Ab-initio structure determination of β-La2WO6

The structure of the low-temperature form of β-La₂WO₆ has been determined from laboratory X-ray, neutron time-of-flight and electron diffraction data. This tungstate crystallizes in the non-centrosymmetric orthorhombic space group (no. 19) P2₁2₁2₁, with Z=8, a=7.5196(1) Å, b=10.3476(1) Å, c=12.7944(2) Å, and a measured density 7.37(1) g cm⁻³. The structure consists of tungsten [WO₆] octahedra and tetrahedral [OLa₄]. Tungsten polyhedra are connected such that [W₂O₁₁]¹⁰⁻ units are formed.     

Crystal Structure and Magnetic Properties of SrCaMnGaO5+δ

The room-temperature crystal structure of the brownmillerite SrCaMnGaO_5+δ (δ=0.035) has been refined from neutron powder diffraction data; space group Ima2, a=15.7817(6), b=5.4925(2), c=5.3196(2)> Å. Mn and Ga occupy 99.0(2)% of the 6- and 4-coordinate sites, respectively. A combination of magnetometry, neutron diffraction and μSR spectroscopy has shown that the compound orders magnetically at 180 K, and that the low-temperature phase has a G-type antiferromagnetic structure, with an ordered magnetic moment of 3.30(2) μ_B per Mn at 2 K. Displaced hysteresis loops provide evidence that the atomic moment has an additional, glassy component. Magnetometry shows that significant short-range magnetic interactions persist above 180 K, and μSR that the spin fluctuations are thermally activated in this temperature region. The compound is an electrical insulator which at 159 K shows an unusually large magnetoresistance of 85% in 6 T, increasing to 90% in 13 T.     

Crystal Structure, Electric and Magnetic Properties of Layered Cobaltite β-NaxCoO2

The layered oxide thermoelectric material β-Na_0.67CoO₂ has been studied by powder neutron diffraction, electric and magnetic measurements. This compound includes an edge-sharing CoO₆ slab and a highly vacant Na⁺ sheet in a unit cell (space group symmetry C2/m, a=4.9023(4) Å, b=2.8280(2) Å, c=5.7198(6) Å and β=105.964(6)° at 300 K). The evaluated formal valence of cobalt ion, +3.33(1), is ascribed to the coexistence of Co³⁺ and Co⁴⁺ in the ratio 2:1. Polycrystalline β-Na_0.67CoO₂, a p-type thermoelectric material, exhibits metallic behavior of the electric resistivity below 300 K. The Curie-Weiss-type magnetic susceptibility indicates antiferromagnetic interactions between magnetic cobalt ions in the edge-sharing CoO₆ slab.     

Synthesis and Crystal Structure of a Novel Borophosphate Compound with Noncentrosymmetric:Cd3BPO7

A new borophosphate compound Cd3 BPO7(Mr = 490.98) has been successfully synthesized by conventional high temperature solid state reaction for the first time and its structure was determined by single-crystal X-ray diffraction. It crystallizes in orthorhombic, space group Pna21, with a = 13.247(3), b = 8.8217(18), c = 5.0792(10), V = 593.56(20)3, Z = 4, Dc = 5.494 g/cm3, λ(CuKα) = 1.5406, F(000) = 880, the final R = 0.0212 and wR = 0.0513 for 1287 observed reflections(I 2σ(I)). In the crystal structure of Cd3 BPO7, the Cd(II) forms a slightly distorted octahedral geometry, BO3 3- and PO4 3- ions linking adjacent Cd(II) ions to form a 3D network. The second harmonic generation(SHG) on powder samples was measured using Kurtz and Perry technique. The result indicates that Cd3 BPO7 exhibits a SHG response 2.2 times larger than that of potassium dihydrogen phosphate(KDP) and is phase-matchable.     

Crystal Structure of a New Form of Sodium Octoborate β-Na2B8O13

Single crystals of a new form of sodium octoborate, β-Na₂B₈O₁₃, were obtained fortuitously from a complex Na₂O-B₂O₃-P₂O₅ mixture, and studied. The compound is monoclinic, space group P2₁/c; the unit cell parameters are a=11.731(4) Å, b=7.880(3) Å, c=10.410(4) Å, β=99.883(3)°; Z=4. The crystal structure was solved from 1653 reflections until R₁=0.0444; it consists of two infinite, independent, and interleaved boron-oxygen networks containing a complex borate anion (B₈O₁₃)²⁻ formed by six BO₃ triangles (Δ) and two BO₄ tetrahedra (T), which can be viewed as a B₅O₁₀ group linked to a B₃O₇ group; this leads to a Fundamental Building Block (FBB) with the shorthand notation: 8: ∞³ [(5:4Δ+T)+(3:2Δ+T)]. This FBB is identical to that described in other octoborates such as α-Na₂B₈O₁₃ and Ag₂B₈O₁₃. However, some subtle differences exist in the interlinking of the octoborate anions found in these three compounds, which explains their different structure and unit cell parameters.     

Crystal Structure and Spectroscopic Properties of a New Ternary Tungstate Li3Ba2Ho3（WO4）8

A single crystal of Li3Ba2Ho3(WO4)8 was obtained from a flux of Li2WO4 under an air atmosphere. The structure of the pure crystal was determined by single-crystal X-ray diffraction method. It crystallizes in the monoclinic system, space group C2/c with a = 5.240(4), b = 12.790(10), c = 19.247(15), β = 91.921(15)°, V = 1289.1(18)3, Z = 2, Mr = 2773.09, Dc = 7.144 g/cm3, μ = 47.732 mm-1, Rint = 0.0693, F(000) = 2340, the final R = 0.0472 and wR = 0.1221 for 1535 observed reflections (I > 2σ(I)). The Li3Ba2Ho3(WO4)8 has a high structure disorder with one 8f site shared by Li(1) and Ho ions with occupancy of 0.25 and 0.75, respectively. The fundamental structure is constituted by distorted square antiprisms Ho/Li(1)O8 with C1 symmetry, distorted Li(2)O6 octahedra and BaO10 polyhedra. The optical properties were investigated by IR and absorption spectroscopy, and the emission cross sections and gain cross sections of 5I7 → 5I8 of Ho3+ were calculated.     

A New Type of Macrocyclic Polyethers and its Compiexing Properties

Crown ethers,first reported by Pedersen in 1967,were found to be capableof forming stable complexes with many salts of alkali and alkaline earth cations.Since then,many new types of macrocyclic polyethers were synthesized andused in chemical and biochemical researches.In 1969 polyoxamacrobicyclic di-amines were introduced by Lehn as potential ionophores.According to Lehn'snomenclature,these compounds are called cryptands.They have three dimen-sional cavities,which can be tailored to the size required by different cations.     

Structure of δ-Bi2O3 from density functional theory: A systematic crystallographic analysis

A systematic crystallographic analysis of the 〈110〉 and 〈111〉 vacancy-ordered structure of cubic δ-Bi₂O₃ obtained from electronic-structure calculations is presented. The ordering of vacancies leads to a doubling of the unit-cell that results in a 2×2×2 fluorite super-structure, with an associated reduction in its space group symmetry from to . The Bi atoms present inside the 〈111〉 vacancy-ordered oxygen sublattice have equal Bi-O bond lengths, whereas, those present inside the 〈110〉 vacancy-ordered oxygen sublattice have three different pairs of Bi-O bond lengths. The specific ionic displacements and electronic charge configurations also depend on the nature of vacancy ordering in the oxygen sub-lattice.     

Low-Frequency Relaxation Phenomena in α-LiIO3: The Nature and Role of Defects

The lithium iodate ionic conduction in the polar c-axis direction is studied between 35 and 470 K for crystals grown in various conditions. So far, to separate processes induced by the nature of electrodes, the impedance spectroscopy technique had been used at room temperature with both metallic and ideally polarizable insulating electrodes, so that a relaxation of space charges was clearly identified. Here, the temperature dependence of the hopping ionic conductivity exhibits quite different activation energies well related to the growth conditions. Following low-temperature Raman and thermodynamic experiments, a new approach based on a vacancy diffusion mechanism is proposed. Experimental conductivity results are then correlated with the valence, size and concentration of extrinsic impurities incorporated during the growth and analyzed by plasma spectroscopy. Finally, a discussion is made on still not well-understood phenomena such as the strong increase of photorefractivity or the enhancement of some Bragg reflections.     

Preparation and Double-Helix Icosahedra Structure of δ-Co2Zn15

The first known example of a regular face-sharing icosahedra helix is presented in the novel crystal structure of δ-Co₂Zn₁₅, solved and refined from single-crystal X-ray and neutron powder diffraction data. The compound δ-Co₂Zn₁₅ is also the first example of an intermetallic compound crystallizing in the acentric hexagonal space group P6₂. The parameters from the single crystal refinement are a=11.292(2) Å, c=7.750(1) Å, Z=4, and R_w=0.025.     

Structure,Electrical and Oxygen Transport Properties of Fe-Doped SrCoO3?δ Perovskites

The structure-property relationship of Fe-doped SrCoO_3-δ was studied. With increase of Fe content in SrCo_1-xFe_xO_3-δ from x=0 to x=0.2, the phase composition changed progressively in the order of hexagonal→brownmillerite (main)+hexagonal→cubic (main)+brownmillerite→single cubic phase. Transition between the hexagonal/brownmillerite phase and the cubic phase took place with variation of the operating conditions, and was associated with remarkable changes in the electrical conductivity and oxygen permeation flux.     

Ab Initio Structure Determination of a New Compound, β-SrGaBO4, from Powder X-Ray Diffraction Data

A new compound, β-SrGaBO₄, has been attained through solid phase transition from α-SrGaBO₄ at high temperatures. Its crystal structure has been determined from powder X-ray diffraction data by direct methods. The refinement was carried out using the Rietveld method and the final refinement converged with Rp=11.42 % and Rwp=15.16 %. It has an orthorhombic P2₁2₁2 space group with cell parameters a=15.3706(2) Å, b=8.9921(1) Å, c=5.9191(1) Å, and Z=8. The structure of β-SrGaBO₄ is built up from Ga₂BO₈ units formed by two GaO₄ tetrahedra and one BO₃ triangle, and Sr₂O₁₂ units formed by two SrO₇ groups. Tetrahedra [GaO₄] are linked by shared O(3) and O(7) atoms to form infinite chains along the c axis.     

Pressure-induced transformations in α- and β-Ge3N4: in situ studies by synchrotron X-ray diffraction

Metastable high-pressure transformations in germanium nitride (α- and β-Ge₃N₄ polymorphs) have been studied by energy- and angle-dispersive synchrotron X-ray diffraction at high pressures in a diamond anvil cell. Between P=22 and 25 GPa, the phenacite-structured β-Ge₃N₄ phase (P6₃/m) undergoes a 7% reduction in unit-cell volume. The densification is primarily concerned with the a-axis parameter, in a plane normal to the hexagonal c-axis. Based on results of previous LDA calculations and Raman spectroscopic studies, we propose that the structural collapse is due to transformation into a new metastable polymorph (δ-Ge₃N₄) that has a unit-cell symmetry based upon P3, that is related to the low-pressure β-Ge₃N₄ phase by concerted displacements of N atoms away from special symmetry sites in the plane normal to the c-axis. No such transformation occurs for α-Ge₃N₄, due to the different stacking of linked GeN₄ layers. All three polymorphs (α-, β- and δ-Ge₃N₄) are based on tetrahedrally coordinated Ge atoms, unlike the spinel-structured γ-Ge₃N₄ phase, that contains octahedrally coordinated Ge⁴⁺. Experimentally determined bulk modulus values for α-Ge₃N₄ (K₀=165(10) GPa, K₀′=3.7(4)) and β-Ge₃N₄ (K₀=185(7) GPa, K₀′=4.4(5)) are in excellent agreement with theoretical predictions. The bulk modulus for the new δ-Ge₃N₄ polymorph is only determined above the β-δ transition pressure (P=24 GPa); K=161(20) GPa, assuming K′=4. Above 45 GPa, both α- and δ-Ge₃N₄ polymorphs become amorphous, as determined by X-ray diffraction and Raman scattering.     

Synthesis and Properties of Pr1-xRbxMnO3 (0.05≤x≤0.08) with Perovskite-Type Structure

Rb-substitued Pr_1-xRb_xMnO₃ (0.05≤x≤0.08) was successfully synthesized by solid state reaction. Powder X-ray diffraction showed that all the compounds were orthorhombic with the space group of Pnma. Spin glass behaviors were observed for all the compounds at low temperature, suggesting the competition between ferromagnetic and antiferromagnetic. The temperature dependence of the resistivity for the compound Pr_0.92Rb_0.08MnO_3.02 at 0 and 2 T magnetic field was also investigated. The compound shows semiconducting behavior, and the band gap is 0.3 eV. The maximum magnetoresistance is about 30% at 2 T magnetic field near 116 K.     

The defect chemistry of La1−ΔMnO3+δ

The effects of oxygen reduction treatments on the magnetic properties of La-deficient manganites, La_1−ΔMnO_3+δ and Sr- and Ca-doped manganites, La_1−xM_xMnO_3+δ (M: Sr, Ca) have been investigated to confirm the contrasting oxygen reduction effects on the magnetization properties. It is found that oxygen reduction treatments in reduced oxygen pressures of 10³- for La_1−ΔMnO_3+δ result in a continuous change in the magnetization but the reduction treatments for La_1−xM_xMnO_3+δ result in a negligible change under the same reduction conditions. To interpret the contrasting behavior of the La-deficient manganites, several possible models have been discussed. Among the models, the most probable model is that vacancies generated by the La deficiency Δ are partially replaced by Δ₂(=Δ−Δ₁?Δ₁) Mn ions to give both La and Mn site vacancies according to the formula La_1−ΔV_ΔMnO_3+δ→{La_1−ΔMn_Δ2V_Δ1}{Mn_1−Δ2V_Δ2}O_3+δ. Details of thermodynamic basis of this model have been presented.     

Synthesis, crystal growth and structure of Mg containing β-rhombohedral boron: MgB17.4

For the first time, single crystals of Mg containing β-rhombohedral boron MgB_17.4 were synthesised from the elements in a Mg/Cu melt at 1600 °C. The crystal structure determined by the refinement of single crystal data (space group R-3m, , , 890 reflections, 123 variables, R₁(F)=0.049, wR₂(I)=0.122) improves and modifies the former structure model derived from earlier investigations on powder samples. Mg is located on four different positions with partial occupation. While the occupation of the sites D (53.3%), E (91%) and F (7.2%) is already known from other boron-rich borides related to β-rhombohedral boron, the occupation of the fourth position (18h, 6.7%) is observed for the first time. Two boron positions show partial occupation. The summation reveals the composition MgB_17.4 and Mg_5.85B_101.9, respectively, confirmed by WDX measurements. The single crystals of MgB_17.4 show the highest Mg content ever found. Preliminary measurements indicate no superconductivity.     

Raman characterization of α- and β-LiFe5O8 prepared through a solid-state reaction pathway

Lithium ferrite, a mixed-inverse spinel of type A_xB_y[A_1−xB_1−y]O₄ was produced through solid state synthesis by calcining a Li₂CO₃/Fe₂O₃ mixture at 900 °C. The presence of both the ordered α-phase and disordered β-phase of LiFe₅O₈ was confirmed by XRD analysis, while formation of the latter was achieved by air quenching from high temperature. Laser Raman analysis was performed on both the α-LiFe₅O₈ and β-LiFe₅O₈ powders in order to achieve a reference set of Raman shifts for the spinel. The strongest, characteristic Raman peaks were determined to be 493, 382, 358, 300, and 263 cm⁻¹ for both phases while smaller peaks at 202 and 236 cm⁻¹ present in the α-phase were diminishing in intensity when the β-phase was present, thus providing unique identifiers for the presence of the disordered ferrite structure. SEM images taken of the synthesized LiFe₅O₈ powders showed particle sizes of less than 300 nm and an even particle size distribution.     

The preparation and crystal structure of (η-C5H5)(OC)3Mo(CH2)3C6H5

The reaction of with p-CH₃C₆H₄S(O)₂O(CH₂)₃C₆H₅ produces (η⁵-C₅H₅)(OC)₃Mo(CH₂)₃C₆H₅. This is only the second structurally characterized organometallic species in which an aromatic moiety is separated by three or more methylene groups. The alkyl chain adopts a staggered conformation, the Mo-C(1)-C(2)-C(3)-C(4) unit is nearly coplanar, and the alkyl chain eclipses the trans-carbonyl group on Mo. NMR evidence indicates that this conformation is preserved in solution.