Ba1−x R x F2 + x Phases (R = Gd-Lu) with distorted fluorite-type structures—products of crystallization of incongruent melts in the BaF2-RF3 Systems (R = Gd-Lu). III. Defect Ba0.75Lu0.25F2.25 structure. A new {Lu8[Ba6F71]} supercluster of defects

The structure of Ba_0.75Lu_0.25F_2.25 crystals grown from melt has been studied by X-ray diffraction analysis (4729 measured reflections, 269 independent reflections with I > σ (I), R = 1.1%, R _w = 0.7%). The crystals are crystallized in the cubic system, sp. gr. Pm $\bar 3$ m, with the lattice parameter a = 5.9870(9) Å. A new complex of defects is singled out—a supercluster of the composition {R ₈[Ba₆F₇₁]}. This supercluster differs from the well-known rare earth octahedral supercluster of the composition {Ba₈[R ₆F_68-69]} because its nucleus is formed not by RE cations but by an alkali earth cation, Ba. The {R ₈[Ba₆F₇₁]} supercluster has a configuration close to that of the [B₁₄F₆₄] fragment of the fluorite structure and can replace the latter isomorphously. The model of the Ba_0.75Lu_0.25F_2.25 crystals consisting of coherently intergrown isostructural microphases having different chemical compositions is characterized by the good agreement of the calculated and experimentally determined occupancies of the F^1? positions. The comparison of the Ba_0.8Yb_0.2F_2.2 (phase studied earlier) and Ba_0.75Lu_0.25F_2.25 structures demonstrates the evolution of the defect structure along the series of rare earths with the corresponding change of the sp. gr. Fm $\bar 3$ m by the sp. gr. Pm $\bar 3$ m.     

Structural study of K<Subscript>0.93</Subscript>Ti<Subscript>0.93</Subscript>Nb<Subscript>0.07</Subscript>OPO<Subscript>4</Subscript> single crystals at 30 K

The structure of a K_0.93Ti_0.93Nb_0.07OPO₄ single crystal is studied at the temperature 30 K. The measurements are performed on a four-circle HUBER-5042 diffractometer with a DISPLEX DE-202 cryostat. Processing of the diffraction data and the preliminary refinement of the model are performed using the ASTRA program package. The final refinement of the structure model is made using the JANA2000 program complex. The refinement shows that the structure of a K_0.93Ti_0.93Nb_0.07OPO₄ crystal at T = 30 K is similar to its structure at room temperature. No phase transitions are revealed. Slight temperature-induced displacements of the potassium positions in the large cavities of the mixed framework are established.     

Nitrone cycloadditions to 1,2-diphenylcyclopropenes and subsequent transformations of the isoxazolidine cycloadducts

1,3-Dipolar cycloaddition of C-aryl,N-aryl (or N-methyl) nitrones with a number of 1,2-diphenylcyclopropenes substituted at the C(3) position occurs with the formation of expected "normal" cycloadducts (with N-methylnitrones) and products of their subsequent transformations. Among them are corresponding alpha-acetophenyl aziridines and tetra (or penta) -arylpyrroles. Aziridines and the normal cycloadducts can be also thermally converted to such arylpyrroles with moderate to good yields. Substitution at the C(3( position of cyclopropenes by an electron acceptor group decreases the reactivity of cyclopropenes.     

An acousto-optical imaging spectrometer for astrophysical measurements

An optical scheme of an acousto-optical imaging spectrometer for observing extended astrophysical objects with line emission spectra is proposed. The use of an additional prism with a specified angular dispersion makes it possible to separate images of an extended object at different emission lines and images generated by minor maxima of the acousto-optical filter transmission function. A prototype of the imaging spectrometer has been designed.     

A monopole mass filter with a hyperbolic V-shaped electrode

In this article we compare the classical monopole mass filter of von Zahn and the monopole mass filter with a hyperbolic V-shaped electrode. The experimental results and those of computer simulation for both mass spectrometers are presented. We show that the replacement of a conventional 90 degrees V-shaped electrode by an electrode with a hyperbolic profile substantially improves the peak shape of any given mass, and increases the mass resolution by a factor of 3-4 and the abundance sensitivity by a factor of 100. The potential of high analytical performance combined with electroforming techniques for electrode manufacture indicate future practical uses of such instruments. Copyright 1999 John Wiley & Sons, Ltd.     

New data on the p + N → [Σ0K+] + N reaction at E p= 70 GeV and the search for exotic baryons

New data for the diffractive reaction p+N→ [Σ⁰ K ⁺] +N at E _p= 70 GeV were obtained with partially upgraded SPHINX setup. The data are in a good agreement with the results of our previous study of this reaction. In the mass spectrum M(Σ⁰ K ⁺) a structure at the threshold region with a mass ∼1810 MeV and a distinct X(2000) peak with M= 1989 ± 6 MeV and γ= 91 ± 20 MeV are observed. Unusual features of the massive X(2000) state (narrow decay width, anomalously large branching ratio for the decay channel with strange particle emission) make it a serious candidate for cryptoexotic pentaquark baryon with hidden strangeness |qqqsˉs>. We also present new results on the narrow threshold structure X(1810) with M= 1807 ± 7 MeV and γ= 62 ± 19 MeV which is produced in the region of very small P ² _T < 0.01 GeV². The possibility of the Coulomb production mechanism for X(1810) is discussed. Received: 28 April 1999     

Localization of Surface Spectra

We study spectral properties of the discrete Laplacian H on the half-space with random boundary condition ; the V(n) are independent random variables on a probability space and λ is the coupling constant. It is known that if the V(n) have densities, then on the interval [-2(d+1), 2(d+1)] (=σ(H ₀), the spectrum of the Dirichlet Laplacian) the spectrum of H is P-a.s. absolutely continuous for all λ [JL1]. Here we show that if the random potential P satisfies the assumption of Aizenman–Molchanov [AM], then there are constants λ _d and Λ _d such that for |λ|<lambda; _d and |λ|> Λ _d the spectrum of H outside σ(H ₀) is P-a.s. pure point with exponentially decaying eigenfunctions. Received: 3 December 1998 / Accepted: 27 May 1999     

A three body approach to study the structural properties of 2-n halo nuclei and the search for Efimov states

The discovery of neutron rich isotopes of the lightest elements on the neutron drip line exhibiting a halo structure has opened up new vistas in research activities. The novel structural features associated with the halo phenomena have been the subject for extensive theoretical and experimental investigations in recent times. In this talk, I propose to present a broad overview of the recent developments in this field, bringing out the striking features which show that a large number oflight nuclei near the neutron drip line are characterized by a clear separation between a ‘normal’ core nucleus and a loosely bound low density veil of neutrons. Specifically, the two neutron halos offer a natural premises, from a theoretical standpoint, to employ three body techniques for studying their detailed structural properties. A considerable part of the talk will be devoted to report and highlight the results on a number of light halo nuclei such as ¹¹Li, ¹¹Be, ¹⁹B and ²²C on which we have been carrying out investigations employing a simple but realistic three body model. These three body systems which have been termed as ‘Borromean’ (i.e while three body systems are bound, the corresponding binary subsystems on the other hand are unbound) are characterized by large spacial extension and very low separation energy of the neutron. They are, therefore, ideally suited for exploring the possibility of the existence of Efimov states in two neutron halo nuclei. We have recently carried out the three body analyses to predict the possibility of the occurrence of such states on which experimental work at various laboratories is underway.     

α + 12C scattering at 110 MeV: Has exotic 7.65 MeV Hoyle’s state signatures “alpha-condensate” structure?

We present new measurements of the α + ¹²C elastic and inelastic (to the states 4.44, 7.65, and 9.64 MeV) scattering at E _lab = 110 MeV in the wide angular range from ～10° to 175°, which enable us to examine the condensate and cluster properties of the low-lying excited states in ¹²C. We present the diffraction-radius analysis of our data together with a considerable amount of the existing data. The magnitudes of the diffraction radii for the ground and the first excited (4.44 MeV) states are found to be equal, whereas they appear to be enhanced by ～0.6 fm both for 7.65 and 9.64 MeV states. This result shows that the radius of the Hoyle’s 0 ₂ ⁺ , 7.65 MeV state in ¹²C is by a factor of ～1.2–1.3 larger than that of the ground state. It is demonstrated that the direct transfer mechanism of ⁸Be dominates at the largest angles in all four reactions reported here. The configuration corresponding to the transfer of ⁸Be in its ground state (I ^π = 0⁺) with L = 0 turns out to be the most important for the 7.65 MeV state of ¹²C. Evidence of existence of some features of α-condensed structure of the Hoyle’s 0 ₂ ⁺ state in ¹²C was obtained: its enhanced radius and large contribution of α-particle configuration with L = 0.     

Effects of concentration and temperature on viscoelastic properties of aqueous potassium oleate solutions

The rheological properties of semidilute aqueous solutions containing long cylindrical micelles of an anionic surfactant, potassium oleate, are studied. It is shown that, at surfactant concentrations above 1 wt %, the rheological properties of the solutions are adequately described in terms of the simple Maxwell model of a viscoelastic liquid characterized by a single relaxation time. Based on the analysis of normalized dependences of the loss modulus on the storage modulus, the characteristic times of the processes governing the rheological properties of the above systems, i.e., the average breaking time and reptation time of micelles, are estimated. It is found that the breaking time of micelles decreases and relaxation time increases with increasing surfactant concentration due to lengthening of micellar chains.