Use Of Single Crystals Research Articles

High-resolution studies of deuterium by time-domain zero-field NQR

Time-domain zero-field NQR methods are used to determine quadrupole coupling constants and asymmetry parameters for a variety of polycrystalline solids. Results for aliphatic and aromatic groups are reported. Several cases are presented in which sites with small differences in quadrupole coupling constants are resolved, and very small (≤0.1) asymmetry parameters are measured. Structure in the zero-field spectrum of diethylterephthalate due to dipolar couplings between inequivalent methylene deuterons is discussed; computer simulations of the experimental spectrum yield a reasonable assignment of the electric field gradient tensor orientations without the use of single crystals.

Magnetization of ferromagnetic uranium monoselenide single crystals

Magnetization measurements on single crystal USe ( T c ≈ 178 K) indicate large anisotropy, remanence and coercivity. Isofield magnetization curves measured in low fields after first cooling the crystal from above T c in zero field to low temperature, show a peak, as well as jumps upon subsequent heating in a field.

Solid-state reactivity of crystalline hydroquinones with quinone vapour; crystal structures of 2,5-dimethylhydroquinone and 2,3,5,6-tetramethylhydroquinone

Use of single crystals of 2,5-dimethylhydroquinone and 2,3,5,6-tetramethylhydroquinone in studies of their reactions with 1,4-benzoquinone vapour has been found to give information not obtainable from previous studies of reactions of powders. Structures of the two crystalline hydroquinones were determined by X-ray analysis. Crystals of the dimethylhydroquinone are orthorhombic, a= 13.256(2), b= 4.5609(8), c= 11.977(2)Å, Z= 4, space group Pca21; and the structure has been refined to an R factor of 0.0335 on 704 non-zero reflections. Molecules have an anti-arrangement of hydroxy groups and form hydrogen-bonded chains with a motif like that of γ-hydroquinone. Slight differences in the geometry of the hydrogen bonding at the two ends of the molecule are responsible for the polar nature of the structure. Results of both the test for second harmonic generation and a pyroelectric test for non-centrosymmetry are positive with these crystals. The pyroelectric test applied to crystals in which twinning is known to occur shows clear evidence of such twinning and might be useful in certain other cases as a test for twinning.

Preparation and characterization of polymer single crystals for use as adsorbent

Single crystals of polyoxymethylene are flat, homogeneous, and essentially uncharged and offer a large specific surface area. Since they can be prepared reproducibly and in large amounts, they constitute a suitable model substrate in systematic adsorption studies. The crystallization procedure is discussed in some detail. The thickness of the crystals is obtained from EM and SAXS measurements, the results being in excellent mutual agreement. Combining this thickness with the crystal density the geometrical surface area is found to be 150 m 2/g. This is compared with the surface area obtained by BET analysis of nitrogen adsorption (30 m 2/g) and with the surface area that follows from adsorption of polyoxyethylated nonyl phenols from aqueous solution (60 m 2/g). The discrepancy in the results is explained in terms of different degrees of aggregation of POM crystals in the dry state and in suspension. Finally, some preliminary results of albumin and sodium polystyrene sulfonate adsorption are discussed.

Use of single SiTe_2 crystals with a layered structure in optical filter design

A method of constructing an interference filter by combining samples of a layered-structure compound in series is proposed. The use of single crystals of layered-structure compounds has the advantage of perfect crystal surfaces and low absorption coefficients. The possibility of designing an optical filter with predetermined characteristics in any wavelength of the near-infrared region is emphasized. Such a filter shows a relatively high transmittance maximum at the desired wavelength, which depends on the thickness of the samples used, and a controlled half-width. Infrared filters using the layered-structure semiconductor SiTe2 are described.

X-ray fluorescence analysis in the nanogram region with a total reflected and a Bragg polarized primary beam

Improvements of the detection limit in photon induced X-ray fluorescence analysis are obtained by the reduction of the radiation background at constant excitation conditions. Two methods are presented: 1) Bragg-reflection at 2 ν=90° results in a beam linearly polarized and monchromatic. Positioning the detector at an angle 90° to the direction of the polarized beam suppresses the production of scattered photons from sample and sample carrier into the detector. The use of single crystals instead of amorphous scattering materials increases the intensity of the polarized beam, in some cases up to a factor 104. Detection limits and actually used crystal materials and lattice planes for Braggreflection are given in tables. Samples may be of either form, liquid or solid, no special sample preparation is required. 2) The application of X-ray total reflection on the polished and plane surface of a reflector which serves as the sample carrier reduces the radiation background. In the case of X-ray total reflection there is scarcely a penetration of the X-rays into the reflector material. The penetration depth of some 100 A in the reflector-substrate can be compared to an extreme thin foil of same thickness. Therefore the reflector-substrate combines its mechanical strenght, its compact and constant geometric form, its chemical resistance and the possibility of easy handling-with the low background properties of ultra-thin foils. Samples should be liquids where the liquid part of the matrix is evaporated and the remaining trace elements are investigated. Both method are capable of detecting elements in the nanogram region.

Satellite NMR in Cu doped with transition impurities

My students and post-docs have made measurements of the conduction electron spin magnetization density, Mσ(R↘) near iron group atoms (Sc through Ni) in Cu. Mσ(R↘) produces an additional effective local field which shifts the NMR frequency of nearby shells of Cu nuclei relative to Cu nuclei far from all impurities. Resonances of nearby shells appear as weak satellites to the strong resonance of distant Cu nuclei (the main line). Mσ(R↘) at a given site is proportional through known constants to the splitting of the satellite from the main line. We have identified the shell giving rise to the satellite in many cases by use of single crystals. We find theoretically that to a good approximation Mσ(R↘) is proportional to the spin susceptibility χs of the impurity. For CuCr we find a large temperature independent χs, in contrast to CuMn and CuFe. The results lead to a picture of the electronic structure of Cr, Mn, and Fe along the lines of Schrieffer and Hirst as ions with integral numbers of d-electrons, possessing crystal field and spin-orbit couplings.

Proton spin-lattice relaxation experiments on S= Heisenberg antiferromagnetic linear chains

Abstract The dynamical behaviour of the S = 1 2 Heisenberg antiferromagnetic linear-chain system in single crystals CuSeO 4 .5H 2 O is investigated through proton spin-lattice relaxation experiments. This insulator contains besides the linear-chain system with exchange interaction J/k =-0.85 K, a paramagnetic system with θ=0.05 K. The dynamic properties of both magnetic systems have been studied extensively as a function of field (5–15 kOe) and temperature (0.3–20 K) and prove to be quite different. The contributions of these two systems to the relaxation of different protons are separated. The use of single crystals allows a study of T 1 as a function of the direction of the external field. Moreover the large dipolar fields at the proton sites due to the presence of the paramagnetic system, provide the unique opportunity for a chain system to study the relaxation times of different singular resonance lines. The spin-lattice relaxation of protons that interact dominantly with the paramagnetic system is governed by the spectral density of the auto-correlation function of the z component of the paramagnetic electron spins. The relaxation of protons close to the ALC system, however, proves to be governed by the spectral densities of the auto-correlation function of both the parallel and perpendicular component of the nearest ALC electron spin, φ z 0 (ω n ) and φ + 0 (ω n ). A study of the angular dependence of T 1 shows that these spectral densities of the ALC system exhibit different field and temperature dependencies. Particularly the difference in the temperature dependence is striking: φ z 0 (ω n ) decreases towards lower temperatures, whereas φ + 0 (ω n ) is temperature independent.

Temperature and field dependences of the magnetic and magnetooptical properties of single crystal dysprosium iron garnet

The magnetization and the circular magnetic birefringence (CMB) or Faraday rotation φ F measurements in dysprosium iron garnet (DyIG) are reported in detail in the temperature range 2.6-300 K and under magnetic field H a up to 26 kOe applied along either the easy axis |111| or the intermediate |110| direction. Typical linear field dependences of the magnetization and CMB are observed and their spontaneous values are obtained by extrapolating these evolutions to zero applied field. It is shown that the use of single crystals avoids the treatment adopted for polycrystalline samples by Pauthenet who extrapolated to infinite fields which may be doubtful. The anisotropic character of the magnetic properties and of the Faraday rotation appears to be very important below 100°K. The rare-earth contribution to the total CMB is determined and the field dependence of the total Faraday rotation is discussed from the paramagnetic evolution of the Dy3+ions. Similar temperature evolutions are observed for the quantities χ-1and |\Delta\phi_{F}/\DeltaH_{a}|^{-1} where χ is the magnetic susceptibility. Our experimental data, corresponding to these quantities, show some disagreement from the free ion Curie-Weiss's law as observed previously in polycrystalline samples. New values for the paramagnetic Curie temperature and the molecular field parameters of the Dy3+sublattice are deduced from both the magnetic and magnetooptical data.

Electron diffraction patterns and extinction distances in rutile TiO2

The primary transmission electron diffraction patterns for rutile (TiO2) have been obtained with the use of single crystals and a standard goniometer stage. The patterns have been indexed and the occurrence of spots is explained. Kikuchi patterns and extinction distances have been also computed. These data will be useful for electron-microscope observations of rutile-type oxides and difluorides.

Locations of oxygen, nitrogen and carbon atoms in vanadium determined by neutron diffraction

The occupation sites of oxygen, nitrogen, and carbon atoms dissolved interstitially in vanadium have been determined by means of neutron diffraction with use of single crystals of VO 0.032, VN 0.013 and VC 0.006. It is revealed that the interstitial atoms occupy, randomly, the octahedral sites in the b.c.c. host lattice of the three crystals. Neutron diffraction is advantageous for the present purpose, since the coherent scattering amplitudes of the solute atoms are much larger than that of the vanadium atom.

Analysis of the crystal structure of poly(ethylene sulfide) by electron diffraction

The crystal structure of poly(ethylene sulfide) has been determined by electron diffraction. The analysis was based on the combined use of single crystal and fiber diffraction data. The refined str...

Image Contrast Of Amorphous Carbonaceous Materials

We have previously reported a method for preparing extremely thin films of graphite single crystals for use as support films (1). Usefulness of these films, in particular, for obtaining high resolution images of phase objects, was demonstrated by imaging (with bright field mode of CTEM) surface steps with heights of less than 10Å on graphite films (2) and also single atoms of tungsten (3). In this study using the same graphite substrates we have made some observations of amorphous materials to understand the image contrasts in terms of microstructure.Although image contrasts of thin evaporated carbon films, even if prepared as thin as possible, do not show measurable dependence on film thicknesses, we chose as test objects the extremely thin films of contamination which grew discontinuously on the substrate films during specimen observation (4). Image contrasts of these films were then compared with those of known structures such as surface steps and a single layer of graphite crystal.

Recrystallization

In many areas of recrystallization our knowledge is limited to phenomenological descriptions. The nature of the driving forces is well known. The atomic mechanisms of grain boundary motion and nucleation, however, are only little understood: first, mainly because of our ignorance of grain-boundary structure, and second mainly because of inherent experimental difficulties. Since recrystallization depends greatly upon small amounts of foreign elements and their state of solution and precipitation, a great variety of phenomena can be obtained in the cases of the recrystallization of non-high-purity metals and of alloys. The understanding of these interrelations is only just beginning. Additionally, processes such as texture formation or dynamic recrystallization need still further clarification. Further work, therefore, both on clarification of the physical mechanisms and on the phenomena of more complex and technological materials, although laborious seems to be very worthwhile. For the first area, an extreme control of purity and, largely, the use of single crystals seems to be necessary. The second area requires detailed analysis of the different co-acting processes.

Nuclear Electric Quadrupole Interaction of Ir Nuclei in Ferromagnetic Dilute Alloys of Iron and Nickel

NMR of Ir in cubic Fe Ir and Ni Ir alloys shows nuclear quadrupole splitting. By the use of single crystals, it was shown that the principal value of the quadrupole interaction along the direction of the magnetization is almost independent of the orientation of the magnetization with respect to the crystallographic axes. This quadrupole interaction is explained by electric field gradient induced by the electronic spin through spin-orbit coupling, with the assumption that the spin-orbit coupling and splitting of d e and d γ bands by cubic crystalline field at the impurity site are smaller than the width of the bands. For other impurity neclei in iron or nickel, splitting of NMR lines by quadrupole interaction was not clearly observed and the reason why it was so is discussed.

Coulometric investigation on the use of single crystals of sodium chloride as a primary standard

The use of single crystals of sodium chloride as a primary standard substance for argentimetry is described. The water on the surface and the purity of the crystal are measured by coulometric titration methods. The adsorbed water amounts to 0.48 μg cm −2 when the sample is dried in a desiccator containing magnesium perchlorate at room temperature for about 3 h. The loss in weight of the crystal is negligible when it is heated at about 600°. The purities of three crystals, by precise coulometric titrations, are 99,988,99.993 and 99.995%, with standard deviations of 0.0143, 0.0138 and 0.0124%, respectively. It can be concluded that the crystal is useful as a primary standard.

Apparatus for bending crystals at constant strain rate

A description is given of a device which uses the thermal expansion of a metal rod to bend at a constant strain rate alkali halide single crystals for use in piezoelectric and internal friction studies. Plastic strains of approximately 10−2 have been produced in potassium chloride and lithium fluoride at strain rates of about 10−7 s−1.

Production of thin metallic single crystals

A technique is described for obtaining thin metallic single crystals for use in a variety of measurements. The method has been used successfully to make gallium and indium single crystal plates as thin as 25 μm.

A comparison of TLD and film for personnel dosimetry.

Abstract The film badge is today the most widely used personnel monitoring device for want of a more reliable and accurate system. Recent evaluations of the performance of the available commercial film badge services report a wide variation in both the accuracy and consistency of the reported exposures. The best accuracy that one can obtain for X- and gamma-radiation appears to be –50 to +200%. This paper reports the feasibility of using the thermoluminescence of single crystals of LiF (TLD-100) ‡ to measure exposure levels of X- and gamma-radiation encountered in the personnel dosimetry range. The use of single crystals greatly reduces the contribution of the non-radiation induced thermoluminescence. The variation in response of individual crystals was taken into account by calibration of the crystal in place at the time of read-out. LiF single crystals in lucite capsules were attached to film badge holders and given known test exposures from X- and gamma-ray sources and mixtures of these radiations. The performance of the TLD system is compared with the measured values reported by the film badge suppliers. No correction was made for the slight energy dependence of LiF at the low keV exposures. The over-all accuracy of the TLD system is about ± 30% for X- and gamma-ray energies above 25 keV effective.

ИЗМЕРЕНИЕ ИНТЕНСИВНОСТЕЙ ДИФРАКЦИЙ ПОЛИКРИСТАЛЛИЧЕСКИ Х ВЕЩЕСТВ, ОСОБЕННО У БОЛЬШИХ УГЛОВ БРЭГГА

The paper describes a simple experimental method for interplacing the background at diffractions having high Bragg angles, based on the use of single crystals of the same material.