Production of single-crystal K3C60

A simplified technique for synthesizing high-quality millimeter-sized single-crystals of K₃C₆₀ is reported. The preparation method consists of a two-step process where pure C₆₀ crystals are first grown and later doped with potassium vapor. The undoped crystals are grown using an open-tube vapor transport with a flowing helium gas carrier. Doping occurs in a highvacuum environment inside a specialized quartz tube which allows phase purity to be monitored via magnetization measurements of the superconducting state. Utilizing the aforementioned procedure, one can attain samples with magnetically determined superconducting transition widths (10–90% diamagnetic shielding) of less than 1 K.     

K3C60晶体中钾离子的平衡位置

分析八面体空隙中心碱金属的相互作用势能, 研究碱金属沿[111]、[110]和[100]三个不同方向移动时, 势能的变化情况, 结果表明, 对于K3C60晶体, K+离子在三个不同方向上都存在非中心平衡位置, 而且在三个不同方向上, 非中心平衡位置相对中心的偏移量不同, 在[111]轴上为0.98, 在[110]轴方向上为0.78, 在[100]方向上为0.56, 在[111]轴方向的非中心的势能是最小值, 为-508.59meV.通过对计算结果的分析, 认为非中心平衡位置的出现应归因于短程相互作用, 其中与C60的相互作用是最主要的, 其势能最低点对应的位置不在中心;三个不同方向的非中心平衡位置偏离中心距离不同, 是因为与之对应晶面上的C60面密度不同.     

Band Gaps of Na2C60 and K4C60

respectively.     

Interpretation of anomalous normal state optical conductivity of K 3 C 60 fullerides

The observed frequency dependent optical response of alkali–metal-doped fulleride superconductors (T _c ?≈?19 K) has been theoretically analysed. The calculations of the optical conductivity, σ(ω), have been made within the two-component schemes: one is the coherent Drude carriers (electrons) responsible for superconductivity and the other is incoherent motion of carriers from one atom to other atom of C₆₀ molecule to a pairing between Drude carriers. The approach accounts for the anomalies reported (frequency dependence of optical conductivity) in the optical measurements for the normal state. The model has only one free parameter, the relaxation rate. The frequency dependent relaxation rates are expressed in terms of memory functions. The coherent Drude carriers form a sharp peak at zero frequency and a long tail at higher frequencies, i.e. in the infrared region. However, the hopping of carriers from one atom to the other (incoherent motion of doped electrons) yields a peak value in the optical conductivity centred at mid-infrared region. It is found that both the Drude and hopping carriers will contribute to the optical process of conduction in the K₃C₆₀ and shows similar results on optical conductivity in the mid-infrared as well as infrared frequency regions as those revealed from experiments.     

Rb3C60单晶薄膜制备及Rb3C60/C60相界面稳定性研究

通过控制蒸发源电流、源和样品间距及扩散时间,室温下在C60单晶(111)解理面上制备出Rb3C60.用低温同步辐射角分辨光电子谱测量了样品的能带结构.观察到的能带色散表明样品为单晶薄膜.用X光电子谱首次研究了Rb3C60与C60相界面的稳定性.结果表明温度低于约420 K时界面是稳定的.     

The single-crystal elastic constants of cubic (3C) SiC to 1000° C

Experimental measurements of the polycrystalline elastic moduli of the cubic (3C) beta polytype of SiC at elevated temperatures and the room-temperature single-crystal elastic constants were combined through equations that relate the two to determine the stiffnesses C_ij and the compliances S_ij to 1000° C. The results demonstrate a general method for estimating the elevated temperature single-crystal constants of cubic crystals and illustrate that the cubic (3C) beta polytype of SiC becomes more elastically anisotropic at elevated temperatures.     

Normal-incidence reflectance of crystalline K(6)C(60), Rb(6)C(60), and Cs(6)C(60)

The structure factors and normal-incidence reflectances of the alkali-metal-doped fulleride crystals M(6)C(60) (where M = K, Rb, or Cs) were calculated, and the peak reflectances are in the 2-20% range for incident wavelengths of 9-17 ?, which indicates that C(60) may be a promising transmissive (spacer) material for normal-incidence mirrors that have relatively high normal-incidence reflectance in the x-ray region.     

The viscosity of Na2CO3 and K2CO3 aqueous solutions in the range 20–60°C

Results of new relative measurements of the density and viscosity of aqueous solutions of Na₂CO₃ and K₂CO₃ at atmospheric pressure are presented. The temperature ranged from 20 to 60°C, and the composition ranged from pure water to near saturation. All the data have been correlated with the aid of double polynomials. Owing to some evidence of chemical attack of the borosilicate viscometers, the accuracy of the viscosity measurements is estimated to be ±1%, whereas that of density is estimated to be ±0.01%.     

Au电极作用下C60、2C60与4C60富勒烯分子的电子传输特性

采用扩展的Hückel方法与格林函数方法,研究了Au电极作用下,C60富勒烯、2C60和4C60聚合体分子的电子结构与导电性,并对它们的电子结构与电子输运特性进行了对比.研究结果表明,C60、2C60或4C60富勒烯分子与Au电极 "接触"后,其HOMO、LUMO间的能隙减小;C60、2C60或4C60分子与Au电极之间的结合既有共价键的成分,又有离子键的成分,其中,C60、4C60分子与Au电极结合的离子键特征更为明显;三种富勒烯分子的电子输运性能依次具有C60>2C60>4C60分子的顺序.     

Pairing Mechanism and Superconductivity of Rb3C60 Fullerides

The nature of electron pairing mechanism and the superconducting transition temperature (T _c ) of alkali metal (Rb) doped fullerenes is studied within the framework of strong coupling theory. Chemical substitution of alkali metal in the parent compound introduces free electrons in the lowest unoccupied molecular orbital and for Rb₃C₆₀, the band is filled up to the Fermi level. The intercage interactions between the adjacent C₆₀ cages and expansion of lattice due to the intercalation of Rb atoms are investigated using nearest-neighbor interactions. The free electrons in lowest molecular orbital are coupled with intermolecular phonons. The renormalized Coulomb repulsive parameter * and the attractive coupling strength are obtained for the intermolecular phonon frequency _er. T _c is then estimated as 8.6 K, which is lower as compared with the published data of 30 K. The electrons also couple with the intramolecular phonons and introducing them in ad hoc way, T _c enhances to 34 K. Analytical results on T _c allow one to visualise the relative interplay between the strength of inter- and intramolecular phonons. To illustrate the usefulness of the above approach the carbon isotope effect exponent and the energy gap ratio are estimated which are consistent with the experimental data. The present analysis reveals that both low frequency intermolecular and high frequency intramolecular phonons participate in the pairing mechanism, T _c mainly arises from high frequency intramolecular phonons.     

The Dark Reaction of C60 and of C70 with Molecular Oxygen at Atmospheric Pressure and Temperatures between 300 K and 800 K

The solid fullerenes C₆₀ and C₇₀ react in deliberate and adventitious situations with molecular oxygen in a wide range of temperatures. Using thin films and polycrystalline bulk samples with well-defined structures and chemical histories we investigated the complex process from molecular intercalation over atomic adduct formation to deep oxidation and polymerisation which together describe the oxidation reaction. A combination of photoemission (UPS, XPS) photoabsorption (XAS), FT-IR, temperature-programmed and isothermal gravimetric measurements and DSC allowed to identify the existence and gradual interconversion of the three types of solid products besides the gas phase products CO and CO₂. Both fullerenes react along the same reaction path. The difference of the molecular structures results in different activation barriers in the initial step of intercalation and in the final step of cage-opening. The overall reactivity of both fullerenes is quite similar at temperatures above the gasification onset of 570 K. The formation of the various adduct compounds was found to exert a detectable influence upon the overall molecular shape of the fullerenes and a small effect on the electronic structure was observed. The only moderate differences in electronic and geometric structures of pristine and initially oxidised fullerenes precludes a pronounced molecule-by-molecule reaction control and allows the topochemistry of the intercalation to control the shape of the reaction interface. This control is less effective for C₇₀ than for the C₆₀ fullerene.     

The Dark Reaction of C60 and of C70 with Molecular Oxygen at Atmospheric Pressure and Temperatures between 300 K and 800 K

Abstract

The solid fullerenes C₆₀ and C₇₀ react in deliberate and adventitious situations with molecular oxygen in a wide range of temperatures. Using thin films and polycrystalline bulk samples with well-defined structures and chemical histories we investigated the complex process from molecular intercalation over atomic adduct formation to deep oxidation and polymerisation which together describe the oxidation reaction. A combination of photoemission (UPS, XPS) photoabsorption (XAS), FT-IR, temperature-programmed and isothermal gravimetric measurements and DSC allowed to identify the existence and gradual interconversion of the three types of solid products besides the gas phase products CO and CO₂. Both fullerenes react along the same reaction path. The difference of the molecular structures results in different activation barriers in the initial step of intercalation and in the final step of cage-opening. The overall reactivity of both fullerenes is quite similar at temperatures above the gasification onset of 570 K. The formation of the various adduct compounds was found to exert a detectable influence upon the overall molecular shape of the fullerenes and a small effect on the electronic structure was observed. The only moderate differences in electronic and geometric structures of pristine and initially oxidised fullerenes precludes a pronounced molecule-by-molecule reaction control and allows the topochemistry of the intercalation to control the shape of the reaction interface. This control is less effective for C₇₀ than for the C₆₀ fullerene.     

The Dark Reaction of C60 and of C70 with Molecular Oxygen at Atmospheric Pressure and Temperatures between 300 K and 800 K

The solid fullerenes C₆₀ and C₇₀ react in deliberate and adventitious situations with molecular oxygen in a wide range of temperatures. Using thin films and polycrystalline bulk samples with well-defined structures and chemical histories we investigated the complex process from molecular intercalation over atomic adduct formation to deep oxidation and polymerisation which together describe the oxidation reaction. A combination of photoemission (UPS, XPS) photoabsorption (XAS), FT-IR, temperature-programmed and isothermal gravimetric measurements and DSC allowed to identify the existence and gradual interconversion of the three types of solid products besides the gas phase products CO and CO₂. Both fullerenes react along the same reaction path. The difference of the molecular structures results in different activation barriers in the initial step of intercalation and in the final step of cage-opening. The overall reactivity of both fiiHerenes is quite similar at temperatures above the gasification onset of 570 K. The formation of the various adduct compounds was found to exert a detectable influence upon the overall molecular shape of the fiiHerenes and a small effect on the electronic structure was observed. The only moderate differences in electronic and geometric structures of pristine and initially oxidised fullerenes precludes a pronounced molecule-by-molecule reaction control and allows the topochemistry of the intercalation to control the shape of the reaction interface. This control is less effective for C₇₀ than for the C₆₀ fullerene.     

Production and characterization of single-crystal FeCo nanowires inside carbon nanotubes

We describe the synthesis of novel monocrystalline FeCo nanowires encapsulated inside multiwalled carbon nanotubes (MWNTs). These FeCo nanowires exhibit homogeneous Fe and Co concentrations and do not contain an external oxide layer due to the presence of insulating nanotube layers. The method involves the aerosol thermolysis of toluene-ferrocene-cobaltocene solutions in inert atmospheres. The materials have been carefully characterized using state-of-the-art high-resolution transmission electron microscopy (HRTEM), electron-energy-loss spectroscopy (EELS), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), electron diffraction, HREELS-STM elemental mapping, X-ray powder diffraction, and SQUID magnetometry. We noted that the formation of FeCo alloys occurs at relatively low pyrolytic temperatures (e.g., 650-750 degrees C). These single-crystal nanowires, which have not been reported hitherto, always exhibit the FeCo (110) plane parallel to the carbon nanotube axis. The FeCo nanomaterials have shown large coercive fields at room temperature (e.g., 900 Oe). We envisage that these aligned ferromagnetic nanowires could be used in the fabrication of high-density magnetic storage devices and magnetic composites.     

On the Evaporation of C60 in Vacuum and Inert Gases at Temperatures Between 830 K and 1050 K

Abstract

The binary diffusion coefficient D_AB of subliming C₆₀ in He, N₂ and Ar gas has been determined at a gas pressure between 5 and 10 kPa. It resulted D_AB/(cm^2/s)=D_o(P_o/P_t(T/T_o)ⁿ as a function of the total pressure P_t of the vapor phase and temperature T. At T_o=273 K and P_o=1.0133×10⁵ P_a, the values D_o = (0.059±0.004) cm²/s, (0.011±0.003) cm²/s, (0.012±0.007) cm²/s, n = 1.77±0.06, 2.02±0.18, 1.77±0.37 were obtained for He, N₂, Ar and (830-1000) K, (800-1020) K, (875-1095) K, respectively. Only 40 wt% of the initial C₆₀ material yielded reliable D_AB where the vapor pressure of C₆₀ followed log₁₀(P/Pa) = -(8976±60)/T/K+(11.05±0.07) for T between 640 and 1055 K.     

On the Evaporation of C60 in Vacuum and Inert Gases at Temperatures Between 830 K and 1050 K

The binary diffusion coefficient D_AB of subliming C₆₀ in He, N₂ and Ar gas has been determined at a gas pressure between 5 and 10 kPa. It resulted D_AB/(cm^2/s)=D_o(P_o/P_t(T/T_o)ⁿ as a function of the total pressure P_t of the vapor phase and temperature T. At T_o=273 K and P_o=1.0133×10⁵ P_a, the values D_o = (0.059±0.004) cm²/s, (0.011±0.003) cm²/s, (0.012±0.007) cm²/s, n = 1.77±0.06, 2.02±0.18, 1.77±0.37 were obtained for He, N₂, Ar and (830-1000) K, (800-1020) K, (875-1095) K, respectively. Only 40 wt% of the initial C₆₀ material yielded reliable D_AB where the vapor pressure of C₆₀ followed log₁₀(P/Pa) = -(8976±60)/T/K+(11.05±0.07) for T between 640 and 1055 K.     

Intramolecular Negative-Effect in the Alkali-Metal-Doped C60; A2C60 and A4C60

It is investigated theoretically why alkali-metal-doped C₆₀, A_xC₆₀, is a nonmagnetic semiconductor for x=2 and 4. We find that this is the consequence of the intramolecular negative- U effect due to the strong electron-phonon interaction and the weak Hund coupling between conduction electrons.     

Temperature Dependence of Local Structure of Rb3C60 Superconductor

EXAFS studies on Rb₃C₆₀ are made as a function of temperature from 6K to 300 K. It is found that the distance r_Rb-c and the second, third and fourth cumulant terms of Debye-Waller factors, σ(2), σ_c(3) and σ_c(4), show the anomalous behavior near T_c.