Electric field induced structure formation in free standing ferroelectric films

Free standing ferroelectric films in a rotating electric field have been investigated. Depending upon the applied voltage and the period of the field rotation different structure formations could be observed. At high fields, a homogeneous orientation is obtained. In the case of very low fields, a schlieren texture results. Under special conditions, a stable ring system appears. The dynamics of these rings have been studied. The dependence of the structure formation on the relation between the impulse length of the applied field and the reorientation time is discussed.     

[PPh3Cl]⊕[ReCl4(N2S2)]⊖ Synthese,IR-Spektrum und Kristallstruktur

[PPh₃Cl]^⊕[ReCl₄(N₂S₂)]^?; Synthese, IR-Spektrum und Kristall Strukture The title compound is prepared by the reaction of [ReCl₃(NSCl)₂(POCl₃)] with triphenylphosphane; it formes black crystals. The crystal structure determination was solved with X-ray methods (2861 observed independent reflexions, R = 0.038). The compound crystallizes monoclinic in the space group P2₁/n with four formula units per unit cell. The structure consists of [PPh₃Cl]^⊕ cations and [ReCl₄(N₂S₂)]^? anions, in which the rhenium atom is surrounded octahedrally by four chlorine ligands and the N atoms of a ReN₂S₂-five-membered ring in cis-position (symmetry C_2v). The Re? N bond lengths (181 pm) and the NS bond lengths (152 pm) are in the range of double bonds; the S? S distance is very long (253 pm). The i.r. spectrum is reported.     

Komplexe des Rheniums mit planarer ReN2S2-Fünfringstruktur Synthesen und Kristallstrukturen von AsPh4[ReCl4(N2S2)] und PPh4[ReBr4(N2S2)]

Complexes of Rhenium with Planar ReN₂S₂ Rings. Syntheses and Crystal Structures of AsPh₄[ReCl₄(N₂S₂)] and PPh₄[ReBr₄(N₂S₂)] The complex [ReCl₄(N₂S₂)]^? can be obtained as PPh₄^⊕ or AsPh₄^⊕ salt by the action of S(NSiMe₃)₂ and of diphenylacetylene, respectively, on the chlorothionitrene complex [ReCl₄(NSCl)₂]^?. Another method of synthesis is the reaction of [ReCl₃(NSCl)₂(POCl₃)] with SbPh₃. [ReBr₃(N₂S₂)]₂ is obtained from excess Me₃SiBr and [ReCl₃(NSCl)₂(POCl₃)]. The anionic complex [ReBr₄(N₂S₂)]^? forms from either [ReCl₄(NSCl)₂]^? or [ReCl₄(N₂S₂)]^? with Me₃SiBr. All compounds are black, diamagnetic, and sensitive to moisture; the PPh₄^⊕ and AsPh₄^⊕ salts are soluble in CH₂Cl₂ and CH₂Br₂. The IR spectra are reported. The crystal structures of AsPh,₄[ReCl₄(N₂S₂)] and PPh₄[ReBr₄(N₂S₂)] were determined by X-ray diffraction. AsPh₄[ReCl₄(N₂S₂)]: space group P2/n, Z = 2, a = 1244.5, b = 1429.3, c = 791.1 pm, γ = 96.89° (1715 observed reflexions, R = 0.082). PPh₄[ReBr₄[ReBr₄(N₂S₂)]: space group P2₁/n, Z = 4, a = 961.7, b = 1397.4, c = 2205.7 pm, β = 102.10° (1787 observed reflexions, R = 0.073). In both compounds the [ReX₄(N₂S₂)]? anions have the same type of structure, the Re atoms forming part of planar ReN₂S₂ rings; the bond lengths are ReN 177 pm, NS 152 pm, and SS 259 for the chloro compound and ReN 184 pm, NS 153 pm, and SS 264 pm for the bromo compound. In AsPh₄[ReCl₄(N₂S₂)] the cations are stacked to form columns in the c-direction; in PPh₄[ReBr₄(N₂S₂)], there is considerable distortion form this packing principle.     

M?ssbauer effect in superconducting tin-europium-molybdenum-sulfides

Mössbauer spectra of¹¹⁹Sn in Sn_1–z Eu _z Mo₆S₈ (z=0, 1/3, 2/3) have been measured in the temperature range 4.9 KT293 K. The spectra consist of a quadrupole doublett with unequal intensities at all temperatures. The Debye-Waller factor, the isomeric shift and the asymmetry of the intensity of the quadrupole components show irregularities between 50 K and 110 K which are interpreted as arising from a structural phase transition. The temperature dependence of the Debye-Waller factor can be approximated by a simple phonon spectrum. No quantitative explanation can be given for the large values of the asymmetry. Evidence is presented that this behavior is connected with the properties of the librational modes of the Mo₆S₈-units in the crystal.     

Cyano-modified pre-coated plates — a medium polar modification for high performance thin-layer chromatography

Summary The introduction of a cyano-modified, pre-coated layer substantially widens the selectivity of stationary phases in thin-layer chromatography. This is a moderately polar sorbent based on silica gel 60, which can be used both in adsorption chromatography and in reversed-phase chromatography. This new pre-coated layer is particularly suitable for separation of steroids, alkaloids and derivatized amino acids. The possibility of separating habitforming drugs and preservatives in the presence of ionpair reagents is also discussed.     

Revisiting the form of dead time corrections for neutron coincidence counting

A standard nondestructive assay technique for determining the mass of plutonium in an item is passive neutron coincidence counting. In passive neutron coincidence counting, the dead time or rate loss corrections for the singles and doubles neutron counting rates are routinely made using empirical relationships that are based on the design and performance of the individual counter used to make the measurements. The empirical methods were developed ahead of any supporting theory for dead time losses and have worked well to date for the majority of safeguards measurement scenarios. Modern applications using highly efficient systems with short neutron lifetimes together with the challenges posed by highly multiplying items mean dead time corrections of higher fidelity are needed. While many attempts have been made to develop dead time corrections that are based on the physical principles of the measurements being performed, these corrections have often been found to be difficult to implement in a real system. For instance, Matthes and Haas developed an approach in 1985 which did not gain favor largely because the form of the doubles correction apparently required numerical integration which was difficult to implement with the computer technology of that time. A recent review into the approach that was developed by Matthes and Haas has determined that a straightforward analytical expression can be derived for the doubles correction that is similar to the singles rate correction that was developed in their original work. An analysis of the expressions is presented here to show how they relate to the traditional empirical methods. Further, we illustrate their implications and limitations. For instance the empirical methods do not address within burst losses i.e. rate related losses due to the correlated neutron bursts from fission, whereas the Matthes and Hass expression for singles counting does exhibit such an effect.     

Grey level on axis computer holograms for incoherent image processing

We present a method for the simple production of a pair of real non-negative pupil functions p?₁,p?₂, that act together to yield a bipolar system spread function P>(x) = |p₁|p² ? |p₂|p². The subtraction can be perf ormed by electronic means.     

Endliche auflösbare Gruppen mit normalem ℱ- Injektor II

Ohne ZusammenfassungDiese Arbeit ist Teil der Dissertation des Autors, die am Fachbereich Mathematik der Johannes-Gutenberg-Universität in Mainz eingereicht wurde (D77).