Investigations of alkali doped Fe2O3--V2O5 catalysts by transmission and conversion electron Mössbauer spectroscopy

Fe₂O₃--V₂O₅ catalysts doped with Cs₂SO₄ (molar ratio: V:Fe:Cs=1:0.74:0.06) were found to be a rather inhomogeneous mixture of various crystalline and amorphous iron vanadate phases. After calcination in air the catalyst was divided into three different parts which were analyzed separately revealing the formation of FeVO₄ in the top and bottom fraction of the crucible and Fe₂V₄O₁₃ in the middle fraction. As compared to the well crystallized FeVO₄ reference sample, the quadrupole splittings of FeVO₄ in the Cs-doped catalysts were larger pointing to more distorted iron sites which were assumed to be responsible for high catalytic selectivities. In contrast, the quadrupole splittings of FeVO₄ in the less selective K- and Rb-doped Fe₂O₃--V₂O₅ catalysts were smaller. Additional components in the bottom fraction were also α-Fe₂O₃ and Fe_1-xS. As indicated by the CEMS spectra the latter is located preferentially on the surface of the catalyst particles. This revised version was published online in August 2006 with corrections to the Cover Date.     

Positron-lifetime study of vacancy annealing in neutron-irradiated GaAs

Positron-lifetime measurements have been used to study the annealing of vacancies in neutron-irradiated GaAs. The vacancies which are interpreted as defects in the Ga sublattice disappear in a single annealing stage (at 500°C in GaAs doped with Si or Zn, and at 600°C in Cr-doped GaAs).     

Effect of Ga doping on magneto-transport properties in colossal magnetoresistive La0.7Ca0.3Mn1−xGaxO3 (0<x<0.1)

Samples of La_0.7Ca_0.3Mn_1−xGa_xO₃ with x=0, 0.025, 0.05 and 0.10 were prepared by standard solid-state reaction. They were first characterized chemically, including the microstructure. The magnetic properties and various transport properties, i.e. the electrical resistivity, magnetoresistivity (for a field below 8 T), thermoelectric power and thermal conductivity measured each time on the same sample, are reported. The markedly different behaviour of the x=0.1 sample from those with a smaller Ga content, is discussed. The dilution of the Mn³⁺/Mn⁴⁺ interactions with Ga doping considerably reduces the ferromagnetic double exchange interaction within the manganese lattice leading to a decrease of the Curie temperature. The polaron binding energy varies from 224 to 243 meV with increased Ga doping.     

An example of a superproportional graph

In the theory of random graphs, several classes of graphs occur that are exceptional cases in limit theorems for subgraph counts. The purpose of this article is to show the existence of graphs in one of these classes, by providing an explicit, computer generated, example. We also show that the class is closed under complementation. © 1994 John Wiley & Sons, Inc.     

Aqua­(phthalocyaninato)magnesium n‐propyl­amine disolvate

The crystals of the title compound, [Mg(C₃₂H₁₆N₈)(H₂O)]·2C₃H₉N, are built up from MgPc(H₂O) [Pc is phthalo­cyaninate(2−)] and n‐propyl­amine mol­ecules that inter­act via O—H⋯N hydrogen bonds. The MgPc(H₂O) mol­ecule is non‐planar. The central Mg atom is coordinated by the four equatorial isoindole N atoms of the Pc ring system and by the O atom of an axial water mol­ecule. The Mg atom is displaced by 0.509 (1) Å from the N₄ plane towards the water O atom. MgPc(H₂O)·2(n‐propyl­amine) mol­ecules related by the inversion centre are linked by N—H⋯O hydrogen bonds to form a dimeric aggregate.     

Convex ordering criteria for Lévy processes

Modelling financial and insurance time series with Lévy processes or with exponential Lévy processes is a relevant actual practice and an active area of research. It allows qualitatively and quantitatively good adaptation to the empirical statistical properties of asset returns. Due to model incompleteness it is a problem of considerable interest to determine the dependence of option prices in these models on the choice of pricing measures and to establish nontrivial price bounds. In this paper we review and extend ordering results of stochastic and convex type for this class of models. We also extend the ordering results to processes with independent increments (PII) and present several examples and applications as to α-stable processes, NIG-processes, GH-distributions, and others. Criteria are given for the Lévy measures which imply corresponding comparison results for European type options in (exponential) Lévy models.