Doping mechanism of C60 fullerene by sodium and potassium

ESR spectra of localized states of KC₆₀ and K₂C₆₀ and conducting electrons of K₃C₆₀ were recorded in the process of doping C₆₀ by potassium. The ESR spectra of K _x C₆₀ (x < 3) and Na _x C₆₀ correspond to the semiconductive state of the samples.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2190–2195, November, 1995.This work was financially supported by the Russian Foundation for Basic Research (Project No. 93-03-4101). The work was performed under terms of the interindustry program Fullerenes and Atomic Clusters.     

X-ray spectra and electronic structure of K x C60

Highly resolved CK_α spectra of the compounds K₃C₆₀ and K₆C₆₀ synthesized by thermal decomposition of potassium azide in a vacuum chamber of an X-ray tube were obtained. The possibility of monitoring the extent of intercalation of fullerenes by the CK_α spectra is demonstrated. The results are compared with the MNDO calculations for K₆C₆₀. The data are used to construct a diagram of the structure of the highest occupied and C1s levels in the series C₆₀−K₃C₆₀−K₆C₆₀. Deceased. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 3, pp. 514–519, May–June, 1996.     

Rotation dynamics of C60 molecules in organic superconductor K3C60

The rotation dynamics of C₆₀ molecules in organic superconductor K₃C₆₀ has been investigated from the viewpoint of intramolecular interaction. It is determined that the rotation of C₆₀ at mom temperature has been frozen up within a small region of rotation angle (0°–50°), and pointed out that the reason for the freeze is the physical interaction rather than the geometrical hindrance. The computations of the interactions for alkali-doped compounds A_3-x A′ _x C₆₀ (x = 1, 2, 3; A, A′ = K, Rb, Cs) other than K₃C₆₀ have also been camed out. Fmm the obtained results, it is seen that the superconducting transition temperatures T, are strongly connected with the interactions in them, and this observation is consistent with the discovery of the correlation between T_c, and lattice constants a. Project supported by the National Natural Science Foundation of China.     

Interaction of platinum fulleride C60Pt with deuterium

X-ray photoelectron spectra and catalytic properties of platinum fulleride C₆₀Pt were studied. The value of the bond energy of Pt4f_7/2 (72.4 eV) found for platinum in the starting C₆₀Pt suggests a partial charge transfer from Pt to C₆₀. The interaction of solid platinum fulleride with gaseous deuterium leads to the formation of fullerene deuterides C₆₀D _x and Pt clusters. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 999–1002, May, 1999.     

Positron annihilation in C60 and K6C60

The positron density distributions in C₆₀ and K₆C₆₀ have been evaluated using the positron lifetime and Doppler-broadening spectroscopy for the annihilation radiation.In C₆₀, positrons are distributed in the interstitial sites between the C₆₀ molecules,which has been demonstrated by measurements of the temperature dependence of the Doppler-broadening of the annihilation radiation. On the other hand, the positron density distribution must be greatly changed in K₆C₆₀, because positrons are repelled by Coulomb interactions by the positively charged K atoms. It has been observed that there is an extremely short lifetime and a small Doppler-broadening component for the positron annihilation in K₆C₆₀. This component is considered to reflect the positron annihilation inside a C₆₀ molecule.     

Possible explanation of unstable superconducting phase in KxC60 with Tc=21 K

EPR and MMMA methods revealed two K₃C₆₀ phases, the unstable one with T_c=21.0 K containing C¹⁻₆₀ centers and a stable one with T_c=18.0 K constituting of C₆₀ molecules in the form of C³⁻₆₀ ions. The metastable phase appears only in the first stage of potassium intercalation of fullerene at the C₆₀/K₃C₆₀ phase boundary. This phase is being transformed to a stable one on further doping to K₃C₆₀. The appearance of a metastable phase is related to the instability of the fullerene C₆₀ fcc structure during the process of octahedral sites being filled by potassium ions. The critical temperature T_c shift upwards from 18.5 to 21.0 K is ascribed to the possible process of potassium transient back charge transfer valency, i.e. K₃¹⁺C₆₀³⁻↔K₂¹⁺K¹⁻C₆₀¹⁻ [3K¹⁺+C³⁻₆₀⇔2K¹⁺+K¹⁻+C₆₀¹⁻], which could enlarge the unit cell necessary to shift a superconducting critical temerature according to T_c on volume dependence.     

Towards the selective formation of specific isomers of fullerenes: T - and p-dependence in the yield of various isomers of fullerenes C60–C84

Systematic fractional change in the yield of various isomers of fullerenes was revealed to strongly depend on temperature of a buffer gas. A new kinetic consideration is proposed for understanding the observed temperature- and pressure-dependence of yield of fullerenes. The model consists of three competitive reactions in consideration of plausible behaviors of a precursor, (1) decomposition into smaller fragments, (2) isomerization leading to formation of a stable fullerene cage, and (3) growth into a larger carbon cluster. Arrhenius activation energy of formation of stable fullerenes was determined to be 0.8 eV for both C₆₀ and C₇₀, while a higher energy of 2.0?3.3 eV for seven different isomers of higher fullerenes ranging from C₇₆ to C₈₄. Correlation in the activation energy is noted for a series of higher fullerenes with different sizes, suggesting the existence of a specific precursor in their formation processes.     

New complexes of fullerences C60 and C70 with CoII and MnII tetraphenylporphyrinates and their ESR study

Complexes of fullerenes C₆₀ and C₇₀ with cobalt(II) and manganese(II) tetraphenylporphyrinates of compositions Mn(TPP)·(C₆₀)₂(CS₂)_1.5 (1), Mn(TPP)·C₇₀(CS₂) _x , wherex<=1.25 (2), Co(TPP)·C₆₀(CS₂)_0.5 (3), and Co(TPP)·C₇₀(CS₂) _x , wherex<0.25 (4), were synthesized and studied by ESR spectroscopy. At 77 K, complexes1 and2 have singlet ESR spectra characteristic of the low-spin (S=1/2) state of Mn^II, withg=2.002 and linewidths of 250 G and 300 G, respectively, and differing significantly from that of the initial Mn^II(TPP) (g ₁=5.9 andg=2.0,S=5/2). The spectra of complexes1 and2 exposed to oxygen exhibit hyperfine structure due to interaction with⁵⁵Mn and¹⁴N nuclei. The ESR spectra of complexes3 and4 are asymmetric (<g>=2.4, ΔH _pp=(500–600) G), which is due to the overlap of parallel and perpendicular spectral components. The absence of ESR signals from C₆₀ ^.− and C₇₀ ^.− radical anions makes it possible to conclude that the formation of complexes1–4 is not accompanied by electron transfer from Co(TPP) and Mn(TPP) to fullerences C₆₀ and C₇₀. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 722–725, April, 1999.     

Structure and properties of superconducting and nonsuperconducting alkali-metal fullerides axc60 (A = Na,K, Rb,or Cs)

Shielding diamagnetism and X-ray powder diffraction were used to characterize alkali-metal fullerides of sodium, potassium, rubidium, and cesium. The superconducting phase in both the potassium and rubidium systems has the composition A₃C₆₀ and a face centered cubic structure with alkali metal filling all octahedral and tetrahedral sites of close packed C₆₀, layers. High pressure magnetic susceptibility measurements on K₃C₆₀ showed a negative pressure dependence on the superconducting transition temperature of −0.78 K/kbar. No evidence for superconductivity was observed in either the sodium or cesium systems, even though Na₃C₆₀ appears isostructural with K₃C₆₀ and Rb₃C₆₀.     

High resolution photoemission study of condensed layers of nitrogen and carbon monoxide

High resolution (0.09 eV) UPS spectra have been obtained of condensed films of N₂ and CO. All spectral features are broadened by ? 0.6 eV upon condensation. The origin of this broadening is discussed. The difference in linewidths for all equivalent levels, Δε_CON2 ～ 0.1 to 0.2 eV can be understood in terms of a hole-dipole multiphonon excitation mechanism. Photoemission from what is believed o be the a³Π excited neutral state of CO has been detected in the solid phase for the first time.     

Raman characterization of C60 salts electroformed in aqueous media

Electrochemical intercalation of C₆₀ films from aqueous LiOH, KOH, NaOH, RbOH and CsOH solutions was studied by electrochemical methods and Raman spectroscopy. The intercalation of these alkali metals cations gives rise to reduction peaks at different applied potentials depending on the chemical nature of the cation used. Reduction of the C₆₀ films was observed to rather different extents depending on the composition of the working solution. Electrochemical and Raman experiments show that K⁺ and Cs⁺ ions form the most stables and active salts, while Li⁺, Na⁺ and Rb⁺ are not significantly intercalated. Furthermore, Raman characterization of the K⁺ and Cs⁺ doped films leads to the identification of the its main components, namely the K₂C₆₀ and CsC₆₀ species.     

X-ray absorption spectroscopy of rare earth intermediate oxides

The M_IV–V and L_III absorption spectra (between 850 and 7500 eV) of intermediate rare earth oxides (Pr₇O₁₂ and Pr₉O₁₆) were studied. These oxides required careful preparation and handling in order to assure their composition.The spectra are characterized by multiplet features that are interpreted as having contributions from both trivalent and tetravalent sites. In the M_IV–V spectra the appearance of distinct multiplet lines and additional weaker features demonstrate clearly the increasing ratio of the tetravalent sites as the oxygen content increases from Pr₇O₁₂ to PrO₂. Similar behavior has been observed for the CeO_x and TbO_x systems. These observations show that in these oxides, the trivalent and tetravalent sites are inequivalent and that the evidence of valence transition is seen in the appearance of the complex spectral features originating from the tetravalent sites.     

Investigation on the interaction on and the rotation of C60 in alkali-doped complexes AXA′3-XC60 (X = 1, 2, 3; A,A′ = alkali)

The interaction on and the rotation of C₆₀ in akali-doped C₆₀ solids, A_XA′_3-XC₆₀ (X = 1, 2, 3; A, A′ = alkali), have been calculated with Buckingham potential model. The results show that the total interaction on C₆₀ changes dramatically when the pure C₆₀ solid is alkali-doped into K₃C₆₀. The interaction on C₆₀ in K₃C₆₀ is about 20 times greater than that in pure C₆₀. And the main component in the former, occupying > 90% is electrostatic, while in the latter, the main components, occupying > 90%, are dispersive and repulsive. The results also show that in contrast to the whole-region free rapid rotation of C₆₀ molecule in its pure solid, the rotation of C₆₀ in K₃C₆₀ is mostly forbidden due to a 10 times increase (reaching about 300 kJ/mol) in potential barrier, except for the region from 0° to 50° where a broad, smooth, and shallow potential well exists. Calculations for alkali-doped complexes other than K₃C₆₀, i.e., A_XA′_3-XC₆₀ (X = 1, 2, 3; A, A′ = K, Rb, Cs), come to the same conclusion. Finally, an interesting and meaningful result is that the superconducting transition temperatures of A_XA′_3-XC₆₀ (X = 1, 2, 3; A, A′ = K, Rb, Cs) change inversely with the total interactions on C₆₀. © 1995 John Wiley & Sons, Inc.     

Mössbauer spectra,magnetic and electrical behavior of Ba1+xFe2S4 phases

The Mössbauer spectra, and magnetic and electrical properties of Ba_1+xFe₂S₄ infinitely adaptive phases with 0.074 ≤ x ≤ 0.142 and of BaFe₂S₄ were studied. The properties are highly anisotropic because of the presence in the structure of one-dimensional infinite chains of edge sharing FeS₄ tetrahedra. BaFe₂S₄ is a semiconductor, E_g = 0.66 eV; magnetic susceptibility can be fit by a one-dimensional Heisenberg model with spin $\text{5}\text{2}$ and $\text{J}\text{k}\text{= ?30°}\text{K}$. The Ba_1+xFe₂S₄ phases have Curie-Weiss behavior with an effective moment of about 2 B.M. The moment increases with x. These phases are metallic. The Mössbauer isomer shift varies linearly with valence, increasing with increasing x. The single quadrupole split absorption line characteristic of these compounds disappears at about 270°K and a complex spectrum consisting of overlapping hyperfine patterns appears at lower temperatures. Magnetic short-range ordering is responsible for this behavior although the susceptibility in this temperature range does not reflect this effect.     

Synthesis,Structure, and Theoretical Study of Trifluoromethyl Derivatives of C84(22) Fullerene

Trifluoromethylation of higher fullerene mixtures with CF₃I was performed in ampoules at 400 to 420 and 550 to 560 °C. HPLC separation followed by crystal growth and X‐ray diffraction studies allowed the structure elucidation of nine CF₃ derivatives of D₂‐C₈₄ (isomer 22). Molecular structures of two isomers of C₈₄(22)(CF₃)₁₂, two isomers of C₈₄(22)(CF₃)₁₄, four isomers of C₈₄(22)(CF₃)₁₆, and one isomer of C₈₄(22)(CF₃)₂₀ were discussed in terms of their addition patterns and relative formation energies. DFT calculations were also used to predict the most stable molecular structures of lower CF₃ derivatives, C₈₄(22)(CF₃)_2–10. It was found that the addition of CF₃ groups to C₈₄(22) is governed by two rules: additions can only occur at para positions of C₆(CF₃)₂ hexagons and no additions can occur at triple‐hexagon‐junction positions on the fullerene cage.     

The phase diagram of fullerene C60 at high temperatures and pressures

Experimental and literature data were used to calculate the Gibbs energies of polymerized C₆₀ phases and construct the equilibrium T-p phase diagram of fullerene C₆₀ at temperatures from 0 to 1000 K and pressures from 0 to 8 GPa. The diagram contains stability regions of the orthorhombic, tetragonal, and rhombohedral polymerized C₆₀ phases and primitive cubic (PC) and face-centered cubic (FCC) nonpolymerized C₆₀ phases. The orthorhombic phase (linear polymer) is an equilibrium phase at 298 K and 1 bar and in the adjacent region. The equilibrium line observed experimentally (FCC C₆₀—orthorhombic phase) is well described by the phase diagram. The optimum temperatures and pressures of the synthesis of polymerized phases are determined by kinetic rather than thermodynamic parameters.     

Preparation and properties of polyethoxysilsesquioxane‐C60 hybrids

Free‐standing films of C₆₀‐polyethoxysiloxane hybrids were prepared, and their optical limiting properties were evaluated. Triethoxysilylated C₆₀, with a formula of H₃C₆₀[Si(OEt)₃]₃, was synthesized by the hydrolysilylation of C₆₀ with triethoxysilane in the presence of platinum catalyst. C₆₀‐polyethoxysiloxanes were prepared by a cohydrolytic polycondensation of triethoxysilylated C₆₀ with tetraethoxysilane in a molar ratio of Si/C₆₀ = 10–1000 under nitrogen flow. The molecular weight of C₆₀‐polyethoxysiloxane increased with a decrease of Si/C₆₀. Transparent and flexible free‐standing films were prepared by aging an ethanol solution of C₆₀‐polyethoxysiloxane at 80 °C for 6–8 days. The mechanical strength and Young's modulus increased with a decrease in Si/C₆₀. These free‐standing films showed an optical limiting property, for which the threshold value decreased from 1163 mJ/cm² (Si/C₆₀ = 1000) to 130 mJ/cm² (Si/C₆₀ = 10) with a decrease of Si/C₆₀. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3273–3279, 2007     

Structures and phase transitions in (K<Subscript>1-x</Subscript>Rb<Subscript>x</Subscript>)<Subscript>2</Subscript>SeO<Subscript>4</Subscript>

Summary The phase diagram of the mixed crystal (K_1-xRb_x)₂SeO₄ was determined by means of thermal analysis and neutron scattering experiments. The hexagonal to orthorhombic phase transition line exists for any x. The normal-incommensurate phase transition temperature decreases continuously with increasing Rb content. However, the incommensurate-commensurate phase transition was not observed except for K₂SeO₄. According to the clear softening of the Σ₂- Σ ₃ phonon branches and the finite frequency at 0 K for x>0.34, an existence of the hypothetical phase transition was confirmed.     

Radical reactions of C84

Pulse radiolysis and steady-state radiolysis experiments describing the radical and electron transfer reactions of C₈₄ are reported here for the first time. C₈₄ reacts readily with radiolytically generated chloromethyl (^•CCl₃) and trichloromethylperoxyl (CCl₃OO^•) radicals in CCl₄. The formation of the radical adduct has been confirmed from its characteristic absorption in the UV (320 nm) and visible (480 nm). Radical-induced oxidation in 1,2-dichloroethane (1,2-DCE) resulted in a short lived transient absorbing at 920 nm. Reduction of C₈₄ in toluene/2-propanol/acetone could be conveniently followed by formation of an absorption band with an absorption maximum at 960 nm.     

Physicochemical Properties and Structure of SiC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>:Fe Films Grown From a Gas Mixture of Ferrocene,Hydrogen and 1,1,3,3,5,5-Hexamethylcyclotrisilazane

Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), and EXAFS spectroscopy are used to study the composition and structure of SiC_xN_y:Fe films obtained by chemical vapor deposition (CVD) in the Fe–Si–C–N–H system from a mixture of hydrogen, ferrocene (C₅H₅)₂Fe, and organosilicon compound 1,1,3,3,5,5–hexamethylcyclotrisilazane (HMCTS) C₆H₂₁N₃Si₃. The films are deposited under low pressures (LPCVD) at 1123–1273 K, and their phase composition at 300–1300 K is predicted using thermodynamic modeling. The obtained films are nanocomposites with amorphous matrices containing α-Fe crystallites and carbon clusters with a size of 5–10 nm.