Chemical vs. electrochemical extraction of lithium from the Li-excess Li(1.10)Mn(1.90)O4 spinel followed by NMR and DRX techniques.

Lithium extraction from the Li-excess Li1.10Mn1.90O4 spinel has been performed by chemical and electrochemical methods in aqueous and in organic media, respectively. De-lithiated samples have been investigated by XRD, SEM, TG, (7)Li and (1)H MAS-NMR techniques. The comparative study has allowed demonstrating that the intermediate de-intercalated samples prepared during the chemical extraction by acid titration are similar to those prepared by the electrochemical way in a non-aqueous electrolyte. LiMn2O4 based spinel with a tailored de-lithiation degree can be prepared as a single phase by controlling the pH used in chemical extraction. (7)Li MAS-NMR spectroscopy has been used to follow the influence of the manganese oxidation state on tetra and octahedral Li-signals detected in Li-extracted samples. The oxidation of Mn(III) ions goes parallel to the partial dissolution of the spinel, following Hunter's mechanism. Based on this mechanism, a generalized chemical reaction has been proposed to explain the formation of intermediate Li(+) de-intercalated samples during acid treatment in aqueous media. By the (1)H MAS NMR study, no evidence of Li-H topotactic exchange in the bulk of the acid treated material was found.

Synergy of diffraction and spectroscopic techniques to unveil the crystal structure of antimonic acid.

The elusive crystal structure of the so-called 'antimonic acid' has been investigated by means of robust and state-of-the-art techniques. The synergic results of solid-state magic-angle spinning nuclear magnetic resonance spectroscopy and a combined Rietveld refinement from synchrotron X-ray and neutron powder diffraction data reveal that this compound contains two types of protons, in a pyrochlore-type structure of stoichiometric formula (H3O)1.20(7)H0.77(9)Sb2O6. Some protons belong to heavily delocalized H3O+ subunits, while some H+ are directly bonded to the oxygen atoms of the covalent framework of the pyrochlore structure, with O-H distances close to 1 Å. A proton diffusion mechanism is proposed relying on percolation pathways determined by bond-valence energy landscape analysis. X-ray absorption spectroscopy results corroborate the structural data around Sb5+ ions at short-range order. Thermogravimetric analysis and differential scanning calorimetry endorsed the conclusions on the water content within antimonic acid. Additional 0.7 water molecules per formula were assessed as moisture water by thermal analysis.

Influence of metakaolin purities on potassium geopolymer formulation: The existence of several networks.

Geopolymer materials are obtained by the alkaline activation of aluminosilicate sources, the best of which is metakaolin. However, every raw material is different, and very few comparative studies have been done on different metakaolin sources. The aim of this work is to develop methods for the prediction of the working properties of geopolymer materials based on the reactivity of the metakaolin employed. Infrared spectroscopy showed direct relationships between the wettability, the Si/Al ratio and the kinetics of conversion of Si-O-Si bonds to Si-O-Al bonds. Moreover, it was demonstrated that the presence of impurities and the reactivity of the metakaolin can generate the formation of one or several networks. Finally, a descriptive model of the mechanism of geopolymer formation was proposed that takes into account the quality of metakaolin used.

Structure and properties of bioactive eutectic glasses based on the Ca3(PO4)2-CaSiO3-CaMg(SiO3)2 system.

Taking into account the phase equilibrium relationships within the Ca3(PO4)2-CaSiO3-CaMg(SiO3)2 ternary system, three bioactive glasses with a eutectic composition and analogous amounts of Ca3(PO4)2 (∼40 wt.%) have been prepared. The structure of the glasses was investigated by 31P and 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The glasses exhibited thermal expansion coefficients (50-600 °C) of 11.8-13.3×10(-6) °C(-1), a glass transition temperature of 790-720 °C and a softening temperature of 811-750 °C. The mechanical properties of the glasses were as follows: bending strength ∼100 MPa, Young's modulus 94-83 GPa, Vickers microhardness 7.1-4.1 GPa and toughness 0.8 MPa m1/2. The bioactive properties were discussed in terms of their structure deduced by MAS-NMR spectroscopy and the field strength of the network modifiers (Mg2+ and Ca2+). A knowledge of the glass structure was important in predicting its bioactivity.