Local structure of glassy lithium phosphorus oxynitride thin films: a combined experimental and ab initio approach

Lithium phosphorus oxynitride (LiPON) is an amorphous solid-state lithium ion conductor displaying exemplary cyclability against lithium metal anodes. There is no definitive explanation for this stability due to the limited understanding of the structure of LiPON. We provide a structural model of RF-sputtered LiPON via experimental and computational spectroscopic methods. Information about the short-range structure results from 1D and 2D solid-state nuclear magnetic resonance experiments investigating chemical shift anisotropy and dipolar interactions. These results are compared with first principles chemical shielding calculations of Li-P-O/N crystals and ab initio molecular dynamics-generated amorphous LiPON models to unequivocally identify the glassy structure as primarily isolated phosphate monomers with N incorporated in both apical and as bridging sites in phosphate dimers. Structural results suggest LiPON's stability is a result of its glassy character. Free-standing LiPON films are produced that exhibit a high degree of flexibility highlighting the unique mechanical properties of glassy materials

The path from root input to mineral-associated soil carbon is dictated by habitat-specific microbial traits and soil moisture

Soil microorganisms help transform plant inputs into mineral-associated soil organic carbon (SOC) – the largest and slowest-cycling pool of organic carbon on land. However, the microbial traits that influence this process are widely debated. While current theory and biogeochemical models have settled on carbon-use efficiency (CUE) and growth rate as positive predictors of mineral-associated SOC, empirical tests are sparse, with contradictory observations. Using 13C-labeling of an annual grass (Avena barbata) under two moisture regimes, we found that microbial traits associated with formation of 13C-mineral-associated SOC varied by soil habitat, as did active microbial taxa and SOC chemical composition. In the rhizosphere, bacterial-dominated communities with fast growth, high biomass, and high extracellular polymeric substance (EPS) production were positively associated with 13C-mineral-associated SOC. In contrast, the detritusphere held communities dominated by fungi and more filamentous bacteria, and with greater exoenzyme activity; there, 13C-mineral-associated SOC was associated with slower microbial growth and lower microbial biomass. CUE was a negative predictor of 13C-mineral-associated SOC in both habitats. Using 13C-quantitative stable isotope probing, we found that the majority of 13C assimilation in the rhizosphere and detritusphere at week 12 of the experiment was performed by very few bacterial and fungal taxa (3–5% of the total taxa that assimilated 13C). Several complementary chemical analyses (13C-NMR, FTICR-MS, and STXM-NEXAFS) suggested that SOC in the rhizosphere had a more oxidized chemical signature, while SOC in the detritusphere had a less oxidized, more lignin-like chemical signature. Our findings challenge current theory by demonstrating that microbial traits linked with mineral-associated SOC are not universal, but vary with soil habitat and moisture conditions, and are shaped by a small number of active taxa. Emerging SOC models that explicitly reflect these interactions may better predict SOC storage, since climate change causes shifts in soil moisture regimes and the ratio of living versus decaying roots

Detecting Reactive Products in Carbon Capture Polymers with Chemical Shift Anisotropy and Machine Learning

Aminopolymers are attractive sorbents for CO2 direct air capture applications as their amines readily react with atmospheric levels of CO2 to form chemisorbed species. The identity of the chemisorbed species varies upon experimental conditions like amine chemistry, support material, CO2 loading, and humidity, forming a variety of carbonyl-type sites. 13C solid-state nuclear magnetic resonance (NMR) is often used to help elucidate the identity of the chemisorbed species however the chemical shift range for carbonyl sites is small and comparable to observed chemisorbed 13C peak widths. Herein, application of a 2D chemical shift anisotropy (CSA) recoupling pulse sequence (ROCSA) is used to obtain CSA tensor values at each isotropic chemical shift, overcoming the isotropic peak resolution limitation. CSA tensor values describe the local chemical environment and can readily differentiate between chemisorbed products. To aid this experimental technique, we also developed a k-nearest-neighbor (KNN) classification model to distinguish chemisorbed compounds via their CSA tensor parameters. The combination of 2D CSA measurements coupled with a KNN classification model enhances the ability to accurately identify chemisorbed products especially in the case of mixtures. This methodology is demonstrated on poly(ethylenimine) in a solid-support γ-Al2O3 exposed to CO2 followed by incomplete regeneration at 100 °C and shows a mixture of strongly bound chemisorbed products, ammonium carbamate and urea

Structural modifications induced by Na+/K+ ion exchange in silicate glasses: A multinuclear NMR spectroscopic study

The structural mechanisms of stress relaxation during Na+/K+ ion exchange is studied in a variety of Na silicate glasses with and without alkaline-earth modifiers, using one and two-dimensional 23Na and 29Si nuclear magnetic resonance (NMR) spectroscopy. The results suggest that significant structural modifications accompany the Na+/K+ ion exchange process in the form of a shortening of the average Na[sbnd]O and Si[sbnd]NBO distances and an opening of the Si[sbnd]O[sbnd]Si angles without any detectable change in the Qn speciation. These trends are similar to those observed in analogous mixed Na,K glasses derived via the melt-quench route, with increasing K:Na ratio. Consequently, the ion-exchange-induced reorganization of the glass network is accompanied by a partial relaxation of the stress generated by the exchange of smaller Na+ by the larger K+ and better accommodation of the latter ion as a modifier.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Na+/K+ ion exchange in silicate glasses: Results from 17O 3QMAS NMR

The effect of Na+/K+ ion exchange on the distribution of the modifier ions and on their interaction with the oxygen atoms in the network is investigated in ternary Na,Mg- and Na,Ca- silicate glasses using 17O triple-quantum magic-angle-spinning nuclear magnetic resonance spectroscopy. The results indicate that the K+ ions preferentially exchange with the Na+ ions present in {Na,Ca}-NBO and {Na,Mg}-NBO environments, while the Na-NBO environment is not significantly affected. Remelting of the ion-exchanged glasses leads to a homogenization of the modifier ion distribution around the NBO sites. The dynamical site preference for the Na+/K+ ion exchange has far-reaching implications in our understanding of the alkali ion transport and more specifically of the mixed-alkali effect in silicate glasses.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

LiSi3As6 and Li2SiAs2 with flexible SiAs2 polyanions: synthesis, structure, bonding, and ionic conductivity

Two novel ternary phases, LiSi3As6 and Li2SiAs2, have been synthesized and characterized. Both phases have an identical Si : As ratio of 1 : 2 providing insight on how layers of the parent phase SiAs2 accommodate excess electrons from Li cations to form Si–As anionic frameworks. LiSi3As6 exhibits a variety of bonding schemes involving Si–Si and As–As bonds, as well as corner-sharing SiAs4 tetrahedra, while Li2SiAs2 is isostructural to the previously reported Li2SiP2, with adamantane-like Si4As10 units connected into 3D framework. LiSi3As6 and Li2SiAs2 are predicted to be indirect semiconductors which was experimentally confirmed by optical properties characterization. Li2SiAs2 exhibits low thermal conductivity of 1.20 W m−1 K−1 at 300 K in combination with a room temperature ionic conductivity of 7 × 10−6 S cm−1, an order of magnitude greater than that of the phosphide and nitride analogues, indicating its potential as a solid-state Li-ion conductor.</p