Metastable Nitric Acid Trihydrate in Ice Clouds.

The composition of high-altitude ice clouds is still a matter of intense discussion. The constituents in question are ice and nitric acid hydrates, but the exact phase composition of clouds and its formation mechanisms are still unknown. In this work, conclusive evidence for a long-predicted phase, alpha-nitric acid trihydrate (alpha-NAT), is presented. This phase was characterized by a combination of X-ray and neutron diffraction experiments, allowing a convincing structure solution. Furthermore, vibrational spectra (infrared and inelastic neutron scattering) were recorded and compared with theoretical calculations. A strong interaction between water ice and alpha-NAT was found, which explains the experimental spectra and the phase-transition kinetics. On the basis of these results, we propose a new three-step mechanism for NAT formation in high-altitude ice clouds.

Spectroscopic, structural and magnetic investigations of iron(II) complexes based on 1-isopropyl- and 1-isobutyl-substituted tetrazole ligands.

Two partly new [Fe(intz)6](BF4)2 complexes with n = 3 and 4 (i3tz = 1-isopropyl-1H-tetrazole, i4tz = 1-isobutyl-1H-tetrazole) were synthesized and characterised by X-ray powder diffraction, magnetic susceptibility measurements, vibrational, electronic and 57Fe-Mößbauer spectroscopy as well as DSC. The [Fe(i3tz)6](BF4)2 complex was re-investigated and shows an incomplete spin transition at Tc âˆ¼ 109 K, while the [Fe(i4tz)6](BF4)2 features a complete but rather gradual spin transition with T½ = 223 K. In the lack of suitable crystals of [Fe(intz)6](BF4)2 with n = 3 and 4, we synthesized crystals of [Ni(intz)6](BF4)2 with n = 3 and 4, determined their X-ray crystal structures, and proved them to be homeotypic with the respective Fe-complexes by X-ray powder diffraction. DSC measurements showed an endothermic peak for the [Fe(i3tz)6](BF4)2 around T = 260 K not corresponding to a spin transition and suggesting a structural phase transition at this temperature. A well-developed peak at Tp = 225 K matches the spin-transition temperature T½ = 223 K for [Fe(i4tz)6](BF4)2.

The relevance of interfaces for oxide ion transport in yttria stabilized zirconia (YSZ) thin films.

Thin YSZ films were prepared on magnesia, sapphire and strontium titanate (STO) single crystals using pulsed laser deposition and, for comparison, by a sol-gel method on STO. The bulk and interfacial mass and charge transport properties of these films were investigated by complementary impedance spectroscopy and tracer diffusion measurements. In this context, a novel two-step tracer diffusion experiment is introduced. For YSZ films on sapphire and magnesia, grain bulk conductivities similar to those of polycrystalline samples were measured in most cases. Strongly blocking grain boundaries could be identified by impedance measurements. The films on sapphire and magnesia also exhibited good agreement between effective transport properties of impedance and tracer measurements. YSZ layers on strontium titanate single crystals, on the other hand, showed a strongly increased effective conductivity in impedance studies. However, in tracer diffusion experiments this could be unambiguously attributed to conduction in the substrate while the diffusion coefficient of YSZ on STO was comparable to that of YSZ films on other substrates. Moreover, the tracer diffusion experiments did not indicate any significant increase of oxide ion mobility on a free YSZ surface compared to a Pt|YSZ interface.

Investigation of the oxygen exchange mechanism on Pt|yttria stabilized zirconia at intermediate temperatures: Surface path versus bulk path.

The oxygen exchange kinetics of platinum on yttria-stabilized zirconia (YSZ) was investigated by means of geometrically well-defined Pt microelectrodes. By variation of electrode size and temperature it was possible to separate two temperature regimes with different geometry dependencies of the polarization resistance. At higher temperatures (550-700 °C) an elementary step located close to the three phase boundary (TPB) with an activation energy of ∼1.6 eV was identified as rate limiting. At lower temperatures (300-400 °C) the rate limiting elementary step is related to the electrode area and exhibited a very low activation energy in the order of 0.2 eV. From these observations two parallel pathways for electrochemical oxygen exchange are concluded.The nature of these two elementary steps is discussed in terms of equivalent circuits. Two combinations of parallel rate limiting reaction steps are found to explain the observed geometry dependencies: (i) Diffusion through an impurity phase at the TPB in parallel to diffusion of oxygen through platinum - most likely along Pt grain boundaries - as area-related process. (ii) Co-limitation of oxygen diffusion along the Pt|YSZ interface and charge transfer at the interface with a short decay length of the corresponding transmission line (as TPB-related process) in parallel to oxygen diffusion through platinum.

Alumni Profiles.

Former PhD students, post-docs and junior researchers of the Section de chimie et biochimie now holding positions at different universities and private companies remember the time they spent in Geneva and give an account of how this has set off and influenced their careers.

A hexagonal 2a,c superstructure of Ba7F12Cl2.

Hydrothermal synthesis yielded crystals of heptabarium dodecafluoride dichloride, Ba7F12Cl2, displaying a new 2a,c hexagonal superstructure with P-6 symmetry. The superstructure results from the disorder of Ba2+ cations over two adjacent tricapped trigonal prismatic sites located in channels parallel to the c axis.