BaCoxZn2-xFe16O27六角铁氧体电子结构与介电特性的第一性原理研究

提出了W型六角铁氧体BaCo_xZn_2-xFe₁₆O₂₇的晶体结构模型,并通过基于密度泛函理论框架下Hubbard参数U修正的广义梯度近似密度泛函理论方法研究了该材料的基态电子结构、磁性和静电介电性特性.Co和Zn共掺杂引起BaFe₁₈O₂₇的导电性从半金属转换到半导体.BaCo_xZn_2-xFe₁₆O₂₇的能隙随x增加而增加,品格常数和原胞磁矩随之而变小.介电常数计算表明,BaCo_xZn_2-xFe₁₆O₂₇的静电介电常数随x增加而增加,在6.2—7.2范围而且显示各向异性.Bom电荷计算分析表明Co和Zn本身极化对材料的介电常数和其各向异性影响不大.     

Convex combinations of quadrant dependent copulas

It is well known that quadrant dependent (QD) random variables are also quadrant dependent in expectation (QDE). Recent literature has offered examples rigorously establishing the fact that there are QDE random variables which are not QD. The examples are based on convex combinations of specially chosen QD copulas: one negatively QD and another positively QD. In this paper we establish general results that determine when convex combinations of arbitrary QD copulas give rise to negatively or positively QD/QDE copulas. In addition to being an interesting mathematical exercise, the established results are helpful when modeling insurance and financial portfolios.     

Raman Spectrum of n-Propylamine in Cyclohexane

Polarized (Isotropic) Raman spectra of the N-H valence region of n-propylamine have been recorded as a function of concentration in cyclohexane. Statistical analysis of the data using non-linear least squares techniques revealed the presence of a fourth spectral band in this region, not observed in an earlier IR study. This latter peak has been assigned to vibrations of monomers in the liquid. The resolved spectral parameters are consistent with a picture of diminishing molecular association upon dilution of the amine in this non-polar solvent.     

Convenient One‐Pot Synthesis of Vinyl Ethers from Phenyl 2‐Hydroxyalkyl Selenides

Vinyl ethers were prepared with good yields in a one‐pot, two‐step transformation by O‐alkylation reaction of phenyl 2‐hydroxyalkyl selenides with primary or secondary organic halides followed by oxidation elimination with 30% hydrogen peroxide.     

Novel Traceless Liquid‐Phase Synthesis of Coumarin Derivatives on Poly(Ethylene Glycol) Support

Coumarin derivatives were prepared by the von Pechmann reaction of PEG‐bound acetoacetate reagent with phenols in the presence of TiCl₄ in excellent yield and purity with a facile workup procedure. The polymer reagent could be recycled two to four times without diminishing the yield or purity.