Synthesis, X-ray diffraction study, and ionic conductivity of new AB2O6 pyrochlores

The oxides A(Ti_0.5Te_1.5)O₆ (A = K, Rb, Cs, Tl), A(Ti_0.5W_1.5)O₆ (A = Rb, Cs, Tl), and Cs(B_0.5W_1.5)O₆ (B = Zr, Hf) have been obtained as polycrystalline powders giving X-ray diffraction patterns characteristic of defect cubic pyrochlores, space group (No. 227), Z = 8. The best discrepancy R factors, from 0.0265 for Rb(Ti_0.5Te_1.5)O₆ to 0.0554 for Cs(Zr_0.5W_1.5)O₆, were obtained for the B cations randomly distributed at 16(d), A ions at one quarter of 32(e), and oxygen atoms at 48(f) positions. A linear relationship is observed between the a unit cell parameters and the ionic radii of the A cations, as well as the average ionic radii of the B atoms. The results of electrical resistivity measurements for A(Ti_0.5Te_1.5)O₆ (A = K, Rb, Cs, Tl) are given.     

Determination of trace elements in Brazilian rice grains and in biological reference materials by neutron activation analysis

Instrumental neutron activation analysis was applied to the determination of the elements Na, K, Br, As, Rb, Zn, Co, Fe and Sc in Brazilian rice samples and in biological standards. Hg and Se concentrations were determined by using a simple radiochemical separation. The chemical procedure was carried out by means of distillation of Hg and Se in HBr medium and subsequent precipitation of selenium by sodium metabisulfite and mercury by thioacetamide. The accuracy of the instrumental and radiochemical methods was evaluated by means of analysis of the Reference Materials NBS-Bovine Liver, Bowen's Kale and NBS-Rice Flour.     

Electrohydrodynamic instability in some nematic cyanobiphenyls in an a.c. electric field

Electrohydrodynamic instability in homeotropically oriented nematic samples of 4'-n-octyl-4-cyano-biphenyl and 4'n-alkyloxy-4-cyanobiphenyl, (n = 8.9) have been studied in an a.c. electric field. The domain patterns during the instability in these compounds in a very low frequency a.c. field are very similar to those in a d.c. field. The domain patterns observed at higher frequencies have been identified as 'maltese crosses' or 'crossed isogyres'. The electro-convective 'isotropic' flows near the electrode play an important role in the observed instability.     

Combined supercritical-fluid chromatography/mass spectrometry in the analysis of diuron in plasma using on-line phase-system switching

On-line sample pretreatment by means of the phase-system switching approach is an interesting technique for the analysis of aqueous samples, e.g., plasma, by means of supercritical-fluid chromatography. In order to analyse plasma samples the following analytical procedure is used. The plasma sample is injected on to a short precolumn, which is washed with water and subsequently dried with nitrogen. Next, the solutes are desorbed with the supercritical mobile phase, analysed with packed-column supercritical-fluid chromatography and detected with either a UV detector or a mass spectrometer, equipped with a moving-belt interface. The herbicide diuron is selected as a test compound to study the feasibility of this approach. Using a selective detector the procedure is sufficiently sensitive to detect diuron in plasma, but not appropriate to detect the diuron metabolites in a post-mortem plasma sample. These have been identified with liquid chromatography/mass spectrometry. The detection limit of diuron in plasma using the procedure described is about 30 ng/mL.     

On the continuation for variational inequalities depending on an eigenvalue parameter

In this paper we generalize recent theoretical results on the local continuation of parameter-dependent non-linear variational inequalities. The variational inequalities are rather general and describe, for example, the buckling of beams, plates or shells subject to obstacles. Under a technical hypothesis that is satisfied by the simply supported beam, we obtain the existence of a continuation of both the solution and the eigenvalue with respect to a local parameter. A numerical continuation method is presented that easily overcomes turning points. Numerical results are presented for the non-linear beam.     

Control of small phase arrays to form a prescribed amplitude-phase distribution

Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 32, No. 9, pp. 1132–1136, September, 1989.