ESR Studies on C60.OX and C60.HMTTEF

Abstract

The electron spin resonance (esr) of C₆₀.O_x and C₆₀.HMTTEF (hexamethylentetratellurafulvalene) has been investigated at 9.36 GHz as a function of temperature. T (298 T 4 K). C₆₀.O_x shows an esr absorption of equal ‘g’ value to that of C₆₀ exposed to O₂ and light but is more intense. The C₆₀ in its pure form is in a singlet state. The impurity sites introduced by O₂ produce the esr absorption. The Curie - Weiss plots of inverse esr absorption intensity versus temperature indicate an antiferromagnetic TH = 50 for air exposed C₆₀ and 90K for C₆₀O_x. From the esr intensity at room temperature, the calculated number of free spins (S = 1/2) is =1/3 per mole of C_60-O_x. In case of C₆₀.HMTTEF, there is a very weak esr absorption at room temperature suggesting that the room temperature form is diamagnetic with very small charge transfer between C₆₀ and HMTTEF. This conclusion is consistent with the structure and magnetic susceptibility of this cocrystal. As the temperature is lowered, the equilibrium: AD <=> A?^? + D?⁺ is displaced towards the formation of free radical species, A?^? and D?⁺.     

Pressure dependence of superconductivity in Ba4C60

In Ba_xC₆₀ superconductivity (sc) at T_c=(6.8±0.2) K is associated with the x=4 phase. Here we present pressure studies of T_c of a Ba_xC₆₀ sample (dT_c/dp= –1.9 K/GPa) which exhibits a large volume fraction of the x=4 phase. In the alkali-fullerides K₆₀C₆₀ and Rb₃C₆₀ T_c is governed by the electronic density of states N(E_F) (T_c exp{–1/N(E_F)V}). The dependence of N(E_F) on volume can be approximated by N(E_F)=N₀ d _eff ³, where d_eff is the surface to surface distance of neighboured C₆₀-molecules. This relationship is corroborated by (T_C,d)-data of various compounds of the form A_3–xB_xC₆₀ (A: K, Rb; B: K, Rb, Cs; x2). Strong deviation from this behavior is found for the alkaline-fulleride superconductor Ba₄C₆₀. This deviation is attributed to the hybridization of alkaline-metal s-, d-states and C₆₀ -states.     

Enhancement of the quantum efficiency of radiative recombination of the fullerene C70 in a toluene solution

An investigation of the spectra and quantum efficiency of photoluminescence provided direct evidence of the presence of C₇₀ aggregates in a toluene solution. Toluene solutions of C₆₀ and C₇₀ in the proportion 1 g/l and artificial mixtures of these were studied. The hypothesis is put forward that mixed aggregates of C₆₀ and C₇₀ exist. Pis’ma Zh. Tekh. Fiz. 24, 70–75 (November 26, 1998)     

Synthesis and Characterization of Some Organic Cations Buckminsterfulleride Salts

Abstract

Six new organic cations (Triphenylmethane dye, Hemicyanine dyes, calix[4]arene derivative) buckminsterfulleride salts were prepared by metathetical reaction. the products were characterized by UV-NIR, ESI-MS, ESR and FT-IR. the ESR spectra show a typical C₆₀ anion radical signal and dependence on countercations and temperature, the linewidth has an abrupt increasing at about the THF melting point and the ESR signal diminishes gradually when the temperature below 170K. the possible explanation is that the THF crystallization results in exclusion of solute from the lattice and leads to C₆₀ anion aggregation, which promotes line broadening and diamagnetic dimer of (C₆₀)₂ ^2? forming.     

Electron paramagnetic resonance of Cu2+ and VO2+ ions in phosphate glasses

Electron spin resonance (ESR) spectra ofx(CuO · V₂O₅ (1 –x) (Na₂O · P₂O₅) andx(CuO · 2V₂O₅) (1 –x) (Na₂O · P₂O₅) glasses for 0 x 40 have been studied at the X-band and at 300 K. It is found that forx 5, both Cu²⁺ and VO²⁺ are present, mostly as isolated species. Forx 10, broad resonance lines atg = 2.1524 forx(CuO · V₂O₅) (1 –x) (Na₂O · P₂O₅) and atg = 2.1448 forx(CuO · 2V₂O₅) (1 –x) (Na₂O · P₂O₅) are observed which may be mainly due to dipole-dipole type interaction between transition metal (TM) ions. Spin Hamiltonian parameters of TM ions have been calculated. Optical spectra of the sodium phosphate glasses doped with single TM ions have also been studied. The theoretical optical basicity, A_th, of these doped glasses has been calculated. It is found that for VO²⁺ ionsg ,g and increase whileA ,P and g _|/g decrease with increase in A_th. However, no significant change is observed in the spin Hamiltonian parameters of Cu²⁺ with the change in A_th.     

Influence of Structural Effects on the Superconductivity of Ammonia‐Doped NaK2C60

Abstract

The role of structural effects on the anomalous superconducting properties of (NH₃) _x NaK₂C₆₀ fullerides was investigated using ²³Na and ²H NMR. In the metallic (NH₃) _x NaK₂C₆₀ (0.5 < x < 1) compounds, the ²³Na quadrupolar splitting is observed to be independent from ammonia concentration x which, on the other hand, substantially affects the superconducting transition temperature. The marginal influence of sodium cation displacement in the superconducting properties is confirmed also by the absence of any electric field gradients in the recently synthesized nonmagnetic insulator (NH₃)₂NaK₂C₆₀. ²H NMR measurements on deuterated samples enabled us to study the ND₃ reorientational dynamics, indicating a hampered ammonia rotation in the insulating (x = 2) case, where the Na⁺ ions become centered.     

Nepheline gradient solid solutions

Nepheline solid solutions having high thermal expansion coefficients ranging from 1.2 to 1.8×10⁻⁵°C⁻¹ were prepared from the systems NaAlSiO₄-LiAlSiO₄ and NaAlSi₄-KAlSiO₄. The thermal expansion coefficient decreased by substituting Li⁺ for Na⁺, and increased by substituting K⁺ for Na⁺. By forming a series of the nepheline solid solutions in a single ceramic body, a functionally gradient material with respect to thermal expansion was prepared. The thermal expansion coefficient should gradually vary with position according to the composition x in Na_1−x M_xAlSiCO₄ (M=Li and K).     

Surface degradation behaviour of sodium borophosphate glass in aqueous media: Some studies

The degradation behaviour of phosphate glass with nominal composition, 40Na₂O-10BaO-xB₂O₃-(50-x)P₂O₅, where 0 ≤ x ≤ 20 mol%, was studied in water, HCl and NaOH solutions at room temperature to 60°C for different periods extending up to 300 h. These glasses were synthesized by conventional melt-quench technique. Dissolution rates were found to increase with B₂O₃ content in the glass. The dissolution rates for the glass having 10 mol% B₂O₃ were found to be 0·002 g/cm² and 0·015 g/cm² in distilled water and 5% NaOH solution, respectively, at room temperature after 225 h of total immersion period, whereas it increased considerably to 0·32 g/cm² in 5% NaOH at 60°C after 225 h. However, glass samples with x = 15 and 20 mol% B₂O₃ were dissolved in 5% HCl solution after 5 h immersion. The degradation behaviour has been correlated with the structural features present in the glass. The optical microscopy of the corroded surface revealed that the corrosion mechanism were different in acid and alkali media.     

Crystal and Molecular Structure of a New Complex of [60]Fullerene, 2(C60)·2(TMTSF)·(C6H6)

Abstract

A new molecular complex of [60]fullerene with composition 2(C₆₀)·2(TMTSF)·(C₆H₆) was synthesized. The structure and composition of the complex were found by an x-ray study. Crystal data: 2(C₆₀)2(C₁₀H₁₂Se₄)(C₆H₆), M = 2415.4, monoclinic, a = 19.388(4), b = 13.410(2), c = 32.467(6) A, β = 92.71(2)°, V = 8432(3) [Adot]³, space group P2₁/n, Z = 4, d_calc = 1.903 g/cm³, R = 0.0606. The crystal structure was shown to be layered with the alternating layers of three types. Two of them have the same composition (C₆₀, TMTSF) but different interorientation of molecules in a layer and different number of shortened contacts C…C and C…Se. The third layer consists of benzene molecules. The energy of intermolecular interactions C₆₀…. TMTSF was estimated by ab initio calculations. The TMTSF molecule has a “boat” conformation.     

Pulsed Laser Processing of Carbon and Silicon Clusters and Thin Films

Abstract

Several new processes have been developed for the preparation of fullerenes and thin films by using a pulsed excimer laser. The irradiation of a pulsed KrF excimer laser beam onto a C₆₀ powder target produced single phase C₆₀ thin films when the laser energy fluence was in the range between 40 and 50mJ/cm². By atomic force microscopy, the laser-deposited C₆₀ thin film was verified to have a surface far smoother that the surfaces of films produced by the conventional evaporation method. The stainless steel rods coated with this film exhibited an excellent tribological property. Cluster formation from SiC and other carbides MC_n(M=Ti,W,B) was investigated by laser desorption time-of-flight mass spectrometry. No clear indication was observed for the production of such clusters as (SiC)₆₀ and (M_xC_60-x) from the sintered targets directly as well as from the films laser deposited from the targets. However, C₆₀ and C₇₀ were found to exists in the laser-deposited films, indicating a new applicability of pulsed laser processing for segregative cluster synthesis from solid solution. Preliminiary results on thin film deposition via pulsed ablation of (Ba,Na)_xSi₄₆ clathrate were also presented.     

Intercalation properties and electrochemical reduction of chromium oxide chloride

Intercalation complexes M_x(DMSO)_y[Cr_1?x⁺³ Cr_x⁺²OCl] have been obtained by cathodic reduction of chromium oxide chloride in a solution of M⁺ (Li⁺, Na⁺, K⁺) in dimethyl-sulfoxide (DMSO). From galvanostatic experiments upper limits for x were determined to be x = 0.12 for Li⁺ and x = 0.05 for Na⁺ and K⁺. Correlations between the degree of reduction (x), ionic radii of M⁺, amount of solvent intercalation (y), lattice expansion and M⁺-coordination are discussed.Chromium oxide chloride samples with either chloride deficiency or Cl^? by O^2? replacement showed a degree of reduction of only x = 0.05 upon Li⁺-intercalation.     

High Temperature Vaporization and Thermodynamic Properties of the Potassium - C60 Phases

Abstract

Knudsen cell mass spectrometry and simultaneous Knudsen-torsion effusion technique were used to study K/C₆₀ binary system. Activities of potassium and/or C₆₀ were measured in all heterogeneous regions of the K-C₆₀ phase diagram and the Gibbs energies of formation of K_xC₆₀(x=l, 3, 4, 6) were calculated. Gaseous species KC₆₀(g) was identified and the Gibbs energy of formation of KC₆₀(g) was calculated directly from the experimental data.     

Systematic hardness measurements on mixed and doped crystals of rubidium halides

Efforts are made to improve the hardness of rubidium halide crystals by (i) solid solution hardening and (ii) impurity hardening. Systematic microhardness measurements have been made on rubidium halide mixed crystals (RbBr-RbI and KI-RbI) and rubidium halide crystals doped with Sr²⁺ ions. The composition dependence of the hardness of mixed crystals follows the law ΔH _v =K x (1− x),where ΔH _v is the enhancement in hardness,K a constant andx and (1 −x) the concentrations of the first and second component of the mixed crystals, respectively. The hardness of doped crystals increases with the concentrationC of the dopant according to the law, ΔH _v+6 =k C ^m ,wherek andm are constants. The relative efficacy of the two methods of hardening is discussed.     

Synthesis and characterization of new organic metals formed by interaction of FeCl3 with polyacetylene (CH)x and poly(para)phenylene (C6H4)x

The reaction of FeCl₃ dissolved in dry nitromethane with polyacetylene, (CH)_x, results in the formation of p-type conducting polymers (σ = 780 ohm^?1 cm^?1). IR spectra of (CH)_x lightly doped with FeCl₃ exhibit the formation of two new bands characteristic of other p-type dopants of (CH)_x. Mössbauer spectroscopy shows that the anion formed in the reaction is a high spin Fe^II complex. The doping causes significant change in (CH)_x interchain distances as evidenced by X-ray diffraction. Similar reaction occurs between poly(para)phenylene, (C₆H₄)_x and FeCl₃ causing the increase of the conductivity of compressed poly(para)phenylene powder to metallic regime. The reaction mechanism is more complex than in the case of (CH)_x since Mössbauer spectroscopy shows the existence of two types of Fe^II iron ions.     

Magnetic Properties of Some Fullerene Compounds

Abstract

Magnetic properties of some fullerene intercalation compounds are detailed. A few examples are presented including properties of 1) Acceptors: MoF₆ From magnetic measurements the existence of positively charged C₆₀ (C₆₀ ⁺) can be inferred 2) Donnors: Yb_xC₆₀ and Eu₃C₆₀ compounds. Magnetic properties of (Yb_xC₆₀) are dominated by crystal field effects. A high field magnetic transition (17 T at 4 K) occurs in Eu₃C₆₀ associated with a large hysteresis of the magnetization and relaxation effects. This complex magnetic behaviour is attribued to Eu²⁺.     

Spectroscopic Investigations of M(CO)5-C60 (M = W,MO) Complexes: Precursors for Metal Fullerides

Abstract

Photochemical reactions of M(CO)₆ (M = W, Mo) with C₆₀ in solution yield η:²-complexes of M(CO)₅ with C₆₀. The complexes have been characterised by IR, UV/VIS, NMR and DSC. They do not show any orientational ordering down to 12 K and all the infrared bandwidths remain the same down to this temperature. The complexes can be decomposed thermally or photochemically yielding metal fullerides, which show characteristic reduction in peak width in the variable temperature IR spectra due to orientational ordering. Transitions are manifested in calorimetric studies also. Metal → C₆₀ charge-transfer is observed in IR and XPS. A high temperature IR study of the C₆₀-W(CO)₅ complex reveals sequential elimination of the carbonyls yielding MC₆₀. The study shows that carbonyl complexes can be used as precursors to make transition metal fullerides.     

Conductivite ionique dans les systems Na1+xZr2−xLx(PO4)3 (L = Cr,Yb)

Ionic conductivity measurements in the solid solution Na_1+xZr_2?xL_x(PO₄)₃ (L = Cr, Yb) have been carried out. The materials have a Nasicon-type structure in a 0 ? x ? x^max._L range (x^max._Cr = 2.0 and x^max._Yb = 1.9). A small monoclinic distortion appears at low temperature for Na₃Cr₂(PO₄)₃. As in the Na_1+xZr₂P_3?xSi_xO₁₂ system a strong increase of the conductivity with rising x has been observed. The results are discussed in connection with temperature and structural parameters.     

Neutron diffraction study on the medium and short-range order of ternary chalcogenide glasses

Glasses from Ge_xAs_40–xS₆₀, Ge_xAs_40–xSe₆₀, Ge_xSb_40–xS₆₀ and Ge_xSb_40–xSe₆₀ families have been investigated by neutron diffraction. The well expressed first sharp diffraction peak of the neutron spectra at Q = 10–14 nm^–1 has been explained by a pseudo-binary model of the structure of studied chalcogenide glasses. The amplitude of this interference maximum increases with Ge-content in all investigated samples. Experimental radial distribution functions show that the basic structural units, GeS(Se)₄ tetrahedra and As(Sb)S₃ or As(Sb)Se₃ pyramids persist in glassy state for the whole range of studied compositions. Crystal-like model distribution functions have been used to describe the short-range order observed. A shortening of heteropolar bonds in S-containing glasses as well as an increase of heteropolar bonds length in Se-containing glasses has been found.     

Synthesis and structural studies of Na<Subscript>2</Subscript>O-ZnO-ZnF<Subscript>2</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> oxyfluoride glasses

This paper describes the synthesis and spectroscopic studies of the glass system, 20Na₂O-(20-x) ZnO-xZnF₂-60B₂O₃(x = 0, 5, 10, 15, 20), prepared by melt quenching method. The analyses of DSC and XRD did not show the crystallinity of the glass sample. ¹¹B MAS-NMR shows the presence of sharp peak around −14 ppm. From the IR studies, the broadening of the peak around 1200–1400 and 800–1100 cm⁻¹ shows the presence of mixed linkages like B-O-B, B-O-Zn in the network.     

Vibrational Spectra of Chloro‐ and Bromofullerenes

Abstract

Infrared and Raman scattering spectra of the chloro‐ and bromofullerenes C₆₀Cl₆, C₆₀Br₆, C₆₀Br₈, C₆₀Br₂₄, C₇₀Cl₁₀, and C₇₀Br₁₀ have been studied both experimentally and theoretically employing first‐principle calculations. Complete vibrational assignment for these molecules is proposed.