Band 225, Heft 7 -

A method for locating in an unknown one-dimensional structure (or structure projection) a resonant scattering atom from reflection intensities measured at two wavelengths is presented in terms of the parameter space concept. Even with rather few one-dimensional data the method permits high resolution (well beyond that of Fourier or Patterson calculations). In presence of some hundred ‘light’ atoms, a resonant scattering ‘heavy’ atom can be retrieved from a small number of proposed positions, even if reflection intensities with maximum relative errors of up to 20 or 30% are used. The same applies to structures composed of up to about 50 ‘normal’ atoms with scattering powers similar to that of the resonant scatterer. Further, the method allows for combining the results from sub-sets of a harmonic reflection data sequence, and there is no need to include the lowest reflection orders. Results for some hypothetical model structures both with and without inversion centre are presented and discussed. Also, the advantage of the technique compared to conventional one-dimensional difference Patterson calculations is illustrated and possible applications of the method and extensions to several resonant scatterers are adressed.

[Ba] 1.30 [(Ru,Cu)O 3 ] crystallizes in a modulated composite crystal structure previously observed for [Ba] x [(Pt,Cu)O 3 ], x = 1.317. The composite structure is refined in one centrosymmetric and one non-centrosymmetric model in the superspace groups P -31 c (00 γ ) and P 31 c (00 γ ), respectively, with a = 10.135(1) Å, c = 4.2902(3) Å and q = 1.601(1) c *. The centrosymmetric case was refined using Crenel functions. The first substructure is formed by Ba cations located in channels of the second substructure, which consists of one-dimensional chains of face-sharing polyhedra ∞ 1 [O 3/2 (Ru,Cu)O 3/2 ]: alternating blocks of mostly three octahedra and trigonal prisms. DC-susceptibility measurements result in typical spin-glass behavior, an effective magnetic moment of μ eff = 2.55μ B , and a Weiss temperature of θ p = -202 K.

The new metal-rich phosphide Sm 15 Ir 33 P 26 was obtained from a reaction of the elements in a lead flux (1 : 2 : 2 : 30 molar ratio, 1370 K, slow cooling rate). Sm 15 Ir 33 P 26 adopts a new structure type which was characterized by powder and single crystal X-ray data: C 2/ m , a = 4720.6(8), b = 392.87(9), c = 1329.2(2) pm, β = 95.478(1)°, wR 2 = 0.0376, 3757 F 2 values and 227 variables. The iridium and phosphorus atoms build up a complex three-dimensional [Ir 33 P 26 ] δ – polyanionic network in which the samarium atoms fill diverse larger cavities with coordination numbers of 15–17. The seventeen crystallographically independent iridium atoms have between 3 or 4 phosphorus neighbors in distorted trigonal-, square-planar, or tetrahedral coordination. All phosphorus atoms are isolated and most of them fill tri-capped trigonal prisms. The Sm 15 Ir 33 P 26 structure is an intergrowth variant of ThCr 2 Si 2 , SrPtSb, CeMg 2 Si 2 , and TiNiSi-related slabs.

Despite high mosaicity and complex micro structures in pyroxenes from Mars average occupancies and average atomic positions of pigeonite were determined from X-ray single crystal and neutron and X-ray powder diffraction data. The chemical composition of selected single crystals varied between En 39 Fs 55 Wo 6 and En 60 Fs 34 Wo 6 . The average composition of the whole sample was determined from powder diffraction to be about En 43 Fs 51 Wo 6 . The ordering of the cations on the M1 and M2 positions, expressed by a partitioning or distribution coefficient K D , in the pigeonite structure was determined and a closure temperature of about 540 °C was estimated for crystals stemming from the core of our specimen. General trends of the crystal chemistry with K D are compared with literature data. From in-situ high temperature neutron powder data, however, it is deduced that a simple intra-phase-ordering process is not adequate, since diffusion processes between intimately intergrown augite and pigeonite phases at temperatures above 600 °C also take place. This makes the usual interpretation of structural data towards the prior history of the martian sample material doubtful.

A synthetic strontium isomorph (Sr 2 ZnSi 2 O 7 ) of the hardystonite structure (Ca 2 ZnSi 2 O 7 ) was investigated with X-ray powder diffraction. The average structure (S.G. P -42 1 m ), a = 8.0007(1) Å, c = 5.1722(1) Å, was refined by Rietveld method and a R F 2 = 0.0216 was achieved using 394 independent reflections. The results show that Sr-hardystonite has a melilite-type structure with no incommensurate modulations. Its geometrical and bonding features are consistent with trends defined by the other so-called normal and modulated melilite structures.

Room temperature X-ray powder diffraction data were used to investigate modulated crystal structures of two monoclinic polyphosphates of crystal form C , In(PO 3 ) 3 and Y(PO 3 ) 3 , by the Rietveld refinement. The structures were refined with the program JANA2006 in the superspace groups Xc (0 β 0)0, β = 1/3, for commensurately modulated In(PO 3 ) 3 , and Cc (0 β 0)0, β = 0.36564(7), for incommensurately modulated Y(PO 3 ) 3 . The positional modulations were described with the first-order harmonic functions. The Rietveld refinement was performed with soft restraints on bond lengths and valence angles in PO 4 tetrahedra, as well as on angles in MO 6 octahedra. No initial information about modulation parameters was used. A good agreement of the observed and calculated intensities of main and satellite reflections was achieved for both compounds. Obtained modulated structures of In(PO 3 ) 3 and Y(PO 3 ) 3 are close to published single crystal data.