Exploring the Limits of Fe-Rich Chemistries in Na-Based CaFe2O4-Type Postspinel Oxides.

Structural trends, physical properties, and electrochemical performances of the NaFexRu2-xO4 system have been investigated. Synthesis attempts using both conventional solid-state routes and high-pressure methods were explored for the compositional range 1.0 ≤ x ≤ 1.67. Based on Rietveld refinements against powder X-ray diffraction data, analyses of 57Fe Mössbauer spectroscopy data, and elemental analysis by electron microprobe, the existence of a confined compositional solid solution (1 ≤ x ≤ 1.3) adopting the CaFe2O4-type postspinel structure is demonstrated. This is contrasted with the NaFexTi2-xO4 system, for which no evidence of a solid solution is observed. However, for all explored synthetic routes of NaFexRu2-xO4 compositions, a trivalent iron oxidation state is maintained. Structural analysis and qualitative bond valence energy landscape models reveal that the progressive integration of iron into the postspinel framework results in narrowed sodium ion diffusion channels, restricting electrochemical deintercalation of sodium. Consequently, the CaFe2O4-type iron-rich compounds explored in this study demonstrate limited potential as positive electrode materials for sodium batteries. It is expected that this fundamental insight will help guide the exploration of alternative NaM2O4-based positive electrode materials with similar structure types.

Geometric considerations of the monoclinic-rutile structural transition in VO2.

The mechanism of the displacive phase transition in VO2 near the transition temperature is discussed in terms of a geometrical approach, combining simple calculations based on the Brown's band valence model and in situ X-ray diffraction experimental results. Considering that the structural origin is well linked to the electrostatic potential optimization as in a Peierls model, our geometrical calculations and experimental studies are in agreement and suggest that VO2 phase transition is the consequence of very short atomic shifts mainly associated to a decrease of the 2nd sphere coulombic interactions. Hence, at a given temperature, the allotropic form (monoclinic versus rutile form) offering the largest unit-cell volume is stabilized over the lower unit-cell volume allotropic, while the transition occurs at the intercept of the unit cell variation versus temperature of the two forms, which exhibit significantly different thermal expansion coefficients.

Structural transformations of the La2-xPrxNiO4+δ system probed by high-resolution synchrotron and neutron powder diffraction.

Compositions in the La2-xPrxNiO4+δ series offer an attractive balance of chemical stability and electrochemical performance for use as cathode materials in solid oxide fuel cells (SOFCs). A detailed crystallographic study of this system has been performed, combining both high resolution synchrotron and neutron powder diffraction data, in order to investigate structural details of the series as a function of composition, temperature and oxygen over-stoichiometry. The monoclinic structure (space group F2/m) of ambient temperature Pr-rich compositions for 1.0 < x ≤ 2.0 is discussed in terms of octahedra tilt arrangements and possible long-range structural modulations. In situ synchrotron diffraction experiments and TEM are employed to examine the role of temperature and interstitial oxygen on these structural distortions. With increasing La substitution, a region of mixed monoclinic and tetragonal phases is described for 0.5 ≤ x ≤ 1.0. La-Rich compositions are found to be single phase tetragonal (F4/mmm for 0 < x < 0.5) or orthorhombic (Fmmm for x = 0). Possible origins and electrochemical property consequences of the refined structural trends are considered.

Structure determination of Ba5AlF13 by coupling electron, synchrotron and neutron powder diffraction, solid-state NMR and ab initio calculations.

The room temperature structure of Ba5AlF13 has been investigated by coupling electron, synchrotron and neutron powder diffraction, solid-state high-resolution NMR (19F and 27Al) and first principles calculations. An initial structural model has been obtained from electron and synchrotron powder diffraction data, and its main features have been confirmed by one- and two-dimensional NMR measurements. However, DFT GIPAW calculations of the 19F isotropic shieldings revealed an inaccurate location of one fluorine site (F3, site 8a), which exhibited unusual long F-Ba distances. The atomic arrangement was reinvestigated using neutron powder diffraction data. Subsequent Fourier maps showed that this fluorine atom occupies a crystallographic site of lower symmetry (32e) with partial occupancy (25%). GIPAW computations of the NMR parameters validate the refined structural model, ruling out the presence of local static disorder and indicating that the partial occupancy of this F site reflects a local motional process. Visualisation of the dynamic process was then obtained from the Rietveld refinement of neutron diffraction data using an anharmonic description of the displacement parameters to account for the thermal motion of the mobile fluorine. The whole ensemble of powder diffraction and NMR data, coupled with first principles calculations, allowed drawing an accurate structural model of Ba5AlF13, including site-specific dynamical disorder in the fluorine sub-network.

Reaction of Pb(II) and Zn(II) with Ethyl Linoleate To Form Structured Hybrid Inorganic-Organic Complexes: A Model for Degradation in Historic Paint Films.

To investigate soap formation in drying oils in historic paints, the reaction between metal acetates (K+, Zn2+, Pb2+) and ethyl linoleate (EL) was studied using optical microscopy, X-ray powder diffraction, and electron microscopy. Pb(II) and Zn(II) react rapidly with EL to form highly structured, spherulitic, luminescent crystallites that aggregate. Evidence from Fourier transform infrared (FTIR) and scanning electron microscopy/energy dispersive X-ray analysis and high-resolution synchrotron powder X-ray diffraction indicates that these are organic-inorganic hybrid complexes or coordination polymers. FTIR absorbance peaks at ca. 1540 cm-1 for Pb(II) and ca. 1580 cm-1 for Zn(II) are consistent with the formation of carboxylate complexes. The complexes formed offer insight into the degradation processes observed in oil paint films, suggesting that soap formation is rapid when metal ions are solubilized and can occur with unsaturated fatty acids that are present in fresh oils. These complexes may account for the atypical luminescence observed in lead-containing cured oil paint films.

Nonstoichiometric control of tunnel-filling order, thermal expansion, and dielectric relaxation in tetragonal tungsten Bronzes Ba0.5-xTaO3-x.

Ordering of interpolated Ba(2+) chains and alternate Ta-O rows (TaO)(3+) in the pentagonal tunnels of tetragonal tungsten bronzes (TTB) is controlled by the nonstoichiometry in the highly nonstoichiometric Ba0.5-xTaO3-x system. In Ba0.22TaO2.72, the filling of Ba(2+) and (TaO)(3+) groups is partially ordered along the ab-plane of the simple TTB structure, resulting in a √2-type TTB superstructure (Pbmm), while in Ba0.175TaO2.675, the pentagonal tunnel filling is completely ordered along the b-axis of the simple TTB structure, leading to a triple TTB superstructure (P21212). Both superstructures show completely empty square tunnels favoring Ba(2+) conduction and feature unusual accommodation of Ta(5+) cations in the small triangular tunnels. In contrast with stoichiometric Ba6GaTa9O30, which shows linear thermal expansion of the cell parameters and monotonic decrease of permittivity with temperature within 100-800 K, these TTB superstructures and slightly nonstoichiometric simple TTB Ba0.4TaO2.9 display abnormally broad and frequency-dependent extrinsic dielectric relaxations in 10(3)-10(5) Hz above room temperature, a linear deviation of the c-axis thermal expansion around 600 K, and high dielectric permittivity ∼60-95 at 1 MHz at room temperature.

Localization of oxygen interstitials in CeSrGa3O(7+δ) melilite.

The solubility of Ce in the La(1-x)Ce(x)SrGa3O(7+δ) and La(1.54-x)Ce(x)Sr0.46Ga3O(7.27+δ) melilites was investigated, along with the thermal redox stability in air of these melilites and the conductivity variation associated with oxidization of Ce(3+) into Ce(4+). Under CO reducing atmosphere, the La in LaSrGa3O7 may be completely substituted by Ce to form the La(1-x)Ce(x)SrGa3O(7+δ) solid solution, which is stable in air to ∼600 °C when x ≥ 0.6. On the other side, the La(1.54-x)Ce(x)Sr0.46Ga3O(7.27+δ) compositions displayed much lower Ce solubility (x ≤ 0.1), irrespective of the synthesis atmosphere. In the as-made La(1-x)CexSrGa3O(7+δ), the conductivity increased with the cerium content, due to the enhanced electronic conduction arising from the 4f electrons in Ce(3+) cations. At 600 °C, CeSrGa3O(7+δ) showed a conductivity of ∼10(-4) S/cm in air, nearly 4 orders of magnitude higher than that of LaSrGa3O7. The oxidation of Ce(3+) into Ce(4+) in CeSrGa3O(7+δ) slightly reduced the conductivity, and the oxygen excess did not result in apparent increase of oxide ion conduction in CeSrGa3O(7+δ). The Ce doping in air also reduced the interstitial oxide ion conductivity of La1.54Sr0.46Ga3O7.27. Neutron powder diffraction study on CeSrGa3O7.39 composition revealed that the extra oxygen is incorporated in the four-linked GaO4 polyhedral environment, leading to distorted GaO5 trigonal bipyramid. The stabilization and low mobility of interstitial oxygen atoms in CeSrGa3O(7+δ), in contrast with those in La(1+x)Sr(1-x)Ga3O(7+0.5x), may be correlated with the cationic size contraction from the oxidation of Ce(3+) to Ce(4+). These results provide a new comprehensive understanding of the accommodation and conduction mechanism of the oxygen interstitials in the melilite structure.

How Lewis acidity of the cationic framework affects KNaNbOF(5) polymorphism.

The valence matching principle is used to explain the loss of inversion symmetry in the noncentrosymmetric (NCS) polymorph of KNaNbOF5 in comparison to its centrosymmetric (CS) polymorph. The [NbOF5](2-) anion has five contacts to both potassium and sodium in the NCS polymorph, whereas in the CS polymorph there are only four contacts to potassium and six contacts to sodium. The lower average Lewis acidity of the cationic framework in the NCS polymorph relative to the CS polymorph reflects the loss of inversion symmetry. This lower average Lewis acidity is achieved during hydrothermal synthesis with a potassium-rich solution when the K:Na ratio in the reaction is greater than 1:1, as the Lewis acidity of potassium is lower than that of sodium. The contrasting coordination environments are manifested in secondary distortions that weaken the primary Nb═O interaction and lengthen the Nb═O bond in the NCS polymorph. An unusual heat-induced phase transition from the CS to the NCS polymorph was studied with in situ powder X-ray diffraction. The transition to the NCS polymorph upon cooling occurs through an intermediate phase(s).

Structure and magnetism of the A site scandium perovskite (Sc0.94Mn0.06)Mn0.65Ni0.35O3 synthesized at high pressure.

Scandium perovskite (Sc0.94Mn0.06)Mn0.65Ni0.35O3, synthesized at high pressure and high temperature, has a triclinic structure (space group ) at room temperature and ambient pressure with a √2ap×√2ap×2ap structure with α≈90(°),ß≈89(°),γ≈90(°). Magnetic measurements show that the material displays Curie-Weiss behaviour above 50 K with C=2.11 emu K mol(-1) (µeff=4.11 µB per formula unit) and θ=-95.27 K. Bond valence sum analysis of the crystal structure shows that manganese is present in three different oxidation states (+2, +3, +4), with the +2 oxidation state on the A site resulting in a highly tilted perovskite structure (average tilt 21.2(°) compared with 15.7(°) calculated for LaCaMnNbO6), giving the formula (Sc3+(0.94)Mn2+(0.06))(Mn4+(0.41)Mn3+(0.09))(Mn3+(0.15)Ni2+(0.35))O3.

Defect structure, phase separation, and electrical properties of nonstoichiometric tetragonal tungsten bronze Ba(0.5-x)TaO(3-x).

New insight into the defect chemistry of the tetragonal tungsten bronze (TTB) Ba(0.5-x)TaO(3-x) is established here, which is shown to adapt to a continuous and extensive range of both cationic and anionic defect stoichiometries. The highly nonstoichiometric TTB Ba(0.5-x)TaO(3-x) (x = 0.25-0.325) compositions are stabilized via the interpolation of Ba(2+) cations and (TaO)(3+) groups into pentagonal tunnels, forming distinct Ba chains and alternate Ta-O rows in the pentagonal tunnels along the c axis. The slightly nonstoichiometric Ba(0.5-x)TaO(3-x) (x = 0-0.1) compositions incorporate framework oxygen and tunnel cation deficiencies in the TTB structure. These two mechanisms result in phase separation within the 0.1< x < 0.25 nonstoichiometric range, resulting in two closely related (TaO)(3+)-containing and (TaO)(3+)-free TTB phases. The highly nonstoichiometric (TaO)(3+)-containing phase exhibits Ba(2+) cationic migration. The incorporation of (TaO)(3+) units into the pentagonal tunnel and the local relaxation of the octahedral framework around the (TaO)(3+) units are revealed by diffraction data analysis and are shown to affect the transport and polarization properties of these compositions.

Crystal structures of spin-Jahn-Teller-ordered MgCr2O4 and ZnCr2O4.

Magnetic ordering in the geometrically frustrated magnetic oxide spinels MgCr2O4 and ZnCr2O4 is accompanied by a structural change that helps to relieve the frustration. Analysis of high-resolution synchrotron x-ray scattering reveals that the low-temperature structures are well described by a two-phase model of tetragonal I41/amd and orthorhombic Fddd symmetries. The Cr4 tetrahedra of the pyrochlore lattice are distorted at these low-temperatures, with the Fddd phase displaying larger distortions than the I41/amd phase. The spin-Jahn-Teller distortion is approximately one order of magnitude smaller than is observed in first-order Jahn-Teller spinels such as NiCr2O4 and CuCr2O4. In analogy with NiCr2O4 and CuCr2O4, we further suggest that the precise nature of magnetic ordering can itself provide a second driving force for structural change.

High-pressure synthesis, structure, and photoluminescence of a new KSbO3-type bismuth germanate Bi3Ge3O10.5.

A new Bi(3)Ge(3)O(10.5) compound has been synthesized under high pressure, P = 7 GPa, and 700 °C. Instead of the pyrochlore that is normally stabilized under high pressure, the Bi(3)Ge(3)O(10.5) crystallizes in a KSbO(3)-ype crystal structure. The crystal structure has been refined by the Rietveld method from synchrotron X-ray diffraction data. Moreover, we have also characterized the Bi(3)Ge(3)O(10.5) by X-ray photoelectron spectroscopy, photoluminescence, and specific heat.

Synthesis and structure resolution of RbLaF4.

The synthesis and structure resolution of RbLaF(4) are described. RbLaF(4) is synthesized by solid-state reaction between RbF and LaF(3) at 425 °C under a nonoxidizing atmosphere. Its crystal structure has been resolved by combining neutron and synchrotron powder diffraction data refinements (Pnma,a = 6.46281(2) Å, b = 3.86498(1) Å, c = 16.17629(4) Å, Z = 4). One-dimensional (87)Rb, (139)La, and (19)F MAS NMR spectra have been recorded and are in agreement with the proposed structural model. Assignment of the (19)F resonances is performed on the basis of both (19)F-(139)La J-coupling multiplet patterns observed in a heteronuclear DQ-filtered J-resolved spectrum and (19)F-(87)Rb HMQC MAS experiments. DFT calculations of both the (19)F isotropic chemical shieldings and the (87)Rb, (139)La electric field gradient tensors using the GIPAW and PAW methods implemented in the CASTEP code are in good agreement with the experimental values and support the proposed structural model. Finally, the conductivity of RbLaF(4) and luminescence properties of Eu-doped LaRbF(4) are investigated.

Structure resolution of Ba5Al3F19 and investigation of fluorine ion dynamics by synchrotron powder diffraction, variable-temperature solid-state NMR, and quantum computations.

The room temperature structure of Ba(5)Al(3)F(19) has been solved using electron microscopy and synchrotron powder diffraction data. One-dimensional (1D) (27)Al and ultrafast magic-angle-spinning (MAS) (19)F NMR spectra have been recorded and are in agreement with the proposed structural model for Ba(5)Al(3)F(19). The (19)F isotropic chemical shift and (27)Al quadrupolar parameters have been calculated using the CASTEP code from the experimental and density functional theory geometry-optimized structures. After optimization, the calculated NMR parameters of both the (19)F and (27)Al nuclei show improved consistency with the experimental values, demonstrating that the geometry optimization step is necessary to obtain more accurate and reliable structural data. This also enables a complete and unambiguous assignment of the (19)F MAS NMR spectrum of Ba(5)Al(3)F(19). Variable-temperature 1D MAS (19)F NMR experiments have been carried out, showing the occurrence of fluorine ion mobility. Complementary insights were obtained from both two-dimensional (2D) exchange and 2D double-quantum dipolar recoupling NMR experiments, and a detailed analysis of the anionic motion in Ba(5)Al(3)F(19) is proposed, including the distinction between reorientational processes and chemical exchange involving bond breaking and re-formation.

Phase separation and suppression of the structural and magnetic transitions in superconducting doped iron tellurides, Fe(1+x)Te(1-y)S(y).

Single crystal and powder samples of the series of iron chalcogenide superconductors with nominal composition, Fe((1.15))Te((1-)y)S(y), are found to form for 0 ≤ y ≤ 0.15. They crystallize in the tetragonal anti-PbO structure, which is composed of layers of edge-shared Fe(Te, S)(4) tetrahedra. For y = 0, Fe(1+x)Te (x ≈ 0.12(1)) is nonsuperconducting and undergoes a tetragonal (P4/nmm) to monoclinic (P2(1)/m) structural transition at ∼65 K, associated with the onset of commensurate antiferromagnetic order at q = (0.5 0 0.5). We show that on sulfur substitution, Fe(1+x)Te(1-y)S(y) becomes orthorhombic (Pmmn) at low temperature for 0 ≤ y ≤ 0.015, where the greatly suppressed magnetic scattering is now incommensurate at q = (0.5-δ 0 0.5) and possesses short ranged magnetic correlations that are well fitted with a two-dimensional Warren peak shape. At much higher concentrations of S (y ≥ 0.075), there is suppression of both the structural and magnetic transitions and a superconducting transition at 9 K is observed. Between these two composition regimes, there exists a region of phase separation (0.025 ≤ y ≤ 0.05), where the low temperature neutron diffraction data is best refined with a model containing both the tetragonal and orthorhombic phases. The increase in the amount of sulfur is found to be associated with a reduction in interstitial iron, x. Microprobe analysis of a single crystal of composition Fe((1.123(5)))Te((0.948(4)))S((0.052(4))) confirms the presence of compositional variation within the crystals, rationalizing the observed phase separation.

Frustration of magnetic and ferroelectric long-range order in Bi(2)Mn(4/3)Ni(2/3)O(6).

The slight incommensurate modulation of the structure of Bi(2)Mn(4/3)Ni(2/3)O(6) is sufficient to suppress the electrical polarization which arises in commensurate treatments of the structure, due to antiferroelectric coupling of local polar units of over 900 A(3). The incommensurate structure is produced by the competition between ferroelectric Bi lone pair-driven A site displacement, chemical order of Mn and Ni on the B site, and both charge and orbital order at these transition metals. The interplay between the frustrated polar Bi displacements and the frustrated spin order at the B site, induced by positional disorder, produces magnetodielectric coupling between the incommensurately modulated lattice and the spin-glass-like ground state with an unusual relationship between the magnetocapacitance and the applied field.

Structure and polarization in the high Tc ferroelectric Bi(Zn,Ti)O3-PbTiO3 solid solutions.

Theoretical ab initio and experimental methods are used to investigate the [Bi(Zn1/2Ti1/2)O3]x[PbTiO3]1-x solid solution. We find that hybridization between Zn 4s and 4p and O 2p orbitals allows the formation of short, covalent Zn-O bonds, enabling favorable coupling between A-site and B-site displacements. This leads to unusually large polarization, strong tetragonality, and an elevated ferroelectric to paraelectric phase transition temperature.