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 ß-Na2O2 is observed in the neutron diffraction measurements above 400 °C, and the crystal structure of ß-Na2O2 is determined from neutron diffraction data at 500 °C. α-Na2O2 adapts a hexagonal P62m (no. 189) structure, and ß-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 ß-Na2O2 due to the realignment/rotation of O22- 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 (ß-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.

Structural and Magnetic Studies of ABO4-Type Ruthenium and Osmium Oxides.

Oxides of the form ABO4 with A = K, Rb, Cs and B = Ru and Os have been synthesized and characterized by diffraction and magnetic techniques. For A = K the oxides adopted the tetragonal (I41/a) scheelite structure. RbOsO4, which crystallizes as a scheelite at room temperature, underwent a continuous phase transition to I41/amd near 550 K. RbRuO4 and CsOsO4 were found to crystallize in the orthorhombic (Pnma) pseudoscheelite structure, and both displayed discontinuous phase transitions to I41/a at high temperatures. CsOsO4 was determined to undergo a phase transition to a P21/c structure below 140 K. CsRuO4 crystallizes with a baryte-type structure at room temperature. Upon heating CsRuO4 a first order phase transition to the scheelite structure in I41/a is observed at 400 K. A continuous phase transition is observed to P212121 below 140 K. DC magnetic susceptibility data is consistent with long-range antiferromagnetic ordering at low temperatures for all compounds except for CsOsO4, which is paramagnetic to 2 K. The effective magnetic moments are in agreement with the spin only values for an S = 1/2 quantum magnet. Effective magnetic moments calculated for Os compounds were lower than their Ru counterparts, reflective of an enhanced spin orbit coupling effect. A magnetic structure is proposed for RbRuO4 consisting of predominately antiferromagnetic (AFM) ordering along the 001 direction, with canting of spins in the 100 plane. A small ordered magnetic moment of 0.77 µB was determined.

Controlling Oxygen Defect Formation and Its Effect on Reversible Symmetry Lowering and Disorder-to-Order Phase Transformations in Nonstoichiometric Ternary Uranium Oxides.

In situ synchrotron powder X-ray diffraction measurements have demonstrated that the isostructural AUO4- x ( A = alkaline earth metal cation) oxides CaUO4- x and α-Sr0.4Ca0.6UO4- x undergo a reversible phase transformation under reducing conditions at high temperatures associated with the ordering of in-plane oxygen vacancies resulting in the lowering of symmetry. When rhombohedral (space group R3̅ m) CaUO4- x and α-Sr0.4Ca0.6UO4- x are heated to 450 and 400 °C, respectively, in a hydrogen atmosphere, they undergo a first-order phase transformation to a single phase structure which can be refined against a triclinic model in space group P1̅, δ-CaUO4- x and δ-Sr0.4Ca0.6UO4- x, where the oxygen vacancies are disordered initially. Continued heating results in the appearance of superlattice reflections, indicating the ordering of in-plane oxygen vacancies. Cooling ordered δ-CaUO4- x and δ-Sr0.4Ca0.6UO4- x to near room temperature results in the reformation of the disordered rhombohedral phases. Essential to the transformation is the generation of a critical amount of oxygen vacancies. Once these are formed, the transformation can be accessed continuously through thermal cycling, showing that the transformations are purely thermodynamic in origin. Stoichiometric structures of both oxides can be recovered by heating oxygen deficient CaUO4- x and α-Sr0.4Ca0.6UO4- x under pure oxygen to high temperatures. When heated in air, the amount of oxygen vacancy defects that form in CaUO4- x and α-Sr0.4Ca0.6UO4- x are found to correlate with the A site composition. The inclusion of the larger Sr2+ cation on the A site reduces defect-defect interactions, which increases the amount of defects that can form and lowers their formation temperature. The relative difference in the amount of defects that form can be understood on the basis of oxygen vacancy and U5+ disordering as shown by both ab initio calculations and estimated oxygen vacancy formation energies based on thermodynamic considerations. This difference in defect-defect interactions consequently introduces variations in the long-range ordered anionic lattice of the δ phases despite the isostructural relationship of the α structures of CaUO4- x and Sr0.4Ca0.6UO4- x. These results are discussed with respect to the influence the A site cation has upon anion defect formation and ordering and are also compared to δ-SrUO4- x, the only other material known to be able to undergo a reversible symmetry lowering and disorder-to-order transformation with increasing temperature.

Unexpected Crystallographic Phase Transformation in Nonstoichiometric SrUO4- x: Reversible Oxygen Defect Ordering and Symmetry Lowering with Increasing Temperature.

In situ synchrotron powder X-ray diffraction measurements have demonstrated that SrUO4 undergoes a reversible phase transformation under reducing conditions at high temperatures, associated with the ordering of oxygen defects resulting in a lowering of crystallographic symmetry. When substoichiometric rhombohedral α-SrUO4- x, in space group R3̅ m with disordered in-plane oxygen defects, is heated above 200 °C in a hydrogen atmosphere it undergoes a first order phase transformation to a (disordered) triclinic polymorph, δ-SrUO4- x, in space group P1̅. Continued heating to above 450 °C results in the appearance of superlattice reflections, due to oxygen-vacancy ordering forming an ordered structure δ-SrUO4- x. Cooling δ-SrUO4- x toward room temperature results in the reformation of the rhombohedral phase α-SrUO4- x with disordered defects, confirming the reversibility of the transformation. This suggests that the transformation, resulting from oxygen vacancy ordering, is not a consequence of sample reduction or decomposition, but rather represents a change in the energetics of the system. A strong reducing atmosphere is required to generate a critical amount of oxygen defects in α-SrUO4- x to enable the transformation to δ-SrUO4- x but once formed the transformation between these two phases can be induced by thermal cycling. The structure of δ-SrUO4- x at 1000 °C was determined using symmetry representation analysis, with the additional reflections indexed to a commensurate distortion vector k = ⟨1/4 1/4 3/4⟩. The ordered 2D layered triclinic structure of δ-SrUO4- x can be considered a structural distortion of the disordered 2D layered rhombohedral α-SrUO4- x structure through the preferential rearrangement of the in-plane oxygen vacancies. Ab initio calculations using density functional theory with self-consistently derived Hubbard U parameter support the assigned ordered defect superstructure model. Entropy changes associated with the temperature dependent short-range ordering of the reduced U species are believed to be important and these are discussed with respect to the results of the ab initio calculations.

Pressure-induced anomalies and structural instability in compressed ß-Sb2O3.

Here, we report a high-pressure study of orthorhombic structured ß-Sb2O3 (valentinite) by the combination of synchrotron in situ X-ray diffraction and first-principles theoretical calculations at pressures up to 40.5 GPa. Our results reveal that the metastable ß-Sb2O3 undergoes an isostructural phase transition at high pressure, yielding a distorted ß phase at 7-15 GPa through symmetry breaking and structural distortion as inferred from our XRD analyses and DFT theoretical calculations where pressure-induced elasticity softening is observed at pressures of 7-15 GPa. At pressures higher than 15 GPa, a new high-pressure monoclinic phase is discovered from the current synchrotron X-ray diffraction data. Upon further compression up to ∼33 GPa, the monoclinic Sb2O3 starts to lose its long-range order and forms an amorphous component coexisting with the monoclinic one. To further explore the structural instability and understand the origin of pressure-induced phase transitions in ß-Sb2O3 upon compression, we have performed first-principles calculations to track the evolution of its phonon velocities, density of states and phonon dispersion curves under high pressure. Our results may play an important role in determining the local structures as well as their structural relationship among sesquioxides.

YCa3(CrO)3(BO3)4: A Cr(3+) Kagomé Lattice Compound Showing No Magnetic Order down to 2 K.

We report a new gaudefroyite-type compound YCa3(CrO)3(BO3)4, in which Cr(3+) ions (3d(3), S = 3/2) form an undistorted kagomé lattice. Using a flux agent, the synthesis was significantly accelerated with the typical calcining time reduced from more than 2 weeks to 2 d. The structure of YCa3(CrO)3(BO3)4 was determined by combined Rietveld refinements against X-ray and neutron diffraction data. Symmetry distortion refinement starting from a disordered YCa3(MnO)3(BO3)4 model was applied to avoid overparameterization. There are two ordering models, namely, K2-1 and K2-2, with the space groups P63 (No. 173) and P3̅ (No. 147), respectively, that differ in the [BO3] ordering between different channels (in-phase or out-of-phase). Both models give similarly good fits to the diffraction data. YCa3(CrO)3(BO3)4 is an insulator with the major band gap at Eg = 1.65 eV and a second transition at 1.78 eV. Magnetically, YCa3(CrO)3(BO3)4 is dominated by anti-ferromagnetic exchange along edge-sharing CrO6 octahedral chains perpendicular to the kagomé planes, with Θ ≈ -120 K and µeff ≈ 3.92 µB. The compound shows no spin ordering or freezing down to at least 2 K.

Synthesis, structural characterization, and physical properties of the new transition metal oxyselenide Ce2O2ZnSe2.

The quaternary transition metal oxyselenide Ce2O2ZnSe2 has been shown to adopt a ZrCuSiAs-related structure with Zn(2+) cations in a new ordered arrangement within [ZnSe2](2-) layers. The color of the compound changes as a function of cell volume, which can vary by ∼0.4% under different synthetic conditions. At the highest, intermediate, and lowest cell volumes, the color is yellow-ochre, brown, and black, respectively. The decreased volume is attributed to oxidation of Ce from 3+ to 4+, the extent of which can be controlled by synthetic conditions. Ce2O2ZnSe2 is a semiconductor at all cell volumes with experimental optical band gaps of 2.2, 1.4, and 1.3 eV for high, intermediate, and low cell volume samples, respectively. SQUID measurements show Ce2O2ZnSe2 to be paramagnetic from 2 to 300 K with a negative Weiss temperature of θ = -10 K, suggesting weak antiferromagnetic interactions.

Infinitely Adaptive Transition-Metal Ordering in Ln2O2MSe2-Type Oxychalcogenides.

A number of Ln2O2MSe2 (Ln = La and Ce; M = Fe, Zn, Mn, and Cd) compounds, built from alternating layers of fluorite-like [Ln2O2](2+) sheets and antifluorite-like [MSe2](2-) sheets, have recently been reported in the literatures. The available MSe4/2 tetrahedral sites are half-occupied, and different compositions display different ordering patterns: [MSe2](2-) layers contain MSe4/2 tetrahedra that are exclusively edge-sharing (stripe-like), exclusively corner-sharing (checkerboard-like), or mixtures of both. This paper reports 60 new compositions in this family. We reveal that the transition-metal arrangement can be systematically controlled by either Ln or M doping, leading to an "infinitely adaptive" structural family. We show how this is achieved in La2O2Fe1-xZnxSe2, La2O2Zn1-xMnxSe2, La2O2Mn1-xCdxSe2, Ce2O2Fe1-xZnxSe2, Ce2O2Zn1-xMnxSe2, Ce2O2Mn1-xCdxSe2, La2-yCeyO2FeSe2, La2-yCeyO2ZnSe2, La2-yCeyO2MnSe2, and La2-yCeyO2CdSe2 solid solutions.

Mn- and Yb-Doped BaTiO3-(Na0.5Bi0.5)TiO3 Ferroelectric Relaxor with Low Dielectric Loss.

In this work, a Mn-and Yb-doped BaTiO3-(Na0.5Bi0.5)TiO3 ferroelectric relaxor was designed and prepared. The effects of Mn on the microstructures, dielectric and electrical properties of the ceramics were investigated. The X-ray structural analysis shows a perovskite structure. The SEM images show the homogeneous microstructure of ceramics with an average grain size of about 1 µm. The temperature-dependent permittivity shows relaxor characteristics as Mn-doped. Mn at a low level (x ≤ 0.005) is beneficial for low dielectric loss and high resistivity. The maximum resistivity of ≥3 × 1012 Ω cm and minimum dielectric loss of ≤0.06 can be achieved at x ≤ 0.005. The resistivity of the ceramics follows the Arrhenius law with activation energy decreasing from ~1.31 to 1.01 eV as x increases. With lower Mn dopant, oxygen vacancies and charge carrier concentration partially decrease with Mn doping, which is helpful to improve the insulation resistance and decrease the dielectric loss.

Promising oxonitridosilicate phosphor host Sr3Si2O4N2: synthesis, structure, and luminescence properties activated by Eu2+ and Ce3+/Li+ for pc-LEDs.

A novel oxonitridosilicate phosphor host Sr(3)Si(2)O(4)N(2) was synthesized in N(2)/H(2) (6%) atmosphere by solid state reaction at high temperature using SrCO(3), SiO(2), and Si(3)N(4) as starting materials. The crystal structure was determined by a Rietveld analysis on powder X-ray and neutron diffraction data. Sr(3)Si(2)O(4)N(2) crystallizes in cubic symmetry with space group Pa ̅3, Z = 24, and cell parameter a = 15.6593(1) Å. The structure of Sr(3)Si(2)O(4)N(2) is constructed by isolated and highly corrugated 12 rings which are composed of 12 vertex-sharing [SiO(2)N(2)] tetrahedra with bridging N and terminal O to form three-dimensional tunnels to accommodate the Sr(2+) ions. The calculated band structure shows that Sr(3)Si(2)O(4)N(2) is an indirect semiconductor with a band gap ≈ 2.84 eV, which is close to the experimental value ≈ 2.71 eV from linear extrapolation of the diffuse reflection spectrum. Sr(3-x)Si(2)O(4)N(2):xEu(2+) shows a typical emission band peaking at ~600 nm under 460 nm excitation, which perfectly matches the emission of blue InGaN light-emitting diodes. For Ce(3+)/Li(+)-codoped Sr(3)Si(2)O(4)N(2), one excitation band is in the UV range (280-350 nm) and the other in the UV blue range (380-420 nm), which matches emission of near-UV light-emitting diodes. Emission of Sr(3-2x)Si(2)O(4)N(2):xCe(3+),xLi(+) shows a asymmetric broad band peaking at ~520 nm. The long-wavelength excitation and emission of Eu(2+) and Ce(3+)/Li(+)-doped Sr(3)Si(2)O(4)N(2) make them attractive for applications in phosphor-converted white light-emitting diodes.

Ag2Mo2O7: an oxide solid-state Ag+ electrolyte.

Ag2Mo2O7 powders and micro-crystals were prepared at 400 °C for 24 h and 500 °C for 6 h using solid-state reactions. The Ag2Mo2O7 samples crystalized in a triclinic P1̄ space group with the cell parameters a = 6.0972(1) Å, b = 7.5073(1) Å, c = 7.6779(2) Å, α = 110.43(1)°, ß = 93.17(1)°, γ = 113.51(1)°, and V = 294.17(1) Å3 from Rietveld refinements. Ag2Mo2O7 powder is homogeneous with size of 2-8 µm and the ceramic pellets are in good sintering conditions with a relative density ∼93%. The indirect band gaps E g(i) of Ag2Mo2O7 from reflectance measurements and DFT calculations are 2.63(1) and 1.80 eV. The vibrational modes of Ag2Mo2O7 were investigated by first-principles (DFT) calculations and Raman spectrum measurements with 24 of 33 predicted Raman modes recorded. According to DOS analyses, the valence bands (VB) of Ag2Mo2O7 are mainly constituted of O-2p and Ag-4d orbitals, while the conduction bands (CB) are mainly composed of Mo-4d and the O-2p orbitals. Regarding the impedance analysis, Ag2Mo2O7 is a silver oxide ion electrolyte with a conductivity of ∼5 × 10-4 S cm-1 at 450 °C. The carrier activation energy of Ag2Mo2O7 is 0.88(3) eV from the temperature dependent conductivity measurements.

Synthesis, structure, and characterization of the series BaBi(1-x)Ta(x)O(3) (0

The series BaBi(1-x)Ta(x)O(3) (0

Visible photocatalysis of novel oxime phosphonates: synthesis of ß-aminophosphonates.

A novel type of oxime phosphonate was synthesized and used in the intermolecular cascade radical addition reaction of alkenes to access ß-aminophosphonates via visible-light-driven N-centered iminyl radical-mediated and redox-neutral selective C-P single-bond cleavage in an active phosphorus radical route. The procedure is characterized by its ability to achieve the construction of Csp3-P and Csp3-N bonds without the requirement for oxidants and bases.

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).

Rh(III)-Catalyzed Phosphine Oxide Migration Reactions: Selective Synthesis of 3-Phosphinoylindoles.

3-Phosphinoylindoles are important components of biological active natural products and materials in pharmaceuticals. Herein, a new approach for the synthesis of 3-phosphinoylindoles has been established by a Rh(III)-catalyzed cyclization from readily accessible (2-azidostyryl)diphenylphosphine oxides. This intramolecular transformation occurs through a unique phosphine oxide group migration and offers a straightforward route to rebuild sp2 C-P bond and construct the indole ring in single step.

Autoxidation Photoredox Catalysis for the Synthesis of 2-Phosphinoylindoles.

A new approach to the synthesis of 2-phosphinoylindoles through photoredox catalysis without external oxidants has been developed. Promoted by a ruthenium photoredox catalyst, a broad scope of 2-phosphinoylindoles can be synthesized through phosphinoylation/cyclization of diphenylphosphine oxide at room temperature under irradiation without external oxidants. The estrone skeleton isocyan is also an amenable substrate for this cyclization, yielding a molecule that has potential medicinal applications.

The impact of anion ordering on octahedra distortion and phase transitions in SrTaO2N and BaTaO2N.

In this work we synthesized BaTaO2N and SrTaO2N using a two-step high-temperature solid-state reaction method and analysed the structural distortions, relative to the ideal cubic perovskite structure, according to group theory. From a complete distortion analysis/refinement using high-resolution neutron diffraction data in the temperature range 8 to 613 K, we identified tetragonal structures for BaTaO2N [P4/mmm (No. 123)] and SrTaO2N [I4/mcm (No. 140)]. In contrast to an anion-disordered cubic perovskite (Pm \overline{3}m No. 221) with Ta at the cell center, both systems show a site preference for oxygen anions in the two opposite corners (along the c axis) of the Ta-O/N octahedra rather than the four square corners in the ab plane (Γ3+ occupancy distortion), which induces a tetragonal elongation of the unit cell with the c axis being longer than the a axis. A further Ta-O/N octahedra displacement [R5-(a,0,0), rotation about the c axis] distortion was observed in SrTaO2N. This distortion mode is accompanied by an increased unit-cell distortion that decreases as the temperature increases. Ultimately a second-order phase transition caused by the loss of the R5-(a,0,0) mode was observed at 400-450 K.

Pressure induced polymerization of acetylide anions in CaC2 and 107 fold enhancement of electrical conductivity.

Transformation between different types of carbon-carbon bonding in carbides often results in a dramatic change of physical and chemical properties. Under external pressure, unsaturated carbon atoms form new covalent bonds regardless of the electrostatic repulsion. It was predicted that calcium acetylide (also known as calcium carbide, CaC2) polymerizes to form calcium polyacetylide, calcium polyacenide and calcium graphenide under high pressure. In this work, the phase transitions of CaC2 under external pressure were systematically investigated, and the amorphous phase was studied in detail for the first time. Polycarbide anions like C66- are identified with gas chromatography-mass spectrometry and several other techniques, which evidences the pressure induced polymerization of the acetylide anions and suggests the existence of the polyacenide fragment. Additionally, the process of polymerization is accompanied with a 107 fold enhancement of the electrical conductivity. The polymerization of acetylide anions demonstrates that high pressure compression is a viable route to synthesize novel metal polycarbides and materials with extended carbon networks, while shedding light on the synthesis of more complicated metal organics.

Synthesis, characterisation and properties of rare earth oxyselenides A4O4Se3 (A = Eu, Gd, Tb, Dy, Ho, Er, Yb and Y).

Rare earth oxyselenides A4O4Se3 (A = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Y) were synthesised using solid state reactions and three new structure types (ß, γ, and δ) were observed. A4O4Se3 materials adopt either the α (A = Nd, Sm), ß (A = Eu), γ (A = Gd, Tb) or δ (A = Dy, Ho, Er, Yb, Y) structure depending on the rare earth radius. Each structure type contains alternating [A2O2](2+) and Se(2-)/Se2(2-) layers. Different ordered and disordered arrangements of Se(2-) and [Se-Se](2-) give the Se layer flexibility and lead to the four different structure types observed. The volume coefficients of expansion for A4O4Se3 ranged from +1.746(9) × 10(-5) to +2.237(3) × 10(-5) K(-1) from 12 to 300 K; no structural phase transitions were observed in this temperature range. Diffuse reflection spectra show A4O4Se3 are semiconductors with band gap Eg 1.02-1.46 eV. Gd4O4Se3, Dy4O4Se3, and Tb4O4Se3 samples show antiferromagnetic ordering with Néel temperature, TN, of 7-9 K. DFT calculations confirm the two different valence states of Se(2-) and Se2(2-) in Eu4O4Se3.