C-axis Of Unit Cell Research Articles

Soft chemical deintercalation of silver from delafossites AgFeO2, and AgCrO2

Selective deintercalation of Ag+-ion from AgFeO2 and AgCrO2 with the use of dilute aqueous solutions of NH4Cl was achieved successfully. Based on the ICP analysis, the Ag: Fe and Ag: Cr ratios were 0.9:1 and 0.87:1 in the deintercalated sample of AgFeO2 and AgCrO2, respectively. Retention of both structure and the polytypes were noticed after deintercalation together with the reduction in their unit cell c-axis. Raman spectra of deintercalated samples confirmed the preservation of overall symmetry. XPS results confirmed the existence of Fe (III) and Cr (III) in the Ag+-ion deintercalated samples.

Microstructural differences between Viscose and Lyocell revealed by in-situ studies of wet and dry fibers

Regenerated cellulose fibers manufactured using Lyocell and Viscose processes exhibit differences in their structural response to mechanical deformation. Here, we study the effect of stretching and stress relaxation on the orientation of crystal and amorphous phases. Our results show that on stretching, orientation in both crystal and amorphous phases increases linearly with strain, correlating with the increase in stress and with the stretching of the crystalline unit cell along the c-axis. On holding after stretching to a particular strain, the stress relaxes logarithmically in time, correlating with a decrease in the strain along the c-axis of the crystal unit cell. The stress relaxation is also correlated with a logarithmic increase in amorphous orientation, while crystalline orientation stays constant. We attribute the stress development during stretching to deformation of the crystal unit cell, while crystal reorientation in the fiber direction results in increase in the crystalline orientation parameter. On holding the fiber at a fixed total strain, the stress relaxes as strain is transferred from crystal to amorphous regions. Thus, the strain on the unit cell c-axis decreases and amorphous orientation increases. There are quantitative differences between the rate of increase in amorphous phase orientation during stress relaxation for Lyocell and Viscose fibers. For dry fibers, Lyocell shows a slower increase in orientation during stress relaxation. On wetting the fibers, their structural response changes qualitatively. We combine wide angle x ray diffraction and birefringence experiments with our model to infer that that on stretching the wet fiber, the crystalline phase is neither strained nor oriented. However, orientation develops in the amorphous phase. During stress relaxation in wet condition, Lyocell fibers shows a faster increase in amorphous orientation than Viscose fibers, in line with the comparison of relaxation time spectra for wet Viscose and Lyocell fibers.

Magnetic ordering of Ho6Co2Ga-type {Gd, Tb, Dy}6Co2.2Al0.8 and Tb6Co2Al compounds by magnetization and neutron diffraction study

The magnetic ordering of polycrystalline Ho6Co2Ga-type Gd6Co2.2Al0.8, Tb6Co2.2Al0.8, Tb6Co2Al and Dy6Co2.2Al0.8 (space group Immm, N 71, oI36) has been established using magnetic measurements and the magnetic structure of Tb6Co2.2Al0.8 has been resolved by the neutron diffraction study. Gd6Co2.2Al0.8 exhibits field sensitive ferro-antiferromagnetic ordering, meanwhile Tb6Co2.2Al0.8, Tb6Co2Al and Dy6Co2.2Al0.8 are antiferromagnets with metamagnetic transformation of magnetic ordering. Gd6Co2.2Al0.8 is soft ferromagnet, whereas Tb6Co2.2Al0.8, Tb6Co2Al and Dy6Co2.2Al0.8 exhibit low-temperature weak permanent magnet properties. From these compounds, Gd6Co2.2Al0.8 demonstrates largest magnetocaloric effect with magnetic entropy change of −10.89 J/kg⋅K at 75 K for a field change of 50 kOe (relative cooling power of ~760 J/kg).Neutron diffraction study indicates non-collinear abc-antiferromagnetic ordering of Tb6Co2.2Al0.8 in zero applied magnetic field. Below 63 K and down to 52 K, Tb6Co2.2Al0.8 exhibits antiferromagnetic ordering with magnetic space group I′22′2′ (wave vector K0 = [0, 0, 0]). Below 52 K and down to 1.5 K, the magnetic structure of Tb6Co2.2Al0.8 is sum of magnetic component with P21′212′ magnetic space group (wave vector K0 = [0, 0, 0]) and square-modulated antiferromagnetic component along c-axis of unit cell with K1 = [±1/4, 0, ±1/4] wave vector.

Temperature driven anomalous unit-cell c-axis shifts in highly oriented pyrolytic graphite measured at the magic-angle

Small misfit-angles in the grain boundary of HOPG interfaces have been recently reported to act as a source of unusual superconductive effects, with significant changes in the electronic-band-structure. These observations may imply that possible structural-transitions can exist with the variation of the temperature. In this letter we report the presence of temperature driven structural-transitions in the temperature range from 298.15 to 673.15 K measured under magic-angle HOPG (0.8°,±0.2°, first magic-angle). Novel memory effects consisting in an anomalous reversible linear increase of the c-axis unit cell values of HOPG with the increase of the temperature are described.

3D Maps of Mineral Composition and Hydroxyapatite Orientation in Fossil Bone Samples Obtained by X-ray Diffraction Computed Tomography

Whether hydroxyapatite (HA) orientation in fossilised bone samples can be non-destructively retrieved and used to determine the arrangement of the bone matrix and the location of muscle attachments (entheses), is a question of high relevance to palaeontology, as it facilitates a detailed understanding of the (micro-)anatomy of extinct species with no damage to the precious fossil specimens. Here, we report studies of two fossil bone samples, specifically the tibia of a 300-million-year-old tetrapod, Discosauriscus austriacus, and the humerus of a 370-million-year-old lobe-finned fish, Eusthenopteron foordi, using XRD-CT – a combination of X-ray diffraction (XRD) and computed tomography (CT). Reconstructed 3D images showing the spatial mineral distributions and the local orientation of HA were obtained. For Discosauriscus austriacus, details of the muscle attachments could be discerned. For Eusthenopteron foordi, the gross details of the preferred orientation of HA were deduced using three tomographic datasets obtained with orthogonally oriented rotation axes. For both samples, the HA in the bone matrix exhibited preferred orientation, with the unit cell c-axis of the HA crystallites tending to be parallel with the bone surface. In summary, we have demonstrated that XRD-CT combined with an intuitive reconstruction procedure is becoming a powerful tool for studying palaeontological samples.

Crystal alignment of caffeine deposited onto single crystal surfaces via hot-wall epitaxy.

Defined crystal growth is highly demanded for technological applications but also fundamental research. Within this work, the crystal growth of the asymmetric molecule caffeine was studied on single crystalline surfaces of muscovite mica, sodium chloride and potassium chloride. While elongated needle-like crystals grow on muscovite mica and sodium chloride, smaller individual "bird-like" structures were observed on potassium chloride. Depending on the surface type and temperature, the disk-shaped caffeine molecules prefer either an edge-on or flat-on orientation with respect to the surface, but in each case, a defined crystallographic relation between the surface and caffeine crystallites was determined by using the X-ray pole figure technique. On muscovite mica and sodium chloride, needle-like crystallites with edge-on oriented molecules aligned mainly with the unit cell c-axis (which coincides with the long needle axis) along the [1-10]mica, [100]mica, [110]mica and [110]NaCl, [1-10]NaCl directions, respectively. Crystals consisting of flat-on oriented molecules on KCl showed also defined alignments with respect to the substrate, but due to the altered molecule-substrate contact, the b-axis aligned along [110]KCl and [1-10]KCl. Growth at elevated temperatures enabled changes in the crystal growth whereby more defined structures formed on NaCl. On KCl, the bird-like structures remained very similar, while caffeine on the mica surface at elevated temperatures resulted in even additional texture forming with the caffeine molecules now also favoring a flat-on orientation with respect to the surface. The systematic variation of various system parameters demonstrates how sensitive the growth behavior of caffeine on this variety of substrates is.

Effects of pressure on the electronic structures of LaOFeP

Abstract We studied the electronic structures of LaOFeP under applied pressure using first-principles calculations. The electronic density of states at the Fermi level decreases continuously with increasing pressure. The electron branches of Fermi surfaces are rather robust to pressure, while the hole branches change significantly. Two hole surfaces shrink into small ellipsoid-like surfaces and disappear finally, at which the applied pressure is ∼ 74.7 GPa . The pressure response can be understood by the band structures around the Fermi level. Comparative studies reveal that the disappearance of hole surfaces is mainly due to the compression of the FeP layer along the c-axis of unit cell.

Fabrication of Chelate-Setting Cement Using Silicon-Substituted Hydroxyapatite and its Property

We have developed novel hydroxyapatite (HAp) cement, “chelate-setting HAp cement” on the basis of chelate bonding of inositol hexaphosphate (IP6). In order to create the cement with enhanced bioactivity, we used a silicon-substituted hydroxyapatite (Si-HAp) as a starting material instead of pure HAp powder. The Si-HAp powders were prepared via an aqueous precipitation reaction and surface-modified with IP6 (IP6-Si-HAp). The Si-HAp were synthesized with desired Si contents (0, 0.4, 0.8, 1.6, and 2.4 mass%). Regardless of the amount of Si, the XRD patterns and FT-IR spectra of the powders were similar to each other. Additionally, chemical composition of Si-HAp powders were the almost same as the nominal one. The substitution of Si resulted in a decrease in the a-axis and increase in the c-axis of unit cell of HAp. While, the compressive strength of IP6-Si-HAp cements was higher than that of Si-HAp cements. These results suggest that surface-modification of any powders with IP6 is effective for enhancement of mechanical property. Comparison of mechanical property between HAp and Si-HAp cement specimens revealed that the substitution of lower levels of Si into0 HAp lattice did not greatly influence compressive strength. However, the substitution of high dose of Si (over 2.4 mass%) reduced the compressive strength. Therefore, to fabricate the chelate-setting cements with enhanced bioactivity, it is necessary to control the amount of Si.

Growth by MOCVD of (001) Bi-2223 superconducting thin films on (001) and (110) MgO substrates

Superconducting c-axis thin films of Bi2Sr2Ca2Cu3O10 (Bi-2223) were grown by MOCVD on (001) and (110) MgO substrates. Films on both substrates show, as revealed by AFM investigations, similar morphology composed of regular rectangular grains and relatively low roughness of about two or three half c-axis unit cell order. AFM images also suggest that films might have in-plane epitaxy. The films have zero resistance critical temperatures of 95.1K for the (001) MgO substrate and 75K for the (110) MgO substrate, respectively. Considering very large films-substrate lattice mismatch between (110) MgO and (ab)-plane of the superconducting phase the growth of indicated films is a surprise.

The crystal structure and electronic properties of a new metastable non-stoichiometricBaAl4-type compound crystallized from amorphousLa6Ni34Ge60 alloy

A new metastable La–Ge–Ni ternaryBaAl4-type(ThCr2Si2-type) compound, of which the space group isI4/mmm is synthesized. It is obtained by a polymorphic transformation from anLa6Ni34Ge60 amorphous alloy on crystallizing. The formula of the compound is(La0.3Ge0.7)(Ni0.85Ge0.15)2Ge2. This indicates that it is highly non-stoichiometric compared to the stoichiometricLaNi2Ge2. It is foundthat the c-axis lattice parameter of this compound is much longer than that ofLaNi2Ge2. It should be notedthat the longer c-axis unit cell parameter is attributable only to the longer interlayer distance between Ge site andNi site layers. The temperature dependences of electrical resistivity and thermoelectric power of the(La0.3Ge0.7)(Ni0.85Ge0.15)2Ge2 compoundand La6Ni34Ge60 amorphous alloy are also clarified. The comparison of these electronic properties betweenthe two materials indicates that sp-electrons mainly contribute to the density of statesaround the Fermi level of this compound.

Characterization of individual threading dislocations in GaN using ballistic electron emission microscopy.

Threading dislocations (TDs) of molecular beam epitaxy grown GaN film were studied with ultrahigh vacuum ballistic electron emission microscopy in order to quantify any fixed negative charge at identifiable TDs, with approximately 3 nm spatial and approximately 10 meV local barrier resolution. In contrast to several prior studies, we find no indication of fixed negative dislocation charge at specific TD structures, with a conservative upper limit of approximately 0.25 e(-) per c-axis unit cell. We do observe evidence of positive surface charge at TDs and at GaN step edges, which may be due to local piezoelectric fields.

An X-ray diffraction and modelling study of short chain branch location within the structure of polyethylene

The issue of whether short chain branches in polyethylene are tolerated within the crystalline component, or rejected from it, is long-standing and controversial. We have re-examined this issue by combining new X-ray diffraction techniques with molecular modelling. The X-ray diffraction patterns were recorded from a series of polyethylenes covering a broad range of branch types, contents and distributions, in both unoriented and fibre sample forms. This paper is the third and final in a sequence investigating the structure of this series. After unit cell parameters had been determined by whole pattern fitting, relative intensity ratios were measured to detect any systematic changes as functions of branch type, content and distribution. Previous studies have investigated only changes in the unit cell parameters. The influence of paracrystalline distortions was taken into account. Significant changes were found in three intensity ratios, which decreased as a function of branch concentration and increased as a function of the degree of heterogeneity (blockiness) in branch distribution. Molecular modelling showed that only branch inclusion was compatible with the experimental intensity changes. The modelling indicated that the branches were incorporated into the crystalline lattice via distortion of the neighbouring chains, mainly in the direction parallel to the unit cell a axis direction. The length of the branch backbone fitted into the lattice by lying approximately parallel to the unit cell c-axis (the main chain direction), emulating a section of a main chain.Another finding from the modelling concerned the extent of inclusion in the more highly branched samples (typically 15–20SCB/1000C, linear low density polyethylene). For models of these samples, branches could only be tolerated within a unit cell having the experimental values of unit cell parameters at concentrations around 3SCB/1000C, substantially lower than in the bulk material. At higher incorporated branch concentrations, crystalline packing of the chains was not maintained during model minimisation. For such polyethylenes, branches must therefore be partitioned between the crystalline and amorphous regions. These modelling findings provide a theoretical basis for the recent, similar experimental findings from solid-state 13C nuclear magnetic resonance spectroscopy.

Cation disorder in superconducting HgBa 2CaCu 2O 6+ δ films prepared on SrTiO 3 substrates

Hg–Ba–Ca–Cu–O thin films prepared on SrTiO 3 (100) substrates generally exhibit shorter c-axis unit cell lengths and lower zero-resistance superconducting transition temperatures as compared to bulk. Microstructural studies of HgBa 2CaCu 2O 6+ δ films based on X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) revealed shorter BaO–CuO 2 interplanar distances and longer CuO 2–CuO 2 average distances as compared to those reported for bulk compounds. It is suggested that the observed cationic disorder causes smaller c-axis lattice parameter and is probably induced by the lattice mismatch between film and substrate. The observed atomic rearrangements are expected to influence superconducting properties of the Hg-based films.

Pulsed laser deposition of YBa 2Cu 3O 7− δ and NdBa 2Cu 3O 7− δ thin films: a comparative study

Thin films of YBa 2Cu 3O 7− δ and NdBa 2Cu 3O 7− δ were grown by pulsed laser deposition. The statistical methods of experimental design and regression analysis were used to correlate the electrical and morphological properties of the films to the growth conditions. For optimised transition temperatures the deposition parameter settings of target–substrate distance, oxygen pressure and laser energy density were found to differ significantly for YBa 2Cu 3O 7− δ and NdBa 2Cu 3O 7− δ . While the maximum transition temperatures obtained were similar (∼90 K) for the two materials, all NdBa 2Cu 3O 7− δ films had a roughness comparable to the c-axis unit cell dimension whereas the YBa 2Cu 3O 7− δ films had a surface roughness which varied between 5–17 nm depending on the growth conditions. In addition, there were differences in the average size and density of non-stoichiometric outgrowths on the two types of film. These differences we relate to a difference in growth mode. Our atomic force microscope and scanning tunnelling microscope studies suggest a 3D screw dislocation mediated growth for YBa 2Cu 3O 7− δ and a 2D layer-by-layer process for NdBa 2Cu 3O 7− δ .

On the Structural, Acidic and Catalytic Properties of Zeolite SUZ-4

Highly crystalline SUZ-4 zeolite was synthesized and its acidic and catalytic properties were investigated using FTIR spectroscopy and n-hexane conversion. Based on the analysis of the published crystallographic data, it is concluded that SUZ-4 zeolite has a three-dimensional pore system that includes ten-membered ring channels parallel to unit cell c-axis which are intersected by two arrays of eight-membered ring channels running in the plane [001]. Experimental results reported here support this conclusion. About 45% of acid sites (bridging hydroxyls) are located in the large channels of SUZ-4 zeolite, while the remaining sites occupy positions in nonplanar double eight-membered rings. The bridging hydroxyls in SUZ-4 zeolite are more accessible for guest molecules (n-hexane and isobutane) than those in ferrierite. Both zeolites show similar strength of acid sites and have similar protolytic cracking activity. The hydrogen transfer activity of ferrierite is observed to be higher than that of the SUZ-4 ze...

The growth mechanism of pulsed laser deposited and DC magnetron sputtered high T c YBa 2Cu 3O 7 thin films

The investigation of growth mechanism of pulsed laser deposition (PLD) and magnetron sputtered high T c YBa 2Cu 3O 7(YBCO) thin films were carried out by atomic force microscopy (AFM). Observations showed that the nucleation and growth of the YBCO thin films have the customary island and coalescence mode. For PLD films, both screw dislocation islands and terrace islands occurred, but for the magnetron sputtered thin films only. screw dislocation islands were present. All the islands consist of many steps with a height of one or multiple c-axis unit cell, which indicated that the film grow unit cell by unit cell.

Growth mechanism studies of YBa 2Cu 3O 7−x superconducting films by atomic force and scanning tunneling microscopes

Systematic atomic force and scanning tunneling microscopic studies of the growth mechanisms of YBa 2Cu 3O 7−x films prepared by laser ablation have been carried out. In addition to the well known spiral growth features, layer-by-layer growth of crystal grains has been observed on c-oriented films frown mainly on ZrO 2 substrates. The majority of measured single terrace steps in the spiral growth grains correspond to c-axis unit cell. While, most of the layer heights for layer-by-layer growth grains are close to a half or a multiple of the unit cell. The changes of spiral growth pattern with the film thickness have also been studied.

Ion inclusion into Langmuir–Blodgett films studied on a quartz crystal microbalance

The potential of a simple quartz crystal microbalance (QCM) system for the study of ion inclusion into Langmuir–Blodgett (LB) films is demonstrated for cadmium docosanoate multilayers over a range of pH values which cover the range of unbound acid films to the high pH salt films. The reproducibility of the deposition process, the number of ions bound per fatty acid molecule and the inclusion of water into the film are measured as a function of pH and compared with values from X-ray reflection measurements. The Microscopic drying process and its relevance to film repreoducibility are also investigated. There is a slight, significant change in the areal density of the fatty acid component as acid films are converted to fully bound ionic soap films, in addition to the well documented change in the unit-cell c-axis.

Growth of high quality BiSrCaCuO single crystals by the self-flux method with a temperature-holding process

We prepared the BiSrCaCuO(Bi-based) single crystals by the self-flux method with a temperature-holding process. The plate-like Bi-based single crystals with the size of about 2 x 2mm 2 were prepared at the holding temperature of 840°C. From the X-ray diffraction patterns, we found that the c-axis unit cell length of the crystal ( C 0=39 A ̊ ) was longer than that of an 80K phase Bi-based single crystal.

Transmission electron microscopy studies of thin films of YBa 2Cu 3O 7-x

The microstructure of thin films of YBa 2Cu 3O 7-x deposited on MgO substrates by a Metal Organic Chemical Vapour Deposition method has been analysed by transmission electron microscopy. Films are highly textured with the unit cell c-axis of the film perpendicular to the surface. Some of the films contain a proportion of a-axis oriented grains. In films containing a high proportion of this type of grains, cross-sectional observations show that the insertion of the grains in the matrix is likely associated with a nucleation-growth process. Images also show the presence in the c-oriented matrix of stacking faults perpendicular to c and allow to analyse relaxation of the thin layer at the film/substrate interface.