Atomic structure and enormous anisotropy of thermal expansion in NiSi single crystals. I. Refinement of structure models

The atomic structure of NiSi single crystals has been studied by the methods of X-ray diffraction analysis at 295 and 418 K. The refinement of the structure models with due regard for anharmonicity of atomic vibrations revealed the pronounced anisotropy of thermal vibrations and the considerable contribution of the fourth-order anharmonicity. The maps of the probability density function of atomic displacements and single-particle potentials of both atoms were constructed.     

Temperature-induced changes in physical properties and lattice symmetry of nickel crystals

It has been shown that the changes in some physical properties of nickel crystals can be explained on the assumption of the existence of an isotropic magnetic phase within the temperature range 470 < T < 631 K, whose thermodynamic properties are determined by the exchange interaction alone. At T _is ≈ 470 K, this phase should be transformed into an anisotropic magnetic phase because of the cooperative effect of relativistic interactions. This provides a consistent interpretation of the changes in the symmetry and the number of physical characteristics of the crystal in the framework of the Landau theory within the temperature range 273 < T < 650 K.     

Atomic structure of Cd1 − x Zn x Te solid solution single crystals and structural prerequisites of their ferroelectricity

Precision X-ray structural investigation of Cd_{1 ? x} Zn _x Te single crystals (x = 0.04, 0.10, 0.21, 0.30, 0.40) is performed at temperatures below and above the ferroelectric phase transition. The regularities associated with the increase in the concentration of Zn atoms in solid solutions are revealed. It is established that the unit cell parameter of the crystals varies according to the Vegard law. The thermal parameters of Te and Zn atoms are strongly overestimated within the model of a sphalerite-type average structure. It is found that the positions of the Zn cations are split and the anion sublattice is disordered in such a way that the Te anions are predominantly displaced along the threefold axes. The Zn atoms can serve as off-center impurities. This phenomenon offers strong possibilities for manifesting the ferroelectric properties in the solid solutions under investigation.     

Magnetocrystallographic anisotropy in nickel-zinc and nickel-manganese ferrite single crystals

Temperature dependence of the first-order magnetocrystallographic anisotropy constant in nickel-zinc and nickel-manganese ferrite single crystals grown using the Verneuil method has been investigated. The anisotropy constant K₁ was measured by the torque in the temperature range 77° to 500°K. The results obtained are discussed on the basis of the one-ion (single-ionic) model taking into account the ionic distribution of two-valence metals on the sublattices.     

Plasticity of high purity iron single crystals (II) surface observations

Slip traces were studied on high purity iron single crystals after deformation in tension in the temperature range from 4.2 to 473 K (the same crystals were used for the work-hardening study described in Part I of this paper). By evaluation of Nomarski interference contrast and by method of repolishing of the surface before incremental loading detailed quantitative information was obtained about slip line pattern in various work-hardening stages. An improved model of slip processes in stage III of the work-hardening curve is presented.     

New model of the bond diffractometer for precise determination of lattice parameters and thermal expansion of single crystals

A new model of diffractometer for very precise measurements (1 ppm accuracy) of lattice parameters and thermal expansion is presented. Special attention is paid to the temperature control and stability in a wide range from liquid nitrogen up to 800 K. Two examples of the application of the diffractometer in a study of solid state phase transitions are given.     

Intercalation and anisotropy of optical absorption in MoRe0.005Se1.995 single crystals

The optical energy gap of MoRe_0.005Se_1.995 single crystal has been measured at room temperature near the fundamental absorption method. The incident light was polarized along c‐axis of the crystals. Two and three dimensional models are adopted for generating the results. Direct as well as indirect transitions are involved in the absorption process. The indirect transition was found to be allowed with two phonons involved in the process. The three dimensional model could be used to describe the optical properties of the single crystal of MoRe_0.005Se_1.995. Also, the optical energy gaps depend upon the amount of the intercalating Re material, which show the anisotropy of the chemical bonds. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of USb2 single crystals and their structural perfection

USb₂ single crystals were grown by three methods; I — Chemical vapour transport with iodine as transporting reagent, II — Crystallization from U Sb liquid solution, III — Crystallization from U Sb Sn liquid solution. The morphology of growth and results of X-ray topography examination of crystal surface are given.     

Effect of different metal ions on structural,thermal, spectroscopic and optical properties of ATCC and ATMC single crystals

A novel metal‐organic coordination complex nonlinear optical crystals, tri‐allylthiourea cadmium chloride [(CdCl₂(AT)₃] and tri‐allylthiourea mercury chloride [(HgCl₂(AT)₃] abbreviated as ATCC, ATMC (AT is Allylthiourea i.e.,CH₂=CHCH₂NHCSNH₂) has been synthesized and grown as single crystals. It was synthesized in deionised water and further recrystallized to improve its purity. Single crystals of the allylthiourea co‐ordination complex nonlinear optical crystals tri allylthiourea cadmium chloride (ATCC) with dimensions of 14x14x10 mm³ and tri allylthiourea mercury chloride (ATMC) with dimensions of 15x15x12 mm³ were grown successfully from aqueous solution by solvent evaporation as well as by temperature lowering method. It exhibits powder SHG efficiencies higher than that of a well known organic NLO crystal Urea. The solubility of the as grown crystals was estimated from the aqueous solution and the effect of different metal ions on the grown crystals, structural, thermal, spectral and optical properties were analyzed. XRD studies the reveals the same structure of both materials. Influence of the different central metal (Cd and Hg) atoms, changing the thermal properties of the materials when NLO complexes formed with the common ligand allylthiourea. The metal co‐ordination was confirmed form the spectroscopic analysis. From the UV transmittance studies, red shift was from the transparency cut‐off wavelengths. The value is 285nm for ATCC is and is 335nm ATMC, Non‐linear an optical study confirms the suitabilities of the as grown crystals for the non linear optical applications. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low-temperature X-ray studies of crystal-lattice parameters and thermal expansion of KTiOPO4 crystals

The unit-cell parameters a, b, and c of KTiOPO₄ crystals have been measured by the X-ray diffraction method in the temperature range 80–320 K. The parameters obtained were used to determine the thermal expansion coefficients α_[100], α_[010], and α_[001] along the principal crystallographic axes. It was established that thermal expansion in the crystals is essentially anisotropic and that α_[010] > α _[100], whereas α_[001] is close to zero.     

Shape of thermal etch pits and crystal structure in tellurium crystals

During the systematic study of thermal etching of tellurium crystals, various shaped thermal etch pits were observed on the {101 0} cleavage faces of this crystal. An attempt has been made to explain the shape of non-dislocation and dislocation etch pits. A simple model based on crystal structure and bonding of atoms has been suggested to explain the shape of etch pits.     

Stability of dislocations in MgO single crystals at high temperatures

The dislocation structures of MgO single crystals annealed at 1900 °C for 3 hrs and at 2000°C for 1 hr were observed electron microscopically, and it was found that the following reactions took place at high temperatures: a/2 〈11 0〉 + a/2 [110], = a [100], a/2 [110] + a/2 〈11 01〉 = a/2 [011]. The resultant dislocations were sessile. Their interaction with impurities would make the subgrain boundaries stable.     

Modeling of the defect structure in dislocation-free silicon single crystals

A mathematical model of the formation of primary grown-in microdefects on the basis of dissociation diffusion is presented. Cases of “vacancy-oxygen” (V + O) and “carbon-interstitial” (C + I) interaction near the crystallization front are considered for dislocation-free Si single crystals grown by the floating-zone and Czochralski methods. The approximate analytical expressions obtained by setting 1D and 2D temperature fields in a crystal are in good agreement with the heterogeneous mechanism of formation of grown-in microdefects.     

Change in dislocation distribution of KCl single crystals during thermal cyclic annealing

The change in dislocation distribution of KCl single crystals during thermal cyclic annealing was studied by etch pit method. The results obtained are summarized as follows. (1) In contrast to the case in Cu (KITAJIMA et al.; HASEGAWA et al.), the dislocation distribution in KCl crystals became non-uniform with an increase in the cyclic annealing time, i.e., in the region near the surface, many sub-boundaries were formed, but in the interior, the density of dislocations and sub-boundaries decreased. The cyclic annealing is more effective in reducing the dislocation density than the isothermal annealing. (2) The longer the period of thermal cycle, the more effective in reducing the dislocation density. (3) The larger the temperature difference between the maximum and minimum during one cycle, the more effective in reducing the dislocation density. (4) In the Sr-doped crystals, the thermal cyclic annealing was also effective in reducing the dislocation density within subgrains but sub-boundary length per unit area slightly increased by the cyclic annealing. (5) When the samples were annealed under the atmosphere of Cl₂, the dislocation density remarkably increased against our expectation, although the scattering centres had disappeared by this treatment.     

Growth and characterization of high purity single crystals of barium ferrite

High purity single crystals of barium ferrite have been grown with the use of a BaO flux. The detected residual impurities have a total concentration of 130 ppm (wt), of which 100ppm is due to platinum. While the crystals exhibit occassional inclusions of a barium rich phase, the predominant microstructural defects are voids, which exhibit two geometries; one spherical and one hexagonal. Magnetic and resistivity measurements have yielded values which are comparable to those reported in the literature. Room temperature effective linewidth measurements at 10 GHz indicate that eddy current losses are dominant for disk shaped samples thicker than ≈0.3 mm. Room temperature FMR linewidth measurementson a sample 0.04 mm thick exhibit a linear frequency response between 55 and 90 GHz, which is consistent with a Landau-Lifshitz damping mechanism.     

Brittle fracture of single crystals and active structure of cleavage surfaces

By the method of vacuum decoration at an electronmicroscopic scale with the use of lowangle electron diffraction the dependence of brittle fracture of alkali halide crystals and of the active structure of cleavage surfaces on the character of ordered distribution of point defects in crystals has been studied. The occurrence of cleavage steps reflects the presence in crystal volume, where brittle fracture took place, of a high degree of periodicity of point defects mainly in one crystallographic direction. The smooth areas of the cleavage surface reflect the existence in these places of a periodic distribution of point defects in two mutually perpendicular crystallographic directions. Coincidence of periods between the cleavage steps with the periods in the arrangement of point defects on smooth areas of the cleavage surface allows the steps to be regarded not only as elements of the geometric microrelief but primarily as electrically active linear elements of the crystal surface, reflecting the linear arrangement of point defects. Together with cleavage steps exhibiting a stable in time activity there arise cleavage steps with an extremely high, but rather shortlived activity being the consequence of the brittle fracture process proper. The electrically active linear elements of the point defect lattice – linear elements of the potential relief of the surface – play a most important role in heterogeneous processes, in particular crystallization processes.     

Synthesis,structural and thermal studies of tetrathioureacopper(I) chloride crystals

Tetrathioureacopper(I) chloride, hereafter abbreviated as TCC, was synthesised and single crystals were obtained from saturated aqueous solution by slow evaporation (solution growth) method at room temperature. The crystals obtained are bright, colourless and transparent having well defined external faces. The grown crystals were characterized through elemental analysis, single crystal X‐ray diffraction study, thermal analysis, electron spin resonance spectroscopy and Fourier Transform infrared spectroscopy. The elemental analysis confirms the stoichiometry of the compound. The single crystal diffraction studies indicate that TCC crystallises in the tetragonal lattice and the unit cell parameters are a = b = 13.4082 Å, c = 13.8074 Å, V = 2482.29 ų, α = β = γ = 90°. Space group and the number of molecules per unit cell (Z) are found to be P4₁2₁2 and 8 respectively. The TG curve of the sample shows a prolonged decomposition from 210 to 628.3 °C, from which the decomposition pattern has been formulated. The endothermic peaks in the DTA curve indicate melting and decomposition of the compound at 165.2 and 633.8 °C respectively. An exothermic peak in high temperature DSC indicates a phase transition in the compound at 274.8 °C. Thermal anomalies observed in the low temperature DSC at –163.3, –152.0, –141.5, –108.3, 1.0 and 12.1 °C in the heating run and –157.1 and –153.9 °C in the cooling run reveal first order phase transitions in the crystal. The peaks observed at –146.2 °C in both the heating and cooling runs suggest occurrence of a second order phase transition in this compound. The IR spectroscopic data were used to assign the characteristic vibrational frequencies of various groups present in the compound. The ESR study confirms that the copper is in the +1 oxidation state in the complex. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Impurity induced structural phase transformations in melt grown single crystals of lead iodide

Lead iodide is a wide‐band gap and highly resistive semiconductor considered to be a promising room temperature nuclear detector. The phenomenon of polytypism is posing interesting problems of phase transformations among its polytypic modifications and formation of polytypic admixture during growth due to native impurities. Transformations have also been observed even when the material is stored for few months that causes deterioration in functioning of the PbI₂ devices. Lead iodide has been purified and single crystals were grown using zone‐refining system. The observed phase transformations during growth and storage have been explained in the light of distortion of [PbI₆]^4‐ octahedron due to impurities present in the material and the known crystal structures of PbI₂. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)