Comparative Raman study of the Ti complex Cp2Ti(η2-C60) · C6H5CH3 and TixC60 films

Raman spectra from the first Ti fullerene complex Cp ₂Ti(η²-C₆₀) · C₆H₅CH₃ are presented. Compared to spectra of pure C₆₀, the spectra of the Ti complex exhibit a number of new peaks due to the symmetry lowering for C₆₀. The A _g(2) mode is downshifted by 12 cm⁻¹ compared to C₆₀, which corresponds to a charge transfer of one electron per Ti-C₆₀ bond. This value (6 cm⁻¹ for one transferred electron) is identical to the downshift of the A _g(2) mode in alkali metal fullerides with ionic bonding. The spectra of Cp ₂Ti(η²-C₆₀) · C₆H₅CH₃ were compared to the spectra of evaporated Ti_xC₆₀ films. The A _g(2) mode in Ti₄C₆₀ showed a downshift of about 25 cm⁻¹ compared to pure C₆₀, which corresponds to a charge transfer of one electron per Ti atom; this is similar to the ionic alkali metal fullerides and different from η²-C₆₀-type bonding. From Fizika Tverdogo Tela, Vol. 44, No. 3, 2002, pp. 483–485. Original English Text Copyright ? 2002 by Talyzin, Jansson, Usatov, Burlakov, Shur, Novikov. This article was submitted by the authors in English.     

CORAL: QSPR models for solubility of [C60] and [C70] fullerene derivatives

Quantitative structure–property relationships (QSPRs) between the molecular structure of [C₆₀] and [C₇₀] fullerene derivatives and their solubility in chlorobenzene (mg/mL) have been established by means of CORAL (CORrelations And Logic) freeware. The CORAL models are based on representation of the molecular structure by simplified molecular input line entry system (SMILES). Three random splits into the training and the external validation sets have been examined. The ranges of statistical characteristics of these models are as follows: n = 18, r ² = 0.748–0.815, s = 15.1 –17.5 (mg/mL), F = 47–71 (training set); n = 9, r ² = 0.806–0.936, s = 12.5–17.5 (mg/mL), F = 29–103 (validation set).     

Polymeric sheets in Mg4C60

We present preliminary results on the preparation and structure of Mg₄C₆₀, a new fulleride polymer. A series of Mg_xC₆₀ compositions (0<x<6) were prepared by solid state reaction. While most samples were multiphase, a single phase material was obtained at the nominal composition Mg₄C₆₀. The X-ray diffraction pattern of Mg₄C₆₀ was successfully indexed and fitted to a rhombohedral structure based on two-dimensional polymeric sheets of C₆₀ ions. This result extends the family of fulleride polymers to the salts of multiply charged cations and thus opens a new direction in this field.     

Thermal stability of cubane C8H8

The reasons for the anomalously high thermal stability of cubane C₈H₈ and the mechanisms of its decomposition are studied by numerically simulating the dynamics of this metastable cluster at T = 1050–2000 K using a tight-binding potential. The decomposition activation energy is found from the temperature dependence of the cubane lifetime obtained from the numerical experiment; this energy is fairly high, E _a = 1.9 ± 0.1 eV. The decomposition products are, as a rule, either C₆H₆ and C₂H₂ molecules or the isomer C₈H₈ with a lower energy. Original Russian Text ? M.M. Maslov, D.A. Lobanov, A.I. Podlivaev, L.A. Openov, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 3, pp. 609–612.     

Energetics and structural stability of Cs3C60

Using the ab initio pseudopotential total-energy method and the density-functional theory, we study the energetics of face-centered-cubic Cs₃C₆₀ which is a material of great interest as a possible high transition-temperature superconductor. At the optimized lattice constant the volume per C₆₀ is found to be smaller than the the most-stable hexagon-coordination A15 phase, while the total energy of the fcc phase is about 0.9 eV higher than the A15 phase. These results indicate that a low-temperature and high-pressure synthesis method might be a possible way to produce the fcc Cs₃C₆₀ phase. In addition, it is also found that the A15 Cs₃C₆₀ should show a phase transformation from a hexagon-coordination phase to a pentagon-coordination phase under hydrostatic pressure.     

Structural, elastic, and electronic properties of icosahedral boron subcarbides (B12C3, B13C2), subnitride B12N2, and suboxide B12O2 from data of SCC-DFTB calculations

The structural, elastic, and electronic properties of a series of icosahedral phases, such as boron subcarbides B₁₂C₃ and B₁₃C₂, subnitride B₁₂N₂, and suboxide B₁₂O₂, have been studied in the framework of the SCC-DFTB method. It has been found that the B₁₂C₂ and B₁₃C₂ phases manifest metal-like properties, while B₁₂C₃ and B₁₂O₂ are semiconductors. The estimates have shown that the insertion of 2p atoms (C, N, or O) into intericosahedral pores of elemental boron can cause both a decrease in its elastic modulus (an increase in the compressibility of B₁₂N₂) and a sharp increase in the modulus B (in subcarbides B₁₂C₃ and B₁₂BCC). On the other hand, the insertion of 2p atoms into α-B₁₂ will favor an increase in its hardness (suboxide B₁₂O₂ will have a maximum hardness).     

Site selectivities of alkali ions in CsRb2C60 and Cs2RbC60 superconductors

The temperature-dependent site selectivities of Cs and Rb ions in CsRb₂C₆₀ and Cs₂RbC₆₀ superconductors are computed using the free energy obtained from the configuration entropy and the total energy calculated using the ab initio pseudopotential density-functional theory. It is found that in CsRb₂C₆₀ the smaller Rb ions can occupy a considerable number of large octahedral interstitial sites at high temperatures, however, the transition to random occupation never takes place. For Cs₂RbC₆₀ the Cs occupancy of the large octahedral interstitial sites is almost always 100%, and, interestingly, the two tetrahedral interstitial sites are randomly occupied by Cs and Rb ions even at very low temperatures.     

Metastable surface phase for NaxC60

Na_xC₆₀ thin films with Na concentration 0 ? x ? 3 were investigated using angle-dependent photoelectron spectroscopy. For low doping we observed two distinct fulleride phases coexisting with regions of pristine C₆₀. One of these fulleride phase is predominantly formed close to the surface and disappears after annealing and further addition of Na. At higher doping a phase with stoichiometry of x = 3 is formed.     

Optical studies of phase transitions in the (NH4)3VO2F4 crystal

(NH₄)₃VO₂F₄ crystals were grown, and polarization-optical studies and measurements of birefringence were conducted on crystal plates of various cuts over a wide temperature range. Phase transitions were detected at temperatures T _1↑ = 417 K, T _3↑ = 211 K, and T _4↑ = 205 K (on heating) and at T _1↓ = 413 K, T _3↓ = 210 K, and T _4↓ = 200 K (on cooling). The transitions are accompanied by anomalies of the birefringence and by twinning. The sequence of changes in the phase symmetry is assumed to be as follows: cubic Fm m ↔ orthorhombic Immm (I222₁) ↔ monoclinic 112/m) ↔ triclinic P . Near temperatures T ₂ ≈ 240–250 K, an additional anomaly of the birefringence is observed, with the crystal retaining the orthorhombic symmetry. Original Russian Text ? S.V. Mel’nikova, A G. Kocharova, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 3, pp. 562–564.     

Electronic structure and the infrared absorption and Raman spectra of the semiconductor clusters C24, B12N12, Si12C12, Zn12O12, and Ga12N12

The optimized configurations, electronic structures, charge transfers, band gaps, total energies, cohesive energies, electron density maps, infrared absorption spectra, Raman spectra, and relevant modes of natural acoustic vibrations for the semiconductor clusters C₂₄, B₁₂N₁₂, Si₁₂C₁₂, Zn₁₂O₁₂, and Ga₁₂N₁₂ are calculated using the ab initio Hartree-Fock method in the 6–31G basis set. Original Russian Text ? V.V. Pokropivny, L.I. Ovsyannikova, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 3, pp. 535–542.     

Synthesis, crystal structure, thermal and dielectric properties of bis(p-phenylenediammonium) chloride hexachlorobismuthate(III) monohydrate [C6H4(NH3)2]2ClBiCl6.H2O

Synthesis, crystal structure, and dielectric properties of [C₆H₄(NH₃)₂]₂ClBiCl₆.H₂O are reported. The compound crystallizes in the monoclinic system with space group P2₁/n. The unit cell dimensions are a = 9.836(5), b = 19.582(5), c = 13.082(5) ?, β = 104.731(5)° with Z = 4. The atomic arrangement can be described by an alternation of organic and inorganic layers. The anionic layer is built up of octahedral of [BiCl₆]^3- arranged in sandwich between the organic layers. The crystal packing is governed by means of the ionic N–H···Cl hydrogen bonds, forming a three-dimensional network. The dielectric properties have been investigated at temperature range from 297 to 410 K at various frequencies (10 Hz–100 kHz). Dielectric studies were performed to confirm results obtained with thermal analysis. The evolution of dielectric constant as a function of temperature and frequency of single crystal has been investigated in order to determine some related parameters.     

Yb2.75C60价带光电子能谱

用角积分紫外光电子能谱技术测量了Yb_2.75C₆₀薄膜的价带电子态密度分布.相纯Yb_2.75C₆₀样品通过C1s芯态x射线电子谱峰的位移表征.结果表明Yb_2.75C₆₀是半导体，在费米能级处几乎没有电子态分布.Yb 6s电子态和C₆₀LUMO能带的杂化效应不可忽略，有部分Yb 6s电子分布在Yb-C₆₀杂化能带上. 关键词： 2.75C₆₀')" href="#">Yb_2.75C₆₀ 电子能谱 电子态密度     

K3Ba3C60在不同温度下的Raman光谱研究

研究了K₃Ba₃C₆₀在不同温度下的Raman光谱.发现Raman光谱随温度的变化发生有规律的变化.随着温度的降低,所有模的线宽和强度的变化情况与纯的C₆₀的情况明显不同.径向Ag(2)模的频率在20K到室温之间有一个反常大的向高频漂移(9cm^-1),这表明在C₆₀分子和掺杂离子之间存在着轨道杂化.另外Raman光谱中Ag(1)模两个分量的相对强度随着温度的降低发生有规律的变化. 关键词： 高频漂移 轨道杂化     

Recovery of the metallic state and quantum oscillations in a dielectric salt of the (ET)8[Hg4X12(C6H5Y)2] family at X = Y = Br under compression

The quasi-two-dimensional organic conductor (ET)₈[Hg₄Br₁₂(C₆H₅Br)₂] (ET is bis(ethylenedithio)tetrathiafulvalene) undergoes a metal-insulator phase transition at T ∼ 170 K. It has been shown that a pressure of 8 kbar suppresses this transition and holds a metallic state to liquid-helium temperatures. Shubnikov—de Haas quantum oscillations with the single frequency F = 215 T have been observed in a magnetic field up to 15 T. The properties of this salt are compared to the properties of the (ET)₈[Hg₄Cl₁₂(C₆H₅Cl)₂] salt of the same family. Time annealing provides an important effect on the conducting properties of the salt under investigation. Original Russian Text ? R.B. Lyubovskii, S.I. Pesotskii, R.N. Lyubovskaya, E.I. Zhilyaeva, O.A. Bogdanova, 2009, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2009, Vol. 89, No. 11, pp. 685–687.     

Electron-vibrational energy exchange in collisions of Ar atoms in the 3P2 metastable state with N2(X1S g+ ) molecules

Rate constants for electron-vibrational energy exchange Ar(³ P ₂) + N₂(X ¹Σ _g ⁺, ν = 0) → Ar(¹ S ₀) + N₂(C ³Π _u , ν′), where ν′ = 0, 1, 2, were calculated. Calculations were performed taking into account the presence of a resonance in electron scattering by N₂(X ¹Σ _g ⁺). As a result, the interaction of Ar(³ P ₂) with N₂(X ¹Σ _g ⁺, ν = 0) was characterized by attraction and, in the end, intersection of electron-vibrational potential surfaces correlating with Ar(³ P ₂) + N₂(X ¹Σ _g ⁺, ν = 0) and Ar(¹ S ₀) + N₂(C ³Π _u , ν′) at interparticle distances of 2.5–3.5 ?. Exchange interaction at which electron-vibrational transitions in the region of intersection of electron-vibrational transitions in the region of intersection of electron-vibrational potential surfaces accompanied by spin exchange were induced was calculated by the asymptotic method. The rate constants determined at 300–600 K were on the order of 10⁻¹¹−10⁻¹² cm³/s and weakly increased as the temperature grew. Mainly the C ³Π _u , ν′ = 0 state of the N₂ molecule was populated. The calculation results were in satisfactory agreement with the experimental data obtained at 300 K.     

Structural mechanisms of proton conduction in MemHn(XO4)(m + n)/2 crystals

Investigations of K₃H(SO₄)₂ samples, which belong to the family of Me _m H _n (XO₄)_{(m + n)/2} (Me = K, Cs, Rb, NH₄; X = S, Se, As) crystals, revealed temperature anomalies in the dielectric and optical properties and in the heat capacity upon phase transitions. X-ray diffraction studies confirmed that K₃H(SO₄)₂ crystals undergo a structural phase transition from the monoclinic phase to the trigonal phase C2/c → R m and that the high proton conductivity in the high-temperature phases of crystals belonging to this family is due to the formation of a qualitatively new system of hydrogen bonds. Original Russian Text ? A.A. Simonov, I.P. Makarova, V.V. Grebenev, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 8, pp. 1477–1479.     

Giant dielectric relaxation in SrTiO3-SrMg1/3Nb2/3O3 and SrTiO3-SrSc1/2Ta1/2O3 solid solutions

Ceramic samples of (1−x)SrTiO₃-xSrMg_1/3Nb_2/3O₃ and (1−x)SrTiO₃-xSrSc_1/2Ta_1/2O₃ were prepared, and their dielectric properties were studied at x=0.005–0.15 and 0.01–0.1, respectively, at frequencies 10 Hz–1 MHz and at temperatures 4.2–350 K. A giant dielectric relaxation was observed in the temperature range 150–300 K, and not so strong but well-developed relaxation was found in the temperature range 20–90 K. The activation energy U and the relaxation time τ₀ were determined to be 0.21–0.3 eV and from 10⁻¹¹ to 10⁻¹² s for the high-temperature relaxation and 0.01–0.02 eV and 10⁻⁸–10⁻¹⁰ s for the low-temperature relaxation, respectively. The additional local charge compensation of the heterovalent impurities Mg²⁺ and Nb⁵⁺ (or Sc³⁺ and Ta⁵⁺) by free charge carriers or the host ion vacancies is suggested to be the underlying physical mechanism of the relaxation phenomena. On the basis of this mechanism, the Maxwell-Wagner model and the model of reorienting dipole centers Mg²⁺ (or Sc³⁺) associated with the oxygen vacancy are proposed to explain the high-temperature relaxation with some arguments in favor of the latter model. The polaron-like model with the Nb⁵⁺-Ti³⁺ center is suggested as the origin of the low-temperature relaxation. The reasons for the absence of ferroelectric phase transitions in the solid solutions under study are also discussed. From Fizika Tverdogo Tela, Vol. 44, No. 11, 2002, pp. 1948–1957. Original English Text Copyright ? 2002 by Lemanov, Sotnikov, Smirnova, Weihnacht. This article was submitted by the authors in English.     

X-ray diffraction and magnetic susceptibility of sodium fullerides NaxC60

The X-ray diffraction (XRD), magnetic susceptibility and electron spin resonance (ESR) measurements have been carried out for Na_xC₆₀. The XRD profiles with x<4 can be assigned to a face-centered cubic (fcc) lattice, while those with 4≦x to a hexagonal one. The temperature dependence of magnetic susceptibility χ for Na_xC₆₀ using SQUID was fitted to the Curie law, and estimated temperature-independent component χ₀. The composition x dependence of the χ₀ for Na_xC₆₀ shows two maxima at around x=3 and x=10, and minimum at x=6. The absence of Pauli contribution at x=6 was confirmed using ESR. A trace of superconducting transition at 14 K has been found for some Na_xC₆₀ specimens with 8<x<9.     

Solvent-induced optical properties of C60

Different C₆₀ aggregates, i.e. nanoparticles, clusters of nanoparticles and microcrystals in room-temperature solutions, are reported to account for the colors of fluorescence emissions centered at 440, 575 and 700 nm, respectively. And the configurations of C₆₀ aggregation created in solutions are revealed to be closely associated with the characteristic interactions between C₆₀ and solvent molecules. On this basis, aggregation behaviors and thus induced optical properties of C₆₀ have been tentatively controlled through adopting solvent mixtures.     

Carbon-13 Isotopic Species of H2C3, H2C4, and H2C5: High-Resolution Rotational Spectra

The microwave rotational spectra of the carbon-13 isotopic species of H₂C₃, H₂C₄, and H₂C₅ have been observed in a pulsed supersonic molecular beam by Fourier transform microwave spectroscopy. At high resolution all of the rotational lines exhibit hyperfine structure produced by the magnetic interaction between the nuclear spin of ¹³C and the overall rotation of the molecule. The component of the nuclear spin-rotation tensor along the a-inertial axis is large for most isotopic species, especially at the carbene carbon; at this position C_aa is two to three times larger than at other substituted positions along the chain. In contrast to both H₂C₃ and H₂C₃, in H₂C₄C_aa exhibits a pronounced alternation along the carbon chain backbone. Following detection of the five carbon-13 isotopic species and D₂C₅, an experimental structure (r₀) has been determined to high accuracy for H₂C₅.