Hk0 Diffraction Research Articles

Simultaneous intercalation of lithium, potassium and strontium into graphite in molten salts medium

The LiCl–KCl molten salts method has been used in order to intercalate strontium into graphite. Combination of X-Ray Diffraction (XRD) experiments and ion beam analyses attests the obtaining of a monophasic sample. Its chemical composition corresponds to the Li0.2K0.6Sr0.4C6 formula, revealing the synthesis of a quaternary Graphite Intercalation Compound (GIC), in agreement with the high repeat distance of 640 pm determined from its 00l XRD and with the 1D electronic density profile. The very high-quality data obtained by hk0 diffraction and rotating crystal method lead to a 2D organization of the intercalated sheet described by a large hexagonal unit cell containing 78 carbon atoms. Such quaternary phase including simultaneously alkali and alkaline-earth metals is unique and can only be prepared from molten salts method, opening new perspectives concerning graphite-based two-dimensional materials.

The crystal structure of seeligerite, Pb3IO4Cl3, a rare Pb-I-oxychloride from the San Rafael mine, Sierra Gorda, Chile

AbstractThe crystal structure of seeligerite, Pb3IO4Cl3, from the San Rafael mine, Sierra Gorda, Chile, was solved in the space group Cmm2, and refined to R = 3.07%. The unit-cell parameters are: a = 7.971(2), b = 7.976(2), c = 27.341(5) Å, V = 1738.3(6) Å3 and Z = 8. The crystal structure consists of a stacking sequence along [001] of square-net layers of O atoms and square-net layers of Cl atoms with Pb+ and I+ cations located in the voids of the packing. As is typical of cations with a stereoactive lone-pair of electrons, Pb2+ and I5+ adopt strongly-asymmetrical configurations. Pb2+ cations occur in a variety of coordination polyhedra, ranging from anticubes and monocapped anticubes to pyramidal ‘one-sided’ coordinations. I5+ is coordinated by a square of four oxygen atoms: I1 and I3 exhibit a ‘one-sided’ coordination, whereas I2 has square-planar coordination.The TEM investigation has revealed additional superlattice reflections (which were not registered by X-ray diffraction (XRD)) in the hk0 diffraction pattern of seeligerite based upon a 0.158 Å-1 square net, which can be interpreted as arising from a 20-cation super-sheet motif (12.6 Å x 12.6 Å), likely related to a further level of Pb-I order superimposed upon the 8-site motif identified by XRD.

Single crystal morphology and structural data of a series of polyesters derived from 1,8-octanediol

The morphology of solution grown single crystals of a series of polyesters derived from 1,8-octanediol and either an even or an odd dicarboxylic acid has been investigated. Different crystallization conditions (solvent and temperature) were tested for these polyesters whose dicarboxylate unit varied from pimelate to sebacate. Crystals showed a lozenge morphology with a degree of truncation which for a given solvent changed with the crystallization temperature. Curvature of growth faces, striations on the lamellar surface and a bilayered morphology were features that were observed in some cases. Polyethylene decoration highlighted a crystal sectorization and suggested a molecular folding parallel to the crystal growth faces. The studied polyesters always showed a single crystalline structure, according to electron and X-ray diffraction data. A practically all trans molecular conformation could always be deduced. Unit cell geometry changed from orthorhombic to monoclinic and the molecular content was variable in the studied series (two or four molecular segments). Lamellae gave rise to well-defined electron diffraction patterns that allowed the setting angle of molecular segments to be determined. A good agreement between simulated and experimental hk0 diffraction patterns was always found. Packing along the chain axis direction was completely defined for the sebacate derivative.

Brinrobertsite: a new R1 interstratified pyrophyllite/smectite-like clay mineral: characterization and geological origin

AbstractBrinrobertsite, an ordered, mixed-layered, dioctahedral pyrophyllite-smectite (P/S), occurs in a metabentonite in the Ordovician Nant Ffrancon Formation near Bangor, N Wales. It comprises ~30% of the metabentonite, in association with quartz (~50%) and chlorite (clinochlore; 20%) which replaced glass shards and fine-grained glass matrix. Transmission electron microscopy (TEM) images show sequences of dominant ~24 Å 001 lattice fringes inferred to correspond to 2:1 layers with alternate pyrophyllite-like (low-charge) and smectite-like (higher-charge) interlayers (i.e. R1 ordering). The hk0 diffraction patterns are mostly hexanets with some spotty circles, implying that layers are largely coherently related, but with some turbostratic stacking. Collective data show that d100 = 5.2, b = 9.1, and d001 = 24–25 Å, assuming monoclinic or pseudomonoclinic symmetry. The composition, as determined by energy dispersive spectral analysis, is (Na0.22K0.07Ca0.06)(Al3.81Mg0.08Fe0.08)(Si7.84 Al0.16)O20(OH)4·3.54H2O, as consistent with the sum of the compositions of pyrophyllite-like and smectite-like units. Water content was determined by DTA/TGA analysis. The powder diffraction patterns have a principal peak with d001 = 24.4 Å. Patterns of air-dried and glycol-saturated brinrobertsite, including Na- and Ca-saturated and untreated samples, were modelled satisfactorily as R1-ordered P/S by the program NEWMOD-for-Windows. The unique composition of brinrobertsite relative to R1 IS, which is ubiquitous in metabentonites, was caused by leaching of alkalis and alkaline-earth elements by hydrothermal fluids associated perhaps with a nearby intrusion, as demonstrated by bulk-chemical analyses of the metabentonite. The crystal structure is modelled as having Al/Si distributions symmetrical by reflection across interlayers. This causes all 2:1 layers to be equivalent in having one tetrahedral sheet with little or no Al, and the other with significant Al substitution, giving rise to alternate high- and low-charge interlayers. Geological evidence suggests that brinrobertsite is a back-reacted product of hydrothermal alteration in the sequence: glass → pyrophyllite → brinrobertsite.

Phase coexistence in the deuteron glass Rb0.9(ND4)0.1D2AsO4 proven by neutron diffraction

Neutron diffraction technique has been used to study the transitions from the paraelectric (PE) phase to the ferroelectric (FE) one in the deuteron glass crystal Rb0.9(ND4)0.1D2AsO4 (DRADA-10). The crystal shows PE hk0 diffraction spots splitting into spot patterns from domains, 4 spots from 4 FE domain types. A more complex phase behavior that the one corresponding to a typical PE-FE phase transition has been found. Coexistence of FE and PE features together with incommensurate correlations in the low temperature phase has been discovered. The intensity of the spots associated with the FE domains indicates the fraction of the crystal in the low-temperature phase. For DRADA-10 there is a fairly wide temperature range (7-10 K) over which these spots coexisted with spots remaining from the paraelectric phase

XAS and XRD Studies on Graphite Intercalation Compounds of Re2O7

Abstract Re2O7 was used as starting material for the preparation of pure low-stage graphite intercalation compounds (GICs). X-ray absorption measurements were carried out at the Re LIII and L1 absorption edges. The EXAFS analysis resulted in a coordination number of four and a Re-O bond length of 173.5 pm for the GICs. In the Re L1 XANES the analysis of the strong pre peak confirmed a slight distorted tetrahedral symmetry. Further XANES features indicated a variation of long-range order for the intercalate. Therefore the XAS measurements determined the tetrahedral perrhenate ion as the intercalated species. The majority of synthesized compounds showed sharp hk0 diffraction patterns. Only some compounds showed first diffuse rings and after several days sharp reflections of an ordered “in-plane” lattice. The analysis of the hk0 diffraction pattern yielded a lattice with the constants a = 652 pm, b = 764 pm, γ = 100°, δ a = 0.6° and δ b = 19.4°.

Lamellar and whisker single crystals of poly(2,6‐oxynaphthoate)

AbstractElectron diffraction from single crystal lamellae and whiskers of poly(2,6‐oxynaphthoate) reveals the presence of at least 3 unit cells. The equatorial reflections in the patterns from the whiskers correspond to the dominant (phase I) hk0 diffraction pattern from the lamellae; phase I is monoclinic with 2 chemical repeats per physical repeat. The intensity distributions in the hk0 patterns of phase I and II resemble those of the same phases in poly(p‐oxybenzoate). The hk0 reflections of phase III suggest a common internapthalene unit spacing, but variable lateral (and possibly axial) shifts; apparently related orthorhombic and monoclinic patterns, with variable γ*, are observed. At elevated temperature, above the crystalliquid crystal transition (ca. 330°C), quadrant reflections are retained; the change in the hk0 pattern from any given crystal is gradual, extending over some 40°C. Above the liquid crystal‐liquid crystal transition (ca. 460°C) the pattern can be interpreted in terms of nematic or possibly smectic A packing. © 1992 John Wiley & Sons, Inc.

Some general aspects of the hk0 diffraction patterns of graphite intercalation compounds

A series of monochromatic Laue photographs (MLPs) showing the hk0 diffraction patterns of some graphite intercalation compounds (GICs) is presented. Due to the sixfold symmetry of the graphene layers and the domain structure of GICs they represent an n-fold superposition (n=1, 2, 3 or 6) of the patterns which is produced by a domain with a single rotational orientation. This “domain twinning” often produces aesthetically attractive patterns. The number n depends on the shape of the unit cell of the intercalate and the rotational orientation between the unit cell axes of the in-plane lattices of the guest and the host.

Structural investigations of the graphite intercalation compounds of the dichlorides of the II B-elements (Zn, Cd, Hg)

An account of our structural investigations of the Graphite Intercalation Compounds of ZnCl 2, CdCl 2 and HgCl 2 is given. In the case of zinc chloride-graphite X-ray absorption spectra show that the Zn atoms in the intercalated layers are coordinated tetrahedrally. For cadmium chloride-graphite hk0 diffraction patterns clearly show that the sandwich structure of a layer as it is observed in pure CdCl 2 is conserved. For mercuric chloride-graphite, monochromatic Laue photographs show an oblique in-plane unit cell of the intercalate.

Synthesis and characterization of the graphite intercalation compounds of bismuth trichloride

The intercalation of bismuth trichloride into graphite via the vapor phase has been investigated as a function of the following parameters: temperature of the graphite, vapor pressure of the metal chloride, and pressure of chlorine. Pure-stage compounds with stage index 2, 3, and 4 are easily obtained at high pressures of chlorine (600 mbar). It was not possible to produce a stage 1 compound. With decreasing pressure of chlorine the intercalate content of the compounds becomes smaller until, at the threshold pressure of chlorine (20 mbar), no intercalation is observed anymore. All compounds were characterized by weight uptake and X-ray diffraction. In the case of the pure-stage compounds, the bismuth: chlorine ratio was determined by X-ray fluorescence as 1:3.2 ± 0.1. All compounds that were prepared at a high pressure of chlorine exhibit the same hk0 diffraction pattern. The reflections can be indexed by a rectangular in-plane lattice. The lattice constants indicate a structural relationship between the pure and the intercalated bismuth trichloride. Compounds prepared at low pressures of chlorine exhibit nonreproducible hk0 diffraction patterns.

Layer-disordered wurtzite (ZnS-2H): diffuse X-ray scattering recorded by c-axis precession photography

A novel type of X-ray diffraction pattern results when a layer-disordered crystal is photographed by X-ray precession using the stacking direction as the precession axis and no layer-line screen. The 10l, 20l, 21l and symmetry-equivalent rows of diffuse scattering of layer-disordered wurtzite (ZnS-2H) are represented by compound loops superimposed on the zero-level hk0 diffraction pattern. Closed loops result because the reciprocal-lattice rows associated with diffuse scattering remain in contact with the sphere of reflection during the precession cycle. Formulae for dimensional parameters of the compound loops of diffuse scattering are derived from the sphere-of-reflection construction.

Study of domains and domain walls in ferroelastic BiVO4

The domain structure of pure ferroelastic BiVO4 has been studied by TEM and electron diffraction. Parallel domain walls w' have been imaged in dark field conditions as alternating black and white lines. Angles between domain walls, between domain walls and the [100] axis and the rotation angle between the (hk0) reciprocal planes of two adjacent ferroelastic domains were measured and are in good agreement with calculated values. The multiple splitting of the hk0 diffraction spots when both w' walls are present simultaneously is discussed.

Stacking variations in cancrinite minerals

TEM reveals (0001) stacking faults and periodic superstructures in cancrinite-related minerals, afghanite and franzinite, which are interpreted as aperiodic or periodic displacements of the hexagonal [(Si,AI)60~2] ring units. Franzinite also shows a superstructure when the structure is projected onto the xy plane; satellites occur in the hk0 diffraction patterns.