Structural chemistry of magnéli phases TinO2n−1 (4 ≤ n ≤ 9): IV. Superstructure in Ti4O7 at 140 K

The structure of Ti₄O₇ was reexamined at 298, 140, and 115 K. The room-temperature and the 115 K structures are essentially as previously reported. Two new observations were made: a considerable cell-parameter change at the lower transition with a 0.3% increase in cell volume on cooling and the existence of a fivefold superstructure at 140 K only. The 140 K phase with cell parameters a = 6.918(1), b = 11.142(2), c = 15.127(3) α = 90.64(1), β = 92.79(1), γ = 91.45(1) in space group $\text{P}\text{1}$ corresponds to long-range order of the Ti valences. It shows alternate Ti³⁺ and Ti⁴⁺ slabs parallel to (1 0 $\text{2}$) and a few valences with intermediate values at the contact between the slabs with the overall appearance of a modulated structure. Pairing of Ti³⁺ ions is observed. Similar to Ti₆O₁₁, expansion and contraction of the TiTi distances in the rutile-like chains is correlated with the electrostatic repulsion of the Ti ions so that the 3⁺ slabs are on the whole contracted along c_rutile while the 4⁺ slabs are expanded. The previous interpretation of the 140 K phase as a “liquid of bipolarons” should therefore be revised.     

Chalcogenide-halides of niobium (V). 1. Gas-phase structures of NbOBr(3), NbSBr(3), and NbSCl(3). 2. Matrix infrared spectra and vibrational force fields of NbOBr(3), NbSBr(3), NbSCl(3), and NbOCl(3)

The molecular structures of NbOBr(3), NbSCl(3), and NbSBr(3) have been determined by gas-phase electron diffraction (GED) at nozzle-tip temperatures of 250 degrees C, taking into account the possible presence of NbOCl(3) as a contaminant in the NbSCl(3) sample and NbOBr(3) in the NbSBr(3) sample. The experimental data are consistent with trigonal-pyramidal molecules having C(3)(v)() symmetry. Infrared spectra of molecules trapped in argon or nitrogen matrices were recorded and exhibit the characteristic fundamental stretching modes for C(3)(v)() species. Well resolved isotopic fine structure ((35)Cl and (37)Cl) was observed for NbSCl(3), and for NbOCl(3) which occurred as an impurity in the NbSCl(3) spectra. Quantum mechanical calculations of the structures and vibrational frequencies of the four YNbX(3) molecules (Y = O, S; X = Cl, Br) were carried out at several levels of theory, most importantly B3LYP DFT with either the Stuttgart RSC ECP or Hay-Wadt (n + 1) ECP VDZ basis set for Nb and the 6-311G basis set for the nonmetal atoms. Theoretical values for the bond lengths are 0.01-0.04 A longer than the experimental ones of type r(a), in accord with general experience, but the bond angles with theoretical minus experimental differences of only 1.0-1.5 degrees are notably accurate. Symmetrized force fields were also calculated. The experimental bond lengths (r(g)/A) and angles ( 90 degree angle (alpha)()/deg) with estimated 2sigma uncertainties from GED are as follows. NbOBr(3): r(Nb=O) = 1.694(7), r(Nb-Br) = 2.429(2), 90 degree angle (O=Nb-Br) = 107.3(5), 90 degree angle (Br-Nb-Br) = 111.5(5). NbSBr(3): r(Nb=S) = 2.134(10), r(Nb-Br) = 2.408(4), 90 degree angle (S=Nb-Br) = 106.6(7), 90 degree angle (Br-Nb-Br) = 112.2(6). NbSCl(3): r(Nb=S) = 2.120(10),r(Nb-Cl) = 2.271(6), 90 degree angle (S=Nb-Cl) = 107.8(12), 90 degree angle (Cl-Nb-Cl) = 111.1(11).     

Monochlorogallane: physical properties and structure of the gaseous molecule H2Ga(mu-Cl)2GaH2 as determined by vibrational, electron diffraction, and ab initio studies

Monochlorogallane, synthesized by the metathesis of gallium(III) chloride with an excess of trimethylsilane at ca. 250 K, has been characterized by chemical analysis, by its IR, Raman, and 1H NMR spectra, and by the products of its reaction with trimethylamine. The vibrational spectra of the vapor species isolated in solid Ar, N2, or CH4 matrixes at ca. 12 K imply the presence of only one species, viz. the dimer with an equilibrium structure conforming to D2h symmetry. The structure of this molecule has been determined by gas-phase electron diffraction (GED) measurements augmented by the results of ab initio molecular orbital calculations. An equilibrium structure with D2h symmetry has been assumed in the analysis of the electron diffraction pattern. However, as the molecule has a very low frequency Ga(mu-Cl)2Ga ring-puckering mode, a dynamic model was used to describe it with the aid of a set of pseudoconformers spaced at even intervals (deltadelta = 5 degrees, deltamax, = 20 delta) around the ring-puckering angle delta and Boltzmann-weighted according to a quartic potential V(delta) = V4delta4 + V2delta2. The differences in bond distances and angles between the different pseudoconformers were constrained to the values derived from the ab initio calculations employing second-order Moller-Plesset (MP2) methods (with all the electrons included in the correlation calculations) and a 6-311G(d) basis set. The results for the weighted average of the principal distances (ralpha) and angles (相似文献   

Spectrographic determination of metallic impurities in PuO2

Summary An emission spectrographic method has been developed for the estimation of 22 metallic impurities in plutonium dioxide using carrier-distillation technique Silver chloride at 5% concentration in a charge of 100 mg has been used as the carrier. Mixed oxide standards containing 10% PuO₂ have been used for the analysis of matching composition of plutonium samples. Precision and accuracy determinations of the synthetic samples indicate 7–16% deviation for most of the elements. The work described includes analysis of reference samples and matrix effect studies. The method is suitable for the chemical quality control analysis of plutonium oxide to be used for Fast Breeder Test Reactor (FBTR) fuel.

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Spektralanalytische Bestimmung von metallischen Verunreinigungen in PuO₂

Zusammenfassung Mit dem Verfahren der Trägerdestillation können 22 Elemente in Plutoniumdioxid spektralanalytisch erfaßt werden. Als Träger dient 5% Silberchlorid in je 100 mg. Mischoxid-Standards mit 10% PuO₂ wurden verwendet. Bei der Untersuchung von Genauigkeit und Reproduzierbarkeit synthetischer Proben wurden für die meisten Elemente Abweichungen von 7–16% festgestellt. Matrixeinflüsse wurden ebenfalls untersucht sowie Analysenergebnisse für bestimmte Bezugsstandards mitgeteilt. Das Verfahren eignet sich für die Qualitätskontrolle von PuO₂-Brennstoffen für schnelle Brutreaktoren.

     

Analysis of volatile contaminants in vegetable oils by headspace solid-phase microextraction with carboxen-based fibres

The headspace solid-phase microextraction (HS-SPME) efficiencies from vegetable oil of the recently available Carboxen-poly(dimethylsiloxane) (PDMS) and divinylbenzene-Carboxen-PDMS fibres were found to be much greater than those of the PDMS fibre for a number of volatile contaminants. Using these Carboxen-based fibres, the commonly used HS-SPME equilibration times for aqueous matrices of 30-45 min at room temperature for a number of halogenated and aromatic analytes with volatilities ranging from 1,1-dichloroethylene to hexachlorobenzene were found to be insufficient for the effective extraction of the less volatile analytes from vegetable oil. HS-SPME at 100 degrees C for 45 min, followed by rapid cooling to 0 degrees C with a 10 min continuing extraction, however, significantly increased the SPME efficiencies for the less volatile analytes. Spiking solutions were prepared in vegetable oil instead of methanol as the latter was found to displace analytes from the Carboxen material. Using either of the Carboxen-based fibres and SPME at 100 degrees C, all the target analytes could be determined at low or sub-microg kg(-1) with repeatability < or =10%, even though an equilibrium SPME of the less volatile analytes was not achieved.     

Compleximetric determination of calcium in impure calcium carbonate and limestone

Calcium is determined in impure calcium carbonate and limestone samples by titrating with 0.01 M disodium EGTA at pH 12 in the presence of at least 0.6 mg of magnesium and a maximum of 500 μg of iron(III), using a pH 12.5 sodium hydroxide-potassium cyanide-sodium sulfide buffer and Calcon indicator. The results of such titrations are compared with those obtained by titrating at pH 10 the calcium perchlorate solutions derived from calcium oxalate, and with those of a modified LEWIS AND MELNICK method. The results for magnesium (% MgO) obtained by difference are in fair agreement. Magnesium can be titrated compleximetrically as magnesium perchlorate, but the reagent blank must be determined.     

Gamma-ray spectroscopy of 192–195Po

Prompt and delayed γ-rays have been observed from very neutron deficient ^192–195Po nuclei by using the recoil-decay tagging (RDT) and recoil gating techniques. The yrast levels up to the (10⁺) state in the ¹⁹²Po were identified for the first time. Comprehensive data for ¹⁹⁴Po rendered it possible to extend the yrast line and to observe several positive and negative parity non-yrast states. In the odd-mass isotopes ¹⁹³Po and ¹⁹⁵Po, favoured and unfavoured states on top of the 13/2⁺ state have been identified. The results are discussed within the simple vibrator and rotor pictures as well as in the framework of coexisting spherical and deformed-intruder structures. Received: 5 July 1999 / Revised version: 7 September 1999     

Infrared imagery of an air/CO2 axisymmetric jet

This experiment uses an infrared imaging system to investigate a subsonic, non-isoenergetic, air/CO₂ axisymmetric jet. The classical limitations of using IR imagery with hot gases are presented and a novel approach to overcome these limitations is proposed. The results suggest that radial and axial irradiant profiles measured with the IR imager, when non-dimensionalized, collapse onto curves of similarity. This behavior could allow temperature, velocity, and concentration profiles to be deduced from the IR image.     

First observation of excited states in 184Pb: spectroscopy beyond the neutron mid-shell

Excited states have been identified for the first time in ¹⁸⁴Pb, the first even-even Pb isotope beyond the 82 < N < 126 mid-shell, using the recoil-decay tagging (RDT) technique. A collective band built on the first-excited 2⁺ state has been observed. This resembles those seen in ^186,188Pb and the Hg isotones, and can thus be associated with a prolate-deformed shape. Variable moment of inertia (VMI) fits of the prolate 0⁺ level energies in ^184,186,188Pb indicate that the minimum appears at N= 103, the same neutron number at which the corresponding minimum in Hg isotopes is observed. Received: 19 May 1998 / Revised version: 1 July 1998