Efficacious symmetry-adapted atomic displacement method for lattice dynamical studies

The crystallinity effects on scaling properties of photoinduced modes in crystalline silver nanoprisms with C3v symmetry are studied using a realistic atomistic model and group theory. Among all vibrational modes, photoinduced modes can be identified as those vibrational modes which possess larger in-phase radial atomic displacement and can be projected out by the projected density of states method. We found that the properties of vibrations in silver nanoprisms strongly depend on the particle's aspect ratio (bisector length over thickness). By considering crystallinity of silver nanoprisms, the dominant modes with the in-plane oscillation become several closely spaced modes, and become obvious for nanoprisms with a smaller aspect ratio. The oscillation spectra show that the dominant planar modes are insensitive to thickness change. On the contrary, the atomic displacements show significantly different patterns for nanoprisms of different thicknesses. We also found that, for nanoprisms with same aspect ratio that is larger than 4, the vibrational properties of dominant modes exhibit scaling similarity. By using a simple linear transformation, the vibration frequencies for large-sized nanoprisms of aspect ratio 6 can be obtained by a corresponding scaling factor. The calculated results are in good agreement with experimental data.

The amorphization of silicon due to atomic displacement during ion implantation has been simulated. A model based on Monte Carlo calculation reproduces very well the depth profile of atomic mixing and displacement length of host silicon atoms reported by previous experiments. The critical displacement in the depth direction for amorphization has been determined to be 5 Å. This average threshold value is shown to be universal for identification of amorphous regions in silicon for a wide range of implantation conditions involving different doping species, acceleration energies, and doses.

Tuning of CeO2 buffer layers for coated superconductors through doping

Superconducting copper fluoride ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{1.3}{\mathrm{Cu}(\mathrm{O},\mathrm{F})}_{4+\ensuremath{\delta}}$ with ${T}_{c}=55$ K had been synthesized via the solid phase fluorination of ${\mathrm{La}}_{0.7}$${\mathrm{Sr}}_{1.3}$Cu${\mathrm{O}}_{3.25}$ with Cu${\mathrm{F}}_{2}$ at relatively low temperature. The fluoride is a derivative of its precursor, but its lattice constant a is expanded from 3.7547(5) to 3.9368(7) \AA{}, while its lattice parameter $c$ is expanded from 12.896(3) to 13.075(4) \AA{}. The obvious difference in lattice parameters and crystal structure seems to distinguish this compound from the superconducting ${\mathrm{Sr}}_{2}\mathrm{Cu}{\mathrm{O}}_{2}{\mathrm{F}}_{4+\ensuremath{\delta}}$ reported in a recent study. It gives strong evidence for superconductivity with reduced Cu${\mathrm{O}}_{2}$ sheets while in the presence of apical oxygen or fluorine.

The positions of Higher Order Laue Zone (HOLZ) Lines, visible in the bright field discs of Convergent Beam Electron Diffraction (CBED) patterns, are frequently used to determine the lattice parameters or strain present in phases observed in the Analytical Electron Microscope (AEM). The technique usually relies upon the use of a “standard” sample of known lattice parameter that has the same crystal structure as the unknown phase and is of similar average atomic number. The relative movement of particular HOLZ lines in the unknown phase with respect to the standard are used to calculate the difference in lattice parameters between the standard and the unknown. The thickness of the samples is not taken into consideration. This paper describes the results of both computer simulations and cold stage experiments in the AEM which lead us to conclude that the HOLZ lines are not independent of thickness, particularly in the presence of highly dynamical zero layer diffraction effects.

Force constants and dipole moment derivatives have been computed for the molecules CH3 X(X=F, Cl, Br, I) using MO wavefunctions with pseudo-potentials for the interaction between the inner shell and valence electrons. The values obtained at the SCF approximation level from a set of gaussian valence orbitals contracted in double-zeta form and enriched with polarization functions compare well with experimental assignments of force and interaction constants, as well as integrated intensities from infrared data. The transferability of atomic force fields and polar tensors (second derivatives of total energies and first derivatives of dipole moments with respect to atomic displacements in cartesian coordinates) is discussed.

Synchrotron x-radiation in a double-crystal geometry has been used for topography and rocking-curve measurements to study imperfections in single-crystal HP/HT synthetic diamonds grown from (111) seeds by the reconstitution technique. The influence that clusters of small opaque particles (sometimes called `pin-point-like inclusions') have upon lattice parameter differences, as well as upon relative lattice tilts within each sample, was of particular interest in this study. Rocking curves measured with a large beam diameter (illuminating entire specimens) gave widths (FWHM) in the range 5 - 30 seconds of arc in a non-dispersive Bragg - Bragg diffraction geometry in which good diamond crystals give and for which the calculated theoretical minimum width is under . Rocking curves were also recorded for small regions of each crystal, using a translatable pin-hole, and these showed that some diamonds had relatively narrow rocking curves in selected areas (of diameter). Epochs of crystal growth were recorded in series of x-ray topographs, taken at intervals of 2.5 seconds of arc. Such series were taken for all four principal azimuths (, 90, 180 and ) of the incident beam by rotating the sample about the normal to the Bragg plane. This procedure yielded important information about lattice parameter distribution and lattice tilt across entire samples. For the samples heavily populated with pin-point-like inclusions it was concluded that there is a possible slight variation in lattice parameter values in some regions of the crystal but most of it falls below the adopted angular step resolution, although better accuracy can be achieved. Relative tilt measurements revealed that there are large misorientations of those parts of the crystal which are close to external outcrops of {111} boundaries. In this case of (111) oriented diamond samples, three separate regions with different tilts can be attributed to the three parts of the crystal close to optically visible {111} growth-sector boundaries.

Small Pb inclusions in Al have been studied by a number of investigators because the alloy system offers the possibility of observing the processes of melting and solidification directly. Both solids are fee, and the mutual solubility of solid Pb and Al is negligible. Despite a large difference in lattice parameter, it has been found that inclusions follow a parallel-cube orientation relationship and their equilibrium shape is a cuboctahedron, bounded by ﹛111﹜ and ﹛100﹜ facets [1]. Following Herring, the relative extent of the two types of facet directly indicates a ratio of interfacial energies γl00/γ111- However, recent investigations have shown that for inclusions in the range of a few to a few tens of nanometers the equilibrium shape becomes a function of size [2]. In the present work, this size dependence of the equilibrium shape has been investigated further. Al alloys with about lat.% Pb were prepared by rapid solidification or by ion implantation, and equilibrated by annealing at about 300°C.

The behaviour of misfit dislocations introduced to accommodate differences in lattice parameter at the interface between two epitaxially oriented crystals of different materials is studied as a function of the degree of interdiffusion of the constituents. The forces considered to be acting on the dislocations are the elastic interaction force between misfit dislocations, the homogeneous force arising from the difference in atomic volume of the constituents, and the osmotic force produced by the net vacancy flux accompanying interdiffusion. In the elasticity calculations, image forces and end effects, including buckling terms, are analysed. The results are compared with prior treatments of related problems, all of which have involved different boundary conditions, and none of which has included considerations if the osmotic force together with the other forces.

Strained-layer semiconductor films offer tremendous potential with regards to optoelectronic applications for high speed communications, mobile communications, sensing, and novel logic devices. It is an unfortunate reality that many of the possible film/substrate combinations that could be exploited technologically are off limits because of large differences in lattice parameters, chemical compatibilities, or thermal expansion rates. These mechanical, chemical, and thermal incompatibilities manifest themselves primarily in terms of lattice defects such as dislocations and antiphase boundaries, and in some cases through enhanced surface roughness. An additional limitation, from a production point of view, is money. Device manufacturers as a rule want the cheapest substrate possible. Freeing the heteroepitaxial world of the bonds of (near) lattice matching would vastly expand the types of working devices that could be grown. As a result, a great deal of effort has been expended finding schemes to integrate dissimilar film/substrate materials while preserving the perfection of the film layer. One such scheme receiving significant attention lately is the so-called compliant substrate approach.

High purity β—SiC films have been epitaxially grown on Si (100) and off-axis Si (100) substrates. Conventional chemical vapor deposition at 1633K and 1 atm using SiH4 and C2H4 reactants in an H2 carrier gas was employed for this growth. Large differences in lattice parameter and thermal coefficients of expansion between the film and the substrate were partially compensated for by growing aninitial “conversion layer.” Plan view and cross—sectional transmission electron microscopy were utilized to study the conversion layer and the defects present in the films. A high density of stacking faults and dislocations were present in allthe films grown on Si, however, antiphase boundaries were eliminated when certain off—axis Si (100) substrates were employed. The effects of misorientation angle and substrate preannealing were investigated. The electrical properties of the β-SiC films were also studied using differential capacitance—voltage and Hall effect measurements as well as the I—V characteristics of Au—β—SiC Schottky diodes. It was found that the elimination of antiphase boundaries reduced leakage current but did not significantly effect carrier concentration or Hall electron mobility. Finally, the I—V characteristics of a Metal—Semiconductor—Field—Effect—Transistor (MESFET) have been evaluated for a β—SiC filmon an on—axis Si (100) substrate.

Deposition of silver on NaCl, KCl, and LiF with 20 Å · s−1 in vacuum (5 · 10−6 torr) and electronographical determination of changes of lattice constant. Precision of measurement better than ± 0.004 Å (reflections {400} and {220}). Thickness of films from 50 to 500 Å with intervals of 10 Å. The lattice parameter of films till to 115 Å in thickness did'nt differ from the parameter of bulk silver (4.086 Å). At about 175 Å the parameter reached minimum (about 4.00 Å) and took again the parameter of bulk material with the film thickness of 215 Å. With polycrystals no changes of lattice parameters occurred. – The phenomenon is connected to growth mechanism, two‐dimensional and three‐dimensional nucleation and to the differences in lattice parameters of substrate and deposit.

Heterojunctions GaAs–AlxGa1−xAs involved in the elaboration of IR laser diodes have been studied. The difference in lattice parameter between the GaAs substrate and the aluminum-substituted epitaxic layer AlxGa1−xAs has been measured accurately on a double-crystal spectrometer for a series of compositions. These data coupled with radius of curvature determination have permitted calculation of the stress in the layer and the bulk lattice parameter of AlxGa1−xAs. characterization of the defects introduced during the liquid-phase epitaxy has been performed by X-ray topography.

Magnetic investigations of organic conductors, β′-type [Pd(dmit)2]2, have been performed by Electron Paramagnetic Resonance (EPR) measurements. We found that most of them except one compound underwent antiferromagnetic transitions. Although they are isostructural with little differences in lattice parameters, their spin–spin correlations and antiferromagnetic transition temperatures show strong counter ion dependence. The EPR g-values of Pd(dmit)2 cannot be explained within the framework of isolated radical description which is a good approximation for conventional organic conductors. The electronic structures of a series of molecular conductors based on Pd(dmit)2 at ambient pressure are discussed from microscopic points of view.

Determination of lattice parameter and elastic properties of porous silicon by X-ray diffraction