Ba2[Ni3N2]: a low-valent nitridonickelate-synthesis, crystal structure, and physical properties

The ternary alkaline-earth nitridonickelate Ba2[Ni3N2] (Ba2[NiI2Ni0N2]) was prepared by the reaction of mixtures of Ba2N and Ni in nitrogen gas of ambient back-pressure at 1173 K. The crystal structure determined by X-ray single-crystal and powder diffraction methods as well as from neutron diffraction data at various temperatures between 2 and 298 K is orthorhombic (Cmca (no. 64), 298 K: a=715.27(18) pm, b=1032.99(21) pm, c=740.12(20) pm) and provides the first example of a nitridonickelate with a two-dimensional complex anion. The Ni2 atom is described with a split position and the corresponding superstructure variants are investigated by theoretical full-potential nonorthogonal local-orbital calculations (FPLO). The average oxidation state of Ni in Ba2[Ni3N2] is +0.67, the lowest average value observed in nitridonickelates so far. Investigations of the physical properties demonstrate that Ba2[Ni3N2] acts as a "poor" metal with a large resistivity of approximately 2.7 mOmega cm at 300 K and exhibits low-dimensional magnetism with antiferromagnetic ordering at T approximately 90 K. XAS spectra correspond with low-valent Ni states.     

Poly(2,5-dimercapto-1,3,4-thiadiazole) as a cathode for rechargeable lithium batteries with dramatically improved performance

Organosulfur compounds with multiple thiol groups are promising for high gravimetric energy density electrochemical energy storage. We have synthesized a poly(2,5-dimercapto-1,3,4-thiadiazole) (PDMcT)/poly(3,4-ethylenedioxythiophene) (PEDOT) composite cathode for lithium-ion batteries with a new method and investigated its electrochemical behavior by charge/discharge cycles and cyclic voltammetry (CV) in an ether-based electrolyte. Based on a comparison of the electrochemical performance with a carbonate-based electrolyte, we found a much higher discharge capacity, but also a very attractive cycling performance of PDMcT by using a tetra(ethylene glycol) dimethyl ether (TEGDME)-based electrolyte. The first discharge capacity of the as-synthesized PDMcT/PEDOT composite approached 210 mAh g(-1) in the TEGDME-based electrolyte. CV results clearly show that the redox reactions of PDMcT are highly reversible in this TEGDME-based electrolyte. The reversible capacity remained around 120 mAh g(-1) after 20 charge/discharge cycles. With improved cycling performance and very low cost, PDMcT could become a very promising cathode material when combined with a TEGDME-based electrolyte. The poor capacity in the carbonate-based electrolyte is a consequence of the irreversible reaction of the DMcT monomer and dimer with the solvent, emphasizing the importance of electrolyte chemistry when studying molecular-based battery materials.     

Structural investigations on YbRh2Si2: from the atomic to the macroscopic length scale

YbRh2Si2 has advanced to a prototype material for investigating physics related to the Kondo effect. An optimization of the synthesis resulted in single crystals of extraordinary crystalline quality. At the atomic scale, we utilize scanning tunneling microscopy to study the topography of cleaved single crystals. A structural and chemical analysis was conducted by highly accurate x-ray diffraction and wavelength dispersive x-ray spectroscopy measurements. The latter indicate a homogeneity range of the YbRh2Si2 phase between approximately 40.0–40.2 at.% Rh. For our high-quality samples the number of defects found on the atomic scale (of the order of 0.3% of the visible lattice sites) is in quantitative agreement with a very small off-stoichiometry within this homogeneity range. Comparing our results for these samples allows an assignment of the structural defects observed at the cleaved surfaces to Rh occupying Si sites and, even less numerous Si in Rh sites. Such an analysis is hampered for samples of lesser quality, but there seem to be numerous empty Si-sites. Based on these observations the results of scanning tunneling spectroscopy can be analyzed in further detail and provide insight into the Kondo physics.     

New Rare‐Earth Metal Germanides RE2Ge9 (RE = Nd,Sm) by Thermal Decomposition of High‐Pressure Phases REGe5

The rare‐earth metal germanides RE₂Ge₉ (RE = Nd, Sm) have been prepared by thermal decomposition of the metastable high‐pressure phases REGe₅ at ambient pressure. The compounds adopt an orthorhombic unit cell with a = 396.34(4) pm; b = 954.05(8) pm and c = 1238.4(1) pm for Nd₂Ge₉ and a = 395.46(7) pm; b = 946.4(2) pm and c = 1232.1(3) pm for Sm₂Ge₉. Crystal structure refinements reveal space group Pmmn (No. 59) for Nd₂Ge₉. The atomic pattern resembles an ordered defect variety of the pentagermanide motif REGe₅ (RE = La; Nd, Sm, Gd, Tb) comprising corrugated germanium layers. These condense into a three‐dimensional network interconnected by eight‐coordinated germanium atoms. The resulting framework channels along [100] enclose the neodymium atoms. With respect to the atomic arrangement of the pentagermanides, half of the interlayer germanium atoms are eliminated in an ordered way so that occupied and empty germanium columns alternate along [001]. The rare‐earth metal atoms of both types of compounds, REGe₅ and RE₂Ge₉, exhibit the electronic states 4f ³ and 4f ⁵ (oxidation state +3) for neodymium and samarium, respectively, evidencing that the modification of the germanium network leaves the electron configuration of the metal atoms unaffected.     

An empirical model predicting the viscosity of highly concentrated, bimodal dispersions with colloidal interactions

The relationship between particle size distribution and viscosity of concentrated dispersions is of great industrial importance, since it is the key to get high solids dispersions or suspensions. The problem is treated here experimentally as well as theoretically for the special case of strongly interacting colloidal particles. An empirical model based on a generalized Quemada equation is used to describe η as a function of volume fraction for mono- as well as multimodal dispersions. The pre-factor η˜ accounts for the shear rate dependence of η and does not affect the shape of the η vs φ curves. It is shown here for the first time that colloidal interactions do not show up in the maximum packing parameter and φ_max can be calculated from the particle size distribution without further knowledge of the interactions among the suspended particles. On the other hand, the exponent ɛ is controlled by the interactions among the particles. Starting from a limiting value of 2 for non-interacting either colloidal or non-colloidal particles, ɛ generally increases strongly with decreasing particle size. For a given particle system it then can be expressed as a function of the number average particle diameter. As a consequence, the viscosity of bimodal dispersions varies not only with the size ratio of large to small particles, but also depends on the absolute particle size going through a minimum as the size ratio increases. Furthermore, the well-known viscosity minimum for bimodal dispersions with volumetric mixing ratios of around 30/70 of small to large particles is shown to vanish if colloidal interactions contribute significantly. Received: 7 June 2000/Accepted: 12 February 2001     

Excited-State Kinetics of an Air-Stable Cyclometalated Iron(II) Complex

The complex class [Fe(N^N^C)(N^N^N)]⁺ with an Earth-abundant metal ion has been repeatedly suggested as a chromophore and potential photosensitizer on the basis of quantum chemical calculations. Synthesis and photophysical properties of the parent complex [Fe(pbpy)(tpy)]⁺ (Hpbpy=6-phenyl-2,2′-bipyridine and tpy=2,2′:6′,2′′-terpyridine) of this new chromophore class are now reported. Ground-state characterization by X-ray diffraction, electrochemistry, spectroelectrochemistry, UV/Vis, and X-ray spectroscopy in combination with DFT calculations proves the high impact of the cyclometalating ligand on the electronic structure. The photophysical properties are significantly improved compared to the prototypical [Fe(tpy)₂]²⁺ complex. In particular, the metal-to-ligand absorption extends into the near-IR and the ³MLCT lifetime increases by 5.5, whereas the metal-centered excited triplet state is very short-lived.     

A measurement of differential cross-sections and nucleon structure functions in charged-current neutrino interactions on iron

A high-statistics measurement of the differential cross-sections for neutrino-iron scattering in the wide-band neutrino beam at the CERN SPS is presented. Nucleon structure functions are extracted and theirQ ² evolution is compared with the predictions of quantum chromodynamics.     

Unbinding of three strings in the Born-Oppenheimer approximation

The unbinding transition of three strings, fluctuating without overhangs in two dimensions, is considered in the Born-Oppenheimer approximation. The calculation, which is extremely simple, gives some insight into the physical considerations that determine the universality class. In the case of short-range pair interactions the unbinding transition is predicted to be universal, with the same critical exponents as in the unbinding of two strings. For strings bound by short-range three-body interactions the unbinding transition is nonuniversal. Both of these predictions are in agreement with recent exact studies.Dedicated to H. Wagner     

Mössbauer studies of polymers in the glassy state and near their glass transition

Mössbauer absorption spectra of glassy block and copolymers with lowT _G containing ferrocene have been recorded in a wide temperature range (4–330 K) to investigate the dynamical processes of the system, in the glassy state and around the glass transition. An inhomogeneous broadening due to conformational substates with different hyperfine parameters found at lower temperatures becomes motionally narrowed above 40 K. Whereas in the glassy state both the block and the copolymers reveal the same vibrational behaviour, we found differences in the vicinity of their respective glass transitions. For the block polymer an anomalous decrease of the recoilfree fractionf _A was found above the static glass transition. In agreement with neutron data on other polymers we attribute this behaviour to an onset of motions of the ferrocene units of higher than 10¹⁰ Hz. For the copolymers, however, a departure of lnf _A from linearity was detected already far below the staticT _G. We propose that this is caused by residual solvents which allow an easier cooperative rearrangement of the segments even belowT _G. First attempts to interpret the Mössbauer data by means of the predictions of the mode coupling theory are reported.