Reflections In Diffraction Patterns Research Articles

Comparative study of the structural and magnetic properties of Mn1/3NbS2 and Cr1/3NbS2

We compare the magnetism of single crystals of the intercalated transition metal dichalcogenides ${\mathrm{Cr}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$ and ${\mathrm{Mn}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$ using techniques such as dc and ac susceptibility and Lorentz transmission electron microscopy (LTEM). We present a detailed structural investigation of these materials using electron and single-crystal x-ray diffraction measurements to show how substitutional disorder and stacking faults can manifest in ${\mathrm{Cr}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$ and ${\mathrm{Mn}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$, and give rise to additional superlattice reflections in diffraction patterns acquired from ${\mathrm{Mn}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$. Magnetic susceptibility and LTEM measurements show ${\mathrm{Cr}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$ displays chiral helimagnetism below its magnetic ordering temperature (${T}_{\mathrm{C}}$) of 111 K, while there is no evidence that ${\mathrm{Mn}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$ exhibits helimagnetic ordering below its transition temperature ${T}_{\mathrm{C}}=45$ K.

Physicochemical investigation of nanopowders prepared by laser ablation of crystalline silicon in water

In this work, an investigation of SiOx nanopowders synthesised by the laser ablation of crystalline silicon in water, evaporation of solvent and heat treatment is presented. The material is obtained for the first time by laser ablation in large quantities, which allows the use of a variety of physicochemical methods. The chemical composition and structure of the nanopowders were analysed before and after calcination in air and/or argon from 200 to 1000°С. The particles prepared by laser ablation of a silicon target in water contain an amorphous and/or crystalline silicon core and a silicon oxide shell. Calcination results in the disappearance of crystalline silicon reflections in diffraction patterns and the appearance of oxygen vacancies in the silica. These materials are potentially important for such applications as optics, sensors and catalysis.

Peculiar reflections in diffraction patterns as indicators of structural type and quality

Reflections serving as indicators of the types of packets forming crystal structures of many layered semiconductors have been revealed in diffraction patterns. It is found that the values l for the strongest reflection in series 000l and 00l, as well as the next to the strongest reflection in series $hh\bar 2\bar hl$ (h = const) and 0kl (k = const) for hexagonal and monoclinic structures, respectively, determine the number of polyhedral (Tand O) cation-filled layers per cell and indicate the types of packets TOT $TO\bar TE$ , $TO\bar T\bar TE$ , $TOO\bar TE$ , $TTO\bar T\bar TE$ , $OOE_1 TO\bar TE_1 $ and OOE $OOE_1 TO\bar TE_2 TO\bar TE_1 $ , where T and $\bar T$ are inversely oriented tetrahedra, O is an octahedron, E is an empty interpacket layer, and E1 and E2 are partially filled (to less than 1/3) interpacket layers.

Mullite and mullite-type crystal structures

Abstract Synthetic mullites (Al4+2 x Si2 –2 x O10 – x ) belong to one of the most important groups of ceramic materials. Their crystal structure consists of chains of edge-sharing AlO6 octahedra interconnected by tetrahedral double chains of TO4 (T = Al, Si) tetrahedra. The occurrence of oxygen vacancies is associated with the formation of triclusters of TO4 groups. The distribution of oxygen vacancies and the distribution of Al and Si on the tetrahedral sites are partially ordered yielding an incommensurately modulated crystal structure with satellite reflections in diffraction patterns. The number of symmetrically independent distribution patterns of oxygen vacancies is given for a 2 × 2 × 2 and a 3 × 3 × 2 superstructure model. Many other compounds have chain structures similar to mullite. These compounds are assigned to subgroups, all derived from a hypothetical tetragonal aristotype structure.

Modelling X-ray Diffraction From The Myosin Superlattice Of Vertebrate Muscle

Muscular force is generated by molecular interactions between the contractile proteins actin and myosin. The myosin filaments in the sarcomere of vertebrate muscle pack on a triangular array into which the actin filaments are interdigitated. High resolution studies of the actin-myosin interactions are performed by x-ray fiber diffraction analysis of whole muscle fibers. In most vertebrate muscles however, the myosin filaments pack in a so-called “superlattice” arrangement that involves a semi-random distribution of two filament rotations. The unknown effects of this disorder on diffraction by muscle fibres have so far prevented a rigorous analysis of x-ray fiber diffraction patterns in terms of the structure of this complex system. We report a quantitative model of the disorder and its incorporation into calculations of x-ray fiber diffraction patterns from model structures. This allows rapid calculation of the diffraction and does not involve numerical averaging over the disorder. Calculations show that the disorder modulates the Bragg reflections in diffraction patterns and introduces diffuse diffraction. The results of this analysis will allow the effects of the disorder to be included in muscle structure refinement programs, allowing more accurate structure determination from x-ray fiber diffraction data.

On twinning and microstructures in calcite and dolomite

Apparent superlattice reflections obtained in electron diffraction patterns of modulated calcite and dolomite have been ascribed to local domains of various different metastable Ca-Mg ordering schemes that are not known as macroscopic phases. We show that the type “c” reflections in diffraction patterns of supposed superstructures can in fact be produced by superposition of diffraction from the host crystal and that from domains that are in an orientation related to the host by twinning on (104). From details of the additional reflections present, we deduce that the carbonate anions are orientationally disordered in the twin nanodomains, which have the R3̅m space group of high-temperature disordered calcite. This twinning can explain the diffraction ascribed to type “γ”/”μ”/”ν” superstructures, and resolves controversies over the occurrence of these purportedly different superstructures. The relationships between composition, orientational order of the carbonates, molar volume and known macroscopic structures, and the possibility of interfacial strain reduction by static disorder in the twin domain, are discussed. We stress the importance of checking for presence of twinned nanodomains using microdiffraction before attributing an apparent superstructure modulation to local cation ordering. High-resolution imaging may not be diagnostic, since overlap of small domains and host matrix can give Moiré patterns that resemble superlattice fringes.

The Essential Light Chain N-terminal Extension Alters Force and Fiber Kinetics in Mouse Cardiac Muscle

The functional significance of the actin-binding region at the N terminus of the cardiac myosin essential light chain (ELC) remains elusive. In a previous experiment, the endogenous ventricular ELC was replaced with a protein containing a 10-amino acid deletion at positions 5-14 (ELC1vDelta5-14, referred to as 1vDelta5-14), a region that interacts with actin. 1vDelta5-14 mice showed no discernable mutant phenotype in skinned ventricular strips. However, because the myofilament lattice swells upon skinning, the mutant phenotype may have been concealed by the inability of the ELC to reach the actin-binding site. Using the same mouse model, we repeated earlier measurements and performed additional experiments on skinned strips osmotically compressed to the intact lattice spacing as determined by x-ray diffraction. 1vDelta5-14 mice exhibited decreased maximum isometric tension without a change in calcium sensitivity. The decreased force was most evident in 5-6-month-old mice compared with 13-15-month-old mice and may account for the greater ventricular wall thickness in young 1vDelta5-14 mice compared with age-matched controls. No differences were observed in unloaded shortening velocity at maximum calcium activation. However, 1vDelta5-14 mice exhibited a significant difference in the frequency at which minimum complex modulus amplitude occurred, indicating a change in cross-bridge kinetics. We hypothesize that the ELC N-terminal extension interaction with actin inhibits the reversal of the power stroke, thereby increasing isometric force. Our results strongly suggest that an interaction between residues 5-14 of the ELC N terminus and the C-terminal residues of actin enhances cardiac performance.

Electron microscopic study of the dehydration of diaspore

The thermal transformation of natural, single-crystal diaspore (D) to corundum (C) and water has been analyzed in detail by means of transmission electron microscopy (TEM). Pieces of single-crystal diaspore were partially dehydrated at temperatures ranging from 400 to 600 °C and were prepared in cross-section for TEM studies. The transformation of diaspore was studied at the faces (100) D , (010) D , and (001) D , with special attention to the microstructural details at the reaction interface. Irrespective of the transformation direction, the dehydration product is highly porous. Nanometer-sized pores form two-dimensional arrays arranged parallel to the basal plane of corundum forming lamellae that are separated by thin regions of dense corundum. The periodicity of the lamellar structure is 3.7 nm, which causes distinct satellite reflections in diffraction patterns in the case of dehydration at moderate temperatures ( During transformation, the diaspore crystal cleaves parallel to (010) D , and micro-cracks are developed parallel to (100) D and to (001) D in the corundum due to the misfit of the lattices. The entire crack system as well as the lamellar pore system play an important role in the dehydration process because they serve as effective transport paths for the water vapor. The rapid dehydration at the (010) D faces is attributed to fast-propagating cracks along [010] D . The slow transformation processes along [100] D and [001] D are discussed in terms of difficulties in crack propagation and in generating suitable crack systems, respectively.

Arithmetic properties of spectra produced by Farey hierarchies of approximants

We discuss the consequences of the hierarchical nature of series of approximants of aperiodic crystals on their diffraction patterns and spectra of elementary excitations. We show how a linear form defined on Z3 can be used to order Bragg reflections in diffraction patterns according to their amplitudes, and gaps in spectra of elementary excitations according to their widths, for all the structures in the hierarchy. Bragg peaks amplitudes and gap widths are projective functions on P(Z3), recursively defined on 2D Farey sets (generalization of Farey series).

Periodic superorder and disorder of decagonal Al[sbnd]Ni[sbnd]Co and Al[sbnd]Co[sbnd]Cu([sbnd]Si) phases

Commensurate satellite reflections in diffraction patterns of decagonal Al-Ni-Co and Al-Cu-Co phases indicate a periodic superorder on the quasiperiodic substructure. From the intensity characteristics of these satellites a box-like modulation function is derived which may be visualized in different ways including approximant phases. An interpretation by fine-scale periodic lamella microdomains matches closely high-resolution electron microscopy observations and characteristic structural features of the average structure.

Hydrogen-induced phases in AISI 310 type steel

Hydrogen-induced phases in stable austenitic stainless steels which do not undergo any transformations by cold work or low temperature cooling have been an object of several studies. As was shown in a previous study, alloying with nitrogen increases the stability of Cr18Ni16Mn10 steel with respect to hydrogen-induced [epsilon][sub H] formation and, at nitrogen contents of more than 0.5 wt. %, the [epsilon][sub H] phase is no longer formed. As nitrogen retards the HE of austenitic stainless steels, it is important to test its stabilizing effect in steels of different compositions. The present paper aims to study the formation and disappearance of hydrogen-induced phases in AISI 310 type steel during hydrogen charging and outgassing. This study concludes that alloying with nitrogen retards the formation of the phase [epsilon][sub H] that is caused by hydrogen charging; and the change of [gamma] reflections in diffraction patterns during the aging of hydrogen-charged steels does not confirm either the occurrence of a modulated structure or the existence of the [gamma][sub H] phase ([gamma]*) as a pseudo-hydride precipitated through spinodal decomposition of hydrogenated austenite.

Effect of oxygen on the stability of 1-D Lps in Cu3Pd

The variety of ordered phases exhibited by Cu-Pd alloys has led many investigators to study it both experimentally and theoretically. Recently, we have determined the Cu-rich portion of the Cu-Pd phase diagram (Fig. 1) and indicated that the region denoted as 1-d LPS actually represents a large number of single phase fields of long period superstructures (LPS) separated by narrow two phase regions in agreement with theoretical calculations. Each single phase field would correspond to an LPS with a characteristic modulation period, which can be measured from the splitting of the 001 reflection in diffraction patterns. High resolution electron microscopy (HREM) studies on Cu-Pd alloys have revealed that the antiphase boundaries (APB) associated with the LPS formed at low temperatures exhibit a sharp character, while those in LPS equilibrated at high temperatures are diffuse. In a recent theoretical study on LPS, Ceder et al. pointed out that the pair interaction potential in reciprocal space, V(k), associated with Cu-Pd alloys shows a rather deep minimum which arises as a result of flat sections on the Fermi surface. Ordering concentration waves located away from the minimum of V(k) would quickly disappear at higher temperatures because of their higher contribution to the configuration energy, thus leading to the formation of a smooth modulation profile. Consequently, in selected area diffraction patterns (SAD), only first order satellites are expected. On the other hand, at lower temperatures, where the contribution from the ordering concentration waves away from the minimum in V(k) is also significant, higher order satellites should be observable in SAD patterns.

E-beam melted icosahedral Al–Mn surface layers: Microstructures and phason defects

The microstructure of icosahedral Al(Mn) in melt-quenched surface alloys has been characterized with TEM. Single-phase alloys with 20 at. % Mn show angular misorientations (up to ∼7°) across internal boundaries radiating from the nucleation centers of grains. Lowering the Mn content results in fcc Al at grain boundaries and between branches of the icosahedral grains; the presence of Al-rich liquid around growing grains apparently eliminates the misorientations seen with 20 at. % Mn. The grains have a dense distribution of dislocation-like defects, but have sufficient orientational order to exhibit well-defined diffraction contours. Weak reflections in diffraction patterns from small areas (0.7 μm) of grains exhibit deviations from exact icosahedral symmetry positions which were analyzed to evaluate a phason strain tensor and characterize misordering defects in the quasicrystalline structure. A negative matrix element (m ≍–0.05) for a tensor axis along a twofold icosahedral axis accounts for most of the distortions in these patterns and those from other melt-quenched icosahedral Al(Mn) and Al(Mn, Si) alloys; this dominant element lowers the symmetry to D2. This axis is nearly perpendicular to the growth direction, which suggests that the misordering is due to rapid growth during the quench; however, the D2 distortion may also result from an elastic instability predicted for icosahedral phases. Distortions in Al(Cr, Ru) can be fit with a similar tensor but with m>0. Thus the phason character identified here may apply to other aluminum-transition metal icosahedral phases as well.

Analysis of a displacive superlattice in nickel arsenide

Abstract The structural analysis by convergent-beam electron diffraction is described for a weak orthorhombic superlattice observed in both mineral and synthetic samples of NiAs. The three equivalent directions for the commensurate wavevector, q = ⅓(a∗ + b∗), defined a domain structure within the hexagonal subcell. Within a single domain, the symmetry around the c axis was reduced from 6mm to 2mm. The internal structure of higher-order Laue-zone reflections in diffraction patterns recorded at 100 K was used to associate a specific phonon eigenvector or condensed soft mode with the atomic displacement pattern.

Characterization of the Ni/NiO, Interface Region in Oxidized High Purity Nickel by Transmission Electron Microscopy

ABSTRACTNi/NiO interface structures were investigated using TEM, and the observed structures were compared with current heterophase interface models. Relative magnitudes of Ni/NiO interfacial energies were obtained from measurements of dihedral angles at triple grain junctions between Ni and NiO grains. Extra reflections in diffraction patterns from oxide grains adjacent to the Ni/NiO interface were compared with kinematical structure factor calculations for several proposed structures.

Structural distortions in the low-temperature phase ofNbO2

At about 810\ifmmode^\circ\else\textdegree\fi{}C niobium dioxide undergoes an apparently second-order transition between a high-temperature (HT) rutile structure and a low-temperature (LT) phase whose structure is slightly distorted from rutile. The primary order parameters of the transition are the amplitudes of sinusoidal displacement waves with wave vectors ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{p}\ensuremath{\equiv}(\frac{1}{4},\ifmmode\pm\else\textpm\fi{}\frac{1}{4},\frac{1}{2})$. These waves give rise to a series of superlattice reflections in diffraction patterns obtained with the LT phase. Additional superlattice reflections occur at wave vectors which are related by the rutile point group to ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{m}\ensuremath{\equiv}(\frac{1}{2},\frac{1}{2},0)$ and ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{x}\ensuremath{\equiv}(\frac{1}{2},0,0)$. The latter reflections may be diffraction harmonics or they may represent the presence of secondary distortions in the LT phase. A detailed neutron diffraction study at room temperature has allowed the LT structure to be determined and has demonstrated that both primary and secondary distortions are required for an adequate description of this phase. The general symmetry properties of the primary distortion are discussed and this distortion is discribed in terms of a simple model involving dimerization of $c$-axis chains of Nb atoms.

On the Origin of Stacking Faults in Pollycrystalline Thin Evaporated Films of Some Face-Centered Cubic Metals

In an earlier publication, extra reflections in the transmission electron diffraction patterns of pollycrystalline thin films of some of the face-centered cubic metals have been attributed to stacking faults in these films, (K. Bahadur and P. V. Sastry; Proc. Phys. Soc., London, 78 , (1961) 594). In this paper, an attempt has been made to determine the mechanism whereby the stacking faults get introduced in these films. The precise conditions of evaporation, which give rise to extra reflections in the diffraction patterns of these films, have been studied. It has been shown that the stacking faults arise as a result of misfits occurring at the interface where the nuclei of condensation join together.

Forbidden Reflections in Electron Diffraction Patterns of Evaporated Tin Films

Previous work has shown the existence of some structurally forbidden reflections in the electron diffraction patterns of (100) oriented polycrystalline films. These films were deposited under ultra-high vacuum conditions. A continuation of this work has resulted in the observation of these and other forbidden reflections in the diffraction patterns of two other kinds of films: (001) oriented single crystals containing deformation twins and (100) oriented single crystals. The existence of these strong reflections is probably due to strong dynamic interactions between the electron beam and the crystal lattice.