Sites and Mobility of Lithium along the Li1+xTi2-xInx(PO4)3 (0 ≤ x ≤ 2) Series Deduced by XRD, NMR, and Impedance Spectroscopy

The structure and Li conductivity has been investigated in the Li1+xTi2-xInx(PO4)3 (0 ≤ x ≤ 2) series prepared by the ceramic route at 900 °C. The XRD patterns of 0 ≤ x ≤ 0.2 samples show the presence of rhombohedral (S.G. R3̅c); those of 0.2 ≤ x ≤ 1 samples display both rhombohedral and orthorhombic (S.G. Pbca), and 1 ≤ x ≤ 2 samples exhibit only monoclinic (S.G. P21/n) phases. At intermediate compositions, the secondary LiTiPO5 phase was detected. The Rietveld analysis of XRD patterns was used to deduce unit-cell parameters, chemical composition, and percentage of phases. The amount of In3+, deduced from structural refinements of three phases, was confirmed by 31P MAS NMR spectroscopy. The Li mobility was investigated by 7Li MAS NMR and impedance spectroscopies. The Li conductivity increased with the Li content in rhombohedral but decreased in orthorhombic, increasing again in monoclinic samples. The maximum conductivity was obtained in the rhombohedral x = 0.2 sample (σb = 1.9 × 10-3 S·cm-1), with an activation energy Eb = 0.27 eV. In this composition, the overall Li conductivity was σov = 1.7 × 10-4 S·cm-1 and Eov = 0.32 eV, making this composition a potential solid electrolyte for all-solid-state batteries. Another maximum conductivity was detected in the monoclinic x ∼ 1.25 sample (σov = 1.4 × 10-5 S·cm-1), with an activation energy Eov = 0.39 eV. Structural models deduced with the Rietveld technique were used to analyze the conduction channels and justify the transport properties of different Li1+xTi2-x Inx(PO4)3 phases.

29Si Magic Angle Spinning Nuclear Magnetic Resonance, Fourier-Transform Infrared, and Monte Carlo Study of Synthetic Tetrasilicic Magnesium Mica Solid Solutions

The parallel 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) and Fourier-transform infrared study of synthetic micas made it possible to compare structural features of the tetrasilicic magnesium mica K(Mg2.5□0.5) Si4O10(OH)2 (TMM) and their K(Mg3)(Si3.5Mg0.5)O10(OH)2 (TMMA) and K(Mg3)(Si3.5Be0.5)O10(OH)2 (TMMB) derivatives. In the TMM mica, SiO4 tetrahedra are elongated in the plane ab and shortened along the c* direction with respect to those of the phlogopite (Phl) K(Mg3)(Si3Al)O10(OH)2. The substitution of Si4+ by R2+ (Mg2+ or Be2+) produces, besides the 29Si MAS NMR signal of Si (3Si) at -91.2 ppm, new components at -84.4 or -87.5 ppm that correspond to Si (2Si1Mg) or Si(2Si1Be) environments. Tetrahedral cation distributions in TMM/TMMA, TMM/TMMB solid solutions are investigated with respect to the TMM/Phl series by means of NMR and Monte Carlo simulations, concluding that divalent Mg2+ and Be2+ are further dispersed than trivalent Al3+ cations in tetrahedral sheets of micas. In three analyzed series, cation distributions display features between those of the homogeneous dispersion of charges of phlogopites and the maximum dispersion of charges of TMM derivatives. In three series, the location of charge deficits that compensate K+ cations changes from octahedral in TMM to tetrahedral sheets in phlogopite and TMMA and TMMB derivatives.

Self-diffusion in polycrystalline Li1+x Ti2-x Al x (PO4)3 (0.2 ≤ x ≤ 0.4) samples followed by 7Li PFG (pulse field gradient) NMR spectroscopy

Short and long range lithium motions in powder Li_1+x Ti_2-x Al _x (PO₄)₃ (LTAP) NASICON compounds prepared by ceramic (x = 0.2 and 0.4) and sol-gel (x = 0.3 and 0.4) routes are discussed. ND diffraction and MAS-NMR spectroscopy were previously used to investigate structural features of these compounds. In particular, Fourier map differences showed that the amount of Li atoms allocated at M3 increases at the expense of M1 sites when the Li content increases. In this work, PFG-NMR results show that diffusion coefficients rise with the amount of lithium and temperature. The restricted diffusion inside NASICON particles is compared with "free" diffusion processes. At 300 K, diffusion coefficients D _PFG ∼ 5 × 10^-12 m² s^-1 have been deduced in ceramic x = 0.2 and 0.4 samples, decreasing with diffusion time Δ used in PFG experiments. In sol-gel samples, diffusion coefficients are near those of ceramic samples, but decrease faster with diffusion Δ times, as a consequence of the Li confinement inside sub-micrometric crystallites. The NMR spin-echo signal displays minima at specific q(γgδ) values that are related to the crystallite size. From R _dif ∼ q _m ^-1 distances, calculated from the position of minima, and from diffusion coefficients deduced for high Δ values, the mean crystallite size was estimated. Finally, from the temperature dependence of conductivity and diffusion coefficients, the activation energy and charge carriers concentrations were determined.

Hybrid Cements: Mechanical Properties, Microstructure and Radiological Behavior

The use of more eco-efficient cements in concretes is one of the keys to ensuring construction industry sustainability. Such eco-efficient binders often contain large but variable proportions of industrial waste or by-products in their composition, many of which may be naturally occurring radioactive materials (NORMs). This study explored the application of a new gamma spectrometric method for measuring radionuclide activity in hybrid alkali-activated cements from solid 5 cm cubic specimens rather than powder samples. The research involved assessing the effect of significant variables such as the nature of the alkaline activator, reaction time and curing conditions to relate the microstructures identified to the radiological behavior observed. The findings showed that varying the inputs generated pastes with similar reaction products (C-S-H, C-A-S-H and (N,C)-A-S-H) but different microstructures. The new gamma spectrometric method for measuring radioactivity in solid 5 cm cubic specimens in alkaline pastes was found to be valid. The variables involved in hybrid cement activation were shown to have no impact on specimen radioactive content. The powder samples, however, emanated ²²²Rn (a descendent of ²²⁶Ra), possibly due to the deformation taking place in fly ash structure during alkaline activation. Further research would be required to explain that finding.

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)H_0.77(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.

The structure of kaliophilite KAlSiO4, a long-lasting crystallographic problem

Kaliophilite is a feldspathoid mineral found in two Italian magmatic provinces and represents one of the 12 known phases with composition close to KAlSiO₄. Despite its apparently simple formula, the structure of this mineral revealed extremely complex and resisted structure solution for more than a century. Samples from the Vesuvius-Monte Somma and Alban Hills volcanic areas were analyzed through a multi-technique approach, and finally the crystal structure of kaliophilite was solved using 3D electron diffraction and refined against X-ray diffraction data of a twinned crystal. Results were also ascertained by the Rietveld method using synchrotron powder intensities. It was found that kaliophilite crystallizes in space group P3 with unit-cell parameters a = 27.0597 (16), c = 8.5587 (6) Å, V = 5427.3 (7) ų and Z = 54. The kaliophilite framework is a variant of the tridymite topology, with alternating SiO₄ and AlO₄ tetrahedra forming sheets of six-membered rings (6³ nets), which are connected along [001] by sharing the apical oxygen atoms. Considering the up (U) and down (D) orientations of the linking vertex, kaliophilite is the first framework that contains three different ring topologies: nine (1-3-5) (UDUDUD) rings, six (1-2-3) (UUUDDD) rings and twelve (1-2-4) (UUDUDD) rings. This results in a relatively open (19.9 tetrahedra nm^-3) channel system with multiple connections between the double six-ring cavities. Such a framework requires a surprisingly large unit cell, 27 times larger than the cell of kalsilite, the simplest phase with the same composition. The occurrence of some Na for K substitution (3-10%) may be related to the characteristic structural features of kaliophilite. Micro-twinning, pseudo-symmetries and anisotropic hkl-dependent peak broadening were also detected, and they may account for the elusive character of the kaliophilite crystal structure.

Water-Free Proton Conduction in Discotic Pyridylpyrazolate-based Pt(II) and Pd(II) Metallomesogens

In this work we report on water-free proton conductivity in liquid-crystal pyridylpyrazolate-based Pt(II) and Pd(II) complexes [M(pz(R(n,n)py))2] (pz(R(n,n)py) = 3-(3,5-dialkyloxyphenyl)-5-(pyridin-2-yl)pyrazolate, R(n,n) = C6H3(OCnH2n+1)2; n = 4, 12, 16, M = Pd; n = 12, M = Pt) with potential application as electrolyte materials in proton exchange membrane fuel cells. The columnar ordering of the complexes in the liquid-crystalline phase opens nanochannels, which are used for fast proton exchange as detected by impedance spectroscopy and NMR. The NMR spectra indicate that the proton conduction mechanism is associated with a novel C-H···N proton transfer, which persists above the clearing point of the material. The highest conductivity of ∼0.5 μS cm(-1) at 180 °C with an activation energy of 1.2 eV is found for the Pt(II) compound in the mesophase. The Pd(II) complexes with different chain length (n = 4, 12, and 16) show lower conductivity but smaller activation energies, in the range of 0.74-0.93 eV.

Synthesis and characterization of NaNiF3·3H2O: an unusual ordered variant of the ReO3 type

A new hydrated sodium nickel fluoride with nominal composition NaNiF3·3H2O was synthesized using an aqueous solution route. Its structure was solved by means of ab initio methods from powder X-ray diffraction and neutron diffraction data. NaNiF3·3H2O crystallizes in the cubic crystal system, space group Pn3̅ with a = 7.91968(4) Å. The framework, derived from the ReO3 structure type, is built from NaX6 and NiX6 (X = O, F) corner-shared octahedra, in which F and O atoms are randomly distributed on a single anion site. The 2a × 2a × 2a superstructure arises from the strict alternate three-dimensional linking of NaX6 and NiX6 octahedra together with the simultaneous tilts of the octahedra from the cube axis (φ = 31.1°), with a significant participation of hydrogen bonding. NaNiF3·3H2O corresponds to a fully cation-ordered variant of the In(OH)3 structure, easily recognizable when formulated as NaNi(XH)6 (X = O, F). It constitutes one of the rare examples for the a(+)a(+)a(+) tilting scheme with 1:1 cation ordering in perovskite-related compounds. The Curie-like magnetic behavior well-reflects the isolated paramagnetic Ni(2+) centers without worth mentioning interactions. While X-ray and neutron diffraction data evidence Na/Ni order in combination with O/F disorder as a main feature of this fluoride, results from Raman and magic-angle spinning NMR spectroscopies support the existence of specific anion arrangements in isolated square windows identified in structural refinements. In particular, formation of water molecules derives from unfavorable FH bond formation.

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.

Ionic mobility in Nasicon-type LiMIV2(PO4)3 materials followed by 7Li NMR spectroscopy

Lithium mobility in LiM2(PO4)3 compounds, with M= Ge, Ti, Sn, Zr and Hf, has been investigated by 7Li Nuclear Magnetic Resonance (NMR) spectroscopy in the temperature range 100-500 K. From the analysis of 7Li NMR quadrupole interactions (CQ and η parameters), Li sites occupancy and exchange processes between structural sites have been studied. Below 250K, Li ions are preferentially located at M1 sites in rhombohedral phases, but occupy M12 sites in triclinic ones. At increasing temperatures, Li mobility has been deduced from spin-spin () and spin-lattice relaxation () rates. In this analysis, the presence of two relaxation mechanisms in plots has been associated with departures of conductivity from the Arrhenius behavior. At high temperatures, residence times at M12−T11−T11−T1 and M12 sites become similar and conductivity significantly increase. This superionic state can be achieved by enlarged order-disorder transformations in rhombohedral phases, or by sharp first order transitions in triclinic ones. Results described in the LiTi2(PO4)3 sample have been compared with those obtained in rhombohedral Li1+xTi2-xAlx(PO4)3 and LiTi2-xZrx(PO4)3 series showing respectively higher and lower conductivities. In the case of Li1.2Ti1.8Al0.2(PO4)3, displaying the highest reported conductivity, NMR results are discussed in relation with those obtained by Neutron Diffraction (ND) and Impedance Spectroscopy (IS). Diffusion coefficients determined by NMR Pulse Field Gradient (PFG) technique are similar to those deduced from Impedance Spectroscopy and NMR relaxation data.

Structural characterization and NMR study of NaNbWO(6) and its proton-exchanged derivatives

The structural characterization of NaNbWO(6), prepared by the ceramic route, has been performed. Electron diffraction has shown the presence of two related phases in a 1:1 ratio, whose lattice parameters correspond to those of the well-known tetragonal tungsten bronzes (TTB) and those of a monoclinically distorted phase. In addition to basic unit cells, the morphology of the two phases has been found to be similar, but they present a slight difference in the W/Nb ratio. (1)H and (23)Na magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectra of NaNbWO(6) and its proton-exchanged derivatives have been interpreted on the basis of the ideal TTB structure. The average structure and the morphology remain unchanged in Na(1-x)H(x)NbWO(6) derivatives. (1)H and (23)Na MAS-NMR spectroscopies have been used to monitor changes produced during exchange processes. It has been shown that the exchange of Na ions is mainly produced, but not exclusively, at tetragonal channels. However, a large amount of Na ions at the pentagonal channels do not exchange with protons, suggesting that these ions are needed to stabilize the TTB-like structure. A tentative distribution of sodium ions in the most-exchanged oxide, deduced from NMR results, approximately (Na(0.46))(p)(Na(0.08))(s)H(0.46)NbWO(6), has been proposed. NMR spectra of Na(1-x)H(x)NbWO(6) indicate that two different OH groups are formed upon exchanging. The study of samples hydrated with D(2)O allowed us to conclude that deuterons of adsorbed water exchange with protons of the two OH groups. The proton-deuteron exchange is slow at room temperature but is strongly enhanced at 90 degrees C. This observation relates to the proton conductivity displayed by exchanged products under a humid atmosphere.

Layered microporous tin(iv) bisphosphonates

This article reports the hydrothermal synthesis and characterization of two new series of porous tin(IV) phosphonophenoxyphenylphosphonates with controlled pore size distributions, using as precursor the 4-(4'-phosphonophenoxy)phenyl phosphonic acid, [H2O3P-C6H4]2-O. Supermicroporous solids (S(BET), 300-400 m2 g(-1)) were obtained employing n-alcohol (C1-C6)-water mixtures (solvents ratio 1 : 1), in the presence of hydrofluoric acid. X-Ray powder diffraction shows that these compounds are semi-crystalline and the local environments around the phosphorus and tin elements have been studied by 31P and 119Sn MAS-NMR spectroscopy, respectively. The microstructure (particle sizes and shapes) of these phosphonates has been analyzed by scanning and transmission electron microscopy. This study shows that the microstructures of single-ligand (for instance tin(IV) phenylphosphonate) and cross-linked tin(IV) bisphosphonates are different. Tin(IV) phenylphosphonate crystallizes as micron-sized spheres, theta approximately 1-2 microm, formed by the aggregation of nanospheres, whereas tin(IV) bisphosphonates crystallize as microparticles larger than 20 microm. The textural properties of these porous solids were characterized by N2 and CO2 sorption isotherms. The key result of this work is that maxima of pore size distributions smoothly shift from 12 to 16 angstroms upon increasing the chain length of the alcohol. The microporosity of tin(IV) bisphosphonates is compatible with a double role played by the phosphonate groups acting as a pillar between adjacent layers and as a component of the hybrid organic-inorganic layers.