Fluorine abundances in planetary nebulae

We have determined fluorine abundances from the F II 4789 and F IV 4060 nebular emission lines for a sample of planetary nebulae (PNe). Our results show that fluorine is generally overabundant in PNe, thus providing new evidence for the synthesis of fluorine in asymptotic giant branch (AGB) stars. [F/O] is found to be positively correlated with the C/O abundance ratio, in agreement with the predictions of theoretical models of fluorine production in thermally pulsing AGB stars. A large enhancement of fluorine is observed in the Wolf-Rayet PN NGC 40, suggesting that high mass-loss rates probably favor the survival of fluorine.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter

The effect of fluorine on viscosities in the system Na2O-Al2O3-SiO2: implications for phonolites, trachytes and rhyolites

The effect of fluorine on melt viscosities of five compositions in the system Na2O-Al2O3- SiO2h as been investigateda t one atmospherea nd 1000-1600'Cb y concentric-cylinder viscometry. The compositions chosen were albite, jadeite and nepheline on the join NaAlOlSiO2 and two others of the join at 75 mole percent SiO2, one peralkaline and one peraluminous. All melt viscosities were independent of shear rate over two orders of magnitude, indicating Newtonian behavior. All viscosity-temperature relationships were Arrhenian within error. Fluorine reduces the viscosities and activation energies of all melts investigated. The viscosity-reducing power of fluorine increases with the SiO2 content of melts on the join NaAlO2-SiO2 and is a maximum at Na/Al (molar) = I for melts containing 75 mole percent SiO2. Fluorine and water have similar effects on aluminosilicate melt viscosities, probably due to depolymerization of these melts by replacement of Si-O-(Si, Al) bridges with Si-OH and Si-F bonds, respectively. Evidence from slag systems shows that fluorine also reduces the viscosity of depolymerized silicate melts. The viscous flow of phonolites, trachytes and rhyolites will be strongly afected by fluorine. It appears that fluorine contents of igneous rocks may be combined with water in calculation schemes for determining the viscosity of natural melts

On the origin of fluorine in the Milky Way

The main astrophysical factories of fluorine (19F) are thought to be Type II supernovae, Wolf-Rayet stars, and the asymptotic giant branch (AGB) of intermediate mass stars. We present a model for the chemical evolution of fluorine in the Milky Way using a semi-analytic multi-zone chemical evolution model. For the first time, we demonstrate quantitatively the impact of fluorine nucleosynthesis in Wolf-Rayet and AGB stars. The inclusion of these latter two fluorine production sites provides a possible solution to the long-standing discrepancy between model predictions and the fluorine abundances observed in Milky Way giants. Finally, fluorine is discussed as a possible probe of the role of supernovae and intermediate mass stars in the chemical evolution history of the globular cluster omega Centauri.Comment: 7 pages, 4 figures. MNRAS in pres

Chemical diffusion of fluorine in melts in the system Na2OAl2O3SiO2

The volatilization of fluorine from three melts in the system Na2OAl2O3SiO2 has been investigated at 1 atm pressure and 1200–1400°C. The melts chosen have base compositions corresponding to albite, jadeite and a peraluminous melt with 75 mole % SiO2. Melt spheres were suspended from platinum loops in a vertical tube furnace in a flow of oxygen gas, then quenched, sectioned and analysed by electron microprobe. The microprobe scans indicate that transport of fluorine to the melt-vapor interface is by binary, concentration-independent interdiffusion of fluorine and oxygen. FO interdiffusivity increases in the order albite < peraluminous < jadeite. There is no simple reciprocal relationship between FO interdiffusivity and melt viscosity. Comparison with data on high-pressure interdiffusivity of fluorine and oxygen in jadeite melt indicates that FO interdiffusivity increases with pressure from 0.001 to 10 kbar while the activation energy remains unchanged. Fluorine chemical diffusivity in albite melt is substantially lower than H2O chemical diffusivity in obsidian melts suggesting that different diffusive mechanisms are responsible for the transport of F and H2O in igneous melts. Fluorine diffuses in albite melt via an anionic exchange with oxygen whereas water probably diffuses in obsidian melt via an alkali exchange mechanism

Electronic Structures and Optical Properties of Partially and Fully Fluorinated Graphene

In this letter we study the electronic structures and optical properties of partially and fully fluorinated graphene by a combination of abinitio G0W0 calculations and large-scale multi-orbital tight-binding simulations. We find that for partially fluorinated graphene, the appearance of paired fluorine atoms is more favorable than unpaired atoms. We also show that different types of structural disorder, such as carbon vacancies, fluorine vacancies, fluorine vacancy-clusters and fluorine armchair- and zigzag-clusters, will introduce different types of midgap states and extra excitations within the optical gap. Furthermore we argue that the local formation of $sp^3$ bonds upon fluorination can be distinguished from other disorder inducing mechanisms which do not destroy the $sp^2$ hybrid orbitals by measuring the polarization rotation of passing polarized light.Comment: Final version appeared in Phys. Rev. Let

Chemical diffusion of fluorine in jadeite melt at high pressure

The chemical diffusion of fluorine in jadeite melt has been investigated from 10 to 15 kbars and 1200 to 1400°C using diffusion couples of Jadeite melt and fluorine-bearing jadeite melt (6.3 wt.% F). The diffusion profile data indicate that the diffusion process is concentration-independent, binary, F-O interdiffusion. The F-O interdiffusion coefficient ranges from 1.3 × 10−7 to 7.1 × 10−7 cm2/sec and is much larger than those obtained by Kushiro (1983) for Si-Ge and Al-Ga interdimision in jadeitic melts. The Arrhenius activation energy of diffusion is in the range of 36 to 39 kcal/mole as compared with 19 kcal/mole for fluorine tracer diffusion in a lime-aluminosilicate melt. The diffusivity and activation energy of F-O interdiffusion vary slightly with pressure, but the pressure dependence of F-O, Al-Ga and Si-Ge interdiffusion may be related to the relative volumes of the interdiffusing species for each pair. The magnitude of chemical diffusivity of fluorine is comparable to that of the chemical diffusivity of water in obsidian melts. The diffusivities of various cations are significantly increased by the addition of fluorine or water to a silicate melt. This fact, combined with the high diffusivity of fluorine, suggests that the F− ion is the principal diffusing species in dry aluminosilicate melts and that dissolved fluorine will accelerate chemical equilibration in dry igneous melts

Effects of water and fluorine on the viscosity of albite melt at high pressure: a preliminary investigation

The viscosities of fluorine- and water-bearing melts based on albite composition have been determined at 7.5, 15 and 22.5 kbar by the falling-sphere method. All melt viscosities decrease isothermally with increasing pressure. At 1200°C the viscosity of the fluorine-bearing melt (albite + 5.8 wt.% fluorine substituted for oxygen, denoted AbF2O−1) decreases from5000 ± 750P at7.5kbar to1600 ± 240P at22.5kbar. At 1400°C the viscosity of this melt decreases from1300 ± 200P at7.5kbar to430 ± 65P at22.5kbar. At 1400°C the viscosity of albite + 2.79 wt.% water (denoted AbH2O) decreases from650 ± 100P at7.5kbar to400 ± 60P at22.5kbar. Fluorine (as F2O−1) and water strongly decrease the viscosity of albite melt over the entire range of investigated pressures. The ratio of the effects of 5.8 wt.% fluorine [F/(F + O)molar = 0.10] and 2.79 wt.% water [OH/(OH + O)molar = 0.10] on the log of melt viscosity [Δ log η(AbF2O−1)/Δ log η(AbH2O)] equals0.90 ± 0.05, 0.84 ± 0.05and0.97 ± 0.05at7.5, 15and22.5kbar, respectively. Comparison with available data on the high-pressure viscosity of albite melt indicates that both F2O−1 and H2O maintain their viscosity-reducing roles to lower crustal pressures. The difference between the viscosities of melts of albite, AbF2O−1 and AbH2O, may be explained in terms of the relatively depolymerized structures of AbF2O−1 and AbH2O melts. The depolymerization of albite melt by the addition of water results from the formation of SiOH bonds. The depolymerization of albite melt by F2O−1 substitution results from the formation of non-bridging oxygens associated with network-modifying aluminum cations that are formed upon fluorine solution. The strong viscosity-reducing effects of water and fluorine in albite melt at pressures corresponding to the mid- to lower continental crust indicate that these two components will strongly influence the dynamic behavior of anatectic melts during initial magma coalescence and restite-melt segregation

WDX-Analysis of the New Superconductors RO(1-x)F(x)FeAs and Its Consequences on the Electronic Phase Diagram

Polycrystalline samples of RO1-xFxFeAs (0 < x < 0.25) (R = La, Sm, Gd) were investigated by wavelength-dispersive X-ray spectroscopy (WDX) in the electron microscope to determine the composition of the samples, in particular the fluorine content. It was found that the measured fluorine content can deviate considerably from the initial weight. In the lanthanum compound LaO1-xFxFeAs, we found good agreement mainly for x > 0.05, but for x < 0.05 the fluorine hardly goes into the sample. For the samarium compound we measured less than half the fluorine in the sample as initially weighed at all fluorine concentrations. These measured values are taken into account when drawing the electronic phase diagrams of LaO1-xFxFeAs and SmO1-xFxFeAs. This leads to a more consistent picture of both of the diagrams in comparison to the plot of the initial weight.Comment: 5 pages, 4 figures, Accepted for publication in Journal of Superconductivity and Novel Magnetis