Showing papers in "Acta Crystallographica in 1993"

Multipole Analysis of the Electron Density in Triphylite LiFePO4 using X-ray Diffraction Data

Abstract: The electron density distribution and 3d-orbital electron occupancies for the Fe atom in synthetic triphylite, LiFePO 4 , have been analysed using single-crystal X-ray diffraction data measured at T=298 K with Mo Kα (λ=0.71069 A) radiation to a resolution corresponding to (sin θ max /λ)=1.078 A -1 . Measurements of 3265 reflections gave 944 unique data [R int (I)=0.036] with I>2σ(I). For an atomic multipole density model fitted by least-squares methods R(F)=0.0174 for all unique reflections. The Fe atom 3d-orbital occupancies have been derived from the multipole population coefficients using point-group-specific relations

Synchrotron X-ray study of the electron density in a-Al2O3

Abstract: Structure factors for synthetic haematite, α-Fe 2 O 3 , have been measured for two small crystals using focused λ=0.7 A synchrotron radiation. The structure factors from the two data sets are consistent. Approximate symmetry in the concordant densities, related more closely to the Fe-Fe geometry than to the nearest-neighbour Fe-O interactions, is similar to that in the corundum α-Al 2 O 3 structure. Deformation density maxima are located at the midpoint of the Fe-Fe vector along the c axis, on a common face for O-octahedra, perpendicular to c. Maxima also occur at the midpoint of the Fe-Fe vector bisecting the edges of the O-octahedra. These results are in accordance with theoretical predictions for metal-metal bonding

Structure refinement of the incommensurate composite crystal Sr_ TiS_3 through the Rietvld analysis process

Abstract: The structure of the incommensurate composite crystal of the trigonal sulfide Sr 1.145 TiS 3 , M r =244.42, has been analyzed on the basis of the four-dimensional superspace group P R3m 1s using powder X-ray diffraction data and the Rietveld analysis process. The unit cell and other crystal data are a 1 =a 2 =11.5108 (2), a 3 =2.99094 (5) A, a=[000.57227 (2)], V=343.19 A 3 , D x =3.548 Mg m -3 and Z=3. R wp was 0.104 on an incommensurate modulated structure model with 47 structural parameters. The crystal is composed of columns of face-shared polyhedra, (TiS 6/2 )∞, and rows of Sr, and both are parallel to the c axis

The mechanism of iron uptake by transferrins: the structure of an 18kDa NII-domain fragment from duck ovotransferrin at 2.3 Å resolution

Abstract: The molecular structure of an iron-containing 18 kDa fragment of duck ovotransferrin, obtained by proteolysis of the intact protein, has been elucidated by protein crystallographic techniques at 2.3 A resolution. This structure supports a mechanism of iron uptake in the intact protein whereby the binding of the synergistic (bi)carbonate anion is followed by binding of the metal with the lobe in the open configuration. These stages are then followed by domain closure in which the aspartic acid residue plays a further key role, by forming an interdomain hydrogen-bond interaction in addition to serving as a ligand to the iron. This essential dual role is highlighted by model building studies on the C-terminal lobe of a known human variant. In this variant a mutation of a glycine by an arginine residue enables the aspartic acid to form an ion pair and reduce its effectiveness for both metal binding and domain closure. The X-ray structure of the 18 kDa fragment strongly suggests that the histidine residue present at the iron binding site of the intact protein and arising from the second interdomain connecting strand has been removed during the preparative proteolysis.

Electron Density in Potassium Tetrachloropalladate (K2PdCl4) From Synchrotron Radiation

Abstract: The difference electron density has been evaluated for potassium tetrachloropalladate, K2PdC14, using synchrotron radiation at 0.7 and 0 . 9 ~ at room temperature. A similar data set was measured with Mo Ka radiation. The Ap maps for the synchrotron data sets are consistent, and agree approximately with the Mo Ka map away from the Pd nucleus. There is marked depletion of electrons near the Pd core, and substantial movement of electron density near the structural cavities. The Pd-atom charge is approximately + 2, but the K-atom charge is significantly negative. These charges result from transfer of electron density from tightly packed to more open regions of the structure, and do not necessarily reflect atomic electronegativities. K2PdCI4, P4/mmm, Z = 1, /z0.7 = 4.1784, /Zo.9 = 8.6075 m m i, F(000) = 152, T= 293 K, Mr=326.4, a=7.0750(5) , c = 4.1158 (5) ~, V = 206.02 (4) .~3, D,. = 2.631 Mg m-3

Tables of Coincidence Orientations for Ordered Tetragonal L lo Alloys for a Range of Axial Ratios

Abstract: In this paper we develop and apply techniques for computation of CSL, DSCL and step-vector data for grain boundaries in tetragonal materials for a range of axial ratios. This has application to L10 alloys including TiA1, which is a candidate for lightweight high-temperature structural applications. Our results are compared with others and found to be more accurate and complete. We provide data for a wider range of axial ratios than those considered by previous workers. We have also derived equivalent quaternions for tetragonal crystals in tetragonalcrystal coordinates and listed conditions for selecting a unique reduced rotation in tetragonal-crystal coordinates so that a disorientation description becomes available.

Jahn-Teller distortion of the electron density in a-nickel sulfate hexahydrate

Abstract: The electron density at room temperature has been determined for α-nickel sulfate hexahydrate, α-NiSO 4 .6H 2 O: tetragonal, P4 1 2 1 2, M r =262.86, a=6.783 (1), c=18.288 (2) A, V=841.4 (3) A 3 , Z=4, D x =2.075 Mg m -3 , Mo Kα, λ=0.71069 A, μ=2.58 mm -1 , F(000)=544. R=0.023, wR=0.026 from 1864 independent reflections using X-ray diffraction data from a previous study of its structure, absolute chirality and optical activity [Stadnicka, Glazer & Koralewski (1987). Acta Cryst. B43, 319-325]

Arithmetic equivalence of point groups for quasiperdiodic structures

Abstract: Necessary and sufficient conditions are formulated for an n-dimensional arithmetic point group such that it may be the symmetry group of a d-dimensional quasiperiodic but not periodic, i.e. incommensurate, structure with Fourier modulus of rank n. Only point groups leaving invariant a d-dimensional subspace (the physical space) are considered. For an arithmetic point group describing an incommensurate structure, all equivalent choices for the internal space are related by the normalizer in Gl (n, \\bb Z) of the point group. Also, the conditions on arithmetic equivalence of two point groups allowing an incommensurate structure are discussed. These conditions yield a further partition of the arithmetic crystal classes.

X-ray study of the electron density in magnesite, MgCO3

Abstract: The electron density in a naturally faced mineral crystal of MgCO 3 was re-measured with Mo Kα (λ= 0.71073 A) diffraction data, using extinction corrections that minimize differences between equivalent reflection intensities. The deformation electron density has 0.66 e A -3 , density maxima in the C-O bonds and 0.35 e A -3 maxima at the O-atom lone pairs. Space group R3c, hexagonal, M r =84.31, a= 4.635 (1), c=15.023 (2) A, V=279.5 (1) A 3 , Z=6, D x =3.005 Mg m -3 , μ (Mo Kα)=0.48 mm -1 , F(000)=252, T=293 K, R=0.022, wR=0.025, S= 5.87 for 332 unique reflections