Probing the isomer,fluorination and bond effects in C<Subscript>3</Subscript>H<Subscript>6</Subscript>, cyclo-C<Subscript>3</Subscript>H<Subscript>6</Subscript> and C<Subscript>3</Subscript>F<Subscript>6</Subscript> molecules using electron impact

We have carried out experimental and theoretical studies on electron scattering from the C₃H₆ isomers and C₃F₆ molecules and we report on total, differential as well as theoretical integral elastic cross-sections for these molecules. Vibrational excitation functions are also presented for the typical vibrational peaks in C₃H₆ and cyclo-C₃H₆ for the angle of 90^○, impact energy range of 1–16 eV and loss energies of 0.12 eV and 0.13 eV, respectively. In the cross-sections, clear differences in peak positions and magnitudes between the C₃H₆ isomers can be viewed as the isomer effect. The same is observed between C₃H₆ and C₃F₆ in a clear manifestation of the fluorination effect. The resemblance of the π^* shape resonance in the cross-sections, observed at about 2.2 eV for C₃H₆ and 3.5 eV for C₃F₆, to those in C₂H₄ and C₂F₄ clearly points to the effect of the double bond in the molecular structures for these molecules. Theoretical analysis is performed to provide rationales for the scattering dynamics.     

Thermodynamic properties and structure of oxyfluorides Rb<Subscript>2</Subscript>KMoO<Subscript>3</Subscript>F<Subscript>3</Subscript> and K<Subscript>2</Subscript>NaMoO<Subscript>3</Subscript>F<Subscript>3</Subscript>

According to the results of calorimetric and structural studies, the Fm{ie1202-1}m phase in K₂NaMoO₃F₃ remains stable at least to 100 K. No ferroelectric transformation assumed earlier has been revealed in a series of Rb₂KMoO₃F₃ samples prepared using various technologies. Only a phase transition of nonferroelectric origin has been observed near 195 K, and its thermodynamic characteristics have been determined. An analysis of the stability of the cubic structure of molybdenum fluorine-oxygen elpasolites-cryolites has been performed in the framework of the hypothesis on strengths of interatomic bonds. The barocaloric effect in Rb₂KMoO₃F₃ has been estimated.     

Cluster <Emphasis Type="Italic">ab initio</Emphasis> calculations for the C<Subscript>60</Subscript>F<Subscript>24</Subscript>, C<Subscript>60</Subscript>Cl<Subscript>24</Subscript>, and C<Subscript>60</Subscript>Br<Subscript>24</Subscript> halofullerenes

This paper reports on ab initio cluster calculations of the equilibrium geometry, electronic structure, and vibrational properties of the C₆₀, C₆₀F₂₄, C₆₀Cl₂₄, C₆₀Br₂₄ molecules.     

Phase transition in YbAl<Subscript>3</Subscript>C<Subscript>3</Subscript>

¹⁷⁰Yb M?ssbauer spectroscopy, temperature dependent X-ray, magnetisation and specific heat data are presented in the hexagonal intermetallic YbAl₃C₃, in order to shed light on the isostructural transition occurring near 80 K and to investigate the electronic state of the Yb ion above and below the transition. In the low temperature phase, we find that there occurs an atomic rearrangement in the hexagonal unit cell, leading to a strong symmetry lowering at the Yb site. We show that no magnetic ordering of the Yb³⁺ moments occurs down to 0.04 K, and we discuss this finding in terms of 4f-conduction electron hybridisation and geometric frustration.     

X-ray emission and X-ray photoelectron spectroscopic studies of fullerene fluoride C<Subscript>60</Subscript>F<Subscript>24</Subscript>

The structure of the fullerence fluoride C₆₀F₂₄ of the T _h symmetry contains two types of chemically different carbon atoms, namely, atoms of isolated double bonds and atoms of CF groups. X-ray photoelectron and x-ray emission spectroscopic studies of C₆₀F₂₄ revealed a difference in the widths of the x-ray bands corresponding to these types of atoms. Nonempirical quantum-chemical calculations performed for C₅₉NF ₂₄ ⁺ ions with a hole in the C 1s core level of the fullerence fluoride showed that the difference in the bandwidths may be due to the fact that the vibrational states of the system are different when 1s electrons are removed from chemically nonequivalent atoms.     

Stepwise computational synthesis of fullerene C<Subscript>60</Subscript> derivatives. Fluorinated fullerenes C<Subscript>60</Subscript>F<Subscript>2<Emphasis Type="Italic">k</Emphasis></Subscript>

Reactions of fullerene C₆₀ with atomic fluorine are studied by the unrestricted broken spin symmetry Hartree-Fock (UBS HF) approach implemented in semiempirical codes based on the AMI technique. The calculations are focused on a successive addition of a fluorine atom to the fullerene cage following the indication of the highest chemical susceptibility of the cage atom, which is calculated at each step. The proposed computational synthesis is based on the effectively unpaired-electron concept of the chemical susceptibility of fullerene atoms. The obtained results are analyzed from the standpoints of energy, symmetry, and the composition abundance. A good fitting of the data to experimental findings proves a creative role of the suggested synthetic methodology.     

Vibrational spectra of disordered oxyfluoride (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>WO<Subscript>3</Subscript>F<Subscript>3</Subscript>

Comparative analysis of the IR absorption and Raman scattering spectra of a polycrystalline sample of perovskite-type oxyfluoride (NH₄)₃WO₃F₃ has been performed in the frequency range 370–4000 cm^?1 at temperatures from 92 to 303 K, including the transition between the orientationally disordered cubic and low-symmetry phases. The conformation of WO₃F₃ octahedral groups is established and transitional anomalies of the internal modes of these groups and ammonium ions are revealed. Comparative analysis of the IR and Raman spectra suggests that the phase transition under study is mainly related to the ordering of octahedral groups and formation of W-O…H-N hydrogen bonds.     

EPR spectra of aerosol particles formed by pyrolysis of C<Subscript>3</Subscript>H<Subscript>8</Subscript> plus Ar and C<Subscript>3</Subscript>H<Subscript>8</Subscript> plus Fe(CO)<Subscript>5</Subscript> plus Ar mixtures in a flow reactor

The paper is devoted to electron paramagnetic resonance investigation of nanoparticles and aggregates of nanoparticles formed by pyrolysis of propane as well as a mixture of propane and iron pentacarbonyl. The measurement showed that the pyrolysis of the C₃H₈ plus Ar mixture results in the formation of a carbonaceous phase (phase I), which is quite different from that formed by the C₃H₈ plus Fe(CO)₅ plus Ar mixture (phase II). In phase I there is a strong oxygen effect for as-prepared samples; 75% of spins are accessible to the environmental gas via the interconnected system of microvoids and microchannels. In phase II there was a weak oxygen effect for the as-prepared samples. However, after exposition of phase II to air for 160 h, the properties of phase II have become about the same as that of phase I. A strong oxygen effect was observed for the air-exposed phase II. The line width for phase II increases monotonically with the iron content in the sample. This increase is probably related to the dipole-dipole interactions between the radical centers and the iron atoms distributed throughout the carbon matrix.     

Metastable quasi-one-dimensional ensembles of C<Subscript>8</Subscript> clusters

The geometric and energy characteristics of quasi-one-dimensional ensembles of metastable carbon clusters C₈ are calculated using the molecular dynamics method with an empirical interatomic potential. The decay activation energy and the lifetime of the metastable state are determined.     

Upconversion excitations in Pr<Superscript>3+</Superscript>-doped BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> crystal

We report the orange-to-blue and infrared-(IR)-to-blue wavelengths upconversion luminescence in Pr³⁺:BaY₂F₈ crystals. Mechanism of the orange light upconversion into blue ³P₀ state emission was confirmed to be energy transfer between two Pr³⁺ ions in the ¹D₂ state. IR-to-blue upconversion has only been observed under two different color IR pumping. The first resonant step was the ³H₄→¹G₄ ground state absorption transition, and the second resonant transition was the excited state absorption from the ¹G₄ to ¹I₆ and ³P_J levels. A comparison of the efficiency of the IR-to-blue upconversion in several praseodymium activated host is presented and discussed. A model of the IR pumped upconversion praseodymium blue laser is presented and the population inversion conditions are calculated.     

Density functional study of Pu<Subscript>2</Subscript>C<Subscript>3</Subscript>

The structural, magnetic, electronic, vibrational, thermodynamic and elastic properties of plutonium sesquicarbide (Pu₂C₃) are investigated based on density functional theory. The use of the Hubbard term to describe the 5f electrons of plutonium is discussed according the lattice parameters and magnetism. The calculated lattice constants, magnetism and density of states agree well with the experimental data or other theoretical calculations. The Pu-C bonds of Pu₂C₃ have a mixture of covalent character and ionic character, while covalent character is stronger than ionic character. The phonon frequencies and the assignment of infrared-active, Raman-active and silent modes at Γ point are obtained. Furthermore, the enthalpy difference H-H₂₉₈, entropy S, heat capacity and linear thermal expansion coefficient α of Pu₂C₃ have been calculated and compared with the available data. Lastly, the calculated elastic properties predict that Pu₂C₃ is ductile metal. In addition, the effect of spin-orbit coupling on the structural, magnetic, and electronic properties of Pu₂C₃ has been discussed. We hope that our results can provide a useful reference for further theoretical and experimental research on Pu₂C₃.     

Effective production of Xe<Subscript>2</Subscript>I excimer molecules by high-energy charged particles in Xe containing a small amount of C<Subscript>3</Subscript>F<Subscript>7</Subscript>I

An anomalously high efficiency of generating Xe₂I* excimer molecules in dense Xe–C₃F₇I gaseous mixtures with a small amount of C₃F₇I that are excited by a pulsed beam of fast electrons is discovered. The electron energy is 150 keV, and the beam current amplitude and duration are, respectively, 5 A and 5 ns. The temporal–spectral characteristics of spontaneous radiation from XeI* and Xe2I* excimer molecules are measured. Also, the luminescence times of the upper level for the B–X transition in the XeI* molecule (λ_max = 253 nm) and the upper level for the 4²Γ–1²Γ transition in the Xe₂I* molecule (λ = 352 nm), as well as the rate constants of quenching these levels by the gaseous mixture components, are determined. Based on the characteristics of the track structure of a high-energy plasma produced by charged particles in the dense gaseous medium, a model of plasma-chemical processes leading to the formation of XeI* (λ_max = 253 nm) and Xe₂I* (λ = 352 nmnm) excimer molecules in a Xe–C₃F₇I mixture with a small amount of an iodine atom donor is suggested.     

VUV luminescence of Er<Superscript>3+</Superscript> ions in LiYF<Subscript>4</Subscript> and BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> crystals

Vacuum ultraviolet luminescence of Er³⁺ ions in LiYF4 and BaY₂F₈ crystals has been investigated. It is revealed that under excitation by 193 nm radiation from an ArF excimer laser the interconfigurational 5d–4f radiative transitions in Er³⁺ ions are observed. It is shown that from the LiYF₄:Er crystal only the spin-forbidden luminescence (λ = 165 nm) is detected, whereas both the spin-forbidden (λ = 169 nm) and spin-allowed (λ = 160.5 nm) components are observed from the BaY₂F₈:Er crystal.     

A new allotropic form of carbon [C<Subscript>28</Subscript>]<Subscript>n</Subscript> based on fullerene C<Subscript>20</Subscript> and cubic cluster C<Subscript>8</Subscript> and Si and Ge analogs of this form: Computer simulation

The structure of a new allotropic form of carbon [C₂₈]_n having a simple cubic lattice and space group \(Pm \bar 3\) is proposed. The geometrical parameters of the building block of such a hypothetic crystal are preliminarily determined from DFT-PBE calculations of the cluster C₈@(C₂₀)₈ and the polyhedral hydrocarbon molecule C₈@(C₂₀H₁₃)₈, in which the centers of the cubic clusters C₈ coincide with the centers of the cluster C₈@(C₂₀)₈ and of the molecule C₈@(C₂₀H₁₃)₈, respectively, and dodecahedral C₂₀ carbon cages are located at the vertices of a cube. The energy of dissociation of the cluster C₈@(C₂₀)₈ into a cubic cluster C₈ and eight dodecahedral clusters C₂₀ is calculated to be 1482 kcal/mol, and the energy of each C₈-C₂₀ bond is equal to 74.2 kcal/mol. The structure of the [C₂₈]_n crystal is refined using the DFT-PBE96/FLAPW method and optimized geometry. Calculations show that the crystal is a dielectric with an energy gap of 3.3 eV. The lattice parameter a of the crystal is equal to 5.6 Å, and its density is 3.0 g/cm³. The possible existence of analogous allotropic forms of elements Si and Ge is discussed. A method is proposed for designing a hypothetic allotropic form [C₂₈]_n from C₂₀(CH₃)₈ molecules with T _h symmetry.     

Equilibrium state of C<Subscript>60</Subscript>, C<Subscript>70</Subscript>, and C<Subscript>72</Subscript> nanoclusters and local defects of the molecular skeleton

The stability of C₆₀ and C₇₀ fullerenes and C₆₀ and C₇₂ nanotubes devoid of 2–12 atoms of the cluster skeleton was theoretically studied. It was established that C_n molecules with an even number of atoms remain stable, which was confirmed by experimental studies of monomolecular decay of clusters with the number of atoms n≥30. The change in the internuclear distances and in the ionization potential of nanoclusters was determined depending on the number of eliminated atoms. Such defects were shown to decrease the ionization potential of nanoclusters by 0.5–0.8 eV. The electron spectrum was calculated within the Harrison semiempirical tight-binding model in the Goodwin modification. A new parametrization of interatomic matrix elements of the Hamiltonian and atomic terms for carbon nanoclusters was suggested.     

Molecular dynamics study of the reaction C<Subscript>3</Subscript> + H <Subscript>3</Subscript><Superscript>+</Superscript>

Both a quantum molecular dynamic method and high level ab initio calculations (MP2, CCSD(T)) have been used to investigate the mechanism of the C₃ + H₃⁺ reaction, which is part of the ion chemistry in interstellar clouds. Furthermore statistic initial orientations in collision simulations have been set up in order to determinate reaction cross-sections and rate coefficients of all occurring reaction channels. Our analysis shows that the revealed mechanism is strongly determined by dynamic effects.     

Actively mode-locked Tm-Ho:LiYF<Subscript>4</Subscript> and Tm-Ho:BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> lasers

We report on the generation of mode-locking pulse trains with high average output powers from diode-pumped Tm-Ho:LiYF₄ and Tm-Ho:BaY₂F₈ lasers emitting at around 2 μm. The highest output power of 365 mW was obtained with the Tm-Ho:YLF₄ laser, whereas the shortest pulse duration of 120 ps and the widest tunability range of 59 nm was achieved with the Tm-Ho:BaY₂F₈ laser. PACS 42.55.Xi; 42.60.Fc; 42.72.Ai; 42.55.Rz; 42.70.Hj     

Three types of photomagnetic effects in Sp<Subscript>3</Subscript>Cr(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>3</Subscript> crystals

Three different photomagnetic effects caused by ultraviolet light in paramagnetic crystals based on molecules of spiropyrans (Sp) Sp₃Cr(C₂O₄)₃ and SpI have been revealed and separated: (1) in the high-temperature range (30–300 K), the photomagnetic effect in Sp₃Cr(C₂O₄)₃ is determined by the charge transfer between chromium ions and spiropyran molecules; (2) in the low-temperature range (2 K), the photomagnetic effect in Sp₃Cr(C₂O₄)₃ is due to the photoisomerization of spiropyran molecules, the change in the crystal field, and the splitting of the levels of Cr³⁺ ions in zero field; and (3) in the temperature range 2–20 K, the generation of radiation-induced paramagnetic defects contributes to the magnetic moment of the organic sublattice Sp⁺.     

On the interaction of self-assembled C<Subscript>60</Subscript>F<Subscript>18</Subscript> polar molecules with the Ni(100) surface

The current work is dedicated to investigation of the interaction between self-assembled polar molecules of fullerene fluoride C₆₀F₁₈ with the chemically active surface Ni(100) under radiation and heat treatments. X-ray photoelectron spectroscopy is used in combination with quantum-chemical simulation. For the first time, the transformation of an as-deposited dielectric continuous 2D thin film to a 3D island-type assembly with molecular ordering within the islands is shown to take place. The degree of coverage of the Ni surface by C₆₀F₁₈ islands (0.6–0.7) and their height (~6 nm) are estimated. Quantum-chemical simulation shows that the chemisorption energy of the C₆₀F₁₈ molecule on the Ni surface equals ~6.6 eV and fluorine atoms are located at a distance of 1.9 Å above the Ni surface. The results of the investigation provide an opportunity to create nanoscale ordered structures with local changes in the work function.