Synthesis, Structure, and Properties of 2O-BaPtO3, a Phase Derived from Hexagonal Perovskite

We have made the compound 2O-BaPtO3 by high-pressure, high-temperature synthesis, determined its structure, and tested its catalytic activity. Compounds of the same stoichiometry have been reported and tentatively identified as hexagonal perovskites, and although no structural model was ever established, 2O-BaPtO3 is clearly different and, to the best of our knowledge, unique. It features continuous chains of face-sharing PtO6 octahedra, like the well-known 2H hexagonal perovskite type, but with a staggered offset between the chains that breaks hexagonal symmetry and disrupts the close-packed array of A = Ba and X = O that is a defining characteristic of ABX3 perovskites. We investigated this structure and its stability vs the conventional 2H form using X-ray and neutron diffraction, X-ray absorption spectroscopy, and ab initio calculations. Catalytic testing of 2O-BaPtO3 showed that it is active for hydrogen evolution.

Competing Magnetic Interactions and the Role of Unpaired 4f Electrons in Oxygen-Deficient Perovskites Ba3RFe2O7.5 (R = Y, Dy)

Oxygen-deficient perovskite compounds with the general formula Ba₃RFe₂O_7.5 present a good opportunity to study competing magnetic interactions between Fe³⁺ 3d cations with and without the involvement of unpaired 4f electrons on R³⁺ cations. From analysis of neutron powder diffraction data, complemented by ab initio density functional theory calculations, we determined the magnetic ground states when R³⁺ = Y³⁺ (non-magnetic) and Dy³⁺ (4f⁹). They both adopt complex long-range ordered antiferromagnetic structures below T_N = 6.6 and 14.5 K, respectively, with the same magnetic space group C_a2/c (BNS #15.91). However, the dominant influence of f-electron magnetism is clear in temperature dependence and differences between the size of the ordered moments on the two crystallographically independent Fe sites, one of which is enhanced by R-O-Fe superexchange in the Dy compound, while the other is frustrated by it. The Dy compound also shows evidence for temperature- and field-dependent transitions with hysteresis, indicating a field-induced ferromagnetic component below T_N.

Tunable Polymer Nanoreactors from RAFT Polymerization-Induced Self-Assembly: Fabrication of Nanostructured Carbon-Coated Anatase as Battery Anode Materials with Variable Morphology and Porosity

We demonstrate a modular synthesis approach to yield mesoporous carbon-coated anatase (denoted as TiO₂/C) nanostructures. Combining polymerization-induced self-assembly (PISA) and reversible addition-fragmentation chain-transfer (RAFT) dispersion polymerization enabled the fabrication of uniform core-shell polymeric nanoreactors with tunable morphologies. The nanoreactors comprised of a poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) shell and a poly(benzyl methacrylate) (PBzMA) core. We selected worm-like and vesicular morphologies to guide the nanostructuring of a TiO₂ precursor, namely, titanium(IV) bis(ammonium lactato)dihydroxide (TALH). Subsequent carbonization yielded nanocrystalline anatase and simultaneously introduced a porous carbon framework, which also suppressed the crystal growth (∼5 nm crystallites). The as-prepared TiO₂/C materials comprised of a porous structure, with large specific surface areas (>85 m²/g) and various carbon contents (20-30 wt %). As anode components in lithium-ion batteries, our TiO₂/C nanomaterials improved the cycling stability, facilitated high overall capacities, and minimized the capacity loss compared to both their sans carbon and commercial anatase analogues.

Phase Formation and Degradation of Na2ZrO3 under CO2 Cycling Studied by Ex Situ and In Situ Diffraction

We positively identified and quantified the solid-state phases involved in the carbonation/decarbonation cycle of Na₂ZrO₃ when used for carbon capture. Previous work had only qualitatively inferred the phases present using diffraction pattern matching and thermogravimetric analysis. Here, we used the Rietveld-refinement method to analyze synchrotron X-ray and neutron powder diffraction data from samples treated ex situ. We then confirmed and extended our findings by in situ diffraction using a purpose-built gas-flow apparatus. This allowed us to resolve discrepancies in the earlier literature concerning which phases are present during the carbonation and regeneration processes. A key finding is the simultaneous presence of the monoclinic and tetragonal phases of ZrO₂ and that the "metastable" tetragonal phase is favored by smaller particles and can reincorporate into the bulk but the stable monoclinic phase does not. The result will help optimize the cycling of Na₂ZrO₃.

Hydrogen-Bonding 2D Coordination Polymer for Enzyme-Free Electrochemical Glucose Sensing

Regular detection of blood glucose levels is a critical indicator for effective diabetes management. Owing to the intrinsic highly sensitive nature of enzymes, the performance of enzymatic glucose sensors is...

Crystal and Magnetic Structures of Monoclinic FeOHSO4

The crystal and magnetic structures and properties of the monoclinic form of the iron hydroxysulfate FeOHSO₄ were investigated by magnetometry and neutron powder diffraction. The space group C2/c was confirmed, and the proton position was located close to that predicted by ab initio calculations. The collinear antiferromagnetic k(0,0,0) structure forming below the Néel temperature T_N ∼ 125 K is described by the C2'/c' (No. 15.89) magnetic space group, with the moments along the b axis. Overall, FeOHSO₄ is isostructural to FeSO₄F in terms of both the crystal and magnetic structures.

Synthesis of 12β-Methyl-18-nor-bile Acids

Decoupling the roles of the farnesoid X nuclear receptor and Takeda G-protein-coupled bile acid receptor 5 is essential for the development of novel bile acid therapeutics targeting metabolic and neurodegenerative diseases. Herein, we describe the synthesis of 12β-methyl-18-nor-bile acids which may serve as probes in the search for new bile acid analogues with clinical applicability. A Nametkin-type rearrangement was applied to protected cholic acid derivatives, giving rise to tetra-substituted Δ13,14- and Δ13,17-unsaturated 12β-methyl-18-nor-bile acid intermediates (24a and 25a). Subsequent catalytic hydrogenation and deprotection yielded 12β-methyl-18-nor-chenodeoxycholic acid (27a) and its 17-epi-epimer (28a) as the two major reaction products. Optimization of the synthetic sequence enabled a chromatography-free route to prepare these bile acids at a multi-gram scale. In addition, the first cis-C-D ring-junctured bile acid and a new 14(13 → 12)-abeo-bile acid are described. Furthermore, deuteration experiments were performed to provide mechanistic insights into the formation of the formal anti-hydrogenation product 12β-methyl-18-nor-chenodeoxycholic acid (27a).

Structure Evolution of Na2O2 from Room Temperature to 500 °C

Na₂O₂ is one of the possible discharge products from sodium-air batteries. Here, we report the evolution of the structure of Na₂O₂ from room temperature to 500 °C using variable-temperature neutron and synchrotron X-ray powder diffraction. A phase transition from α-Na₂O₂ to β-Na₂O₂ is observed in the neutron diffraction measurements above 400 °C, and the crystal structure of β-Na₂O₂ is determined from neutron diffraction data at 500 °C. α-Na₂O₂ adapts a hexagonal P62m (no. 189) structure, and β-Na₂O₂ adapts a tetragonal I4₁/acd (no. 142) structure. The thermal expansion coefficients of α-Na₂O₂ are a = 2.98(1) × 10^-5 K^-1, c = 2.89(1) × 10^-5 K^-1, and V = 8.96(1) × 10^-5 K^-1 up to 400 °C, and a ∼10% volume expansion occurs during the phase transition from α-Na₂O₂ to β-Na₂O₂ due to the realignment/rotation of O₂^2- groups. Both phases are electronic insulators according to DFT calculations with band gaps (both indirect) of 1.75 eV (α-Na₂O₂) and 2.56 eV (β-Na₂O₂). An impedance analysis from room temperature to 400 °C revealed a significant enhancement of the conductivity at T ≥ 275 °C. α-Na₂O₂ shows a higher conductivity (∼10 times at T ≤ 275 °C and ∼3 times at T > 275 °C) in O₂ compared to in Ar. We confirmed, by dielectric analysis, that this enhanced conductivity is dominated by ionic conduction.

Alkali Metal-Modified P2 NaxMnO2: Crystal Structure and Application in Sodium-Ion Batteries

Sodium-ion batteries (NIBs) are an emerging alternative to lithium-ion batteries because of the abundance of sodium resources and their potentially lower cost. Here we report the Na_0.7MnO₂ solid state synthesized at 1000 °C that shows two distinct phases; one adopts hexagonal P2-type P6₃/mmc space group symmetry, and the other adopts orthorhombic Pbma space group symmetry. The phase ratio of P2 to the orthorhombic phase is 55.0(5):45.0(4). A single-phase P2 structure is found to form at 1000 °C after modification with alkali metals Rb and Cs, while the K-modified form produces an additional minor impurity. The modification is the addition of the alkali elements during synthesis that do not appear to be doped into the crystal structure. As a cathode for NIBs, parent Na_0.7MnO₂ shows a second charge/discharge capacity of 143/134 mAh g^-1, K-modified Na_0.7MnO₂ a capacity of 184/178 mAh g^-1, Rb-modified Na_0.9MnO₂ a capacity of 159/150 mAh g^-1, and Cs-modified Na_0.7MnO₂ a capacity of 171/163 mAh g^-1 between 1.5 and 4.2 V at a current density of 15 mA g^-1. The parent Na_0.7MnO₂ is compared with alkali metal (K, Rb, and Cs)-modified Na_xMnO₂ in terms of surface morphology using scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, scanning electron microscopy, ²³Na solid-state nuclear magnetic resonance, and X-ray photoelectron spectroscopy and in terms of electrochemical performance and structural electrochemical evolution using in situ or operando synchrotron X-ray diffraction.

Interfacial Reactions between Lithium and Grain Boundaries from Anatase TiO2-TUD-1 Electrodes in Lithium-Ion Batteries with Enhanced Capacity Retention

The synergistic incorporation of anatase TiO₂ domains into siliceous TUD-1 was optimized in this work and the resulting sample was implemented as the electrode in lithium-ion batteries. Triethanolamine was used as both the templating and complexing agent, the Si/Ti ratio was controlled, and the formation of Ti-O-Si bridges was optimized, as revealed through Fourier transform infrared spectroscopy, with the porous character of the materials being confirmed with N₂ adsorption-desorption isotherms. The controlled formation of Ti-O-Si bridges resulted in attractive specific charge capacities, high rate capability, and a good retention of capacity. The electrochemical performance of the composite material clearly demonstrates a synergistic effect between pure TiO₂ in its anatase form and the otherwise inactive siliceous TUD-1 matrix. Specific capacities of 300 mA h g^-1 with a retention of 94% were obtained at a current density of 0.1 A g^-1 over 100 cycles. This work showcases the use of bifunctional templating agents in the improvement of the performance and the long-term cyclability of composite electrodes, which can be potentially applied in future synthesis of energy materials.

Mechanistic Insight into Energy-Transfer Dynamics and Color Tunability of Na4 CaSi3 O9 :Tb3+ ,Eu3+ for Warm White LEDs

In this work, a latent energy-transfer process in traditional Eu³⁺ ,Tb³⁺ -doped phosphors is proposed and a new class of Eu³⁺ ,Tb³⁺ -doped Na₄ CaSi₃ O₉ (NCSO) phosphors is presented which is enabled by luminescence decay dynamics that optimize the electron-transfer energy process. Relative to other Eu³⁺ ,Tb³⁺ -doped phosphors, the as-synthesized Eu³⁺ ,Tb³⁺ -doped NCSO phosphors show improved large-scale tunable emission color from green to red upon UV excitation, controlled by the Tb³⁺ /Eu³⁺ doping ratio. Detailed spectroscopic measurements in the vacuum ultraviolet (VUV)/UV/Vis region were used to determine the Eu³⁺ -O^2- charge-transfer energy, 4f-5d transition energies, and the energies of 4f excited multiplets of Eu³⁺ and Tb³⁺ with different 4f^N electronic configurations. The Tb³⁺ →Eu³⁺ energy-transfer pathway in the co-doped sample was systematically investigated, by employing luminescence decay dynamics analysis to elucidate the relevant energy-transfer mechanism in combination with the appropriate model simulation. To demonstrate their application potential, a prototype white-light-emitting diode (WLED) device was successfully fabricated by using the yellow luminescence NCSO:0.03Tb³⁺ , 0.05Eu³⁺ phosphor with high thermal stability and a BaMgAl₁₀ O₁₇ :Eu²⁺ phosphor in combination with a near-UV chip. These findings open up a new avenue to realize and develop multifunctional high-performance phosphors by manipulating the energy-transfer process for practical applications.

Order, Disorder, and Dynamics in Brownmillerite Sr2Fe2O5

The room-temperature structure of brownmillerite-type Sr₂Fe₂O₅ remains controversial, despite numerous published crystallographic studies utilizing X-ray, neutron, and electron diffraction data collected on single-crystalline and powder samples. The main disagreements concern the ordering of twisted FeO₄ tetrahedral chains within and between the layers stacked along the b axis, and the impact of this ordering on oxide-ionic conductivity. Here, we present new data along with a reinterpretation of previously published diffraction images, including the reassignment of satellite reflections, which harmonize the results of past studies in a unified description of tetrahedral chain ordering in Sr₂Fe₂O₅ at length scales relevant to X-ray and neutron diffraction. Implications for the prevailing model of oxide ion transport in this material are also discussed.

In-situ synthesis of Ni-Co-S nanoparticles embedded in novel carbon bowknots and flowers with pseudocapacitance-boosted lithium ion storage

We design a facile approach to prepare a bimetallic transition-metal-sulphide-based 3D hierarchically-ordered porous electrode based on bimetallic metal-organic frameworks (Ni-Co-MOFs) by using confinement growth and in-situ sulphurisation techniques. In the novel resulting architectures, Ni-Co-S nanoparticles are confined in bowknot-like and flower-like carbon networks and are mechanically isolated but electronically well-connected, where the carbon networks with a honeycomb-like feature facilitate electron transfer with uninterrupted conductive channels from all sides. Moreover, these hierarchically-ordered porous structures together with internal voids can accommodate the volume expansion of the embedded Ni-Co-S nanoparticles. The pseudocapacitive behaviours displayed in the NCS@CBs and NCS@CFs occupied a significant portion in the redox processes. Because of these merits, both the as-built bowknot and flower networks show excellent electrochemical properties for lithium storage with superior rate capability and robust cycling stability (994 mAh g^-1 for NCS@CBs and 888 mAh g^-1 for NCS@CFs after 200 cycles). This unique 3D hierarchically-ordered structural design is believed to hold great potential applications in propagable preparation of carbon networks teamed up with sulphide nanocrystals for high energy storage.

Crystal and Magnetic Structures of Melilite-Type Ba2MnSi2O7

Melilite-type Ba₂MnSi₂O₇ was synthesized by a standard powder solid-state reaction route, and its magnetic properties were studied at low temperature. The magnetic structure was found to be C-type pointing along the c axis from neutron powder diffraction, which is different from the G-type ordering previously reported in all other 2-2-4-2 melilites with manganese as the B'-site transition metal. Ab initio (density functional theory) and magnetic dipole-dipole calculations were used to understand the magnetic structure by determining the spin supersuperexchange parameters as well as the relative influence of spin-orbit coupling and the magnetic dipole-dipole interactions.

Squeezing electrons out of 6s2 lone-pairs in perovskite-type oxides

Having identified a set of conditions that predispose a solid-state ionic compound to a pressure-induced valence transition, we investigated a series of Bi(iii) perovskite oxides. We found such a transition below 10 GPa in every case, including one that we synthesised for the first time (double perovskite-type Ba2BiOsO6).

A new tri-nuclear Cu-carbonate cluster utilizing CO2 as a C1-building block - reactive intermediates, a probable mechanism, and EPR and magnetic studies

This work demonstrates that simple copper-bipyridine compounds and atmospheric CO2 react to produce useful/complex materials under appropriate conditions. Starting from a distorted square planar copper(ii) complex, [(tbubpy)CuCl2](tbubpy = 4-tert-butyl-2-(4-tert-butylpyridin-2-yl)pyridine), an air-sensitive, copper(i) complex, [(tbubpy)2CuI][BF4], which exhibits a distorted tetrahedral geometry, was synthesized and characterized. Reactions of [(tbubpy)2CuI][BF4] with CO2 inside a sealed tube and with air were carried out over a week and three weeks, respectively. A new tricopper(ii)-carbonato cluster, [{(tbubpy)2Cu}3(μ-CO3)][PF6]4, was isolated with three distorted octahedral copper(ii) centres bound by a carbonate-bridge formed from atmospheric CO2. NMR and UV-Vis spectroscopic analyses coupled with previous reports point to a multi-step process in the formation of a trinuclear CuII-carbonato cluster that includes the probable involvement of μ-hydroxo-bridged dicopper(ii) type intermediates.

Investigation of K modified P2 Na0.7Mn0.8Mg0.2O2 as a cathode material for sodium-ion batteries

We demonstrate that K addition to P2-Na_0.7Mn_0.8Mg_0.2O₂ results in an inhomogeneous distribution and leads to inferior electrochemical performance relative to the parent.

Experimental and computational study of the magnetic properties of ZrMn2-xCoxGe4O12

Polycrystalline samples in the solid solution ZrMn_2-xCo_xGe₄O₁₂ (x = 0.0, 0.5, 1.0, 1.5 and 2.0) have been prepared using the ceramic method and characterised by a combination of magnetometry, X-ray diffraction and neutron diffraction. They all adopt the space group P4/nbm with a ∼ 9.60, c ∼ 4.82 Å and show long-range magnetic order with transition temperatures, T_C, in the range 2 ≤ T_C/K ≤ 10. The underlying magnetic structure is the same in each case but the ordered spins lie along [001] when x = 0.0 and in the (001) plane for all other compositions. In all cases the magnetically-ordered phase is a weak ferromagnet although the magnitude of the spontaneous magnetisation and the strength of the coercive field are composition-dependent. The magnetic structure can be rationalized by considering the strengths of the interactions along the distinct M-O-Ge-O-M superexchange pathways in the crystal and the observed magnetic structure is entirely consistent with the predictions of ab initio calculations.

Magnetic structure and properties of centrosymmetric twisted-melilite K2CoP2O7

Twisted-melilite dipotassium cobalt pyrophosphate (K₂CoP₂O₇, P4₂/mnm, #136), originally reported by Gabelica-Robert (1981), was synthesized in powder form by a standard solid-state reaction route. The magnetic properties of the material were studied by magnetometry and its magnetic structure determined using neutron powder diffraction for the first time. Below T_N = 11 K, the material adopts a G-type antiferromagnetic structure with moments aligned in the ab-plane (magnetic space group Pn'nm, #58.3.473). Ab initio calculations were performed to examine the isotropic magnetic spin exchange parameters as well as the preferred direction of magnetic moments due to spin-orbit coupling. The relationship between crystal structure geometry and the strength of the magnetic interactions was examined and compared to those of melilite-type Sr₂CoGe₂O₇.

Phononic Structure Engineering: the Realization of Einstein Rattling in Calcium Cobaltate for the Suppression of Thermal Conductivity

Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is fundamentally different from skutterudites, clathrates as well as Na analogue. We have also tentatively studied the phonon band structure of this material by DFT lattice dynamics simulation, showing the relative contribution to phonons in the distinct layers of Ca3Co4O9.

Synthesis, structure and geometrically frustrated magnetism of the layered oxide-stannide compounds Fe(Fe3-xMnx)Si2Sn7O16

Fe4Si2Sn7O16 has a unique crystal structure that contains alternating layers of Fe(2+) ions octahedrally coordinated by O (oxide layer) and Sn (stannide layer), bridged by SiO4 tetrahedra. The formula can be written as FeFe3Si2Sn7O16 to emphasise the distinction between the layers. Here, we report the changes in structure and properties as iron is selectively replaced by manganese in the oxide layer. Solid-state synthesis was used to produce polycrystalline samples of Fe(Fe3-xMnx)Si2Sn7O16 for x≤ 2.55, the structures of which were characterised using high-resolution synchrotron X-ray and neutron powder diffraction. Single-crystal samples were also grown at x = 0.35, 0.95, 2.60 and characterised by single crystal X-ray diffraction. We show that manganese is doped exclusively into the oxide layer, and that this layer contains exclusively magnetically active high-spin M(2+) transition metal cations; while the stannide layer only accommodates non-magnetic low-spin Fe(2+). All samples show clear evidence of geometrically frustrated magnetism, which we associate with the fact that the topology of the high-spin M(2+) ions in the oxide layer describes a perfect kagomé lattice. Despite this frustration, the x = 0 and x = 2.55 samples undergo long-range antiferromagnetic ordering transitions at 3.0 K and 2.5 K, respectively.

Ionothermal Synthesis of High-Voltage Alluaudite Na2+2xFe2-x(SO4)3 Sodium Insertion Compound: Structural, Electronic, and Magnetic Insights

Exploring future cathode materials for sodium-ion batteries, alluaudite class of Na2Fe(II)2(SO4)3 has been recently unveiled as a 3.8 V positive insertion candidate (Barpanda et al. Nat. Commun. 2014, 5, 4358). It forms an Fe-based polyanionic compound delivering the highest Fe-redox potential along with excellent rate kinetics and reversibility. However, like all known SO4-based insertion materials, its synthesis is cumbersome that warrants careful processing avoiding any aqueous exposure. Here, an alternate low temperature ionothermal synthesis has been described to produce the alluaudite Na2+2xFe(II)2-x(SO4)3. It marks the first demonstration of solvothermal synthesis of alluaudite Na2+2xM(II)2-x(SO4)3 (M = 3d metals) family of cathodes. Unlike classical solid-state route, this solvothermal route favors sustainable synthesis of homogeneous nanostructured alluaudite products at only 300 °C, the lowest temperature value until date. The current work reports the synthetic aspects of pristine and modified ionothermal synthesis of Na2+2xFe(II)2-x(SO4)3 having tunable size (300 nm ∼5 μm) and morphology. It shows antiferromagnetic ordering below 12 K. A reversible capacity in excess of 80 mAh/g was obtained with good rate kinetics and cycling stability over 50 cycles. Using a synergistic approach combining experimental and ab initio DFT analysis, the structural, magnetic, electronic, and electrochemical properties and the structural limitation to extract full capacity have been described.

A New n = 4 Layered Ruddlesden-Popper Phase K(2.5)Bi(2.5)Ti4O13 Showing Stoichiometric Hydration

A new bismuth-containing layered perovskite of the Ruddlesden-Popper type, K(2.5)Bi(2.5)Ti4O13, has been prepared by solid-state synthesis. It has been shown to hydrate to form stoichiometric K(2.5)Bi(2.5)Ti4O13·H2O. Diffraction data show that the structure consists of a quadruple-stacked (n = 4) perovskite layer, with potassium ions occupying the rock salt layer and its next-nearest A site. The hydrated sample was shown to remove the offset between stacked perovskite layers relative to the dehydrated sample. Computational methods show that the hydrated phase consists of intact H2O molecules in a vertical "pillared" arrangement bridging across the interlayer space. Rotations of H2O molecules about the c axis were evident in molecular dynamic calculations, which increased in rotation angle with increasing temperature. In situ diffraction data for the dehydrated phase point to a broad structural phase transition from orthorhombic to tetragonal at ∼600 °C. The relative bismuth-rich composition in the perovskite block results in a higher transition temperature compared to related perovskite structures. Water makes a significant contribution to the dielectric constant, which disappears after dehydration.

Type II Bi1 - xWxO1.5 + 1.5x: a (3 + 3)-dimensional commensurate modulation that stabilizes the fast-ion conducting delta phase of bismuth oxide

The Type II phase in the Bi1 - xWxO1.5 + 1.5x system is shown to have a (3 + 3)-dimensional modulated δ-Bi2O3-related structure, in which the modulation vector ℇ `locks in' to a commensurate value of 1/3. The structure was refined in a 3 × 3 × 3 supercell against single-crystal Laue neutron diffraction data. Ab initio calculations were used to test and optimize the local structure of the oxygen sublattice around a single mixed Bi/W site. The underlying crystal chemistry was shown to be essentially the same as for the recently refined (3 + 3)-dimensional modulated structure of Type II Bi1 - xNbxO1.5 + x (Ling et al., 2013), based on a transition from fluorite-type to pyrochlore-type via the appearance of W4O18 `tetrahedra of octahedra' and chains of corner-sharing WO6 octahedra along 〈110〉F directions. The full range of occupancies on this mixed Bi/W site give a hypothetical solid-solution range bounded by Bi23W4O46.5 (x = 0.148) and Bi22W5O48 (x = 0.185), consistent with previous reports and with our own synthetic and analytical results.

Energy and temperature dependence of rigid unit modes in AlPO₄-5

For structures that can be treated as networks of rigid, corner-connected polyhedra, the dominant distortion modes can be described by so-called rigid unit modes that are close to zero frequency. This type of behaviour is common in zeolitic/zeotypic materials such as the AlPO4 family of compounds and has been suggested by some authors to play a significant role in molecular diffusion within the pores of such compounds. We explore the energy and temperature dependence of these modes in AlPO4-5 using inelastic neutron scattering and heat capacity measurements. Ab initio based computational modelling is also used to assign the observed dynamic behaviour to rigid unit modes. We observe that these rigid unit modes persist down to very low temperatures and show no signs of freezing out.

Long- and short-range structure studies of KBT-KBZ solid-solutions using synchrotron radiation

The relaxor ferroelectric K(0.5)Bi(0.5)TiO(3) has been synthesised in a solid-solution series with K(0.5)Bi(0.5)ZrO(3), as K(0.5)Bi(0.5)Ti1-xZrxO(3). High-resolution synchrotron X-ray powder diffraction and X-ray absorption near edge structure spectroscopy were used to characterise the long-range average and local structural behaviour. Rietveld refinements against diffraction data show that a pseudocubic tetragonal region exists across the whole solid-solution series, with truly cubic symmetry only observed at x = 1 (pure KBZ). Variable-temperature diffraction data for x = 0 (pure KBT) showed a broad ferroelectric transition from tetragonal to cubic symmetry at approximately 683 K with a coexistence of both phases close to that temperature, accompanied by a marked volume contraction. Ti K-edge data showed that Zr doping has a minimal effect on Ti off-centering, and revealed no evidence for local clustering. Metal L-edges showed that Ti(4+) cations remain off-centered with increasing Zr content, while Zr(4+) cations approach a higher-symmetry coordination environment, most likely due to the increased size of the Zr atoms. Although there is a minimal effect on actual Ti-offsets, an effective dilution of these environments by Zr doping leads to a reduction in polar domains and a diminished ferroelectric response.

Neutron Laue diffraction study of the complex low-temperature magnetic behaviour of brownmillerite-type Ca2Fe2O5

The atomic and magnetic structure of brownmillerite Ca2Fe2O5has been refined against single-crystal neutron Laue diffraction data collected at 300, 100 and 10 K under zero-field and low-magnetic field (35 Oe = 35 × 103/4π A m−1) conditions. Ca2Fe2O5is a canted G-type antiferromagnet withPcm′n′ symmetry, the magnetic moments on Fe being directed approximately along the crystallographiccaxis at room temperature. The refinement results show clearly that this magnetic structure persists down toT= 10 K, despite a previous suggestion that an anomalous magnetic susceptibility enhancement observed in Ca2Fe2O5single crystals between 40 and 140 K might signify a reorientation of the antiferromagnetic easy axis fromctoabelow 40 K. Alternative explanations for this susceptibility anomaly are considered in terms of the evidence for partial or short-range loss of order in the anomalous regime, possibly due to the presence of multiple competing sublattice interactions.

An unconventional method for measuring the Tc L3-edge of technetium compounds

Tc L3-edge XANES spectra have been collected on powder samples of SrTcO3 (octahedral Tc(4+)) and NH4TcO4 (tetrahedral Tc(7+)) immobilized in an epoxy resin. Features in the Tc L3-edge XANES spectra are compared with the pre-edge feature of the Tc K-edge as well as other 4d transition metal L3-edges. Evidence of crystal field splitting is obvious in the Tc L3-edge, which is sensitive to the coordination number and oxidation state of the Tc cation. The Tc L3 absorption edge energy difference between SrTcO3 (Tc(4+)) and NH4TcO4 (Tc(7+)) shows that the energy shift at the Tc L3-edge is an effective tool for studying changes in the oxidation states of technetium compounds. The Tc L3-edge spectra are compared with those obtained from Mo and Ru oxide standards with various oxidation states and coordination environments. Most importantly, fitting the Tc L3-edge to component peaks can provide direct evidence of crystal field splitting that cannot be obtained from the Tc K-edge.

Tuning the giant magnetoelastic transition in Ba3BiIr2O9 and Ba3BiRu2O9

We have experimentally investigated the effects of pressure on the magnetoelastic transitions associated with the opening of spin-gaps in Ba3BiIr2O9 and Ba3BiRu2O9. For both compounds, reducing the unit cell volume by either external physical and internal chemical pressure was found to reduce the temperature T(*) of the transition and, to a lesser extent, the magnitude of the associated negative thermal volume expansion. The results yield the latent heat associated with the transitions, -3.34(3) × 10(2) J mol(-1) for Ba3BiIr2O9 and -7.1(5) × 10(2) J mol(-1) for Ba3BiRu2O9. The transition in Ba3BiRu2O9 is significantly more robust than in Ba3BiIr2O9, requiring an order of magnitude higher pressures to achieve the same reduction in T(*). The differing responses of the two compounds points to differences between the 4d and 5d metals and hence to the importance of spin-orbit coupling, which is expected to be much stronger in the Ir compound.

Soft ferromagnetism in mixed valence Sr1−xLaxTi0.5Mn0.5O3 perovskites

The structural, magnetic and electrical properties of the mixed Ti–Mn oxides Sr_1−xLa_xTi_0.5Mn_0.5O₃ (0 ≤ x ≤ 0.5) are reported.

Studying the effects of Zr-doping in (Bi0.5Na0.5)TiO3via diffraction and spectroscopy

The phase transformation in the (Bi_0.5Na_0.5)Ti_1−xZr_xO₃ are characterized using XRD and XANES.