A. I. Boltalin

SYNTHESIS AND CRYSTAL STRUCTURE OF COPPER(II) TRIFLUOROACETATES, CU2(CF3COO)4.2 CH3CN AND CU(CF3COO)2(H2O)4

Cu2(CF3COO)4 · 2 CH3CN (I) and Cu(CF3COO)2(H2O)4 (II) have been prepared by concentrating of acetonitrile and aqueous solutions respectively. According to X-ray data, the complex I consists of binuclear molecules with Cu–O 1.969 A, Cu–N 2.114 A. The Cu…Cu distance was found to be 2.766 A, one of the longest for dimeric structures, apparently, due to the high acidity of trifluoroacetic acid. The coordination environment of Cu atom in II can be described as 4 + 2: 2 Cu–O (H2O) 1.937 A, 2 Cu–O (CF3COO) 1.985 A, 2 Cu–O (H2O) 2.447 A. The mononuclear structure is stabilized by formation of two intra- and six intermolecular hydrogen bonds. Synthese und Kristallstruktur der Kupfer(II)-trifluoroacetat Cu2(CF3COO)4 · 2 CH3CN und Cu(CF3COO)2(H2O)4 Cu2(CF3COO)4 · 2 CH3CN (I) und Cu(CF3COO)2(H2O)4 (II) wurden durch Konzentrieren von Losungen in Acetonitril (I) bzw. Wasser (–10 °C) (II) erhalten. Die Struktur von I besteht aus zweikernigen Komplexen mit Cu–O 1.969 A, Cu–N 2.114 A. Der Cu…Cu-Abstand ist mit 2.766 A fur dimere Strukturen einer der langsten bisher gefundenen, offenbar als Folge der hohen Aciditat der Trifluoressigsaure. Die koordinative Umgebung von Cu in II kann mit 4 + 2 beschrieben werden: 2 Cu–O (H2O) 1.937 A, 2 Cu–O (CF3COO) 1.985 A, 2 Cu–O (H2O) 2.447 A. Die einkernige Struktur wird durch zwei intra- und sechs intermolekulare Wasserstoffbrucken stabilisiert.

Multiple andreev reflections spectroscopy of superconducting LiFeAs single crystals: Anisotropy and temperature behavior of the order parameters

The superconducting state of LiFeAs single crystals with the maximum critical temperature Tc ≈ 17 K in the 111 family has been studied in detail by multiple Andreev reflections (MAR) spectroscopy implemented by the break-junction technique. The three superconducting gaps, ΔΓ = 5.1–6.5 meV, ΔL = 3.8–4.8 meV, and ΔS = 0.9–1.9 meV (at T ≪ Tc), as well as their temperature dependences, have been directly determined in a tunneling experiment with these samples. The anisotropy degrees of the order parameters in the k space have been estimated as <8, ∼12, and ∼20%, respectively. Andreev spectra have been fitted within the extended Kümmel-Gunsenheimer-Nikolsky model with allowance for anisotropy. The relative electron-boson coupling constants in LiFeAs have been determined by approximating the Δ(T) dependences by the system of the two-band Moskalenko and Suhl equations. It has been shown that the densities of states in bands forming ΔΓ and ΔL are approximately the same, intraband pairing dominates in this case, and the interband coupling constants are related as λΓL ≈ λLΓ ≪ λSΓ, λSL.

Synthesis of calcium, strontium, and barium fluorides by thermal decomposition of trifluoroacetates

Alkaline-metal (barium, strontium, and calcium) fluorides with a highly developed specific surface are synthesized by the thermal decomposition of the corresponding metal trifluoroacetates in an inert atmosphere followed by annealing in an oxygen flow at 500°C. The specific surfaces of the synthesized samples are studied by comparative nitrogen adsorption. The resulting specific surface values are 43.67, 36.52, and 23.98 m2/g for CaF2, SrF2, and BaF2, respectively.

Investigation of LiFeAs by means of “break-junction” technique

In our tunneling investigation using Andreev superconductor-normal metal-superconductor contacts on LiFeAs single crystals we observed two reproducible independent subharmonic gap structures at dynamic conductance characteristics. From these results, we can derive the energy of the large superconducting gap ΔL = (2.5–3.4) meV and the small gap ΔS = (0.9–1) meV at T = 4.2 K for the TClocal ≈ (10.5–14) K (the contact area critical temperature which deviation causes the variation of ΔL). The BCS-ratio is found to be 2ΔL/kBTC = 4.6–5.6, whereas 2ΔS/kBTC ≪ 3.52 results from induced superconductivity in the bands with the small gap.

Uniform Patterns of Fe-Vacancy Ordering in the Kx(Fe,Co)(2-y)Se2 Superconductors

The Fe-vacancy ordering patterns in the superconducting KxFe2–ySe2 and nonsuperconducting Kx(Fe,Co)2–ySe2 samples have been investigated by electron diffraction and high angle annular dark field sc...

Alkali metal and ammonium fluoro(trifluoroacetato)metallates M′[M′′3(μ3-F)(CF3COO)6(CF3COOH)3], where M′ = Li, Na, K, NH4, Rb, or Cs and M′′ = Ni or Co. Synthesis and crystal structures

A series of fluoro(trifluoroacetato)metallates were synthesized by crystallization from solutions in trifluoroacetic acid containing nickel(II) or cobalt(II) nitrate hydrates and alkali metal or ammonium fluorides: Li[Ni3(μ3-F)(CF3COO)6(CF3COOH)3](CF3COOH)3 (I), M′[Ni3(μ3-F)(CF3COO)6(CF3COOH)3] (M′ = Na (II), NH4 (IV), Rb (V), and Cs (VI)), NH4[Co3(μ3-F) (CF3COO)6(CF3COOH)3] (III), and Cs[Ni3(μ3-F)(CF3COO)6(CF3COOH)3](CF3COOH)0.5 (VII). The crystal structures of these compounds were determined by single-crystal X-ray diffraction. All structures contain triangular trinuclear complex anions [M3″(μ3-F)(CF3COO)6(CF3COOH)3]− (M″ = Ni, Co) structurally similar to trinuclear 3d metal oxo carboxylate complexes. The three-coordinated F atom is located at the center of the triangle formed by Ni(II) or Co(II) atoms. The metal atoms are linked in pairs by six bridging trifluoroacetate groups located above and below the plane of the [M″3 F] triangle. The oxygen atoms of the axial CF3COOH molecules complete the coordination environment of M″ atoms to an octahedron.

Acid Co(II) and Ni(II) trifluoroacetate complexes: Synthesis and crystal structure

AbstractAnhydrous and partially hydrated acid trinuclear trifluoroacetates of divalent transition metals of the composition [M3(CF3COO)6(CF3COOH)6)](CF3COOH) and [M3(CF3COO)6(CF3COOH)2(H2O)4)](CF3COOH)2, respectively, where M = Co (I, III) Ni (II, IV), were synthesized and studied by X-ray diffraction. Complexes I and II were obtained by crystallization from solutions of M(CF3COO)2 · 4H2O in trifluoroacetic anhydride; complexes III and IV were synthesized under the same conditions with the use of 99% trifluoroacetic acid as a solvent. Crystals I are triclinic: space group

Local structure and hyperfine interactions of 57Fe in NaFeAs studied by Mössbauer spectroscopy.

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