Electronic parameters of Sr2Nb2O7 and chemical bonding

X-ray photoelectron spectroscopy (XPS) measurements were carried out on a strontium pyroniobate (Sr₂Nb₂O₇) powder sample, which was synthesized using standard solid-state method. The binding energy (BE) differences between the O 1s and cation core levels, Δ(O-Nb)=BE(O 1s)-BE(Nb 3d_5/2) and Δ(O-Sr)=BE(O 1s)-BE(Sr 3d_5/2), were used to characterize the valence electron transfer on the formation of the Nb-O and Sr-O bonds. The chemical bonding effects were considered on the basis of our XPS results for Sr₂Nb₂O₇ and earlier published structural and XPS data for other Sr- or Nb-containing oxide compounds. The new data point for Sr₂Nb₂O₇ is consistent with the previously derived relationship for a set of Nb⁵⁺-niobates that Δ(O-Nb) increases with increasing mean Nb-O bond distance, L(Nb-O). A new empirical relationship between Δ(O-Sr) and L(Sr-O) was also obtained. Interestingly, the correlation between Δ(O-Sr) and L(Sr-O) was found to differ from that between Δ(O-Nb) and L(Nb-O). Possible cause for the difference is discussed.     

Some studies on the solid solutions in the systems Sr2Nb2O7Eu2Nb2O7 and Sr2Ta2O7Eu2Ta2O7

Preparation of new solid solutions containing divalent europium have been tried in the systems Eu₂Nb₂O₇Sr₂Nb₂O₇ and Eu₂Ta₂O₇Sr₂Ta₂O₇. These solid solutions described as Eu_2xSr_2(1?x)M₂O₇ (M = Nb and Ta) exist in a pure orthorhombic phase in a limited region of x from 0 to about 0.5. The compounds with compositions close to Eu₂M₂O₇ exist but techniques have not been found to prepare them in pure form.     

Tin-magnesium substitution in Ir3Sn7—structure and chemical bonding in MgxIr3Sn7-x (x=0-1.67)

Well-shaped single crystals of binary Ir₃Sn₇ were obtained from a tin flux (starting composition Ir:Sn=1:10). The magnesium based stannides Mg_xIr₃Sn_7-x (x=0.61-1.67) were synthesized from the elements in glassy carbon crucibles in a water-cooled sample chamber of a high-frequency furnace. The samples were characterized by X-ray diffraction on powders and single crystals. All compounds crystallize with the cubic Ir₃Ge₇ type structure (space group Imm, Z=4). In this structure type the p-block atoms occupy the Wyckoff positions 12d and 16f and form two interpenetrating frameworks consisting of cubes and square antiprisms. The transition metal atoms center the square antiprisms and are arranged in pairs. With increasing magnesium substitution the lattice parameter of Ir₃Sn₇ (935.3 pm) decreases from 934.7 pm (x=0.61) to 930.6 pm (x=1.67) and the Ir-Ir distances decrease from 294 pm (Ir₃Sn₇) to 290 pm (Mg_1.67Ir₃Sn_5.33). In the ternary compounds Mg substitutes Sn on both framework sites. However, the 12d site shows a substantially larger preference for Mg occupation. By performing first-principles calculations we investigated the bonding situation in Ir₃Sn₇ and its alteration upon Mg incorporation. For binary Ir₃Sn₇ there are considerable bonding interactions between Ir and Sn atoms (d-p bonding) and between neighboring Sn atoms on the site 16f (p-p bonding). Both types of interactions diminish when substituting Sn for Mg. This explains the different site preference of Mg in Mg_xIr₃Sn_7−x: Mg occupation of the site 12d retains covalent p-p framework bonding between 16f atoms in the ternary compounds.     

Sr2Ta2O7纳米片的水热合成及其高效光降解甲基橙溶液的性能研究

采用水热法制备了Sr₂Ta₂O₇纳米片,并通过XRD、UV-Vis、SEM和TEM等手段对其进行了表征。以降解甲基橙溶液为反应模型,研究了Sr₂Ta₂O₇纳米片的光催化性能。光催化结果表明,水热法制备的Sr₂Ta₂O₇纳米片具有更高的光催化降解效率,为固相法制备的块体Sr₂Ta₂O₇样品的2倍。二者催化性能上差异主要是由于两种方法制备的样品具有不同的晶粒大小和比表面积造成的。     

CuAl2 revisited: Composition, crystal structure, chemical bonding, compressibility and Raman spectroscopy

The structure of CuAl₂ is usually described as a framework of base condensed tetragonal antiprisms [CuAl_8/4]. The appropriate symmetry governed periodic nodal surface (PNS) divides the space of the structure into two labyrinths. All atoms are located in one labyrinth, whereas the second labyrinth seems to be ‘empty’. The bonding of the CuAl₂ structure was analyzed by the electron localization function (ELF), crystal orbital Hamiltonian population (COHP) analysis and Raman spectroscopy. From the ELF representation it is seen, that the ‘empty’ labyrinth is in fact the place of important covalent interactions. ELF, COHP in combination with high-pressure X-ray diffraction and Raman spectroscopy show that the CuAl₂ structure is described best as a network built of interpenetrating graphite-like nets of three-bonded aluminum atoms with the copper atoms inside the tetragonal-antiprismatic cavities.     

Electron deformation density in titanium diboride chemical bonding in TiB2

Titanium diboride, TiB₂, crystallizes in the AlB₂-type structure, hexagonal P6/mmm. The conventional, free atom crystal structure refinement led to R=2.23%, and including extinction corrections to R=1.58%. Multipole refinements with multipoles up to order four (hexadecapole) reduced the R value to 1.21%. Difference density maps revealed charge deficiencies on the boron sites and broadbands of charge accumulations between the boron atoms indicating a graphitic B-delocalization of the boron sp² hybrid orbitals.     

The binary gallide Eu5Ga9 - crystal structure, chemical bonding and physical properties

The title compound was prepared from the elements by reaction in a sealed tantalum tube at 1320 K followed by slow cooling to 970 K or, alternatively, in glassy carbon crucibles with HF melting. The crystal structure of Eu₅Ga₉ was refined from single-crystal data: Cmcm, a=4.613(1) Å, b=10.902(3) Å, c=26.097(6) Å, Z=4, R_F=0.036, 811 structure factors and 46 variables. The structure is described as a three-dimensional network formed by gallium atoms with europium atoms embedded in the cavities. The bonding analysis (LMTO, ELF) confirmed this representation of the structure. Magnetic susceptibility measurements show Curie-Weiss behavior above 60 K with a magnetic moment per Eu atom of 8.12(1) μ_B, indicating divalent europium. Eu₅Ga₉ orders antiferromagnetically at 19.0(5) K with re-ordering at 6.0(5) K. The electrical resistivity shows a metallic temperature dependence and magnetic scattering. ¹⁵¹Eu Mössbauer spectroscopic experiments are compatible with divalent europium and show complex magnetic hyperfine field splitting below the ordering temperature.     

A new lead Mo(IV) phosphate with a tunnel structure: Pb2Mo2O(PO4)2P2O7

A molybdenum (IV) phosphate containing lead, Pb₂Mo₂(PO₄)2P₂O₇, has been synthesized for the first time. It crystallizes in the space group C2/c with a=14.098(1) Å, b=14.187(2) Å, c=6.5592(4) Å and β=102.08(1)°. Its original tunnel structure, built up of Mo₂O₁₁ bioctahedra, P₂O₇ and PO₄ phosphate groups can be described from the assemblage of [Mo₄P₄O₂₄]_∞ ribbons interconnected through monophosphate groups. The stereoactivity of the 6s² lone pair of Pb²⁺, which is surrounded by nine oxygen atoms, is discussed.     

Li2Si3O7: Crystal structure and Raman spectroscopy

The crystal structure of metastable Li₂Si₃O₇ was determined from single crystal X-ray diffraction data. The orthorhombic crystals were found to adopt space group Pmca with unit cell parameters of , and . The content of the cell is Z=4. The obtained structural model was refined to a R-value of 0.035. The structure exhibits silicate sheets, which can be classified as [Si₆O₁₄] using the silicate nomenclature of Liebau. The layers are build up from zweier single chains running parallel to c. Raman spectra are presented and compared with other silicates. Furthermore, the structure is discussed versus Na₂Si₃O₇.     

Heat capacity, enthalpy and entropy of strontium niobate Sr2Nb2O7 and calcium niobate Ca2Nb2O7

The heat capacity and the enthalpy increments of strontium niobate Sr₂Nb₂O₇ and calcium niobate Ca₂Nb₂O₇ were measured by the relaxation time method (2–300 K), DSC (260–360 K) and drop calorimetry (720–1370 K). Temperature dependencies of the molar heat capacity in the form C_pm = 248.0 + 0.04350T − 3.948 × 10⁶/T² J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and C_pm = 257.2 + 0.03621T − 4.434 × 10⁶/T² J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇ were derived by the least-square method from the experimental data. The molar entropies at 298.15 K, S_m°(298.15 K) = 238.5 ± 1.3 J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and S_m°(298.15 K) = 212.4 ± 1.2 J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇, were evaluated from the low-temperature heat capacity measurements.     

Preparation and structure of NaSr0.5Al2B2O7 and NaCa0.5Al2B2O7

Two compounds NaSr_0.5Al₂B₂O₇ and NaCa_0.5Al₂B₂O₇, have been found to crystallize into a new structure type by Rietveld refinement from X-ray powder diffraction data. Their structure belongs to hexagonal space group P6₃/m, with lattice parameters of , for NaSr_0.5Al₂B₂O₇ and , for NaCa_0.5Al₂B₂O₇, respectively. The structure is built up by [Al₂B₂O₇]²⁻ double layer and Na⁺/Ca²⁺ or Na⁺/Sr²⁺ ions alternatively stacking along the c-axis. The sites in the inter-double layer are fully occupied jointly by Na and Ca or Sr, but the intra-double layer sites are only half occupied solely by Na. A mechanism of the transition of the structure from CaAl₂B₂O₇ to present structure type by replacing only 1% Ca by Na (2%) as observed by Chang and Keszler (Mater. Res. Bull. 33 (1998) 299) is also proposed.     

[Nd(SSCNH2)4]-的电子结构 一种有机硫配位镧系络合物的模型阴离子

本文详细研究了络阴离子{Nd[SSCN(C_2H_5)_2]_4}~-的简化模型[Nd(SSCNH_2)_4]~-的电子结构和化学键合情况。从对键能的贡献看, 钕和硫原子之间的结合主要是离子性的, 但从电荷的重新分布看,应该认为共价结合对成键有一定贡献。σ键起主要作用, 对Nd—S和Nd—S′键来说, π键级只有σ键级的1/5～1/6。根据计算结果分析了为什么至今只合成了很少几种有机硫配位镧系络合物而存在大量稳定无机复合镧系硫化物的原因。建议了几种合成稳定的有机硫配位镧系络合物的可能途径。     

Structural investigation of the Cu2Se-In2Se3-Ga2Se3 phase diagram, X-ray photoemission and optical properties of the Cu1−z(In0.5Ga0.5)1+z/3Se2 compounds

Structures of compounds in the Cu₂Se-In₂Se₃-Ga₂Se₃ system have been investigated through X-ray diffraction. Single crystal structure studies for the so-called stoichiometric compounds Cu(In,Ga)Se₂ (CIGSe) confirm that the chalcopyrite structure (space group I4¯2d) is very flexible and can adapt itself to the substitution of Ga for In. On the other hand a structure modification is evidenced in the Cu_1−z(In_0.5Ga_0.5)_1+z/3Se₂ series when the copper vacancy ratio (z) increases; the chalcopyrite structure turns to a modified-stannite structure (I4¯2m) when z≥0.26. There is a continuous evolution of the structure from Cu_0.74(In_0.5Ga_0.5)_1.09Se₂ to Cu_0.25(In_0.5Ga_0.5)_1.25Se₂ ((i.e. Cu(In_0.5Ga_0.5)₅Se₈), including Cu_0.4(In_0.5Ga_0.5)_1.2Se₂ (i.e. Cu(In_0.5Ga_0.5)₃Se₅). From this single crystal structural investigation, it is definitively clear that no ordered vacancy compound exists in that series. X-ray photoemission spectroscopy study shows for the first time that the surface of powdered Cu_1−z(In_0.5Ga_0.5)_1+z/3Se₂ compounds (z≠0) is more copper-poor than the bulk. The same result has often been observed on CIGSe thin films material for photovoltaic applications. In addition, optical band gaps of these non-stoichiometric compounds increase from 1.2 to 1.4 eV when z varies from 0 to 0.75.     

The Crystal Structure of Sr2TiSi2O8

Sr₂TiSi₂O₈ single crystals were grown by Czochralski pulling and from a high-temperature solution. X-ray diffractometry revealed the modulated crystal structure of Sr₂TiSi₂O₈ to belong to the 5D superspace group P4bm (−α, α, 1/2; α, α, 1/2) with α=0.3. Atomic positions, anisotropic displacement factors and positional modulation parameters for Sr₂TiSi₂O₈ are determined and discussed. The positional modulation is further investigated by electron diffraction and high-resolution transmission electron microscopy. In the latter experiments, the 2D modulation appears to be superimposed by some 1D modulation waves. This effect is discussed in terms of growth conditions.     

Resonant photoemission spectroscopy study of electronic structure of V2O5

The electronic structure of V₂O₅ has been studied by resonant photoemission spectroscopy (RPES) and band structure calculation. The RPES data showed that the valence band (VB) of V₂O₅ is formed by strong hybridization of O2p and V3d states, the contribution from V3d state is predominant in the high binding energy (BE) region of the VB. The approximate charge distributions of O and V ions in V₂O₅ are calculated for the first time as from the RPES data.     

Investigation of crystal structure and associated electronic structure of Sr6BP5O20

Strontium borophosphate phosphate (Sr₆BP₅O₂₀, SrBP), activated by divalent europium ions is a bluish-green phosphor emitting in a broad band with the emission peak near 480 nm. In this paper, we report the crystal structure of SrBP determined from an analysis of the X-ray diffraction pattern of a prismatic single crystal (size 60 μm×50 μm×40 μm). This crystal was chosen from undoped phosphor powder samples prepared for this purpose by solid-state reaction. SrBP is observed to crystallize in a body-centered tetragonal lattice with the lattice parameters and , the associated space group being (space group 120). Using the structural data from this study, we have also calculated its electronic structure using the augmented spherical wave method and the local density approximation (LDA). We show the ordering of the electronic states by the density of states (DOS) and the partial DOS plots. The LDA gives a direct optical band gap at the Γ point of about 5 eV. The significance of the crystal structure and associated electronic structure is discussed with respect to maintenance of this phosphor in Hg-discharge lamps.     

Revisiting Y2Si2O7 and Y2SiO5 polymorphic structures by Y MAS-NMR spectroscopy

This paper describes the ⁸⁹Y MAS-NMR spectra for all the established polymorphs of Y₂Si₂O₇ (y, α, β, γ and δ) and Y₂SiO₅ (X1 and X2). The combination of our spectroscopic data with the structural information published up to now from diffraction data permits the revision and correction of mistakes which appear in the literature. Finally, the influence of different structural factors, such as yttrium coordination number and Y-O distances on the ⁸⁹Y NMR isotropic chemical shift is analyzed.     

The crystallographic and magnetic characteristics of Sr2CrO4 (K2NiF4-type) and Sr10(CrO4)6F2 (apatite-type)

Solid-state reaction between SrCO₃, Cr₂O₃ and SrF₂ has produced the apatite phase Sr₁₀(CrO₄)₆F₂ and Sr₂CrO₄ which adopts the K₂NiF₄-type structure. The reaction outcome was very sensitive to the heating rate with rapid rise times favouring the formation of Sr₂CrO₄, which has been synthesised at ambient pressure for the first time. Powder X-ray diffraction and electron diffraction confirmed that Sr₂CrO₄ adopts a body centred tetragonal cell (space group I4/mmm) with lattice parameters a=3.8357(1) Å and c=12.7169(1) Å, while a combination of neutron and X-ray diffraction verified Sr₁₀(CrO₄)₆F₂ is hexagonal (space group P6₃/m) with lattice parameters a=9.9570(1) Å and c=7.4292(1) Å. X-ray photoelectron spectroscopy and magnetic measurements were used to characterise the oxidation states of chromium contained within these phases.     

Crystallographic and magnetic characterisation of the brownmillerite Sr2Co2O5

Sr₂Co₂O₅ with the perovskite-related brownmillerite structure has been synthesised via quenching, with the orthorhombic unit cell parameters a=5.4639(3) Å, b=15.6486(8) Å and c=5.5667(3) Å based on refinement of neutron powder diffraction data collected at 4 K. Electron microscopy revealed L-R-L-R-intralayer ordering of chain orientations, which require a doubling of the unit cell along the c-parameter, consistent with the assignment of the space group Pcmb. However, on the length scale pertinent to NPD, no long-range order is observed and the disordered space group Imma appears more appropriate. The magnetic structure corresponds to G-type order with a moment of 3.00(4) μ_B directed along [1 0 0].     

Structural relations between weberite and zirconolite polytypes—refinements of doped 3T and 4M Ca2Ta2O7 and 3T CaZrTi2O7

New weberite-type Ca₂Ta₂O₇ and zirconolite-type CaZrTi₂O₇ polytypes have been prepared by doping with Nd/Zr and Th/Al, respectively, and their structures have been refined using single-crystal X-ray diffraction intensity data. The 3T zirconolite polytype, Ca_0.8Ti_1.35Zr_1.3Th_0.15Al_0.4O₇, has a=7.228(1), c=16.805(1) Å. The 3T weberite-type polytype, Ca_1.92Ta_1.92Nd_0.08Zr_0.08O₇, has a=7.356(1), c=18.116(1) Å. Both 3T polytypes have space group P3₁21, Z=6. The 4M Ca₂Ta₂O₇ polytype has the same composition, from electron microprobe analyses, as the 3T polytype, and has cell parameters: a=12.761(1), b=7.358(1), c=24.565(1) Å, β=100.17(1)°, space group C2, Z=16. The structural relationships between the different zirconolite and weberite polytypes are discussed. A consideration of the structures from the viewpoint of anion-centered tetrahedral arrays shows that zirconolite can be considered as an anion-deficient fluorite derivative phase. However, the fluorite-type topology of edge-shared OM₄ tetrahedra is not maintained in the Ca₂Ta₂O₇ weberite-type polytypes, even though they have a fluorite-like fcc packing of metal atoms. One of the oxygen atoms moves from a tetrahedral Ta₃Ca interstice to an adjacent Ta₂Ca₄ octahedral interstice in the weberite polytypes.