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Centimeter-sized single crystals of Ba2LaFeNb4O15 were grown from a high-temperature solution by using LiBO2 flux and a sealed platinum assembly. The obtained single crystals display the same physical properties as their ceramic counterparts. A frequency-dependent dielectric permittivity maximum was found ( Tm = 100 K at 5 kHz), which indicates relaxor behavior. Magnetic susceptibility measurements revealed purely paramagnetic behavior between 10 and 350 K. X-ray diffraction measurements of Ba2LaFeNb4O15 single crystals revealed an incommensurate structure at room temperature with a bidimensional modulation characterized by vectors q1 = ( α, α, 1/2) and q2 = ( α, - α, 1/2) with α = 0.295(1). This crystal growth method offers a promising elaboration route to centimeter-sized crystals of niobate-based compounds, which may not be grown from the pure liquid phase, especially those with a tetragonal tungsten bronze (TTB) structure. [ABSTRACT FROM AUTHOR]

The H2 antagonist cimetidine forms many polymorphs, several of which have resisted structural analysis thus far. Using single-crystal X-ray measurements obtained from synchrotron radiation, the crystal structure of cimetidine form C has been solved. This layered structure crystallizes in space group C2/ c with an unusually large lattice parameter, a = 82.904 Å. The thickness of each layer L is equal to a′ = a/6 = 13.82 Å, and a = 6 a′ originates from a sixfold LLLL′ L′ L′ sequence with L and L′ differing by 0.5 b. This packing is reminiscent of polytypic stacking in metals. A (3 + 1)-dimensional superspace model is derived and used to explain and predict many polytypic modifications. This model is characterized by (i) the (3 + 1)-dimensional symmetry group X2/ c(α0γ)00, where X = 00; (ii) the lattice parameter a′ and modulation vector q = 1/ n( a′*); (iii) the atomic positions of a single molecule of cimetidine form C; (iv) the primary variable, 1/ n. The model reproduces the previously solved structure, the 6M polytype, and generates the related polytypes nM with lattice parameter anM = na′ for n = 1, 2, 3, 4 and 5. A comparison of powder X-ray diffraction patterns available for cimetidine form C with those simulated for the nM polytypes suggests that the powder samples published previously probably contain a mixture of various polytypes. [ABSTRACT FROM AUTHOR]

Hexagonal ferrites represent an extensive family of mixed-layer magnetic materials with periods up to 1500 Å along the stacking direction, probably constituting the largest unit cells in the inorganic realm. The (TS) nT subfamily includes and structures that can be derived from Y ferrite Ba2 M2Fe12O22 ( M = Zn, Fe, Co, Mg, Mn) by introducing stacking faults. A unified (3 + 1)-dimensional superspace model is proposed for all members of the (TS) nT family. The model belongs to the superspace group with , , has a unit cell of the basic structure with a = 5.88, c = 4.84 Å and modulation vector , where n is rational for periodic structures and irrational for the aperiodic ones. The model was tested on calculated data of one of the principal members of the (TS) nT family, the Y ferrite ( ). The fit obtained with the superspace model was excellent. The model allowed a reduction of refinable parameters by 19% with respect to the ordinary refinement without a significant increase of the refinement R values. [ABSTRACT FROM AUTHOR]

3,4-Diphenyl-2a,5a,6,7,8,8a,8b-heptahydro-furo[4,3,2- de]chromen-2-one (1) was prepared as part of a project aimed at the synthesis of polycyclic natural-product-like scaffolds. X-ray analysis of crystals grown from ethanol revealed an incommensurately modulated structure. Data include main and satellite reflections up to second order. The modulation vector was refined using the NADA program. The modulated character of the structure of the organic compound C22H20O3 is interpreted in terms of the intermolecular C—H⋯O hydrogen bonds and close-contact approximation. [ABSTRACT FROM AUTHOR]

Based on the structural data of phases α (hexagonal; 756–972 K), β (monoclinic; 605–751 K), γ (incommensurate, monoclinic; 295 K) and δ (lock-in, monoclinic; 110 K) of sodium carbonate, Na2CO3, we could draw a parallel between the phase transitions and the evolution of the second coordination sphere of the C atoms. The temperature-dependent structures observed in the β phase are reproduced in the incommensurate γ phase as a modulation wave, which relates to the content of the symmetrically equivalent {110} lattice planes in the α phase. By decreasing the temperature, the phase transitions are associated with a stepwise increase in the number of Na ions participating in the second coordination sphere of the C atoms. Over the full temperature range, this number increases from 3 to 7. The C—O distances and the mobility of the O atoms depends on the number of Na ions in the vicinity of the C atoms. [ABSTRACT FROM AUTHOR]

Based on the structural data of phases α (hexagonal; 756–972 K), β (monoclinic; 605–751 K), γ (incommensurate, monoclinic; 295 K) and δ (lock-in, monoclinic; 110 K) of sodium carbonate, Na2CO3, we could draw a parallel between the phase transitions and the evolution of the second coordination sphere of the C atoms. The temperature-dependent structures observed in the β phase are reproduced in the incommensurate γ phase as a modulation wave, which relates to the content of the symmetrically equivalent {110} lattice planes in the α phase. By decreasing the temperature, the phase transitions are associated with a stepwise increase in the number of Na ions participating in the second coordination sphere of the C atoms. Over the full temperature range, this number increases from 3 to 7. The C—O distances and the mobility of the O atoms depends on the number of Na ions in the vicinity of the C atoms. [ABSTRACT FROM AUTHOR]

The incommensurate palmierite-like structure of β-K5Yb(MoO4)4, potassium ytterbium tetramolydate, has been refined in the (3 + 1)-dimensional monoclinic superspace group X2/ m(0ρ0)00, with X = [0 0 0 0; ½½ 0 0; 0 0 ½½; ½½½½] and the unit-cell parameters a = 10.4054 (16), b = 6.1157 (12), c = 19.7751 (18) Å, β = 136.625 (10)°; q = 0.6354 (30) b*. The occupations of the K and Yb atomic positions are described by crenel functions. The structure model reveals a balanced interaction between the atoms of the first and second coordination spheres. It is shown that the third coordination sphere should not be neglected in studies of modulated structures. The ordering of the K and Yb atoms appears to be the driving force for the modulation of all the other atoms. [ABSTRACT FROM AUTHOR]

N-Salicylideneaniline (SA), C13H11NO, belongs to the large family of aromatic Schiff bases. It is of particular importance owing to its reversible photoreactivity. SA forms two photochromic polymorphs, both with two non-coplanar benzene rings. In addition, we have recently discovered a planar polymorph, named the β-polymorph, which will be discussed in a subsequent paper. We report here the structure of the α2-polymorph in the orthorhombic crystal system. This compound exhibits a strong intramolecular O—H…N hydrogen bond and the dihedral angle between the two rings varies with temperature. [ABSTRACT FROM AUTHOR]

In order to apply the molecular dynamics (MD) method to simulate modulated phases in organic materials, a compensating external pressure tensor is proposed to compensate for the deficiencies of the force field applied in the simulation. MD can well reproduce modulated phases that have been measured by diffraction. Mechanisms of incommensurate modulation are revealed from the simulations. Details of the structures relating to the origins and mechanisms giving rise to the formation of modulated phases are presented. [ABSTRACT FROM AUTHOR]

Diaqua (15-crown-5) copper (II) dinitrate is an organic salt presenting an one-dimensional commensurate modulation. The diffraction pattern is characterized by strong Bragg reflections and weaker superstructure reflections (satellites). The structure is described in the superspace formalism as a modulated structure in (3+1) dimensions with occupational and displacive modulation functions. [ABSTRACT FROM AUTHOR]

A molecular dynamics simulation of 4, 4′-dichlorobiphenyl sulfone was carried out in order to study the phase transition from a normal to incommensurate modulated phase observed experimentally. Agreements of simulated and experimental diffraction patterns and lattice constants indicates that the simulation is able to well reproduce the normal-incommensurate phase transition. The incommensurate modulation is found to originate from large-scale fluctuations in the displacements of the molecules. [ABSTRACT FROM AUTHOR]

Quininium (R)-mandelate, C20H25N2O2+·C8H7O3-,is an organic salt with an incommensurately modulated structure. The superspace approach is required for a precise description of the structure at room temperature. In addition to the main reflections, the diffraction pattern also exhibits higher-order satellite reflections. The large number of first- and second-order satellite reflections with relatively strong intensities indicates a significant modulation. No average structure solution could be obtained on the basis of the main reflections only and hence the structure solution was performed in two suitable superstructure approximations, including satellite intensities. Positional modulation functions and modulation functions for the anisotropic atomic displacement parameters were introduced. The modulation originates from a competition between intramolecular and intermolecular forces, which is reflected, in the superspace formalism, in the modification of hydrogen bonds along the internal space variable t. [ABSTRACT FROM AUTHOR]

In order to perform a molecular dynamics (MD) simulation of the incommensurate crystalline structure hexamethylenetetramine suberate (C 6 H 12 N 4 )(HOOC-(CH 2 ) 6 -COOH), we present in a first step the separate simulations of the crystalline structure of each of the two pure components, hexamethylenetetramine (HMT) and suberic acid. The domain decomposition parallel MD program ddgmq is used for this purpose. A second-generation consistent force field (CFF91) is employed to describe the interactions between atoms. Starting from experimental crystal structures, both pure components were heated from low to high temperatures. Our MD results show that the HMT system can be well represented by CFF91. In the case of suberic acid the layered structure of the crystal is largely preserved although deviations in the unit cell lengths from the experimental values are ∼10%. Rather than attempt a complex re-parametrisation of CFF91 we chose to impose a fixed compensating external pressure tensor to correct for the deficiencies of the chosen force field. After optimising this compensating external pressure tensor at one temperature we find that experimental lattice constants and angles can be well reproduced over a range of temperatures. [ABSTRACT FROM AUTHOR]

Using electrodeposition from a bath of molten fluorides single crystals of tetragonal β-tantalum have been obtained for the first time at normal pressure; The unit-cell parameters a = 10.211 (3), c = 5.3064 (10) Å, space group P&macr4;21m. The β-Ta structure belongs to the σ-type Frank-Kasper structures which are typical for binary intermetalliccompounds and β-U. In comparison to the σ-type, additional intercalated Ta atoms (population factor ∼0.01) have been detected between the atoms located in the channels of the structure. The shorter interatomic distances observed between the channel atoms in comparison with the atoms of the framework justify the 'self-hosting' characteristic. β-Ta exhibits common features with the complex tetragonal structures of the high-pressure phases for the elements Rb, Ba, Sr, Bi and Sb. [ABSTRACT FROM AUTHOR]

Discusses a study on the perovskite-type crystal structure of the colourless transparent crystals potassium tantalate, with a high degree of precision. Type of structure of the new cubic potassium tantalate; Differences in the properties; Ability of potassium tantalate to maintain its dielectric properties.

An La2Co1.7 crystals was investigated by single-crystal neutron and X-ray diffraction. The neutron measurement was performed with a Laue white-beam technique at 15 K and room temperature, using a large position-sensitive detector. The X-ray measurements were obtained at room temperature from a CCD detector. The average structure of La2Co1.7 is hexagonal with cell parameters a = 4.885 (1), c = 4.273 (2) Å and space group P63/mmc. The satellites are located at the vertices of small hexagons perpendicular to the c axis. The modulated crystal was indexed assuming a sixfold twinned 3 + 1 dimensional structure with q = (α, 0, γ). The structure was solved in the pseudoorthorhombic cell, with a = 8.461 (1), b = 4.885 (1), c = 4.273 (2) Å, in the superspace group C2/m(α, 0, γ). Owing to space requirements, the Co atoms cannot fit precisely into the octahedral sites of the La h.c.p. (hexagonal close packing). Instead, the Co atoms adopt a different periodicity, which is not commensurate with the periodicity of the La atoms. Two structure models have been refined in order to describe this behavior, one using the sawtooth function for the positional modulation of cobalt and the other describing the structure as a composite system. The chemical composition calculated from the composite model is La2O1.8 (1) with the estimated standard deviation arising from the variation of q for different samples. In both models lanthanum is incommensurately modulated, while the position of cobalt seems not to be affected by any relative periodic displacement. [ABSTRACT FROM AUTHOR]

The structural phases observed in LiKSO4 crystals due to thermotropic transitions have been studied for more than a century. Nowadays many different phases are referenced, but some of the results are still controversial. Structural studies by single-crystal X-ray diffraction from room temperature to 803 K are presented here. Phase II (708 < T < 943 K) is extensively discussed on the basis of ordered and disordered models, using harmonic and anharmonic atomic displacements, and considering a twinned crystal composed of three orthorhombic domains Analyses of the same phase at different temperatures determine the best structure model. [ABSTRACT FROM AUTHOR]

The structure of the title compound, [La(C5H6O4)(C5H7O4)-(H2O)]·H2O, consists of dense layers formed by chains of one-edge-sharing LaO9(H2O) polyhedra, linked together by the glutarate ligand. The three-dimensional polymeric structure, built up through connection of these layers by the hydrogen glutarate ligand, exhibits cavities accommodating a guest water mol­ecule. The lanthanum ion is tenfold coordinated by four glutarates, acting as bridging-chelating carboxyl­ate groups, by three hydrogen glutarates, three times monodentate, and by one water mol­ecule. Its coordination polyhedron is highly distorted and intermediate between a bicapped dodecahedron and a tetracapped trigonal prism. Hydro­gen bonding links the two water mol­ecules and the framework built up from this polynuclear coordination polymer. A very short hydrogen bond, D⋯ A = 2.484 (3) Å, links the proton­ated with the deprotonated acid groups in the hydrogen glutarate. [ABSTRACT FROM AUTHOR]

The organic salt hexamethylenetetramine sebacate presents a 1D modulation with strong satellite reflections, Single crystal X-ray' intensities of main reflections and satellites up to sixth order have been measured at room temperature and used to refine this incommensurate structure. Two incommensurate structure models of hexamethylenetetramine (hexamine) sebacate are proposed and compared. The first is based on displacive atomic modulation functions and the second one uses displacive modulation and occupational step functions. Both models require high-order harmonic terms to describe the strongly nonsinusoidal modulation components. The structural characteristics of hexamine sebacate are similar in the two models. The structure is a layer structure consisting of alternating sheets of hexamine and sebacic acid connected by H bonds. Each layer of sebacic acid molecules consists of close-packed aliphatic chains with their axis slightly tilted with respect to the layer normal. The chains from alternating areas, each of which is characterized by a common orientation of the chains, These characteristics have also been observed in hexamethylenetetramme suberate. [ABSTRACT FROM AUTHOR]