specifically p-toluenesulfonic acid (p-TSA), to facilitate the cross-linking reaction within the organosilicon network. These modifications aim to enhance the protective properties of the sol-gel coatings. EIS and LEIS are used to assess their effectiveness. Films formulated with HMMM demonstrate robust passive protection against corrosion, while those doped with L-Cys and BTA exhibit self-healing and active protection properties, offering a promising alternative to traditional chemical conversion pretreatments based on hexavalent chromium. Funding Sources This work has been supported by the Project PID2022-139920OB-I00 (Ministry of Science, Innovation and Universities, MICINN, Spain).

High resolution NMR spectra of fluorotrimethylsilane

High resolution 19F, 1H, 13C, and 29Si NMR spectra of (CH3)3SiF are reported, including chemical shifts, and coupling constants for all isotopologues.

Solid-state 29Si NMR spectra of pure silica zeolites for the International Zeolite Association Database of Zeolite Structures

Abstract With a view to extending the comprehensiveness of the International Zeolite Association Database of Zeolite Structures, this paper reports initial efforts to incorporate solid-state nuclear magnetic resonance (NMR) spectra in the database. A collection of 51 29Si solid-state NMR spectra of pure silica zeolites with 39 different framework types, gleaned from the published scientific literature over the past four decades, is presented here. High resolution 29Si solid-state NMR spectra showing crystallographic resolution of the Si sites were collected, digitized, analyzed, and incorporated into the online Database of Zeolite Structures and can be freely viewed online in an interactive fashion. In addition, with such a large dataset of 29Si solid-state NMR spectra, various correlations between 29Si chemical shifts and geometric parameters describing the local structure around the Si sites were evaluated, showing that the best correlation results when both Si–O–Si bond angles and Si–O bond lengths are considered. It is anticipated that this addition of solid-state NMR spectra to the Database of Zeolite Structures will be of great interest and usefulness to the international community of zeolite scientists.

Highly reactive nanomineral assembly in soil colloids: Implications for paddy soil carbon storage

Mineral availability for carbon (C) binding is a key regulator of soil C storage, yet little is known about the highly reactive nanomineral assembly in the paddy soil colloids. Here, using high-resolution transmission electron microscopy (HRTEM), solid-state 27Al and 29Si nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS), we investigated the coordination nature of short-range-ordered (SRO) minerals in water-dispersible colloids that were isolated from the paddy soil under different six-year fertilization regimes. Our results showed that organic fertilization not only promoted the transformation of crystalline minerals to SRO phases in the bulk soils but also increased the concentrations of Fe, Al and Si in the soil colloids compared to chemical fertilization alone, and thus enhanced the accumulation of organic C in both the bulk soils and the soil colloids. The HRTEM images revealed that water-dispersible colloids in all soils, regardless of treatment, were composed of crystalline Fe nanominerals (with some Al/Si) and SRO-Al/Si nanominerals (with some Fe) associated with organic C. Furthermore, the combined results from the 27Al and 29Si NMR spectroscopy and XPS not only confirmed the presence of SRO-Al/Si nanoparticles as Si-rich allophane and phytolith but also demonstrated that organic fertilization promoted the transformation of aluminosilicates to SRO-Al/Si nanominerals in soil colloids. Together, these findings indicate that six-year organic fertilization promotes the formation of SRO minerals (e.g., ferrihydrite, Si-rich allophane and Fe-substituted allophane, as well as phytolith) in soils and modulates the assembly of organo-mineral complexes possibly by driving the biogeochemical cycles of Fe, Al, Si and specific organic ligands, thus contributing to the long-term storage of C in paddy soils.

The degree of polymerization and structural disorder in (Mg,Fe)SiO3 glasses and melts: Insights from high-resolution 29Si and 17O solid-state NMR

Probing the degree of polymerization and structural disorder of (Mg,Fe)SiO3 glasses and melts remains a challenging problem, despite their strong implications for the properties of natural volcanic glasses and dense melts in Earth’s surface and interior. Here, we explore the iron-induced changes in silicon and the oxygen configurations of (Mg1−XFeX)SiO3 glasses (Fe3+/ƩFe = 0.22 ± 0.07) with varying XFe [=Fe/(Mg + Fe), containing up to 13.5 wt% FeO] using high-resolution 29Si and 17O MAS, and 2D 17O 3QMAS NMR, unveiling the effect of Fe2+ on the extent of disorder in these glasses. The 29Si MAS NMR spectra show the apparent peak broadening in less polymerized species (i.e., Q1) and an increase in the fraction of highly polymerized (i.e., Q3) species with increasing XFe. These changes imply the selective decrease in NMR signal intensity of Q1 and Q2 due to their spatial proximity to Fe2+ (i.e., paramagnetic effect), and slight increase in the degree of polymerization due to the presence of minor fraction of Fe3+. The 2D 17O NMR spectra revealed that the fraction of Si-O-Si apparently increases with XFe and more significant changes in peak width for Mg-O-Si are observed with varying XFe. The results indicate that Mg-O-Si is more strongly affected by Fe2+ than Si-O-Si, suggesting a moderate degree of preferential proximity between Fe2+ and non-bridging oxygen. The results allow us to conceptualize the scheme to account for paramagnetic interaction between the nuclear spins and Fe2+. The scheme provides a systematic protocol to interpret the NMR spectra of diverse iron-bearing silicate glasses. The current NMR results, in conjunction with the results in the previous study for Fe3+-dominant sodium silicate glasses, suggest that paramagnetic effect depends on the valence state of Fe, indicating the presence of a moderate degree of preferential interaction between Fe2+ and NBO and less polymerized Si units as well as that between Fe3+ and highly polymerized Si units and BO. Though qualitative, the current NMR results for (Mg,Fe)SiO3 glasses shed light on an opportunity to explore the detailed structures of iron-bearing natural volcanic glasses and can improve understanding of the transport properties of the basaltic melts.

Microstructure of β-Dicalcium Silicate after Accelerated Carbonation

The present work presents the microstructure of β-Ca2SiO4 (β-C2S) after accelerated carbonation. The synthesis procedure of β-C2S was examined first, and the crystalline and amorphous structure, the distribution and the pore structure of β-C2S carbonation products were also determined by X-ray diffraction (XRD) quantitative analysis, simultaneous thermal analyzer (TG/DTA), Fourier transform-infrared spectroscopy (FT-IR), high resolution 29Si magic angle spinning nuclear magnetic resonance (29Si NMR), N2-sorption techniques, and scanning electron microscopy (SEM), respectively. Test results indicate that carbonation products are dramatically formed in the initial 2 h. The main carbonation products are crystalline calcite and amorphous three-dimensional network silica gels, which contain nanometer-sized pores. The calcite, silica gels and un-carbonated β-C2S are distributed hierarchically.

Effect of Ge/Si substitutions on the local geometry of Si framework sites in zeolites: A combined high resolution 29Si MAS NMR and DFT/MM study on zeolite Beta polymorph C (BEC)

We employed density functional theory/molecular mechanics (DFT/MM) calculations and 29Si magic-angle spinning (MAS) NMR spectroscopy to investigate the effect of single and multiple Ge/Si substitutions on the 29Si NMR parameters as well as the local geometry of SiO4 tetrahedra of the nearest (Ge-O-Si) and next-nearest (Ge-O-Si-O-Si) neighboring Si atoms. The influences of the Ge/Si substitutions are compared with the effects of the corresponding Al/Si substitutions (i.e., Al-O-Si and Al-O-Si-O-Si, respectively). Zeolite Beta polymorph C (BEC), containing double four-membered rings (D4Rs) and exhibiting three distinguishable T sites in the framework, was chosen for this study as a model of germanium containing zeolites. Our computations give a systematic downshift of the 29Si chemical shift of Si by 1–6 ppm for Ge-O-Si sequences. Furthermore, the contributions of two, three, and four Ge atoms as the nearest neighbors to the downshift of Si are not additive and the calculated downshifts lie in the intervals from 2 to 6 ppm, from 1 to 9 ppm, and from 5 to 11 ppm, respectively. Conversely, the contributions of two, three, and four Al atoms as the nearest neighbors are approximately additive. The downshifts caused by Ge nearest neighbors are less than half compared with the corresponding downshifts caused by Al. Moreover, our calculations show that there are no systematic contributions of Ge and Al as next-nearest neighbors (i.e., Ge-O-Si-O-Si and Al-O-Si-O-Si, respectively) to the 29Si chemical shift of Si, and not even the direction (sign) can be predicted without calculating the corresponding sequence.

Low-temperature synthesis of zircon by soft mechano–chemical activation-assisted sol–gel method

Zircon (i.e., ZrSiO4) was synthesized by a sol–gel route at a low temperature with assistance of soft mechano–chemical activation of derived precursors in a high-energy density stirred bead mill. The reaction of precursors pretreated by soft mechano–chemical at different time in sol–gel process was analyzed. The samples were characterized by X-ray diffraction, Fourier transfer infrared spectroscopy, nitrogen gas adsorption method (BET), thermogravimetry, scanning electron microscopy and high-resolution 29Si nuclear magnetic resonance spectroscopy. The results show that zircon powder with a high crystallinity and a small grain size can be formed via soft mechano–chemical pre-activation and sol–gel route at a lower temperature. The specific surface area of calcined powder increases and the average crystalline size and mean surface particle size decreases with increasing the soft mechano–chemical treatment time. It is indicated that the soft mechano–chemical activation can accelerate the dehydration/dehydroxylation reactions, reduce the bonding degree of Si–O bonds, and break a certain extent of [SiO4]4− tetrahedral three-dimensional network, which is formed in the sol–gel reaction, thus enhancing the reactivity of precursors and facilitating the formation of ZrSiO4 from Zr4+ ions with [SiO4]4− tetrahedra during the subsequent thermal treatment at a lower temperature (i.e., 700 °C). XRD patterns of all the ground precursors fired at 700 °C. Soft mechano–chemical activation of precursor prepared in sol–gel route could decrease the subsequent sintering temperature of ZrSiO4. Compared to the precursor unactivated, ZrSiO4 powder with a high crystallinity was formed at a lower sintering temperature (i.e., 700 °C) for the precursor activated for 6 h.

High-resolution 29Si CP/MAS solid state NMR spectroscopy and DFT investigation on the role of geminal and single silanols in grafting chromium species over Phillips Cr/silica catalyst

Phillips Cr/silica catalyst is industrially important in ethylene polymerization. The high-resolution solid state 1H MAS NMR and 29Si CP/MAS NMR allowed the identification of the type and amount of silanols: geminal vs. single (isolated and vicinal) silanols on Phillips catalysts calcined at different temperatures, which were compared with those of the bare silica gel counterparts. The residual silanols on the catalyst and silica gel samples were all decreased with increasing calcination temperatures from 120 to 800°C. For the catalysts treated at temperatures lower than 300°C, the amount of residual silanol groups were much lower than those of the silica gel counterparts. It suggested that the chromium species were mainly grafted on the silica gel through esterification reaction with surface silanols below 300°C. The geminal silanols almost disappeared on the catalysts at 120°C, while that for the silica gel occurred at 300°C. Further increasing the calcination temperatures from 300 to 800°C, the amount of single silanols were slower decreased for the catalysts than that for the silica gel samples. It indicated that the presence of the grafted chromate species obstructed further removal of the residual single silanols. The role of silanols on the formation of surface chromate species on the well-defined polyoligomericsilsesquioxane (POSS) models containing various types of silanols was theoretically studied by density functional theory (DFT) method. It was shown that one silanol of the geminal pair and another adjacent single silanol was the most thermodynamically favored for grafting chromium species. The priority of the reaction between chromium species and different types of surface silanol groups during calcination for Phillips catalysts were experimentally and theoretically elucidated for the first time.

Effect of the raw material type and the reaction time on the synthesis of halloysite based Zeolite Na-P1

Zeolites are currently one of the most important classes of inorganic materials because of their multiple applications not only as ions exchangers and molecular sieves, but also as catalysts. This works focus the synthesis and the characterization of Zeolite Na-P1 using halloysite (collected near Ain Khemouda, western Tunisia) as the starting material. Two parameters, such as the host materials type (natural or treated) and the reaction time, involved in the synthesis process are investigated. The intermediate phases and final products were characterized by X-ray diffraction, Infrared IR spectroscopy, scanning electron microscopy and high-resolution 29Si and 27Al MAS NMR. Obtained results show that the hydrothermal synthesis from natural and heated-halloysite leads to formation of homogenous Zeolite Na-P1. The difference in the crystallization/transformation time process is explained by the effect of the dissolution rate of the starting materials in sodium hydroxide solution. In the case of heated halloysite, the synthesis reaction with alkali solution occurs very readily and achieved without prior thermal activation at high temperature. The optimal conditions of Zeolite Na-P1 crystallization, from heated-halloysite, are reached at 120°C.

Spying on Fe ions and their role in modified aluminosilicates during the sorption of anions using solid-state NMR spectroscopy

Post-synthetic modifications of zeolites enable their possible use as adsorbents in separation and purification processes. Here, we present a multinuclear (29Si, 27Al, 31P) magnetic resonance study of solid samples of zeolite A (LTA) and hydroxysodalite (HS), their post-synthetic modification using FeSO4 solution, and the results of their analysis by NMR after the anion (PO43−) adsorption process. High-resolution 29Si and 27Al MAS NMR spectra provide valuable structural information on the oxidation of Fe2+ ions to Fe3+ species during the modification process, whereas 27Al 3Q/MAS solid-state NMR spectra of LTA and HS unveil a simultaneous dealumination process characteristic of acid leaching in aluminosilicate systems. 31P solid-state NMR spectra of LTA and HS, collected at “very-fast” spinning speed, revealed a dramatic change in the chemical shift values for the phosphates present, directly characteristic of FePO4 in the case of LTA, and phosphates of alkali and/or alkaline earth metals in the case of HS. Moreover, density functional theoretical (DFT) computations provide satisfactory agreement with the obtained 27Al and 29Si experimental data. This work demonstrates possible NMR approaches to the characterization of complex framework materials and provides key insights into aluminosilicate materials affected by unpaired electron(s).

Structure and Chemical Durability of Lead Crystal Glass.

Silicate glasses containing lead, also called lead crystal glasses, are commonly used as food product containers, in particular for alcoholic beverages. Lead's health hazards require major attention, which can first be investigated through the understanding of Pb release mechanisms in solution. The behavior of a commercial crystal glass containing 10.6 mol % of PbO (28.3 wt %) was studied in a reference solution of 4% acetic acid at 22, 40, and 70 °C at early and advanced stages of reaction. High-resolution solid-state 17O and 29Si NMR was used to probe the local structure of the pristine and, for the first time, of the altered lead crystal glass. Inserted into the vitreous structure between the network formers as Si-O-Pb bonds, Pb does not form Pb-O-Pb clusters which are expected to be more easily leached. A part of K is located near Pb, forming mixed Si-O-(Pb,K) near the nonbridging oxygens. Pb is always released into the solution following a diffusion-controlled dissolution over various periods of time, at a rate between 1 and 2 orders of magnitude lower than the alkalis (K and Na). The preferential release of alkalis is followed by an in situ repolymerization of the silicate network. Pb is only depleted in the outermost part of the alteration layer. In the remaining part, it stays mainly surrounded by Si in a stable structural configuration similar to that of the pristine glass. A simple model is proposed to estimate the Pb concentration as a function of glass surface, solution volume, temperature, and contact time.

Insights into the physicochemical characteristics from vermiculite to silica nanosheets

Mesoporous silica nanosheets (SiNSs) were prepared from acid-treated vermiculite (Verm). The influences of acid concentration and reaction time on the structural and textural properties of acid-treated samples were investigated. The physicochemical characteristics of SiNSs were studied by X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared (FTIR), N2 adsorption-desorption techniques, zeta potential, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and 29Si and 27Al nuclear magnetic resonance (NMR). XRF study indicated that acid-treatment leads to the removal of the aluminum and magnesium from the octahedral sheets along with other impurities. XRD, SEM and HRTEM measurements supported that treatment with moderate concentrated acid and reaction time favor the formation of translucent and monodispersed SiNSs, which have been reported rarely. Acid-activation with 2M HCl for 12h gave a maximum value of specific surface area (764m2 g−1) with zeta potential of −38.9mV. FTIR and NMR spectroscopic analyses demonstrated the formation of active groups (Si-OH groups) on the surface of SiNSs. The SiNSs thus obtained by acid-treatment can be used as promising catalyst supports, adsorbents, and morphology-controlling reagents of nanomaterials.

On the origin of high ionic conductivity in Na-doped SrSiO3.

Understanding the local structure and ion dynamics is at the heart of ion conductor research. This paper reports on high-resolution solid-state 29Si, 23Na, and 17O NMR investigation of the structure, chemical composition, and ion dynamics of a newly discovered fast ion conductor, Na-doped SrSiO3, which exhibited a much higher ionic conductivity than most of current oxide ion conductors. Quantitative analyses reveal that with a small dose (<10 mol%) of Na, the doped Na integrates into the SrSiO3 structure to form Na x Sr1-x SiO3-0.5x , and with >10 mol% Na doping, phase separation occurs, leading to the formation of an amorphous phase β-Na2Si2O5 and a crystalline Sr-rich phase. Variable-temperature 23Na and 17O magic-angle-spinning NMR up to 618 °C have shown significant changes in Na ion dynamics at high temperatures but little oxide ion motion, suggesting that Na ions are responsible for the observed high ionic conductivity. In addition, β-Na2Si2O5 starts to crystallize at temperatures higher than 480 °C with prolonged heating, resulting in reduction in Na+ motion, and thus degradation of ionic conductivity. This study has contributed critical evidence to the understanding of ionic conduction in Na-doped SrSiO3 and demonstrated that multinuclear high-resolution and high-temperature solid-state NMR is a uniquely useful tool for investigating ion conductors at their operating conditions.

Effect of iron content on the structure and disorder of iron-bearing sodium silicate glasses: A high-resolution 29Si and 17O solid-state NMR study

Despite its geochemical importance and implications for the properties of natural magmatic melts, understanding the detailed structure of iron-bearing silicate glasses remains among the outstanding problems in geochemistry. This is mainly because solid-state NMR techniques, one of the most versatile experimental methods to probe the structure of oxide glasses, cannot be fully utilized for exploring the structural details of iron-bearing glasses as the unpaired electrons in Fe induce strong local magnetic fields that mask the original spectroscopic features (i.e., paramagnetic effect). Here, we report high-resolution 29Si and 17O solid-state NMR spectra of iron-bearing sodium silicate glasses (Na2O–Fe2O3–SiO2, Fe3+/ΣFe=0.89±0.04, thus containing both ferric and ferrous iron) with varying XFe2O3 [=Fe2O3/(Na2O+Fe2O3)], containing up to 22.9wt% Fe2O3. This compositional series involves Fe–Na substitution at constant SiO2 contents of 66.7mol% in the glasses. For both nuclides, the NMR spectra exhibit a decrease in the signal intensities and an increase in the peak widths with increasing iron concentration partly because of the paramagnetic effect. Despite the intrinsic difficulties that result from the pronounced paramagnetic effect, the 29Si and 17O NMR results yield structural details regarding the effect of iron content on Q speciation, spatial distribution of iron, and the extent of polymerization in the iron-bearing silicate glasses. The 29Si NMR spectra show an apparent increase in highly polymerized Q species with increasing XFe2O3, suggesting an increase in the degree of melt polymerization. The 17O 3QMAS NMR spectra exhibit well-resolved non-bridging oxygen (NBO, Na–O–Si) and bridging oxygen (BO, Si–O–Si) peaks with varying iron concentration. By replacing Na2O with Fe2O3 (and thus with increasing iron content), the fraction of Na–O–Si decreases. Quantitative consideration of this effect confirms that the degree of polymerization is likely to increase with iron content and that Fe3+ is predominantly a network-former. The 17O NMR spectra suggest a moderate degree of preferential partitioning of iron between NBO and BO clusters. The present results bear strong promise for studying iron-bearing silicate glasses using solid-state NMR techniques, constraining the effect of iron content on the degree of polymerization. The observed changes in atomic structures of iron-bearing sodium silicate glasses will be helpful for unraveling atomic origins of the properties of natural silicate melts.

Low-Temperature High-Resolution Solid-State (cryoMAS) NMR of Han Purple BaCuSi2O6

We present low-temperature (low-T, sub-liquid-N2) high-speed high-resolution 29Si solid-state (cryoMAS) nuclear magnetic resonance studies on a model 2D-BEC quantum magnet BaCuSi2O6, known also as Han Purple. We observe broadened 29Si lines below the well-established 100 K structural phase transition confirming the existence of inhomogeneities at low temperatures. Interestingly, the low-T spectra of \({\mathrm{BaCuSi}}_{2}{\mathrm{O}}_{6}\) closely resemble those of the novel compound Ba2CoSi2O6Cl2 taken at room temperature. This suggests that the Co compound features structural modulations or inhomogeneities already at room temperature. The low-T crystal structure and magnetism of \({\mathrm{BaCuSi}}_{2}{\mathrm{O}}_{6}\) are more complex than previously believed, and deserve further investigation.

Preparation and characterization of sol–gel-modified pineapple leaf fiber/polylactic acid composites

In this study, the fibers generated from agricultural waste-pineapple leaf are obtained through exposure, drying, crushing and sifting. A novel sol–gel method is utilized to modify the pineapple leaf fiber (PALF), which is subsequently treated by a coupling agent. The aim is to improve the compatibility between PALF and the polymer matrix and to enhance the heat resistance and mechanical properties of the composite material. Furthermore, a series of modified PALF/polylactic acid (PLA) composites are prepared. FTIR, high resolution solid-state 13C and 29Si NMR experiments show that the PALF was successfully modified by the silane coupling agent and sol–gel method. Polarizing optical microscopy analysis reveals PLA crystal growth of a sufficiently high density along the polymer-fiber interface. Moreover, the storage and loss moduli of PLA are increased by adding the modified PALF. Apart from the enhancement of the mechanical properties, the incorporation of modified PALF reduces the amount of agricultural waste and extends the application of PLA.

Reversibly meltable layered alkylsiloxanes with melting points controllable by alkyl chain lengths

Meltable layered alkylsiloxanes (CnLSiloxanes) were synthesized from tetraethoxysilane and alkyltrialkoxysilanes with carbon numbers (n) of 12, 16, and 18 via hydrolysis and polycondensation at 100 and 150 °C under basic conditions. Differential scanning calorimetric (DSC) measurements revealed that CnLSiloxanes melted reversibly at −0.8 to 51.3 °C, the melting points being dependent on n. Scanning electron microscope (SEM) images showed that thin plates were stacked. X-ray diffraction (XRD) peaks were observed at angles corresponding to distances (d) of 2.1, 2.5, and 2.8 nm for C12-, C16-, and C18LSiloxanes, respectively. High-resolution solid-state 13C nuclear magnetic resonance (NMR) measurements showed that the organic moieties were alkylsilyl groups with long alkyl chains and that an all-trans conformation was dominant. This was supported by the XRD peak corresponding to a d value of 0.41 nm. High-resolution solid-state 29Si NMR measurements demonstrated the presence of SiO4 and CSiO3 units. A structural model has been proposed for CnLSiloxanes, where siloxane sheets consisting of the SiO4 and CSiO3 units are stacked with the ordered interdigitated monolayer of the alkyl chains in between and are bonded covalently with the alkyl chains.

7Li and 29Si solid state NMR and chemical bonding of La2Li2Si3

The ternary silicide La2Li2Si3 was synthesized from the elements in a sealed niobium tube. La2Li2Si3 was characterized by powder and single crystal X-ray diffraction: Ce2Li2Ge3 type, Cmcm, a = 450.03(8), b = 1880.3(4), c = 689.6(1) pm, wR2 = 0.0178, 597 F2 values, and 26 parameters. The La2Li2Si3 structure contains two crystallographically independent silicon sites, both in slightly distorted trigonal prismatic coordination. The Si1 atoms are located in condensed La6 prisms and form cis–trans chains (two-bonded silicon) with Si1–Si1 distances at 238 and 239 pm, indicating single bond character. The Si2 atoms are isolated within La2Li4 prisms. La2Li2Si3 might be formally considered as an electron precise Zintl phase with an electron partition (2La3+)(2Li+)(2Si12–)(Si24–). Electronic structure calculations show a trend in this direction based on a charge density analysis with large electron localization around the Si1–Si1 chains. The compound is found weakly metallic with chemical bonding reminiscent of LaSi and additional features brought in by Li and Si2. High resolution solid state 7Li and 29Si MAS-NMR spectra are in agreement with the crystal structural information, however, the 29Si resonance shifts observed suggest strong Knight shift contributions, at variance with the Zintl concept. Variable temperature solid state 7Li spectra indicate the absence of motional narrowing on the kHz timescale within the temperature range 300K < T < 400 K.