Structures isomorphes MeX2O4—Evolution structurale entre 2 K et 300 K de l'antimonite FeSb2O4: Elasticité et ordre magnétique anisotropes

Neutron diffraction has been used to study the whole structural evolution of the antimony oxide FeSb₂O₄, from 2 to 300 K. The antiferromagnetic order has been investigated: at 2 K the magnetic moment is M = 3.8 μ_B. An extrapolated Néel temperature T_N = 45 ± 6 K is observed. The function M(T) below T_N is similar to that found in the isomorphous NiSb₂O₄. Magnetostrictive effects are observed. Above 70 K, the thermal expansion tensor is anisotropic with α_a ? α_c. Using the anisotropic temperature factors B_a (Ų), B_c (Ų) at 2 and 300 K, anisotropic Debye temperatures are calculated. Then, using simple approximations, mean force constants F_a, F_c are calculated; they allow to evaluate the anisotropic compressibility coefficients χ_a ～ 0.857 × 10^?11Pa^?1, χ_c ～ 0.467 × 10^?11Pa^?1; the value of the Grüneisen constant is $\text{γ}\text{= 0.33}$.     

Crystal structure of the ceramic superconductor BaPb0.75Bi0.25O3

The ceramic superconductor BaPb_0.75Bi_0.25O₃ is monoclinic atT<430 K with space groupI2/m. Lattice constants and the atomic coordinates result from 20 reflection groups (12<2<120°, =1.1267 Å) collected with a high resolution powder diffractometer using a synchrotron radiation source and a full neutron powder pattern (6<2<140, =1.909 Å). Synchrotron and neutron data were simtultaneously refined with SIMREF, a new profile refinement program, based on the Rietveld method. Group theoretical analysis of the distortion field leading from the ideal perovskite structure toI2/m exhibitsR ₄ ⁺ symmetry. A comparison is made with the corresponding distortions in nonsuperconducting BaBiO₃ where three different symmetry types are identified. The space groupI2/m found for BaPb_0.75Bi_0.25O₃ allows only one type of averaged (Pb/Bi)O₆ octahedron in the structure: this may support valence fluctuations and hence contribute to the mechanism of superconductivity in this particular compound.     

Isotope ratio monitoring gas chromatography/Mass spectrometry of D/H by high temperature conversion isotope ratio mass spectrometry

Of all the elements, hydrogen has the largest naturally occurring variations in the ratio of its stable isotopes (D/H). It is for this reason that there has been a strong desire to add hydrogen to the list of elements amenable to isotope ratio monitoring gas chromatography/mass spectrometry (irm-GC/MS). In irm-GC/MS the sample is entrained in helium as the carrier gas, which is also ionized and separated in the isotope ratio mass spectrometer (IRMS). Because of the low abundance of deuterium in nature, precise and accurate on-line monitoring of D/H ratios with an IRMS requires that low energy helium ions be kept out of the m/z 3 collector, which requires the use of an energy filter. A clean mass 3 (HD(+.)) signal which is independent of a large helium load in the electron impact ion source is essential in order to reach the sensitivity required for D/H analysis of capillary GC peaks. A new IRMS system, the DELTA(plus)XL(trade mark), has been designed for high precision, high accuracy measurements of transient signals of hydrogen gas. It incorporates a retardation lens integrated into the m/z 3 Faraday cup collector. Following GC separation, the hydrogen bound in organic compounds must be quantitatively converted into H(2) gas prior to analysis in the IRMS. Quantitative conversion is achieved by high temperature conversion (TC) at temperatures >1400 degrees C. Measurements of D/H ratios of individual organic compounds in complicated natural mixtures can now be made to a precision of 2 per thousand (delta notation) or, better, with typical sample amounts of approximately 200 ng per compound. Initial applications have focused on compounds of interest to petroleum research (biomarkers and natural gas components), food and flavor control (vanillin and ethanol), and metabolic studies (fatty acids and steroids). Copyright 1999 John Wiley & Sons, Ltd.     

The structure of dehydrated Na zeolite A (SiAl = 1.09) by neutron profile refinement

Contrary to the recent work of J. M. Thomas et al. (J. Chem. Soc. Chem. Commun., 678, 1981), it was found that a high-resolution neutron powder diffraction pattern of a sample of dehydrated Na zeolite A (with $\text{Si}\text{Al}$ ratio 1.09) can be indexed and refined in the cubic groups $\text{Pm}\text{3}\text{m (R}{}_{\mathrm{<mtext>pw</mtext>}}\text{= 11.0\%)}$ (A. K. Cheetham and J. C. Taylor, J. Solid State Chem.21, 253, 1977) and $\text{Fm}\text{3}\text{c (R}{}_{\mathrm{<mtext>pw</mtext>}}\text{= 10.3\%)}$, whereas in the rhombohedral space group $\text{R}\text{3}$ refinement was unsuccessful. For two other samples having $\text{Si}\text{Al}$ ratios of 1.03 and 1.12 the diffraction patterns again showed no evidence of rhombohedral distortion and can be indexed using a cubic unit cell. It appears that the Si, Al ordering of zeolite A, and hence the crystal symmetry, must depend upon the conditions of preparation of the zeolite.     

Extended-ion approximation and pressure shifts inF-band energies

Pressure variation of maximumF-band absorption energies in the halides of lithium, sodium and potassium has been investigated employing the extended-ion approximation for the calculation of theF-electron energy eigenvalues and using values of local compressibility in the neighbourhood of theF-centres which include the effect of vacancy and pressure. The results obtained agree with the experimental results.