Order-disorder and ionic conductivity in calcium nitride-hydride.

Recently nitrogen-hydrogen compounds have successfully been applied as co-catalysts for mild conditions ammonia synthesis. Ca2NH was shown to act as a H2 sink during reaction, with H atoms from its lattice being incorporated into the NH3(g) product. Thus the ionic transport and diffusion properties of the N-H co-catalyst are fundamentally important to understanding and developing such syntheses. Here we show hydride ion conduction in these materials. Two distinct calcium nitride-hydride Ca2NH phases, prepared via different synthetic paths are found to show dramatically different properties. One phase (ß) shows fast hydride ionic conduction properties (0.08 S/cm at 600 °C), on a par with the best binary ionic hydrides and 10 times higher than CaH2, whilst the other (α) is 100 times less conductive. An in situ combined analysis techniques reveals that the effective ß-phase conducts ions via a vacancy-mediated phenomenon in which the charge carrier concentration is dependent on the ion concentration in the secondary site and by extension the vacancy concentration in the main site.

Anion redox as a means to derive layered manganese oxychalcogenides with exotic intergrowth structures.

Topochemistry enables step-by-step conversions of solid-state materials often leading to metastable structures that retain initial structural motifs. Recent advances in this field revealed many examples where relatively bulky anionic constituents were actively involved in redox reactions during (de)intercalation processes. Such reactions are often accompanied by anion-anion bond formation, which heralds possibilities to design novel structure types disparate from known precursors, in a controlled manner. Here we present the multistep conversion of layered oxychalcogenides Sr2MnO2Cu1.5Ch2 (Ch = S, Se) into Cu-deintercalated phases where antifluorite type [Cu1.5Ch2]2.5- slabs collapsed into two-dimensional arrays of chalcogen dimers. The collapse of the chalcogenide layers on deintercalation led to various stacking types of Sr2MnO2Ch2 slabs, which formed polychalcogenide structures unattainable by conventional high-temperature syntheses. Anion-redox topochemistry is demonstrated to be of interest not only for electrochemical applications but also as a means to design complex layered architectures.

Re-investigating the structure-property relationship of the solid electrolytes Li 3-x In1-x Zr x Cl6 and the impact of In-Zr(iv) substitution.

Chloride-based solid electrolytes are considered interesting candidates for catholytes in all-solid-state batteries due to their high electrochemical stability, which allows the use of high-voltage cathodes without protective coatings. Aliovalent Zr(iv) substitution is a widely applicable strategy to increase the ionic conductivity of Li3M(iii)Cl6 solid electrolytes. In this study, we investigate how Zr(iv) substitution affects the structure and ion conduction in Li3-x In1-x Zr x Cl6 (0 ≤ x ≤ 0.5). Rietveld refinement using both X-ray and neutron diffraction is used to make a structural model based on two sets of scattering contrasts. AC-impedance measurements and solid-state NMR relaxometry measurements at multiple Larmor frequencies are used to study the Li-ion dynamics. In this manner the diffusion mechanism and its correlation with the structure are explored and compared to previous studies, advancing the understanding of these complex and difficult to characterize materials. It is found that the diffusion in Li3InCl6 is most likely anisotropic considering the crystal structure and two distinct jump processes found by solid-state NMR. Zr-substitution improves ionic conductivity by tuning the charge carrier concentration, accompanied by small changes in the crystal structure which affect ion transport on short timescales, likely reducing the anisotropy.

Counting the Acid Sites in a Commercial ZSM-5 Zeolite Catalyst.

This work investigates the acid sites in a commercial ZSM-5 zeolite catalyst by a combination of spectroscopic and physical methods. The Brønsted acid sites in such catalysts are associated with the aluminum substituted into the zeolite lattice, which may not be identical to the total aluminum content of the zeolite. Inelastic neutron scattering spectroscopy (INS) directly quantifies the concentrations of Brønsted acid protons, silanol groups, and hydroxyl groups associated with extra-framework aluminum species. The INS measurements show that ∼50% of the total aluminum content of this particular zeolite is extra framework, a conclusion supported by solid-state NMR and ammonia temperature-programmed desorption (TPD) measurements. Evidence for the presence of extra-framework aluminum oxide species is also seen in neutron powder diffraction data from proton- and deuterium-exchanged samples. The differences between results from the different analytical methods are discussed, and the novelty of direct proton counting by INS in this typical commercial catalyst is emphasized.

Enhanced Oxide Ion Conductivity by Ta Doping of Ba3Nb1-xTaxMoO8.5.

Significant oxide ion conductivity has previously been reported for the Ba3M'M″O8.5 family (M' = Nb5+, V5+; M″ = Mo6+, W6+) of cation-deficient hexagonal perovskite derivatives. These systems exhibit considerable structural disorder and competitive occupation of two distinct oxygen positions (O3 site and O2 site), enabling two-dimensional (2D) ionic conductivity within the ab plane of the structure; higher occupation of the tetrahedral O3 site vs the octahedral O2 site is known to be a major factor that promotes oxide ion conductivity. Previous chemical doping studies have shown that substitution of small amounts of the M' or M″ ions can result in significant changes to both the structure and ionic conductivity. Here, we report on the electrical and structural properties of the Ba3Nb1-xTaxMoO8.5 series (x = 0.00, 0.025, 0.050, 0.100). AC impedance measurements show that substitution of Nb5+ with Ta5+ leads to a significant increase in low-temperature (<500 °C) conductivity for x = 0.1. Analysis of neutron and X-ray diffraction (XRD) data confirms that there is a decrease in the M1O4/M1O6 ratio upon increasing x from 0 to 0.1 in Ba3Nb1-xTaxMoO8.5, which would usually coincide with a lowering in the conductivity. However, neutron diffraction results show that Ta doping causes an increase in the oxide ion conductivity as a result of longer M1-O3 bonds and increased polyhedral distortion.

Mechanochemical Synthesis and Structure of Lithium Tetrahaloaluminates, LiAlX4 (X = Cl, Br, I): A Family of Li-Ion Conducting Ternary Halides.

State-of-the-art oxides and sulfides with high Li-ion conductivity and good electrochemical stability are among the most promising candidates for solid-state electrolytes in secondary batteries. Yet emerging halides offer promising alternatives because of their intrinsic low Li+ migration energy barriers, high electrochemical oxidative stability, and beneficial mechanical properties. Mechanochemical synthesis has enabled the characterization of LiAlX4 compounds to be extended and the iodide, LiAlI4, to be synthesized for the first time (monoclinic P21/c, Z = 4; a = 8.0846(1) Å; b = 7.4369(1) Å; c = 14.8890(2) Å; ß = 93.0457(8)°). Of the tetrahaloaluminates, LiAlBr4 exhibited the highest ionic conductivity at room temperature (0.033 mS cm-1), while LiAlCl4 showed a conductivity of 0.17 mS cm-1 at 333 K, coupled with the highest thermal and oxidative stability. Modeling of the diffusion pathways suggests that the Li-ion transport mechanism in each tetrahaloaluminate is closely related and mediated by both halide polarizability and concerted complex anion motions.

Investigation of the Crystal Structure and Ionic Pathways of the Hexagonal Perovskite Derivative Ba3-xVMoO8.5-x.

The hexagonal perovskite derivatives Ba3NbMoO8.5, Ba3NbWO8.5, and Ba3VWO8.5 have recently been reported to exhibit significant oxide ion conductivity. Here, we report the synthesis and crystal structure of the hexagonal perovskite derivative Ba3-xVMoO8.5-x. Rietveld refinement from neutron and X-ray diffraction data show that the cation vacancies are ordered on the M2 site, leading to a structure consisting of palmierite-like layers of M1Ox polyhedra separated by vacant octahedral layers. In contrast to other members of the Ba3M'M″O8.5 family, Ba3-xVMoO8.5-x is not stoichiometric and both barium and oxygen vacancies are present. Although synthesized in air at elevated temperatures, Ba3-xVMoO8.5-x is unstable at lower temperatures, as illustrated by the formation of BaCO3 and BaMoO4 by heat treatment in air at 400 °C. This precludes measurement of the electrical properties. However, bond-valence site energy (BVSE) calculations strongly suggest that oxide ion conductivity is present in Ba3-xVMoO8.5-x.

BiMnPO5 with ferromagnetic Mn2+-(µ-O)2-Mn2+ units: a model for magnetic exchange in edge-linked Mn2+O6 octahedra.

Magnetic interactions within Mn-(µ-O)2-Mn pairs are crucial to the function of some essential enzymes and catalysts, but their nature is unclear. Neutron diffraction reveals that similar units in BiMnPO5 show ferromagnetic coupling which has been rationalized by density functional theory modelling and calculations of magnetic exchange energies. The results are important to many solid state and biological systems.

Spin-ice physics in cadmium cyanide.

Spin-ices are frustrated magnets that support a particularly rich variety of emergent physics. Typically, it is the interplay of magnetic dipole interactions, spin anisotropy, and geometric frustration on the pyrochlore lattice that drives spin-ice formation. The relevant physics occurs at temperatures commensurate with the magnetic interaction strength, which for most systems is 1-5 K. Here, we show that non-magnetic cadmium cyanide, Cd(CN)2, exhibits analogous behaviour to magnetic spin-ices, but does so on a temperature scale that is nearly two orders of magnitude greater. The electric dipole moments of cyanide ions in Cd(CN)2 assume the role of magnetic pseudospins, with the difference in energy scale reflecting the increased strength of electric vs magnetic dipolar interactions. As a result, spin-ice physics influences the structural behaviour of Cd(CN)2 even at room temperature.

Sample Dependence of Magnetism in the Next-Generation Cathode Material LiNi0.8Mn0.1Co0.1O2.

We present a structural and magnetic study of two batches of polycrystalline LiNi0.8Mn0.1Co0.1O2 (commonly known as Li NMC 811), a Ni-rich Li ion battery cathode material, using elemental analysis, X-ray and neutron diffraction, magnetometry, and polarized neutron scattering measurements. We find that the samples, labeled S1 and S2, have the composition Li1-xNi0.9+x-yMnyCo0.1O2, with x = 0.025(2), y = 0.120(2) for S1 and x = 0.002(2), y = 0.094(2) for S2, corresponding to different concentrations of magnetic ions and excess Ni2+ in the Li+ layers. Both samples show a peak in the zero-field-cooled (ZFC) dc susceptibility at 8.0(2) K, but the temperature at which the ZFC and FC (field-cooled) curves deviate is substantially different: 64(2) K for S1 and 122(2) K for S2. The ac susceptibility measurements show that the transition for S1 shifts with frequency whereas no such shift is observed for S2 within the resolution of our measurements. Our results demonstrate the sample dependence of magnetic properties in Li NMC 811, consistent with previous reports on the parent material LiNiO2. We further establish that a combination of experimental techniques is necessary to accurately determine the chemical composition of next-generation battery materials with multiple cations.

Li1.5La1.5MO6 (M = W6+, Te6+) as a new series of lithium-rich double perovskites for all-solid-state lithium-ion batteries.

Solid-state batteries are a proposed route to safely achieving high energy densities, yet this architecture faces challenges arising from interfacial issues between the electrode and solid electrolyte. Here we develop a novel family of double perovskites, Li1.5La1.5MO6 (M = W6+, Te6+), where an uncommon lithium-ion distribution enables macroscopic ion diffusion and tailored design of the composition allows us to switch functionality to either a negative electrode or a solid electrolyte. Introduction of tungsten allows reversible lithium-ion intercalation below 1 V, enabling application as an anode (initial specific capacity >200 mAh g-1 with remarkably low volume change of ∼0.2%). By contrast, substitution of tungsten with tellurium induces redox stability, directing the functionality of the perovskite towards a solid-state electrolyte with electrochemical stability up to 5 V and a low activation energy barrier (<0.2 eV) for microscopic lithium-ion diffusion. Characterisation across multiple length- and time-scales allows interrogation of the structure-property relationships in these materials and preliminary examination of a solid-state cell employing both compositions suggests lattice-matching avenues show promise for all-solid-state batteries.

Structure and thermal expansion of the distorted Prussian blue analogue RbCuCo(CN)6.

The structure and thermal expansion of the Prussian blue analogue RbCuCo(CN)6 has been determined via neutron and X-ray powder diffraction. The system crystallises in Cccm and harbours three coexisting distortions relative to the parent Fm3[combining macron]m structure, which leads to anisotropic thermal expansion with a near-zero component in one direction. The difficulties associated with determining octahedral tilt systems in Prussian blue analogues and related double molecular perovskites are discussed.

High-Pressure Study of the Elpasolite Perovskite La2NiMnO6.

Here we report a high-pressure investigation into the structural and magnetic properties of the double perovskite La2NiMnO6 using neutron scattering over a temperature range of 4.2-300 K at ambient pressure and over a temperature range of 120-1177 K up to a maximum pressure of 6.6 GPa. X-ray diffraction was also used up to a maximum pressure of 64 GPa, over a temperature range of 300-720 K. The sample was found to exist in a mixed rhombohedral/monoclinic symmetry at ambient conditions, the balance of which was found to be strongly temperature- and pressure-dependent. Alternating current magnetometry and X-ray absorption near-edge structure measurements were made at ambient pressure to characterize the sample, suggesting that the transition-metal sites exist in a mixed Ni3+/Mn3+ and Ni2+/Mn4+ state at ambient temperature and pressure. Analysis of the magnetic properties of the sample shows that the Curie temperature can be enhanced by ∼12 K with 2 GPa applied pressure, but it is highly stable at pressures beyond this. We report a pressure-volume-temperature equation of state for this material over this combined temperature and pressure range, with an ambient temperature bulk modulus of ∼179(8) GPa. The previously reported transition from monoclinic to rhombohedral symmetry upon heating to 700 K is seen to be encouraged with applied pressure, transforming fully by ∼1.5 GPa. Raman spectroscopy data were collected up to ∼8 GPa and show no clear changes or discontinuities over the reported phase transition to rhombohedral symmetry or any indication of further changes over the range considered. The ambient-pressure Grüneisen parameter γth was determined to be γth = 2.6 with a Debye temperature of 677 K. The individual modal parameters γj at ambient temperature were also determined from the high-pressure Raman data.

Structure and physical properties of SeCo1-x Mn x O3.

We describe the high-pressure (4 GPa) high-temperature (∼1100 K) synthesis of the solid solution series SeCo1-x Mn x O3 (0 < x < 1) using H2SeO4 and transition metal oxide mixtures sealed in Pt capsules. Neutron powder diffraction has been performed to determine progression of the structure across the solution. All samples crystallise with orthorhombic Pnma symmetry, and octahedral tilting is determined to increase with Mn content. SQUID magnetometry measurements were performed, and reveal that the Néel temperature shifts only by approximately 1 K over the series.

Relationship between the Crystal Structure and Electrical Properties of Oxide Ion Conducting Ba3W1.2Nb0.8O8.6.

The oxide ionic conductor Ba3W1.2Nb0.8O8.6 has been synthesized as part of an investigation into the new class of Ba3M'M''O8.5 (M' = W, Mo; M'' = Nb) oxide-ion conducting hexagonal perovskite derivatives. The substitution of W6+ for Nb5+ in Ba3W1+ xNb1- xO8.5+ x/2 leads to an increase in the oxygen content, which enhances the low-temperature ionic conductivity. However, at 400 °C, the ionic conductivity of Ba3W1.2Nb0.8O8.6 is still significantly lower than the molybdenum compound Ba3MoNbO8.5. Remarkably, at 600 °C the bulk oxide ionic conductivities of Ba3MoNbO8.5, Ba3WNbO8.5, and Ba3W1.2Nb0.8O8.6 are very similar (σb = 0.0022, 0.0017, and 0.0016 S cm-1, respectively). The variable-temperature neutron diffraction results reported here demonstrate that Ba3W1.2Nb0.8O8.6 undergoes a similar structural rearrangement to Ba3MoNbO8.5 above 300 °C, but the ratio of (W/Nb)O4 tetrahedra to (W/Nb)O6 octahedra rises at a faster rate upon heating between 300 and 600 °C. There is a clear relationship between the ionic conductivity of Ba3M'1+ xM''1- xO8.5+ x/2 (M' = W, Mo; M'' = Nb) phases and the number of tetrahedrally coordinated M' and M ″ cations present within the crystal structure.

Bulk phase behavior of lithium imide-metal nitride ammonia decomposition catalysts.

Lithium imide is a promising new catalyst for the production of hydrogen from ammonia. Its catalytic activity has been reported to be significantly enhanced through its use as a composite with various transition metal nitrides. In this work, two of these composite catalysts (with manganese nitride and iron nitride) were examined using in situ neutron and X-ray powder diffraction experiments in order to explore the bulk phases present during ammonia decomposition. Under such conditions, the iron composite was found to be a mixture of lithium imide and iron metal, while the manganese composite contained lithium imide and manganese nitride at low temperatures, and a mixture of lithium imide and two ternary lithium-manganese nitrides (LixMn2-xN and a small proportion of Li7MnN4) at higher temperatures. The results indicate that the bulk formation of a ternary nitride is not necessary for ammonia decomposition in lithium imide-transition metal catalyst systems.

Impact of Interstitial Ni on the Thermoelectric Properties of the Half-Heusler TiNiSn.

TiNiSn is an intensively studied half-Heusler alloy that shows great potential for waste heat recovery. Here, we report on the structures and thermoelectric properties of a series of metal-rich TiNi1+ySn compositions prepared via solid-state reactions and hot pressing. A general relation between the amount of interstitial Ni and lattice parameter is determined from neutron powder diffraction. High-resolution synchrotron X-ray powder diffraction reveals the occurrence of strain broadening upon hot pressing, which is attributed to the metastable arrangement of interstitial Ni. Hall measurements confirm that interstitial Ni causes weak n-type doping and a reduction in carrier mobility, which limits the power factor to 2.5-3 mW m-1 K-2 for these samples. The thermal conductivity was modelled within the Callaway approximation and is quantitively linked to the amount of interstitial Ni, resulting in a predicted value of 12.7 W m-1 K-1 at 323 K for stoichiometric TiNiSn. Interstitial Ni leads to a reduction of the thermal band gap and moves the peak ZT = 0.4 to lower temperatures, thus offering the possibility to engineer a broad ZT plateau. This work adds further insight into the impact of small amounts of interstitial Ni on the thermal and electrical transport of TiNiSn.

Grain-by-Grain Compositional Variations and Interstitial Metals-A New Route toward Achieving High Performance in Half-Heusler Thermoelectrics.

Half-Heusler alloys based on TiNiSn are promising thermoelectric materials characterized by large power factors and good mechanical and thermal stabilities, but they are limited by large thermal conductivities. A variety of strategies have been used to disrupt their thermal transport, including alloying with heavy, generally expensive, elements and nanostructuring, enabling figures of merit, ZT ≥ 1 at elevated temperatures (>773 K). Here, we demonstrate an alternative strategy that is based around the partial segregation of excess Cu leading to grain-by-grain compositional variations, the formation of extruded Cu "wetting layers" between grains, and-most importantly-the presence of statistically distributed interstitials that reduce the thermal conductivity effectively through point-defect scattering. Our best TiNiCuySn (y ≤ 0.1) compositions have a temperature-averaged ZTdevice = 0.3-0.4 and estimated leg power outputs of 6-7 W cm-2 in the 323-773 K temperature range. This is a significant development as these materials were prepared using a straightforward processing method, do not contain any toxic, expensive, or scarce elements, and are therefore promising candidates for large-scale production.

Impact of Nb vacancies and p-type doping of the NbCoSn-NbCoSb half-Heusler thermoelectrics.

The half-Heuslers NbCoSn and NbCoSb have promising thermoelectric properties. Here, an investigation of the NbCo1+ySn1-zSbz (y = 0, 0.05; 0 ≤ z ≤ 1) solid-solution is presented. In addition, the p-type doping of NbCoSn using Ti and Zr substitution is investigated. Rietveld analysis reveals the gradual creation of Nb vacancies to compensate for the n-type doping caused by the substitution of Sb in NbCoSn. This leads to a similar valence electron count (∼18.25) for the NbCo1+ySn1-zSbz samples (z > 0). Mass fluctuation disorder due to the Nb vacancies strongly decreases the lattice thermal conductivity from 10 W m-1 K-1 (z = 0) to 4.5 W m-1 K-1 (z = 0.5, 1). This is accompanied by a transition to degenerate semiconducting behaviour leading to large power factors, S2/ρ = 2.5-3 mW m-1 K-2 and figures of merit, ZT = 0.25-0.33 at 773 K. Ti and Zr can be used to achieve positive Seebeck values, e.g. S = +150 µV K-1 for 20% Zr at 773 K. However, the electrical resistivity, ρ323K = 27-35 mΩ cm, remains too large for these materials to be considered useful p-type materials.

Room Temperature Magnetically Ordered Polar Corundum GaFeO3 Displaying Magnetoelectric Coupling.

The polar corundum structure type offers a route to new room temperature multiferroic materials, as the partial LiNbO3-type cation ordering that breaks inversion symmetry may be combined with long-range magnetic ordering of high spin d5 cations above room temperature in the AFeO3 system. We report the synthesis of a polar corundum GaFeO3 by a high-pressure, high-temperature route and demonstrate that its polarity arises from partial LiNbO3-type cation ordering by complementary use of neutron, X-ray, and electron diffraction methods. In situ neutron diffraction shows that the polar corundum forms directly from AlFeO3-type GaFeO3 under the synthesis conditions. The A3+/Fe3+ cations are shown to be more ordered in polar corundum GaFeO3 than in isostructural ScFeO3. This is explained by DFT calculations which indicate that the extent of ordering is dependent on the configurational entropy available to each system at the very different synthesis temperatures required to form their corundum structures. Polar corundum GaFeO3 exhibits weak ferromagnetism at room temperature that arises from its Fe2O3-like magnetic ordering, which persists to a temperature of 408 K. We demonstrate that the polarity and magnetization are coupled in this system with a measured linear magnetoelectric coupling coefficient of 0.057 ps/m. Such coupling is a prerequisite for potential applications of polar corundum materials in multiferroic/magnetoelectric devices.