Study of mechanical relaxation processes in ladder organosilicon polymers

Dynamic viscoelastic properties of ladder polyorganosiloxanes have been studied by the method of induced resonance oscillations in the temperature range from –150 to 300°C and in the frequency range from 8 to 300 Hz. Two broad maxima of mechanical losses were observed for all the polymers studied independently of their chemical structure, a low-temperature maximum in the –150 to 0°C region and a high-temperature maximum in the 0 to 250°C region. It was found that the introduction of alkyl groups into polyphenylsilsesquioxanes does not noticeably affect the position and nature of the low-temperature relaxation peaks but in certain cases leads to an increase in the dynamic modulus in polyphenylalkylsilsesquioxanes, compared with the modulus of polyphenylsilsesquioxane in the same temperature region. In the high-temperature region, a distinct dependence is observed of the relaxation peak on the length and the content of alkyl groups in the polymer. It has been found that increase in the length and the content of alkyl groups is accompanied by a shift in the peak in the direction of low temperatures and a decrease in the value of the modulus. Hypotheses have been suggested on the nature of the shifts which lead to the relaxation peaks observed in the ladder polyorganosiloxanes.Institute of Heteroorganic Compounds, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 5, pp. 804–809, September–October, 1973.     

Origin of intrinsic Josephson coupling in the cuprates and its relation to order parameter symmetry: An incoherent hopping model

Experiments on the cuprate superconductors demonstrate that these materials may be viewed as a stack of Josephson junctions along the direction normal to the CuO₂ planes (the c-axis). In this paper, we present a model which describes this intrinsic Josephson coupling in terms of incherent quasiparticle hopping along the c-axis arising from wave-function overlap, impurity-assisted hopping, and boson-assised hopping. We use this model to compute the magnitude and temperature T dependence of the resulting Josephson critical current j_c(T) for s- and d-wave superconductors. Contrary to other approaches, d-wave pairing in this model is compatible with an intrinsic Josephson effect at all hole concentrations and leads to j_c(T) T at low T. By parameterizing our theory with c-axis resistivity data from YBa₂Cu₃O_7−δ (YBCO), we estimate j_c(T) for optimally doped and underdoped members of this family. j_c(T) can be measured either directly or indirectly through microwave penetration depth experiments, and current measurements on Bi₂Sr₂CaCu₂O₈ and La_2−xSr_xCuO₄ are found to be consistent with s-wave pairing and the dominance of assisted hopping processes. The situation in YBCO is still unclear, but our estimates suggest that further experiments on this compound would be of great help in elucidating the validity of our model in general and the pairing symmetry in particular.     

Octaboraneyl Complexes of Nickel: Monomers for Redox-Active Coordination Polymers

Herein, we establish the preparation, characterization, and reactivity of a new diphosphine ligand, 1,2-bis(di(3-dicyclohexylboraneyl)propylphosphino)ethane (P₂B^Cy₄), a scaffold that contains four pendant boranes. An entryway into the coordination chemistry of P₂B^Cy₄ is established by using nickel, providing the octaboraneyl complex [Ni(P₂B^Cy₄)₂]—this species contains a boron-rich secondary coordination sphere that reacts readily with Lewis bases. In the case of 4,4′-bipyridine, an air-sensitive coordination polymer is obtained. Characterization of this material by solid-state NMR and EPR spectroscopy reveals the presence of a charge-transfer polymer, which forms as a function of intramolecular Ni→4,4′-bpy electron transfer (ET), providing an array of oxidized nickel sites and reduced 4,4′-bpy radical anion sites. Notably, the related intermolecular reaction between the model fragments [Ni(dnppe)₂] (dnppe=1,2-bis(di-n-propylphosphino)ethane) and a bis(boraneyl)-protected 4,4′-bpy, provides no ET. Overall, the P₂B^Cy₄ fragment provides a unique opportunity for Lewis base activation, in one case allowing for the facile construction of monomers for incorporation into redox-active macromolecules.     

Diffraction Approach in the Scattering Problem for Three Charged Quantum Particles

Mathematical Notes -     

Jahn-Teller effect and the orbital ground state of Cu2+ ions in Eu2CuO4 and La2CuO4 crystals

Local ion displacements and their anharmonicity in Eu₂CuO₄ and La₂CuO₄ crystals are studied by high-precision x-ray diffractometry. A symmetry analysis and the temperature behavior of the probability density function of Cu²⁺ ions reveal a degenerate ground orbital state in the Eu₂CuO₄ crystal (tetragonal doublet d _xz, d _yz). The pattern and anharmonicity of the local displacements are found to be determined not only by lattice vibrations but by 2D antiferromagnetic spin fluctuations in the CuO₂ sheet as well. By contrast, in the La₂CuO₄ crystal the orbital ground state of the Cu²⁺ ion is a singlet (d _z 2). The pattern of Eu³⁺ local displacements and their effect on the properties of the Cu subsystem in the Eu₂CuO₄ crystal are also studied. Fiz. Tverd. Tela (St. Petersburg) 39, 1600–1608 (September 1997)     

Elastic properties of ultrahard fullerites

The elastic properties of C₆₀ fullerite samples synthesized under pressure P=13.0 GPa at high temperatures were investigated using acoustic microscopy. The velocities of longitudinal (c _L=17–26 km/s) and transverse (c _T=7.2–9.6 km/s) elastic waves in the samples were measured. It was established that the longitudinal sound velocity of ultrahard fullerites is higher than that of any other known solid. The bulk modulus of these ultrahard samples is higher than that of diamond and reaches a value greater than 1 TPa. The high bulk modulus, the relatively large shear moduli, and the substantial Poisson ratio indicate that the structure of the ultrahard fullerites is fundamentally different from that of diamond. Zh. éksp. Teor. Fiz. 114, 1365–1374 (October 1998)     

Vacuum arcs for plasma centrifuge isotope enrichment

Requirements for economic plasma centrifuge isotope enrichment are reviewed. A distributed-discharge vacuum arc in chromium, as reported by A.I. Vasin et al. (1979), is described and a qualitative model for its operation is given. Experiments were performed on a modified distributed-discharge vacuum arc in which the parameters appear to be well suited for plasma centrifuge operation. A 2-m-long, 10-cm-diameter fully ionized DC chromium arc was produced with ion density and temperature in the range of 10¹³ cm^-3 and 0.16 eV. Chromium mass throughput rates in the range of 1 g/min are reported, with continuous running times of up to 40 min., limited only by chromium supply