Perfect Single Crystals Research Articles

Dislocation nucleation induced by a shock wave in a perfect crystal: Molecular dynamics simulations and elastic calculations

We study the nucleation mechanism of the defects responsible for plastic flow in a $〈100〉$ fcc perfect single crystal submitted to a shock wave. In the large-scale nonequilibrium molecular dynamics simulation, small dislocation loops are created from thermal fluctuations just behind the shock front, in a narrow region of a few lattice parameters width. Their critical size is measured. The activation energy for the formation of an edge dipole, under high pressure, is computed within the Peierls framework. The elastic constants and the generalized stacking fault energy are computed from the interatomic potential. This model enables a qualitative discussion of the influence of material parameters such as intrinsic and unstable stacking faults versus elastic energy release.

Microstructure characterisation of ALD-grown epitaxial SnO 2 thin films

The microstructures of epitaxial SnO 2 (rutile) thin films deposited by atomic layer deposition on α-Al 2O 3 (0 1 2) substrates at 600°C using either SnCl 4 or SnI 4 as tin precursor have been investigated by X-ray diffraction and transmission electron microscopy. It is shown that the film/substrate interface is flat without any visible additional phase, while the film structure and surface morphology depend on the metal precursor. Typical of the SnCl 4-process films is a rough surface and a high density of defects, including (0 1 1) and (1 0 1) twins and anti-phase boundaries (APBs). In contrast, the SnI 4-process films with their uniform thickness, flat surface and low density of APBs approach the perfect single crystal. Irrespective of the processing, all films have (1 0 1) texture, parallel to the substrate surface, with the epitaxial relationships [0 1 0] SnO 2 ∥ [1 0 0] α-Al 2 O 3 and [1 0 1 ̄ ] SnO 2 ∥[ 1 ̄ 2 ̄ 1] α-Al 2 O 3 .

Habit modification of nearly perfect single crystals of potassium dihydrogen phosphate (KDP) by trivalent manganese ions studied using synchrotron radiation X-ray multiple diffraction in Renninger scanning mode

The X-ray multiple diffraction technique using synchrotron radiation is applied in the preliminary study of the habit modification of KDP samples as induced by incorporation of the trivalent transition metal cation Mn3+. High-resolution Renninger scans of pure and doped KDP were carried out using 400 as the primary reflection, echoing the fact that these impurity species were segregated in the {100} growth sector. The analysis of the Renninger scans of the doped KDP crystals is consistent with the presence of the impurity species chemically bound within the KDP crystal structure, as confirmed through the suppression of the huge observed peak asymmetry, characteristic of perfect crystals. In addition, an extra Renninger-scan peak measured for the doped material is indicative of the impurity atoms occupying interstitial crystallographic sites in the lattice, a result consistent with X-ray standing-wave measurements. Renninger-scan reflection curve widths and lattice parameter measurements reveal the decrease in crystalline perfection (increased mosaic spread, η) and lattice contraction ofca0.4% in theaandclattice directions at the surface plane for the Mn3+-doped KDP samples in comparison with the undoped crystals.

Simulation of mechanical damping in nanostructures

Structure and mechanical damping of nanostructures have been investigated by means of molecular dynamics simulations. Embedded atom method potential was adopted in the simulation. Four copper model samples of nanoparticle were prepared. These were perfect single crystal, single crystal with interstitials, polycrystalline state, and amorphous state. Each system consisted of 5000 atoms. The mechanical damping of these particles was investigated by monitoring the elastic and thermal motion of atoms after the initial deformation of tetragonal and of dilatational strains. The mean elastic strain was calculated to evaluate the elastic damping. The vibration continued for a long time in a single crystal sample, and it quickly damped in polycrystalline and amorphous samples. The mechanical damping was increased by introduction of interstitials. The process of the thermalization was also investigated through the time variation of the velocity distributions. The distribution attained a thermal equilibrium in a short time for the polycrystalline and amorphous samples.

A new type of quantum wells: stacking faults in silicon carbide

We report on a new type of quantum wells with the width as thin as 10Å, which are composed of SiC only, and consequently have ideal interfaces. These quantum wells are actually stacking faults in SiC. Certain types of stacking faults in SiC polytypes create small 3C-like regions, where the stacking sequences along the c-axis become locally cubic in the hexagonal host crystals. Since the conduction band offsets between the cubic and hexagonal polytypes are very large with the conduction band minima of 3C–SiC lower than that of the other polytypes, such thin 3C inclusions can introduce locally lower conduction bands, thus acting as quantum films perpendicular to the c-axis. One mechanism for the occurrence of stacking faults in the perfect SiC single crystals is the motion of partial dislocations in the basal planes, the partial dislocations leaving behind stacking fault regions.

Charge transfer in CdTe at 200 and 300 K

Using a perfect single crystal sample of CdTe grown using PVT method, the electronic charge transfer in the II–VI compound semiconductor CdTe at 200 and 300 K has been evaluated using two different approaches: (1) by solving a quadratic equation involving the observed structure factors of h+ k+ l=4 n+2 type reflections; and (2) by a graphical approach in which the observed and calculated atomic form factors are extrapolated to sin θ/λ=0 , to determine the transferred charge. Precise X-ray structure factors collected using MoK α radiation have been used for the analysis. The results obtained are reasonable and clearly indicate the ionicity by which charge is transferred from Cd to Te in CdTe.

Condon domains in aluminum and lead

Formation of Condon domains was observed in highly perfect single crystals of Al and Pb by the periodic, sharp broadening of the spectral linewidth. Unlike in Be, domains in these metals can arise only via strong anharmonicity of the quantum oscillations, at very low temperatures.

High‐Energy X‐Rays: A tool for Advanced Bulk Investigations in Materials Science and Physics

High‐energy X‐rays between 30 keV and 1MeV, such as provided by modern synchrotron radiation sources as the ESRF and HASYLAB, bear the advantage of high penetration into most materials. Even heavy element compositions can be accessed in their volume. The range of applications is huge and spreads from nuclear spectroscopy to the characterization of metal extrusion under industrial conditions. This article compiles an overview over the most common instrumental diffraction techniques.Modern two‐dimensional detectors are used to obtain rapid overviews in reciprocal space. For example, diffuse scattering investigations benefit from the very flat Ewald sphere as compared to low energies, which allow mapping of several Brillouin zones within one single shot. Diffraction profiles from liquids or amorphous materials can be recorded easily. For materials science purposes, whole sets of Debye–Scherrer rings are registered onto the detector, their diameters and eccentricities or their intensity distribution along the rings relating to anisotropic strain or texture measurements, respectively. At this point we stress the resolution of this technique which has to be carefully taken into account when working on a second generation synchrotron source.Energy‐dispersive studies of local residual strain can be studied by a dedicated three‐circle diffractometer which allows accurately to adjust the scattering angle from a defined gauge volume.Triple axis diffractometry and reciprocal space mapping is introduced and can be employed for highest resolution purposes on single crystal characterization, even under heavy and dense sample environments. Thus, the perfection of single crystals can be mapped and strain fields and superstructures as introduced by the modulation from ultrasonic waves into crystals or epitaxially grown Si/Ge layers can be investigated in detail. Phase transitions as magnetic ordering can be studied directly or through its coupling to the crystal lattice. Time resolved studies are performed stroboscopically from a sub‐nanosecond to a second time scale.The combination of these techniques is a strong issue for the construction and development of future instruments.

Effect of Metastable Phases on the Structural Perfection of Single Crystals of Stable Bismuth Oxide Compounds

It is shown that a detailed understanding of metastable phase relations is crucial for the ability to grow perfect Bi4Si3O12 and Bi4Ge3O12 single crystals by the Czochralski process. The high stability of metastable states in melts of bismuth-oxide-containing systems has a marked effect on the structural perfection of the growing crystals of stable compounds. Crystals containing inclusions of metastable phases are grown from overheated melts.

Microstructure evolution of AlN films deposited under various pressures by RF reactive sputtering

Wurtzite AlN (2H-AlN) films were deposited on p-type Si(100) by RF planar magnetron reactive sputtering under various pressures at a relatively low temperature (350 °C). X-Ray diffraction (XRD) and selected area electron diffraction (SAD) were used to study the microstructural features of the deposited films. XRD results showed that with the decrease in sputtering gas pressure, the preferred orientation of AlN films changed from (100) to (002). SAD results confirmed that a strong (100) texture existed in the films with (100) preferred orientation. Polycrystalline diffraction rings were recorded for the film where preferred orientation was not obvious. The AlN film deposited at 2 mtorr exhibited improved grain orientation along the c-axis. A further decrease in sputtering pressure to 1.4 mtorr produced an AlN film with two-domain structure instead of a perfect single crystal.

InGaAs zone growth single crystal with convex solid–liquid interface toward the melt

We have developed a zone growth method of producing an In x Ga 1− x As single crystal with uniform InAs composition in the direction of the growth. The seed used for this method of zone growth is an InGaAs single crystal grown on a GaAs seed using the vertical gradient freeze (VGF) method. The source for the zone growth method is an InGaAs ternary alloy produced by quenching an InGaAs melt with an InAs composition of 0.3. The convex solid–liquid interface toward the melt is necessary for growing a long InGaAs zone single crystal. We examined ways to control the shape of the solid–liquid interface for the InGaAs zone crystal. We found that: (1) growing an InGaAs crystal in the furnace region, where the temperature gradient in the growth direction changes abruptly, is effective in forming the convex solid–liquid interface toward the melt and (2) lengthening the heat sink attached to a seed increases the efficiency of heat discharge through the crystal and the solid–liquid interface becomes convex. The InGaAs zone crystal having a uniform InAs composition region of 0.3 was a perfect single crystal and was 17 mm long, which is more than twice the length of zone single crystals grown previously. The orientation of the crystal growth was (0 0 1), and the crystal diameter was 15 mm.

Morphology and structure of nylon‐2,14 single crystals from dilute solution

AbstractA perfect single crystal of nylon‐2,14 was prepared from 0.02% (w/v) 1,4‐butanediol solution by a “self‐seeding” technique and isothermal crystallization at 120 and 145 °C. The morphology and structure features were examined by transmission electron microscopy with both image and diffraction modes, atomic force microscopy, and wide‐angle X‐ray diffraction (WAXD). The nylon‐2,14 single crystal grown from 1,4‐butanediol at 145 °C inhabited a lathlike shape with a lamellar thickness of about 9 nm. Electron diffraction and WAXD data indicated that nylon‐2,14 crystallized in a triclinic system with lattice dimensions a = 0.49 nm, b = 0.51 nm, c = 2.23 nm, α = 60.4°, β = 77°, and γ = 59°. The crystal structure is different from that of nylon‐6,6 but similar to that of other members of nylon‐2Y. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1913–1918, 2002

Molecular-dynamics study of martensitic transformations in sintered FeNi nanoparticles

Nanocrystalline metals with grain sizes in the nanometer range often have interesting properties which differ from usual polycrystalline samples or single crystals. Here we report on the first molecular-dynamics simulations of martensitic transformations—i.e. structural transformations from fcc to bcc—in sintered FeNi nanoparticles. The atomic interactions were described by an embedded-atom method (EAM) potential specially designed to model the FeNi system. Simulations were carried out by applying a constant pressure and temperature ensemble (NPT ensemble) to 32 nanoparticles each containing slightly more than 1000 atoms. The nano-particles initially were placed with random crystallographic orientation on an fcc lattice such that the particles attract each other. After relaxation for 0.58 ns at a temperature of 800 K and a pressure of 0 GPa a polycrystalline sample with a density just about two percent larger than that of a perfect single crystal was achieved. Subsequent cooling towards low temperatures allows the study of the temperature-induced martensitic transformation at pre-existing defects.

Nonionic Block Copolymer Synthesis of Large-Pore Cubic Mesoporous Single Crystals by Use of Inorganic Salts

Large-pore cubic (Im3m) mesoporous silica single crystals have been synthesized by using commercial nonionic block copolymers as templates and inorganic salts as additives. These single crystals possess exclusively uniform rhombdodecahedron shapes ( approximately 1 mum) with approximately 100% yield. The mesopore lattice array in each crystal face is solved by TEM, further confirming that these crystals are perfect single crystals.

Development of a horizontally and vertically focused neutron monochromator using stacked elastically bent Si single crystals

A horizontally and vertically focused monochromator has been developed for the 4-axis neutron diffractometer applied for a single crystal structure analysis. Silicon perfect single crystals are bent elastically in order to focus monochromatic neutrons horizontally. The monochromatic beam can also be focused vertically by stacking the horizontally bent crystals. Tilting motion of each stacked bent crystal is controlled independently by stepping pulse motors for optimizing and reproducing perfectly the vertical focusing at the sample position. The intensity of neutrons with a 1.57 Å wavelength monochromatized by the new monochromator increases remarkably, and is comparable to that of pyrolytic graphite monochromator with a 2.44 Å wavelength. The high tunability of the doubly focusing system established in the present study can be adopted easily when obtaining the shorter wavelength of neutrons.

Isomerism in the Terbium(III) Trifluoroacetate Trihydrate Dimer

Precision X‐ray diffraction analysis of a perfect single crystal showed that the molecule of terbium(III) trifluoroacetate trihydrate is a combination of two isomers (monomeric form), Tb(CF3COO)3(H2O)3 and Tb(CF3COO)2(CF3COOH)(H2O)2(OH) with a statistical weight of 0.5. Based on precision X‐ray diffraction analysis, the statistical composition of the compound resulting from neutralization of terbium(III) hydroxide with trifluoroacetic acid is Tb(CF3COO)2.5(CF3COOH)0.5(H2O)2.5(OH)0.5. The presence of the CF3COOH molecule and the OH- group is also confirmed by IR and 1H NMR spectra, respectively. The presence of 18 rather than the anticipated nine Stark components of the 5D4→7F0 transition of the Tb3+ ion can be explained in terms of both a static and a dynamic model. In the static case, the compound can be represented as a combination of equal fractions of two isomers; hence the doubled number of Stark components. In this case, the molecule of the dimer remains statistically centrosymmetric. In the dynamic version, migration of a proton across the dimer from one of the water molecules to the nearest monodentate anion CF3COO- and back leads to an electron density redistribution in the vicinity of each Tb3+ ion within the dimer, the crystallographic equivalence of their positions being preserved. However, X‐ray structure analysis cannot distinguish between these two cases. In any case, the observed isomerism in terbium(III) trifluoroacetate trihydrate removes discrepancies between the data of X‐ray structural and luminescent analyses.

Controllable variation of the intensity of diffracted X-ray beams and double modulation of such beams for transmission and reception of audio information

This paper discusses research devoted to the study of the intensity change of diffracted and transmitted X-ray beams under the action of a temperature gradient or acoustic oscillations, as well as the techniques for `controllable complete reflection' of transmitted X-radiation in the direction of the diffracted beam. Also discussed are results on the double modulation of X-ray beam intensity by acoustic oscillations. A scheme is presented for a practical device for transmission and reception of audio information, in particular speech, with the help of X-ray beams diffracted by perfect single crystals. Finally, there are suggestions for applications of modulated beams in the solution of various problems in physics.

A random distribution (spectrum) of mechanical relaxation/retardation rate constants and its influence on tensile strength of imperfect (real, visco-elastic) polymer fibers in constant strain-rate extension

A random distribution of relaxation/retardation rate constants is derived and incorporated into the thermodynamic theory of strength of imperfect (real) polymer fibers, which is used to analyze empirical polyethylene (fiber) strength data. The results, weighted equally with previous results, give for the perfect polyethylene fiber — the only unique reference state — at 25°C: σc(strength)≈7.5GPa,Kc(modulus)≈325GPa,εc(strain)≈0.023, and Wc(failurework)≈0.087GPa. These numbers represent the best currently available for the characterization constants of a perfect fibrilliform single crystal of finite molecular weight polyethylene. The widths of uncertainty are ca. 15%. Also, these numbers are exactly those calculated with the thermodynamic theory of failure based on stress-induced fusion with a constant heat of fusion. Such figures indicate the general strength of the fusion theory.

Strain-induced structural phase transition of a Ni lattice through dissolving Ta solute atoms

The structural phase transition of a single-crystal Ni lattice upon dissolving Ta solute atoms is investigated by means of molecular-dynamics simulations with a realistic n-body Ni-Ta potential. It is found that when the solute concentration is within 9--19 at. % of Ta, the accumulated strain results in a martensitic phase transition, i.e., face-centered-cubic (fcc) Ni transforms into a face-centered-orthorhombic-(fco) like structure through shearing, and that when the solute concentration is over 21 at. % of Ta, the Ni lattice collapses and turns into an amorphous state. Comparatively, for the case of an initial hcp Ni lattice, the same martensitic and amorphization transitions also take place. The former hcp-fco transition, however, is mainly through atomic rearrangement as well as readjustment of lattice parameters, and the resultant state is almost perfect single crystal with no shearing bands. Besides, the above structural transitions are frequently in association with a dramatic softening in shear elastic moduli.

Emission Mössbauer spectroscopy inTiO2single crystal

An almost perfect single crystal of ${\mathrm{TiO}}_{2}$ was doped by about 50 ppm of ${}^{57}\mathrm{Co}.$ M\"ossbauer spectra were measured versus sample orientation, temperature, and thermal history. It was found that Co occupies both substitutional and interstitial sites being a fast diffuser, while located interstitially. It decays to ${\mathrm{Fe}}^{3+}(S=\frac{5}{2})$ in unperturbed lattice sites, ${\mathrm{Fe}}^{2+}(S=2)$ in lattice sites associated with the ${\mathrm{VO}}^{2\ensuremath{-}}$ vacancy, ${\mathrm{Fe}}^{2+}(S=0)$ in interstitial sites having adjacent ${\mathrm{VO}}^{2\ensuremath{-}},$ and finally to ${\mathrm{Fe}}^{1+}(S=\frac{3}{2})$ in unperturbed interstitial sites. Thermal history of the sample could be erased by heating to about 750 K. Substitutional iron following ${}^{57}\mathrm{Co}$ decay could be observed solely in the host lattice at elevated temperatures. High-temperature data indicate two charge states of iron, i.e., ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}.$ ${\mathrm{Fe}}^{2+}$ with $S=2$ exists in the vicinity of defects and converts gradually to ${\mathrm{Fe}}^{2+}(S=0)$ with the increasing temperature, while ${\mathrm{Fe}}^{3+}(S=\frac{5}{2})$ resides in the unperturbed lattice sites. A host matrix becomes more covalent at very high temperatures as well as slightly anharmonic. No significant diffusivity of the substitutional iron could be seen. The total area under the spectrum follows unusual pattern due to the gradual disappearance of the signal coming from iron located interstitially, i.e., a transfer of iron atoms into fast diffusing interstitials with increasing temperature occurs. All processes were observed to be reversible upon heating/cooling.