True Unit Cell Research Articles

Fast oxide-ion conductors in Bi2O3-V2O5 system: Bi108-xVxO162+x(x=4-9) with 3×3×3 superstructure

In this study, the possibility to stabilize O2-ion conductors in Bi2O3-V2O5 system was investigated. Six pseudo-binary Bi2O3-V2O5 mixtures [3.50 < x(V2O5) < 8.50 mol%] were thermally treated at 1000?C for 1 h. The samples were characterized by XRD, HRTEM/SAED, DTA and EIS techniques. The high-temperature reaction between ? Bi2O3 and V2O5 resulted in formation of microcrystalline single-phase specimens containing the phase based on ?-Bi2O3 if V2O5 content was ? 4.63 mol%. The obtained phases exhibited main diffraction peaks corresponding to the simple cubic ?-Bi2O3 (space group Fm-3m) but Rietveld refinement showed a threefold repeat on a simple cubic sublattice indicating that the true unit cell is 3?3?3 supercell. Within proposed supercell, the octahedrally coordinated V5+ ions fully occupy 4a Wyckoff position and partially occupy 32f. The Bi3+ ions are placed at the rest of 32f and at 24e and 48h with full occupation. In total, 22 % of anionic sites are vacant. The ionic conductivity of phase with the lowest dopant content, i.e. Bi 103V5O167, amounts 0.283 S cm-1 at 800?C with the activation energy of 0.64(5) eV, which is comparable to the undoped ?-Bi2O3 known as the fastest ion conductor.

Crystal structure of the (REE)-uranyl carbonate mineral kamotoite-(Y)

AbstractKamotoite-(Y) is a rare supergene product of uraninite hydration–oxidation weathering and its structure is unknown. Based on single-crystal X-ray diffraction data collected with high-redundancy using a microfocus source, kamotoite-(Y) is monoclinic, has space group P21/n,with a = 12.3525(5), b = 12.9432(5), c = 19.4409(7) Å, β = 99.857(3)°, V = 3069.8(2) Å3 and Z = 4. Crystals are pervasively twinned (two-fold rotation around [0.75 0 0.75]), giving a strongly pseudo-orthorhombic diffractionpattern. The pseudoorthorhombic pattern can be described with an orthorhombic super-cell (transformation matrix 0,1,0/1,0,1/3,0,1), approximately four times larger in volume then a true monoclinic unit cell. This unit-cell is the same as the cell given elsewhere for the structure of bijvoetite-(Y),another (REE)-containing uranyl carbonate. The successful structure solution and refinement (R = 0.044 for 6294 unique observed reflections), carried out using our choice of unit cell, as well as the superstructure refinement and comparison of the original structure data forbijvoetite-(Y) reveal that these two crystal structures are identical. The crystal structure of kamotoite-(Y) consists of electroneutral sheets of the bijvoetite-(Y) uranylanion topology and an interlayer with H2O molecules not-coordinated directly to any metal cation. Despite determinationof the kamotoite-(Y) structure and demonstration that bijvoetite-(Y) has the same structure, the identity of these two minerals cannot be proved without additional study of the holotype material.

The structure of the melamine–cyanuric acid co-crystal

The crystal structure of the melamine:cyanuric acid (CA.M) adduct has been redetermined and the previously reported cell is shown to be incorrect. The true unit cell has approximately twice the volume of the earlier cell. Crystal data for CA.M: monoclinic, space group I2/m, a = 14.8152(19) A, b = 9.6353(18) A, c = 7.0405(9) A, β = 93.194(11)°, V = 1003.5(3) A3, Z = 2. In contrast to the previous report, this contains an ordered array of cyanuric acid and melamine. Hydrogen bonding between the two components is described in detail and precise information about intermolecular distances reported for the first time. The structure contains hydrogen-bonded sheets stacked perpendicular to the crystallographic (101) plane. The molecular geometry of the cyanuric acid and melamine components is described in detail. Possible explanations for the difference between this structure and the previous report are described in the light of quantum chemical calculations.

The crystallographic structure of Panicum Mosaic Virus (PMV).

The structure of Panicum Mosaic Virus (PMV) was determined by X-ray diffraction analysis to 2.9Å resolution. The crystals were of pseudo symmetry F23; the true crystallographic unit cell was of space group P2(1) with a=411.7Å, b=403.9Å and c=412.5Å, with β=89.7°. The asymmetric unit was two entire T=3 virus particles, or 360 protein subunits. The structure was solved by conventional molecular replacement from two distant homologues, Cocksfoot Mottle Virus (CfMV) and Tobacco Necrosis Virus (TNV), of ∼20% sequence identity followed by phase extension. The model was initially refined with exact icosahedral constraints and then with icosahedral restraints. The virus has Ca(++) ions octahedrally coordinated by six aspartic acid residues on quasi threefold axes, which is completely different than for either CfMV or TNV. Amino terminal residues 1-53, 1-49 and 1-21 of the A, B and C subunits, respectively, and the four C-terminal residues (239-242) are not visible in electron density maps. The additional ordered residues of the C chain form a prominent "arm" that intertwines with symmetry equivalent "arms" at icosahedral threefold axes, as was seen in both CfMV and TNV. A 17 nucleotide hairpin segment of genomic RNA is icosahedrally ordered and bound at 60 equivalent sites at quasi twofold A-B subunit interfaces at the interior surface of the capsid. This segment of RNA may serve as a conformational switch for coat protein subunits, as has been proposed for similar RNA segments in other viruses.

Crystallographic, magnetic and dielectric studies of the potential multiferroic cryolite (NH 4) 3FeF 6

The cryolite phase (NH 4) 3FeF 6 has been studied using variable temperature X-ray powder diffraction, together with dielectric and magnetic measurements, in order to resolve its potential multiferroic properties. The X-ray data reveal a direct transition from cubic to triclinic symmetry between 270 and 250 K, consistent with earlier reports of a ferroelectric T C of 264 K. The crystal structure of the low temperature phase has been modelled approximately in a pseudo-tetragonal, triclinic space group F1, a = 8.98258(9) Å, b = 8.99631(9) Å, c = 9.27961(9) Å, α = 90.039(1)°, β = 90.451(1)° and γ = 90.339(1)° (at 100 K) but weak superlattice reflections remain unindexed, suggesting the true unit cell to be larger. Electrical measurements indicted an increase in capacitance above 260 K, however this was accompanied by an increase in dielectric loss and so is inconclusive evidence of a ferroelectric–paraelectric phase transition. Polarisation-field loops were not possible due to overriding leakage. Magnetic susceptibility data show paramagnetic behaviour with no indication of long-range order above 2 K, and only weak antiferromagnetic interactions ( θ ∼ −7 K). This therefore clearly demonstrates the material is not multiferroic.

Phase formation in the Ag2MoO4-CuO-MoO3 system and the crystal structure of the new double molybdate Ag2Cu2(MoO4)3

Subsolidus phase relations in the Ag2MoO4-CuO-MoO3 oxide-salt ternary system were determined. T-x diagram was plotted for the Ag2MoO4-CuMoO4 quasi-binary join. Double molybdate Ag2Cu2(MoO4)3 was found to exist on this join. This compound is a superstructure derived from orthorhombic Li3Fe(MoO4)3. Its structure was solved in terms of a subcell (a = 5.0749(3), b = 11.300(2), c = 18.127(3) A, space group Pnma, Z = 4, R = 0.0678). In the true unit cell, the parameter a is tripled; suggested space group is P212121. A characteristic feature of the Ag2Cu2(MoO4)3 structure is infinite columns (extended along axis a) of face-sharing oxygen octahedra, in which disordered silver atoms are located (Ag(21), Ag(22), and Ag(23)) with various degrees of irregularity of their octahedral coordination and a strong anisotropy of thermal vibrations. Distorted CuO6 octahedra form zigzag ribbons extended in the same direction. MoO4 tetrahedra, which are arranged according to the pseudo-hexagonal law, link the aforementioned major structural elements into a three-dimensional framework. Trigonal-prismatic voids of the framework are occupied by silver atoms Ag (1). Presumably, the disorder of the silver ions in octahedral columns can be responsible for the increased ion conductivity of silver copper molybdate. A partial order of the same ions is the most likely reason for the appearance of superstructure with the tripled unit cell volume.

Superstructure of a phosphor material Ba 3MgSi 2O 8 determined by neutron diffraction data

Ba 3MgSi 2O 8, a phosphor host examined for use in white-light devices and plant-growth lamps, was synthesized at 1225 °C in air. Its crystal structure has been determined and refined by a combined powder X-ray and neutron Rietveld method ( P 3 ¯ , Z=3, a=9.72411(3) Å, c=7.27647(3) Å, V=595.870(5) Å 3; R p/ R wp=3.79%/5.03%, χ 2=4.20). Superstructure reflections, observed only in the neutron diffraction data, provided the means to establish the true unit cell and a chemically reasonable structure. The structure contains three crystallographically distinct Ba atoms—Ba1 resides in a distorted octahedral site with S 6 ( 3 ¯ ) symmetry, Ba2 in a nine-coordinate site with C 3 (3) symmetry, and Ba3 in a ten-coordinate site with C 1 (1) symmetry. The Mg atoms occupy distorted octahedral sites, and the Si atom occupies a distorted tetrahedral site.

Proton and Deuteron Exchange in TTB‐Like Na1.2Nb1.2W0.8O6: Structural Characterization and Spectroscopic Study

AbstractProton‐ and deuteron‐exchange reactions have been performed on Na1.2Nb1.2W0.8O6 with the tetragonal tungsten bronze (TTB) structure. Na+/H+ and Na+/D+ ion exchange occurs when aqueous nitric acid solutions are used as the exchanging agent. Up to two out of the three Na+ ions are readily exchanged to yield crystalline powder samples. The ion‐exchange reaction proceeds topotactically, in other words the basic TTB structure is retained, although X‐ray and electron diffraction reveal that the exchange process produces an orthorhombic supercell which possesses a true unit cell based on a ≈ 2√2aTTB, b ≈ √2aTTB, c = 2TTB. The nature of the hydrogenated species has been analyzed by means of IR and Raman spectroscopy on both parent and exchanged compounds. The results are interpreted in terms of short M–O (M = Nb or W) bonds formed through the trapping of two protons or deuterons close to the oxygen ions of the [MO6] octahedra. The band assigned to the short M–O bonds disappears after deprotonation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

A coupled electron diffraction and rigid unit mode (RUM) study of the crystal chemistry of some zeotypic AlPO 4 compounds

Electron diffraction and lattice dynamical calculations are used to investigate the unit cells, space group symmetries and inherent displacive flexibility of the room-temperature average structures of AlPO 4-8, AlPO 4-16 and AlPO 4-tridymite. The zero-frequency rigid unit modes (RUMs) of the idealized high-symmetry polymorphs thereof are also investigated along with their relationship to the lower-temperature polymorphism of these zeotypic aluminophosphates. The clear presence of G ± 1 2 c * satellite reflections in addition to the Bragg reflections ( G) of the underlying Cmc2 1 parent structure in the case of AlPO 4-8 shows that the true unit cell of the room-temperature polymorph has a doubled c-axis due to a condensed RUM mode. Structured diffuse scattering is also observed which can be related to the thermal excitation of RUM modes. In the case of AlPO 4-tridymite, a complex F1 triclinic polymorph is observed and related to soft RUM modes while, in the case of AlPO 4-16, a soft q= 0 RUM mode is shown to be responsible for an observed phase transition in the case of the all SiO 2 analogue of AlPO 4-16. A large number of additional zero-frequency RUM modes also exist in the case of AlPO 4-16.

High‐pressure metastable phase transitions in β‐Ge3N4 studied by Raman spectroscopy

AbstractWe studied polymorphic transitions occurring in β‐Ge3N4 metastably compressed to high pressures (P = 40 GPa) at room temperature. Previous studies under high‐pressure–high‐temperature conditions have shown that this phase transforms into a newly discovered spinel‐structured material (γ‐Ge3N4) above 10 GPa and 1200°C. However, ab initio theoretical studies have indicated that the phenacite‐structured low‐pressure β‐Ge3N4 polymorph should also undergo a series of metastable transformations upon compression at low temperatures. Here we studied these transformations by in situ Raman spectroscopy in a diamond anvil cell. The phase transitions were first predicted to involve a sequence of second‐order phase changes driven by soft phonon modes at the Brillouin zone centre (the Γ point: q = 0) resulting in displacive motions of the N atoms, causing the symmetry to descend from P63/m through P6 to P3. The two transitions were predicted to occur at P = 20 and 28 GPa. However, it was also noted that a direct first‐order transition between P63/m and P3 phases might occur at P = 23 GPa if order–disorder processes or phonon condensations away from the Brillouin zone centre were considered. Here we present experimental evidence for a phase transition occurring at P = 20 GPa, within β‐Ge3N4 that has been metastably compressed at ambient temperature. From the number of Raman modes observed, this transition most likely corresponds to the direct P63/m–P3 transformation, and it is therefore first order in character, but with a small activation energy barrier as ascertained by little or no hysteresis observed upon decompression. However, when all the minor features appearing in the Raman spectrum are accounted for, the number of observed resonances is greater than the number of zone centre modes expected even for the P3 structure. This indicates that the true unit cell is larger than expected from zone‐centre mode softening, and that order–disorder processes or phonon instabilities at q≠0 must have occurred during the transition. Copyright © 2003 John Wiley & Sons, Ltd.

Thermal dehydration of H3 + xPVxM12 − xO40·yH2O Keggin type heteropolyacids; formation, thermal stability and structure of the anhydrous acids H3PM12O40, of the corresponding anhydrides PM12O38.5 and of a novel trihydrate H3PW12O40·3H2O

The formation and thermal stability of the catalytically important anhydrous acids H3PM12O40 (M = Mo,W,V), of the corresponding anhydrides PM12O38.5 and of a novel first time isolated trihydrate H3PW12O40·3H2O have been studied by high temperature X-ray diffraction (XRD) and thermogravimetry (TG). The corresponding crystal structures were determined from X-ray powder data using the Rietveld method and by deriving three-dimensional structure models from the experimental radial distribution functions. The true unit cell symmetry of the anhydrous acid H3PMo12O40 is rhombohedral, space group R3, a = 11.48 A, α = 87.46°, Z = 2. Its crystal structure is built up from distorted (PMo12O40) Keggin units. The molecular centers of the Keggin units form a rhombohedral distorted cubic body centered lattice. The structure resembles that of the cubic ammonium/potassium salts. However, due to the formation of hydrogen bridges between the polyanions, the orientation of the Keggin units is different. Heat treatment of the anhydrous acid between 673 K and 733 K leads to the release of the constitutional water molecules with the formation of an amorphous anhydride. The anhydride structure is built up from lacunary polyions PMo12O38+ and PMo12O39− in a three dimensional arrangement very similar to that of the corresponding anhydrous acid. However, the mutual orientation of the polyions is largely disordered. Dehydration of H3PW12O40·6H2O between 413 K and 453 K results in the formation of a new cubic acid containing three molecules of water of crystallization. Its structure is closely related to that of the well known hexahydrate. Both structures may be thought of as being made up of two different interpenetrating substructures of anions and cations with space group symmetry Pn3m. The anionic substructures are similar; the cationic substructures, however, are different. In the hexahydrate structure the polyanions are linked by nearly planar H5O2+ dioxonium cations and the oxygen atoms occupy the x,¼,¾ crystallographic sites. The release of water leads to the formation of H3O+ oxonium cations which occupy the regular cationic positions ¼,¾,¾. The molecular formula of the new trihydrate should be written as (H3O+)3PW12O40.

Reappraisal of the space group of bafertisite

Bafertisite, Ba(Fe, Mn)2Ti(Si2O7)O(OH, Cl)2, has been reported as belonging to three space groups, i.e., Pmmn, P21/m, and Cm. The samples of bafertisite from the original locality Bayan Obo, Inner Mongolia, and from Jiangsu Province, China, were reinvestigated by X-ray methods. On the basis of single-crystal photographs, Pmmn could be ruled out and the relationship between a subcell in P21/m and the true cell in Cm deduced. The transformation matrix from the subcell to the true cell is 002/02¯0/101¯. Using the X-ray powder data, the refined true unit cell parameters in space group Cm are: a=10.612(3), b=13.637(7), c=12.464(2) Å, β=119.49(2)° for Bayan Obo sample, and a=10.633(6), b=13.67(1), c=12.465(5) Å, β=119.55(4)° for the Jiangsu sample. The 00l-reflections, which are rather prominent due to preferred orientation, can only be explained by our new unit cell.

Oxide Structures: By Hook Or by Crook

Abstract Sometimes oxides are friendly; x-ray or neutron diffraction solves the structure and (at most) simple HREM does a double check. Sometimes they are not. Problems can range from phase purity (identifiable using EDX) to more subtle issues about the true unit cell and/or atom locations. The latter are often particularaly hard for x-ray or neutron to handle when single crystal data is not available and, for instance, the samples are polyphase or textured. One common problem is that the unit cell is rather larger than originally thought due to a superstructure. While superstructure reflections may be obvious in the microscope due to the very large dynamic range of TED patterns, they may not show in powder patterns. The issue then is to determine the true structure using (often) a combination of HREM and TED plus multislice simulations to confirm the structure.

Cation ordering in lead-molybdenum-vanadium oxychlorides

The crystal chemistry and structure of layered lead oxychloride parkinsonite, ideally Pb7Mo09C12, have been reevaluated in light of TEM observations of natural and synthetic samples and strong analogy with Dla and D022 ordering patterns of Ni-Mo alloys. Dla-like superstructures occur in synthetic parkinsonite and V-bearing parkinsonite because of PbMo and PbV ordering, with ordering stoichiometries Pb9Mo (10-site scheme) and Pb,s V2 (20-site scheme). Natural parkinsonite orders on Pb7Mo (8-site) or Pbl4M02 (16-site) scheme. This ordering involves formation of an incommensurate state that may be associated with a pseudohexagonal motif similar to DOn superstructure in Ni-Mo alloys. It is our contention that X-ray methods usually miss important information about superstructures in these minerals because of very high X-ray absorption. As a result, single-crystal structure refinements can lead to erroneous identification of tetragonal or pseudotetragonal I subcell as true unit cell. Transmission electron microscopy allows examination of extremely thin crystallites and observation of weak superstructure reflections. Crystal-chemical inferences about structure and ordering behavior in these minerals are supported by EXAFS and XANES studies of a synthetic parkinsonite ordered on 9: 1 scheme, Pb9MoOCl2, indicating that M06+ may well be present in fivefold square-pyramidal coordination. INTRODUCTION best fit to observed powder X-ray diffraction pattern is obtained when all Mo is only on sites within The structures of layered lead oxychlorides are Pb2 sublayer (Fig. 1). This is compatible with prefbased on an alternation of litharge-like PbO layers and erential occurrence of V on analogous Pb2-like sites in sheets of Cl atoms (Fig. 1). The main feature of interest kombatite (Cooper and Hawthorne 1994). is wide diversity of elements that partially substitute It seems that X-ray methods often miss very weak for Pb: Si, P, As, V, Mo, and W (Aurivillius 1982). superstructure reflections associated with Pb-X ordering, Charge balance and stable coordination are achieved by recording reflections only from tetragonal subcell. It insertion of 0 atoms between Pb2 layers and creation of is this subcell (Fig. 1) that has been erroneously identified o vacancies within PbO sheets. In kombatite, as true unit cell in case of parkinsonite (Symes et PbI4(V04)209Cl4 (Rouse et al. 1986; Cooper and Hawal. 1994). This may also be case for Si analog thorne 1994), each V5+ is tetrahedrally coordinated, with asisite (Rouse et al. 1988). The latter workers achieved apical 0 atom providing bonding between adjacent an R index of 6.9% (weighted) without locating Si Pb2 layers. Charge balance results in an 0 vacancy in positions. However, attempts to locate Si atoms by a PbO layer. Kombatite and sahlinite, PbI4(As04)209CI4, second difference synthesis failed. They attributed this both contain pentavalent cations and are isostructural failure to the inconsiderable scattering power of ca. one (Rouse and Dunn 1985; Cooper and Hawthorne 1994). Si atom per cell relative to that of Pb atoms. The Asisite, ideally Pb7Si08Cl2 (Rouse et al. 1988), seems to high mass-absorption coefficients for layered lead oxbe isostructural with parkinsonite (Symes et al. 1994). ychloride minerals (e.g., 840 cm-1 for kombatite) can preThe latter authors give formula of natural parkinsonite vent observation of superstructure reflections. It as Pb634Moo890on0802Cl,98 (0 = cation vacancy). Recalwas only by special sample preparation that complex culation of this formula for correct 0 stoichiometry for superstructure of kombatite was solved by Cooper and M06+ substitution (0 = 9 apfu), and excluding cation vaHawthorne (1994). cancies, gives Pb713Mo,oo09Cl,98, which is clearly comIn this paper, we present a reevaluation of crystal patible with general Pb7Xn+0(6+o5n)CI2stoichiometry chemistry of layered lead-molybdenum-vanadium oxyof kombatite, sahlinite, and asisite. Our reinterpretation chlorides primarily on basis of electron diffraction of parkinsonite structure precludes cation vacancies. data (TEM) and X-ray absorption spectroscopy (XAS) Symes et al. (1994) found that, for natural parkinsonite, studies of Mo coordination in synthetic parkinsonite. 0003-004X/96/1 I 12-1350$05.0

Synthesis, structural characterisation and magnetic properties of Gd14W4O33−xNy(0 ⩽ x ⩽ 17 ± 2, 0 ⩽ y ⩽ 9 ± 2), a new fluorite-related oxynitride

A series of crystalline Gd-W oxides, with Gd:W ratios spanning the values 0.1–3.5 has been prepared and treated with a flow of NH3(g) in the temperature range 700–1100°C. Besides the previously reported formation of GdWO3N and Gd2WO3N2, the following new results were obtained: (i) cubic Gd0.1 WO3 upon ammonolysis forms an (oxy)nitride Gd11.1 W(O, N)4 with an f.c.c. lattice: (ii) the previously described Gd2WO3N2 exhibits a solid solubility range Gd3−A W1−1(O,N)x with, approximately, (0 ⩽ x ⩽ 1: (iii) ammonolysis of Gd14W4O33 yields an oxynitride, the composition of which varies with the synthesis temperature according to Gd14W4Oin33−yNy(0 ⩽ x ⩽ 17 ± 2, 0 ⩽ y ⩽ 9 ± 2). All strong diffraction lines, including those of the pure oxide (x = y = 0), can be indexed with a body-centred monoclinic unit cell derived from that of fluorite. For Gd14W4O33 the cell parameters are a = 5.3592(11), b = 3.7295(7), c = 3.8636(8) Å, β = 92.170(2)° and V = 77.17 Å3. However, strong superstructures reflections in the electron diffraction patterns indicate that the true unit cell is considerably larger. A partial structure determination of Gd14W4O33−xNx has been carried out by the Rietveld method. Magnetic susceptibility measurements in the temperature interval 15–300 K show a Curie-Weiss law behaviour for Gd14W4O33−xNxN with μeff = 8.08 ± 0.06 Bohr magnetons per Gd3+ and θ = − 12.4 ± 1.1 K for all compositions.

Crystal chemistry of beryllophosphates: The crystal structure of moraesite, Be2(PO4)(OH) · 4H2O

Abstract Moraesite, Be2(PO4)(OH) · 4H2O, is a hydrated beryllophosphate mineral. The crystal structure of moraesite from Humaita, Brazil has been solved and refined in the space group C2/c, a = 8.553(6), b = 12.319(6), c = 7.155(8) Å, β = 97.93(9)°, up to R = 0.062 for 262 reflections. The structure is characterized by infinite double chains, running along c, formed by [PO4] and [BeO2(OH)H2O] tetrahedra. Within such chains three- and four-membered rings of tetrahedra occur. Additional water molecules, not linked to any cation, fill the large channels of the structure. Within each channel, the hydrogen bonds assume either of two schemes. Evidences were obtained that the true unit cell of moraesite is three times larger, with a b parameter of 36.96 Å and space group symmetry Cc. It is suggested that the ordering of the hydrogen bond system, concerted with minor adjustments of the structure, could be responsible for the triplication of the b axis.

Phase transformations among three polymorphs of Mg2NiH4

Abstract The high temperature form of Mg 2 Ni hydride has an f.c.c. lattice with the metal atoms occupying antifluorite positions. It transforms to the low temperature form at 235°C. In earlier work the structure of the low temperature form was determined from its X-ray powder diffraction data. It can be described as a monoclinic distortion of the high temperature cubic lattice. However, the low temperature phase is usually contaminated with a small amount of an unknown phase which shows a similar powder pattern to the high temperature form. In the present investigation it was revealed that this unknown phase is another polymorph of Mg 2 Ni hydride which is easily transformed into the low temperature form by grinding the powder in a mortar at room temperature. This intermediate phase was found to be stabilized by substituting cobalt for 10 wt.% Ni. This structure can also be described as a distortion of the high temperature cubic lattice and the main peaks were indexed with an orthorhombic cell with a = 6.572 A , b = 4.520 A and c = 4.560 A . However, the presence of several weak peaks indicates that longer-range ordering exists and that the true unit cell is larger than this.

Structure of a Vermiculite-Aniline Intercalate

AbstractA vermiculite-aniline intercalate with a basal spacing of 14.78 Å was investigated by one- and two-dimensional X-ray diffraction methods. The intercalate, prepared by ion exchange between Na-saturated vermiculite from Llano, Texas, and a 1% aniline hydrochloride solution, contains only one aniline cation per single layer cell. A reduced effective cell-charge is believed to be responsible for this. Structure factor calculations were made in space group C2/c and with unit cell dimensions of a = 5.33, b = 9.18, c = 29.78 Å, and β = 97.0°. However, extra reflections in the a*b* plane, which are similar to those in a vermiculite-benzidine intercalate, showed that after aniline intercalation the true unit cell became primitive. The aniline cations are distributed statistically over equivalent crystallographic sites in the interlayer space. The organic molecules are orientated with their planes vertical and their nitrogen atoms over the projected centers of the ditrigonal cavities into which they key. The aniline cations form ordered arrays upon the silicate layers by packing into rows. Perpendicular to [010], populated and vacant rows alternate. Along populated rows aromatic ring planes are alternately parallel and perpendicular to [010]. With small adjustments this model is similar to that of benzidine-vermiculite.

Antiferromagnetism of the FeNb and FeTa MU-phases

The magnetic structure of the antiferromagnetic FeNb and FeTa alloys has been investigated using a variety of techniques including susceptibility. Mössbauer and neutron diffraction measurements. Our results indicate a simple structure in which alternate Kagomé layers of Fe atoms have their moments oriented in opposite directions, approximately perpendicular to the plane of the layers. Positive Weiss temperatures confirm that the dominant nearest neighbour intra-layer interactions are ferromagnetic. Specific heat and low angle neutron scattering observations suggest extensive short-range order above T N. The substantial non-magnetic contribution to the Mössbauer spectra is shown to result from ≈ 25% occupation of the high-coordination ‘A’ sites by Fe atoms with a similar proportion of the low-coordinationB2 Kagomé sites occupied by Nb. A structural neutron super lattice line too weak to have been observed in earlier work using X-rays suggests that the true μ-phase unit cell may be three times larger in dimensions than previously envisaged.

On the Crystal Structure of the β′‐Form of Triglycerides and Structural Changes at the Phase Transitions LIQ. → α → β′ ← β

AbstractBased on X‐ray single crystal data of the β′‐form of triundecanoin the general features of the molecular arrangement in this crystal form have been derived. The unit cell contains eight molecules arranged according to space group P21/c, with alternation of the tilt of the hydrocarbon chains in adjacent bimolecular layers. The structure within the molecular layer is quite similar to that of the β‐form; the main difference being the chain packing. A second β″‐form, β′2, of triundecanoin has been observed and it differs from the one mentioned above in the orientation of the chain packing subcell 0° in relation to the true unit cell. The polymorphic transitions have been followed by recording of the diffraction pattern versus temperatures. The polymorphic transitions α · β′ · β can be regarded mainly as different lateral arrangement of a dimeric unit. In the α‐form the chains are disordered near the methyl end groups, and due to this disorder the structure is closely related to the lamellar liquid‐crystalline phase. It is also possible to classify β′‐forms according to short‐spacing data into a β′1‐type and a β′2‐type, and these groups can be identified also in complex triglycerides.