Perfect Single Crystals Research Articles

Low-temperature pyroelectricity

The current state of the set of problems associated with the study and application of the pyroelectric effect at low (T<30 K) temperatures is outlined. The reasons for the qualitatively different temperature dependences of the total pyroelectric coefficient γσ(T) in linear pyroelectrics and ferroelectrics are discussed. An analysis is given of the reasons why the theoretical temperature dependences obtained for the primary pyroelectric coefficient γe(T) adequately describe the experimental γσ(T) dependences for all materials studied. In this connection, the correctness of determining the secondary pyroelectric coefficient γS(T) from the macroscopically measured coefficients of thermal expansion is considered. The potential of thermodynamically nonequilibrium polar media and low-temperature pyroelectric materials is substantiated. The review consists of the following sections: (1) an introduction; (2) the theory of low-temperature pyroelectricity; (3) experimental procedures; (4) discussion of the experimental data: (A) perfect single crystals, (B) the problem of the secondary pyroelectric coefficient, (C) imperfect single crystals; (5) pyroelectric materials for low-temperature applications; and (6) a conclusion.

Experimental and Numerical Approach of Inter- and Transgranular Stress- and Rotation Heterogeneities in the Plastic Behaviour of a Multicrystal

The introduction of micromechanical parameters in the description of the mechanical behaviour of polycrystalline materials is getting more and more important. In the interest of the study of inter- and transgranular stress and strain heterogeneities inside an assembly of grains, the multicrystal is introduced. The complete microstructure is known which makes possible an experimental and numerical approach which takes into account the complete microstructure. The modelling is done by a finite element simulation meshing the grain morphology. The experimental validation is done by x-ray diffraction and optical microscopy. The multicrystal is elaborated on the base of a coarse grained Nickel based 600 alloy. A long heat treatment close to the melting temperature allowed to get into the regime of grain growing. In the finite element calculations the grains are considered as perfect single crystals with only one crystallographic orientation. Naturally, during the elaboration process materials develop sub-grain structures with slight misorientations called small angle grain boundaries or grain mosaicity. The quality of the crystal can be described by the mosaicity. Depending on elaboration parameters, the intergranular misorientations can be distributed more or less randomly. Due to the local distribution of mosaic blocs, the heterogeneity of the measurement depends on the observation scale. The use of optical microscopy allows the determination of plastic activate zones. The observation of heterogeneities inside these zones needs finer measurement techniques such as x-ray diffraction. The measurement of misorientations by rocking the sample around a crystallographic axis allows to quantify the misorientations. This is even more interesting during plastic slip where, due to the anisotropy of the crystal, the mosaicity develops anisotropically, too. Finally, the microscale parameters like the mosaicity are investigated by keeping in mind the influence on the local stress and strain evolution. The comparison to the finite elements simulation is of great interest in relation to the choice of micromechanical parameters which do not contain anything comparable to a mosaic function.

Indistinct Defect Images in Topographs of Nearly Perfect Aluminum Crystals Just Prior to Appearance of Dislocation Loops

Vacancy generation and annihilation mechanisms by the growth of two types of dislocation loops, the interstitial and the vacancy type, in nearly perfect aluminum single crystals at fairly high temperatures were investigated by synchrotron radiation topography. Topographs were continuously taken with white beam X-ray at an elevated temperature. Just prior to the appearance of the loops, an indistinct image topograph, in which not only defect images but also fringes were unclear, was taken for both types of loops in spite of a short exposure time (<3 s). This phenomenon was interpreted as being due to the scattering of the diffracted X-ray beam by very small clusters of interstitial atoms or vacancies which are invisible on the X-ray topograph. Moreover, some of them grow into interstitial or vacancy type dislocation loops as revealed by X-ray topography.

Optical spectroscopy of excitonic states in CuInSe2

Optical properties of structurally perfect CuInSe2 single crystals were studied in the temperature range of 4.2–300 K with the use of photoluminescence, optical absorption, optical reflection, and wavelength-modulated optical reflection (WMOR). The intense lines of free excitons A (∼1.0414 eV) and B (∼1.0449 eV) with a half-width of ∼0.7 meV at 4.2 K are found to be related to two extrema of valence band split by a crystal field. The excitons emission line C (∼1.2779 eV) in WMOR spectra are related to a lower valence band split-off by spin-orbit interaction. Within the context of the quasi-cubic Hopfield model, the parameters of valence band splitting ΔCF=5.2 meV and ΔSO=234.7 meV defined by the crystal and spin-orbit interaction, respectively, are calculated. In the region of the fundamental absorption edge, the lines of bound excitons are found with a half-width ∼0.3 meV that is indicative of a high quality of grown CuInSe2 crystals.

LP‐5: Late‐News Poster: Nanocrystalline Phosphors for Low Voltage Excitation Applications

AbstractChemical precipitation techniques allow the preparation of a wide variety of materials, in the form of perfect single crystals, to be produced in the size range of 50–100nm. The properties of three such compounds, Y2O3:Eu, Zn2SiO4:Mn and Y2SiO5:Ce, are discussed. Results under electron excitation show that luminescence efficiencies of up to 10 lm.W−1 are attainable under 1–2kV excitation.

New nonlinear optical crystal K_2Al_2B_2O_7

The new nonlinear optical crystal K2Al2B2O7 is discovered with the molecular engineering approach on the basis of anionic group theory. An optically perfect single crystal with space group P321, free of moisture and hygroscopy, is readily grown by the top-seeding flux method. Its transparence range covers 180 to 3600 nm. The refractive indices are measured with the minimum-deviation method, based on which the Sellmeier equation is obtained. The measured nonlinear optical coefficient d11 is 0.45 pm/V. The moderate walk-off angle and angular bandwidth, together with the high optical homogeneity, make it a promising candidate for the fourth- and the fifth-harmonic generation of a Nd:YAG laser.

Crystal growth under heat field rotation conditions

Rotation of the heat field which is applied to the outer walls of a crystallizer or growth crucible, provides a contact-free induction of forced convection (i.e. flow azimuthal component) in a medium of crystallization. Thus, the stirring of a melt or solution is gained without the direct effect of a rotating crystal, or crucible, or various mixers. This allows to simplify crystallization and to escape vibration and other excitations. The circular movement of a heat field by the perimeter of a growing crystal, which proceeds at a certain amplitude and frequency, provides the formation of a structurally perfect single crystal. An original heating furnace provides the rotation of the heat field. Thermal control of this furnace allows maintainance of a necessary temperature in a growth zone with high precision; it is possible to set the amplitude and frequency of thermal oscillation in a wide range of values. This technique was tested for the growth of nonlinear-optical CLBO crystals from a high-temperature melt-solution and KDP crystals from a water solution.

Resolution of the USANS diffractometer at the FRJ-2 research reactor in Jülich

The resolution of a Bonse–Hart setup with two Si(1 1 1) perfect channel cut single crystals is studied. The dimension of the crystals is determined in such a way as to avoid single reflected neutrons to leave the channels. The setup gives the same contrast ratio as the Oak Ridge instrument using a Cd absorber in the longer wall of the crystals. The applicability of the equipment is tested by the measurement of a monodisperse polystyrene latex dispersion model system.

Synthesis of GeO 2 nanorods by carbon nanotubes template

GeO 2 nanorods have been synthesized by means of a carbon nanotubes-confined reaction, carbon nanotubes are used as templates and pure metallic germanium is a reactant instead of the common-used volatile metal or non-metal oxides. The products are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive spectrometer (EDS) as well as X-ray diffraction (XRD). It is found that the GeO 2 nanorods are perfect single hexagonal crystals covered by an amorphous layer of Ge and O, and have diameters of 50–200 nm and lengths of several μm.

Quadrupole Effects on 73Ge NMR Spectra in Isotopically Controlled Ge Single Crystals

NMR spectra of 73Ge (nuclear spin I = 9/2) in perfect single crystals of germanium with different isotopic content were measured at 80, 300, and 450 K. The observed specific line shapes gave evidence of the isotopic disorder, in particular, abnormal broadening of the spectrum was found for the magnetic field directed along the [111] axis. Local lattice deformations in the germanium crystal lattice due to "isotopic disorder" were calculated in the framework of the adiabatic bond charge model. The results were applied to study random non-cubic crystal field interactions with the nuclear quadrupole moments and corresponding effects on NMR spectra. The simulated second moment of the resonance frequency distributions caused by the magnetic dipole-dipole and electric quadrupole interactions are used to analyze the lineshapes, theoretical predictions being in a qualitative agreement with the experimental data.

Polarizer–analyzer optics

The principles behind the design and operation of polarization‐based optics for nuclear resonant scattering of synchrotron radiation are discussed. With perfect single crystals and collimated X‐rays emitted from undulator‐based third‐generation synchrotron radiation sources, polarization‐selective optics with a sensitivity of parts per billion can be obtained. A general approach to optical activity is introduced, and the polarization dependence of the index of refraction is calculated for nuclear forward scattering for a medium with unidirectional symmetry. Some recent experimental results are reviewed and future applications are discussed.

Synthesis, Ion-exchange, Structural Characterization and Adsorption of K, Na-FER Type Zeolite

Synthesis of K, Na -FER type zeolite wasstudied in the reactant system ofK 2O-–Na 2O-–Al 2O 3-–SiO 2-–CO 3-–HCO 3-ndash;H 2O.Sodium silicate, silica sol andfumed silica were tested as the silica source, andsolid aluminum sulfate, aluminum hydroxide andmeta-kaolinite as the alumina source. The startingmaterials, the composition of the reactant, and thesynthesis temperature greatly influence the phasescrystallized. A pure phase of K, Na-FER zeolite washydrothermally prepared at 208 °C with sodiumsilicate and solid aluminum sulfate as startingmaterials. The optimum composition of the reactantfor synthesis of the pure FER zeolite was determined. Chemical analysis, XRD, FT-IR, 29Si and 27AlMAS NMR, TG/DTA, and adsorption of nitrogen, methanoland n-hexane were used to characterize the zeolite andcompared with the reference sample of perfect singlecrystals of siliceous FER zeolite. The content ofK+and Na+in the zeolite decreases graduallywith the times of the treatment ofion-exchange/calcination, leading to an increase in theadsorption capacities of nitrogen and methanol, anda decrease of the loading of n-hexane. The location ofthe K+, the stacking faults, and dealumination of thezeolite framework are discussed based on the ion exchange and adsorption behavior.

73Ge NMR spectra in germanium single crystals with different isotopic composition

We have studied the influence of isotopic disorder on the local deformations in Ge single crystals from both experimental and calculation points of view. The nuclear magnetic resonance (NMR) spectra of73Ge nuclei (the nuclear spin equals 9/2) in perfect single crystals of germanium with different isotopic content were measured at temperatures 80, 300 and 450 K. Abnormal broadening of the spectrum was found to occur when the magnetic field was aligned along the [111] axis of a crystal. The observed specific angular dependence of the quadrupole broadening was attributed to isotopic disorder among atoms of germanium sited around the73Ge NMR probe. Local lattice deformations in germanium crystal lattice due to isotopic impurity atoms were calculated in the framework of the adiabatic bond charge model. The results obtained were applied to study random noncubic crystal field interactions with the nuclear quadrupole moments and corresponding effects in NMR spectra. Simulated second and fourth moments of resonance frequency distributions caused by the magnetic dipole-dipole and electric quadrupole interactions are used to analyze the lineshapes, theoretical predictions agree qualitatively with the experimental data.

Heat transport in fullerite samples

The absolute value of the coefficient of thermal conductivity k( T) of fullerite samples and its temperature dependence strongly depend on the conditions of the sample preparation. In our experiments we have studied the transport properties of perfect single crystals and of pure C 60 powder compacted under different conditions. The compacted samples were pressurized at low temperature (only compacted samples) or polymerized at high pressure and high temperatures.

Synthesis and Characterization of a Polycrystalline Ionic Thin Film by Large-Scale Molecular-Dynamics Simulation

A simulation methodology for the synthesis of polycrystalline, ionic thin films is developed. The method involves the preparation of a polycrystalline substrate onto which a thin film is subsequently grown by crystallization from the melt. A detailed structural analysis of a textured sixteen-grain FeO film, with a grain size of approximately 4.7 nm, shows that the interiors of the grains are almost perfect single crystals with only a very few vacancies and no interstitials. The grains are delineated by 〈001〉 tilt grain boundaries; as expected, the low-angle grain boundaries in the film consist of arrays of dislocations, while the high-angle grain boundaries are relatively narrow and well ordered.

MICROSTRUCTURAL FEATURE OF NANOPHASE Sn-Bi PARTICLES PRODUCED BY AN ELECTROHYDRODYNAMIC TECHNIQUE

Sn-Bi nanometer powders were produced by an electrohydrodynamic technique. The microstructural feature of Sn-Bi nanometer powders was investigated by means of XRD, TEM and HREM. The results show that Sn-Bi nanometer powders produced are composed of tetragonal β-Sn monophase supersaturated solid solution and rhombohedral Bi monophase supersaturated solid solution. Most of the Sn-Bi nanometer powders are single crytals, only a few Sn-Bi nanometer powders are twin crystals. The particles of﹤10nm in diameter are perfect single crystals. There exist dislocations in the particles larger than 10nm in diameter. The formation mechanism of microstructure for Sn-Bi nanometer powders is analyzed.

The lattice contraction of nanometre-sized Sn and Bi particles produced by an electrohydrodynamic technique

Nanometre-sized Sn and Bi particles were produced by an electrohydrodynamic technique. The lattice parameters of the nanometre-sized Sn and Bi particles with different grain sizes were measured by x-ray diffraction. It was found that the lattice parameters a and c for the nanometre-sized Sn and Bi particles are smaller than the equilibrium lattice parameters and of the corresponding Sn and Bi perfect single crystals respectively. With a reduction in grain size, the values of a, c and the unit cell volume V decrease. The results are attributed to a supersaturation of the vacant lattice sites in the nanoscale particles.

Lattice Defects in Undoped CdAs2 Monocrystals

The spectra of optical absorption and photoconductivity of perfect CdAs2 single crystals were investigated near the intrinsic edge and in the extrinsic absorption region. The existence of three donor levels created by structural defects is established. The mechanism of defect formation is suggested: the interstitial Cdi atom in different charge states created the ε1 < 0.02 eV and ε3 ≅ 0.42 eV levels, the Asv vacancies created the ε2 ≅ 0.26 eV level.

Features of the optical absorption of crystals of the fullerene C60 in the region of the orientational phase transition

The absorption coefficient of perfect single crystals of the fullerene C60 is measured in the energy range 1.6–2.1 eV at temperatures from 4.2 to 300 K. An absorption fine structure is discovered in the and is assigned to electronic and vibronic transitions with the production of free excitons and excitons localized on structural defects. It is shown that in the region of the structural phase transition from a face-centered cubic structure to a simple cubic structure the absorption coefficient undergoes a jump, which is associated with an energy shift of the free exciton line toward lower energies. It is discovered that spatial inhomogeneity, which is associated with the growth of the new phase from a finite number of nuclei, appears in the crystal at the time of this transition.

Formation effects and optical absorption of Ag nanocrystals embedded in single crystal SiO2 by implantation

Ag + ions of 200 keV were implanted into single crystal SiO2 at room temperature to five different doses: 5×1015, 2.3×1016, 4.5×1016, 5.6×1016, and 6.7×1016/cm2. With increasing dose, the implanted Ag distributions change from usual Gaussian-type profiles to abnormal bimodal profiles with narrow full width at half maximum, which are associated with Ag nanoparticles forming during high dose implantation. The implanted Ag depth profile evolution with dose can be clearly observed during Rutherford backscattering spectroscopy analysis. The nanoparticles form dual-layer structures at high doses: as far as the dose of 6.7×1016/cm2 is concerned, transmission electron microscopy proves that the shallower implanted layer contains noninteracting small Ag nanoparticles with the diameters of about 7 nm; the deeper layer contains a high density of interacting large nanoparticles with the diameters of about 25 nm. High resolution electron microscopy identifies that the nanoparticles are perfect single crystals. Although plasmon resonance frequency of the Ag nanoparticles formed at relatively low dose agrees well with the Mie’s theoretical prediction, great redshift due to multipole interactions between high density nanoparticles occurs for high doses, moreover, the magnitude of redshift increases with implanted dose.