Td Sites Research Articles

Aluminum-Tailored Energy Level and Morphology of Co3- x Alx O4 Porous Nanosheets toward Highly Efficient Electrocatalysts for Water Oxidation.

Tuning energy levels plays a crucial role in developing cost-effective, earth-abundant, and highly active oxygen evolution catalysts. However, to date, little attention has been paid to the effect of using heteroatom-occupied lattice sites on the energy level to engineer electrocatalytic activity. In order to explore heteroatom-engineered energy levels of spinel Co3 O4 for highly-effective oxygen electrocatalysts, herein Al atoms are directly introduced into the crystal lattice by occupying the Co2+ ions in the tetrahedral sites and Co3+ ions in the octahedral sites (denoted as Co2+ Td and Co3+ Oh , respectively). Experimental and theoretical simulations demonstrate that Al3+ ions substituting Co2+ Td and Co3+ Oh active sites, especially Al3+ ions occupying the Co2+ Td sites, optimizes the adsorption, activation, and desorption features of intermediate species during oxygen evolution reaction (OER) processes. As a result, the optimized Co1.75 Al1.25 O4 nanosheet exhibit unprecedented OER activity with an ultralow overpotential of 248 mV to deliver a current of 10 mA cm-2 , among the best Co-based OER electrocatalysts. This work should not only provide fundamental understanding of the effect of Al-occupied different Co sites in Co3-x Alx O4 composites on OER performance, but also inspire the design of low-cost, earth-abundant, and high-active electrocatalysts toward water oxidation.

Synthesis and Characterization of Copper doped Nickel ferrite Prepared by Sol-gel Method

Abstract Nano-crystalline AFe2O4 are technologically important spinel ferrite, having extra ordinary magnetic, chemical and electrical properties. In these ferrospinels materials , NiFe2O4 a inverse spinel is mainly fascinating because of their ferro-magnetism, in which magnetization originate due to the anti-parallel spins among metal ions; Fe3+ ions at tetrahedral sides(Td) and Ni2+ ions at octahedral sides(Oh). In the present study NiFe2O4 and copper doped Ni(1-X)CuXFe2O4(Ni0.8Cu0.2Fe2O4) were synthesized by sol-gel process using polyvinyl alcohol (PVA). PVA provided the polymeric network which helps to restrict cations mobility, maintain local stoichiometry and hinder the growth of particle size of ferrite. The existence of Cubic, spinel-type and crystalline phase was revealed by X-ray Diffraction (XRD). Surface morphology and composition of prepared materials were confirmed by Scanning Electron Microscopy (SEM) and Energy dispersive X-ray analysis (EDAX). Fourier Transform Infrared Spectroscopy (FT-IR), was carried out to know the vibration spectra of materials. XRD results reveal the spinel structure of nanocrystalline NiFe2O4 with the space group of Fd3m. FTIR spectra show the characteristic band in the range of 400-600cm-1 related to Oh and Td sites. SEM results show soft agglomerated particles with different shapes and uniform size distribution.

Impact of La3+ and Y3+ ion substitutions on structural, magnetic and microwave properties of Ni0.3Cu0.3Zn0.4Fe2O4 nanospinel ferrites synthesized via sonochemical route.

In the current study, Ni0.4Cu0.2Zn0.4LaxYxFe2−xO4 (x = 0.00 − 0.10) nanospinel ferrites (NSFs) were fabricated via an ultrasonic irradiation route. The creation of single phase of spinel nanoferrites (NSFs) was investigated by X-ray powder diffractometry (XRD) and selected area diffraction pattern (SAED). The cubic morphology of all samples was confirmed by scanning and transmission electron microscopies (SEM and TEM) respectively. The UV-Vis investigations provided the direct optical energy band gap values in a narrow photon energy interval of 1.87–1.92 eV. The 57Fe Mössbauer spectroscopy analysis explained that the hyperfine magnetic fields of Octahedral (Oh) and Tetrahedral (Td) sites decreased with substitution. The paramagnetic properties of NPs decrease with increase of content of doped ions. Investigations of magnetic properties reveal a superparamagnetic nature at 300 K and soft ferromagnetic trait at 10 K. The Ms (saturation magnetization) and Mr (remanence) decrease and the Hc (coercivity) increases slightly with La3+ and Y3+ substitution. The observed magnetic traits are deeply discussed in relation with the morphology, structure, magnetic moments and cation distributions. The microwave characterization of the prepared NSFs showed that, dissipation (i.e., absorption) of incoming microwave energy occurs at a single frequency, for each sample, lying between 7 and 10.5 GHz. The reflection losses (RL) at these frequencies range from −30 to −40 dB and the mechanism of which is explained in the framework of dipolar relaxation and spin rotation. The best microwave properties were obtained with a LaY concentration of x = 0.08 having an RL of −40 dB @ 10.5 GHz and an absorption bandwidth of 8.4 GHz @ −10 dB. With these high values of RL and absorbing bandwidth, LaY doped NiCuZn NSF products would be promising candidates for radar absorbing materials in the X-band.

Cation distribution and magnetic properties in ultrathin(Ni1–xCox)Fe2O4(x=0–1)layers on Si(111) studied by soft x-ray magnetic circular dichroism

We study the electronic structure and the magnetic properties of epitaxial (Ni1-xCox)Fe2O4(111) layers (x = 0 - 1) with thicknesses d = 1.7 - 5.2 nm grown on Al2O3(111)/Si(111) structures, to achieve a high value of inversion parameter y, which is the inverse-to-normal spinel-structure ratio, and hence to obtain good magnetic properties even when the thickness is thin enough for electron tunneling as a spin filter. We revealed the crystallographic (octahedral Oh or tetrahedral Td) sites and the valences of the Fe, Co, and Ni cations using experimental soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism spectra and configuration-interaction cluster-model calculation. In all the (Ni1-xCox)Fe2O4 layers with d = about 4 nm, all Ni cations occupy the Ni2+ (Oh) site, whereas Co cations occupy the three different Co2+ (Oh), Co2+ (Td), and Co3+ (Oh) sites with constant occupancies. According to these features, the occupancy of the Fe3+ (Oh) cations decreases and that of the Fe3+ (Td) cations increases with decreasing x. Consequently, we obtained a systematic increase of y with decreasing x and achieved the highest y value of 0.91 for the NiFe2O4 layer with d = 3.5 nm. From the d dependences of y and magnetization in the d range of 1.7 - 5.2 nm, a magnetically dead layer is present near the NiFe2O4/Al2O3 interface, but its influence on the magnetization was significantly suppressed compared with the case of CoFe2O4 layers reported previously [Y. K. Wakabayasi et al., Phys. Rev. B 96, 104410 (2017)], due to the high site selectivity of the Ni cations. Since our epitaxial NiFe2O4 layer with d = 3.5 nm has a high y values (0.91) and a reasonably large magnetization (180 emu/cc), it is expected to exhibit a strong spin filter effect, which can be used for efficient spin injection into Si.

Structural and magnetic properties of Al-doped yttrium iron garnet ceramics: 57Fe internal field NMR and Mössbauer spectroscopy study

The structural and magnetic properties of Al substituted yttrium-iron garnet (Y3AlxFe5-xO12, x = 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6 and 1.8) ceramic powders synthesized using solution combustion method were investigated. Post combustion, the samples were calcination at 1045 °C for 6 h and subsequently at 1200 °C for 6 h to obtain phase-pure garnets. X-ray diffraction (XRD) results confirm the formation of garnets with Ia3¯d structure. The occupancy of Y3+ ions in the dodecahedral site and the distribution of Al3+ and Fe3+ ions in the tetrahedral and octahedral sites in the bcc structure of the garnet were confirmed by Rietveld refinement of XRD patterns, Mössbauer spectroscopy and 57Fe internal field NMR spectroscopy. For low Al content, Al3+ ions have preference to occupy tetrahedral (Td) sites than the octahedral (Oh) sites. At higher Al content the distribution of Al tends towards a ratio of 3:2 at the tetrahedral:octahedral site. Increase in Al doping results in the decrease in the lattice parameter due to smaller size of Al3+ as compared to Fe3+ ion. All the studied samples show coral-network-like surface morphology. The saturation magnetization (MS) values decrease from ∼26.94 emu/g to ∼ 0.17 emu/g with increase in Al content from 0.0 to 1.8. Further addition of Al makes the sample paramagnetic at RT. Substitution of non-magnetic Al3+ reduces the saturation magnetization rapidly due to the decrease in the superexchange interaction in the crystal.

(Invited) Realizing the Electro(photo)Chemical Behavior in Liquid Electrolyte through in-Situ Methodology

Metal oxide of spinel family has performed great potential toward oxygen evolution reaction (OER), but the fundamental mechanism of spinel oxides is still far from being completely understood especially for the role of metal ions. Owing to various coordinated sites of divalent/trivalent metals ions and surface condition (morphology and defects), it’s a great challenge to have a fair assessment of electrocatalytic performance of spinel system. Herein, we demonstrated a series of MFe2O4 (M = Fe, Co, Ni, Zn) family with a well-controlled morphology to achieve comprehensive study of electrocatalytic activity toward OER, in which most of the divalent metals (M2+) of MFe2O4 were located at octahedral sites. By utilizing several analyses, we could conclude a universal rule toward the spinel metal oxide for oxygen evolution. The activities for OER in spinel metal oxide system were determined by the occurrence of phase transformation rather than electronic state of metal ions, and this structural transformation could work well in both crystallographic sites (Td and Oh sites). Additionally, the divalent metal ion significantly dominated the formation of oxyhydroxide through an epitaxial relationship which depended on the atomic arrangement in the interface of spinel and metal oxyhydroxide, while trivalent metal ions were likely to be hosts lattices. The metal oxyhydroxide was formed during redox occurrence rather than OER, while the redox occurrence with remarkable increase in resistance and capacitance might result from the structural transformation from spinel to metal oxyhydroxide. We believed that the approaching strategies and information obtained in present study can offer a guide to design a promising electrocatalytic system toward oxygen evolution reaction and other fields.

IMMU-23. CMV pp65 RNA-PULSED DENDRITIC CELL VACCINES FOR PEDIATRIC GLIOBLASTOMA AND MEDULLOBLASTOMA: A PHASE I TRIAL

Recurrent medulloblastoma and malignant glioma are lethal tumors that are virtually incurable. The cytomegalovirus (CMV) antigen pp65 is ubiquitously expressed on medulloblastoma and malignant glioma but not on healthy brain, making this an attractive candidate for targeted therapy, including immunotherapy. A recently completed phase I trial at Duke evaluated autologous CMV pp65 RNA-pulsed dendritic cell (DC) vaccines in newly diagnosed adult glioblastoma patients. Vaccines were most effective when given with GM-CSF after tetanus-diphtheria toxoid (Td) site preconditioning, achieving a median overall survival of over 40 months. Additionally, DC migration to lymph nodes emerged as a strong biologic correlate with progression-free survival. We are now evaluating this platform in children as a single-arm phase I trial. Our hypothesis is that pp65 RNA-pulsed autologous DC vaccines will be well-tolerated and lead to significant DC migration and pp65-specific T cell responses in children with recurrent medulloblastoma or malignant glioma. As the primary objective, we will establish the safety and feasibility of vaccinating pediatric patients with recurrent medulloblastoma or malignant glioma using CMV pp65 RNA-pulsed DCs with GM-CSF and Td preconditioning. Our secondary objectives are to 1.) assess the ability of radiolabeled DCs to track to vaccine-draining lymph nodes, and 2.) measure T-cell responses against pp65 before and after DC vaccination. Time to progression is not a primary endpoint in this study but will be accessioned to derive hypotheses that may relate other data derived in this study with progression.

Investigation of Structural Parameters and Magnetic Properties of Mixed Li-Ni Spinel Ferrites

The solid state method was used to synthesize the mixed spinel ferrites where x is the increments of Ni content. The net magnetization (Mnet) of the prepared samples was calculated according to the suggested cations distribution . The calculated magnetization of tetrahedral (Td) site (MT) unchanged, while the magnetization of octahedral (Oh) site (MO) and Mnet decreased due to increasing Ni2+ ions. The measured values of Mnet showed the same behavior as the calculated values, which decreased exponentially with increasing of the Ni concentrations. In addition, Mnet increased with increasing of applied magnetic field (H), which showed a normal behavior for the ferrites system. The ratio of FeT/FeO indicated that the relative number of the Fe3+ ions on Oh sites decreased with increasing Ni content. The ionic radii of Td sites ( ) unchanged where the radii of Oh sites ( ) increased with increasing of the Ni2+ ions. The values of the average ionic radii increased with increasing of Ni2+ ions. The expected values of lattice parameter ( ) increased, where the values of oxygen parameter and decreased with more incorporation of Ni content. All the interionic distances (bond lengths (BLs)) changed due to increasing of Ni contents. The bond angles , , , and were calculated. The values of , and increased and the decreased with increasing of Ni2+ ions. The relative permeability ( ) increased with increasing of the filed H and decreased with the increase in in composition x.Substitution of the Ni2+ ions has a tremendous influence on the properties of Li-spinel ferrite such the magnetic properties.

Diatoms as an indicator for tile drainage flow in a German lowland catchment

BackgroundThe separation of runoff components within a model simulation is of great importance for a successful implementation of management measures. Diatoms could be a promising indicator for tile drainage flow due to their diverse preferences to different aquatic habitats. In this study, we collected diatom samples of 9 sites (4 tile drainage, TD, and 5 river sites, Ri) in a German lowland catchment at a weekly or biweekly time step from March to July 2013 with the aim of testing the suitability of diatoms for tile drainage flow, which is typical for lowland catchment.ResultsPlanothidium lanceolatum, Ulnaria biceps, and Navicula gregaria dominated in TD sites with relative abundances of 22.2, 21.5, and 10.9%, respectively. For Ri sites, the most abundant species was Navicula lanceolata (20.5%), followed by Ulnaria biceps (12.9%), Cyclotella meneghiniana (9.5%), and Planothidium lanceolatum (9.3%). Compared with Ri sites, TD had a lower diatom density, biomass, species richness, and percentage of Aquatic/Riparian diatoms (AqRi%). However, the proportion of Riparian diatoms (RiZo%) increased at TD. Indicator value method (IndVal) revealed that the two groups (Ri and TD) were characterized by different indicator species. Fifteen taxa, including Cocconeis placentula, Cyclotella meneghiniana, N. lanceolata, and U. biceps, were significant indicators for Ri sites. Planothidium lanceolatum, Achnanthidium minutissimum, and Navicula gregaria were significant indicators for TD sites.ConclusionA pronounced variation was found in the species lists of diatom community between Ri and TD water body types associated with different indicator species. With respect to hydrograph separation, these findings highlight the suitability of diatoms as an indicator for tile drainage flow. However, spatial and temporal variations of diatoms should be considered in future surveys.

Theoretical investigation of the cathodoluminescence spectra of Co-doped ZnO nanowires

The morphology, structural and optical properties of the Co2+ ions doped in ZnO nanowires were investigated by S. Geburt et al. (2013) [1]. The cathodoluminescence CL spectrum is composed of the ZnO near-band-edge NBE (ultraviolet emission), green emission band attributed to the recombination of carriers at intrinsic ZnO defects and sharp red emission assigned to the Co2+ intra-3d luminescence. The photoluminescence excitation PLE spectrum shows the direct transitions from the ground state of Co2+ occupying Td site to the higher excited states. The visible region provides important information about the electronic structure of the Co2+ doped zinc oxide nanowires. This work investigates an interpretation of the origin of the ultraviolet NBE and discusses the defects involved in the broad green emission band. A theoretical crystal-field analysis of the visible lines, associated to the Co2+ ion transition occupying Td site symmetry in ZnO host crystal, is also realized which leads to a satisfactory correlation between experimental and calculated energy levels. The electronic structure was compared to the structure reported for Co2+ ion doped in ZnO nanocrystalline (nanorods, nanoparticles) and bulk crystals. The quantum confinement effect of ZnO nanowires is interpreted.

Spectroscopic Evidence for Superexchange in the Ferrimagnetic Spinel FeCr2S4

Low-energy orbital excitations in spinel FeCr2S4 have been studied element specifically at both Fe and Cr 2p–3d resonances by resonant inelastic X-ray scattering (RIXS). As a first step X-ray absorption spectroscopy measurements confirmed that iron, located at the tetrahedral (Td) sites of the spinel structure, is in a Fe2+ (d6) state and chromium, located at the octahedral (Oh) sites, is in a Cr3+ (d3) state. We have identified RIXS spectral features with the help of crystal-field simulations that account consistently for on-site dd excitations and provide 10Dq = 0.30 ± 0.05 eV for Fe Td sites and 10Dq = 1.60 ± 0.05 eV for Cr Oh sites. The giant magneto-optical Kerr effect previously observed for photons with 0.3 eV can be unambiguously attributed to the lowest possible dd excitations on the iron site Fe(e3t23) → Fe(e2t24). A 1.6 eV energy-loss feature, measured in the Fe 2p–3d RIXS spectra, is found to be a two-site orbital excitation directly related to superexchange (SE) interaction between Fe and Cr ...

A Mesoporous Cobalt Aluminate Spinel Catalyst for Nonoxidative Propane Dehydrogenation

AbstractA mesoporous CoAl2O4 spinel (Co‐Al) is synthesized by a one‐step evaporation‐induced self‐assembly (EISA) method. N2 physisorption and TEM are used to demonstrate the presence of mesopores within the Co‐Al material. The spinel crystal structure of Co‐Al, in which Co occupies tetrahedral (Td) sites, is confirmed by using XRD and UV/Vis spectroscopy. In nonoxidative propane dehydrogenation at 550 °C, a propane conversion of approximately 8 % is observed for Co‐Al with a >80 % propylene selectivity, which corresponds to a turnover frequency of 5.1 h−1 based on an estimation of the number of active Co sites by using NH3 temperature‐programmed desorption. A much higher propane conversion rate and a circa 80 % propylene selectivity is observed upon reaction at 600 °C. Continuous deactivation of the catalyst is observed for Co‐Al at this elevated temperature. In situ X‐ray absorption spectroscopy results suggest that Co remains as a Td Co2+ species under the reaction conditions. The Td Co2+ sites within the Co‐Al material are thus proposed to act as Lewis acidic active sites; this acidity is verified using IR spectroscopy with pyridine as a probe molecule.

Active Tetrahedral Iron Sites of γ-Fe2O3 Catalyzing NO Reduction by NH3

Maghemite (γ-Fe2O3) with a spinel structure, consisting of tetrahedral Fe3+ (Fe3+Td) and octahedral Fe3+ (Fe3+Oh) sites, has been intensively investigated as an environmentally benign catalyst for selective catalytic reduction (SCR) of NOx with NH3. In most cases, Fe3+Oh sites were regarded as catalytically active sites (CASs). Here we identify the CASs in SCR by substituting Fe3+Oh or Fe3+Td sites of γ-Fe2O3 with catalytically inactive Ti4+ or Zn2+, respectively. The SCR activity of γ-Fe2O3 is preserved after Ti4+ doping but drastically decreases when the catalyst is doped with Zn2+, demonstrating that Fe3+Td sites serve as CASs in SCR. Synchrotron X-ray absorption spectra coupled with density functional theory calculations reveal that the transfer of an electron from inactive Fe2+ to active Fe3+ in the tetrahedral site is easier than that in the octahedral site, making the tetrahedral iron sites active in SCR.

FT-IR Studies of Nickel Substituted Polycrystalline Zinc Spinel Ferrites for Structural and Vibrational Investigations

FT-IR spectra of Ni1-sZnsFe2O4 spinel ferrite, s changed by 0.2 according to 0.0 s 1.0, have been analyzed in the frequency range (350−1000) cm-1. Six polycrystalline ferrites samples were synthesized using the conventional standard double sintering ceramic method. Two main absorption bands were observed, their positions were found to be strongly dependent on s-value. The high frequency band in the range 550-600 cm−1 and a low frequency band at around 400 cm−1 were assigned to tetrahedral Td and octahedral Oh sites, respectively, of spinel lattice. Force constant (FC) was calculated for Tdand Oh sites and was found to decrease with increasing Zn ions. Threshold frequency nth for the electronic transition was determined and found to increase with increasing Zn ions. Cations distribution for the prepared mixed ferrite was concluded based on the FT-IR spectra. The ionic radii for each site were correlated to the cations distribution of the given ferrite.

Cation Exchange of Anisotropic-Shaped Magnetite Nanoparticles Generates High-Relaxivity Contrast Agents for Liver Tumor Imaging

Cation exchange is a powerful means to adjust the properties of nanocrystals through composition change with morphology retention. Herein, we demonstrate that cation exchange can engineer the composition of iron oxide nanocrystals to dramatically improve their contrast ability in magnetic resonance imaging (MRI). We successfully construct manganese and zinc engineered iron oxide nanoparticles with diverse shapes (sphere, cube, and octapod) by facile cation exchange reactions. Extended X-ray absorption fine structure (EXAFS) study indicates that Mn2+ and Zn2+ ions are doped into the crystal lattice of ferrite, and more importantly, most of them are distributed in Td sites of ferrite. These engineered shaped-anisotropic iron oxide nanoparticles exhibit both high saturated magnetization and large effective boundary radii, which leads to remarkable transverse relaxivity (r2), for example, 754.2 mM–1 s–1 for zinc engineered octapod iron oxide nanoparticles. These engineered iron oxide nanoparticles, as high-pe...

Effect of the grafting agent on the structure and catalytic performance of Ti-MCM-22

Ti-MCM-22 prepared by grafting with Ti(OEt)4 has more Ti(iv) at Td sites than those prepared with Ti(OPr)4 and Ti(OBu)4, and gives higher catalytic activities in cyclohexene and cyclooctene epoxidation using t-butylhydrogen peroxide as the oxidant.

In Operando Identification of Geometrical-Site-Dependent Water Oxidation Activity of Spinel Co3O4.

Spinel Co3O4, comprising two types of cobalt ions: one Co(2+) in the tetrahedral site (Co(2+)(Td)) and the other two Co(3+) in the octahedral site (Co(3+)(Oh)), has been widely explored as a promising oxygen evolution reaction (OER) catalyst for water electrolysis. However, the roles of two geometrical cobalt ions toward the OER have remained elusive. Here, we investigated the geometrical-site-dependent OER activity of Co3O4 catalyst by substituting Co(2+)(Td) and Co(3+)(Oh) with inactive Zn(2+) and Al(3+), respectively. Following a thorough in operando analysis by electrochemical impedance spectroscopy and X-ray absorption spectroscopy, it was revealed that Co(2+)Td site is responsible for the formation of cobalt oxyhydroxide (CoOOH), which acted as the active site for water oxidation.

Effects of defect on the storage and diffusion of Na and Mg interstitials in Si anode

The effects of 90° partial dislocation on the performance of Si anode in Na-ion batteries (NIBs) and Mg-ion batteries (MIBs) are studied by the multi-scale simulation method. In defect-free Si crystal, Na and Mg interstitials prefer to stay at tetrahedral (Td) site and the migration path is Td-Hex-Td. Because of the insufficient binding strength of Na (−0.79 eV) and Mg (−0.969 eV), perfect Si crystal is not suitable for the storage of them. In the presence of 90° partial dislocation, its spacious cores may promote the binding energies of Na and Mg to 0.423 eV and 0.4 eV, respectively. This leads to the insertion of Na and Mg into Si anode possible. Moreover, it is thermodynamically and kinetically favorable for Na and Mg diffusion into the 90° partial dislocation core and surrounding areas. They may concentrate in the dislocation core. This core can also provide a fast diffusion avenue for Na (Mg) by lowing the migration barriers from 1.04 eV (1.02 eV) to 0.75 eV (0.804 eV). Therefore, 90° partial dislocation may improve the charge/discharge rate of NIBs and MIBs.

Revealing Conduction Mechanisms in Lithium-Rich Garnets Using Aliovalent Dopants

Lithium stuffed garnets such as Li7La3Zr2O12 (LLZO) show promise as solid state lithium ion conductors for use in high energy density storage applications due to their chemical and thermal stability and electrochemical performance versus current technology electrolytes. Much work has focused on improving the conductivity of these systems by optimizing the lithium content in the cubic garnet phase rather than the less conducting tetragonal polymorph1. Despite extensive studies on these materials, there is uncertainty as to the preferred migration of Li; whether the ions move from one octahedral (Oh) site to another via a neighbouring tetrahedron (Oh-Td-Oh path), or hop directly across Oh sites, bypassing the Td site (Oh-Oh path), is still up for debate2,3. We seek to confirm the lattice site occupancy of the mobile lithium ions and their associated conduction pathways by varying the lithium-substituted dopant species (Ga, Zn, Ge), thereby blocking Li movement through one of the two paths. The effect of dopant charge and ionic radius on cubic phase stabilization (as a result of Li concentration) and lattice strain is studied. Moreover, the moisture sensitivity of Li-conductor ceramics has been recently associated with a deterioration of Li transport4 and density of the material, of great importance in battery applications5. In this study, we use an argon-supplied glove box coupled to a furnace for processing and synthesis, preventing proton exchange with Li, thus avoiding the creation of proton charge carriers and improving the densification to above 90% relative density. This is fundamental to analyze the real Li-conduction mechanism and optimize the Li conductivity in garnets. To study the degradation process of lithium displacement by protons in moisture, and the effect of the protons on the ionic conductivity, we conduct isotopic exchange experiments on the samples using H2O and D2O followed by secondary ion mass spectroscopy. The total conductivity of the samples is found by AC impedance in a range of humidities and temperatures. X-ray diffraction and neutron diffraction are employed to determine the structural features, specifically to localize the dopants, Li and protons and to analyse the conduction pathways as a function of the dopant. References 1J. Awaka et al.(2009) J. Solid State Chem. 182, 2046-2052 2 Han, J. et al. (2012) Chem. Commun., 48 (79), 9840–9842 3 Xu, M. et al. (2012) Phys .Rev. B, 85, 052301 4Aguesse, F. et al. (2014) Adv. Mater. Interfaces, 1 (7), 1300143 5 Bernuy-Lopez, C. et al. (2014) Chem. Mater., 26 (12), 3610-3617 Figure 1

Revealing the Li Conduction Pathway Using Dopants at the Li Sites in Stuffed Garnets

Lithium-rich garnets such as Li7La3Zr2O12 (LLZO) show promise as solid state lithium ion conductors for use in high energy density storage applications due to their chemical and thermal stability and electrochemical performance versus current technology electrolytes. Much work has focused on improving the conductivity of these systems by maximizing the lithium content but also through the introduction of lattice vacancies created by the addition of cation dopants (such as Ga1) in the Li sites. Additionally, the importance of stabilizing the cubic garnet phase as opposed to the less conducting tetragonal polymorph has been acknowledged and pursued in the search for high conductivities2. There is uncertainty with regard to the specific Li ion conduction mechanism in these systems however it is widely agreed that the Li ions in LLZO adopt tetrahedral (Td) and octahedral (Oh) positions in the lattice and that the conduction pathway is by a face-sharing network of these sites3, 4. It is yet unclear if the Li ions move from one Oh site to another via a neighbouring Td site (Oh-Td-Oh path), or whether they hop directly across Oh sites, bypassing the Td site (Oh-Oh path), and how this pathway is influenced by the lattice site occupancy of Li and dopants. The sensitivity to air and moisture during synthesis5as well as phase transformations of the garnet structure further complicate the picture. In this study, we determine the exact nature of the effect of dopants on the conductivity by investigating the dopant species Zn(II), Ga(III) and Ge(IV). An understanding of the lattice position adopted by these cations as determined by their size and/or charge will be achieved through analysis of the synthesized samples by neutron diffraction and solid state NMR. By tracking the dopant ion position, the corresponding Li conduction pathway can be found and related to that required for optimum conductivity. We also find the critical Li ion to vacancy ratio for optimum ionic conduction, by varying the stoichiometry of each dopant of a particular oxidation number and measuring the resultant electrolyte ionic conductivity using an AC impedance technique. Lattice strain effects will be a final consideration- how does the size of the dopant ion restrict or enhance the diffusion pathway of the Li ions, and is there a blocking effect seen above certain dopant concentrations? Control of the atmospheric conditions is achieved throughout by the use of an Argon-supplied glovebox coupled with a furnace (1600°C), preventing exchange of Li with H+during synthesis and processing. This has proven to improve the densification of the electrolyte and increase the Li-conductivity by between 1 and 3 orders of magnitude. Figure 1. a) Arrangement of polyhedra in the garnet Ga-doped LLZO cubic structure and b) Possible Li-pathways in the Td-Oh network.1 References [1] Bernuy-Lopez, C. et al. (2014) Chem. Mater., 26 (12), 3610-3617 2 J. Awaka et al.(2009) J. Solid State Chem.182, 2046–2052 3 Han, J. et al. (2012) Chem. Commun., 48 (79), 9840–9842 4 Xu, M., et al. (2012) Phys .Rev. B, 85, 052301 5 Aguesse, F. et al. (2014) Adv. Mater. Interfaces, 1 (7), 1300143 Figure 1