6Li nuclear magnetic resonance chemical shifts, coordination number and relaxation in crystalline and glassy silicates

Revealing the Local Structure and Dynamics of the Solid Li Ion Conductor Li3P5O14

The development of fast Li ion-conducting materials for use as solid electrolytes that provide sufficient electrochemical stability against electrode materials is paramount for the future of all-solid-state batteries. Advances on these fast ionic materials are dependent on building structure-ionic mobility-function relationships. Here, we exploit a series of multinuclear and multidimensional nuclear magnetic resonance (NMR) approaches, including 6Li and 31P magic angle spinning (MAS), in conjunction with density functional theory (DFT) to provide a detailed understanding of the local structure of the ultraphosphate Li3P5O14, a promising candidate for an oxide-based Li ion conductor that has been shown to be a highly conductive, energetically favorable, and electrochemically stable potential solid electrolyte. We have reported a comprehensive assignment of the ultraphosphate layer and layered Li6O16 26- chains through 31P and 6Li MAS NMR, respectively, in conjunction with DFT. The chemical shift anisotropy of the eight resonances with the lowest 31P chemical shift is significantly lower than that of the 12 remaining resonances, suggesting the phosphate bonding nature of these P sites being one that bridges to three other phosphate groups. We employed a number of complementary 6,7Li NMR techniques, including MAS variable-temperature line narrowing spectra, spin-alignment echo (SAE) NMR, and relaxometry, to quantify the lithium ion dynamics in Li3P5O14. Detailed analysis of the diffusion-induced spin-lattice relaxation data allowed for experimental verification of the three-dimensional Li diffusion previously proposed computationally. The 6Li NMR relaxation rates suggest sites Li1 and Li5 (the only five-coordinate Li site) are the most mobile and are adjacent to one another, both in the a-b plane (intralayer) and on the c-axis (interlayer). As shown in the 6Li-6Li exchange spectroscopy NMR spectra, sites Li1 and Li5 likely exchange with one another both between adjacent layered Li6O16 26- chains and through the center of the P12O36 12- rings forming the three-dimensional pathway. The understanding of the Li ion mobility pathways in high-performing solid electrolytes outlines a route for further development of such materials to improve their performance.

Toward Understanding of the Li-Ion Migration Pathways in the Lithium Aluminum Sulfides Li3AlS3 and Li4.3AlS3.3Cl0.7 via 6,7Li Solid-State Nuclear Magnetic Resonance Spectroscopy

Li-containing materials providing fast ion transport pathways are fundamental in Li solid electrolytes and the future of all-solid-state batteries. Understanding these pathways, which usually benefit from structural disorder and cation/anion substitution, is paramount for further developments in next-generation Li solid electrolytes. Here, we exploit a range of variable temperature ⁶Li and ⁷Li nuclear magnetic resonance approaches to determine Li-ion mobility pathways, quantify Li-ion jump rates, and subsequently identify the limiting factors for Li-ion diffusion in Li₃AlS₃ and chlorine-doped analogue Li_4.3AlS_3.3Cl_0.7. Static ⁷Li NMR line narrowing spectra of Li₃AlS₃ show the existence of both mobile and immobile Li ions, with the latter limiting long-range translational ion diffusion, while in Li_4.3AlS_3.3Cl_0.7, a single type of fast-moving ion is present and responsible for the higher conductivity of this phase. ⁶Li-⁶Li exchange spectroscopy spectra of Li₃AlS₃ reveal that the slower moving ions hop between non-equivalent Li positions in different structural layers. The absence of the immobile ions in Li_4.3AlS_3.3Cl_0.7, as revealed from ⁷Li line narrowing experiments, suggests an increased rate of ion exchange between the layers in this phase compared with Li₃AlS₃. Detailed analysis of spin-lattice relaxation data allows extraction of Li-ion jump rates that are significantly increased for the doped material and identify Li mobility pathways in both materials to be three-dimensional. The identification of factors limiting long-range translational Li diffusion and understanding the effects of structural modification (such as anion substitution) on Li-ion mobility provide a framework for the further development of more highly conductive Li solid electrolytes.

Crystal Structure and Bond-Valence Investigation of Nitrogen-Stabilized LiAl5O8 Spinels

An in-depth insight into the effect of nitrogen substitution on structural stabilization is important for the design of new spinel-type oxynitride materials with tailored properties. In this work, the crystal structures of ordered and disordered LiAl₅O₈ obtained by slow cooling and rapid quenching, respectively, were analyzed by a X-ray diffraction (XRD) Rietveld refinement and OccQP program. The variation in the bonding state of atoms in the two compounds was explored by the bond valence model, which revealed that the instability of spinel-type LiAl₅O₈ crystal structure at room temperature is mainly due to the severe under-bonding of the tetrahedrally coordinated Al cations. With the partial substitution of oxygen with nitrogen in LiAl₅O₈, a series of the nitrogen-stabilized spinel Li_yAl_(16+x-y)/3O_8-xN_x (0 < x < 0.5, 0 < y < 1) was successfully prepared. The crystal structures were systematically investigated by the powder XRD structural refinement combined with ⁷Li and ²⁷Al magic-angle spinning nuclear magnetic resonance. All the Li⁺ ions entered the octahedra, while the Al resonances may be composed of multiple non-equivalent Al sites. The structural stability of spinel Li_yAl_(16+x-y)/3O_8-xN_x at ambient temperature was attributed to the cationic vacancies and high valence generated by the N ions, which alleviated the under-bonding state of the tetrahedral Al-O bond. This work provides a new perspective for understanding the composition-structure relationship in spinel compounds with multiple disorders.

Iridate Li8IrO6: An Antiferromagnetic Insulator

A polycrystalline iridate Li₈IrO₆ material was prepared via heating Li₂O and IrO₂ starting materials in a sealed quartz tube at 650 °C for 48 h. The structure was determined from Rietveld refinement of room-temperature powder neutron diffraction data. Li₈IrO₆ adopts the nonpolar space group R3̅ with Li atoms occupying the tetrahedral and octahedral sites, which is supported by the electron diffraction and solid-state ⁷Li NMR. This results in a crystal structure consisting of LiO₄ tetrahedral layers alternating with mixed IrO₆ and LiO₆ octahedral layers along the crystallographic c-axis. The +4 oxidation state of Ir⁴⁺ was confirmed by near-edge X-ray absorption spectroscopy. An in situ synchrotron X-ray diffraction study of Li₈IrO₆ indicates that the sample is stable up to 1000 °C and exhibits no structural transitions. Magnetic measurements suggest long-range antiferromagnetic ordering with a Néel temperature (T_N) of 4 K, which is corroborated by heat capacity measurements. The localized effective moment μ_eff (Ir) = 1.73 μ_B and insulating character indicate that Li₈IrO₆ is a correlated insulator. First-principles calculations support the nonpolar crystal structure and reveal the insulating behavior both in paramagnetic and antiferromagnetic states.

Tailoring of White Luminescence in a NaLi3 SiO4 :Eu2+ Phosphor Containing Broad-Band Defect-Induced Charge-Transfer Emission

Single-component materials with white-light emission are ideal for lighting applications. However, it is very challenging to achieve white luminescence in single-dopant activated solid phosphors. Herein, white NaLi₃ Si_{1- x} O₄ :Eu²⁺ materials are designed via defect engineering and synthesized by reducing the Si content (0.15 ≤ x ≤ 0.25). Stochiometric NaLi₃ SiO₄ :Eu²⁺ exhibits a narrow-band blue emission at 469 nm, ascribed to the 5d → 4f transition of Eu²⁺ at highly symmetric cuboid Na sites, while samples with Si content reduced by 15-25% display white emission with two peaks at 472 nm and 585 nm. The newly appeared broadband yellow peak arises from charge-transfer transitions involving Eu²⁺ and nearby defects, as verified by an unusual bandwidth, a large Stokes shift, and a long decay time. A single-component white light-emitting diode device is fabricated by employing a white phosphor to demonstrate a color-rendering index of 82.9. This result provides a new design strategy for single-component white-light materials with broad-band defect-induced charge-transfer emission.

Computationally Guided Discovery of the Sulfide Li3AlS3 in the Li-Al-S Phase Field: Structure and Lithium Conductivity

With the goal of finding new lithium solid electrolytes by a combined computational-experimental method, the exploration of the Li-Al-O-S phase field resulted in the discovery of a new sulfide Li₃AlS₃. The structure of the new phase was determined through an approach combining synchrotron X-ray and neutron diffraction with ⁶Li and ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy and revealed to be a highly ordered cationic polyhedral network within a sulfide anion hcp-type sublattice. The originality of the structure relies on the presence of Al₂S₆ repeating dimer units consisting of two edge-shared Al tetrahedra. We find that, in this structure type consisting of alternating tetrahedral layers with Li-only polyhedra layers, the formation of these dimers is constrained by the Al/S ratio of 1/3. Moreover, by comparing this structure to similar phases such as Li₅AlS₄ and Li_4.4Al_0.2Ge_0.3S₄ ((Al + Ge)/S = 1/4), we discovered that the AlS₄ dimers not only influence atomic displacements and Li polyhedral distortions but also determine the overall Li polyhedral arrangement within the hcp lattice, leading to the presence of highly ordered vacancies in both the tetrahedral and Li-only layer. AC impedance measurements revealed a low lithium mobility, which is strongly impacted by the presence of ordered vacancies. Finally, a composition-structure-property relationship understanding was developed to explain the extent of lithium mobility in this structure type.

A Simple Coulombic Model for 31P NMR Spectra of Cluster-Encapsulated Phosphorus Atoms

While sophisticated computational methods can predict ³¹P NMR spectra of phosphorus atoms encapsulated within Keggin-derived heteropoly tungstate and molybdate cluster anions, calculated and experimental chemical shift values typically deviate considerably from one another. Motivated by the observation that experimentally determined ³¹P chemical shift values within a series of water-soluble plenary and metal-cation substituted lacunary Keggin anions, [PM ⁿW₁₁O₃₉]^{(7- n)-} (M ⁿ = Ag⁺, Zn²⁺, Nb⁵⁺, W⁶⁺) and [(PW₁₁O₃₉)₂M ⁿ]^{(14- n)-} (M ⁿ = Y³⁺, Zr⁴⁺), varied as a linear function of the oxidation states, n, of the complexed M ⁿ cations, a linear correlation was sought between observed chemical shift values and the net Coulombic forces experienced by the encapsulated phosphorus atoms. The Coulombic model based on Shannon radii, published electronegativity values, and bond angles from X-ray crystallographic data remarkably accounted for the relative ³¹P chemical shift values of phosphorus atoms in over 50 metal-oxide cluster anions, including large structures comprised of up to four Keggin-derived fragments with an overall R² value of 0.974. With the model being applied here to three cluster anions whose ³¹P chemical shift values are reported here for the first time, predicted and experimental values differed by only ±0.4 ppm.

Evaluating lithium diffusion mechanisms in the complex spinel Li2NiGe3O8

Li⁺-ion diffusion in the all-solid-state battery electrolyte candidate material Li₂NiGe₃O₈ was investigated and D_Li = 3.89 × 10⁻¹² cm² s⁻¹ calculated.

Mechanochemical Induced Structure Transformations in Lithium Titanates: A Detailed PXRD and 6Li MAS NMR Study

Lithium titanates are used in various applications, such as anode materials for lithium intercalation (Li4Ti5O12) or breeding materials in fusion reactors (Li2TiO3). Here, we report the formation of nano-crystalline lithium titanates by a mechanochemical approach and present a deeper insight into their structural characteristics by X-ray diffraction (XRD) and solid-state NMR spectroscopy. The compounds were synthesized in a high-energy planetary ball mill with varying milling parameters and different grinding tools. NaCl type Li2TiO3 (α-Li2TiO3) was formed by dry milling of lithium hydroxide with titania (rutile or anatase) and by a milling induced structure transformation of monoclinic β-Li2TiO3 or spinel type Li4Ti5O12. Heating of mechanochemical prepared α-Li2TiO3 induces a phase transformation to the monoclinic phase similar to hydrothermal reaction products, but a higher thermal stability was observed for the mechanochemical formed product. Microstructure and crystallographic structure were characterized by XRD via Rietveld analysis. Detailed phase analysis shows the formation of the cubic phase from the various educts. A set of two lattice parameters for α-Li2TiO3 was refined, depending on the presence of OH− during the milling process. An average crystallite size of less than 15 nm was observed for the mechanochemical generated products. The local Li environment detected by 6Li NMR revealed Li defects in the form of tetrahedral instead of octahedral site occupation. Subsequent adjustment of the structural model for Rietveld refinement leads to better fits, supporting this interpretation.

The crystal structure and luminescence properties of a novel green-yellow emitting Ca1.5Mg0.5Si1−xLixO4−δ:Ce3+ phosphor with high quantum efficiency and thermal stability

The CMSL:0.005Ce sample cracked into a soft fine powder when the temperature decreased to ∼330 °C.

Identification of electrochemical reaction products in lithium–oxygen cells with 7Li nutation spectroscopy

In the Li–O2 battery system, it is has been shown to be challenging to differentiate the discharge products or determine the electrolyte stability with direct 7Li NMR. Defined 7Li quadrupole lineshapes are not observed for cycled cathodes. Here, 7Li nutation NMR is demonstrated to be an effective method for the identification of Li2O2 in cycled cathodes. The 7Li quadrupole interaction of Li2O2 (35 kHz) and Li2CO3 (120 kHz) are of similar magnitude to typically radiofrequency fields (ranging from 40 to 60 kHz). The 7Li nutation frequency will therefore be influenced by both interactions. The discharge products of the cycled cathodes were determined by comparing the 7Li nutation frequencies of the cycled cathodes to the 7Li nutation frequency of the pristine materials when the applied radiofrequency field was 30 kHz. Li2CO3 was determined to be the main discharge product in the propylene carbonate/dimethyl carbonate and trimethyl phosphate electrolyte systems, since the 7Li nutation frequencies of the cathodes corresponded to the 7Li nutation frequency of pristine Li2CO3. The 7Li nutation frequency of the tetraethylene glycol dimethyl ether cathode was between the 7Li nutation frequencies of both pristine Li2O2 and pristine Li2CO3, indicating that both Li2O2 and Li2CO3 were discharge products influencing the observed nutation frequency. From 7Li nutation NMR the novel trimethyl phosphate electrolyte was determined to be an unsuitable Li–O2 electrolyte, as the fast 7Li nutation frequency indicated that Li2O2 was not a primary discharge species. With 17O NMR, Li2CO3 was confirmed to be a main discharge product formed with the trimethyl phosphate electrolyte.

NMR crystallography of monovalent cations in inorganic matrixes: Li+ siting and the local structure of Li+ sites in ferrierites

A new approach to the determination of the Li⁺ siting and the local structure of Li⁺ sites in zeolites is reported.

Lithium conductivity in glasses of the Li2O–Al2O3–SiO2system

An Arrhenius plot for DC electrical conductivity of fully polymerized and depolymerized Li aluminosilicate glasses is one way to reflect the effect of local structurevs.lithium content.

The use of 6Li{7Li}-REDOR NMR spectroscopy to compare the ionic conductivities of solid-state lithium ion electrolytes

Garnet-like solid-state electrolyte materials for lithium ion batteries are promising replacements for the currently-used liquid electrolytes. This work compares the temperature dependent Li(+) ion hopping rate in Li6BaLa2M2O12 (M = Ta, Nb) using solid-state (6)Li{(7)Li}-REDOR NMR. The slope of the (6)Li{(7)Li}-REDOR curve is highly temperature dependent in these two phases, and a comparison of the changes of the REDOR slopes as a function of temperature has been used to evaluate the Li(+) ion dynamics. Our results indicate that the Nb phase has a higher overall ionic conductivity in the range of 247 K to 350 K, as well as a higher activation energy for lithium ion hopping than the Ta counterpart. For appropriate relative timescales of the dipolar couplings and ion transport processes, this is shown to be a facile method to compare ion dynamics among similar structures.

Monitoring the Electrochemical Processes in the Lithium-Air Battery by Solid State NMR Spectroscopy

A multi-nuclear solid-state NMR approach is employed to investigate the lithium-air battery, to monitor the evolution of the electrochemical products formed during cycling, and to gain insight into processes affecting capacity fading. While lithium peroxide is identified by 17O solid state NMR (ssNMR) as the predominant product in the first discharge in 1,2-dimethoxyethane (DME) based electrolytes, it reacts with the carbon cathode surface to form carbonate during the charging process. 13C ssNMR provides evidence for carbonate formation on the surface of the carbon cathode, the carbonate being removed at high charging voltages in the first cycle, but accumulating in later cycles. Small amounts of lithium hydroxide and formate are also detected in discharged cathodes and while the hydroxide formation is reversible, the formate persists and accumulates in the cathode upon further cycling. The results indicate that the rechargeability of the battery is limited by both the electrolyte and the carbon cathode stability. The utility of ssNMR spectroscopy in directly detecting product formation and decomposition within the battery is demonstrated, a necessary step in the assessment of new electrolytes, catalysts, and cathode materials for the development of a viable lithium-oxygen battery.

Carbon substitution for oxygen in silicates in planetary interiors

Amorphous silicon oxycarbide polymer-derived ceramics (PDCs), synthesized from organometallic precursors, contain carbon- and silica-rich nanodomains, the latter with extensive substitution of carbon for oxygen, linking Si-centered SiO(x)C(4-x) tetrahedra. Calorimetric studies demonstrated these PDCs to be thermodynamically more stable than a mixture of SiO2, C, and silicon carbide. Here, we show by multinuclear NMR spectroscopy that substitution of C for O is also attained in PDCs with depolymerized silica-rich domains containing lithium, associated with SiO(x)C(4-x) tetrahedra with nonbridging oxygen. We suggest that significant (several percent) substitution of C for O could occur in more complex geological silicate melts/glasses in contact with graphite at moderate pressure and high temperature and may be thermodynamically far more accessible than C for Si substitution. Carbon incorporation will change the local structure and may affect physical properties, such as viscosity. Analogous carbon substitution at grain boundaries, at defect sites, or as equilibrium states in nominally acarbonaceous crystalline silicates, even if present at levels at 10-100 ppm, might form an extensive and hitherto hidden reservoir of carbon in the lower crust and mantle.

Tailor-made development of fast Li ion conducting garnet-like solid electrolytes

This paper reports a novel approach to designing advanced solid Li ion electrolytes for application in various solid state ionic devices, including Li ion secondary batteries, gas sensors, and electrochromic displays. The employed methodology involves a solid-solution reaction between the two best-known fast Li ion conductors in the garnet-family of compounds Li(6)BaLa(2)M(2)O(12) (M = Nb, Ta) and Li(7)La(3)Zr(2)O(12). Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), AC impedance, and (7)Li nuclear magnetic resonance (Li NMR) spectroscopy were employed to characterize phase formation, morphology, ionic conductivity, and Li ion coordination in Li(6.5)La(2.5)BaZrMO(12). PXRD shows for formation of a cubic garnet-like structure and AC impedance data is consistent with other known solid Li ion electrolytes. Li(6.5)La(2.5)BaZrTaO(12) exhibits a fast Li ion conductivity of about 6 x 10(-3) S cm(-1) at 100 degrees C, which is comparable to that of currently employed organic polymer electrolytes value at room temperature. The Nb analogue shows an order of magnitude lower ionic conductivity than that of the corresponding Ta member, which is consistent with the trend in garnet-type electrolytes reported in the literature. Samples sintered at 1100 degrees C shows the highest electrical conductivity compared to that of 900 degrees C. (7)Li MAS NMR shows a sharp single peak at 0 ppm with respect to LiCl, which may be attributed to fast migration of ions between various sites in the garnets, and also suggesting average distributions of Li ions at average octahedral coordination in Li(6.5)La(2.5)BaZrMO(12). The present work together with literature used to establish very important fundamental relationship of functional property-Li concentration-crystal structure-Li diffusion coefficient in the garnet family of Li ion electrolytes.

NMR investigations of Li(+) ion dynamics in the NASICON ionic conductors [Formula: see text]

NMR studies of (7)Li and (31)P nuclei are reported in the 150-900 K temperature range for the [Formula: see text] NASICON compounds with x = 0.8, 0.7, 0.6 and 0.3. Magic angle spinning (MAS mode) experiments were performed at room temperature on the (7)Li and (31)P nuclei. The linewidth and the spin lattice relaxation times of these nuclei are studied versus temperature in the static mode. The spectra recorded in the MAS mode show that the (7)Li ions occupy three chemical sites, the occupation of which being very sensitive to the x values but not sensitive to the coexistence of the two varieties [Formula: see text] and [Formula: see text] observed at room temperature in compounds with x≤0.5. On the other hand, the (31)P nucleus MAS spectra are very sensitive to lithium content but also to the variety coexistence. T(1) measurements were performed in a static mode on the (7)Li and (31)P nuclei. In all the compounds, the (7)Li spin lattice relaxation time exhibits two branches with several minima, indicating the complex dynamics for this ion. One of these minima appears in the same temperature range as the minimum of the (31)P nucleus T(1), strongly suggesting a cross-relaxation process between these nuclei. T(1ρ) measurements on (7)Li (static mode) allow us to show a slow motion different from the one probed by the T(1). The analysis of the T(1ρ) behaviour with temperature and composition allows us to ascribe the motion probed by this time to the oxygen ion motion which monitors the opening and closing of the lithium pathways. A qualitative interpretation of the (7)Li  T(1) results is done; it takes into account the cross-relaxation phenomena between (31)P and (7)Li and quadrupolar fluctuations.

Spatial distribution of lithium ions in glasses studied by 7Li{6Li} spin echo double resonance

This manuscript introduces 7Li{6Li} spin echo double resonance (SEDOR) spectroscopy as a novel approach for studying the spatial distribution of lithium ions in solid electrolytes. Theoretical simulations using density operator theory as well as experimental validation on the model compound lithium carbonate reveal that this method affords a selective measurement of 7Li-6Li heteronuclear dipole-dipole couplings. Dipolar second moments characterizing internuclear lithium-lithium interactions have been measured in lithium silicate (Li2O)(x)(SiO2)(1-x), (0.1 < or = x < or = 0.4) and lithium borate (Li2O)(x)(B2O3)(1-x), (0.1 < or = x < or = 0.3) glasses. The results indicate that the spatial distributions of the lithium ions in these two glass systems are decidedly different. In the lithium silicate glass system, the results give clear evidence of strong cation clustering for x < or = 0.3, providing quantitative support for a previously proposed model of a one-dimensional channel structure. In contrast, in the lithium borate glass system, the cations seem to be more or less randomly distributed. Nevertheless, an observed superlinear dependence of M2(7Li-6Li) as a function of ion concentration indicates subtle changes of the lithium arrangement principles, which are discussed in relation to the previously proposed ring structure of borate glasses.

Simulations of molecular dynamics in solid-state NMR spectra of spin-1 nuclei including effects of CSA- and EFG-terms up to second order

By numerical simulations MAS and QCPMG methods for acquiring spectra of spin-1 nuclei were compared in order to determine the most sensitive experiment for analysis of molecular dynamics. To comply with the large quadrupolar constants for 14N and the CSA reported for 6Li both of these interactions are included up to second order. For 2H and 6Li both QCPMG and single-pulse MAS experiments were suitable for dynamics studies whereas the single-pulse MAS experiment were the method of choice for investigation of 14N dynamics for C(Q)'s larger than 750kHz at 14.1T. This property prohibits excitation of the 14N lineshape using either single hard or softer composite rf-pulses. Focusing on 14N it was demonstrated that the centerband lineshape is sensitive toward both off-MAS and CSA effects. In addition, excitation by real-time pulses showed that proper lineshapes corresponding to a site with a C(Q) of 3MHz may be excited by a very short pulse.

Heterogeneities in sol–gel-derived paramagnetics-doped forsterites and willemites — Electron microprobe analysis and stretched-exponential 29Si MAS NMR spin–lattice relaxation studies

We report the synthesis and analysis of sol–gel-derived samples of forsterite (Mg2SiO4) and willemite (Zn2SiO4), doped with paramagnetic Cu2+, Ni2+, and Co2+, at a range of dopant concentrations. Electron probe microanalysis and backscattered electron imaging show the presence of major micrometre-scale heterogeneities in the distribution of paramagnetic centres. Despite the inhomogeneities, the 29Si NMR spin–lattice relaxation behaviour is well-behaved and is consistent with the stretched-exponential expression Mz(t) = Mz(∞){1 – a exp[–(t/T′)n]}. The exponent n is 0.5 within the experimental error in some samples. This value is consistent with relaxation by immobile isolated paramagnetic impurities with negligible 29Si spin diffusion from the impurity centres, but careful curve fitting confirms that n is significantly larger than 0.5 in other samples. Relaxation efficiency is highly dependent on the dopant ion and its concentration. Although the purely empirical stretched-exponential function does not provide a unique physical picture, it is noteworthy that it is sufficiently robust to describe spin–lattice relaxation even in highly inhomogeneous systems. Spin–lattice relaxation is a useful probe of paramagnetics-doped solid samples, but NMR does not provide information on homogeneity. Careful sample characterization on the micrometre scale is highly desirable, as a complement to NMR studies.Key words: MAS NMR, spin–lattice relaxation, 29Si, forsterite, willemite, stretched-exponential relaxation, sol–gel, minor-component heterogeneity, backscattered electron analysis.

The mechanism of Li-ion transport in the garnet Li5La3Nb2O12

We present a detailed study on the exact location and dynamics of Li ions in the garnet-type material Li(5)La(3)Nb(2)O(12) employing advanced solid state NMR strategies. Applying temperature-dependent (7)Li-NMR, (6)Li-MAS-NMR, (6)Li-{(7)Li}-CPMAS-NMR, (6)Li-{(7)Li}-CPMAS-REDOR-NMR as well as 2D-(6)Li-{(7)Li}-CPMAS-Exchange-NMR spectroscopy, we were able to quantify the distribution of the Li cations among the various possible sites within the garnet-type structure and to identify intrinsic details of Li migration. The results indicate a sensitive dependence of the distribution of Li cations among the tetrahedral and octahedral sites on the temperature of the final annealing process. This distribution profoundly affects the mobility of the Li cations within the garnet-type framework structure. Extended Li mobility at ambient temperature is only possible if the majority of the Li cations is accommodated in the octahedral sites, as observed for the sample annealed at 900 degrees C. Octahedrally-coordinated Li cations could be identified as the mobile Li species, whereas the tetrahedral sites seem to act as a trap for the Li cations, rendering the tetrahedrally-coordinated Li cations immobile on the time scale of the NMR experiments.

Distinguishing surface versus buried cation sites in aluminosilicate mesoporous materials

Mesoporous MCM-41 aluminosilicates were prepared through direct synthesis and surface grafting resulting in the incorporation of aluminum into the pore walls and onto the wall surface, respectively. 7Li and 23Na NMR studies of ion-exchanged Li and Na-Al-MCM-41 were able to distinguish between cations in the surface region and those buried deeper in the pore walls. Thus it was demonstrated that most of the cations in the grafted Al-MCM-41 locate in the surface region, whereas the cations in the synthesized Al-MCM-41 are distributed throughout the pore walls. The NMR spectra of dehydrated Li- and Na-MCM-41 resemble those of glassy materials, reflecting the amorphous nature of this class of mesoporous materials. 7Li NMR studies of dehydrated Li-Al-MCM-41 prepared from direct synthesis in the presence of oxygen showed that most of the Li+ cations are not accessible to O2, while the Li+ cations in Al-grafted Li-Al-MCM-41 are accessible, which also confirms their locations. This study provides valuable insights for the understanding of the structure and properties of aluminosilicate mesoporous materials.

Two phase morphology limits lithium diffusion in TiO(2)(anatase): a (7)Li MAS NMR study

7Li magic angle spinning solid-state nuclear magnetic resonance is applied to investigate the lithium local environment and lithium ion mobility in tetragonal anatase TiO(2) and orthorhombic lithium titanate Li(0.6)TiO(2). Upon lithium insertion, an increasing fraction of the material changes its crystallographic structure from anatase TiO(2) to lithium titanate Li(0.6)TiO(2). Phase separation occurs, and as a result, the Li-rich lithium titanate phase is coexisting with the Li-poor TiO(2) phase containing only small Li amounts approximately equal to 0.01. In both the anatase and the lithium titanate lattice, Li is found to be hopping over the available sites with activation energies of 0.2 and 0.09 eV, respectively. This leads to rapid microscopic diffusion rates at room temperature (D(micr) = 4.7 x 10(-12) cm(2)s(-1) in anatase and D(micr) = 1.3 x 10(-11) cm(2)s(-1) in lithium titanate). However, macroscopic intercalation data show activation energies of approximately 0.5 eV and smaller diffusion coefficients. We suggest that the diffusion through the phase boundary is determining the activation energy of the overall diffusion and the overall diffusion rate itself. The chemical shift of lithium in anatase is independent of temperature up to approximately 250 K but decreases at higher temperatures, reflecting a change in the 3d conduction electron densities. The Li mobility becomes prominent from this same temperature showing that such electronic effects possibly facilitate the mobility.

23Na chemical shifts and local structure in crystalline, glassy, and molten sodium borates and germanates

A simple correlation between average Na-O bond length and 23Na isotropic chemical shift in crystalline germanates and borates has been established, similar to existing correlations for sodium in silicates and carbonates. This empirical trend is discussed in terms of a decreasing paramagnetic contribution to the chemical shift with increasing average bond length. The correlation is then applied to data for sodium borate and germanate glasses and melts from room temperature to 1200 degrees C, where both structural and compositional effects on the chemical shift are apparent.