ESR and conductivity studies of doped polyacetylene

Low temperature ESR spectra of cis-rich and trans-rich polyacetylene, lightly doped with pentafluorides (AsF₅, SbF₅) exhibit two clearly distinguishable ESR lines. Detailed studies of these two lines as a function of temperature, doping concentration and microwave power indicate that one of these lines originates from localized spins, the other from conduction electrons. The concentration of the localized spins, generally speaking, decreases with doping. The Pauli-like susceptibility associated with the conduction electrons changes from 1.5×10^–7 to 15×10^–7 emu·cm³/mole for various doping levels and correlates with the conductivity of the same samples. The longitudinal relaxation time of the localized spins in doped trans-rich was found to beT ₁100 s at low temperatures, consistent with values of others in undoped trans rich. The temperature dependence of the homogeneous transverse relaxationT ₂ of the localized spins in doped trans-rich polyacetylene is similar to that of undoped trans polyacetylene. These facts show that the localized spins originate from undoped regions of the doped polyacetylene, perhaps due to inhomogenous doping. Our results are consistent with either formation of metallic islands, or, probably more, with impurity band conduction in strongly disordered semiconductors.     

Electrical transport properties of polyacetylene tetrachloroferrate - [CH(FeCl4)y]x]

Conductivity and thermopower measurements of polyacetylene doped with FeCl₄ are reported. The conductivity changes over 10 orders of magnitude and reaches the maximum value of 200 Ω^-1 cm^-1 at y = 0.07. The thermopower reveals the semiconductor to metal transition at y ? 0.002, with high and temperature independent Seebeck coefficient in the dilute limit and a metallic dependence in the heavily doped samples.     

13C high-resolution NMR study of doped polyacetylene

¹³C high resolution NMR spectra of doped trans polyacetylene, (CH)_x, have been measured. A spectrum of (CH)_x doped with AsF₅ shows a downfield shift, while that with potassium an upfield shift. The charge distribution and the existence of sp³ hybridized carbon on halogen-doped (CH)_x chains are discussed.     

Polyacetylene doped with iron trichloride : Improved conductivity and stability

Trans polyacetylene (CH)_x has been doped with FeCl₃ solutions in Nitromethane. In the “in-situ” EPR experiment we have observed the evolution of the EPR spectra with time during the doping process. That enabled us to gain information about the rate of dopant diffusion in solid material. Both EPR and conductivity measurements show that the doped material is reasonably stable even in the presence of air. Raman spectroscopic studies show the usual features observed in doped trans polyacetylene.     

Hyperfine interaction of polyacetylene intercalated with iron halide anions: 57Fe Mössbauer and EPR spectroscopy studies

⁵⁷Fe Mössbauer spectroscopy and EPR spectroscopy were applied to study the chemical nature of the intercalant in polyacetylene oxidized (doped) chemically with FeCl₃ and FeBr₃. For the (CH) _x/FeCl₃ system the Mossbauer lattice temperature (θ_M = 89 K) was estimated from the temperature dependence of the recoil-free fraction. The strong influence of the high Fe^III state of the dopant is clearly evidenced through EPR spectra of both systems: while the chloride system shows a superposition of a narrow and a broad lines, the bromide one exhibits only a narrow line, whose intensity decreases with increasing dopant concentration.     

纯有机聚合物磁性的模型研究

通过一个简单的紧束缚模型,初步研究了共轭有机聚合物的磁性问题.其中包括侧链基团与主链的共轭性对主链键序波的作用,电荷密度波的形成,次近邻跃迁和电子相互作用的影响等.特别针对有关聚乙炔侧链悬挂有机自由基团(CH₂)的具高自旋基态磁性聚合物(C₃H₃)_n的第一性原理计算结果,给出物理的解释和理解. 关键词：     

Far infrared optical properties of MoCl5 doped CIS polyacetylene

Far infrared transmission spectra of MoCl₅ homogeneously doped polyacetylene are reported. They show that in this spectral range the optical index is frequency independent but increases strongly with increasing doping level. IR conductivity is compared to d.c. conductivity.     

Insulator-to-metal transition in doped polyacetylene

A field-theoretic model of the insulator-to-metal transition in doped polyacetylene is presented. The likelihood of an inhomogeneous distribution of charge in the (CH)_x fibrils is stressed.     

Anisotropic optical properties of pure and doped polyacetylene

Optical absorption and reflection measurements have been carried out on pure and doped polyacetylene films. The absorption data are consistent with a model of (CH)_X as a direct gap quasi-l d semiconductor; doping with AsF₅ or iodine introduces absorption within the gap, but appears to leave the interband transition intact. The anisotropic reflectance from partially aligned films and the increased optical anisotropy upon doping provide evidence of quasi-l d behavior of the semiconducting pure polymer and the metallic doped polymer.     

Alignment of iodine species in stretched polyacetylene films

The¹²⁹I Mössbauer spectroscopy has been applied to iodine-doped highly conducting polyacetylene. For the unstretched polyacetylene films, the iodine species of I^?, I ₃ ^? and I ₅ ^? are observed. The polyiodides have symmetrical charge population. In addition to these anions, a small amount of iodines is covalently bonding to the carbon atoms, breaking the double bonds of the main chain. For the stretched polyacetylene films, the measurements were done under the condition that the stretch direction was parallel or perpendicular to the direction of incident ψ-rays. The relative change of the intensities of the Mössbauer quadrupole-split lines reveals the alignment of the polyiodine anions in the stretched film.     

Conductivity and Hall effect measurements in doped polyacetylene

Conductivity and Hall effect measurements are reported for polyacetylene films heavily doped with AsF₅. The Hall coefficient is anomalously small, however, it is p-type and relatively temperature insensitive in the range 4–300 K indicative of metallic conduction. The effects of film degration by heating and air exposure are also investigated.     

Mössbauer spectroscopic study on polyacetylene doped with iodine

The ¹²⁹I Mössbauer effect has been applied to polyacetylene films of cis-(CHI_0.23)_x as prepared and cis-(CHI_0.20)_x led under dynamic evacuation. It has been clarified that both films contain linear I^?₅ and I^?₃ with symmetric charge population and that the fraction of I^?₃ increases after evacuation. The mechanism of electrical conductivity in iodine-doped polyacetylene has been discussed on the basis of the chemical form and the dynamic behavior of the anion species.     

Band structure of polyacetylene, (CH)x

The one-electron energy bands and densities of states of polyacetylene in both cis- and trans-conformations have been investigated. The principal issue addressed is whether the itinerant picture alone is sufficient to explain the experimental properties of this material. We conclude that the one-electron model provides an excellent zeroth-order explanation of current observations of optical and transport effects in both pure and doped forms of this unusual polymer.     

Magnetoresistance and Corbino resistance of iodine doped-polyacetylene

The collinear magneto- and coaxial Corbino resistance of iodine doped polyacetylene have been measured at 4.2 K for doping levels from 7% to 30%. From a comparison of the two sets of data, the “carrier mobility” has been determined. It ranges between 85 and 180 cm²/Vsec, falling with increasing iodine concentration.     

碘掺杂聚乙炔电子自旋共振及导电性的研究

用四探针法测量掺碘聚乙炔薄膜[CH(I₃)_y]_x,当y=1.0％,4.2％和8.3％时面电阻随温度的变化,发现它们可用变程跳跃(VRH)模型很好的描述。用电子自旋共振(ESR)谱仪研究了不同温度下低浓度碘掺杂聚乙炔的自旋磁化率、峰峰宽等。并给出相应的理论解释。 关键词：     

PMR investigation of the structure of native and iodine doped polyacetylene films

The second moment of the proton magnetic resonance line of native cis- polyacetylene, thermally isomerized trans-polyacetylene, and iodine doped initially cis-polyacetylene has been measured by a pulse technique. The results differ qualitatively and quantitatively from previous reports. They are discussed in terms of the proposed crystalline structures of (CH)_x.     

从头计算法计算Yb3+掺杂钽酸盐的晶体场参数和能级结构

介绍了一种基于从头计算的DV-Xα方法和有效哈密顿量模型,它可以计算晶体中掺杂离子的晶体场参数和旋轨耦合参数,尤其适合计算低对称性的晶体.对于低对称性的晶体,参数的数目比能级的数目多,因此通过实验能级拟合确定所有的参数不太准确,而从头计算法可以准确地确定所有的晶体场参数和旋轨耦合参数.首先用这种模型计算了Yb3+掺杂GdTaO4晶体中的晶体场参数和旋轨耦合参数,然后给出了Yb3+在GdTaO4中的能级结构,并分析了Yb3+:GdTaO4的发射谱形成一个连续的发射带.这有利于激光的调谐和锁模激光输出,预言了Yb3+:GdTaO4有望成为新型全固态超短脉冲激光工作物质.同样用这种模型分别计算了Yb3+掺杂YTaO4和ScTaO4中的晶体场参数和旋轨耦合参数,并给出了Yb3+在YTaO4和ScTaO4中的能级结构,得到了与Yb3+:GdTaO4晶体类似的结论.     

Proton NMR studies of the structure of trans-(CH)x

The proton NMR line width in undoped and AsF₅ doped polyacetylene shows an abrupt change as a function of temperature, indicative of motional narrowing. The characteristic temperature for the onset of narrowing decreases with increasing AsF₅ concentration. The results are discussed in terms of large amplitude librational or translational motions of the trans-(CH)_x chains. The magnitudes obtained for the second moment at temperatures below and above the narrowing transition are used to obtain information on the chain packing of trans-(CH)_x within the three-dimensional crystal structure.     

Calculation of exciton-neutral and exciton-charged impurities elastic scattering cross-sections in Cu2O

The scattering cross-sections for exciton-neutral and exciton-charged impurities have been calculated for Cu₂O doped with Ag⁺ ions. The cross-section for exciton-neutral impurity is in good agreement with that obtained previously by the authors from the experimental n = 1 line width in the yellow series of this material.