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Special Positions (special + position)
Selected AbstractsGovernment administrative burdens on SMEs in East Africa: reviewing issues and actionsECONOMIC AFFAIRS, Issue 2 2001Fiona Macculloch The important macroeconomic reforms achieved in East African economies (Kenya, Tanzania and Uganda) during the late 1980s and early 1990s have failed to deliver the magnitude of private sector growth and increased employment expected. Governments in the region have begun to recognize that lower-level policies and administrative procedures impose significant constraints on private sector development, stemming primarily from the command and control bureaucracies that characterised colonial governance. There are three priority areas for administrative reform: business licensing and registration, tax and customs procedures and specialised approvals. Also discussed are the problems of the special position of the informal sector, the impact of corruption and access to commercial justice. [source] "The Pacific Era Has Arrived": Transnational Education among Japanese Americans, 1932,1941HISTORY OF EDUCATION QUARTERLY, Issue 1 2003Eiichiro Azuma Looking back on the two years at Keisen Girls'School, I am so grateful for the opportunity to have been able to study here, Our teachers have taught us that it was mistaken if we simply aspired to mimic the ways ofJapanese woman. Cognizant of our special position as Americans of Japanese ancestry, we must instead strive to promote the U.S.-Japan friendship. Furthermore, we must adapt the merits of the Japanese spirit [that we have acquired here] to our Americanism. Back in the United States, we will dedicate ourselves to the good of our own society as best possible citizens, cooperating with Americans of other races and learning from each other, Such is the mission of the Nisei as a bridge between Japan and the United States,one that we have come to appreciate [through our schooling in Japan].1 [source] Sulfamates and their therapeutic potentialMEDICINAL RESEARCH REVIEWS, Issue 2 2005Jean-Yves Winum Abstract Starting from the very simple molecule sulfamic acid, O -substituted-, N -substituted-, or di-/tri-substituted sulfamates may be obtained, which show specific biological activities which were or started to be exploited for the design of many types of therapeutic agents. Among them, sulfamate inhibitors of aminoacyl-tRNA synthetases (aaRSs) were recently reported, constituting completely new classes of antibiotics, useful in the fight of drug-resistant infections. Anti-viral agents incorporating sulfamate moieties have also been obtained, with at least two types of such derivatives investigated: the nucleoside/nucleotide human immunodeficiency virus (HIV) reverse transcriptase inhibitors, and the HIV protease inhibitors (PIs). In the increasing armamentarium of anti-cancer drugs, the sulfamates occupy a special position, with at least two important targets evidenced so far: the steroid sulfatases (STSs) and the carbonic anhydrases (CAs). An impressing number of inhibitors of STSs of the sulfamate type have been reported in the last years, with several compounds, such as 667COUMATE among others, progressing to clinical trials for the treatment of hormone-dependent tumors (breast and prostate cancers). This field is rapidly evolving, with many types of new inhibitors being constantly reported and designed in such a way as to increase their anti-tumor properties, and decrease undesired features (for example, estrogenicity, a problem encountered with the first generation such inhibitors, such as EMATE). Among the many isozymes of CAs, at least two, CA IX and CA XII, are highly overexpressed in tumors, being generally absent in the normal tissues. Inhibition of tumor-associated CAs was hypothesized to lead to novel therapeutic approaches for the treatment of cancer. Many sulfamates act as very potent (low nanomolar) CA inhibitors. The X-ray crystal structure of the best-studied isozyme, CA II, with three sulfamates (sulfamic acid, topiramate, and EMATE) has recently been reported, which allowed for a rationale drug design of new inhibitors. Indeed, low nanomolar CA IX inhibitors of the sulfamate type have been reported, although such compounds also act as efficient inhibitors of isozymes CA I and II, which are not associated with tumors. A large number of anti-convulsant sulfamates have been described, with one such compound, topiramate, being widely used clinically as anti-epileptic drug. By taking into consideration a side effect of topiramate, an anti-epileptic drug leading to weight loss in some patients, it has recently been proposed to use this drug and related sulfamates for the treatment of obesity. The rationale of this use is based on the inhibition of the mitochondrial CA isozyme, CA V, involved in lipogenesis. Some sulfamates were also shown to possess potent inhibitory activity against acyl coenzyme A:cholesterol acyltransferase, an enzyme involved in cholesterol metabolism. One such agent, avasimibe, is in advanced clinical trials for the treatment of hyperlipidemia and atherosclerosis. Thus, the sulfamate moiety offers very attractive possibilities for the drug design of various pharmacological agents, which are on one hand due to the relative ease with which such compounds are synthesized, and on the other one, due to the fact that biological activity of most of them is impressive. © 2004 Wiley Periodicals, Inc. [source] Dallapiccola's Early Synthesis: No. 1, ,Vespro, Tutto Riporti', from Cinque Frammenti di SaffoMUSIC ANALYSIS, Issue 1-2 2006Ben Earle ABSTRACT Recent Italian commentary on the twelve-note music of Luigi Dallapiccola (1904,75) has tended to reject the characteristic image of this composer's work that was established during his lifetime. Notions of a ,typically Italian lyricism' or a ,Mediterranean serialism' have been jettisoned in favour of an emphasis on the neo-Webernian ,rigour' of his technique. Such revisionism is misguided. Not only does this new approach exhibit a narrow formalism, but it also downplays the very elements that have granted Dallapiccola's twelve-note music its special position on the periphery of the twentieth-century repertory. By means of detailed analysis of Dallapiccola's first fully dodecaphonic work, the first of the Cinque frammenti di Saffo (1942), this article mounts a defence of the old critical line. Rather than merely highlighting instances of serial ,rigour', the aim is to provide an appreciation of the richness and complexity of the synthesis of stylistic, technical and expressive elements in Dallapiccola's music of this period. [source] The geomorphic work of George Leslie Adkin (1888,1964) and glaciation in the Tararua Range, North Island, New ZealandNEW ZEALAND GEOGRAPHER, Issue 1 2007M. S. Brook Abstract:, Research initiated in 1909 by G. Leslie Adkin (1888,1964) suggested Park Valley in the Tararua Range was glaciated during the Late Quaternary, based on the ,U-shaped' cross-profiles in the uppermost parts of several valleys. Findings were published, but were not met with universal acceptance. Adkin's work remained the only glacial research undertaken on the North Island's axial ranges until the latter part of the 20th century. Adkin holds a special position in New Zealand, because although he worked full-time as a farmer he published nearly 40 articles in scientific journals on topics as varied as M,ori archaeology and geomorphology. [source] Substitutional and positional disorder in Sr2.88Cu3.12(PO4)4ACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2010Ljiljana Karanovi The title compound, a hydrothermally synthesized strontium copper(II) phosphate(V) (2.88/3.12/4), is isotypic with Sr3Cu3(PO4)4, obtained previously by solid-state reaction, but not with Sr3Cu3(PO4)4, obtained previously by the hydrothermal method. A surplus of copper was observed by both structural and chemical analysis, and the formula obtained by the structural analysis is in full agreement with results of the EDX (energy-dispersive X-ray diffraction) analysis. The structure consists of layers of Cu3O12 groups which are linked via the PO4 tetrahedra. The Cu3O12 groups are formed by one Cu1O4 and two Cu2O5 coordination polyhedra sharing corners. The central Cu1 atom of the Cu3O12 group is located at an inversion centre (special position 2a). The unique structural feature of the title compound is the presence of 12% Cu in the Sr1 site (special position 2b, site symmetry ). Moreover, disordered Sr2 atoms were observed: a main site (Sr2a, 90%) and a less occupied site (Sr2b, 10%) are displaced by 0.48,(3),Å along the b axis. Such substitutional and positional disorder was not observed previously in similar compounds. [source] Poly[hexaaquabis(,4 -4-carboxybenzenesulfonato)bis(,3 -4-carboxybenzenesulfonato)calcium(II)dipotassium(I)]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2009G. Prochniak This study presents the coordination modes and crystal organization of a calcium,potassium coordination polymer, poly[hexaaquabis(,4 -4-carboxybenzenesulfonato-,4O1:O1,:O1,,:O4)bis(,3 -4-carboxybenzenesulfonato-,2O1:O1,)calcium(II)dipotassium(I)], [CaK2(C7H5O5S)4(H2O)6]n, displaying a novel two-dimensional framework. The potassium ion is seven-coordinated by four sulfonate and one carboxyl O atom located on five different acid ligands, two of which are unique, and by two symmetry-independent water O atoms. A pair of close potassium ions share two inversion-related sulfonate O-atom sites to form a dimeric K2O12 unit, which is extended into a one-dimensional array along the a -axis direction. The six-coordinate Ca2+ ion occupies a special position (),at (0, , ) and is surrounded by four sulfonate O atoms from two inversion-related pairs of unique acid monoanions and by two O atoms from aqua ligands. The compound displays a layered structure, with K2O12 and CaO6 polyhedra in the layers and aromatic linkers between the layers. The three-dimensional scaffold is open, with nano-sized channels along the c axis. [source] Lithium barium silicate, Li2BaSiO4, from synchrotron powder dataACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2009Jinyoung Kim The structure of lithium barium silicate, Li2BaSiO4, has been determined from synchrotron radiation powder data. The title compound was synthesized by high-temperature solid-state reaction and crystallizes in the hexagonal space group P63cm. It contains two Li atoms, one Ba atom (both site symmetry ..m on special position 6c), two Si atoms [on special positions 4b (site symmetry 3..) and 2a (site symmetry 3.m)] and four O atoms (one on general position 12d, and three on special positions 6c, 4b and 2a). The basic units of the structure are (Li6SiO13)5, units, each comprising seven tetrahedra sharing edges and vertices. These basic units are connected by sharing corners parallel to [001] and through sharing (SiO4)4, tetrahedra in (001). The relationship between the structures and luminescence properties of Li2SrSiO4, Li2CaSiO4 and the title compound is discussed. [source] Chromium-based clinopyroxene-type germanates NaCrGe2O6 and LiCrGe2O6 at 298,KACTA CRYSTALLOGRAPHICA SECTION C, Issue 12 2008Günther J. Redhammer The structure analyses of sodium chromium digermanate, NaCrGe2O6, (I), and lithium chromium digermanate, LiCrGe2O6, (II), provide important structural information for the clinopyroxene family, and form part of our ongoing studies on the phase transitions and magnetic properties of clinopyroxenes. (I) shows C2/c symmetry at 298,K, contains one Na, one Cr (both site symmetry 2 on special position 4e), one Ge and three O-atom positions (on general positions 8f) and displays the well known clinopyroxene topology. The basic units of the structure of (I) are infinite zigzag chains of edge-sharing Cr3+O6 octahedra (M1 site), infinite chains of corner-sharing GeO4 tetrahedra, connected to the M1 chains by common corners, and Na sites occupying interstitial space. (II) was found to have P21/c symmetry at 298,K. The structure contains one Na, one Cr, two distinct Ge and six O-atom positions, all on general positions 4e. The general topology of the structure of (II) is similar to that of (I); however, the loss of the twofold symmetry makes it possible for two distinct tetrahedral chains, having different conformation states, to exist. While sodium is (6+2)-fold coordinated, lithium displays a pure sixfold coordination. Structural details are given and chemical comparison is made between silicate and germanate chromium-based clinopyroxenes. [source] Monoclinic PZN-8%PT [Pb(Zn0.3066Nb0.6133Ti0.08)O3] at 4,KACTA CRYSTALLOGRAPHICA SECTION C, Issue 12 2007Jennifer S. Forrester The structure of the relaxor ferroelectric Pb(Zn0.3066Nb0.6133Ti0.08)O3 (lead zinc niobium titanium trioxide), known as PZN-8%PT, was determined at 4,K from very high resolution neutron powder diffraction data. The material is known for its extraordinary piezoelectric properties, which are closely linked to the structure. Pseudo-cubic lattice parameters have led to considerable controversy over the symmetry of the structure. We find the structure to be monoclinic in the space group Cm (No. 8), with the Zn, Nb and Ti cations sharing the octahedrally coordinated B site (site symmetry m, special position 2a) and Pb occupying the 12-coordinate A site (site symmetry m, special position 2a). O atoms occupy a disorted octahedron around the B site (site symmetry m and special position 2a, and site symmetry 1 and general position 4b). Atomic coordinates have been determined for the first time, allowing the direction of spontaneous polarization to be visualized. [source] A new pillared lithium bismuth phosphate, LiBi7.37P3O19, with elliptical channelsACTA CRYSTALLOGRAPHICA SECTION C, Issue 10 2007Nachiappan Arumugam The structure of a new lithium bismuth phosphate, LiBi7.37P3O19, consists of infinite OBi4 edge-sharing tetrahedral chains in the ac plane, which form Bi2O2 layers parallel to the b axis. They are sandwiched between PO4 tetrahedral and Bi polyhedral layers. The PO4,Bi,PO4 layers are bridged by columns formed by one Bi polyhedron flanked on each side by LiO4 tetrahedra. This bridging Bi atom lies on a twofold axis, special position 4e of the C2/c space group. This arrangement creates pillared open elliptical channels parallel to [010]. [source] Calcium chloride rhenate(VII) dihydrateACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2007Urszula Jarek The crystal structure of calcium chloride rhenate(VII) dihydrate, CaCl(ReO4)·2H2O, investigated at 85,K, consists of calcium cations, chloride anions, rhenate(VII) anions and water molecules. In the nearly tetrahedral rhenate(VII) anion, all constituent atoms lie on special positions of m2m (Re) and m (O) site symmetries. The Cl, anion and water O atom lie on special positions of m2m and 2 site symmetries, respectively. The Ca2+ ion, also on a special position (m2m), is eight-coordinated in a distorted square-antiprismatic coordination mode. The crystal has a layered structure stabilized by Ca,O coordination bonds and O,H...Cl hydrogen bonds. [source] Ca7.96Cu0.04Ge5O18: a new calcium germanate with GeO4 and Ge3O10 unitsACTA CRYSTALLOGRAPHICA SECTION C, Issue 11 2006Günther J. Redhammer The title compound, octacalcium copper pentagermanium octadecaoxide, represents a new intermediate phase between CaO and GeO2, and has not previously been reported in the literature. The structure consists of three different Ge sites, two of them on general 8d positions, site symmetry 1, one on special position 4d, site symmetry 2. Three of the five Ca sites occur on 8d positions, site symmtery 1, one Ca is on 4b with site symmetry and one Ca is on 4c with site symmetry 2. All nine O atoms have symmetry 1 (8d position). By sharing common edges, the Ca sites form infinite bands parallel to the c axis, and these bands are interconnected by isolated GeO4 and Ge3O10 units. These (100) layers are stacked along a in an ABAB, sequence, with the B layer being inverted and displaced along b/2. [source] FOX, `free objects for crystallography': a modular approach to ab initio structure determination from powder diffractionJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2002Vincent Favre-Nicolin A new program has been developed for ab initio crystal structure determination from powder diffraction data (X-ray and neutron). It uses global-optimization algorithms to solve the structure by performing trials in direct space. It is a modular program, capable of using several criteria for evaluating each trial configuration (e.g. multi-pattern). It is also modular in the description of the crystal content, with the possibility of describing building blocks in the sample, such as polyhedra or molecules, and with automatic adaptive handling of special positions and sharing of identical atoms between neighbouring building blocks. It can therefore find the correct structure without any assumption about the connectivity of the building blocks and is suitable for any kind of material. Several optimization algorithms (simulated annealing, parallel tempering) are available, with the possibility of choosing the convergence criterion as a combination of available cost functions. This program is freely available for Linux and Windows platforms; it is also fully `open source', which, combined with an object-oriented design and a complete developer documentation, ensures its future evolution. [source] Al14Ba8La26.3Ru18Sr53.7O167: a variant of cubic perovskite with isolated RuO6 unitsACTA CRYSTALLOGRAPHICA SECTION C, Issue 5 2010F. 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] C,H..., interactions in cocrystals of bis(trimethylsilyl)acetylene and diphenylacetylene with benzeneACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2010Frank Meyer-Wegner We present here the crystal structures of two acetylene derivatives cocrystallized with benzene, namely bis(trimethylsilyl)acetylene benzene solvate, C8H18Si2·C6H6, (I), and diphenylacetylene benzene solvate, C14H10·C6H6, (II). In (I), both molecules belong to the symmetry point group C2h and are located about special positions with site symmetry 2/m. In (II), both molecules show crystallographic inversion symmetry. In both structures, there are C,H..., contacts between aromatic H atoms and the ,-electrons of the triple bond. In addition to these, in (II) there are C,H..., contacts between aromatic H atoms and the ,-electron cloud of the benzene molecules. [source] K3TaF8 from laboratory X-ray powder dataACTA CRYSTALLOGRAPHICA SECTION C, Issue 2 2010ubomír Smr The crystal structure of tripotassium octafluoridotantalate, K3TaF8, determined from laboratory powder diffraction data by the simulated annealing method and refined by total energy minimization in the solid state, is built from discrete potassium cations, fluoride anions and monocapped trigonal,prismatic [TaF7]2, ions. All six atoms in the asymmetric unit are in special positions of the P63mc space group: the Ta and one F atom in the 2b (3m) sites, the K and two F atoms in the 6c (m) sites, and one F atom in the 2a (3m) site. The structure consists of face-sharing K6 octahedra with a fluoride anion at the center of each octahedron, forming chains of composition [FK3]2+ running along [001] with isolated [TaF7]2, trigonal prisms in between. The structure of the title compound is different from the reported structure of Na3TaF8 and represents a new structure type. [source] Dipotassium trimanganese(II) tetrakis(hydrogenphosphite), K2[Mn3(HPO3)4]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 7 2009Farida Hamchaoui The title compound is a new mixed alkali/3d metal phosphite. It exhibits a layered structure formed by linear Mn3O12 trimer units which contain face-sharing MnO6 octahedra interconnected by (HPO3)2, phosphite oxoanions. The K+ cations located between the anionic [Mn3(HPO3)4]2, sheets are ninefold coordinated. The presence of the alkaline ion leads to the highest symmetry and shortest interlayer distance compared with two previous compounds showing the same anionic framework and having ammonium salts as cations. The compound crystallizes in the space group Rm, with two crystallographically independent Mn atoms occupying sites of m and 3m symmetry. All the other atoms, except for the phosphite O atoms, are located on special positions with 3m symmetry. [source] Poly[[diaqua(,3 -2,2-dimethylmalonato)cadmium(II)] tetrahydrate]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 7 2009Ming-Lin Guo In the title complex, {[Cd(C5H6O4)(H2O)2]·4H2O}n, the dimethylmalonate,cadmium metal,organic framework co-exists with an extended structure of water molecules, which resembles a sodalite-type framework. In the asymmetric unit, there are five independent solvent water molecules, two of which are in special positions. The Cd atoms are eight-coordinated in a distorted square-antiprismatic geometry by six O atoms of three different dimethylmalonate groups and by two water molecules, and form a two-dimensional honeycomb layer parallel to the bc plane. Two such layers sandwich the hydrogen-bonded water layer, which has a sodalite-type structure with truncated sodalite units composed of coordinated and solvent water molecules. This work is the first example of a dimethylmalonate cadmium complex containing truncated sodalite-type water clusters. [source] La(Ni2/3Nb1/3)O3 by neutron powder diffractionACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2009Julian R. Tolchard Lanthanum nickel niobium trioxide has been synthesized and its structure refined for the first time. It was found to be a member of the family of technologically important `double perovskites', crystallizing in the monoclinic space group P21/n. The structure is characterized by a strong orthorhombic pseudosymmetry and a concurrent exhibition of both 1:1 B -cation ordering and a,a,c+ -type tilting of the (Ni/Nb)O6 structural units. Trivalent lanthanum resides on the perovskite A site, which is strongly distorted owing to the tilting of the (Ni/Nb)O6 sublattice. Ordering of divalent nickel and pentavalent niobium on the B sublattice is described in terms of two twofold special positions (2c and 2d), with nickel taking almost complete occupancy of the 2d site and the 2c position being occupied by a statistical distribution of nickel and niobium. [source] Lithium barium silicate, Li2BaSiO4, from synchrotron powder dataACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2009Jinyoung Kim The structure of lithium barium silicate, Li2BaSiO4, has been determined from synchrotron radiation powder data. The title compound was synthesized by high-temperature solid-state reaction and crystallizes in the hexagonal space group P63cm. It contains two Li atoms, one Ba atom (both site symmetry ..m on special position 6c), two Si atoms [on special positions 4b (site symmetry 3..) and 2a (site symmetry 3.m)] and four O atoms (one on general position 12d, and three on special positions 6c, 4b and 2a). The basic units of the structure are (Li6SiO13)5, units, each comprising seven tetrahedra sharing edges and vertices. These basic units are connected by sharing corners parallel to [001] and through sharing (SiO4)4, tetrahedra in (001). The relationship between the structures and luminescence properties of Li2SrSiO4, Li2CaSiO4 and the title compound is discussed. [source] Poly[diaqua(,-4,4,-bipyridine-,2N:N,)bis(,-cyanido-,2C:N)bis(cyanido-,C)nickel(II)copper(II)]: a metal,organic cyanide-bridged frameworkACTA CRYSTALLOGRAPHICA SECTION C, Issue 6 2008Olha Sereda The structure of the title compound, [NiCu(CN)4(C10H8N2)(H2O)2]n or [{Cu(H2O)2}(,-C10H8N2)(,-CN)2{Ni(CN)2}]n, was shown to be a metal,organic cyanide-bridged framework, composed essentially of ,Cu,4,4,-bpy,Cu,4,4,-bpy,Cu, chains (4,4,-bpy is 4,4,-bipyridine) linked by [Ni(CN)4]2, anions. Both metal atoms sit on special positions; the CuII atom occupies an inversion center, while the NiII atom of the cyanometallate sits on a twofold axis. The 4,4,-bpy ligand is also situated about a center of symmetry, located at the center of the bridging C,C bond. The scientific impact of this structure lies in the unique manner in which the framework is built up. The arrangement of the ,Cu,4,4,-bpy,Cu,4,4,-bpy,Cu, chains, which are mutually perpendicular and non-intersecting, creates large channels running parallel to the c axis. Within these channels, the [Ni(CN)4]2, anions coordinate to successive CuII atoms, forming zigzag ,Cu,N[triple-bond]C,Ni,C[triple-bond]N,Cu, chains. In this manner, a three-dimensional framework structure is constructed. To the authors' knowledge, this arrangement has not been observed in any of the many copper(II),4,4,-bipyridine framework complexes synthesized to date. The coordination environment of the CuII atom is completed by two water molecules. The framework is further strengthened by O,H...N hydrogen bonds involving the water molecules and the symmetry-equivalent nonbridging cyanide N atoms. [source] L'arséniate Na3Fe2(AsO4)3: étude structurale de la forme basse température et simulation des propriétés de conduction des cations alcalinsACTA CRYSTALLOGRAPHICA SECTION C, Issue 5 2008Najoua Ouerfelli The crystal structure of the low-temperature garnet-like form of trisodium diiron(III) triarsenate, Na3Fe2(AsO4)3, exhibits a three-dimensional framework with small tunnels running along the [111] direction, in which the Na+ cations are located. This study demonstrates the structural origins of the different ionic conductivities of the low- and high-temperature forms. Sodium conduction properties are simulated by means of the bond-valence-sum (BVS) model; the correlations between the low- and high-temperature crystal structures are discussed. The As, Fe and Na atoms lie on special positions (Wyckoff symbols 24d, 16a and 24c, respectively). [source] Three hexafluoridoiridates(IV), Ca[IrF6]·2H2O, Sr[IrF6]·2H2O and Ba[IrF6]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 11 2007Anton I. Smolentsev The structures of the hexafluoridoiridates(IV) of calcium, Ca[IrF6]·2H2O [calcium hexafluoridoiridate(IV) dihydrate], strontium, Sr[IrF6]·2H2O [strontium hexafluoridoiridate(IV) dihydrate], and barium, Ba[IrF6] [barium hexafluoridoiridate(IV)], have been determined by single-crystal X-ray analysis. The first two compounds are isomorphous. Their metal cations are eight-coordinated in a distorted square-antiprismatic coordination environment, and their anions are represented by an almost ideal octahedron. These two structures can be described as frameworks in which all atoms occupy general positions. Sr[RhF6] and Ba[RhF6] have a different space group (, from powder diffraction data) but similar cell dimensions. The structures are very close to that of Ba[IrF6]. The cation is in a cuboctahedral coordination. The metal atoms are located on special positions of symmetry, while the F atoms are in general positions. [source] Strontium tetrafluoridoborate and barium tetrafluoridoborateACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2007Tina Buni In Sr(BF4)2, which is isomorphous with the previously published Ca(BF4)2, the metal atom possesses a coordination number of 8 with a square-antiprismatic environment. Each tetrafluoridoborate anion is bonded to four metal centers. In the barium derivative, the metal center, with symmetry 2/m, is surrounded by 14 F atoms. The B atom and two of the three independent F atoms occupy special positions with symmetry m. Each anion is connected to five Ba atoms. This structure differs significantly from an earlier published structure of Ba(BF4)2 [published as Ba2(BF4)4; Lin, Cheng, Chen & Huang (1998). Jiegon Huaxue, 17, 245]. The radial distribution functions for the present Ba(BF4)2 and earlier Ba2(BF4)4 structures differ significantly. [source] Calcium chloride rhenate(VII) dihydrateACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2007Urszula Jarek The crystal structure of calcium chloride rhenate(VII) dihydrate, CaCl(ReO4)·2H2O, investigated at 85,K, consists of calcium cations, chloride anions, rhenate(VII) anions and water molecules. In the nearly tetrahedral rhenate(VII) anion, all constituent atoms lie on special positions of m2m (Re) and m (O) site symmetries. The Cl, anion and water O atom lie on special positions of m2m and 2 site symmetries, respectively. The Ca2+ ion, also on a special position (m2m), is eight-coordinated in a distorted square-antiprismatic coordination mode. The crystal has a layered structure stabilized by Ca,O coordination bonds and O,H...Cl hydrogen bonds. [source] A new layered perovskite, KSrNb2O6F, by powder neutron diffractionACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2007Chung-Yul Yoo The structure of a new layered oxyfluoride, viz. potassium strontium diniobium hexaoxide fluoride, KSrNb2O6F, was refined from powder neutron diffraction data in the orthorhombic space group Immm. The oxyfluoride compound is an n = 2 member of the Dion,Jacobson-type family of general formula A[A,n,1BnX3n+1], which consists of double layered perovskite slabs, [SrNb2O6F],, between which K+ ions are located. Within the perovskite slabs, the NbO5F octahedra are significantly distorted and tilted about the a axis. A bond-valence-sum calculation gives evidence for O/F ordering in KSrNb2O6F, with the F, ions located in the central sites of the corner-sharing NbO5F octahedra along the b axis. All atoms lie on special positions, namely Nb on m, Sr on mmm, K on m2m, F on mm2, and O on sites of symmetry m and m2m. [source] Orientational disorder of [Mg(H2O)6] octahedra in the novel magnesium selenite hydrate Mg(SeO3)·7.5H2OACTA CRYSTALLOGRAPHICA SECTION C, Issue 1 2007Michel Fleck The crystal structure of magnesium selenite 7.5-hydrate, Mg(SeO3)·7.5H2O (space group P63/mmc), is characterized by two crystallographically distinct [Mg(H2O)6]2+ octahedra, one of which is disordered over two different orientations. The selenite groups and water molecules (with partially disordered H atoms) bridge the octahedra via hydrogen bonds. All the atoms are located on special positions, except for one water molecule. [source] Poly[[di-,3 -acetato-di-,2 -aqua-diaqua-di-,3 -hydroxo-tricopper(II)] naphthalene-1,5-disulfonate]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 12 2006Li-Li Kong The crystal structure of the title complex, {[Cu3(C2H3O2)2(OH)2(H2O)4](C10H6O6S2)}n, is built of infinite polymeric cationic {[Cu3(C2H3O2)2(H2O)4(OH)2]2+}n chains stretching along the a axis, with naphthalene-1,5-disulfonate (1,5-nds) anions in between. One independent CuII cation and the 1,5-nds anion occupy special positions on crystallographic inversion centres. Each CuII cation has an octahedral coordination environment formed by two carboxyl O atoms, two hydroxo O atoms and two water molecules. The carboxylate and hydroxo groups perform a bridging function, linking adjacent Cu atoms in the chain, with a shortest Cu,Cu distance of 2.990,(3),Å. The chains are further linked into a three-dimensional supramolecular framework via hydrogen-bonding interactions involving the sulfonate groups of the 1,5-nds dianions. [source] catena -Poly[[[triaquasulfatocobalt(II)]-,-4,4,-bipyridine] ethane-1,2-diol solvate]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2006Wen-Jie Lu In the title compound, {[Co(SO4)(C10H8N2)(H2O)3]·C2H6O2}n, each CoII center is octahedrally coordinated by two N atoms from two bridging 4,4,-bipyridine (bipy) ligands and four O atoms, one from a monodentate sulfate ligand and three from aqua ligands. The bipy ligands occupy special positions of site symmetry and bridge adjacent cobalt(II) centers to form one-dimensional linear coordination chains. Adjacent chains are arranged in a cross-like fashion around the mid-point of the bipy ligands, resulting in a three-dimensional supramolecular array. [source] | |||||||||||||