The effect of MgO/TiO2 on structural and crystallization behavior of near invert phosphate-based glasses

Varying formulations in the glass system of 40P2O5─(24 − x)MgO─(16 + x)CaO─(20 − y)Na2O─yTiO2 (where 0 ≤ x ≤ 22 and y = 0 or 1) were prepared via melt-quenching. The structure of the glasses was confirmed by X-ray diffraction (XRD), Fourier transform infrared (FTIR), micro Raman and solid-state nuclear magnetic resonance (NMR) spectroscopies. The thermal properties and the activation energy of crystallization (Ec) were measured using thermal analysis and the Kissinger equation, respectively. The glass forming ability of the formulations investigated was seen to decrease with reducing MgO content down to 8 mol% and the glass stability region also decreased from 106 to 90°C with decreasing MgO content. The activation energy of crystallization (Ec) values also decreased from 248 (for 24 mol% MgO glass) to 229 kJ/mol (for the 8 mol% MgO content) with the replacement of MgO by CaO for glasses with no TiO2. The formulations containing less than 8 mol% MgO without TiO2 showed a strong tendency toward crystallization. However, the addition of 1 mol% TiO2 in place of Na2O for these glasses with less than 8 mol% MgO content, inhibited their crystallization tendency. Glasses containing 8 mol% MgO with 1 mol% TiO2 revealed a 12°C higher glass transition temperature, a 14°C increase in glass stability against crystallization and a 38 kJ/mol increase in Ec in comparison to their non TiO2 containing counterpart. NMR spectroscopy revealed that all of the formulations contained almost equal percentages of Q1 and Q2 species. However, FTIR and Raman spectroscopies showed that the local structure of the glasses had been altered with addition of 1 mol% TiO2, which acted as a network modifier, impeding crystallization by increasing the cross-linking between phosphate chains and consequently leading to increased Ec as well as their glass forming ability

Effects of Total Resources, Resource Ratios, and Species Richness on Algal Productivity and Evenness at Both Metacommunity and Local Scales

The study of the interrelationship between productivity and biodiversity is a major research field in ecology. Theory predicts that if essential resources are heterogeneously distributed across a metacommunity, single species may dominate productivity in individual metacommunity patches, but a mixture of species will maximize productivity across the whole metacommunity. It also predicts that a balanced supply of resources within local patches should favor species coexistence, whereas resource imbalance would favor the dominance of one species. We performed an experiment with five freshwater algal species to study the effects of total supply of resources, their ratios, and species richness on biovolume production and evenness at the scale of both local patches and metacommunities. Generally, algal biovolume increased, whereas algal resource use efficiency (RUE) and evenness decreased with increasing total supply of resources in mixed communities containing all five species. In contrast to predictions for biovolume production, the species mixtures did not outperform all monocultures at the scale of metacommunities. In other words, we observed no general transgressive overyielding. However, RUE was always higher in mixtures than predicted from monocultures, and analyses indicate that resource partitioning or facilitation in mixtures resulted in higher-than-expected productivity at high resource supply. Contrasting our predictions for the local scale, balanced supply of resources did not generally favor higher local evenness, however lowest evenness was confined to patches with the most imbalanced supply. Thus, our study provides mixed support for recent theoretical advancements to understand biodiversity-productivity relationships

Carbon-13 chemical shift tensors of disaccharides: measurement, computation and assignment.

A recently developed chemical shift anisotropy amplification solid-state nuclear magnetic resonance (NMR) experiment is applied to the measurement of the chemical shift tensors in three disaccharides: sucrose, maltose, and trehalose. The measured tensor principal values are compared with those calculated from first principles using density functional theory within the planewave-pseudopotential approach. In addition, a method of assigning poorly dispersed NMR spectra, based on comparing experimental and calculated shift anisotropies as well as isotropic shifts, is demonstrated

Muon spin relaxation studies of lithium nitridometallate battery materials : muon trapping and lithium ion diffusion

Lithium nitride has a unique layered structure and the highest reported Li+ ion conductivity for a crystalline material. The conductivity is highly anisotropic, with an intralayer contribution within the graphitic [Li2N] planes dominant at ambient temperature. In this paper transverse- and zero-field muon spin relaxation (mu SR) studies on Li3N and two novel paramagnetic derivatives Li3-x-yNixN with x = 0.36 and 0.57 are reported. These new materials have potential as anodes in rechargeable lithium batteries. The decrease in the muon depolarization rate observed above 180 K for the three materials is shown to arise from motional narrowing due to intralayer Li+ diffusion. The increase in the measured activation energy with x for Li3-x-yNixN suggests that the reduction in the layer spacing that results at high substitution levels is responsible for raising the energy barrier to Li+ jumps, despite the concomitant expansion of the [Li2N] plane. In addition, the onset of interlayer diffusion appears at lower temperatures in Ni-substituted derivatives than in the parent Li3N. The muons themselves are quasi-static, most probably located in a 4h site between the [Li2N] plane and the Li(1)/Ni layer. This is similar to the Li+ interstitial position identified by molecular dynamics simulations as an intermediate for an exchange mechanism for interlayer diffusion. Finally, mu SR gives no evidence for the formation of the muonium equivalent of the hydrogen defects thought to play an important role in intralayer diffusion in Li3N. These results demonstrate that mu SR can be used to obtain diffusion coefficients and activation energies for Li+ transport even in paramagnetic materials where NMR Studies are complicated by strong interactions with the electronic moments

Structure, stoichiometry and transport properties of lithium copper nitride battery materials: combined NMR and powder neutron diffraction studies

A combined NMR and neutron diffraction study has been carried out on three Li<sub>3−x−y</sub>Cu<sub>x</sub>N materials with <i>x</i> = 0.17, <i>x</i> = 0.29 and <i>x</i> = 0.36. Neutron diffraction indicates that the samples retain the P6/mmm space group of the parent Li<sub>3</sub>N with Cu located only on Li(1) sites. The lattice parameters vary smoothly with <i>x</i> in a similar fashion to Li<sub>3−x−y</sub>Ni<sub>x</sub>N, but the Li(2) vacancy concentration for the Cu-substituted materials is negligible. This structural model is confirmed by wideline 7Li NMR spectra at 193 K which show three different local environments for the Li(1) site, resulting from the substitution of neighbouring Li atoms in the Li(1) layer by Cu. Since the Cu-substituted materials are only very weakly paramagnetic, variable temperature 7Li wideline NMR spectra can be used to measure diffusion coefficients and activation energies. These indicate anisotropic Li<sup>+</sup> diffusion similar to the parent Li<sub>3</sub>N with transport confined to the [Li<sub>2</sub>N] plane at low temperature and exchange between Li(1) and Li(2) sites dominant at high temperature. For the intra-layer process the diffusion coefficients at room temperature are comparable to Li<sub>3</sub>N and Li<sub>3−x−y</sub>Ni<sub>x</sub>N, while <i>E</i><sub>a</sub> decreases as <i>x</i> increases in contrast to the opposite trend in Ni-substituted materials. For the inter-layer process <i>E</i><sub>a</sub> decreases only slightly as <i>x</i> increases, but the diffusion coefficients at room temperature increase rapidly with <i>x</i>

Distinguishing hydrogen bonding networks in alpha-D-galactose using NMR experiments and first principles calculations

First principles calculations and solid-state NMR experiments are used to distinguish between possible hydrogen bonding networks in α-d-galactose. In contrast to 13C, the 1H chemical shift parameters show differences which are sufficient to allow the correct network to be identified by comparison with experiments which make use of modern homonuclear decoupling schemes. In addition, clear linear correlations are established between both 1H chemical shift and chemical shift anisotropy, and hydrogen bond length. © 2010 Elsevier B.V. All rights reserved

Through-space contributions to two-dimensional double-quantum J correlation NMR spectra of magic-angle-spinning solids

A routinely used assumption when interpreting two-dimensional NMR spectra obtained with a commonly used double-quantum (DQ) magic-angle-spining (MAS) pulse sequence referred to as the refocused incredible natural abundance double-quantum transfer experiment (INADEQUATE) [A. Lesage, M. Bardet, and L. Emsley, J. Am. Chem. Soc. 121, 10987 (1999)] has been that correlation peaks are only observed for pairs of nuclei with a through-bond connectivity. The validity of this assumption is addressed here by theory, experiment, and computer simulations. If the isotropic chemical shifts of the two nuclei are different and the MAS frequency is far from rotational resonance, the theoretical description demonstrates that DQ correlation peaks are indeed indicative of a J coupling. However, if the isotropic chemical shifts are the same, it is shown that DQ peaks can appear for pairs of nuclei even in the absence of a through-bond J coupling. These peaks appear in the specific case of a pair of nuclei with a nonzero through-space dipole-dipole coupling and chemical shift anisotropy tensors having different principal magnitudes or orientations, provided that the MAS frequency is comparable to or smaller than, the chemical shift anisotropies. Experimental P-31 Spectra recorded on a sample of TiP2O7 and computer simulations show that the magnitude of these anomalous peaks increases with increasing B-0 magnetic field and that they decrease with increasing MAS frequency. This behavior is explained theoretically. (c) 2005 American Institute of Physics