STGR Model and Relation between Growth Rate and Polytypism

Effect of crystal growth rate on polytypism has been explained using STGR model. It has been suggested that during the process of growth, polytypes are formed from a suitable combination of basic units in accordance with the rate of growth. At a slower rate the most probable polytype is predicted to be different from the one at faster rate.     

On the occurrence of polytypism in single crystal CdTe thin films

Cadmium telluride thin films have been found to exhibit polytypism. The polytypes are formed when the as grown amorphous CdTe thin films undergo amorphous to crystalline transformation. The transformed single crystal regions correspond to different polytypes. Besides the well known zinc blende type 3 C cubic phase and less often found wurtzite type 2 H phase, four new polytypes (5 H, 6 H, 6 R and 15 R) the only ones known to-date have been found in the present investigation. In addition to the new polytypes, a new structural variant has also been found. This has the same ‘c’ parameter as that of the 2 H phase but has its ‘a’ lattice parameter as ‘a₀ \documentclass{article}\pagestyle{empty}\begin{document}$ a_{\rm o} \sqrt {3} $\end{document}’ (a₀ being the common lattice parameter of the polytypes). A feasible mechanism making the formation of polytypes intelligible has been suggested.     

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)     

A Statistical Thermodynamics and Growth Restrictions (STGR) Model of Polytypism with Special Reference to Zinc Sulfide

A model to explain polytypism based upon classical statistics and the idea of minimum sandwiches of basic phases has been developed. Presence of two or more phases under suitable thermodynamic conditions and restrictions imposed by statistical, structural, and growth factors have been elaborated. A new statistical approach is used and a criterion to find most probable polytypes of a compound has been evolved. Logical implications of the model are found in good agreement with the experimental data.     

Desymmetrization of OD Structures

It has been shown that the symmetry and structure of individual OD layers may depend on their actual stacking in a (polytypic) OD structure, leading to significant deviations from the ideal OD symmetry. This phenomenon has been called “desymmetrization of OD structures”; its significance for polytypism and for crystallochemistry is discussed.     

Temperature dependence of the tetragonal distortion in I–III–VI2 compounds

On the basis of free energy considerations with anharmonic contributions included a model is developed for the temperature dependence of the tetragonal distortion Δ of the I–III–VI₂ compounds. Agreement with experimental data can be only achieved if compounds with Δ < 0.05 and Δ > 0.05 are considered separately. It is suggested that this result is related to the structural order-disorder behaviour of the I–III–VI₂ compounds.     

Polymorphic–polytypic transition induced in crystals by interaction of spirals and 2D growth mechanisms

The relationship between crystal polymorphism and polytypism can be revealed by surface patterns through the interlacing of the growth spirals. Simple high-symmetry structures as SiC, ZnS, CdI2 and more complex low-symmetry layered structures as n-paraffins, n-alcohols and micas are concerned with polymorphic–polytypic transition. In this paper, we will show for the first time, through in situ AFM observations and X-ray diffractometry, that a protein polymorph (P2₁2₁2₁α-amylase) locally changes, during growth, to a monoclinic P2₁ polytype, thanks to the screw dislocation activity. The interplay between spiral steps and 2D nuclei of the polytypes coexisting in the same crystalline individual allows to foresee the consequences on the crystal quality. The discussion is extended to other mineral and biological molecules and a new general rule is proposed to explain the interactions between surface patterns and the bulk crystal structure.     

Synthesis and Properties of a New Kind of One-Dimensional Conductors 17. Studies on monomeric and polymeric phthalocyaninato-iron(II) and -cobalt(II) complexes

A new approach for a one-dimensional organic conductor with a chemically bonded backbone is presented. The proposed compounds consist of a linear arrangement of transition metal atoms, e.g. Fe, Co, bridged by linear bidentate π-electron containing ligands, e.g. pyrazine, 4,4'-bipyridine, and 1,4-diisocyanobenzene. This structure is stabilized by the tetradentate planar phthalocyanine molecule complexing each octahedrally coordinated central atom in its equatorial plane. The synthesis and characterization of the monomeric, dimeric and polymeric compounds PcML/PcML₂, PcM-L-PcM and [PcML]_n are described and a comparison of their electrical conductivity as a function of their structural features is given in detail. The electrical conduction of the monomeric units increases after polymerisation by a factor of 10⁷ without doping. In the second part soluble tetrasubstituted phthalocyaninato(IVB)-metalloxanes are described and the effect of homogeneous and heterogeneous iodination on conductivity is discussed.     

Distortion Isomerism of Cu(II) Chloride Adducts with Bis(2-benzimidazolyl)ethane. Synthesis,Characterization, X-ray Structures and Spectroscopy of Four Different Isomers

3D structures and spectroscopic properties are reported of four compounds with the ligand bis(2-benzimidazolyl)ethane (hereafter abbreviated as dbz) attached to CuCl₂, all having the general formula [Cu(dbz)Cl₂](Hb) _x . (in which Hb = EtOH or MeOH and x = 1/2 or 1). The X-ray crystal structure has been solved from these four slightly different compounds, namely: green α-[CuCl₂dbz](C₂H₅OH)_1/2 (1), the red compound β-[CuCl₂dbz](C₂H₅OH)_1/2 (2), which both have two slightly different units in the unit cell, the red α-[CuCl₂dbz](CH₃OH) (3) and a blue–green compound β-[CuCl₂dbz](CH₃OH) (4). The geometry around the Cu(II) anion is distorted tetrahedral for all four compounds, with chromophores consisting of two nitrogen atoms of the bidentate chelating dbz molecule and two chloride anions. The unit cells of compound 1 and 2 consist of two chemically identical, but crystallographically different units, while compounds 3 and 4 each have only one independent CuCl₂-(dbz) unit. The major differences are observed in the dihedral angles NCuN–ClCuCl, which vary from 29.3 to 77.1° for the four compounds. The differences are related to different packing effects, ring–ring stacking and H-bond interaction, due to the two different alcohols used. In fact these four compounds represent a new range of examples of distortion isomerism in pseudo-tetrahedrally coordinated species. Characterisation of the four compounds has been completed by IR, EPR and LF spectroscopy.     

Low-temperature Region of the In–Se System

A study of the low-temperature region of the In–Se system has been performed by means of isothermal annealing of diffusion couples as well as by X-ray, microscopic and microprobe analyses. Four binary compounds have been observed – In₄Se₃, InSe, In₆Se₇ and In₂Se₃. Original experimental results about the crystal structure and electrical conductivities of the binary compounds have been compared with available literature data or calculated values. A DSC study of the compound InSe has revealed that it melts peritecticly at about 618 °C. The diffusion controlled transition from one In-Se compound to another can be achieved without kinetic difficulties, following the phase arrangement from the equilibrium phase diagram. There are indications that small deviations from the exact stoichiometric ratios are possible.     

Liquid phase epitaxy of garnets

A short introduction to the technique of liquid phase epitaxy of single crystal thin films of magnetic garnets is given with the emphasis on the relative merits of vertical and horizontal dipping. For vertical dipping results are given for the thickness profiles from top to bottom of the samples and the inhomogeneities within the film thickness. The thickness profile is explained by fluid motion caused by density segregation. The inhomogeneities are explained by growth rate changes due to transient growth at the beginning and nearly steady state growth when the diffusion boundary layer has been formed. It is shown under special conditions these inhomogeneities in composition in LaGa : YIG films can be an advantage and can increase the domain wall velocity. These effects are compared to films grown by horzontal dipping with rotation. Using spin wave resonance it is shown that this process does not produce as homegeneous films as expected. Explanation of this is given in terms of fluid flow during the growth. Because literature data on the diffusion coefficient D has a wide spread of values, an experiment is described in which D has been measured independent of the model using radioactive tracers. These results, combined with the model that the garnet does not diffuse as a molecule but as species like Y₂O₃ and Fe₂O₃ and at the growing interface react to form garnet, has lead to a new insight into the solvation of these species in the liquid. Magnetic bubble films of the type CaGe substituted iron garnets are used in large numbers for devices. This makes the reproducibility an important factor. To increase the yields of usuable material it has been found necessary to make the magnetic parameter H_collapse independent of the growth temperature because of the thermal constants in LPE furnaces. This has been achieved by combining CaGe melts with Ga. Results are given for a 3 μm bubble material. The LPE technique can be used for non-magnetic garnet materials as well as iron garnets. These materials are usually substituted with rare-earths. The incorporation of Ce into Y₃Al₅O₁₂ is a difficult case. How this can be achieved and difficulties encountered are explained in the last part of the paper.     

Melt-Growth of Single Crystals of Cadmium Iodide and Study of Their Polytypsim by X-ray Diffraction

Single crystals of cadmium iodide have been grown from melt using Bridgman-Stockbarger system and their polytypism studied by X-ray diffraction. The initially grown crystals were found to stick to the walls of the ampoule and had black specks of impurity suspended in them. A systematic study has been carried out to determine the effect of degree of vacuum, material of the ampoule and quality of raw material on crystal growth. It has been concluded and later verified using zone-refined material that the observed sticking essentially stems from the presence of impurities in the raw material. From X-ray diffraction it is found that all crystals belong to the most common polytype 4H alone.     

On the computer simulation of many-beam electron microscopical images of phyllo-silicates

The polytypism of pyrophyllite as a special phyllo-silicate is investigated by means of high resolution electron microscopy (HREM), supported by computer simulation. Electron micrographs of appropriate specimen orientations provide crystal structure images, which reveal the actual stacking sequence of the silicate layers. Two crystallographic models (2M₁- and 1Tc-type) are applied for the numerical simulation of the image contrast. The process of the electron scattering in the crystal as well as the influence of the imaging parameters are described by means of multi-slice image calculations. The strong effect of through-focusing to the image contrast of the silicate structure is pointed out.     

Chemical transformations in the W-Al2O3 system under pressure P = 1 bar near the melting point of aluminum oxide

The W-Al₂O₃ system has been considered at a temperature of 2400 K and a pressure of 1 bar. The main chemical processes providing the interaction between the components of the system have been determined. It is shown that evaporation of Al₂O₃ into the gas phase gives rise to numerous reactions, which involve not only tungsten but also Al₂O₃ melt. It is concluded that such interactions can be reduced by decreasing the Al₂O₃ evaporation, which can be done by increasing the inert gas pressure. This approach makes it possible both to optimize the parameters of sapphire crystal growth and increase the lifetime of a tungsten heater and other units of crystallization systems.     

Influence of Naphthalene Moiety on Mesomorphism

Compounds containing naphthalene moiety are synthesized and the liquid crystalline properties studied by texture observations. The mesomorphic properties of these compounds are compared with related compounds to understand the influence of broad units such as naphthalene on mesomorphism. It is observed that there is a limit to which the end alkoxy group can be extended in a mesogen. The method of preparation, and properties of these compounds are given.     

X‐ray powder diffraction structural characterization of iron(III) chloro‐ and bromo‐complexes of triphenylphosphine oxide

Iron (III) complexes of the composition [FeX₂(OPPh₃)₄][FeX₄], where X is Cl or Br were synthesised by several methods. It was found that each particular synthesis has yielded solids with slightly different powder patterns and also their comparison to those of calculated from single crystal data showed remarkable differences. A series of constrained Rietveld refinements has been performed on the title complexes to explain these differences. In a course of Rietveld refinement it was found that preferred orientation is not the only factor affecting the shape of the pattern and atoms parameters were included into refinement too. The results of Rietveld refinement allowed to conclude that the structures of compounds prepared by different methods are identical. In addition, it was shown that also in the case of rather complicated structure like the title complexes the powder diffraction method can lead to reasonable results.     

Fonction Dielectrique Complete D'un Cristal Moleculaire: Polarisabilite Moleculaire et Absorption Infrarouge

In the special case of a molecular crystal, it has been shown that the background of the infrared absorption spectra is not only a function of the dielectric constant at high frequency: the pertinent formulation introduces the entity (ε) _fc in which the contribution of all the optical modes have been taken into account. An analytical expression for (ε) _fc is suggested and make possible the determination of the oscillators strengths with a good accuracy. Calculations have been performed for polycrystalline samples of H.M.T and H.M.T.D. This new parameter has no effect upon the damping constants and slightly affects the static dielectric constant. On the other hand, its influence is clearly established for the infrared reflexion spectra.

The relation between microscopic molecular polarizability α(ω) and macroscopic dielectric function ε(ω) has been given. From the analytical expression of ε(ω), both real and imaginary parts of α(ω) have been expressed in terms of frequency. All the characteristic points of them have been calculated for spectral ranges corresponding to molecular crystal infrared absorption. Application to the F₂ vi-brational modes of solid H.M.T has been given. In other respects the contribution of the nuclei to the static molecular polarizability has been estimated with a good accuracy.     

Growth surface topography of kaolinite-group minerals and their simulation based on the regular sequence of enantiomorphic layers

The regularities of manifestation of polytypism in a number of kaolinite-group minerals (kaolinite, dickite, halloysite, and nacrite) have been investigated by transmission electron microscopy and vacuum decoration. Growth patterns of elementary layers with a thickness of 7 Å, individual for each polytype, were observed on (001) faces of microcrystals. Specific features of the growth patterns of polytypes have been revealed by comparing them with simulated patterns constructed based on a packing of regularly alternating right- and left-handed (enantiomorphic) kaolinite layers. The new approach to the consideration of the polytypism of kaolinite minerals is substantiated by the absence of symmetry elements in the 7-Å-thick layer, which determines their structure; the formation of enantiomorphic forms of kaolinite; the presence of grazing-reflection planes in the growth patterns; and the structure of polytypes with a two-layer period. Packing of enantiomorphic layers may yield eight structures, two of which correspond to the right- and left-handed forms of kaolinite, one is for dickite, two are for halloysite, and three are for nacrite. It is shown that the simulated and real growth patterns of these minerals are in good correspondence.     

Structural investigation of SnO-B2O3 glasses by solid-state NMR and X-ray photoelectron spectroscopy

Local structure of the SnO-B₂O₃ glasses was investigated using several spectroscopic techniques. ¹¹B MAS-NMR spectra suggested that BO₄ tetrahedral units maximized at around the composition with 50 mol% SnO. The BO₄ units were still present at compositions with high SnO content (67 mol% SnO), suggesting that SnO acted not only as a network modifier but also as a network former. O1s photoelectron spectra revealed that the addition of small amounts of SnO formed non-bridging oxygens (NBO) (B-O?Sn) and the amounts of NBO increased with an increase in SnO content. ¹¹⁹Sn Mössbauer spectra indicated that Sn was present only as Sn(II) in the glasses. The structure of the SnO-B₂O₃ glasses was compared with that of conventional alkali borate glasses and lead borate glasses. The thermal and viscous properties of these glasses were discussed on the basis of the glass structure revealed in the present study.     

Peculiarities of the behavior of the W–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system in a controlled reducing atmosphere

The W–Al₂O₃ system at T = 2400 K and standard pressure (controlled Ar + H₂ atmosphere) has been calculated by stochastic simulation. It is shown that the presence of hydrogen leads to the formation of aluminum hydrides, hydrogen oxides, and aluminum hydroxides; the compounds from the two latter groups (except for water) can interact directly with tungsten. The main chemical reactions occurring in the system are determined, based on which a conclusion about the cyclic character of the processes is drawn. Some recommendations on the composition and pressure of controlled atmosphere for growing sapphire crystals are given.