Guanidinate-stabilized monomeric hafnium amide complexes as promising precursors for MOCVD of HfO2

Novel guanidinato complexes of hafnium [Hf{eta2-(iPrN)2CNR2}2(NR2)2] (R2 = Et2, 1; Et, Me, 2; Me2, 3), synthesized by insertion reactions of N,N'-diisopropylcarbodiimide into the M-N bonds of homologous hafnium amide complexes 1-3 and {[mu2-NC(NMe2)2][NC(NMe2)2]2HfCl}2 (4) using a salt metathesis reaction, are reported. Single-crystal X-ray diffraction analysis revealed that compounds 1-3 were monomers, while compound 4 was found to be a dimer. The observed fluxional behavior of compounds 1-3 was studied in detail using variable-temperature and two-dimensional NMR techniques. The thermal characteristics of compounds 1-3 seem promising for HfO2 thin films by vapor deposition techniques. Metal-organic chemical vapor deposition experiments with compound 2 as the precursor resulted in smooth, uniform, and stoichiometric HfO2 thin films at relatively low deposition temperatures. The basic properties of HfO2 thin films were characterized in some detail.     

On dielectrics of polymer electrolytes studied by impedance spectroscopy

We present a phenomenological view on dielectric relaxation in polymer electrolytes in the frequency range where conductivity is independent of frequency. Polymer electrolytes are seen as molecular mixtures of an organic polymer and an inorganic salt. The discussion applies also to ionic liquids. The following is based on systems with poly(ethylene oxide) (PEO) comprising the lithium perchlorate salt (LiClO₄) and also pure low-molecular PEO. In those systems, dipole-dipole interactions form an association/dissociation equilibrium which rules properties of the system in the low-frequency region. It turns out that effective concentration, c _S, of relaxing species provides a suitable variable for discussing electrochemical behavior of the electrolytes. Quantity c _S is proportional to the ratio of DC conductivity and mobility. Polymer salt mixtures form weak electrolytes. However, diffusion coefficient and corresponding molar conductivity display the typical (c _S)^1/2 dependence as well known from strong electrolytes, due to the low effective concentration c _S.     

Der austausch des cyclopentadienylliganden im methylcyclopentadienylmangan(I)tricarbonyl. Untersuchungen zur konkurrierenden komplexbildung mit methylsubstituierten benzolen

Exchange of the cyclopentadienyl ligand in methylcyclopentadienylmanganese(I) tricarbonyl with methyl-substituted benzenes is catalysed by AlCl₃ + HCl and leads to the cations [arene-Mn(CO)₃]⁺. The methylbenzenes isomerize and disproportionate during the reaction. The distribution of the arenes between the solution and the complex ions has been determined. The different complexing abilities of the arenes lead to the proposal of an associative mechanism for the ligand exchange.     

Synthese neuer mono- und dihalogenierter Salicylsäurederivate

Syntheses of 3- and 4-monohalogenated (2a, b; 3a–c; 4a–e) and 3,4-dihalogenated (4f–l) ethyl 5-cyano-salicylates are described. Nitration of the ethyl 5-cyano-4-halo-salicylates3a, b leads to the ethyl 5-cyano-4-halo-3-nitro-salicylates5a, b. Hydrolysis of the ester and cyano group as well as reactions of various chloroformates, N,N-dimethylcarbamyl chloride and acetic anhydride with the hydroxyl group have been studied.

     

Supraleitung in Seltenerdmetall-Carbidhalogeniden des Typs SE2X2C2

Superconductivity in Rare Earth Metal Carbide Halides of the Type SE₂X₂C₂ The metallic nature of the carbide halides Y₂X₂C₂ is due to Y? C covalency. The superconductivity of the compounds is attributed to a pairwise attraction of conduction electrons by C₂-π* states at the Fermi level. The hypothesis is followed by experiments and band structure calculations. – Neutron powder diffraction reveals d(C? C) = 128(1) pm for Y₂Br₂C₂. X-ray single crystal investigations on Y₂Br₂C₂ and Y₂I_1.5Br_0.5C₂ show a characteristic variation of the coordination of the C₂ unit. Systematic changes of the average halide radius in Y₂(X,X′)₂C₂ (X,X′ = Br, Cl I, Cl and I, Br) lead to a monotonic increase of T_c = 2.3 K (X = Cl) via T_c = 5.05 K (X = Br) to a maximum T_c = 11.2 K for Y₂I_1.6Br_0.4C₂. No isotope effect for ¹²C/¹³C could be detected. Photoelectron spectra of Y₂Br₂C₂ (excitation energies between 40 and 140 eV) are compared with the results of band structure calculations (LMTO, E.H.). The electronic structure reveals two bands crossing the Fermi level. One of them has C₂-π*-Y-d_xz,yz character and exhibits a saddle-point at E_F. The other intersects the Fermi level with large dispersion and has exclusively Y-d character at the crossing point. The results are discussed with respect to theoretical models (van Hove singularity, local pairs and itinerant electrons).     

Substituted rod-like aromatic polyamides: Synthesis and structure-property relations

The synthesis and structure-property relations of a number of novel substituted paralinked aromatic homopolyamides and copolyamides are described. The synthesis of the polyamides was carried out by polycondensation of activated N,N'-bis-(trimethylsilyl) substitued aromatic diamines and aromatic diacid chlorides. In order to improve the solubility and to lower melting temperatures, novel arylsubstituted terephthalic acids moieties, such as p-terphenyl-2,5-dicarboxylic acid and o-terphenyl-2,5-dicarboxylic acid, were used in combination with substituted and noncoplanar diamines. Depending on the chemical structure, polyamides with very high solubility (up to 40% w/w) in polar aprotic solvents such as N,N-dimethylacetamide without the addition of inorganic salts were obtained. Lyotropic liquid crystalline behavior was observed for the first time in polyamides which contain noncoplanar biphenylene units.     

Micromechanical Approach of Martensitic Transformation Problem in Steels

To determine the mechanical behavior of material involving the martensitic phase transformation (for example, steels like 100Cr6), a representative volume element (RVE) model including phase transformation criterion is desireable at micromechanical approach. A framework combining the Eshelby's inclusion theory as well as continuum mechanics with phase-transformation (PT) critical condition at RVE model is presented briefly. And application of this model to estimate the critical aspect ratio of martensitic plate or lath inside homogeneneous stress field is also included, where the RVE can be under uniaxial tension/compression or pure shear loading case. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)