Bonding, ion mobility, and rate-limiting steps in deintercalation reactions with ThCr2Si2-type KNi2Se2

Here, we study the nature of metal-metal bonding in the ThCr(2)Si(2) structure type by probing the rate-limiting steps in the oxidative deintercalation of KNi(2)Se(2). For low extents of oxidation, alkali ions are removed exclusively to form K(1-x)Ni(2)Se(2). For greater extents of oxidation, the rate of the reaction decreases dramatically, concomitant with the extraction of both potassium and nickel to form K(1-x)Ni(2-y)Se(2). The appreciable mobility of transition metal ions is unexpected, but illustrates the relative energy scales of different defects in the ThCr(2)Si(2) structure type. Furthermore, the fully oxidized compounds, K(0.25)Ni(1.5)Se(2), spontaneously convert from the tetrahedral [NiSe(4)]-containing ThCr(2)Si(2) structure to a vacancy-ordered NiAs structure with [NiSe(6)] octahedra. From analysis of the atom positions and kinetic data, we have determined that this transformation occurs by a continuous, low-energy pathway via subtle displacements of Ni atoms and buckling of the Se sublattice. These results have profound implications for our understanding of the stability, mobility, and reactivity of ions in materials.     

Chemical pressure and rare-earth orbital contributions in mixed rare-earth silicides La(5-x)Y(x)Si4 (0 ≤ x ≤ 5)

A crystallographic study and theoretical analysis of the structural and La/Y site preferences in the La(5-x)Y(x)Si(4) (0 ≤ x ≤ 5) series prepared by high-temperature methods is presented. At room temperature, La-rich La(5-x)Y(x)Si(4) phases with x ≤ 3.0 exhibit the tetragonal Zr(5)Si(4)-type structure (space group P4(1)2(1)2, Z = 4, Pearson symbol tP36), which contains only Si-Si dimers. On the other hand, Y-rich phases with x = 4.0 and 4.5 adopt the orthorhombic Gd(5)Si(4)-type structure (space group Pnma, Z = 4, Pearson symbol oP36), also with Si-Si dimers, whereas Y(5)Si(4) forms the monoclinic Gd(5)Si(2)Ge(2) structure (space group P2(1)/c, Z = 4, Pearson symbol mP36), which exhibits 50% "broken" Si-Si dimers. Local and long-range structural relationships among the tetragonal, orthorhombic, and monoclinic structures are discussed. Refinements from single crystal X-ray diffraction studies of the three independent sites for La or Y atoms in the asymmetric unit reveal partial mixing of these elements, with clearly different preferences for these two elements. First-principles electronic structure calculations, used to investigate the La/Y site preferences and structural trends in the La(5-x)Y(x)Si(4) series, indicate that long- and short-range structural features are controlled largely by atomic sizes. La 5d and Y 4d orbitals, however, generate distinct, yet subtle effects on the electronic density of states curves, and influence characteristics of Si-Si bonding in these phases.     

Eight-coordinated arsenic in the Zintl phases RbCd4As3 and RbZn4As3: synthesis and structural characterization

Reported are two new series of Zintl phases, ACd(4)Pn(3) and AZn(4)Pn(3) (A = Na, K, Rb, Cs; Pn = As, P), whose structures feature complex atomic arrangements based on four- and eight-coordinated arsenic and phosphorus. A total of 12 compounds have been synthesized from the corresponding elements via high temperature reactions, and their structures have been established by X-ray diffraction. RbCd(4)As(3), KCd(4)As(3), NaCd(4)As(3), NaZn(4)As(3), KCd(4)P(3), and KZn(4)P(3) crystallize with a new rhombohedral structure (space group R3m, Z = 3, Pearson symbol hR24), while the isoelectronic RbZn(4)As(3), CsCd(4)As(3), CsZn(4)As(3), KZn(4)As(3), CsZn(4)P(3), and RbZn(4)P(3) adopt the tetragonal KCu(4)S(3)-type structure (space group P4/mmm, Z = 1, Pearson symbol tP8). Both structures are very closely related to the ubiquitous CaAl(2)Si(2) and ThCr(2)Si(2) structure types, and the corresponding relationships are discussed. The experimental results have been complemented by linear muffin-tin orbital (LMTO) tight-binding band structure calculations. Preliminary transport properties measurements on polycrystalline samples suggest that the compounds of these families could be promising thermoelectric materials.     

Pressure-induced isostructural phase transition and correlation of FeAs coordination with the superconducting properties of 111-type Na(1-x)FeAs

The effect of pressure on the crystalline structure and superconducting transition temperature (T(c)) of the 111-type Na(1-x)FeAs system using in situ high-pressure synchrotron X-ray powder diffraction and diamond anvil cell techniques is studied. A pressure-induced tetragonal to tetragonal isostructural phase transition was found. The systematic evolution of the FeAs(4) tetrahedron as a function of pressure based on Rietveld refinements on the powder X-ray diffraction patterns was obtained. The nonmonotonic T(c)(P) behavior of Na(1-x)FeAs is found to correlate with the anomalies of the distance between the anion (As) and the iron layer as well as the bond angle of As-Fe-As for the two tetragonal phases. This behavior provides the key structural information in understanding the origin of the pressure dependence of T(c) for 111-type iron pnictide superconductors. A pressure-induced structural phase transition is also observed at 20 GPa.     

Gd5-xYxTt4 (Tt = Si or Ge): effect of metal substitution on structure, bonding, and magnetism

A crystallographic study and theoretical assessment of the Gd/Y site preferences in the Gd 5- x Y x Tt 4 ( Tt = Si, Ge) series prepared by high-temperature methods is presented. All structures for the Gd 5- x Y x Si 4 system belong to the orthorhombic, Gd 5Si 4-type (space group Pnma). For the Gd 5- x Y x Ge 4 system, phases with x < 3.6 and x >or= 4.4 adopt the orthorhombic, Sm 5Ge 4-type structure. For the composition range of 3.6 相似文献   

A metal-insulator transition in R2O2Bi with an unusual Bi2- square net (R = rare earth or Y)

A series of tetragonal ThCr(2)Si(2)-type compounds, R(2)O(2)Bi (R = rare earth or Y), are synthesized in which an unusual Bi(2-) anion forms a square net layer that is sandwiched between (R(2)O(2))(2+) fluorite layers. Two-dimensional (2D) electronic bands around the Fermi energy are predominantly composed of 6p(x)6p(y) orbitals in the Bi(2-) square net, which contains a positive hole per Bi(2-) ion. The decrease in the size of the square net caused by reducing the size of the R ion enhances the electrical conductivity because of the hole, resulting in a "chemical pressure"-induced metal-insulator transition.     

Metallothermische Reduktion der Tribromide und -iodide von Thulium und Ytterbium mit Alkalimetallen

Metallothermic Reduction of the Tribromides and -iodides of Thulium and Ytterbium with Alkali Metals Metallothermic reduction of the trihalides MX, (M = Tm, Yb; X = Br, I) with equimolar amounts of alkali metal (A = Li? Cs; 700–800°C, 7 d, tantalum capsules) results in the formation of the dihalides MX, (MI, with the Cd1,-type of structure, MBr, with the SrI₂- or α-PbO₂-type of structure) in the case of lithium and sodium. With A = K, Rb, Cs variants of the perovskitetype structure with the composition AMX, are obtained. They crystallize with the undistorted cubic CaTiO₃-type (CsYbBr₃), with the tetragonal NaNbO₃-11-type (CsTmBr₃) or with the orthorhombic GdFeO₃-type (CsMI₃, RbMX₃ and KMBr₃). In KTmI, and KYbI₃, corner- and edge-connected [MI₆] octahedra form layers which are connected via K⁺ in analogy to a stuffed PuBr_3? or the FeUS₃-type structure (orthorhombic, Cmcm). Single crystals of Rb₄YbI₆ are obtained as a by product upon the reduction of YbI₃ with rubidium. According to their K₄CdCl₆-type structure (trigonal, R3 c), isolated [YbI₆] octahedra are the main structural feature.     

Synthesis and properties of new multinary silicides R5Mg5Fe4Al(x)Si(18-x) (R = Gd, Dy, Y, x ≈ 12) grown in Mg/Al flux

Reactions of iron, silicon, and R = Gd, Dy, or Y in 1:1 Mg/Al mixed flux produce well-formed crystals of R(5)Mg(5)Fe(4)Al(x)Si(18-x) (x ≈ 12). These phases have a new structure type in tetragonal space group P4/mmm (a = 11.655(2) ?, c = 4.0668(8) ?, Z = 1 and R(1) = 0.0155 for the Dy analogue). The structure features two rare earth sites and one iron site; the latter is in monocapped trigonal prismatic coordination surrounded by silicon and aluminum atoms. Siting of Al and Si was investigated using bond length analysis and (27)Al and (29)Si MAS NMR studies. The magnetic properties are determined by the R elements, with the Gd and Dy analogues exhibiting antiferromagnetic ordering at T(N) = 11.9 and 6.9 K respectively; both phases exhibit complex metamagnetic behavior with varying field.     

The high-pressure phase transition of TiS2 from first-principles calculations

The structural stability of TiS₂ under high pressures has been investigated by using first-principles plane-wave pseudopotential density functional theory within the local density approximation (LDA). The obtained results predict that TiS₂ undergoes a pressure-induced first-order phase transition from its trigonal 1T-type structure to orthorhombic cotunnite-type structure at 16.20 GPa. The calculated transition pressure agrees quite well with the experimental finding of 20.7 GPa. The equation of state determined from our calculated results yields bulk moduli of 58.91 and 118.10 GPa for the 1T-type and cotunnite-type phases, respectively. This indicates higher incompressibility of the high-pressure phase of TiS₂. In addition, the electronic structures of the two phases of TiS₂ are also calculated and discussed. The results suggest the structural phase transition of TiS₂ at high pressure is followed by a semimetal to metal electronic transition.     

An <Emphasis Type="Italic">Ab Initio</Emphasis> Study of Electronic Structure of Lithium Metaborate

The electronic structure of lithium metaborate in monoclinic and tetragonal phases is studied using the density functional theory (DFT) method. The band spectra, total and partial densities of states are calculated for both modifications. Deformation electron density maps in LiBO₂ crystals are obtained. Participation of oxygen atoms in chemical bonding due to trigonal BO₃ and tetragonal BO₄ groups in monoclinic and tetragonal phases, respectively, is studied.     

Rare-earth metal-rich antimonides: syntheses, structures, and properties of Tm(3)Sb and Lu(7)Sb(3)

The two most metal-rich lanthanide antimony phases known were obtained from high-temperature solid state syntheses, that for Tm3Sb being of greater difficulty because of its apparent incongruent melting. The Tm3Sb phase crystallizes in the tetragonal space group P42/n (No. 86) with a Ti3P-type (Pearson: tP32) structure, a = 12.2294(5) Angstrom, c = 5.9852(5) Angstrom, and Z = 8. The phase Lu7Sb3 exhibits a Sc7As3-type tetragonal structure, I4/mcm (No. 140) (tI56), with a = 15.5974(7) Angstrom, c = 8.8130(7) Angstrom, and Z = 8. Both structures are described in terms of compact arrays of condensed chains of metal polyhedra (tetrahedral, tetrahedral star, trigonal prismatic, cubic) together with six- to nine-coordinate Sb in metal polyhedra. Magnetic susceptibility data on the paramagnetic Tm3Sb are also reported.     

Crystal structure, coloring problem and magnetism of Gd(5-x)Zr(x)Si4

Ternary Gd(5-x)Zr(x)Si(4) silicides were synthesized by arc melting of the constituent elements and subsequent heat treatments. The Gd(5-x)Zr(x)Si(4) phases adopt the orthorhombic Gd(5)Si(4)-type (space group Pnma) structure for x≤ 0.25 and the tetragonal Zr(5)Si(4)-type (space group P4(1)2(1)2) structure for x≥ 1.0, respectively. The samples with intermediate compositions contain two phases. Single-crystal X-ray diffraction reveals a preferential site occupancy for Zr on the three metal sites in the order of M3 > M2 > M1. Size arguments based on the local coordination environments suggest that the larger Gd atoms preferentially occupy the larger M1 site, while the smaller Zr atoms tend to occupy the smaller metal sites, M2 and M3. Tight-binding linear-muffin-tin orbital calculations illustrate a role of the metal-silicon bonds in the metal site occupation. An increase in the valence electron concentration through the Zr substitution weakens the Si-Si interactions but enhances the metal-silicon and metal-metal interactions. The Curie temperature of Gd(5-x)Zr(x)Si(4) decreases gradually with the increasing Zr content.     

An application of the "coloring problem": structure-composition-bonding relationships in the magnetocaloric materials LaFe13-xSix

The LaFe(13)-(x)Si(x) (1.0 < or = x < or = 5.0) series is studied experimentally and theoretically to gain possible understanding for the relationships among geometrical structure, chemical composition, magnetic behavior, and physical properties as related to the magnetocaloric effect in these compounds. As the Si concentration increases, LaFe(13)-(x)Si(x) exhibits a structural transformation from the cubic NaZn(13) structure type to a tetragonal derivative due primarily to preferential ordering of Fe and Si atoms. At room temperature, LaFe(13)-(x)Si(x) crystallize in the cubic structure for the range 1 < or = x < or = 2.6 and in the tetragonal for 3.2 < or = x < or = 5. In the range 2.6 < or = x < or = 3.2, it shows a two-phase mixture. Temperature-dependent single-crystal X-ray diffraction experiments near the corresponding Curie temperatures were performed on the room-temperature cubic phases to examine the origin of the large isothermal magnetic entropy changes. A thorough statistical and structural analysis of the data indicates that the noncentrosymmetric F43c space group provides a more adequate atomic arrangement than the centrosymmetric Fm3c space group. This change in space group leads to divergence for specific sets of Fe-Fe distances below the Curie temperature that arises from tilting of Fe-centered [Fe(12)-(x)Si(x)] icosahedra. The noncentrosymmetric space group also agrees with the predominance of icosahedral clusters lacking local inversion symmetry. From extended Hückel and tight-binding linear muffin-tin orbital (TB-LMTO) electronic structure calculations on various model structures, the F43c model is more energetically favorable than the Fm3c model. Extended Hückel calculations on various icosahedral [Fe(12)-(n)Si(n)] (n = 1-5) clusters and TB-LMTO calculations on "LaFe(13)," LaFe(11)Si(2), and LaFe(9)Si(4) have also been carried out to study the effects of a main group element (Si) on stabilizing the cubic NaZn(13)-type structure, influencing the transformation between cubic and tetragonal symmetries, and to study relationships among their chemical bonding and magnetic properties.     

Ternary early-transition-metal palladium pnictides Zr3Pd4P3, Hf3Pd4P3, HfPdSb, and Nb5Pd4P4

Several ternary palladium pnictides of the early transition metals have been prepared by arc-melting of the elemental metals and the binary pnictides ZrP, HfP, HfSb2, or NbP, and their structures have been determined by X-ray diffraction methods. The phosphides M3Pd4P3 (M = Zr, Hf) adopt a new structure type (Pearson symbol oP40), crystallizing in the orthorhombic space group Pnma with Z = 4 and unit cell parameters of a = 16.387(2), b = 3.8258(5), and c = 9.979(1) A for Zr3Pd4P3 and a = 16.340(2), b = 3.7867(3), and c = 9.954(1) A for Hf3Pd4P3. The antimonide HfPdSb was identified by powder X-ray diffraction (orthorhombic, Pnma, Z = 4, a = 6.754(1) A, b = 4.204(1) A, and c = 7.701(2) A) and confirmed to be isostructural to ZrPdSb, which adopts the TiNiSi-type structure. The phosphide Nb5Pd4P4 adopts the Nb5Cu4Si4-type structure, crystallizing in the tetragonal space group I4/m with Z = 2, a = 10.306(1) A, and c = 3.6372(5) A. Coordination geometries of pentacapped pentagonal prisms for the early transition metal, tetracapped distorted tetragonal prisms for Pd, and tricapped trigonal prisms for the pnicogen are found in the three structures; tetracapped tetragonal prisms for Nb are also found in Nb5-Pd4P4. In common with many metal-rich compounds whose metal-to-nonmetal ratio is equal or close to 2:1, the variety of structures formed by these ternary palladium pnictides arises from the differing connectivity of pnicogen-filled trigonal prisms. Pnicogen-pnicogen bonds are absent in these structures, but metal-metal bonds (in addition to metal-pnicogen bonds) are important interactions, as verified by extended Hückel band structure calculations on Zr3Pd4P3.     

Experimental and theoretical investigation on the correlation between aqueous precursors structure and crystalline phases of zirconia

Nanometer zirconia powders were prepared by the precipitation method at different pHs and different reaction temperatures. X-ray results show that monoclinic zirconia is favored at pH 4 while tetragonal zirconia is favored at pH 9.5 at room temperature, and monoclinic zirconia is also favored at pH 9.5 and 70 °C reaction temperature, with the slow addition of alkali. Four models of zirconium complexes were applied to simulate the structural monomers in different pH solutions. Geometric parameters and Mulliken charge population were calculated by optimizing these complexes using the density functional theory (DFT/B3LYP). Theoretical analyses show that if Model I ([Zr(OH)₂(H₂O)₄]²⁺ monomers) is favored in the aqueous precursor solution, it will be preferentially polymerized into monoclinic precursor structure irrespective of slow or quick alkali addition. Contrarily, if Model IV ([Zr(OH)₇]³⁻ monomers) is major in the aqueous precursor solution, tetragonal precursor structures are favored irrespective of slow or quick alkali addition. When Model II ([Zr(OH)₄(H₂O)₂]⁰ monomers) and Model III ([Zr(OH)₆]²⁻ monomers), respectively, predominate in the aqueous precursor solution, they will be preferentially polymerized into tetragonal precursor structure at slow alkali addition, however, for quick alkali addition, they will be preferentially polymerized into monoclinic precursor structure. Our theoretical models well explain the present experimental results as well as previous experimental results, and allow building up a correlation between aqueous precursor structures and crystalline phases of zirconia.     

Synthesis and Crystal Structure of Ho_4S_3Si_2O_7

1 INTRODUCTION Halide fluxes are excellent media for growing single crystals of chalcogenides[1~5]. On our research in the Ln-Cu-Zn-Se system to find new phases and to search for potential infrared ceramic materials, two new compounds, KHo2CuSe4 and KEr2CuSe4, were synthesized from the reaction of the precursor with KCl flux[6]. In an attempt to synthesize the homologous sulfide by the same method using KBr as flux in a sealed evacuated quartz tube, single crystals of Ho4S3Si2O7 w…     

柠檬酸溶胶-凝胶法制备的Ce1-xZrxO2: 结构及其氧移动性

采用 XRF、XRD、Raman、XPS、H₂-TPR 以及与氩离子刻蚀相结合的XPS等表征技术对柠檬酸溶胶-凝胶法制备的Ce_1-xZr_xO₂ (0≤x≤1)样品的结构及其氧移动性进行了研究. 结果表明, Ce_1-xZr_xO₂ 样品的晶型结构对其中氧的移动性有明显影响. 当x≤0.15 时, Ce_1-xZr_xO₂ 以立方CeO₂相 Ce-Zr-O 固溶体存在, 随着Zr含量的逐渐增加, CeO₂晶胞体积减小、氧空位浓度增加, 氧移动性逐渐增强; 当x＞0.15时, 形成四方ZrO₂相和立方CeO₂相Ce-Zr-O固溶体的混合物, 随着Zr含量的逐渐增加, 四方ZrO₂相的含量增加、氧空位浓度减小, 氧移动性逐渐减弱. 因此, Ce_0.852Zr_0.152O₂样品具有最高的氧移动性.     

Metastable 11 K Superconductor Na(1-y)Fe(2-x)As(2)

The topochemical deintercalation of Na(+) ions from solid NaFeAs at room temperature in THF with iodine yields the superconducting phase Na(1-y)Fe(2-x)As(2) (T(c) ≈ 11 K). This metastable iron arsenide decomposes at 120 °C and is not accessible by high-temperature solid-state synthesis. X-ray powder diffraction confirms the ThCr(2)Si(2)-type structure, but reveals very small coherently scattering domains with a mean composition Na(0.9(2))Fe(1.7(1))As(2). HRTEM investigations show crystalline as well as strongly distorted areas with planar defects. The latter are probably due to sodium loss and disorder which is also detected by (23)Na solid state NMR. The (57)Fe-M?ssbauer spectrum of Na(1-y)Fe(2-x)As(2) shows one type of iron atoms in tetrahedral coordination. All results point to one crystallographic phase with very small domains due to fluctuations of the chemical composition. From electronic reasons we suggest the superconducting phase is presumably NaFe(2)As(2) in the ordered fractions of the sample.     

Compressibility and structural behavior of pure and Fe-doped SnO2 nanocrystals

We have performed high-pressure synchrotron X-ray diffraction experiments on nanoparticles of pure tin dioxide (particle size ∼30 nm) and 10 mol % Fe-doped tin dioxide (particle size ∼18 nm). The structural behavior of undoped tin dioxide nanoparticles has been studied up to 32 GPa, while the Fe-doped tin dioxide nanoparticles have been studied only up to 19 GPa. We have found that both samples present at ∼13 GPa a second-order structural phase transition from the ambient pressure tetragonal rutile-type structure (P4₂/mnm) to an orthorhombic CaCl₂-type structure (space group Pnnm). No phase coexistence was observed for this transition. Additionally, pure SnO₂ presents a phase transition to a cubic structure at ∼24 GPa. The evolution of the lattice parameters with pressure and the room-temperature equations of state are reported for the different phases. The reported results suggest that the partial substitution of Sn by Fe induces an enhancement of the bulk modulus of SnO₂. Results are compared with previous studies on bulk and nanocrystalline SnO₂. The effects of pressure on Sn-O bonds are also analyzed.