Octahedron

Distribution by Scientific Domains
Distribution within Chemistry


Selected Abstracts


Flattening the B6H2- 6 Octahedron.

CHEMINFORM, Issue 49 2003
Ab initio Prediction of a New Family of Planar All-Boron Aromatic Molecules.
No abstract is available for this article. [source]


Synthesis and structure of a new one-dimensional cobalt complex with dicyanamide and 4-picolyl choride bridges

CRYSTAL RESEARCH AND TECHNOLOGY, Issue 4 2006
Hongxia Pei
Abstract The synthesis and structure of the 1D cobalt (II) complex, [Co(L)2(dca)2] (1) (dca = dicyanamide, C2N3,, L = 4-picolyl choride) is reported. Complex 1 crystallized in triclinic system, space group P -1, with cell dimensions of a = 7.291(2) Å, b = 7.481(2) Å, c = 9.007(3) Å, , =104.444(4)°, , = 96.971(4)°, , =102.618(4)°, V = 456.1(2) Å3, Z = 1, Dc = 1.624 Mg/m3. In complex 1, Co (II) is 6-coordinated by N atoms of four dca ligands and two L ligands. The centrosymmetric CoN6 chromophore is an axially elongated octahedron that has Co-N distances ranging from 2.122(3) to 2.154(3) Å. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Dinuclear Manganese and Cobalt Complexes with Cyclic Polyoxovanadate Ligands: Synthesis and Characterization of [Mn2V10O30]6, and [Co2(H2O)2V10O30]6,

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 34 2009
Shinnosuke Inami
Abstract An all-inorganic complex, [Mn2{(VO3)5}2]6, (1), was synthesized, and the structure determination reveals a dinuclear manganese complex coordinated by two cyclic pentavanadate ligands. The cyclic pentavanadate units sandwich the edge-sharing octahedral dimanganese core through coordination of the oxido group of the pentavanadate. A dinuclear cobalt complex with a cyclic decavanadate, [Co2(OH2)2(VO3)10]6, (2), was also synthesized. The structure analysis reveals a dinuclear cobalt complex with a macrocyclic decavanadate, which is composed of 10 VO4 units joined by the vertex sharings. The CoO6 octahedrons are edge-shared, with each cobalt octahedron coordinated to five oxido groups from the decavanadate. The remaining site is occupied by water. The coordinated water molecules are supported with hydrogen bonds in two directions. Complex 2 in acetonitrile shows no reactivity with dioxygen even at low temperature, and the cyclic voltammogram of 2 shows no redox chemistry in acetonitrile. Complex 2 exhibits chromism by water exposure both in the solid state and in acetonitrile. Complex 2 is green,yellow in color, and the addition of water causes the complex to turn brown. After heating the sample, it returns to its original color in a reversible manner. The EXAFS data in acetonitrile is also reported and is consistent with the solid-state structure. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]


Alkali Metal (Li+,Cs+) Salts with Hexafluorochromate(V), Hexafluorochromate(IV), Pentafluorochromate(IV), and Undecafluorodichromate(IV) Anions

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2008
Zoran Mazej
Abstract The compounds ACrF6 (A = Li,Cs) were prepared by photochemical reactions of AF/CrF3 mixtures in anhydrous HF with elemental F2 at ambient temperature. The crystal structures of compounds ACrF6 (A = K,Cs) are analogous to that of KOsF6, and NaCrF6 exhibits polymorphism. The trigonal phase (II) can be classified to have the well-known LiSbF6 type of structure, while the crystal structure of the orthorhombic modification (I) appears to be a new structure-type. Thermal decomposition of the ACrF6 salts produce ACrF5(A = Rb, Cs), ACrF5/A2CrF6 (A = K), or A2CrF6 (A = Li, Na). These compounds undergo partial solvolysis in anhydrous HF with precipitation of CrF4. From the remaining solutions of the [CrF6]2, anions and dissolved AF (A = Li,Cs), single crystals of ACrF5 (A = K,Cs), A2CrF6·2HF (A = Na, K), A2CrF6·4HF (A = Rb, Cs), Li2CrF6, and K3Cr2F11·2HF were grown, and their crystal structures determined. The main structural feature of the ACrF5 compounds is the infinite zig-zag [CrF5]nn, chain of distorted [CrF6] octahedra joined by cis vertices. The crystal structures of A2CrF6·2HF (A = Na, K) and A2CrF6·4HF (A = Rb, Cs) consist of distorted [CrF6]2, octahedra involved in moderate to strong hydrogen bonding with HF molecules, while two A+ cations compensate the negative charge of each octahedron. In Na2CrF6·2HF, two neighboring HF molecules are involved in moderate to strong hydrogen bonding with each other. (HF)2 dimers with a parallelogram structure are formed. The mutual interactions in the crystal structure of K2CrF6·2HF differ from those found in Na2CrF6·2HF. In the former, each HF molecule interacts with the [CrF6]2, anion and three K+ cations. A2CrF6·4HF compounds of Rb and Cs are isostructural. Their structures consist of A+ cations and [CrF6]2, anions involved in hydrogen bonding with two sets of HF molecules in the trans position. The crystal structure of K3Cr2F11·2HF reveals a rare case of the [M2F11]3, anion. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]


A New Spherical Metallacryptate Compound [Na{Cu6(Thr)8(H2O)2(ClO4)4}]·ClO4·5,H2O: Magnetic Properties and DFT Calculations

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 13 2005
Sheng-Chang Xiang
Abstract The hexanuclear copper(II) complex with threoninato acid has been synthesized. Its structure can be described as an octahedron cage with D2h symmetry in which six copper ions are bound by eight threoninato acids with a [3.11223130] coordination mode and one Na+ cation being captured within the center of the cage. In contrast with other hexanuclear copper compounds containing amino acids, the title compound has a prolate Cu6 octahedron with the longest axial distance and a rectangle equatorial plane, as well as special coordinated perchlorate ions. Compared with classic cryptate, hexanuclear copper(II) compounds with amino acids can be regarded as a new topologic type of spherical macrotricyclic metallacryptates [2,2,2,2] whose cages have a high selectivity for sodium ions. The analysis of magnetic susceptibility data shows that the threoninato compound has a ground state with spin S = 3. The computing coupling constant between the equatorial Cu centers and the axial ones is 4.4 cm,1,calculated by using DFT methods for a model compound. This is close to three known experimental values of 1.39, 0.56 or 0.43 cm,1 for complexes with 4-hydroxy- L -prolinato, L -prolinato or L -threoninato acid as ligands, respectively. The dominant ferromagnetic interactions for these complexes can be essentially attributed to the orthogonality between the magnetic orbitals, dxz or dyz orbitals for the equatorial CuII centers and d orbitals for the axial ones. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]


Shape-Controlled Synthesis of Pd Nanocrystals in Aqueous Solutions

ADVANCED FUNCTIONAL MATERIALS, Issue 2 2009
Byungkwon Lim
Abstract This article provides an overview of recent developments regarding synthesis of Pd nanocrystals with well-controlled shapes in aqueous solutions. In a solution-phase synthesis, the final shape taken by a nanocrystal is determined by the twin structures of seeds and the growth rates of different crystallographic facets. Here, the maneuvering of these factors in an aqueous system to achieve shape control for Pd nanocrystals is discussed. L -ascorbic acid, citric acid, and poly(vinyl pyrrolidone) are tested for manipulating the reduction kinetics, with citric acid and Br, ions used as capping agents to selectively promote the formation of {111} and {100} facets, respectively. The distribution of single-crystal versus multiple-twinned seeds can be further manipulated by employing or blocking oxidative etching. The shapes obtained for the Pd nanocrystals include truncated octahedron, icosahedron, octahedron, decahedron, hexagonal and triangular plates, rectangular bar, and cube. The ability to control the shape of Pd nanocrystals provides a great opportunity to systematically investigate their catalytic, electrical, and plasmonic properties. [source]


Facile Synthesis and Shape Evolution of Single-Crystal Cuprous Oxide

ADVANCED MATERIALS, Issue 20 2009
Xudong Liang
Various morphologies of single-crystal Cu2O such as cube, octahedron, {110} truncated octahedron, and microrhombic dodecahedron with {110} surfaces are prepared in high yield through a facile solution-based one-step reduction method in an alkaline H2O/ethanol/oleic acid system in the presence of d-(+)-glucose. The formation mechanism of these microcrystals has been clarified as the synergic effect of oriented attachment and ripening mechanism. [source]


Raman scattering investigations of the low-temperature phase transition of NaNbO3

JOURNAL OF RAMAN SPECTROSCOPY, Issue 12 2006
S. J. Lin
Abstract In this work, we report the low-temperature phase transition of sodium niobate (NaNbO3) ceramics at the antiferroelectric,ferroelectric phase studied by the Raman scattering technique over the temperature range 10,300 K. The spectral shape indicates that the phase transition occurs at 160 K on the cooling cycle and at 260 K on heating. The recorded spectra are discussed following the separation of modes into the internal stretching modes of the NbO6 octahedron and Na+ cation translational modes. The 62 and 75 cm,1 peaks are proposed as the translational modes of Na+ cation against the NbO6 octahedron. Copyright © 2006 John Wiley & Sons, Ltd. [source]


Pressure- and temperature-dependent Raman studies of KNbW2O9 hexagonal tungsten bronze

JOURNAL OF RAMAN SPECTROSCOPY, Issue 3 2003
czka
Abstract Pressure- and temperature-dependent Raman studies were performed on KNbW2O9 ferroelectric hexagonal tungsten bronze single crystal. The results suggest that some structural changes, possibly phase transitions, connected with tilting of the WO6 octahedra occur in this material at around 290,350 K and 1.0,1.4 GPa. No changes in the spectra could be observed at the 523 and 543,553 K phase transitions. The data indicate that these phase transitions lead to only weak changes in the vibrational properties of KNbW2O9, which were not observed owing to the large bandwidth of the Raman bands. The Raman studies suggest, therefore, that the structure of the unknown phase, existing below 553 K, differs slightly form the orthorhombic structure present above 553 K. The pressure-dependent study revealed that the 52 cm,1 band, assigned to external vibrations of the WO6 octahedron, consists at ambient temperature and pressure of two overlapping bands. These bands become well separated at high pressure. Copyright © 2003 John Wiley & Sons, Ltd. [source]


Neutron Powder Diffraction Study of a Phase Transition in La0.68(Ti0.95Al0.05)O3

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2006
Roushown Ali
Crystal structures and structural changes of the compound La0.68(Ti0.95Al0.05)O3 have been studied using neutron powder diffraction data and the Rietveld method in the temperature range from 25° to 592°C. The Rietveld profile-fitting analyses of the neutron data and the synchrotron diffraction profile revealed that the crystal symmetry of the low-temperature phase of La0.68(Ti0.95Al0.05)O3 is orthorhombic Cmmm (2ap× 2ap× 2ap; p: pseudo-cubic perovskite). The unit-cell and structural parameters were successfully refined with the orthorhombic Cmmm for the intensity data measured at 25°, 182°, and 286°C, and with the tetragonal P4/mmm (ap×ap× 2ap) for intensity data obtained at 388° and 592°C. The P4/mmm -to- Cmmm phase transition was found to be induced by tilting of the (TiAl)O6 octahedron. The tilt angle decreased with increasing temperature, reaching 0° at the Cmmm,P4/mmm transition temperature. [source]


Oxygen Position, Octahedral Distortion, and Bond-Valence Parameter from Bond Lengths in Ti1,xSnxO2 (0 ,x, 1)

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2000
Toshiya Hirata
Based on the virtual crystal approximation (or Vegard's law), the bond lengths of Ti1,xSnxO2 were deduced from those of TiO2 and SnO2, to allow the oxygen position and octahedral distortion to be determined as a function of x. The oxygen positional parameter (u) increased linearly when the Sn4+ cation (which has a larger ionic radius) was substituted for the Ti4+ cation, whereas the octahedral distortion exhibited a nonlinear decay with increasing x in Ti1,xSnxO2. At the same time, the bond-valence parameter, which relates bond valence to bond length, so that the central atom in the octahedron can retain a constant valence of +4.0, exhibited a correlation with u for Ti1,xSnxO2. The present results indicate that the different phonon/physical properties of TiO2 and SnO2 and/or their dependence on x in Ti1,xSnxO2 can be associated with different octahedral distortions. [source]


Alternating Copolymerization of Carbon Dioxide and Propylene Oxide Catalyzed by Cobalt Schiff Base Complex

MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 15 2009
Yongsheng Niu
Abstract Cobalt 2,4-dinitrophenolate (complex 1) based upon a N,N,O,O -tetradentate Schiff base ligand framework was prepared. X-ray diffraction analysis confirmed that complex 1 was triclinic species with a six-coordinated central cobalt octahedron in the solid. Asymmetric alternating copolymerization of carbon dioxide (CO2) with racemic propylene oxide (rac -PO) proceeded effectively by complex 1 in conjunction with (4-dimethylamino)pyridine (DMAP), yielding a perfectly alternating and bimodal molecular weight distribution PO/CO2 poly(propylene carbonate) (PPC) with a small amount of cyclic carbonate byproducts. [source]


Hydrogen bonds and local symmetry in the crystal structure of gibbsite

MAGNETIC RESONANCE IN CHEMISTRY, Issue 11 2010
Anastasia Vyalikh
Abstract First-principles quantum mechanical calculations of NMR chemical shifts and quadrupolar parameters have been carried out to assign the 27Al MAS NMR resonances in gibbsite. The 27Al NMR spectrum shows two signals for octahedral aluminum revealing two aluminum sites coordinated by six hydroxyl groups each, although the crystallographic positions of the two Al sites show little difference. The presence of two distinguished 27Al NMR resonances characterized by rather similar chemical shifts but quadrupolar coupling constants differing by roughly a factor of two is explained by different character of the hydrogen bonds, in which the hydroxyls forming the corresponding octahedron around each aluminum site, are involved. The Al-I site characterized by a CQ = 4.6 MHz is surrounded by OHgroups participating in four intralayer and two interlayer hydrogen bonds, while the Al-II site with the smaller quadrupolar constant (2.2 MHz) is coordinated by hydroxides, of which two point toward the intralayer cavities and four OH-bonds are aligned toward the interlayer gallery. In high-resolution solid-state 1H CRAMPS (combination of rotation and multiple-pulse spectroscopy) four signals with an intensity ratio of 1:2:2:1 are resolved which allow to distinguish six nonequivalent hydrogen sites reported in the gibbsite crystal structure and to ascribe them to two types of structural OH groups associated with intralayer and interlayer hydrogen bonds. This study can be applied to characterize the gibbsite-like layer,intergallery interactions associated with hydrogen bonding in the more complex systems, such as synthetic aluminum layered double hydroxides. Copyright © 2010 John Wiley & Sons, Ltd. [source]


Theoretical study of electron paramagnetic resonance spectra and local structure of trigonal [Cr(H2O)6]3+ complex in GASH:Cr3+ and AlCl3,·,6H2O:Cr3+ systems at different temperatures

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2010
Yan-Fang Li
Abstract The electron paramagnetic resonance (EPR) parameters and local structure of the octahedral [Cr(H2O)6]3+ complex for Cr3+ doped in GASH and AlCl3,·,6H2O crystals with trigonal symmetry have been studied on the basis of the 120,×,120 complete energy matrices, respectively. By simulating the EPR and optical spectra, the local structure distortion parameters are determined in the temperature range of 4.2,297,K. The calculated results show that the local structure of [Cr(H2O)6]3+ octahedron exhibits an elongated distortion and the value of distortion depends on the temperature. Furthermore, the interrelation between the g -factors and orbit-reduction factor k has been studied, and the influence of the local structure on the EPR parameters has also been discussed. [source]


Theoretical studies of the spin Hamiltonian parameters and the local structure for the orthorhombic V4+ in CaYAlO4

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2008
Chuan-Ji Fu
Abstract The spin Hamiltonian parameters (g -factors and the hyperfine structure constants) and the local structure for the orthorhombic V4+ in CaYAlO4 are theoretically studied from the perturbation formulas of these parameters for a 3d1 ion in orthorhombically distorted octahedra. Based on the present studies, this center is explicitly assigned to V4+ occupying the Al3+ site, associated with one next-nearest-neighbor Mg2+ replacing the host Al3+ (i.e. MgAl) in the ab -plane due to the charge compensation. Since the effective charge of the compensator is negative (,e), the ligand O2, intervening in the impurity V4+ and the MgAl would undergo a small displacement (,0.08 Å) towards the central ion because of the electrostatic repulsion. The local structure of this center can be described as elongation of the oxygen octahedron along the c- axis and the slight orthorhombic distortion in the perpendicular direction. The theoretical spin Hamiltonian parameters and the optical transitions based on the above local structure show reasonable agreement with the experimental data. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Structural analysis of lithium lanthanum titanate with perovskite structure

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2009
Koji Ohara
Abstract Neutron and high-energy X-ray diffraction analysis of polycrystalline La4/3-xLi3xTi2O6 have been performed to clarify the extent of disorder of the distribution of La and Li ions and to understand the relation of these distributions to ionic conduction. The distributions of the La and Li ions in a 10 × 10 × 20 cubic box (i.e., 10 × 10 × 10 unit cell) super-structure, in which Ti and O atoms are fixed onto their regular sites, were obtained by the reverse Monte Carlo (RMC) structural modelling of both diffraction data sets. When the occupancy of La ions in the planes perpendicular to the c-axis is analysed, one can find a La-rich and La-poor layers alternating, which is consistent with the results of earlier Rietveld analysis (Stramare et al., Chem. Mater. 15, 3974 (2003) [1]). Of particular interest, the Li ions are found mainly on the interstitial sites between the O-3 triangle plane of the TiO6 octahedron and a La ion, which is different from the earlier work (Yashima et al., J. Am. Chem. Soc. 127, 3491 (2005) [2]). (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


On the real crystal octahedra

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 6 2002
Yury L. Voytekhovsky
A real crystal octahedron is defined as any polyhedron bounded, at least, by some of four pairs of parallel planes being in a standard crystallographic orientation with arbitrary distances between them. All the combinatorially non-equivalent shapes (30 in total) are found and characterized by 2-subordination symbols, automorphism group orders and symmetry point groups. The results are discussed with respect to the diamond crystal morphology. [source]


Diffraction line profiles from polydisperse crystalline systems

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 5 2001
Paolo Scardi
Diffraction patterns for polydisperse systems of crystalline grains of cubic materials were calculated considering some common grain shapes: sphere, cube, tetrahedron and octahedron. Analytical expressions for the Fourier transforms and corresponding column-length distributions were calculated for the various crystal shapes considering two representative examples of size-distribution functions: lognormal and Poisson. Results are illustrated by means of pattern simulations for a f.c.c. material. Line-broadening anisotropy owing to the different crystal shapes is discussed. The proposed approach is quite general and can be used for any given crystallite shape and different distribution functions; moreover, the Fourier transform formalism allows the introduction in the line-profile expression of other contributions to line broadening in a relatively easy and straightforward way. [source]


Re-investigation of the structure and crystal chemistry of the Bi2O3,W2O6 `type (Ib)' solid solution using single-crystal neutron and synchrotron X-ray diffraction

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2010
Neeraj Sharma
Single crystals of composition Bi35.66W4.34O66.51 (or Bi8.2WO15.3, bismuth tungsten oxide), within the type (Ib) solid-solution region of the Bi2O3,WO3 system, were synthesized using the floating-zone furnace method. Synchrotron X-ray and neutron single-crystal diffraction data were used to confirm the previously tentative assignment of the room-temperature space group as I41. Fourier analysis of the combined X-ray and neutron datasets was used to elucidate and refine fully the cation and anion arrays for the first time. The mixed cation site M1 is shown to be coordinated by eight O atoms in an irregular cube when M = Bi, and by six O atoms in an octahedron when M = W. The resulting disorder in the average structure around M1 is discussed in the context of experimentally observed oxide-ion conductivity. [source]


Orientational disorder and phase transitions in crystals of dioxofluoromolybdate, (NH4)2MoO2F4

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2010
Anatoly A. Udovenko
Dioxotetrafluoromolybdate, (NH4)2MoO2F4, was synthesized in a single-crystal form and its structures [(I) at 297,K and (II) at 223,K] were determined by X-ray diffraction. Two independent states of a cis -MoO2F4 octahedron are characteristic of static and dynamic disorder in structure (I). The dynamically disordered Mo atom is displaced from the symmetry axis producing four possible orientations of an anion that allow O and F atoms to be identified in separate orientations owing to the inherent differences between the Mo,O and Mo,F bonding. After the phase transition at lower temperature, (I) transforms into the statically disordered structure (II) with three possible orientations of the cis -MoO2F4 octahedron. In this case, it also seemed possible to distinguish between O and F atoms on a local scale. H atoms of two independent NH4 groups in (II) which form bifurcated N,H...F(O) hydrogen bonds were localized. [source]


Octahedral tilting in cation-ordered Jahn,Teller distorted perovskites , a group-theoretical analysis

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2010
Christopher J. Howard
Computer-based group-theoretical methods are used to enumerate structures arising in A2BB,X6 perovskites, with either rock-salt or checkerboard ordering of the B and B, cations, under the additional assumption that one of these two cations is Jahn,Teller active and thereby induces a distortion of the BX6 (or B,X6) octahedron. The requirement to match the pattern of Jahn,Teller distortions to the cation ordering implies that the corresponding irreducible representations should be associated with the same point in the Brillouin zone. Effects of BX6 (and B,X6) octahedral tilting are included in the usual way. Finally, an analysis is presented of more complex models of ordering and distortion as might lead to the doubling of the long axis of the common Pnma perovskite, observed in systems such as Pr1,,,xCaxMnO3 (x, 0.5). The structural hierarchies derived in this work should prove useful in interpreting experimental results. [source]


Disorder in crystals of dioxofluorotungstates, (NH4)2WO2F4 and Rb2WO2F4

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2008
Anatoly A. Udovenko
Dioxotetrafluorotungstates (NH4)2WO2F4 [(I) at 297,K and (II) at 133,K] and Rb2WO2F4 (III) were synthesized in a single-crystal form and their structures were determined by X-ray diffraction. Two independent states of the cis -WO2F4 octahedron are characteristic of static and dynamic disorder in structure (I). Dynamically disordered W2 is displaced from the symmetry axis producing four possible orientations of anion that permits O and F atoms to be identified in separate orientations owing to the inherent differences between W,O and W,F bonding. After the phase transition at lower temperature (201,K), (I) transforms into the twin structure (II) with complete O/F ordering. Structure (III) is characterized by full O/F static disorder without any phase transitions at lower temperature. [source]


Orientational disorder and phase transitions in crystals of (NH4)2NbOF5

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2008
Anatoly A. Udovenko
Ammonium oxopentafluoroniobate, (NH4)2NbOF5, was synthesized in a single-crystal form and the structures of its different phases were determined by X-ray diffraction at three temperatures: phase (I) at 297,K, phase (II) at 233,K and phase (III) at 198,K. The distorted [NbOF5]2, octahedra are of similar geometry in all three structures, with the central atom shifted towards the O atom. The structure of (I) is disordered, with three spatial orientations of the [NbOF5]2, octahedron related by a jump rotation around the pseudo-threefold local axis such that the disorder observed is of a dynamic nature. As the temperature decreases, the compound undergoes two phase transitions. The first is accompanied by full anionic ordering and partial ordering of the ammonium groups (phase II). The structure of (III) is completely ordered. The F and O atoms in the structures investigated were identified via the Nb,X (X = O and F) distances. The crystals of all three phases are twinned. [source]


Quantitative description of the tilt of distorted octahedra in ABX3 structures

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2007
Rafael Tamazyan
A description of the tilt of octahedra in ABX3 perovskite-related structures is proposed that can be used to extract the unique values for the tilt parameters ,, , and , of ABX3 structures with regular and distorted octahedra up to the point symmetry , from atomic coordinates and lattice parameters. The geometry of the BX6 octahedron is described by three B,X bond lengths (r1, r2, r3) and three X,B,X bond angles (,12, ,13 and ,23) or alternatively by a local strain tensor together with an average B,X bond length. Connections between the proposed method and Glazer's tilt system are discussed. The method is used to analyze structural transformations of I2/c, Pbnm and Immm structures. The proposed description allows the analysis of group,subgroup relations for the ABX3 structures with distorted octahedra, in terms of octahedral deformations and tilting. The method might also be of interest in the study of the phase transitions in the family of ABX3 structures. [source]


Single-crystal structure refinement of NaTiSi2O6 clinopyroxene at low temperatures (298,<,T,<,100,K)

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2003
Günther J. Redhammer
The alkali-metal clinopyroxene NaTi3+Si2O6, one of the rare compounds with trivalent titanium, was synthesized at high temperature/high pressure and subsequently investigated by single-crystal X-ray diffraction methods between 298 and 100,K. One main difference between the high- and the low-temperature form is the sudden appearance of two different Ti3+,Ti3+ interatomic distances within the infinite chain of the TiO6 octahedra just below 197 K. This change can be seen as direct evidence for the formation of Ti,Ti singlet pairs in the low-temperature phase. Mean Ti,O bond lengths smoothly decrease with decreasing temperature and the phase transition is associated with a slight jump in the Ti,O bond length. The break in symmetry, however, causes distinct variations, especially with respect to the two Ti,Oapex bond lengths, but also with respect to the four Ti,O bonds in the equatorial plane of the octahedron. The TiO6 octahedron appears to be stretched in the chain direction with a slightly larger elongation in the P low-temperature phase compared with the C2/c high-temperature phase. Polyhedral distortion parameters such as bond-length distortion and octahedral angle variance suggest the TiO6 octahedron in P to be closer to the geometry of an ideal octahedron than in C2/c. Mean Na,O bond lengths decrease with decreasing temperature and the variations in individual Na,O bond lengths are the result of variations in the geometry of the octahedral site. The tetrahedral site acts as a rigid unit, which does not show pronounced changes upon cooling and through the phase transitions. There are neither large changes in bond lengths and angles nor in polyhedral distortion parameters, for the tetrahedral site, when they are plotted. In contrast with the C2/c,P21/c phase transition, found especially in LiMSi2O6 clinopyroxenes, no very large variations are found for the tetrahedral bridging angle. Thus, it is concluded that the main factor inducing the phase transition and controlling the structural variations is the M1 octahedral site. [source]


The incommensurate structure of K3In(PO4)2

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2003
Alla Arakcheeva
The incommensurately modulated structure of K3In(PO4)2 has been solved and refined. The origin of the modulation relates to the ordering of K cations within the hexagonal close packing of the PO4 anions. The driving forces for the modulation of the other cations are In,P and K,P interactions. The modulation of O atoms of rigid PO4 units follows the cations in order to stabilize the InO6 octahedron. It is shown that the previously published three-dimensional structure refined from powder diffraction data obtained at room temperature is an average structure. Therefore the incommensurately modulated phase of K3In(PO4)2 is the only one that has been unequivocally identified at room temperature. The origin of the modulation is discussed in comparison with the structures of Na3InP2, ,- and ,-Na3In(PO4)2, Na3Fe(PO4)2 and Rb3In(PO4)2. [source]


A novel In3O16 fragment in Cs3In3(PO4)4

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 7 2010
Igor V. Zatovsky
The double phosphate Cs3In3(PO4)4, prepared by a flux technique, features a fragment of composition In3O16 formed by three corner-sharing InO6 polyhedra. The central In atom resides on a twofold rotation axis, while the other two In atoms are on general positions. The O atoms in this fragment also belong to PO4 tetrahedra, which link the structure into an overall three-dimensional anionic In,O,P network that is penetrated by tunnels running along c. Two independent Cs+ cations reside inside the tunnels, one of which sits on a centre of inversion. In general, the organization of the framework is similar to that of K3In3(PO4)4, which also contains an In3O16 fragment. However, in the latter case the unit consists of one InO7 polyhedron and one InO6 polyhedron sharing an edge, with a third InO6 octahedron connected via a shared corner. Calculations of the Voronoi,Dirichlet polyhedra of the alkali metals give coordination schemes for Cs of [9+2] and [8+4] ( symmetry), and for K of [8+1], [7+2] and [7+2]. This structural analysis shows that the coordination requirements of the alkali metals residing inside the tunnels cause the difference in the In3O16 geometry. [source]


K3TaF8 from laboratory X-ray powder data

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 2 2010
ubomír 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]


Uncommon isonicotinamide supramolecular synthons in copper(II) complexes directed by nitrate and perchlorate anions

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2009
Marijana ?akovi
The title compounds, trans -diaquabis(nitrato-,O)bis(pyridine-4-carboxamide-,N1)copper(II), [Cu(NO3)2(C6H6N2O)2(H2O)2], (I), and trans -diaquatetrakis(pyridine-4-carboxamide-,N1)copper(II) bis(perchlorate), [Cu(C6H6N2O)4(H2O)2](ClO4)2, (II), are composed of mononuclear coordination entities involving CuII ions and isonicotinamide. In (I), the centrosymmetric tetragonally distorted octahedral copper(II) environment contains trans -related isonicotinamide and water molecules in the equatorial plane and two nitrate ions occupying the axial sites. In (II), the equatorial plane of the C2 -symmetric distorted octahedron is built up of four isonicotinamide ligands, while water molecules occupy the axial positions. The complex molecules of (I) and (II) are linked into three-dimensional supramolecular frameworks by O,H...O and N,H...O hydrogen bonds. The nitrate and perchlorate ions are building blocks that disturb the robust R22(8) amide supramolecular motif commonly found in crystal structures of copper,isonicotinamide complexes. [source]


Diaquadichloridobis(1H -imidazole)manganese(II) at 100 K

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 6 2009
Barbara Hachu
The mononuclear title complex, [MnCl2(C3H4N2)2(H2O)2], is located on a crystallographic inversion center. The MnII ion is coordinated by two imidazole ligands [Mn,N = 2.2080,(9),Å], two Cl atoms [Mn,Cl = 2.5747,(3),Å] and two water molecules [Mn,O = 2.2064,(8),Å]. These six monodentate ligands define an octahedron with almost ideal angles: the adjacent N,Mn,O, N,Mn,Cl and O,Mn,Cl angles are 90.56,(3), 92.04,(2) and 90.21,(2)°, respectively. Hydrogen bonds between the coordinated water molecules and Cl atoms form a two-dimensional network parallel to (100) involving R42(8) rings. The two-dimensional networks link into a three-dimensional framework through weaker N,H...Cl interactions. Thermogravimetric analysis results are in accordance with the water-coordinated character of the substance and its dehydration in two successive steps. [source]