Li- and Mg-doping into icosahedral boron crystals, α- and β-rhombohedral boron, targeting high-temperature superconductivity: structure and electronic states

The possibility of high T_C superconductivity is suggested for lithium- and magnesium-doped icosahedral boron crystals, α- and β-rhombohedral boron. The doping of these elements was attempted by a vapor diffusion processing. Both lithium and magnesium are hardly doped into the α-rhombohedral boron, although small amounts of metallic parts are found in the sample. In only one Li-doped sample, the metallic part contained 0.02 vol% of the superconductive phase (T_C∼36 K). Magnesium was successfully doped into β-rhombohedral boron homogeneously up to 4 at% (Mg_4.1B₁₀₅), although considerable amount of impurity silicon was introduced together with magnesium. The structures of the doped samples were analyzed assuming co-doping of magnesium and silicon. The relation between the site occupancies of the dopants and the lattice expansion is discussed. The estimation of the density of states near the Fermi energy by EELS and magnetic susceptibility measurements suggested a metal transition of the β-rhombohedral boron by the doping of magnesium and silicon. The relation between the metal transition and the intrinsic acceptor level is also discussed.     

Icosahedral B12, macropolyhedral boranes, β-rhombohedral boron and boron-rich solids

The preference for icosahedral B₁₂ amongst polyhedral boranes and elemental boron is explained based on an optimization of overlap model. The ingenious ways in which elemental boron and boron-rich solids achieve icosahedron-related structures are explained by a fragment approach. The Jemmis mno rules are used to get the electron requirements. The extra occupancies and vacancies in β-rhombohedral structures are shown to be inevitable results of electron requirements. The detailed understanding of the structure suggests ways of doping β-rhombohedral boron with metals for desired properties. Theoretical studies of model β-rhombohedral solids with metal dopings provide support for the analysis.     

High-energy-resolution electron energy-loss spectroscopy study of the electronic structure of Cu- and Mg-Si-doped β-rhombohedral boron crystals

Electronic structures of Cu and Mg-Si-doped β-rhombohedral boron (β-r-B) crystals were studied by using a high-energy-resolution electron energy-loss spectroscopy microscope. Boron 1s electron excitation spectra, which show the density of states of the conduction bands, of the crystals were obtained from single crystalline areas of 100 nm in diameter. The spectrum of Cu-doped β-r-B showed a chemical shift to a lower binding energy side. It means an electron transfers from the doped Cu atoms to B atoms. The intensity distributions of the spectrum was almost the same as that of the non-doped β-r-B, which suggests that all of the doped electrons occupy the intrinsic acceptor level just above the valence bands. The spectrum of Mg-Si-doped β-r-B showed not only a chemical shift to a lower binding energy side but also a sharp intensity increase at the onset with a width of an energy resolution of the experiment. The sharp onset may be assigned to a Fermi edge. It indicates that the doped electrons fill up the acceptor level and occupy the conduction bands forming the Fermi edge, a metallization of β-r-B by the Mg-Si-doping.     

On the diffusion of free carriers in β-rhombohedral boron

To determine the diffusion of untrapped carriers in β-rhombohedral boron, we constructed a feedback pico-ammeter based on pulse integration technique. This enabled measuring deviations from the bias in a 10⁹ Ω sample in the order of 1 nA with 0.7 ms time resolution. For the first time, we obtained the drift velocity of optically generated untrapped electron-hole pairs 106(20) cm s⁻¹ yielding for the band-determined diffusion coefficient and for the carrier mobility . Fitting Fick's second law to the measured trap-determined dispersion of carriers yields the ambipolar diffusion coefficient D^*=0.043(14) and 0.28(10) cm² s⁻¹ at 260 and 340 K, respectively. The thermal activation energy of 0.18 eV agrees with the well-known trapping levels in β-rhombohedral boron.     

Physical-mechanical properties of Zr-(Re)-doped β-rhombohedral boron

Effect of doping with Zr(Re) on the structure and physical-mechanical properties of β-rhombohedral boron has been studied. In all specimens p-type conductivity was found. Internal friction and dynamic shear modulus of the specimens were investigated at frequencies of torsion oscillations (0.5-5 Hz) in the temperature range 80<T<1000 K. The increase of Zr(Re) concentration in the samples results in increase of their hole concentration, this increasing and shifting the observed IF maxima to lower temperatures; activation energy of the maxima and frequency factor of the relaxation processes decrease by 10-15%. Effects of change of the structure-sensitive properties observed in Zr-(Re)-doped boron are analyzed in view of changes of activation energy necessary for the motion of twinning boundaries and stacking faults.     

Refinement of α-CsB9O14 crystal structure

The structure of α-CsB₉O₁₄ was re-examined because the first determination corresponded to a poor reliability factor (12.9%). Single crystals were obtained by heating, melting and slow cooling a stoichiometric mixture (1:4) of β-Cs[B₅O₆(OH)₄]·2H₂O and H₃BO₃. This compound crystallizes in the non-centrosymmetric orthorhombic space group P222₁ (and not P4₁22) with the following parameters: a=8.732(2)Å, b=8.767(3)Å, c=15.736(4)Å, V=1204.6(6)ų, Z=4; after taking into consideration twinning, the structure was refined from 3188 reflections until R₁=0.0304. It consists of two infinite, interleaved three-dimensional boron-oxygen frameworks of the Fundamental Building Blocks formed by two B₃O₆ and one B₃O₇ groups; its shorthand notation is 9:∞³[(3:2Δ+T)+2(3:3Δ)] (Δ, triangle BO₃ and T, tetrahedron BO₄). Knowledge of the correct space group and the structure of α-CsB₉O₁₄ may help in the study of its physical properties, especially the non-linear optical ones.     

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.     

Synthesis and crystal structure of β-hexanitrohexaazaisowurtzitane

A polycyclic caged compound with high strain—hexanitrohexaazaisowurtzitane (HNIW)—has been synthesized via a three-step reaction: condensation, hydrogenolysis debenzylation and nitrolysis, starting with benzylamine and glyoxal. HNIW is the most powerful high energy density compound (HEDC) ever tested. β-HNIW possesses a caged structure consisting of two five-membered rings and one six-membered ring with a nitro group attached to each of the six bridging nitrogens. The nitro group lies basically within a plane. The lengths of C—C bonds of β-HNIW range from 0. 156 nm to 0.159 nm, 0.002–0.005 nm longer than the sp³ C-C bond. The β-HNIW’s crystal belongs to orthorhombic system and space groupPca2₁ with parameters:a = 0.9670 (2),b = 1.1616 (2),c = 1.3032 (3) nm;V = 1.4638(5) nm³,Z = 4; D_c = 1.989 g/cm³ and D_m = 1.982 g/cm³. Project supported by the Advanced Research Funds (12060451867) from the Commission of Science and Technology for National Defence.     

Synthesis and crystal structure of β-hexanitrohexaazaisowurtzitane

A polycyclic caged compound with high strain——hexanitrohexaazaisowurtzitane (HNIW)——has been synthesized via a three-step reaction: condensation, hydrogenolysis debenzylation and nitrolysis, starting with benzylmine and glyoxal. HNIW is the most powerful high energy density compound (HEDC) ever tested. β-HNIW possesses a caged structure consisting of two five-membered rings and one six-membered ring with a nitro group attached to each of the six bridging nitrogens. The nitro group lies basically within a plane. The lengths of C--C bonds of β-HNIW range from 0.156 nm to 0.159 nm, 0.002--0.005 nm longer than the sp~3 C-C bond. The β-HNIW's crystal belongs to orthorhombic system and space group Pca2_1 with parameters: a=0.9670 (2), b=1.1616 (2), c=1.3032 (3) nm; V=1.4638(5) nm~3, Z=4; D_c=1.989 g/cm~3 and D_m=1.982 g/cm~3.     

The crystal structure of π-ErBO3: New single-crystal data for an old problem

Single crystals of the orthoborate π-ErBO₃ were synthesized from Er₂O₃ and B₂O₃ under high-pressure/high-temperature conditions of 2 GPa and 800 °C in a Walker-type multianvil apparatus. The crystal structure was determined on the basis of single-crystal X-ray diffraction data, collected at room temperature. The title compound crystallizes in the monoclinic pseudowollastonite-type structure, space group C2/c, with the lattice parameters a=1128.4(2) pm, b=652.6(2) pm, c=954.0(2) pm, and β=112.8(1)° (R₁=0.0124 and wR₂=0.0404 for all data).     

Synthesis, crystal growth, and structure of Ta3Al2CoC—An ordered quarternary cubic η-carbide and the first single crystal study of a η-carbide

Single crystals of the new carbide Ta₃Al₂CoC were synthesised from metallic melt and characterized by XRD, EDX and WDX measurements. The crystal structure of Ta₃Al₂CoC was refined on the basis of single crystal data (cF112, Fd3¯m, a=11.6153(13) Å, Z=16, 169 reflections, 13 parameters, R₁(F)=0.0315, wR₂(F²)=0.0857). Ta₃Al₂CoC belongs to the great family of η-carbides or which are important components for cermets. Its crystal structure is characterised by TaC₆-octahedra, which are connected to a three-dimensional net. Co and Al have icosahedral surroundings without contacts to C-atoms. All positions show full occupation. Ta₃Al₂CoC represents the first η-carbide with a complete structure refinement on the basis of single crystal data.     

Synthesis and X-ray crystal structure of 3-deoxy-3-fluoro-β-D-allopyranoside

The synthesis of a novel compound, 3-deoxy-3-fluoro--d-allopyranoside, is described. It involves blocking the 4- and 6-positions of allose to cause fluorination only at the 3-position. This six-step synthesis gave pure crystalline compound in a 5% overall yield. The crystal structure, determined by X-ray diffraction, confirmed the chemical formula and configuration of the product. It showed that the C-F bond isaxial, while the C-(OH) bonds areequatorial. The overall conformation of this fluoroallose is similar to that of allose (Acta Cryst.1984,C40, 1863) (with OH instead of F), but the packing in the two crystals is different in the region of C3, the site of chemical variation.     

Synthesis, X-ray crystal structure, and reactivity of Pd2(μ-dotpm)2 (dotpm = bis(di-ortho-tolylphosphino)methane)

Alkylation of PdCl₂(dotpm) (dotpm = bis(di-ortho-tolylphosphino)methane) with n-butyllithium produces the binuclear Pd(0) complex Pd₂(μ-dotpm)₂ and the elimination byproducts 1-butene, cis-2-butene, trans-2-butene, butane, and octane. The dibutyl complex, Pd(dotpm)(n-Bu)₂, is presumed to be the reaction intermediate. The crystal structure of Pd₂(μ-dotpm)₂ reveals that the methylene groups of the bridging dotpm ligands are located on opposite sides of the Pd₂P₄ unit, forming an 8-membered ring that is in an elongated chair conformation. The four phosphorus atoms are not coplanar, and the P1-P2-P3-P4 ring has a torsion angle of 13.8°, which minimizes the spatial interactions among the o-tolyl rings. The Pd-Pd bond distance is 2.8560(6) Å, which indicates that there is a weak “closed-shell” bonding interaction between the d¹⁰-d¹⁰ metal centers. Each palladium atom has a nearly linear geometry, and the eight methyl groups of the dotpm ligands shield the open coordination sites on the metal centers. Four methyl groups shield the metal atoms above and below the Pd₂P₄ ring cavity, and four methyl groups block the open metal sites outside of the Pd₂P₄ ring. The Pd₂(μ-dotpm)₂ complex readily undergoes oxidative addition of dichloromethane to form the rigid A-frame complex Pd₂Cl₂(μ-CH₂)(μ-dotpm)₂.     

Ni7−δSnTe2: Modulated crystal structure refinement, electronic structure and anisotropy of electroconductivity

Single crystals of Ni_7−δSnTe₂ were grown during re-crystallization of the presynthesized powder in a two zone furnace. The modulated structure was solved and refined in the (3+2)-dimensional superspace group I4/mmm(0-α0, α00)0.ss.mm with lattice parameters a=3.759(1) and c=19.410(2) Å (measured at 153 K) and Z=2. Satellite reflections observed in the diffraction images can be assigned to the incommensurate modulation vectors q₁=da* and q₂=db* with d=0.410(1). The composition resulting from X-ray structure refinement is Ni_5.81SnTe₂. The structure model has been also developed in the orthorhombic (3+1)-dimensional superspace group Immm(α00)00s assuming twinning according to [110], giving thus the composition Ni_5.79SnTe₂. The origin of the modulation can be attributed to a variation of the occupancy of the Ni(3) site in Ni/Te slabs of the structure. Band structure calculations on a commensurate approximant and single crystal electrical resistivity measurements reveal anisotropic metallic conductivity for this compound.     

Volatile complexes of Pd(II) with methoxy-β-iminoketones: Synthesis,properties, and crystal and molecular structure

This paper describes a procedure for the synthesis of two new volatile complexes, Pd(L¹)₂ and Pd(L²)₂, from sterically hindered methoxy-β-iminoketones, where HL¹ = C(CH₃)₂(OCH₃)-C(NH)-CH₂-C(O)-C(CH₃)₃; HL² = C(CH₃)₂(OCH₃)-C(NH)-CH₂-C(O)-CH(CH₃)₂. Element analysis and IR spectral data are given. The results of full X-ray crystal structure analysis of the complexes are reported. The compounds have molecular structures; the crystals of the complexes have different symmetry groups and unit cell dimensions. The Pd(L¹)₂ complex molecule has a nonplanar structure; the Pd(L²)₂ complex has a cis-structure. The geometrical characteristics obtained for the coordination units are as follows: the Pd-O and Pd-N bond lengths and N-Pd-O chelate angles were estimated at 1.960 Å, 93.7° for Pd(L¹)₂, and 1.984 Å, 1.976 Å, 92.4° for Pd(L²)₂.     

Crystal growth, crystal structure of new polymorphic modification, β-Bi2B8O15 and thermal expansion of α-Bi2B8O15

Single crystals of α- and β-polymorphs of Bi₂B₈O₁₅ were grown by Czochralski method from a charge of the stoichiometric composition. The crystal structure of β-Bi₂B₈O₁₅ was solved by direct methods from a twinned crystal and refined to R1=0.081 (wR=0.198) on the basis of 1584 unique observed reflections (I>2σ(I)). The compound is triclinic, space group , a=4.3159(8), b=6. 4604(12), c=22.485(4) Å, α=87.094(15)°, β=86.538(15)°, γ=74.420(14)°, V=602.40(19) Å³, Z=2. The B-O layered anion of β-Bi₂B₈O₁₅ is topologically identical to the anion of α-Bi₂B₈O₁₅ but the orientation of neighboring layers is different. Thermal expansion of α-Bi₂B₈O₁₅ has been investigated by X-ray powder diffraction in air in temperature range from 20 to 700 °C. It is strongly anisotropic, which can be explained by the hinge mechanism applied to chains of Bi-O polyhedra. While the anisotropy of thermal expansion is rather high, the volume thermal expansion coefficient α_V=40×10⁶ °C⁻¹ for α-Bi₂B₈O₁₅ is close to those of other bismuth borates.     

Synthesis, crystal and electronic structure, and theoretical investigations of charge-transfer, optical, and bonding properties of a [VF6]3? anion compound

Abstract  

A green-colored V(III) compound, imidazolium hexafluorovanadium(III), [C₃H₅N₂)]₃[VF₆], has been prepared and characterized. The geometric and electronic structure, together with charge-transfer, optical, and bonding properties, were thoroughly investigated by X-ray crystallography, density functional theory (DFT), and time-dependent density functional theory (TDDFT) calculations. The low-energy charge-transfer bands responsible for its green color may be theoretically assigned to a F(2p) → V(4p) ligand-to-metal charge-transfer transition. The ligand-field charge-transfer bands (d → d bands) occur at a lower energy region; they are too weak and will be obscured by the quite intense ligand-to-metal bands in the optical spectrum. Partial density of states analysis clearly shows that the nature of metal–ligand interactions in [VF₆]³⁻ is mainly ionic.     

Synthesis, structure and ring-opening polymerization of macrocyclic arylates containing phthalic unit

A series of maerocyclic arylate diraers have been selectively synthesized by an interfacial polyconden-sation of o-phthaloyldichloride with bisphenols A combination of GPC,FAB-MS,1H and 13C NMR unambiguously confirmed the cyclic nature Although single-crystal X-ray analysis of two such macrocycles reveals no severe strain on the cyclic structures,these macrocycles can undergo facile melt polymerization to give high molecular weight polyary-lates.     

Synthesis and x-ray crystal structure of a stable α-chlorooxetane

The Barton modification of the Hunsdiecker reaction is the key step in the preparation of 3,5-anhydro-5R-chloro-1,2-O-isopropylidenexylofuranose (1), a stable α-chlorooxetane.