The Jahn-Teller distortion and cation ordering in the perovskite Sr2MnSbO6

The perovskite Sr₂MnSbO₆ has been synthesized using conventional ceramic techniques and structurally characterized using high-resolution powder X-ray and neutron diffraction. The structure is tetragonal in space group I4/m. The octahedra were found to feature Jahn-Teller (JT) distortion due to the presence of Mn³⁺, and this is identified as strongly contributing to the octahedral tilting. Evidence for B-site cation ordering is presented however there is extensive anti-site disorder. The disordering of the Mn³⁺ and Sb⁵⁺ cations is believed to be a result of the similar size of these two cations and the polarizability of the Sb⁵⁺ cation. The structure was found to undergo a transition to cubic symmetry at 521 °C with removal of the octahedral tilting leading to the quenching of the JT distortion. This phase transition was found to be continuous and tricritical in nature.     

Luminescent lanthanide selenites and tellurites decorated by MoO4 tetrahedra or MoO6 octahedra: Nd2MoSe2O10, Gd2MoSe3O12, La2MoTe3O12, and Nd2MoTe3O12

Solid state reactions of lanthanide oxide, MoO3 and SeO2 (or TeO2) at high temperature in an evacuated quartz tube lead to four new Ln-Mo-Se(Te)-O quaternary phases with four different types of structures, namely, Nd2MoSe2O10, Gd2MoSe3O12, La2MoTe3O12, and Nd2MoTe3O12. The structure of Nd2MoSe2O10 features a 3D architecture built by the intergrowth of the Nd-Se-O layers with the Nd-Mo-O layers. The structure of Gd2MoSe3O12 contains a 3D network of gadolinium selenite with the MoO6 octahedra occupying the cavities of the structure. The structure of La2MoTe3O12 features a 3D network of La2(Te3O8)2+ with the tunnels along the a axis occupied by the MoO4 tetrahedra. Nd2MoTe3O12 features a 2D layer built by the lanthanide ions interconnected by tellurite groups and ditellurite groups, with the MoO4 tetrahedra as the interlayer pendant groups. Room temperature and low temperature luminescent studies indicate that Nd2MoSe2O10 and Nd2MoTe3O12 exhibit strong luminescence in the near-IR region.     

Reactivity of aqua coordinated monoporphyrinate lanthanide complexes: synthetic, structural and photoluminescent studies of lanthanide porphyrinate dimers

Dimerization of monoporphyrinate lanthanide complexes [Yb(Por)(H(2)O)(3)]Cl, (Por = TTP(2-), TMPP(2-) and TPP(2-)) in the presence of sterically hindered tripodal ligand, zinc Schiff-base, dilute HCl, K(2)CO(3) solution, 4,4'-bipyridine (bipy), and basic 8-hydroxyquinaldine (HQ) solution was observed in CH(2)Cl(2) at room temperature. Six neutral dimeric lanthanide porphyrinate complexes, [Yb(TTP)(mu-OH)](2)(mu-THF) (1), [Yb(TMPP)(mu-OH)(H(2)O)](2) (2), [Yb(TPP)(mu-OH)(mu-H(2)O)](2) (4), [Yb(TMPP)(mu-Cl)(H(2)O)](2) (5), [Yb(TMPP)(mu-OH)](2)(THF) (6) and [Yb(TPP)](2)(mu-OH)(mu-Q) (7), were obtained. X-Ray diffraction studies showed that for the dimers, the two lanthanide ions were bridged by OH(-), Cl(-) or H(2)O. Photoluminescent studies showed that the porphyrinate dianion acted as an antenna, transferred its absorbed visible energy to the lanthanide ion and enabled the latter emitting in the near-infrared (NIR) region. In general, the NIR emission is more intense for the dimers than for the monomers, and the NIR emission intensity decreases as the number of O-H oscillators present in the molecule increases.     

Electrocatalytic proton reduction by phosphido-bridged diiron carbonyl compounds: distant relations to the H-cluster?

Intermediates formed during reduction of Fe(2)(mu-PPh(2))(2)(CO)(6) (1) in the presence of protons have been identified by spectroelectrochemical, continuous-flow, and interrupted-flow techniques. The mechanism for electrocatalytic proton reduction suggested by these observations yields digital simulation of the voltammetry in close agreement with measurements conducted in THF over a range of acid concentrations. The mechanism for electrocatalytic proton reduction involves initial formation of the dianion, 1(2-), which is doubly protonated prior to further reduction and dihydrogen elimination. The IR spectra of the singly and doubly protonated forms of 1(2-) indicate structures corresponding to [FeH(CO)(3)(mu-PPh(2))(2)Fe(CO)(3)](-) (1H-) and FeH(CO)(3)(mu-PPh(2))(2)FeH(CO)(3) (1H(2)). The thiolato and dithiolato analogues of 1 exhibit electrocatalytic proton reduction associated with the two-electron reduction step, and this implies that the corresponding two-electron reduced doubly protonated species is unstable with respect to dihydrogen elimination. The stability of 1H(2) is most likely to be due to the weak interactions between the iron centers of the flattened [2Fe2P] core. Whereas 1H(2) is stable in the absence of a reducing potential, 1H- rearranges rapidly to a product previously described as [Fe(2)(mu-PPh(2))(mu-CO)(PHPh(2))(CO)(5)](-) (1H-(W)). Another protonation product of 1(2-), previously formulated as [Fe(2)(mu-PPh(2))(2)(mu-CO)H(CO)(5)](-), has been reformulated as [Fe(2)(mu-PPh(2))(mu-CO)(CO)(6)](-) (2) on the basis of a range of spectroscopic measurements. Solution EXAFS measurements of 1, 1(2-), 1H-(W), and 2 are reported, and these yield model-independent Fe-Fe distances of 2.61 (1), 3.58 (1(2-)), 2.58 (1H-(W)), and 2.59 A (2). The presence of an Fe-Fe bond for both 1H-(W) and 2 is a key aspect of the proposed structures, and this strongly supports the deductions based on spectroscopic evidence. The fits of the solution EXAFS to different structural models give statistics in agreement with the proposed structures.     

Synthesis, structures and phase transitions in the double perovskites Sr2−xCaxCrNbO6

The synthesis and crystal structures of nine members of the rock-salt ordered double perovskites Sr_2−xCa_xCrNbO₆ is presented. The crystal structures of the end members of the series Sr₂CrNbO₆ and Ca₂CrNbO₆ were refined using powder neutron diffraction data and are cubic in and monoclinic in P2₁/n, respectively, in both cases there being considerable anti-site Cr-Nb mixing. Variable temperature and/or composition studies suggest a direct first-order P2₁/n to transition, a suggestion supported by selected area electron diffraction studies.     

Highly hydrophilic and nonionic poly(2-vinyloxazoline)-grafted silica: a novel organic phase for high-selectivity hydrophilic interaction chromatography

A new hydrophilic and nonionic poly(2-vinyloxazoline)-grafted silica (Sil-VOX _n ) phase was synthesized and applied for the separation of nucleosides and nucleobases in hydrophilic interaction chromatography (HILIC). Polymerization and immobilization onto silica were confirmed by using characterization techniques including ¹H NMR spectroscopy, elemental analysis, and diffuse reflectance infrared Fourier transform spectroscopy. The hydrophilicity or wettability of Sil-VOX _n was observed by measuring the contact angle (59.9°). The chromatographic results were compared with those obtained with a conventional HILIC silica column. The Sil-VOX _n phase showed much better separation of polar test analytes than the silica column, and the elution order was different. Differences in selectivity between these two columns indicate that the stationary phase cannot function merely as an inert support for a water layer into which the solutes are partitioned from the bulk mobile phase. To elucidate the interaction mechanism, the separation of dihydroxybenzene isomers was performed on both columns in normal-phase liquid chromatography. Sil-VOX _n was very sensitive to the dipole moments of the positional isomers of polycyclic aromatic compounds in normal-phase liquid chromatography. The interaction mechanism for Sil-VOX _n in HILIC separation is also described.

Improving prediction market forecasts by detecting and correcting possible over-reaction to price movements

We examine the impact of price trends on the accuracy of forecasts from prediction markets. In particular, we study an electronic betting exchange market and construct independent variables from market price (odds) time series from 6058 individual markets (a dataset consisting of over 8.4 million price points). Using a conditional logit model, we find that a systematic relationship exists between trends in odds and the accuracy of odds-implied event probabilities; the relationship is consistent with participants over-reacting to price movements. In particular, in different time segments of the market, increasing and decreasing odds lead, respectively, to under- and over-estimation of odds-implied probabilities. We develop a methodology to detect and correct the erroneous forecasts associated with these trends in odds in order to considerably improve the quality of forecasts generated in prediction markets.     

Structural investigation of mM Ni(II) complex isomers using transmission XAFS: the significance of model development

High‐accuracy transmission XAFS determined using the hybrid technique has been used to refine the geometries of bis(N‐n‐propyl‐salicylaldiminato) nickel(II) (n‐pr Ni) and bis(N‐i‐propyl‐salicylaldiminato) nickel(II) (i‐pr Ni) complexes which have approximately square planar and tetrahedral metal coordination. Multiple‐scattering formalisms embedded in FEFF were used for XAFS modelling of the complexes. Here it is shown that an IFEFFIT‐like package using weighting from experimental uncertainty converges to a well defined XAFS model. Structural refinement of (i‐pr Ni) was found to yield a distorted tetrahedral geometry providing an excellent fit, χ_r² = 2.94. The structure of (n‐pr Ni) is best modelled with a distorted square planar geometry, χ_r² = 3.27. This study demonstrates the insight that can be obtained from the propagation of uncertainty in XAFS analysis and the consequent confidence which can be obtained in hypothesis testing and in analysis of alternate structures ab initio. It also demonstrates the limitations of this (or any other) data set by defining the point at which signal becomes embedded in noise or amplified uncertainty, and hence can justify the use of a particular k‐range for one data set or a different range for another. It is demonstrated that, with careful attention to data collection, including the correction of systematic errors with statistical analysis of uncertainty (the hybrid method), it is possible to obtain reliable structural information from dilute solutions using transmission XAFS data.     

High‐accuracy X‐ray absorption spectra from mM solutions of nickel (II) complexes with multiple solutions using transmission XAS

A new approach is introduced for determining X‐ray absorption spectroscopy (XAS) spectra on absolute and relative scales using multiple solutions with different concentrations by the characterization and correction of experimental systematics. This hybrid technique is a development of standard X‐ray absorption fine structure (XAFS) along the lines of the high‐accuracy X‐ray extended range technique (XERT) but with applicability to solutions, dilute systems and cold cell environments. This methodology has been applied to determining absolute XAS of bis(N‐n‐propyl‐salicylaldiminato) nickel(II) and bis(N‐i‐propyl‐salicylaldiminato) nickel(II) complexes with square planar and tetrahedral structures in 15 mM and 1.5 mM dilute solutions. It is demonstrated that transmission XAS from dilute systems can provide excellent X‐ray absorption near‐edge structure (XANES) and XAFS spectra, and that transmission measurements can provide accurate measurement of subtle differences including coordination geometries. For the first time, (transmission) XAS of the isomers have been determined from low‐concentration solutions on an absolute scale with a 1–5% accuracy, and with relative precision of 0.1% to 0.2% in the active XANES and XAFS regions after inclusion of systematic corrections.     

Combustion-synthesized sodium manganese (cobalt) oxides as cathodes for sodium ion batteries

We report on the electrochemical properties of layered manganese oxides, with and without cobalt substituents, as cathodes in sodium ion batteries. We fabricated sub-micrometre-sized particles of Na_0.7MnO_2?+?z and Na_0.7Co_0.11Mn_0.89O_2?+?z via combustion synthesis. X-ray diffraction revealed the same layered hexagonal P2-type bronze structure with high crystallinity for both materials. Potentiostatic and galvanostatic charge/discharge cycles in the range 1.5–3.8 V vs. Na | Na⁺ were performed to identify potential-dependent phase transitions, capacity, and capacity retention. After charging to 3.8 V, both materials had an initial discharge capacity of 138 mA?h?g^?1 at a rate of 0.3 C. For the 20th cycle, those values reduced to 75 and 92 mA?h?g^?1 for Co-free and Co-doped samples, respectively. Our findings indicate that earlier works probably underestimated the potential of (doped) P2-type Na_0.7MnO_2?+?z as cathode material for sodium ion batteries in terms of capacity and cycle stability. Apart from doping, a simple optimization parameter seems to be the particle size of the active material.