Dispersion, ionic bonding and vibrational shifts in phospho-aluminosilicate glasses.

Understanding the relationships between structure and properties of aluminosilicate glasses is of interest in magmatic studies as well as for glass applications as mechanical or optical components. Glass properties may be tailored by the incorporation of additional elements, and here we studied the effect of phosphate incorporation on refractive index and the degree of ionic bonding in aluminosilicate glasses. The studied glasses in the system SiO2-Al2O3-Na2O-P2O5 had a metaluminous composition (Al:Na = 1) with the content of SiO2 ranging from 50 to 70 mol% and of P2O5 from 0 to 7.5 mol%. Refractive index was measured at four wavelengths from visible to near-infrared and found to decrease both with increasing P2O5 content (at the expense of NaAlO2) and with increasing SiO2 content (by substitution of SiO4 for AlO4 groups). This trend correlated with a decrease in density while, additionally, the formation of Al-O-P bonds with an SiO2-like structure may account for this change. The degree of ionic bonding, assessed via optical basicity and oxygen polarisability, decreased with increasing P2O5 and SiO2 content. Despite the complexity of the studied glasses, oxygen polarisability and optical basicity were found to follow Duffy's empirical equation for simple oxide glasses. In the high frequency infrared and Raman spectra, band shifts were observed with increasing P2O5 and SiO2 content. They indicated changing average bond strength of the glass network and showed a linear correlation with optical basicity.

Lithium-ion Mobility in Li6 B18 (Li3 N) and Li Vacancy Tuning in the Solid Solution Li6 B18 (Li3 N)1-x (Li2 O)x.

All-solid-state batteries are promising candidates for safe energy-storage systems due to non-flammable solid electrolytes and the possibility to use metallic lithium as an anode. Thus, there is a challenge to design new solid electrolytes and to understand the principles of ion conduction on an atomic scale. We report on a new concept for compounds with high lithium ion mobility based on a rigid open-framework boron structure. The host-guest structure Li6 B18 (Li3 N) comprises large hexagonal pores filled with ∞ 1 [ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ Li7 N] strands that represent a perfect cutout from the structure of α-Li3 N. Variable-temperature 7 Li NMR spectroscopy reveals a very high Li mobility in the template phase with a remarkably low activation energy below 19 kJ mol-1 and thus much lower than pristine Li3 N. The formation of the solid solution of Li6 B18 (Li3 N) and Li6 B18 (Li2 O) over the complete compositional range allows the tuning of lithium defects in the template structure that is not possible for pristine Li3 N and Li2 O.

Structural characterization of a new fluorophosphotellurite glass system.

While phosphotellurite glasses have superior properties over SiO2-based glasses for many applications in optoelectronics and photonic devices, their high hydroxyl content limits their use in the mid-infrared range. This drawback can be overcome by fluoride addition to the formulation. In this work, we report the preparation, optical, and structural characterization of new glasses in the ternary system TeO2-xNaF-NaPO3 having the compositions 0.8TeO2-0.2[xNaF-(1 - x)NaPO3] and 0.6TeO2-0.4[xNaF-(1 - x)NaPO3] (0 ≤ x ≤ 1) obtained by the traditional melt-quenching method and labeled as T8NNx and T6NNx, respectively. Differential scanning calorimetry (DSC) reveals high thermal stability against crystallization, with Tx-Tg varying from 80 to 130 °C, depending on fluoride/phosphate ratios. Raman spectroscopy suggests that the network connectivity increases with increasing phosphate concentration. 125Te, 23Na, 31P, and 19F NMR spectroscopy provides detailed structural information about Te-O-P, Te-F, Te-O-Te, P-O-P, and P-F linkages and the charge compensation mechanism for the sodium ions. The present study is the first comprehensive structural characterization of a fluorophosphotellurite glass system.

Influence of Phosphate on Network Connectivity and Glass Transition in Highly Polymerized Aluminosilicate Glasses.

Melt-derived metaluminous (Al/Na = 1) aluminosilicate glasses in the system SiO2-Al2O3-Na2O-P2O5 were prepared with P2O5 and SiO2 contents varying from 0 to 7.5 and 50 to 70 mol %, respectively. The glass structure was investigated by X-ray absorption near edge structure, far- and medium-infrared, and polarized Raman spectroscopic techniques. The results indicate the incorporation of phosphate into the aluminosilicate network not only as partially depolymerized groups but also as fully polymerized groups charge-balanced by aluminate units in Al-O-P bonds. A new analysis method based on polarized Raman spectra in the bending frequency range indicates a preference of phosphate to reorganize the smallest ring structures. Changes in the glass transition temperature with the increase in phosphate content were found to be consistent with the depolymerization of the network structure shown by spectroscopy. By contrast, increasing the silica content by substituting SiO4 for AlO4 tetrahedra, while keeping the phosphate content constant, was found to have a negligible effect on network polymerization. Still, the glass transition temperature decreased and correlated with a far-infrared sodium band shift to higher frequency. This was interpreted as local changes in bond strength caused by complex interactions between the different network formers and sodium ions.

Li5 SnP3 - a Member of the Series Li10+4x Sn2-x P6 for x=0 Comprising the Fast Lithium-Ion Conductors Li8 SnP4 (x=0.5) and Li14 SnP6 (x=1).

The targeted search for suitable solid-state ionic conductors requires a certain understanding of the conduction mechanism and the correlation of the structures and the resulting properties of the material. Thus, the investigation of various ionic conductors with respect to their structural composition is crucial for the design of next-generation materials as demanded. We report here on Li5 SnP3 which completes with x=0 the series Li10+4x Sn2-x P6 of the fast lithium-ion conductors α- and ß-Li8 SnP4 (x=0.5) and Li14 SnP6 (x=1). Synthesis, crystal structure determination by single-crystal and powder X-ray diffraction methods, as well as 6 Li, 31 P and 119 Sn MAS NMR and temperature-dependent 7 Li NMR spectroscopy together with electrochemical impedance studies are reported. The correlation between the ionic conductivity and the occupation of octahedral and tetrahedral sites in a close-packed array of P atoms in the series of compounds is discussed. We conclude from this series that in order to receive fast ion conductors a partial occupation of the octahedral vacancies seems to be crucial.

In-situ reaction monitoring of a mechanochemical ball mill reaction with solid state NMR.

We present an approach towards the in situ solid state NMR monitoring of mechanochemical reactions in a ball mill. A miniaturized vibration ball mill is integrated into the measuring coil of a home-built solid state NMR probe, allowing for static solid state NMR measurements during the mechanochemical reaction within the vessel. The setup allows to quantitatively follow the product evolution of a prototypical mechanochemical reaction, the formation of zinc phenylphosphonate from zinc acetate and phenylphosphonic acid. MAS NMR investigations on the final reaction mixture confirmed a reaction yield of 89% in a typical example. Thus, NMR spectroscopy may in the future provide complementary information about reaction mechanisms of mechanochemical reactions and team up with other analytical methods which have been employed to follow reactions in situ, such as Raman spectroscopy or X-ray diffraction.

Structural Role of Phosphate in Metaluminous Sodium Aluminosilicate Glasses As Studied by Solid State NMR Spectroscopy.

In this contribution we present a detailed study of the effect of the addition of small to intermediate amounts of P2O5 (up to 7.5 mol %) on the network organization of metaluminous sodium aluminosilicate glasses employing a range of advanced solid state NMR methodologies. The combined results from MAS, MQMAS (multiple quantum MAS), or MAT (magic angle turning) NMR spectroscopy and a variety of dipolar based NMR experiments-27Al{31P}-, 27Al{29Si}-, 29Si{31P}-, and 31P{29Si}-REDOR (rotational echo double resonance) NMR spectroscopy as well as 31P{27Al}- and 29Si{27Al}-REAPDOR (rotational echo adiabatic passage double resonance) NMR-allow for a detailed analysis of the network organization adopted by these glasses. Phosphate is found as QP2, QP3, and QP4 (with the superscript denoting the number of bridging oxygens), the QP4 units can be safely identified with the help of 31P MAT NMR experiments. Al exclusively adopts a 4-fold coordination. The withdrawal of a fraction of the sodium cations from AlO4 units that is needed for charge compensation of the QP2 units necessitates an alternative charge compensation scheme for these AlO4 units via formation of QP4 units or oxygen triclusters. The dipolar NMR experiments suggest a strong preference of P for Al with an average value of ca. 2.4 P-O-Al connections per phosphate tetrahedron. P is thus mainly integrated into the network via P-O-Al bonding, the formation of Si-O-P bonding plays only a minor role.

Fast Lithium Ion Conduction in Lithium Phosphidoaluminates.

Solid electrolyte materials are crucial for the development of high-energy-density all-solid-state batteries (ASSB) using a nonflammable electrolyte. In order to retain a low lithium-ion transfer resistance, fast lithium ion conducting solid electrolytes are required. We report on the novel superionic conductor Li9 AlP4 which is easily synthesised from the elements via ball-milling and subsequent annealing at moderate temperatures and which is characterized by single-crystal and powder X-ray diffraction. This representative of the novel compound class of lithium phosphidoaluminates has, as an undoped material, a remarkable fast ionic conductivity of 3 mS cm-1 and a low activation energy of 29 kJ mol-1 as determined by impedance spectroscopy. Temperature-dependent 7 Li NMR spectroscopy supports the fast lithium motion. In addition, Li9 AlP4 combines a very high lithium content with a very low theoretical density of 1.703 g cm-3 . The distribution of the Li atoms over the diverse crystallographic positions between the [AlP4 ]9- tetrahedra is analyzed by means of DFT calculations.

Solid state NMR at very high temperatures.

Whereas high resolution NMR at temperatures up to 550 K can be routinely performed selecting from a variety of commercially available NMR hardware, experiments in the high temperature regime, defined here as T > 550 K, have been restricted to just a few specialized laboratories. In this contribution we present important developments of high temperature NMR over the last decades. Various methods to achieve high resolution high temperature NMR, including resistive heating, laser-assisted heating and inductive heating, are presented and their specific advantages and disadvantages discussed. The various ways of temperature monitoring including the use of chemical shift thermometers or T1 thermometers are reviewed. In the last section, some typical application examples from the field of oxidic glasses and melts are given.

Long-term entrapment and temperature-controlled-release of SF6 gas in metal-organic frameworks (MOFs).

In this work, a metal-organic framework (MOF), namely MFU-4, which is comprised of zinc cations and benzotriazolate ligands, was used to entrap SF6 gas molecules inside its pores, and thus a new scheme for long-term leakproof storage of dangerous gasses is demonstrated. The SF6 gas was introduced into the pores at an elevated gas pressure and temperature. Upon cooling down and release of the gas pressure, we discovered that the gas was well-trapped inside the pores and did not leak out - not even after two months of exposure to air at room temperature. The material was thoroughly analyzed before and after the loading as well as after given periods of time (1, 3, 7, 14 or 60 days) after the loading. The studies included powder X-ray diffraction measurements, thermogravimetric analysis, Fourier-transform infrared spectroscopy, scanning electron microscopy, 19F nuclear magnetic resonance spectroscopy and computational simulations. In addition, the possibility to release the gas guest by applying elevated temperature, vacuum and acid-induced framework decomposition was also investigated. The controlled gas release using elevated temperature has the additional benefit that the host MOF can be reused for further gas capture cycles.

Fast Ionic Conductivity in the Most Lithium-Rich Phosphidosilicate Li14SiP6.

Solid electrolytes with superionic conductivity are required as a main component for all-solid-state batteries. Here we present a novel solid electrolyte with three-dimensional conducting pathways based on "lithium-rich" phosphidosilicates with ionic conductivity of σ > 10-3 S cm-1 at room temperature and activation energy of 30-32 kJ mol-1 expanding the recently introduced family of lithium phosphidotetrelates. Aiming toward higher lithium ion conductivities, systematic investigations of lithium phosphidosilicates gave access to the so far lithium-richest compound within this class of materials. The crystalline material (space group Fm3m), which shows reversible thermal phase transitions, can be readily obtained by ball mill synthesis from the elements followed by moderate thermal treatment of the mixture. Lithium diffusion pathways via both tetrahedral and octahedral voids are analyzed by temperature-dependent powder neutron diffraction measurements in combination with maximum entropy method and DFT calculations. Moreover, the lithium ion mobility structurally indicated by a disordered Li/Si occupancy in the tetrahedral voids plus partially filled octahedral voids is studied by temperature-dependent impedance and 7Li NMR spectroscopy.

Phase Separation and Nanocrystallization in KF-ZnF2-SiO2 Glasses: Lessons from Solid-State NMR.

In the present paper, we studied the structural evolution of the glass matrix and formation of crystalline fluoride phases for 15KF-15ZnF2-70SiO2 (glass A) and 25KF-25ZnF2-50SiO2 (glass B) glasses upon thermal treatment employing transmission electron microscopy, X-ray diffraction (XRD), and solid-state NMR. The first step marks a phase separation into an SiO2 rich phase and a phase enriched with Zn, K, and fluoride. As shown by XRD, depending on the nominal compositions, K2SiF6 (for 15KF-15ZnF2-70SiO2) and KZnF3 (for 25KF-25ZnF2-50SiO2) are found as the first crystalline phases being formed. Upon longer heat treatment, ZnF2 is additionally formed in both cases. Surprisingly, a significant amount of SiF62- units is detected employing 29Si-MAS and 29Si{19F}CPMAS-NMR spectroscopy even in the base glass, which is completely amorphous according to the X-ray results. The vast majority of Si, however, is found in an exclusive SiO4 environment as Q4, Q3, or Q2 units. The large fraction of nonbridging oxygens per SiO4 tetrahedron (ca. 0.7 for glass A and 0.65 for glass B) indicates quite large fluorine loss during glass synthesis (approx. 60-80%). Employing dipolar NMR techniques, i.e., 29Si-{19F}-REDOR (rotational echo double resonance) and 29Si{19F}CPMAS-NMR (cross polarization magic angle spinning), the presence of Si-F connectivity (in excess of the identified SiF62- units) could be ruled out. The observed differences in the 29Si-19F heteronuclear dipole couplings between the Si-Q4 and Si Q3 units-as determined by REDOR NMR-are compatible with the assumption of a core-shell structure with a mixed cation fluoride as the core and an SiO2-enriched shell.

The Color of the Elements: A Combined Experimental and Theoretical Electron Density Study of ScB2 C2.

The chemical or physical control parameters for the onset of superconductivity in MB2 C2 hetero-graphene materials are unclear. This is mainly due to the almost ubiquitous positional B/C disorder, rendering the description of real structures of borocarbides into one of the most challenging problems in materials science. We will show that high-resolution X-ray diffraction data provides all the essential information to decode even complex coloring problems due to B/C disorder. Electron density studies and subsequent analyses of the fine structure of the Laplacian of the electron density resolves the local electronic structure of ScB2 C2 at sub-atomic resolution and allows for an unequivocal identification of all atoms involved in the coloring scenario. This information could finally be used to identify the electron deficient character of the B/C layers in ScB2 C2 and to synthesize the first bimetallic hetero-metallocene with lithium and scandium atoms embedded in the pentagonal and heptagonal voids, respectively.

The First Alkaline-Earth Fluorooxoborate Ba[B4 O6 F2 ]-Characterisation and Doping with Eu2.

The very first alkaline-earth fluorooxoborate Ba[B4 O6 F2 ] was synthesised by solid state methods starting from Ba(BF4 )2 , ß-BaB2 O4 , and B2 O3 . The crystal structure derived from single-crystal X-ray diffraction (P21 /n, a=6.6384(2) Å, b=7.6733(3) Å, c=11.3385(4) Å, ß=91.281(2)°, Z=4, Rint =0.0269, R1 =0.018, wR2 =0.034) comprises layers of BO3 F tetrahedra condensed through triangular BO3 units according to the descriptor 2Δ2□:<Δ2□>Δ. The extraordinary thirteen-fold coordination of barium by oxygen and fluorine leads to interesting optical properties of a sample doped with divalent europium, where a 4f→4f emission was recorded around 359 nm together with a broad emission band of a 5d→4f emission peaking at 366 nm. The compound is further characterised by IR-, Raman-, and solid-state NMR-spectroscopic methods. Moreover, DFT calculations as well as TGA and DSC measurements were performed.

Lithium Ion Mobility in Lithium Phosphidosilicates: Crystal Structure, 7 Li, 29 Si, and 31 P†MAS NMR Spectroscopy, and Impedance Spectroscopy of Li8 SiP4 and Li2 SiP2.

The need to improve electrodes and Li-ion conducting materials for rechargeable all-solid-state batteries has drawn enhanced attention to the investigation of lithium-rich compounds. The study of the ternary system Li-Si-P revealed a series of new compounds, two of which, Li8 SiP4 and Li2 SiP2 , are presented. Both phases represent members of a new family of Li ion conductors that display Li ion conductivity in the range from 1.15(7)×10-6 Scm-1 at 0 °C to 1.2(2)×10-4 Scm-1 at 75 °C (Li8 SiP4 ) and from 6.1(7)×10-8 Scm-1 at 0 °C to 6(1)×10-6 Scm-1 at 75 °C (Li2 SiP2 ), as determined by impedance measurements. Temperature-dependent solid-state 7 Li NMR spectroscopy revealed low activation energies of about 36 kJ mol-1 for Li8 SiP4 and about 47 kJ mol-1 for Li2 SiP2 . Both compounds were structurally characterized by X-ray diffraction analysis (single crystal and powder methods) and by 7 Li, 29 Si, and 31 P MAS NMR spectroscopy. Both phases consist of tetrahedral SiP4 anions and Li counterions. Li8 SiP4 contains isolated SiP4 units surrounded by Li atoms, while Li2 SiP2 comprises a three-dimensional network based on corner-sharing SiP4 tetrahedra, with the Li ions located in cavities and channels.

Inorganic Double Helices in Semiconducting SnIP.

SnIP is the first atomic-scale double helical semiconductor featuring a 1.86 eV bandgap, high structural and mechanical flexibility, and reasonable thermal stability up to 600 K. It is accessible on a gram scale and consists of a racemic mixture of right- and left-handed double helices composed by [SnI] and [P] helices. SnIP nanorods <20 nm in diameter can be accessed mechanically and chemically within minutes.

High-temperature MAS-NMR at high spinning speeds.

A low cost version to enable high temperature MAS NMR experiments at temperatures of up to 700°C and spinning speeds of up to 10kHz is presented. The method relies on inductive heating using a metal coated rotor insert. The metal coating is accomplished via a two step process involving physical vapor deposition and galvanization.

Substitution of Lithium for Magnesium, Zinc, and Aluminum in Li15 Si4 : Crystal Structures, Thermodynamic Properties, as well as (6) Li and (7) Li†NMR Spectroscopy of Li15 Si4 and Li15-x Mx Si4 (M=Mg, Zn, and Al).

An investigation into the substitution effects in Li15 Si4 , which is discussed as metastable phase that forms during electrochemical charging and discharging cycles in silicon anode materials, is presented. The novel partial substitution of lithium by magnesium and zinc is reported and the results are compared to those obtained for aluminum substitution. The new lithium silicides Li14 MgSi4 (1) and Li14.05 Zn0.95 Si4 (2) were synthesized by high-temperature reactions and their crystal structures were determined from single-crystal data. The magnetic properties and thermodynamic stabilities were investigated and compared with those of Li14.25 Al0.75 Si4 (3). The substitution of a small amount of Li in metastable Li15 Si4 for more electron-rich metals, such as Mg, Zn, or Al, leads to a vast increase in the thermodynamic stability of the resulting ternary compounds. The (6,7) Li NMR chemical shift and spin relaxation time T1 -NMR spectroscopy behavior at low temperatures indicate an increasing contribution of the conduction electrons to these NMR spectroscopy parameters in the series for 1-3. However, the increasing thermal stability of the new ternary phases is accompanied by a decrease in Li diffusivity, with 2 exhibiting the lowest activation energy for Li mobility with values of 56, 60, and 62 kJ mol(-1) for 2, Li14.25 Al0.75 Si14 , and 1, respectively. The influence of the metastable property of Li15 Si4 on NMR spectroscopy experiments is highlighted.

Direct determination of ionic transference numbers in ionic liquids by electrophoretic NMR.

Charge transport in ionic liquids is a phenomenon of utmost interest for electrochemical (e.g. battery) applications, but also of high complexity, involving transport of ion pairs, charged clusters and single ions. Molecular understanding is limited due to unknown contributions of cations, anions and clusters to the conductivity. Here, we perform electrophoretic NMR to determine electrophoretic mobilities of cations and anions in seven different ionic liquids. For the first time, mobilities in the range down to 10(-10) m(2) V(-1) s(-1) are determined. The ionic transference number, i.e. the fractional contribution of an ionic species to overall conductivity, strongly depends on cation and anion structure and its values show that structurally very similar ionic liquids can have cation- or anion-dominated conductivity. Transference numbers of cations, for example, vary from 40% to 58%. The results further prove the relevance of asymmetric clusters like [CationXAnionY](X-Y), X ≠ Y, for charge transport in ionic liquids.

31P NMR characterisation of phosphate fragments during dissolution of calcium sodium phosphate glasses.

Phosphate glasses in the system P2O5-CaO-Na2O dissolve in aqueous solutions, and their solubility can be varied by changing the glass composition. This makes them of interest for use as controlled release materials, e.g. as degradable implants, devices for the release of trace elements or as fertilizers, but in order to tailor glass solubility to meet specific requirements, we need to further our understanding of their dissolution behaviour and mechanism. The structure of P2O5-CaO-Na2O glasses (P2O5 between 55 and 35 mol%; glass structure analysed by 31P MAS NMR) changed from a network (55 mol% P2O5) to short chains (35 mol%) with decreasing phosphate content. Solubility in Tris buffer showed significant differences with phosphate content and glass structure; dissolution varied between 90% (50 mol% P2O5) and 15% (35 mol%) at 24 h. Glasses with high phosphate contents significantly lowered the pH of the solution, while glasses with low phosphate contents did not. Glasses consisting of a phosphate network dissolved by a mechanism involving P-O-P bond hydrolysis, as no Q3 groups but increasing concentrations of Q0 (orthophosphate) were found in solution by solution 31P NMR. Glasses consisting of chains, by contrast, can dissolve by hydration of entire chains, but hydrolysis also occurred, resulting in formation of Q0 and small ring structures. This occurrence of hydrolysis (and thus formation of P-OH groups, which can be deprotonated) caused the pH decrease and explains the variation in solution pH with structure.