Characterization Of Large Single Crystals Research Articles

Alignment facility and software for single-crystal time-of-flight neutron spectroscopy.

An instrument and software algorithm are described for the purpose of characterization of large single crystals at the Alignment Facility of the ISIS spallation neutron source. A method for both characterizing the quality of the sample and aligning it in a particular scattering plane is introduced. A software package written for this instrument is presented, and its utility is demonstrated by way of an example of the structural characterization of large single crystals of Pb(Mg1/3Nb2/3)O3. Extensions and modifications of characterization instruments for future improved beamlines are suggested. It is hoped that this software will be used by the neutron community for pre-characterizing large single crystals for spectroscopy experiments and that in the future such a facility will be included as part of the spectroscopy suite at other spallation neutron sources.

Anisotropic charge transport in large single crystals of π-conjugated organic molecules

The electronic properties of organic semiconductors depend strongly on the nature of the molecules, their conjugation and conformation, their mutual distance and the orientation between adjacent molecules. Variations of intramolecular distances and conformation disturb the conjugation and perturb the delocalization of charges. As a result, the mobility considerably decreases compared to that of a covalently well-organized crystal. Here, we present electrical characterization of large single crystals made of the regioregular octamer of 3-hexyl-thiophene (3HT)8 using a conductive-atomic force microscope (C-AFM) in air. We find a large anisotropy in the conduction with charge mobility values depending on the crystallographic orientation of the single crystal. The smaller conduction is in the direction of π-π stacking (along the long axis of the single crystal) with a mobility value in the order of 10(-3) cm(2) V(-1) s(-1), and the larger one is along the molecular axis (in the direction normal to the single crystal surface) with a mobility value in the order of 0.5 cm(2) V(-1) s(-1). The measured current-voltage (I-V) curves showed that along the molecular axis, the current followed an exponential dependence corresponding to an injection mode. In the π-π stacking direction, the current exhibits a space charge limited current (SCLC) behavior, which allows us to estimate the charge carrier mobility.

Growth and characterization of large single crystals of l-serine methyl ester hydrochloride

Single crystal of l-serine methyl ester hydrochloride (LSMEHCl), was grown using slow evaporation solution growth technique. The grown crystal was confirmed by single crystal X-ray diffraction and the presence of functional groups were identified by FT-IR analysis. From the Optical transmission spectra of the grown crystal the optical energy band gap is found to be 5 eV and the refractive indices ( n x , n y and n z ) were found using Brewster's angle method. The melting point of the material obtained using melting point apparatus is 168 °C. The thermal stability of the crystal was investigated. The second harmonic generation was tested with different particle sizes of powdered LSMEHCl by Kurtz–Perry powder method using Nd:YAG laser, which established the existence of phase matching.

Synthesis and characterization of large single crystals of NpPd 3 by flux method

We report on the growth by flux method of large single crystals of hexagonal NpPd 3. Samples with linear size up to 3 mm were obtained using lead as flux medium. Only polycrystalline samples of NpPd 3 have been previously studied and in particular no information is available on the anisotropy of its physical properties. Considering the double hexagonal closely packed structure of NpPd 3, important differences in the physical properties measured along the c-axis and in the basal plane can be anticipated. Preliminary magnetic susceptibility measurements performed on NpPd 3 single crystal are compared to previous measurements made on polycrystalline samples. The availability of NpPd 3 single crystals opens new perspectives in the understanding of the magnetic and electronic properties of this system.

Production and characterization of large single crystals made of ferromagnetic shape memory alloys Ni–Mn–Ga

Ferromagnetic shape memory alloys (MSMA) are well-known smart materials for actuation applications, due to their large shape change in applied magnetic fields. Off-stoichiometric Ni–Mn–Ga single crystals of MSMA exhibit a magnetic-field induced strain of about 10%. In the past decade a lot of effort has been put into understanding and optimizing the properties of Ni-Mn-Ga alloys in terms of the magnetic shape memory effect, starting with the production of single crystals and going all the way to heat treatment and magneto-mechanical training.Here we focus on and present the production and characterization of single crystal Ni–Mn–Ga alloys with off-stoichiometric composition. The molten material was cast into preheated ceramic shells and directionally solidified in a Bridgman-type process. Cylindrical single crystal bars of diameters up to 48 mm and a length of 110 mm were obtained by using suitable grain selectors. Only small misorientations between the single crystal c-axis and the bar axis were found in these bars. The crystals were characterized in terms of composition, segregation, crystal orientation and microstructural features by means of SEM/EDS, EBSD and light microscopy.

Growth and dielectric characterization of large single crystals of GaAsO 4, a novel piezoelectric material

Gallium arsenate (GaAsO 4) is a new α-quartz-type piezoelectric material. Large single crystals (8 mm along the c-direction) were grown for the first time by hydrothermal methods. The different crystal faces were indexed by X-ray diffraction. The crystal quality was characterized by infrared measurements. X- and Z-cut crystals were prepared in order to perform dielectric measurements. The dielectric constants of GaAsO 4 ( ε′ 11=8.5, ε′ 33=8.6) are the highest found among α-quartz type materials. A linear relationship has been established between a structural property, the tetrahedral tilt angle δ, and the dielectric constants for the well-known α-quartz homeotypes. These results indicate that gallium arsenate should have the highest piezoelectric coupling coefficient of any material of this family.

Synthesis and Characterization of Large Single Crystals of Silicon and Germanium Clathrate-II Compounds and a New Tin Compound with Clathrate Layers

ABSTRACTWe have synthesized large single crystals of clathrate-II compounds with frameworks of silicon and germanium by employing mixed alkali metal countercations. The combinations of alkali metals are rationally selected in order to fit the different cages of the clathrate-II structure. This approach leads to the following stoichiometric and fully “stuffed” compounds: Cs8Na16Si136, Cs8Na16Ge136, Rb8Na16Si136 and Rb8Na16Ge136. The structures and the corresponding Si-Si and Ge-Ge distances are elucidated and established with high accuracy from extensive single crystal X-ray diffraction work. The compounds are stoichiometric, metallic, and are very stable at a variety of extreme conditions such as heat, concentrated acids, hydrothermal treatment etc. No evidence was found for vacancies in the silicon and germanium networks or partial occupancies of the alkali metal sites. The stoichiometry of these fully “stuffed” clathrates is consistent with the measured temperature independent Pauli paramagnetism, supported also by the conductivity measurements on single crystals and thermopower measurements on pellets. A new compound with novel clathrate-like structure forms when small and large cations are combined with tin. The new materials, A6Na18Sn46 (A = K, Rb, Cs), are made of clathrate layers and the interlayer space filled with Sn4-tetrahedra and alkali-metal cations. Its formula can be rationalized as A6Na6Sn34 + 3·Na4Sn4 (one clathrate layer and three tin tetrahedra). The compound is stable in air and is being currently tested at other conditions. Detailed measurements of its transport properties are under way.

Growth and characterization of large single crystals of the intermetallic compound Li-Ga (Zintl)

Single crystals of the β-LiGa (Zintl) intermetallic compound were grown under carefully observed conditions of chemical purity. The three crystals separated from the specially designed boron nitride crucible are believed to be parts of one single crystalline domain of 55 mm in length and 13 mm in diameter. Phase identification was done by X-ray diffraction. The easy cleavage plane was the (111) plane. The lattice parameter was found to be 6.17 Å, corresponding to a composition of Li 0.45Ga 0.55 in the final product.

Growth and characterization of large single crystals of LiGa Zintl intermetallic compound

Single crystals of the beta-LiGa Zintl intermetallic compound were grown under carefully observed conditions of chemical purity. The three crystals separated from the specially designed boron nitride crucible are believed to be parts of one single crystalline domain with 5.5 cm length and 1.3 cm diameter. Phase identification was done by X-ray diffraction. The easy cleavage plane was the (111) plane. The lattice parameter was found to be 6.17 Å, corresponding to a composition Li 0.45Ga 0.55 for the final product. The crystals are being used for studies of the electronic structure of valence and conduction bands of the material by angle resolved photoemission and inverse photoemission techniques.

Growth and characterization of thallium arsenic selenide crystals for nonlinear optical applications

The synthesis, growth, and characterization of large single crystals of the efficient nonlinear optical sulfosalt material thallium arsenic selenide (TAS) are reviewed. Recent significant improvements in crystal growth and purification technology have enabled us to grow high optical quality crystals as measured by etchpit, x-ray topographic, and birefringent interferometric characterization methods. The excellent crystal quality is reflected in the high CO 2 frequency conversion efficiencies obtained with TAS. In addition to quality improvement in 2.5 cm diameter crystals, larger crystals, up to 5 cm in diameter, have now been grown by a modified Bridgman technique. Etching of crystals grown under a high axial thermal gradient indicated a higher defect density near the crystal surface than at the corresponding central region. In addition, high defect densities were found at the seed-crystal joint. This suggests that the etchpits are related to thermal stresses induced by the temperature distribution in a growing crystal. X-ray topographic analyses confirmed the etching studies, and also revealed a subgrain structure in some crystals which is related to the crystal stress distribution and to seeding defects. Birefringent interference patterns, a measure of optical uniformity, show a direct correlation between subgrain structure and optical homogeneity of the crystals. Improved growth control has eliminated these defects. The nonlinear optical susceptibilities of TAS are large, nearly three times those of proustite. Crystals grown in our laboratory have shown high values of second, third, and fourth harmonic conversion efficiency of the output from a CO 2 laser. Fifth harmonic generation has also been demonstrated. A 4.8 cm long crystal, 25 mm in diameter, was used to double 4.8 μm to 2.4 μm with an efficiency of 6.4% for the relatively low fluence of 8 mJ/cm 2 in an 80 nsec pulse.

The Characterization of Large Single Crystals by High-Voltage X-Ray Laue Photographs

AbstractLarge single crystals can be examined by conventional X-ray diffraction procedures only at their surface or by destructive sectioning. Within the limitations inherent in polychromatic X-ray photography, high-voltage Laue pictures arc shown to give some information on the internal quality of large crystals.Asterism in conventional Laue photographs is contrasted, with streaks due to geometric effects in Laue patterns of large crystals. Detail within the streaks reveals subgrain structure. A primary extinction effect can be used as striking proof of good crystals being capable of scattering coherently over large distances.