Octahedral Holes Research Articles

Darstellung und Struktur einiger Gemischtvalenter ternärer Tantalnitride mit Lithium und Magnesium

Li 2− x Ta 2+ x N 4 (ca. 0.2< x<ca. 1) and Mg 2.6− x Ta 1.3+ x N 4 (ca. 0.3< x<ca. 1) were synthesized by reaction of Li 3N, or either Mg 3N 2 or Mg(NH 2) 2, with Ta 3N 5. The structure of Li 2− x Ta 2+ x N 4 was determined by single-crystal and that of Mg 2.6− x Ta 1.3+ x N 4 by powder X-ray diffraction. The crystals of both compounds are cubic ( Fm3 m; Z=1; a≈4.31 A ̊ (Li 2− x Ta 2+ x N 4) or a≈4.36 A ̊ (Mg 2.6− x Ta 1.3+ x N 4)) with an NaCl structure (cations disordered). Li 2− x Ta 2+ x N 4 ( x=0.4) can be described as a disordered high temperature phase of Li 2Ta 3N 5. Experiments aimed at attaining the tantalum-rich phase boundary for Li 2− x Ta 2+ x N 4 with x=1 (LiTa 3N 4) and for Mg 2.6− x Ta 1.3+ x N 4 with x=1. 6 (MgTa 3N 4) produced the new hexagonal phases Li 1− x Ta 3+ x N 4 (ca. 0.0⩽ x⩽ca. 0.02; P6 3/ mcm; Z=3; a=5.180 A ̊ ; c=10.343 A ̊ (for x=0.02)) and Mg 1− x Ta 2+ x N 3 (ca. 0.0 ⩽ x⩽ca. 0.06; P6 3/ mcm; Z=4; a=5.205 A ̊ ; c=10.425 A ̊ (for x=0.06)). Powder X-ray and neutron diffraction data were used for the structure determination. The structures are of a filled up 2H-MoS 2 type and contain (TaN 2) 3− layers between which Li + and Ta 5+ or Mg 2+ and Ta 5+ occupy octahedral holes surrounded by nitrogen.

Self‐Consistent‐Field potential‐energy surfaces for hydrogen atom pairs within small palladium clusters

AbstractAn ab initio electronic structure study is presented of hydrogen–hydrogen interactions in an electronic environment perturbed by the presence of palladium atom clusters. In particular, we investigated changes in the interatomic potential when the atomic centers are trapped inside an fcc palladium octahedral hole and when they are separated from each other by a (111) plane of palladium atoms. The (111) plane was modeled with a cluster of three palladium atoms. Self‐consistent field (SCF) level calculations were performed, and palladium atom pseudopotentials were employed to make the systems studied computationally tractable. For pairs of atoms placed within the octahedral hole, various lines of approach were considered [along the (100), (110), and (111) directions]. Lattice deformations and electronic excitations were examined for their effect on the interatomic potential.

Hydrogen-stabilized Mg 2RhH 1.1 with filled Ti 2Ni-type structure

Mg 2RhH 1.1 is the first example for a hydrogen stabilized binary metal compound with filled Ti 2Ni-type structure. Its composition was refined from X-ray and neutron powder diffraction data on the deuteride and found to be Mg 2RhD 1.1. The deuterium atoms in the structure occupy octahedral holes formed by Mg atoms and tetrahedral holes formed by Mg and/or Rh atoms. Retrieval of hydrogen by desorption destabilizes the structure and leads to a hitherto unknown binary metal compound of composition Mg 2Rh which crystallizes with the Ti 2Pd-type structure, a branch of the MoSi 2-type structure.

Structure of Ta3N5 at 16 K by time-of-flight neutron diffraction

Polycrystalline tritantalum pentanitride (Ta3Ns) has been prepared from the reaction of TaC15 with ammonia, and its structure has been refined from time-of-flight neutron diffraction data. M,. = 612-88, orthorhombic, Cmcm, a = 3.8862 (1), b = 10.2118 (2), c = 10.2624 (3) A, V = 407.26 (3) A, 3, Z = 4, D,. = 9.99 g cm-3, neutron time-of-flight, Rietveld refinement, wRp, Rp, reduced X 2= 0.061, 0.043, 4.21 for data collected at 16 K. Ta3N5 crystal- lizes with the pseudobrookite (Fe2TiOs) structure. Irregular TaN6 octahedra share edges and corners. Introduction. Several authors have mapped out the phase diagram of the Ta-N system (Brauer & Zapp, 1954; Sch6nberg, 1954b). y-Ta2N crystallizes with close-packed metal atoms; the N atoms fill the octa- hedral sites, e-TaN has the B35 structure type which consists of very densely packed metal atoms; N atoms fill deformed octahedral holes (Brauer & Zapp, 1953; Christensen & Lebech, 1978). Ta4N5 and TasN6 can be prepared as thin films and have been analyzed by electron diffraction (Terao, 1971). At elevated temperatures, 6'-TaNx forms with the rock- salt structure (Gatterer, Dufek, Ettmayer & Kieffer, 1975). Thin films of 6-TaNo.8_09 are reported with the hexagonal WC structure (Brauer & Mohr- Rosenbaum, 1971). Some of these interstitial nitrides

The structure of K3C60 and the mechanism of superconductivity.

Analysis of the interatomic distances in the superconducting substance K3C60 indicates that each of the K atoms in tetrahedral interstices between C60 spheres accepts three electrons from C60, thus becoming quadricovalent; its four bonds resonate among the 24 adjacent carbon atoms to give a strong framework in which the negative charges are localized on these K atoms. The electric current is carried by the motion of positive charges (holes) through the network of C60 spheres and the K atoms in octahedral holes. Superconductivity is favored by the localization of the negative charges on the tetrahedral K atoms and their noninvolvement in valence-bond resonance, decreasing the rate of mutual extinction of electrons and holes.

Structural studies of organometallic compounds in solution: II. Magnesium bromide and iodide in diethyl ether and tetrahydrofuran; an extended X-ray absorption fine structure (EXAFS) and large angle X-ray scattering (LAXS) study

The structures of the magnesium iodide complexes formed in concentrated diethyl ether and tetrahydrofuran solutions have been determined by the large angle X-ray scattering (LAXS) technique. Those of the more dilute solutions of magnesium bromide in diethyl ether and tetrahydrofuran solutions have been determined by the extended X-ray absorption fine structure (EXAFS) method. The investigation of the MgI 2 diethyl ether solution was carried out at 44°C since this solution crystallizes at about 30°C. This solution is probably best considered a melt, the structure of which can be regarded as a close-packing of iodide ions with magnesium ions occupying some of the holes. The coordination around a specific magnesium ion depends on whether it occupies a tetrahedral or an octahedral hole. The average MgI bond distance is 2.75 Å. Solvated MgI + is the dominating complex in MgI 2tetrahydrofuran solution. Magnesium is probably six-coordinate in this complex, and the MgI bond distance is 2.52(5) Å. The MgBr bond distance is 2.49 and 2.66 Å in diethyl ether and tetrahydrofuran, respectively.

Superconductivity in Layered Structures of Rare‐Earth Carbide Halides

Densely packed double layers of metal atoms (M = Y, La, Lu), C2 groups in the octahedral holes formed by them, and layers of halogen atoms (X) over those of the M atoms provides a good description of the structure of the M2C2X2 phase shown on the right. The finding that these two-dimensional metallic systems can be super-conducting, can contribute enormously to an understanding of the attraction between conducting electrons in superconductors.

Formation of a Cluster Sheath around a Central Cluster by a “Self‐Organization Process”: the Mixed Valence Polyoxovanadate [V34O82]10⊖

An ellipsoid shell consisting of 30 linked tetragonal VO5 pyramids surrounding a distorted {V4O4}O4 cube-this is one way to describe the title compound (right:solid circles, V; empty circles, O). This cluster is not only of topological interest, however. Its magnetic properties make it a molecular model of the monoclinic and the tetragonal VO2 phase. If the cluster structure is roughly regarded as a cubic close packing of oxygen atoms with vanadium atoms occupying the octahedral holes, then its relationship to the solid is clear.

A FT-IR and V-UV Spectroscopic Study of Nickel-Containing Hydrotalcite-Like Compounds, [Ni1−xAlx(OH)2](CO3)x/2.nH2O

Abstract Hydrotalcite-like compounds containing Ni(II) and Al(III) cations and with different Ni/Al ratios in the brucite layers have been prepared and studied using FT-IR and V-UV/DR spectroscopies. It has been found that the local environment of the Ni(II) cations is the same in all cases, occupying octahedral holes, but the orientation of the interlayer carbonate anions changes with the Ni/Al ratio.

Crystal structure and temperature-resolved powder diffractometry of Cd 5(OH) 8(NO 3) 2 · 2H 2O

A new cadmium hydroxide nitrate Cd 5(OH) 8(NO 3) 2 · 2H 2O was prepared by an interdiffusion method. It is monoclinic with the cell parameters a = 18.931(3) Å, b = 6.858(2) Å, c = 5.931(1) Å, β = 94.85(2)°; the space group is C2 m with Z = 2. The crystal structure has been solved from single-crystal data by means of Patterson and Fourier synthesis ( R = 0.043, 2218 hkl). The structure, related to the brucite type, is built up from OH − ion layers parallel to (100) with 3 4 of the octahedral holes filled with cadmium atoms. The remaining metal atoms are located above and below the empty octahedral sites; they are sixfold coordinated by three OH −, one water molecule, and one bidentate nitrate group. The thermal decomposition of this compound was investigated by means of TRXD and TG methods. It proceeds in four or three stages, depending upon the environmental atmosphere. Under vacuum, the hemihydrate Cd 5(OH) 8(NO 3) 2 · 0.5H 2O is obtained during the first stage, whereas a new anhydrous hydroxide nitrate is displayed in the second one.

Synthese und Kristallstruktur von (TeCl3 +)(MoOCl4 –) / Synthesis and Crystal Structure of (TeCl3 +)(MoOCl4 –)

(TeCl3)(MoOCl4) is obtained by the reaction of TeCl4 and MoOCl3 in a sealed ampoule at 180 °C. It forms yellow-green moisture sensitive crystals. The crystal structure determination (lattice constants a = 734.8(1), b = 1281.5(3), c = 1172.2(2) pm, β = 94.62(2)°, monoclinic space group P 21/c, 2808 unique reflections for 92 parameters, final R = 0.029) shows that (TeCl3)(MoOCl4) consists of square pyramidal (MoOCl4 -) anions and (TeCl3 +) cations. The (MoOCl4 -) anions are linked by asymmetric chlorine bridges and form centrosymmetric dimers (MoOCl4 -)2 2- with the oxygen atoms located in the trans positions of the chlorine bridges. The coordination sphere for the Mo atom is a deformed octahedron of one oxygen and five chlorine atoms. Each trigonal pyramidal (TeCl3 +) cation is connected by chlorine bridges to two neighboured (MoOCl4)2 2- anions resulting in a strongly deformed octahedral environment of six chlorine atoms for the Te atom. The MoOCl5 and TeCl6 octahedra are linked by edges and corners and form layers parallel to the be plane. Alternatively the structure can be regarded as a close packing of Cl and O atoms with the layer sequence ABAC, A ··· and Mo and Te atoms located in one quater of the octahedral holes.

New precursors in the chemistry of IVB transition metal alkoxides II. Synthesis and molecular structure of Zr 2Co 4(μ 6-O)(μ 2-OC 3H 7) 8(OC 3H 7) 2(acetylacetone) 4

The title compound crystallizes in the triclinic system (space group P 1 ) with parameters: a=1206.9(2), b=1304.7(3), c=1335.0(3) pm, α=106.32(2), β=113.74(1). γ=102.66(1)°. The crystal structure determination reveals discrete hexanuclear molecules. The Zr 2Co 4O 19 inorganic core adopts a cubic close packing of oxygens, the metal atoms occupying octahedral holes. A μ 6 oxygen atom is located at the center of the metal octahedron.

Low energy Auger electron spectroscopy of vanadium nitrides

The low energy (20–40 eV) region of the Auger spectrum of vanadium nitrides has been studied in order to extend and complement earlier investigations of titanium nitride. The method of using the high-energy spectrum as an internal calibration has also been used in this work which is made easier by the absence of peak overlap between the nitrogen and metal peaks. The background subtraction and peak deconvolution techniques have been considerably extended in this paper. Two methods have been used for representing the background in the derivative spectrum. In the first approach the smooth part of the background has been fitted using the derivative of a Gaussian as the fitting function. After integration of the background subtracted spectrum, the step caused by inelastic scattering was removed by the “Shirley step” removal procedure. In the second method we used the stoichiometric nitride spectrum to represent the background for all VN x samples with x < 1. The different methods were in excellent agreement, strengthening our confidence in the results. The resultant spectra clearly contained two peaks, a large “metal” and a much smaller “nitride” feature, which were deconvoluted using the pure metal peak shape for fitting both peaks. As was the case with TiN x , the intensity of the “metal” V(3p)V(3d)V(3d) peak was linear in x, i.e. the number of unoccupied octahedral holes, whereas the “nitride” V(3p)V(3d)N(2p) cross transition was a maximum for x = 0.5, again correlating with the number of adjacent occupied-unoccupied holes in the nitride structure.

A study on α-RuCl3 crystal structure by scanning tunneling microscopy

From X-ray analysis, the conclusions are drawn from averaged molecular informations. Thus, limitations are caused when analyzing systems whose symmetry is reduced due to interatomic interactions. In contrast, scanning tunneling microscopy (STM) directly images atomic scale surface electron density distribution, with a resolution up to fractions of Angstrom units. The crucial point is the correlation between the electron density distribution and the localization of individual atoms, which is reasonable in many cases. Thus, the use of STM images for crystal structure determination may be permitted. We tried to apply RuCl3 - a layered material with semiconductive properties - for such STM studies. From the X-ray analysis it has been assumed that α-form of this compound crystallizes in the monoclinic space group C2/m (AICI3 type). The chlorine atoms form an almost undistorted cubic closed package while Ru occupies 2/3 of the octahedral holes in every second layer building up a plane hexagon net (graphite net). Idealizing the arrangement of the chlorines a hexagonal symmetry would be expected. X-ray structure determination of isotypic compounds e.g. IrBr3 leads only to averaged positions of the metal atoms as there exist extended stacking faults of the metal layers.

A Comparative Study of the Solubility and Thermodynamics of Hydrogen in Pd1‐xREx (RE = Dy, Gd, Sm, and Y; x = 0.05 and 0.08) Solid Solution Alloys

AbstractPressure‐composition‐temperature absorption isotherms have been determined for the Pd1‐xREx (RE = Dy, Gd, Sm, and Y; x = 0.05 and 0.08) solid solution face centered cubic alloys in the pressure and temperature ranges 2 ≦ P(H2)/mbar ≦ 1000 and 473 ≦ T/K ≦ 873 using the manometric method. The partial molar enthalpy of solution of hydrogen at infinite dilution (ΔH̄0H) becomes more exothermic with increasing content of RE. The partial molar excess entropy of hydrogen solution at infinite dilution (ΔS̄E.0H) decreases with xRE. A correlation of the atomic diameter (dRE) of the substituting rare‐earth metals when in solution in palladium with ΔH̄0H and with the metal‐hydrogen interaction shows that the increase in the exothermicity of the Pd1‐xREx solid solution alloys is due to the lattice expansion and an attractive metal‐hydrogen interaction. From the correlation of ΔH̄0H with dRE of Pd1‐xREx (x = 0.05 and 0.075), ΔH̄0H of Pd1‐xREx (RE = Yb, Ho, and Eu; x = 0.05 and 0.075) solid solution alloys have been predicted. The dependence of ΔH̄0H of the “expanded” and “contracted” Pd1‐xMx alloys on the size of the octahedral interstitial hole sites (rh) occupied by hydrogen atoms is discussed.

Electronic structure of palladium clusters: implications for cold fusion

Electronic structure calculations at the ab initio HF/STO-3G level are reported for the octahedral clusters Pd{sub 6}{sup 2{minus}}, Pd{sub 6}{sup 12{minus}}, Pd{sub 6}H{sup {minus}}, and Pd{sub 6}H{sub 2}; these clusters serve as models for hydrogen (deuterium) in Pd metal. Electrostatic potentials, fields, and field gradients at various positions in the neighborhood of the octahedral hole are discussed. Forces sufficient to force two deuterium nuclei significantly closer than in diatomic D{sub 2} appear unlikely.

Solubility of hydrogen in Pd2Er5, Pd2Er3, PdEr, Pd4Er3, and Pd3Er: Thermodynamics of Pd —Er —H Systems

AbstractThe solubility of hydrogen in the intermetallic compounds Pd2Er5, Pd2Er3, PdEr, Pd4Er3, and Pd3Er has been measured by the pressure reduction method in the ranges 1 ≤ P/mbar ≤ 1000 and 473 ≤ T/K ≤ 1148. The partial molar enthalpy of solution at infinite dilution (ΔH0H) and the partial molar excess entropy of solution at infinite dilution (ΔSE0H) have been determined from the α‐phase solubility isotherms. |H0H| is found to decrease with increase of palladium content in the following order: Pd2Er5(‐71.8 kJ ‐ mol−1), Pd2Er3 (–61.0 kJ ‐ mol−1), PdEr (‐18.9 kJ · mol−1), and Pd4Er3 (‐9.2 kJ · mol−1). In Pd3Er, however, ΔH0H is −45.1 kJ · mol−1. The extrapolation of ΔH0H, =f(xEr) in solid solution of erbium in palladium to xEr = 0.25 gives ΔH0H = –37.3 kJ · mol−1, whereas, ΔH0H = –45.1 kJ · mol−1 is observed in Pd3Er, indicating that the formation of the ordered structure results in an additional increase of exothermicity of hydrogen absorption reaction. The increase of exothermicity of hydrogen absorption in Pd3Er when compared to PdEr may be attributed to the smaller octahedral interstitial hole size. ΔSE0H is found to lie between −59.0 and −64.0 J · K−1 · mol−1. Pd2Er5, Pd2Er3, and PdEr show decomposition reactions by which erbium hydride and the next intermetallic compound in the Pd–Er phase diagram with less content of erbium are formed. The decomposition pressure at any temperature is found to increase with increase of palladium content, suggesting that the stability of the compounds increases with increase of palladium content at least till PdEr. The heats of formation of Pd2Er3 and Pd4Er3 have been obtained from the present work and compared with the calculated ones.

LEED crystallographic investigations of two surface structures designated Zr(0001)-(1 × 1)-C

Multiple scattering analyses of LEED intensities have been made for two Zr(0001)-(1 × 1)-C surfaces corresponding to a low and a high coverage of C. For the first, a moderate level of correspondence between experimental and calculated I( E) curves is reached when the C atoms occupy octahedral holes between the first and second metal layers to give a ZrC bond distance of 2.29 Å. In this case the absorbed C causes only a 3.5% increase in the neighbouring ZrZr interlayer spacing. However, the normal incidence study for the high-coverage surface suggests that this spacing is then increased by about 25% over the bulk value; such an expansion is consistent with C incorporation into tetrahedral hole underlayer sites.

Remarks on a ternary phase in the La 2O 3Me 2O 5Li 2O system ( Me= Nb, Ta)

A phase belonging to the ternary system La 2O 3-Li 2O-Me 2O 5 ( Me= Nb, Ta) was prepared and characterized both chemically and structurally. It has cubic symmetry (space group Ia3d), a 0= 13 A ̊ , gross formula La 3Li 5Me 2O 12 and it shows a structure based on the garnet oxygen framework, but with two unusual features. Firstly lithium atoms enter the octahedral holes centered at 1 4 , 1 4 , 1 4 inside the unit cell (elsewhere empty in the normal garnets) and secondly a large trivalent cation like La 3+ is supported for the first time by a garnet-like structure. This could influence possible ferroelectric properties of the material.

Polymorphie von Tellur(IV)-iodid/ Polymorphism of Tellurium(IV) Iodide

Abstract Idiomorphous crystals of five different polymorphic modifications (α-ε) of TeI4 are simultaneously grown from solutions of TeI4 and concentrated HI in methanol by evaporating the solvent at room temperature. δ-TeI4 is the only stable phase at normal conditions. Phase transformations of α-, β-, γ-and ε-TeI4 to the final stage δ-TeI4 take place by heating and run through discrete intermediates. The observed graduations lead to a sequence γ-, β-, α-,ε-TeI4 which indicates “increasing metastability”. The crystal structures of α-TeI4 (trigonal; P3̄ml; a = 4.228(2), c - 6.684(6)Å, Z = 0.5; Dx = 5.10 g/cm 3), β-TeI4 (orthorhombic; Pn21m; a = 6.888(2), b = 14.539(3), c = 16.753(4) Å, Z = 8; Dx = 5.03 g/cm3) and γ-TeI4 (monoclinic; P21/c; a = 11.199(4), b = 13.599(4), c = 22.158(6) Å,β = 98.10(3)°, Z = 16; D, = 5.05 g/cm3) are related to the known crystal structure of δ-TeI4. α-TeI4 (“Te0.5I2”) is an isotype of the 2H-CdI : structure with random distribution of Te over the Cd-positions. The crystal structures of β-, γ-and δ-TeI 4 contain discrete tetrameric molecules “(TeI4)2 (TeI3 +I-)2” which are generated by ordered distributions of Te over 1/4 of the octahedral holes of a 2H-(β) and a 4H-sequence (y,δ ) of the nearly close packed iodine layers. A nearly cubic close packing of Iodine atoms is observed in the crystal structure of ε-TeI4 (tetragonal; I41/amd; a = 16.875(6), c -11.829(5) Å, Z = 16; Dx = 5.01 g/cm 3). The ordered distribution of Te in 1/4 of the octahedral holes leads to tetrameric molecules “(TeI3 +I-)4” in a cubanelike arrangement which, thus far, has been observed only in the crystal structures of TeCl4 and TeBr4 . The Madelung Parts of Lattice Energies, MAPLE, of β-, γ-, δ-and ε-TeI4 are calculated.