稀土催化极性单体配位均聚及与非极性单体共聚合的研究

将极性基团引入大分子链中可改善非极性聚烯烃材料的表面性能,扩展其应用范围甚至带来不可预见的新功能,是市场需求并由企业驱动.与聚合后功能化改性和物理共混方法相比,极性与非极性单体配位共聚合是最直接和简便的方法,适用范围广,并可保持聚烯烃的立构规整度,一直以来,相关研究备受企业和科研工作者瞩目.然而,极性基团通常具有Lewis碱性,容易与Lewis酸性的聚合催化剂强烈螯合而致其毒化,因此,这又是极具挑战性的课题.目前,该领域的研究取得了很大的进展,已经实现了乙烯与很多极性单体的共聚合.今后,将集中解决如何实现极性单体均聚合,提高共聚合活性,特别是极性单体插入率和分布可调节性,保持立体选择性,以及获得高分子量、具有实际应用意义的共聚产物等问题.本文旨在将课题组近年来在极性功能化苯乙烯和共轭双烯烃单体的均聚合及与苯乙烯、乙烯和共轭双烯烃等非极性单体共聚合方面的最新研究成果以及国内外该领域的相关报道进行综合阐述,为读者提供解决上述关键问题采用的研究路线、实施方法和创新性思维.     

First-principles calculations on temperature-dependent elastic constants of rare-earth intermetallic compounds: YAg and YCu

We present the temperature-dependent elastic constants of two ductile rare-earth intermetallic compounds YAg and YCu with CsCl-type B2 structure by using a first-principles approach. The elastic moduli as a function of temperature are predicted from the combination of static volume-dependent elastic constants obtained by the first-principles total-energy method with density-functional theory and the thermal expansion obtained by the first-principles phonon calculations with density-functional perturbation theory. The comparison between our calculated results and the available experimental data for Ag and Cu provides good agreements. In the calculated temperature 0-1000 K, the elastic constants of YAg and YCu follow a normal behavior with temperature that those decrease with increasing temperature, and satisfy the stability conditions for B2 structures. The Cauchy pressure for YAg and YCu as a function of temperature is also discussed, and our results mean that YAg and YCu become more ductile while increasing temperature.     

Magnetic and transport properties of RCr0.3Ge2 (R=Tb, Dy, Ho and Er) compounds

The magnetic and transport properties of ternary rare-earth chromium germanides RCr_0.3Ge₂ (R=Y and Tb-Er) have been determined. X-ray and neutron diffraction studies indicate that these compounds have the CeNiSi₂-type structure (space group Cmcm) [1]. Magnetic measurements reveal the antiferromagnetic ordering below T_N equal to 18.5 K (R=Tb), 11.8 K (Dy), 5.8 K (Ho) and 3.4 K (Er). From the neutron diffraction data the magnetic structures have been determined. For TbCr_0.3Ge₂ and DyCr_0.3Ge₂ at low temperatures the magnetic ordering can be described by two vectors k₁=(,0,0) and k₂=(,0,), and k₁′=(,0,0) and k₂′=(,0,), respectively. In HoCr_0.3Ge₂ and ErCr_0.3Ge₂ the ordering can be described by one propagation vector equal to (,,0) and (0,0,0.4187(2)), respectively. In DyCr_0.3Ge₂ some change in the magnetic ordering is observed at T_t=5.1 K. In temperature range from T_t to T_N the magnetic ordering is given by one propagation vector k=(,0,0). YCr_0.3Ge₂ is a Pauli paramagnet down to 1.72 K which suggests that in the entire RCr_0.3Ge₂ series the Cr atoms do not carry magnetic moments. All compounds studied exhibit metallic character of the electrical conductivity. The temperature dependencies of the lattice parameters reveal strong magnetostriction effect at the respective Nèel temperatures.     

Rare-earth hexaborides nanostructures: Recent advances in materials, characterization and investigations of physical properties

Nanostructured rare-earth hexaborides (REB₆) are promising materials for photonic and electronic applications due to their unique characteristic. These include high melting point, hardness, chemical stability, low work function, low volatility at high temperatures, superconductivity, magnetic properties, efficiency, thermionic emission, and narrow band semiconductivity. This article focuses on recent developments regarding the synthesis, characterization, and applications of REB₆ nanostructures. We first summarize information regarding the classification and crystal chemistry of REB₆. Next, we examine the means by which researchers have successfully synthesized REB₆. We consider the structural properties and morphology of REB₆, and the growth mechanism involved in their fabrication. Finally, we offer suggestions for the use of REB₆ nanostructures in photonic and electronic applications, and identifying four areas for further research.     

Ionic acetamide coordination in its complexes with rare-earth iodides

A series of [Ln(CH₃CONH₂)₄(H₂O)₄]I₃ (Ln?=?rare-earth metal) complexes was completed with seven new compounds (Ln?=?Ce, Pr, Sm, Tb, Tm, Yb, Lu); two (Ln?=?Ce, Tm) were studied by X-ray diffraction. The coordination polyhedron of eight oxygen atoms is a distorted square antiprism. No tetrad effect was found for Ln–O bond lengths. The structure is stabilized by a system of intermolecular hydrogen bonds. The most striking feature of the structures is the recently predicted ionic acetamide coordination. The acetamide molecules are non-planar (the Ln–O–C–N torsion angles are 159–170°) and Ln–O–C bond angles vary in the 146.0–156.8° range.     

Dinuclear rare-earth picrate complexes based on a multidentate amide ligand: syntheses,crystal structures,and luminescence properties

A new amide-based multidentate ligand, N,N′-1,2-ethanediyl-bis{2-[(N,N-diethylcarbamoyl)methoxy]benzamide} (L) reacts with M(Pic)₃?·?6H₂O to give rare-earth picrate complexes [M₂L₂(Pic)₄(H₂O)₂](Pic)₂ (M = La (1), Nd (2), Eu (3), Gd (4), Tb (5), Dy (6), Yb (7), Y (8)). X-ray single-crystal diffraction analyses indicate that dinuclear complexes 3?·?2C₄H₈O₂, 6?·?2C₄H₈O₂, and 8?·?2CH₃CN are isomorphous. Each metal is nine-coordinate by four oxygen atoms of two ligands, four oxygen atoms of two bidentate picrates, and one water molecule with a distorted monocapped square antiprism. With hydrogen bonds between the free picrate anions and the coordination cations the complexes exhibit 2-D layers. The luminescent properties of 3 [Eu₂L₂(Pic)₄(H₂O)₂](Pic)₂ are described and factors that influence luminescent intensities are also discussed.     

Micellization and Conductance Behaviour of Thorium Soap Solutions

Abstract

The critical micelle concentration, degree of dissociation and dissociation constant of dilute solutions of thorium soaps in methanol-benzene mixtures were determined from conductivity measurements. The results showed that these soaps behave as weak electrolytes in dilute solutions below the CMC and the CMC increases in presence of Sudan (IV) dye. The various thermodynamic parameters were also evaluated for both dissociation and micellization processes.     

Structure of ultraphosphate glasses with small rare-earth ions by X-ray diffraction

Rare-earth ultraphosphate glasses with nominal R₂O₃ fractions of 10 and 15 mol% and small ionic radius (large atomic number) R³⁺ ions (R = Tb, Tm, Lu) are measured by X-ray diffraction at a synchrotron with photons of 119 keV (maximum scattering vector 260 nm^− 1). The total correlation functions T(r) show well-resolved R-O and O-O first-neighbor peaks. In contrast to all ultraphosphate RP₅O₁₄ crystals and the ultraphosphate glasses of larger R³⁺ ions, where RO₈ polyhedra (mean R-O coordination numbers of ~ 8 for the glasses) are observed, the R³⁺ ions in glasses with R = Lu, Tm, Tb have mean R-O coordination numbers of ~ 7.5. The R-O first-neighbor peaks extracted from the T(r) functions are compared with those obtained from atomic coordinates of related RP₅O₁₄ and RP₃O₉ crystals. The R-O distances of the ultraphosphate glasses studied are found to fall between those of the two crystals but with tails to the side of longer bonds.     

Neutron Diffraction Study of the p - T Phase Diagram for Erbium

Electrical resistivity measurements performed on a single crystal of erbium as a function of temperature and hydrostatic pressure have provided a preliminary p - T phase diagram. The results have been interpreted in terms of a model for the magnetic structures of Er deduced from neutron diffraction studies at ambient pressure. This model predicted the existence of a magnetic structure with a wave vector of Q =2/7 c * at 4.2 K, when the applied pressure is larger than 3 kbar. This paper reports a neutron diffraction study of erbium made in the temperature range of 4 to 100 K, at pressures between 0.5 and 6 kbar. We have observed the predicted suppression of the low temperature conical ferromagnetic phase and the emergence of a new phase with Q =8/33 c *. The neutron diffraction measurements has enabled us to identify the various phases that develop from the cycloidal phases previously observed at atmospheric pressure.     

Magnetic properties of Nd0.9Dy0.1Fe3(BO3)4

The magnetic properties of trigonal Nd_0.9Dy_0.1Fe₃(BO₃)₄ substituted compound with the competitive Nd-Fe and Dy-Fe exchange interactions have been investigated. It has been shown that in Nd_0.9Dy_0.1Fe₃(BO₃)₄ a spontaneous spin-reorientation transition from an ease-axis state to an easy-plane occurs near 8 K. Anomalies of the magnetization curves are observed in a spin-flop transition induced by the magnetic field B‖c. The calculations were performed using a molecular-field approximation and a crystal-field model for the rare-earth subsystem. Extensive experimental data on the magnetic properties of Nd_0.9Dy_0.1Fe₃(BO₃)₄ have been interpreted and good agreement between theory and experiment has been achieved using the obtained theoretical dependences.