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Wyckoff Position (wyckoff + position)

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Chemistry100%

Selected Abstracts

The new ternary phases of La3(Zn0.874Mg0.126)11 and Ce3(Zn0.863Mg0.137)11

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 3 2010
Volodymyr Pavlyuk
The new ternary intermetallic title compounds, namely trilanthanum undeca(zinc/magnesium), La3(Zn0.874Mg0.126)11, (I), and tricerium undeca(zinc/magnesium), Ce3(Zn0.863Mg0.137)11, (II), are isostructural and crystallize in the orthorhombic La3Al11 structure type. These three phases belong to the same structural family, the representative members of which may be derived from the tetragonal BaAl4 structure type by a combination of internal deformation and multiple substitution. Compared to the structure of La3Al11, in (I), a significant decrease of 11.9% in the unit-cell b axis and an increase in the other two directions, of 3.6% along a and 5.2% along c, are observed. Such an atypical deformation is caused by the closer packing of atoms in the unit cell due to atom shifts that reflect strengthening of metallic-type bonding. This structural change is also manifested in a significant difference in the coordination around the smaller atoms at the 8l Wyckoff position (site symmetry m). The Al atom in La3Al11 is in a tricapped trigonal prismatic environment (coordination number 9), while the Zn atoms in (I) and (II) are situated in a tetragonal antiprism with two added atoms (coordination number 10). [source]

Cr1.45Tl1.87Mo15Se19, a monoclinic variant of the hexagonal In3Mo15Se19 type

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 12 2009
P. Gougeon
The monoclinic compound Cr1.45Tl1.87Mo15Se19 (chromium thallium pentadecamolybdenum nonadecaselenide) represents a variant of the hexagonal In3Mo15Se19 structure type. Its crystal structure consists of an equal mixture of Mo6Se8Se6 and Mo9Se11Se6 cluster units. The Mo and Se atoms of the median plane of the Mo9Se11Se6 unit, as well as three Cr ions, lie on sites with m symmetry (Wyckoff site 2e). The fourth Cr ion is in a 2b Wyckoff position with site symmetry. [source]

Bis[4-(dimethylamino)pyridine-,N1]silver(I) nitrate dihydrate

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2009
Al-shima'a A. Massoud
The title compound, [Ag(C7H10N2)2]NO3·2H2O or [Ag(dmap)2]NO3·2H2O, where dmap is 4-(dimethylamino)pyridine, has a distorted linear coordination geometry around the AgI ion. A novel pattern of water,nitrate hydrogen-bonded anionic strands is formed in the c direction, with the cationic [Ag(dmap)2]+ monomers trapped between them. The AgI ion and the nitrate group atoms, as well as the water molecules (including the H atoms), are on a crystallographic mirror plane (Wyckoff position 4a). The influence of bulky methyl substituents in position 4 of the 4-(dimethylamino)pyridine ligand on packing is discussed. The absolute structure was determined unequivocally. [source]

Sodium citrate dihydrate doped with Mn3+ ions

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2009
Dejan Poleti
Sodium citrate dihydrate doped with Mn3+ ions, namely trisodium(I) managnese(III) citrate(3,) dihydrate, [Na3Mn0.011(C6H5O7)(H2O)2]n, was obtained during attempts to prepare some complex MnIII citrates from a concentrated strong alkaline solution containing Na+, Mn3+ and citrate ions. The compound is isostructural with the recently described Na3(C6H5O7)·2H2O [Fischer & Palladino (2003). Acta Cryst. E59, m1080,m1082]. The essential difference between these two structures is the presence of a very small proportion (0.205,wt%) of Mn3+ ions, which are positioned at the special 4e Wyckoff position in C2/c, where they are in a highly distorted octahedral environment of O atoms from two citrate anions. [source]

Pseudo-merohedrally twinned praseodymium hexa­cyano­ferrate(III) tetrahydrate

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 11 2004
Vratislav Langer
Crystals of the title compound, di­aqua­hexa-,-cyano-ferrate(III)­praseo­dym­ium(III) dihydrate, Pr[Fe(CN)6]·4H2O or [PrFe(CN)6(H2O)2]·2H2O, are twinned with three components. The Pr atom is coordinated by eight atoms, viz. six N and two symmetry-related water O atoms. The Pr polyhedron (Pr has site symmetry m2m, Wyckoff position 4c) is linked to an FeC6 octahedron (Fe located on a site with imposed 2/m symmetry, Wyckoff position 4b) through N atoms, forming an infinite array. The second (symmetry independent) water mol­ecule lies on a mirror plane, is not included in coordination and is weakly hydrogen bonded to N atoms. [source]

Crystal topologies , the achievable and inevitable symmetries

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 3 2009
Georg Thimm
The link between the crystal topology and symmetry is examined, focusing on the conditions under which a structure with a given topology can exhibit a certain symmetry. By defining embeddings for quotient graphs (finite representations of crystal topologies) and the corresponding nets (the graph-theoretical equivalents of structures), a strong relationship between the automorphisms of the quotient graphs and the symmetry of the embedded net is established. This allows one to constrain the relative node positions under the premise that an embedding of a net has a certain symmetry, and allows one to assign nodes to equivalents of Wyckoff positions. Two-dimensional examples as well as known crystal structures are used to illustrate the findings. A comparison with a related publication and a discussion on whether constraints on distances between atoms and on bond angles result in restrictions on symmetry without causing confusion conclude the work. [source]

Al14Ba8La26.3Ru18Sr53.7O167: a variant of cubic perovskite with isolated RuO6 units

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 5 2010
F. J. Zúñiga
The crystal structure of the title aluminium barium lanthanum ruthenium strontium oxide has been solved and refined using neutron powder diffraction to establish the parameters of the oxygen sublattice and then single-crystal X-ray diffraction data for the final refinement. The structure is a cubic modification of the perovskite ABO3 structure type. The refined composition is Ba0.167La0.548Sr1.118Ru0.377Al0.290O3.480, and with respect to the basic perovskite structure type it might be written as (Ba8La13.68Sr34.32)(Al13.92La12.64Ru18.08Sr19.36)O192,x, with x = 24.96. The metal atoms lie on special positions. The A -type sites are occupied by Ba, La and Sr. The Ba atoms are located in a regular cuboctahedral environment, whereas the La and Sr atoms share the same positions with an irregular coordination of O atoms. The B -type sites are divided between two different Wyckoff positions occupied by Ru/Al and La/Sr. Only Al and Ru occupy sites close to the ideal perovskite positions, while La and Sr move away from these positions toward the (111) planes with high Al content. The structure contains isolated RuO6 octahedra, which form tetrahedral substructural units. [source]

Bi4Ge3O12 at the onset of pressure-induced amorphization

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 10 2009
Andrzej Grzechnik
The crystal structure of tetrabismuth tris(germanate), Bi4Ge3O12 (I3d, Z = 4), is stable to at least 7.30,GPa, as demonstrated by hydrostatic single-crystal X-ray diffraction measurements in a diamond anvil cell at room temperature. The highest pressure reached in this study is close to the onset of amorphization at about 8,GPa. The Bi and Ge atoms are located at the 16c (3) and 12a () Wyckoff positions, respectively. The compression mainly affects the distorted BiO6 octahedra, while the GeO4 tetrahedra are relatively rigid. When compared with the values obtained under ambient conditions, the long Bi,O distances decrease with increasing pressure, while the short Bi,O distances do not change. [source]

Synthetic aenigmatite analog Na2(Mn5.26Na0.74)Ge6O20: structure and crystal chemical considerations

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 3 2008
Günther J. Redhammer
Disodium hexamanganese(II,III) germanate is the first aenigmatite-type compound with significant amounts of manganese. Na2(Mn5.26Na0.74)Ge6O20 is triclinic and contains two different Na positions, six Ge positions and 20 O positions (all with site symmetry 1 on general position 2i of space group P). Five out of the seven M positions are also on general position 2i, while the remaining two have site symmetry (Wyckoff positions 1f and 1c). The structure can be described in terms of two different layers, A and B, stacked along the [011] direction. Layer A contains pyroxene-like chains and isolated octahedra, while layer B is built up by slabs of edge-sharing octahedra connected to one another by bands of Na polyhedra. The GeO4 tetrahedra show slight polyhedral distortion and are among the most regular found so far in germanate compounds. The M sites of layer A are occupied by highly charged (trivalent) cations, while in layer B a central pyroxene-like zigzag chain can be identified, which contains divalent (or low-charged) cations. This applies to the aenigmatite-type compounds in general and to the title compound in particular. [source]