Nanostructured Niobium and Titanium Carbonitrides as Electrocatalyst Supports.

Nanostructured niobium-titanium carbonitrides, (Nb,Ti)C1-xNx, with the cubic-rock salt structure are prepared without the use of reactive gases via thermal treatment (700-1200 °C) under nitrogen of mixtures of guanidine carbonate and ammonium niobate (V) oxalate hydrate, with addition of ammonium titanyl oxalate monohydrate as a titanium source. The bulk structure and chemical composition of the materials are characterized using powder X-ray diffraction (XRD) and powder neutron diffraction, elemental homogeneity is studied using energy dispersive spectroscopy (EDS) mapping using transmission electron microscopy (TEM), and surface chemical analysis is examined using X-ray photoelectron spectroscopy (XPS). Nanoscale crystallites of between 10 and 50 nm are observed by TEM, where EDS reveals the homogeneity of metal distribution for the mixed-metal materials. Titanium carbonitrides are found to be air sensitive, reacting with air under ambient conditions, while titanium-niobium carbonitrides are found to degrade in aqueous sulfuric acid. The niobium carbonitrides, however, show some stability toward acidic solutions. Materials are produced with composition NbC1-xNx with x between 0.35 and 0.45, and more carbon-rich materials (x ≈ 0.35) are found as the synthesis temperature is increased, as proven by Rietveld refinement of crystal structure against powder neutron diffraction data. Despite phase purity seen by diffraction and negligible bulk carbon content, XPS shows a complex surface chemistry for the NbC1-xNx materials, with evidence for Nb2O5-like oxide species in a carbon-rich environment. The NbC1-xNx prepared at 900 °C has a surface area around 50 m2 g-1, making it suitable as a catalyst support. Loading with iridium provides a material active for the oxygen evolution reaction in 0.1 M sulfuric acid, with minimal leaching of either Nb or Ir after 1000 cycles.

Structure, Spin Correlations, and Magnetism of the S = 1/2 Square-Lattice Antiferromagnet Sr2CuTe1-xWxO6 (0 ≤ x ≤ 1).

Quantum spin liquids are highly entangled magnetic states with exotic properties. The S = 1/2 square-lattice Heisenberg model is one of the foundational models in frustrated magnetism with a predicted, but never observed, quantum spin liquid state. Isostructural double perovskites Sr2CuTeO6 and Sr2CuWO6 are physical realizations of this model but have distinctly different types of magnetic order and interactions due to a d10/d0 effect. Long-range magnetic order is suppressed in the solid solution Sr2CuTe1-xWxO6 in a wide region of x = 0.05-0.6, where the ground state has been proposed to be a disorder-induced spin liquid. Here, we present a comprehensive neutron scattering study of this system. We show using polarized neutron scattering that the spin liquid-like x = 0.2 and x = 0.5 samples have distinctly different local spin correlations, which suggests that they have different ground states. Low-temperature neutron diffraction measurements of the magnetically ordered W-rich samples reveal magnetic phase separation, which suggests that the previously ignored interlayer coupling between the square planes plays a role in the suppression of magnetic order at x ≈ 0.6. These results highlight the complex magnetism of Sr2CuTe1-xWxO6 and hint at a new quantum critical point between 0.2 < x < 0.4.

Insights into the Rich Polymorphism of the Na+ Ion Conductor Na3PS4 from the Perspective of Variable-Temperature Diffraction and Spectroscopy.

Solid electrolytes are crucial for next-generation solid-state batteries, and Na3PS4 is one of the most promising Na+ conductors for such applications, despite outstanding questions regarding its structural polymorphs. In this contribution, we present a detailed investigation of the evolution in structure and dynamics of Na3PS4 over a wide temperature range 30 < T < 600 °C through combined experimental-computational analysis. Although Bragg diffraction experiments indicate a second-order phase transition from the tetragonal ground state (α, P4̅21 c) to the cubic polymorph (ß, I4̅3m) above ∼250 °C, pair distribution function analysis in real space and Raman spectroscopy indicate remnants of a tetragonal character in the range 250 < T < 500 °C, which we attribute to dynamic local tetragonal distortions. The first-order phase transition to the mesophasic high-temperature polymorph (γ, Fddd) is associated with a sharp volume increase and the onset of liquid-like dynamics for sodium-cations (translational) and thiophosphate-polyanions (rotational) evident by inelastic neutron and Raman spectroscopies, as well as pair-distribution function and molecular dynamics analyses. These results shed light on the rich polymorphism of Na3PS4 and are relevant for a range host of high-performance materials deriving from the Na3PS4 structural archetype.

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.

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.

Redox Chemistry and the Role of Trapped Molecular O2 in Li-Rich Disordered Rocksalt Oxyfluoride Cathodes.

In the search for high energy density cathodes for next-generation lithium-ion batteries, the disordered rocksalt oxyfluorides are receiving significant attention due to their high capacity and lower voltage hysteresis compared with ordered Li-rich layered compounds. However, a deep understanding of these phenomena and their redox chemistry remains incomplete. Using the archetypal oxyfluoride, Li2MnO2F, we show that the oxygen redox process in such materials involves the formation of molecular O2 trapped in the bulk structure of the charged cathode, which is reduced on discharge. The molecular O2 is trapped rigidly within vacancy clusters and exhibits minimal mobility unlike free gaseous O2, making it more characteristic of a solid-like environment. The Mn redox process occurs between octahedral Mn3+ and Mn4+ with no evidence of tetrahedral Mn5+ or Mn7+. We furthermore derive the relationship between local coordination environment and redox potential; this gives rise to the observed overlap in Mn and O redox couples and reveals that the onset potential of oxide ion oxidation is determined by the degree of ionicity around oxygen, which extends models based on linear Li-O-Li configurations. This study advances our fundamental understanding of redox mechanisms in disordered rocksalt oxyfluorides, highlighting their promise as high capacity cathodes.

Using applied pressure to guide materials design: a neutron diffraction study of La2NiO4+δ and Pr2NiO4+δ.

The compression behaviours of La2NiO4+δ and Pr2NiO4+δ have been studied up to a pressure of 2.8 and 2.2 GPa respectively. Using neutron diffraction, the mechanism of compression, and the behaviour of the NiO6 and La/PrO9 polyhedra in these layered perovskite materials have been determined. Their compression mechanisms have then been compared to related materials (La2-xPrxNiO4, Pr2-xNdxNiO4, La2-xSrxNiO4 and Pr2-xCaxNiO4) where the unit-cell volume has been reduced by controlling the composition (x), which acts as an 'effective chemical pressure'. Understanding the effects of both has implications for materials design; pressure can be used to finely tune a property, which theoretically may then be emulated using chemical doping.

The Fluorite-Like Phase Nd5Mo3O16±Î´ in the MoO3-Nd2O3 System: Synthesis, Crystal Structure, and Conducting Properties.

This paper describes a study of the system MoO3-Nd2O3 using a combination of X-ray powder diffraction (XRD), neutron powder diffraction (NPD), thermogravimetric analysis (TGA), and ac impedance spectroscopy (IS). A phase-pure material is observed at a composition of 45.5 mol % Nd2O3, which corresponds to an ideal stoichiometry of Nd5Mo3O16.5. XRD and NPD show that the crystal structure is a superstructure of the fluorite arrangement, with long-range ordering of the two cation species leading to a doubled unit cell parameter. The sample is found to be significantly oxygen deficient, i.e. Nd5Mo3O15.63(4), when it is prepared by a solid-state reaction at 1473 K in air. TGA measurements indicate that the sample loses only minimal mass on heating to 1273 K in O2. IS studies of the mean conductivity under different atmospheres show that the sample is a mixed conductor between ambient temperature and 873 K, with a dominant electronic component at higher temperatures, as demonstrated by measurements under inert atmosphere. NPD measurements indicate that the anion vacancies are preferentially located on the O2 sites, while studies of the temperature dependence performed under an O2 atmosphere to 1273 K show significantly anisotropic thermal parameters of the anions. Together with analysis of the total neutron scattering data, this supports a model of oxygen ions hopping between O2 positions, with a vacancy, rather than interstitial, mechanism for the anion diffusion.

Pair Distribution Function Analysis of Structural Disorder by Nb5+ Inclusion in Ceria: Evidence for Enhanced Oxygen Storage Capacity from Under-Coordinated Oxide.

Partial substitution of Ce4+ by Nb5+ is possible in CeO2 by coinclusion of Na+ to balance the charge, via hydrothermal synthesis in sodium hydroxide solution. Pair distribution function analysis using reverse Monte Carlo refinement reveals that the small pentavalent substituent resides in irregular coordination positions in an average fluorite lattice, displaced away from the ideal cubic coordination toward four oxygens. This results in under-coordinated oxygen, which explains significantly enhanced oxygen storage capacity of the materials of relevance to redox catalysis used in energy and environmental applications.

In situ neutron diffraction study of the formation of Ho2Ge2O7 pyrochlore at high temperature and pressure.

The formation of the spin-ice pyrochlore Ho2Ge2O7 by two different high temperature, high pressure routes has been explored using in situ neutron diffraction. The first route involves the solid-state reaction of Ho2O3 and GeO2, and formation of the pyrochlore phase is observed at 994(27) °C and 3.81(2) GPa, which are significantly milder conditions than those previously reported. The second route involves the hydrothermal synthesis of the tetragonal Ho2Ge2O7 pyrogermanate from Ho(NO3)3·5H2O and GeO2 and its subsequent transformation to the pyrochlore phase, which is observed at 683(23) °C and 3.89(3) GPa. The lowering of the formation temperature of high pressure phases by employment of a precursor of appropriate stoichiometry may also have applications in the wider field of solid-state chemistry.

ζ-Glycine: insight into the mechanism of a polymorphic phase transition.

Glycine is the simplest and most polymorphic amino acid, with five phases having been structurally characterized at atmospheric or high pressure. A sixth form, the elusive ζ phase, was discovered over a decade ago as a short-lived intermediate which formed as the high-pressure ∊ phase transformed to the γ form on decompression. However, its structure has remained unsolved. We now report the structure of the ζ phase, which was trapped at 100 K enabling neutron powder diffraction data to be obtained. The structure was solved using the results of a crystal structure prediction procedure based on fully ab initio energy calculations combined with a genetic algorithm for searching phase space. We show that the fate of ζ-glycine depends on its thermal history: although at room temperature it transforms back to the γ phase, warming the sample from 100 K to room temperature yielded ß-glycine, the least stable of the known ambient-pressure polymorphs.

Orbital Dimer Model for the Spin-Glass State in Y_{2}Mo_{2}O_{7}.

The formation of a spin glass generally requires that magnetic exchange interactions are both frustrated and disordered. Consequently, the origin of spin-glass behavior in Y_{2}Mo_{2}O_{7}-in which magnetic Mo^{4+} ions occupy a frustrated pyrochlore lattice with minimal compositional disorder-has been a longstanding question. Here, we use neutron and x-ray pair-distribution function (PDF) analysis to develop a disorder model that resolves apparent incompatibilities between previously reported PDF, extended x-ray-absorption fine structure spectroscopy, and NMR studies, and provides a new and physical explanation of the exchange disorder responsible for spin-glass formation. We show that Mo^{4+} ions displace according to a local "two-in-two-out" rule on each Mo_{4} tetrahedron, driven by orbital dimerization of Jahn-Teller active Mo^{4+} ions. Long-range orbital order is prevented by the macroscopic degeneracy of dimer coverings permitted by the pyrochlore lattice. Cooperative O^{2-} displacements yield a distribution of Mo-O-Mo angles, which in turn introduces disorder into magnetic interactions. Our study demonstrates experimentally how frustration of atomic displacements can assume the role of compositional disorder in driving a spin-glass transition.

Structure of Nano-sized CeO2 Materials: Combined Scattering and Spectroscopic Investigations.

The structure of several nano-sized ceria, CeO2 , systems was investigated using neutron and X-ray diffraction and X-ray absorption spectroscopy. Whilst both diffraction and total pair distribution functions (PDFs) revealed that in all of the samples the occupancy of both Ce4+ and O2- are very close to the ideal stoichiometry, the analysis using Reverse Monte Carlo technique revealed significant disorder around oxygen atoms in the nano-sized ceria samples in comparison to the highly crystalline NIST standard. In addition, the analysis revealed that the main differences observed in the pair correlations from various X-ray and neutron diffraction techniques were attributable to the particle size of the CeO2 prepared by the reported three methods. Furthermore, detailed analysis of the Ce L3 - and K-edge EXAFS data support this finding; in particular the decrease in higher shell coordination numbers with respect to the NIST standard, is attributed to differences in particle size.

Metastable (Bi, M)2(Fe, Mn, Bi)2O(6+x) (M = Na or K) pyrochlores from hydrothermal synthesis.

The hydrothermal syntheses, structures, and magnetism of two new pyrochlore oxides of compositions (Na0.60Bi1.40)(Fe1.06Mn0.17Bi0.77)O6.87 and (K0.24Bi1.51)(Fe1.07Mn0.15Bi0.78)O6.86 are described. With preparation at 200 °C for 6 h in solutions of sodium or potassium hydroxide, the alkali metals introduced from these mineralizers are essential to the synthesis of the phases. The average long-range order of the pyrochlore structure, with space group Fd3̅m, was investigated and refined against X-ray and neutron diffraction data, and it was shown that disorder is present in both the metal and coordinating oxygen positions, along with metal-mixing across both the A and B sites of the structure. XANES analysis confirms the presence of Mn(4+), mixed valence Bi(3+) and Bi(5+), and Fe(3+), the last also verified by (57)Fe Mössbauer spectroscopy. Magnetic measurements show a lack of long-range magnetic ordering that is typical of geometrically frustrated pyrochlores. The observed glasslike interactions occur at low temperatures, with the onset temperature depending upon the magnitude of the applied external field. Variable temperature X-ray diffraction shows that these pyrochlores are metastable and collapse on heating at ca. 395 °C, which suggests that their formation by conventional solid-state synthesis would be impossible.

Total neutron scattering investigation of the structure of a cobalt gallium oxide spinel prepared by solvothermal oxidation of gallium metal.

A new solvothermal synthesis route to mixed-metal gallium oxides with the spinel structure has been developed for ternary oxides of ideal composition Ga(3-x)M(x)O(4-y) (M=Co, Zn, Ni). The structure of the novel cobalt gallate produced in this manner, Ga(1.767(8))Co(0.973(8))O(3.752(8)), has been determined from total neutron scattering to be a partially defective spinel with mixed-valent cobalt (approximately 25% Co(3+) and 75% Co(2+)) and with vacancies on approximately 6% of oxygen sites. Pair distribution function (PDF) analysis reveals significant local deviations from the average cubic structure, which are attributed to the conflicting coordination preferences of the Co(2+) (potential Jahn-Teller distortion) and Ga(3+) (Ga off-centring). Reverse Monte Carlo (RMC) modelling supports this conclusion since different metal-oxygen bond-distance distributions are found for the two cations in the refined configuration. An investigation of magnetic properties shows evidence of short-range magnetic order and spin-glass-like behaviour, consistent with the structural disorder of the material.

Structures of uncharacterised polymorphs of gallium oxide from total neutron diffraction.

A structural investigation is reported of polymorphs of Ga(2)O(3) that, despite much interest in their properties, have hitherto remained uncharacterised due to structural disorder. The most crystalline sample yet reported of γ-Ga(2)O(3) was prepared by solvothermal oxidation of gallium metal in ethanolamine. Structure refinement using the Rietveld method reveals γ-Ga(2)O(3) has a defect Fd3m spinel structure, while pair distribution function analysis shows that the short-range structure is better modelled with local F43m symmetry. In further solvothermal oxidation reactions a novel gallium oxyhydroxide, Ga(5)O(7)(OH), is formed, the thermal decomposition of which reveals a new, transient gallium oxide polymorph, κ-Ga(2)O(3), before transformation into ß-Ga(2)O(3). In contrast, the thermal decomposition of Ga(NO(3))(3)·9H(2)O first forms ε-Ga(2)O(3) and then ß-Ga(2)O(3). Examination of in situ thermodiffraction data shows that ε-Ga(2)O(3) is always contaminated with ß-Ga(2)O(3) and with this knowledge a model for its structure was deduced and refined--space group P6(3)mc with a ratio of tetrahedral/octahedral gallium of 2.2:1 in close-packed oxide layers. Importantly, thermodiffraction provides no evidence for the existence of the speculated bixbyite structured δ-Ga(2)O(3); at the early stages of thermal decomposition of Ga(NO(3))(3)·9H(2)O the first distinct phase formed is merely small particles of ε-Ga(2)O(3).

Yb3O(OH)6Cl·2H2O: an anion-exchangeable hydroxide with a cationic inorganic framework structure.

The first anion-exchangeable framework hydroxide, Yb(3)O(OH)(6)Cl·2H(2)O, has been synthesized hydrothermally. This material has a three-dimensional cationic ytterbium oxyhydroxide framework with one-dimensional channels running through the structure in which the chloride anions and water molecules are located. The framework is thermally stable below 200 °C and can be reversibly dehydrated and rehydrated with no loss of crystallinity. Additionally, it is able to undergo anion-exchange reactions with small ions such as carbonate, oxalate, and succinate with retention of the framework structure.