Polar-Covalent Bonding Beyond the Zintl Picture in Intermetallic Rare-Earth Germanides

A comparative chemical bonding analysis for the germanides La₂MGe₆ (M=Li, Mg, Al, Zn, Cu, Ag, Pd) and Y₂PdGe₆ is presented, together with the crystal structure determination for M=Li, Mg, Cu, Ag. The studied compounds adopt the two closely related structure types oS72-Ce₂(Ga_0.1Ge_0.9)₇ and mS36-La₂AlGe₆, containing zigzag chains and corrugated layers of Ge atoms bridged by M species, with La/Y atoms located in the biggest cavities. Chemical bonding was studied by means of the quantum chemical position-space techniques QTAIM (quantum theory of atoms in molecules), ELI-D (electron localizability indicator), and their basin intersections. The new penultimate shell correction (PSC0) method was introduced to adapt the ELI-D valence electron count to that expected from the periodic table of the elements. It plays a decisive role to balance the Ge−La polar-covalent interactions against the Ge−M ones. In spite of covalently bonded Ge partial structures formally obeying the Zintl electron count for M=Mg²⁺, Zn²⁺, all the compounds reveal noticeable deviations from the conceptual 8−N picture due to significant polar-covalent interactions of Ge with La and M ≠ Li, Mg atoms. For M=Li, Mg a formulation as a germanolanthanate M[La₂Ge₆] is appropriate. Moreover, the relative Laplacian of ELI-D was discovered to reveal a chemically useful fine structure of the ELI-D distribution being related to polyatomic bonding features. With the aid of this new tool, a consistent picture of La/Y−M interactions for the title compounds was extracted.     

Artificial pinning in thick YBCO films: Pinning potential and c-axis correlation

We have introduced artificial pinning centres in thick (>1 μm) YBCO films grown by Pulsed Laser Deposition using substrate decoration, quasi-multilayers, and target doping approaches. We have found that the frequency dependence of critical current density is consistent with a logarithmic dependence of pinning potential on current density. For most of materials used as nano-dots, artificially-induced pinning centres have a larger potential than natural ones. From angle-dependent in-field transport measurements and from Transmission Electron Microscopy we have found evidence of c-axis correlated pinning centres.     

Microstructured Hydrogel Templates for the Formation of Conductive Gold Nanowire Arrays

Microstructured hydrogel allows for a new template‐guided method to obtain conductive nanowire arrays on a large scale. To generate the template, an imprinting process is used in order to synthesize the hydrogel directly into the grooves of wrinkled polydimethylsiloxane (PDMS). The resulting poly(N‐vinylimidazole)‐based hydrogel is defined by the PDMS stamp in pattern and size. Subsequently, tetrachloroaurate(III) ions from aqueous solution are coordinated within the humps of the N‐vinylimidazole‐containing polymer template and reduced by air plasma. After reduction and development of the gold, to achieve conductive wires, the extension perpendicular to the long axis (width) of the gold strings is considerably reduced compared to the dimension of the parental hydrogel wrinkles (from ≈1 μm down to 200–300 nm). At the same time, the wire‐to‐wire distance and the overall length of the wires is preserved. The PDMS templates and hydrogel structures are analyzed with scanning force microscopy (SFM) and the gold structures via scanning electron microscopy (SEM) and energy‐dispersive X‐ray spectroscopy. The conductivity measurements of the gold nanowires are performed in situ in the SEM, showing highly conductive gold leads. Hence, this method can be regarded as a facile nonlithographic top‐down approach from micrometer‐sized structures to nanometer‐sized features.

     

A direct measurement of the dissociation energy of water

We have performed a direct measurement of one of the most fundamental thermochemical values: the O-H bond energy in water. Using a triple-resonance laser excitation scheme, we excite the molecule through a series of vibrational overtone transitions to access directly the onset of the dissociative continuum. The dissociation energy obtained from our experiments, 41145.94+/-0.15 cm(-1), is approximately 30 times more accurate than the currently accepted value and has important implications for other thermochemical quantities linked to the bond energy of water.     

A2-web immanants

We describe the rank $3$ Temperley–Lieb–Martin algebras in terms of Kuperberg’s $A_2$-webs. We define consistent labelings of webs and use them to describe the coefficients of decompositions into reduced webs. We introduce web immanants, inspired by Temperley–Lieb immanants of Rhoades and Skandera. We show that web immanants are positive when evaluated on totally positive matrices and describe some further properties.     

An example showing that A-lower semi-continuity is essential for minimax continuity theorems

Recently the authors have established continuity properties of minimax values and solution sets for a function of two variables depending on a parameter. Some of these properties hold under the assumption that the multifunction, defining the domains of the second variable, is $A$-lower semi-continuous. This property is stronger than lower semi-continuity, but in several important cases these two properties coincide. This note provides an example demonstrating that in general the $A$-lower semi-continuity assumption cannot be relaxed to lower semi-continuity.     

Reactive poly(glycidyl methacrylate) microspheres prepared by dispersion polymerization

Poly(glycidyl methacrylate) [poly(GMA)] microspheres of narrow size distribution were prepared in a simple one‐step procedure by dispersion radical polymerization. Depending on the solvent used, poly(GMA) particle size could be controlled in the range of 0.5–4 μm by changing the solubility parameter of the reaction mixture. In N,N′‐dimethylformamide (DMF)/methanol mixture, the particle size increased and the size distribution broadened with decreasing initial solubility parameter. While in the DMF/methanol solvent system, hydroxypropyl cellulose (HPC) or cellulose acetate butyrate (CAB) were taken as steric stabilizers of the dispersion polymerization, poly(vinylpyrrolidone) (PVP) was used in alcoholic media. Contrary to the DMF/methanol system, narrow particle size distributions were obtained with PVP‐stabilized polymerizations in ethanolic, methanolic, propan‐1‐olic or butan‐1‐olic medium. Both the particle size and polydispersity were reduced with increasing stabilizer concentration. If lower molecular‐weight PVP was used, larger microspheres were obtained. Poly(GMA) samples prepared in a neat alcoholic medium virtually quantitatively retained oxirane group content after the polymerization. Reactivity of the poly(GMA) microspheres was confirmed by their hydrolysis and aminolysis. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3855–3863, 2000     

Solitary waves on finite-size antiferromagnetic quantum Heisenberg spin rings

Motivated by the successful synthesis of several molecular quantum spin rings we are investigating whether such systems can host magnetic solitary waves. The small size of these spin systems forbids the application of a classical or continuum limit. We therefore investigate whether the time-dependent Schrödinger equation itself permits solitary waves. Example solutions are obtained via complete diagonalization of the underlying Heisenberg Hamiltonian.     

Ce20Mg19Zn81: a new structure type with a giant cubic cell

Icosacerium nonadecamagnesium henoctacontazinc, Ce₂₀Mg₁₉Zn₈₁, synthesized by fritting of the pure elements with subsequent arc melting, crystallizes with an unusually large cubic unit cell [space group F3m, a = 21.1979 (8) Å] and represents a new structure type among the technologically important family of ternary rare earth–transition metal–magnesium intermetallics. The majority of atoms (two Ce and five Zn) display .3m site symmetry, two Ce and one Mg atom occupy three 2.mm positions, one Mg and one Zn have 3m site symmetry, one Mg and three Zn atoms sit in ..m positions, and one Zn atom is in a general position. The Ce₂₀Mg₁₉Zn₈₁ structure can be described using the geometric concept of nested polyhedral units, by which it consists of four different polyhedral units, viz.A (Zn+Zn₄+Zn₄+Zn₁₂+Ce₆), B (Mg+Zn₁₂+Ce₄+Zn₂₄+Ce₄), C (Zn₄+Zn₁₂+Mg₆) and D (Zn₄+Zn₄+Mg₁₂+Ce₆), with the outer construction unit being an octahedron or tetrahedron. All interatomic distances in the structure indicate metallic‐type bonding.     

A mathematical model for lifetime prediction of lacquer-paint coatings in sea water

Main features of the mechanism of lacquer-paint coatings destruction in sea water have been revealed. On this basis, a mathematical model has been developed and boundary value problems of mathematical physics have been formulated to calculate the determining processes on characteristic stages of coatings in sea water. The theoretical evaluation of coatings longevity depending on physical characteristics of the system has been made. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)