Twins in CdMnTe single crystals grown by Bridgman method

Cd_1‐xMn_xTe (x =0.2, CdMnTe) crystal was grown by the vertical Bridgman method, which exhibits a pure zincblende structure in the whole ingot. The major defect, twins, which is fatal to CdMnTe crystal, was analyzed with scanning electron microscopy (SEM), X‐ray energy disperse spectroscopy (XEDS) and optic microscopy on the chemical etching surface. The twins observed in the as‐grown ingot are mainly lamellar ones, which lie on the {111} faces from the first‐to‐freeze region of the ingot and run parallel to the growth axis of the ingot. Coherent twins with {115}_t‐{111}_h orientations when indexed with respect to both the twin and host orientations, are often found to be terminated by {110}_t‐{114}_h lateral twins. Te inclusions with about 20 μm in width are observed to preferentially decorate the lamellar twin boundaries. The origin of the twins, relating to the growth twin and the phase transformation twin, is also discussed in this paper. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High resolution neutron and X‐ray diffraction studies as a function of temperature and electric field of the ferroelectric phase transition of RDP

Neutron and high resolution X‐ray diffraction investigations on perfect single crystals of RbH₂PO₄ (RDP), a hydrogen bonded ferroelectric of KDP type are reported. The results of crystal structure analysis from diffraction data, below and above the paraelectric ‐ ferroelectric phase transition, support a disorder ‐ order character of [PO₄H₂]^‐‐groups. The tetragonal symmetry of the paraelectric phase with the double well potential of the hydrogen atoms obtained by diffraction, results simply from a time‐space average of orthorhombic symmetry. According to the group ‐ subgroup relation between the tetragonal space group I42d and the orthorhombic Fdd2 a short range order of ferroelectric clusters in the tetragonal phase is observed. With decreasing temperature the ferroelectric clusters increase and the long range interaction between their local polarisation vectors leads to the formation of lamellar ferroelectric domains with alternating polarisation directions at T_C = 147 K. From the high resolution X‐ray data it is concluded that below T_C the ferroelastic strain in the (a,b)‐plane leads to micro‐angle grain boundaries at the domain walls. The tilt angle is enhanced by an applied electric field parallel to the ferroelectric axis. The resulting dislocations at the domain walls persist in the paraelectric phase leading to a memory effect for the arrangement of twin lamellae. With increased electric field the phase transition temperature T_C is decreased. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigations on the structural perfection of bridgman-grown PbTe single crystals

It is reported on the possibility of improving the structural perfection by the Bridgman method, shown with the example of the influence on the low-angle grain boundary substructures in PbTe arranged in growth direction. In the employed crystal growth apparatus temperature gradients from 15 to 100°C/cm and solidification rates from 0.33 to 4.2 mm/hr could be achieved. The low-angle grain boundary substructure was characterized by X-ray-topographic and etch investigations on (100)-orientated ingots. The thermal conditions during the growth affect the structural perfection decisively. An axial spot dependence of etch pit density and substructure abundance was observed. By reducing the cooling rate during growth — expressed by the product of temperature gradient G and solidification rate V — it is possible to obtain more perfect PbTe-crystals. It seems that the diameter l of the substructure cells which was lying between 0.5 and 2 mm, is related to the cooling rate G · V by the functional coherency l ≈ 1/√G · V. The choice of different initial melt combinations up to 1 at.% Te-surplus exerted no influence on the abundance of the low-angle grain boundaries. The in literature suggested connection of low-angle grain boundary substructure with phenomena resulting from a constitutional supercooling could not be established.     

Estimation of spontaneous antipolarization of ADP based on magnitude of fourth‐order electrooptic effect

Applying the relationship between the spontaneous birefringence of ADP appearing in the low‐temperature antiferroelectric phase with the spontaneous antipolarization, and taking into consideration the known experimental values for the quadratic and fourth‐order electrooptic effects, an estimate is made of the lower limit of the spontaneous antipolarization. The estimation, which employs the fourth‐order electrooptic effect, gives the antipolarization P = 1.2 · 10^‐2 Cm^‐2, a value comparable in magnitude to the spontaneous polarization observed in ferroelectrics belonging to the KDP family of crystals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Brownmillerites as members of the unified OD family with two-dimensional disorder: A topology and symmetry analysis,the prediction of structures,and the properties of crystals

A topology and symmetry analysis of the structures of the brownmillerite family is performed within the OD theory. Blocks of two types with two-dimensional periodicity (“layers”) are revealed in two main structural types with Ibm2 and Pcmn symmetry. The high local symmetry of one layer is not characteristic of the other layer or of the structure as a whole. This feature determines the possibility of varying the arrangement of pairs of layers and the existence of one-dimensional disorder, as well as of structures with a maximum degree of order, periodic structures, and disordered structures. A symmetry analysis allows one to reveal the second direction of disordering (two-dimensional disorder), which is comparatively uncommon. The structure law of the family, which allows the systematic predicting of commensurate structural models without applying the fourth dimension, is described. Structure-property relationships are considered.     

Characterization and solventless growth of salicylic acid macro‐crystals involving a nitrogen gas flow

We report an original solventless thermal crystallisation method to grow large needle‐like salicylic acid (SA) crystals of 10‐12 mm in length. The method is based on the utilization of nitrogen gaz flow on salicylic acid powder during heating just below melting point temperature for 24 h. Salicylic acid provides one of the best examples of a pharmaceutical substance used for cosmetics whose physical and chemical properties indicate hydrogen‐bond formation between the hydroxyl group and an adjacent oxygen atom of the same molecule. The structure of the crystals is confirmed by single crystal X‐ray diffraction; it is monoclinic, a = 4.93(2) Å, b = 11.23(5) Å, c = 11.56(6) Å, β = 90.77(4)° with the space group P 2₁/c. The macrocrystals formation using this method is new and represents an interesting finding for a wide range of applications to be developed in the fields of biotechnology and photonics (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synchrotron White Beam X-Ray Topography Characterization of Defect Structures in 2,10-Undecanedione/Urea Inclusion Compounds

Abstract

The defect structure of a crystal of the urea inclusion compound (UIC) of 2,10-undecanedione was investigated using Synchrotron White Beam X-ray Topography. X-ray transmission topographs recorded from different regions show that the crystal is divided into several twin domains. Each region in the crystal is revealed on the topographs by orientation contrast arising from the mutual misorientation of adjacent regions. Using a combination of pinhole Laue pattern analysis and topographic orientation contrast analysis, the twin operation was determined to be consistent with an approximately 60° rotation about the orthorhombic c-axis. Possible twin boundary structures are also presented. Other defects such as dislocations and inclusions are also characterized.     

Three‐dimensional numerical investigation of the effects of an open window on the convective heat transfer of an oxide Czochralski system

A three‐dimensional numerical analysis was carried out for a real Czochralski crystal growth furnace containing only gas and without any melt and crystal in order to investigate the effects of a small observation window on the temperature and flow field of the system. For this approach, the induction heating equations, the Navier‐Stokes equation with Boussinesq approximation, the continuity and energy equations have been solved in cylindrical coordinates using the finite element method. It has been found that the flow and thermal fields in the system are obviously three‐dimensional and non‐axisymmetric. The gas enters the system through the window is directed towards the opposite side wall where it is divided into two parts of vertical direction as well as expands in horizontal direction. Consequently, there is a spiral gas flow in the crucible and afterheater which rotates upwards in azimuthal direction along the walls. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rh(I) and Pd(II) complexes of methoxy functionalized heterocyclic carbene: Synthesis and characterization

A new methoxy functionalized 2‐(trichloromethyl)‐1,3‐diarylimidazolidin (6) was synthesized as the precursor for N‐heterocyclic carbene complexes of Pd(II) and Rh(I) by the condensation of N,N'‐bis(2,4‐dimethoxyphenyl)‐1,2‐diaminoethane with chloral. The structures of all compounds have been elucidated by a combination of multinuclear NMR spectroscopy, elemental analysis and in one instance, by single crystal X‐ray diffraction. Compound 8, C₂₇H₃₄N₂O₄ClRh, crystallizes in the triclinic space group P‐1 with cell dimensions a = 9.7642(12)Å, b = 11.1914(11)Å, c = 13.0102(14)Å, α = 104.034(9)°, β = 106.658(9)°, γ = 99.658(9)° with Z = 2. The molecular structure of 8 shows the geometry around the Rh metal to be a slightly distorted square planar. The crystal structure shows the formation of centrosymmetric dimers via intermolecular C‐H...Cl hydrogen bonds. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of the crystal structures of 1,3‐di‐tert‐butyl‐2,3‐dihydro‐1H‐1,3,2‐diazasilol‐2‐ylidene and 1,3‐di‐tert‐butyl‐2,2‐dichloro‐1,3‐diaza‐2‐sila‐4‐cyclopentene

The crystal structures of the title compounds, 1,3‐di‐tert‐butyl‐2,3‐dihydro‐1H‐1,3,2‐diazasilol‐2‐ylidene, C₁₀H₂₀N₂Si ( 1 ) and 1,3‐di‐tert‐butyl‐2,2‐dichloro‐1,3‐diaza‐2‐sila‐4‐cyclopentene, C₁₀H₂₀N₂SiCl2 ( 3 ) were solved and are reported. Compound ( 1 ) crystallized in space group P mmn and each molecule has a mirror plane, which bisects the C‐C backbone of the N‐C‐C‐N framework. Compound ( 1 ) was also found to have a 2‐fold twin component. In compound ( 3 ) the space group P 2₁/m results with the mirror plane passing through the N‐C‐C‐N backbone. We compare these structures with the gas phase determination previously reported for ( 1 ) and the incomplete single crystal data for ( 3 ). (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bandgap characters in GaAs‐based ternary alloys

The existence and origins of the bowing character in the bandgap variation of GaAs‐based ternary alloys are theoretically investigated based on two different computational methods. Within the framework of the virtual crystal approximation (VCA), both the empirical sp³s * tight‐binding (TB) method with, and without, the inclusion of the spin‐orbit coupling effects, and the first‐principle full‐potential linear augmented plane wave (FP‐LAPW) technique are applied on both the common‐cation GaSb_xAs_1‐x and the common‐anion Ga_1‐xIn_xAs alloys. These methods are used to calculate the bandgap energy, the partial and total densities of states and the constituent charge ionicity versus the composition x. The results show that the bowing behavior exists in the case of common‐cation alloys (GaSb_xAs_1‐x) as a manifestation of a competition between the anion atoms (As and Sb) in trapping the made‐available‐cationic charges. The bowing parameter is found to be proportional to the electronegativity characters of the competing anions (χanion). Consistent with this in the case of common‐anion alloys (Ga_1‐xIn_xAs), as due to the lack of anion competition, the bowing is just absent and the variation of bandgap energy is found to be rather linear. The excellent agreement between our theoretical results and recent photoluminescence data has corroborated our claim. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Twin Structure of (La,Nd)GaO3 Solid Solutions

For La_1‐xNd_xGaO₃ crystals the La‐Nd substitution leads to decrease of spontaneous strains and for composition with x≈0.32 the six possible twin states of orthorhombic phase (m3mFmmm) may be degenerated in three twin states inhered in m3mF4/mmm species when a distorted perovskite pseudocell becomes tetragonal. The {110} and {112} reflection twins and axial twins with compositional planes close to (211) and (21‐1) (S‐walls) were identified in La_1‐xNd_xGaO₃ (x=0.07, 0.12, 0.20) solid solutions crystals. All observed twins are typical for crystals with GdFeO₃ type perovskite‐like structure. It has been shown that for x≤0.2 and x≥0.5 orientations of S‐walls weakly depend on La/Nd ratio, whereas in the range of 0.2<x<0.5 they depend strongly on the solid solution composition. The tilt angle between two twin states across twin boundary in La_1‐xNd_xGaO₃ (x<0.6) solid solutions is smaller than that between two twins in pure LaGaO₃ or NdGaO₃.     

RHEED Measurements of AgGaSe2 thin films flash evaporated onto {111} a-GaAs

AgGaSe₂ thin films were prepared by flash evaporation on {111} A-oriented GaAs substrates and investigated by reflection high energy electron diffraction (RHEED). Epitaxial growth takes place in the substrate temperature interval of T_s = 845–920 K. The azimuthal orientation of the deposit on the substrate is discussed, supposing a AgGaSe₂ c/a ratio of 1.82. It is shown that detection of twin formation in the films is possible from RHEED measurements in the <211>-azimuths with respect to the substrate.     

Topology and symmetry analysis of rare earth borocarbides structural family,analogy to hexaferrites and relation to properties

The topology and symmetry analysis was applied to a series of rare earth borocarbide compounds, which have been gaining increasing interest due to their magnetic and thermoelectric properties. Using principles of OD theory, the crystal structures were deconvoluted into L(1) (B₁₂ icosahedra and C‐B‐C chain) layers and L(2) (rare earth and B₆ octahedral) layers. The arrangement of B₁₂ icosahedra in the L(1) layer is equal to close packed spheres, however, symmetry of the B₁₂ block lowers symmetry of the resulting layer from P 6/mmm to P 3m1. Both layers, L(1) and L(2) possess symmetry P 3m1 and the conjugation of L(1) with L(2) layers occurs in accordance with the symmetry elements. No disorder may appear here because of equal symmetry of single layers and layer pairs and it is not a classical OD family. Only the increasing of the amount of one type of layers, namely L(1), provides the structural variations. Close analogy to the hexagonal ferrites family has been found. Topology and symmetry analysis reveals principles in the building up of the structural family, gives an insight into the particular order‐disorder formation mechanism/criteria of these homologous borocarbide compounds and as the result relation to the properties (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dynamics of twin layer broadening in indium

The mobility of plane-parallel twin layer boundaries in indium crystals has been studied in the 235–390 K temperature range under stresses, τ, of (0.25–20) g/mm² (static load) and (35–335) g/mm² (crest values under impact load). For τ/G = (0.2–0.6) × 10⁻⁵ (G is the shear modulus), the process of twin layer broadening has been shown to be thermally activated; the process parameters and their stress dependence have been found. – Possible mechanisms of twin layer broadening have been analysed, and it has been concluded that a pole mechanism is inadequate to treat the results obtained. The twin broadening is considered to be due to nucleation and motion of twinning dislocations along boundaries in each subsequent twinning plane; it has been shown that the thermal activation parameters measured while broadening a twin layer, can be inconsistent with the elementary acts of broadening process (nucleation or motion of twinning dislocations). A deep gap between data on stress dependence of the activation energy of twin layer broadening for indium and calcite crystals and the Sumino theory is explained by the determining influence of the real imperfect structure of specimens on the process studied.     

Transmission electron microscopy observations of twin boundaries and sub-boundary networks in bulk CdZnTe crystals

The microstructure characteristics of the twin boundaries and sub-boundary networks in bulk cadmium zinc telluride (CdZnTe) crystals have been studied by transmission electron microscopy (TEM). Three types of twin boundaries were identified and characterized, which are (i) single straight twin boundary, (ii) tilt twin boundary, and (iii) twin boundary with steps. Boundary dislocations at tilt twin boundary and high dislocation density around steps were observed. The origin of the boundary dislocations is ascribed to the lattice misfit between two tilt grains. The formation of the twin boundary with steps is suggested to be the interaction between the twin boundary and dislocations. Honeycomb-like sub-boundary defects were also observed. The probable reason for the formation of the sub-boundary networks is discussed.     

Preference for fcc atom stacking observed during growth of defect‐free LJ3281 clusters

Growth simulations of 3‐dimensional Lennard–Jones (LJ) clusters/nuclei from the vapor phase were carried out in order to estimate the role of kinetic and energetic effects in determining the internal structure of the clusters. The simulated growth was realized from N = 3281 up to ca. 10000 atoms at two reduced growth temperatures T^* = 0.3 and 0.5 and the vapor atom concentration n_v = 0.002 (in reduced units of LJ system). During growth at the low temperature, clusters evolve from octahedral to an irregular shape, while at the higher one they become spherical. Statistics of newly created structural units shows a strong preference for the formation of fcc units with their number approximately 3‐4 times larger than those of the hcp units. The possible explanation of this preference is by: (1) a larger surface diffusion leading to near spherical shape of clusters, (2) an energetic effect caused by different interaction energy of two fcc or hcp layers located on the neighboring {111} planes, and (3) an entropic effect in the form of disappearance of hcp island on the close‐packed fcc layers. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X‐ray diffraction studies of bone apatite under acid demineralization

The analysis of the X‐ray diffraction line broadening used to determine bone apatite crystallite size and lattice microstrain can provide information about the substructure of the bone mineral under differing real or simulated conditions. The paper discusses modifications in the bioapatite crystals observed in the bone subjected to demineralization in a 0.1 N HCl solution. Planar oriented specimens of mature bone with the analyzed surface normal to the longer bone axis were treated for varied immersion times. The crystallite size and the lattice microstrain were determined simultaneously by Fourier analysis of the X‐ray diffraction line profiles. Both were observed to decrease during the acid demineralization. These findings support the idea that the distribution of lattice imperfections in the bulk of bioapatite crystals is highly nonuniform, with crystallite surface regions richer in imperfections dissolving more readily. In addition, an approach is proposed suitable for rough estimation of the reaction front advancement by demineralization‐induced variations in the integrated intensity or shift in the position of the (002) and (004) diffraction lines. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of X‐ray extinction in crystals with inhomogeniously distributed dislocations

The secondary extinction theory of Zachariasen for mosaic crystals as well as the formalism of Becker&Coppens have been used for substructure analysis (lattice disorientations, block size, density of excess dislocations, etc.) in crystals with inhomogeniously distributed dislocations in the Bragg case of diffraction geometry. In the case of large crystals D_S ≫ Λ_hkl the mean total density ρ_D of randomly distributed dislocations was also estimated taking into account additionally the primary X‐ ray extinction treatment. In this connection two cases are considered: I) pure secondary extinction related to arrangement of dislocation walls and II) mixed extinction in crystals with large subgrains related to randomly distributed dislocations and arrangement of dislocation walls. In order to check the considerations, the experimental and calculated data were compared for Be (model I) and Cu (model II) single crystals. The weakly distorted single crystals of Be and Cu were experimentally investigated in the Bragg case of diffraction geometry using Cu Kα₁ radiation by means of double crystal diffractometer. A new experimental procedure was proposed. Using alternative technique for substructure analysis (for instance EBSD) the reliability of analysis based on extinction phenomenon in weakly distorted single crystals has been checked. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation of three‐dimensional symmetric dendrites of BaWO4 microcrystals

Three‐dimensional symmetric dendrites of BaWO₄ were prepared by performing solvothermal reactions in water‐hexane bilayer solutions that contained a barium‐oleate complex in the hexane layer and sodium tungstate in the water layer. The barium‐oleate complex in hexane was obtained via a phase transfer reaction in which aqueous Ba²⁺ ions pass into the hexane phase with the assistance of sodium oleate, oleic acid, and oleylamine. Each of the BaWO₄ dendrites has a three‐dimensional structure with one trunk along the c‐axis and a series of four branches perpendicular to the trunk. As the reaction temperature or the amount of oleylamine increases, BaWO₄ dendrites with sharper branches and higher order hierarchical structures are formed. The presence of oleylamine plays an important role in the formation of the hierarchical superstructures of BaWO₄ dendrites.