The Nature of One-sided Intergrowths on (001) of Triclinic Centrosymmetric K2Cr2O7

Systematic composite crystal growth can arise when larger crystals of the same type exhibit oriented intergrowth. One example of this is potassium dichromate (P1 ) which SHUBNIKOV first described in 1912. In highly supersaturated solutions, a one-sided roughening of its crystals occurs on (001 ). Electron micrographs show that this is due to the growth of platelet-shaped crystals. Optical and X-ray methods as well as electron channeling and BACK-KIKUCHI diffraction patterns have shown that the growths are predominantly parallel intergrowths, about 10% of which are twins according to [010].     

The Nature of One-sided Intergrowths on (001) of Triclinic Centrosymmetric K2Cr2O7 (II)

SHUBNIKOV first observed in 1912 that centrosymmetric K₂Cr₂O₇ crystals (P1 ) exhibit a growth anomaly in highly supersaturated solutions. The (001) faces of these crystals are always smooth, the parallel opposite faces are always rough. Electron micrographs show these rough surfaces to be crystallites that have grown in stacks. In Part I of this publication, the crystallite orientation was determined using electron-induced diffraction methods. As a rule, these crystallites are parallel intergrowths. Only a small number of crystals near the surface of the host crystal are twinned according to [010]. Part II sought to ascertain the cause of these one-sided intergrowths. To determine the most favourable energetic arrangement of two intergrown individuals, interactions across the intergrowth face as a function of shifts parallel to (001) and to (001 ) were calculated, 64 theoretically possible cases were considered. Only in the [010]-twin and the seldom-occurring [11 0]-twin is an approximation of the 2/m symmetry of the macroscopic twin arrangements also found in the intergrowth structure, and this is the case only when the (001 ) faces of the host and guest crystal are intergrown.     

Defect Structure of Strained Heteroepitaxial In(1—x)Al(x)P Layers Deposited by MOVPE on (001) GaAs Substrates

Weak-beam, large angle convergent beam electron diffraction and high resolution transmission electron microscope experiments have revealed, that after strain relaxation due to plastic deformation dislocation networks can be observed in In_(1—x)Al_(x)P heteroepitaxial layers grown on (001) GaAs substrates under compressive stress. The 60° slip dislocations are mostly dissociated into partials of Shockley type whereas in the particular case of layers grown under tension twins are predominantly formed by successive nucleation and slip of 90° Shockley partials on adjacent {111} glide planes lying inclined to the (001) surface. When a few 90° Shockley partials pile up during extension of twins, then planar incoherent twin boundaries with {112} coincidence planes have been formed during strain relaxation. Due to the space group symmetry ((InAl)P belongs to the space group F4-3m) there is a striking asymmetry in defect formation, i.e. defect nucleation and slip on the planes (111) and (1-1-1) slip of the [1-10] zone are preferred to nucleation and slip on the {111} planes of the [110] zone. Apparently, the occupacy of the atomic sites in the dislocation core with either group-III or group-V atoms is responsible for this behaviour. The nature of the defects implies that their spontaneous nucleation should have taken place at the growing surface. Under tensile strain the 90° Shockley partial is nucleated first and the 30° one trails. Under compressive strain this sequence is reversed. It is evident, for dissociated dislocations lying at the interface always the 30° partial, i.e. the partial with less mobility or with higher friction force, is detained near or directly in the interface. Thus, in layers grown under tension the stacking fault associated with the dissociated 60° dislocation lies inside the GaAs substrate. For layers grown under compression it is located inside the ternary layer.     

Growth Mechanism and Crystal Form of Potassium Dichromate

The structure of potassium dichromate (P-1, Z = 4) consists of two layers parallel to (001), between which no crystallographic symmetry relation exists. These layers are nearly coincident with each other when rotated through 90° about c*. One question remained unanswered: do the crystals grow parallel to {001} with d(002) or d(001) layers? The pseudo-fourfold axis does not run parallel to a crystallographic direction and acts only between two layers. This means that crystal growth parallel to {001} occurs in d(001) layers, which corresponds to the occurrence of one-sided intergrowths on (00-1). The layers are also nearly coincident with each other through pseudo-twofold axes parallel to [110] and [-110]. These axes do not act in the (100) or (010) projections. In the projections along the [110] and [-110] directions, the lattice directions are then extended by a factor of approximately 1.414, and the formula units in the cell of the d(002) layer are doubled. This indicates a further possible growth mechanism. The theoretical growth forms of the crystals were calculated with the Fourier transform method of morphology.     

Crystal growth of mixed‐valence ammonium vanadates

A new method has been developed for the synthesis of mixed‐valence ammonium vanadate crystals. Single crystals of (NH₄)₂V₃O₈ were synthesized on a large scale by hydrothermal reduction of NH₄VO₃ in ethanol‐H₂O solutions in the presence of triblock copolymer Pluronic P123. The crystals are shining thin plates with (001) cleavage planes. Calcination of the (NH₄)₂V₃O₈ crystals at 300°C or above resulted in pure phases of V₂O₅. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Etching Studies on Potassium Dichromate (KBC) Crystals

An investigation of the revelation of dislocations in potassium dichromate (KBC) crystals by etching in water, alcohls, and in inorganic acids mixes with acetone and alcohols is carried out. It is observed that several solutions including water, alcohols and organic acids produce disslocation etch pits on various faces of KBC crystals. Observations of twining and dissolution anisotorpy of (001) faces are also presented. Etching of (001) cleavage faces of same crystals in an etchant composed from 1 part HNO₃ and 3 parts acetone indicates the possible growth of another phase at low temperatures.     

Anisotropy of microhardness and fracture of (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>Ni(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> · 6H<Subscript>2</Subscript>O (ANSH) crystals

The Vickers hardness of the (010) and (001) planes in (NH₄)₂Ni(SO₄)₂ · 6H₂O (ANSH) crystals has been measured. Anisotropy hardness of the first kind is revealed for the (010) plane in ANSH. The hardness anisotropy coefficient k ₂ was determined to be 1.5. The temperature dependence of the microhardness of the (001) face of ANSH crystals was investigated in the temperature range from 20 to 80°C. The character of fracture of the (100), (010), and (001) planes during indentation with a spherical indenter has been qualitatively determined.     

Electric twinning in quartz by temperature gradient

This paper presents a new method for obtaining electric twins in quartz crystal: thermal treatment in a temperature gradient at about 500 °C, that means under phase transition point. + 5X quartz crystal plates for resonators were used; a temperature gradient of 15–25 °C was applied for 4–5 hours along the Y axis (the place length). For + 5X resonators, the diminution of the resonant frequency and the increase of the inductance were plotted as a function of the level of twinning.     

Growth and dielectric properties of Ta2O5 single crystal by the floating zone method

Large Ta₂O₅ single crystal with high‐dielectric permittivity was successfully grown by floating zone (FZ) method under air atmosphere. The grown crystal that has been obtained was typically about 8 mm in diameter and 90 mm in length. The crystal growth parameters were optimized. The crystal symmetry, characterized by means of X‐ray diffraction (XRD), was found to be tetragonal. The relative permittivity and loss tangent along growth and [001] direction were measured in the temperature range between ‐200 °C and 200 °C, which showed a strong dielectric anisotropy. At a frequency of 1 MHz and 20 °C, the dielectric permittivity along the growth direction and [001] direction are 81.17 and 25.04 respectively. The stabilization of high‐temperature phase can explain the dielectric enhancement.     

MOVPE growth of spontaneously ordered (GaIn) and (AlIn)P layers lattice matched to GaAs substrates

A systematic study of the metal-organic vapour-phase epitaxial growth of (GaIn)P and (AlIn)P layers deposited on GaAs substrates with (001) and (110) orientation is presented. Special attention has been paid to the growth on (001)-oriented wafers with different misorientations to the growth direction. The influence of the growth conditions on the properties of the epitaxial layers such as lattice mismatch, alloy composition, photoluminescence (PL) wavelength, FWHMs of PL peaks and atomic ordering is discussed. Layers with mirrorlike surfaces and various degrees of order could be deposited at growth temperatures T_g ranging from 595 °C to 750 °C for (GaIn)P and 720 °C to 800 °C for (AlIn)P. In addition to the influence of T_g on the Ga incorporation during the (GaIn)P growth we found the Ga distribution coefficient k_Ga to be affected by the misorientation of the substrates. k_Ga correlates presumably with the number of kinks and steps on the substrate surface. Transmission electron diffraction (TED) and PL investigations show that the degree of order — often described by the ordering paramter η — depends strongly on T_g the ordering is more pronounced when the layers are deposited on substrates misoriented towards the (1 11) lattice plane. Strong ordering has been observed for (GaIn)P samples grown at 680 °C on substrates 2° misoriented towards the [1 10] direction and at 650 °C on substrates 6° misoriented towards the same direction. For the (AlIn)P samples striking ordering has been found when they were grown at 720 °C.     

Stabilized Ion Cr5+ Paramagnetic Resonance in Potassium Dichromate Single Crystals

Potassium dichromate single crystals were synthesized from aqueous solution by isothermal evaporation of the solvent. The electron paramagnetic resonance (EPR) spectra were studied from Cr⁵⁺ ions, stabilized by 60 min heating of X-irradiated crystals at 120°C. EPR spectrum was observed which consists of a strong central line and four almost equidistant satellites with intensities as weak as 2% from that of the central one. The received EPR spectrum may be described within the experimental errors by the anisotropic spin Hamiltonian, the constants at 77°K being: g∥ = 1.982 ± 0.001, g⊥ = 1.925 ± 0.001, A⊥ = 10.2 ± 0.4 G and A⊥ = 4.0 ± 0.4 G.     

In situ AFM investigation of 2D nucleation on the (100) face of KDP

Surface morphology of the (100) face of potassium dihydrogen phosphate (KDP) crystals which were grown at different supersaturations at 25 °C was investigated by in situ atomic force microscopy (AFM). Various AFM images of 2D nucleation under different growth conditions were presented. It is found that the growth of KDP is controlled by polynuclear nucleation mechanism at the high supersaturation. With reduction of the supersaturation, the growth velocity of 2D nuclei becomes very slow and shows typical anisotropy. It is found that the process of coalescence of 2D nuclei does not lead to defect. The experiments show that the growth mechanism for KDP at 25 °C changes between step flow and 2D nucleation in the supersaturation range of 4.5‐5%. The triangular nuclei which are close to equilateral triangle are observed in the experiment at the supersaturation σ = 6% for the first time, showing typical anisotropic growth. Through observing the dissolution of 2D nuclei, the dissolving process can be regarded as the reverse process of growth. We also find that the microcrystals landing on the surface at σ = 9% would grow and coalesce with each other and there is no observable defect in the coalescence. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Slip band formation during bending of gap wafers

{001} LEC-GaP:S wafers were deformed between 470 °C and 640 °C. {111} slip planes of maximum resolved shear stress were activated. Heights and densities of slip steps have been measured by optical and electron microscopy. At lower temperatures deformation proceeds mainly by growth of step heights whereas at higher temperatures bending is accomplished by the increase of slip band density rather than of height.     

Evolution of crystal structure of Zn:N films under high temperature

Zinc‐nitrogen (Zn:N) compound thin film was prepared from a pure metallic Zn target by rf magnetron sputtering at ambient temperature under the mixture of nitrogen and argon gases with the ratio of 1:1. High temperature x‐ray diffraction (HTXRD) measurement under vacuum was used to examine the evolution of structural properties of the Zn:N film. At ambient temperature, the (002), (100), and (101) planes corresponding to Zn structure were observed while at higher temperature, the left shifts corresponding to the increase of lattice constants a and c of Zn were observed. At temperatures of 320 °C, 481 °C and 554 °C, the (222), (321) and (400) planes corresponding to Zn₃N₂ structure were observed with the decrease in the intensity amplitudes of the peaks belonging to the Zn structure. The results indicate the gradual transformation of the Zn₃N₂ phase in the Zn:N films at temperature greater than 320 °C. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Microhardness and the Toughness Anisotropy of Paratellurite Single Crystals in the Tr — 650 °C Temperature Range

The temperature hardness (H_v) and toughness (K_1C) dependence of paratellurite (100), (010), (001) planes is investigated in the T_r −650 °C range. The correlation between the anisotropy degree of Young modulus and H_v is not found. The presence of 11 kind H_v and K_1C anisotropy is shown.     

Growth and Properties of Tetragonal Tungsten Bronze Type Potassium Lithium Niobate Single Crystals

Tetragonal tungsten bronze type potassium lithium niobate single crystals with the Nb/Li ratio larger than 3 are grown by the flux pulling method in our laboratory, and it dose not crack when cooling through the paraelectric/ferroelectric phase transition. Crystal growth is studied in two orientations with growth along [100] and [001], and the latter's quality is superior to that of the former. The lattice constants are a = 1.2575 nm and c = 0.3997 nm at 298 K. A Curie temperature is about 480 ± 3°C.     

On the influence of the pre-ageing temperature on the reversion behaviour of an Al-4.5 at.% Zn-2.0 at.% Mg alloy

The reversion behaviour of an Al-Zn(4.5)-Mg(2.0) alloy was investigated by SAXS in dependence of the pre-ageing temperature, T_pre, (ranging between 60°C and 100°C) and the reversion temperature, T_rev, (120°C till 200°C) starting with precipitates having a radius of (1.2 ± 0.1) nm and (1.5 ± 0.1) nm, respectively. During the reversion treatment applied up to T_rev = 160°C three stages could be distinctly distinguished, namely the dissolution of unstable zones, the growth of the stable zones on the expense of the dissolved one, and at last the coarsening of the precipitates by the OSTWALD-ripening process, where the structure changes become independent of the pre-history (start radius). The change of T_pre from 60°C to 100°C does not influence the structure changes going on at T_rev, that means between 60°C and 100°C the same type of G.P. zones grows.     

Hydrothermal analcime synthesis at 10 MPa isobar

Hydrothermal analcime synthesis at 10 MPa isobar and 100, 200, and 300 °C isotherms was performed. As synthesis initial components the minerals nepheline, albite, natrolite, jadeite, wairakite and the mixtures nepheline + albite and nepheline + natrolite were used. 1 M Na₂SiO₃, 1 M NaCl and 0.1 M NaOH (pH = 14, 7, 13) solutions were applied as the hydrothermal medium. On evaluating the synthesis products by means of X-ray diffraction analysis and X-ray microanalysis (EDAX) it was estimated that different analcime structural modifications originate especially at temperatures above 200 °C in neutral and alkaline medium from all the mentioned minerals. Analcime with a higher symmetry of the unit cell of crystals form at temperatures above 250 °C particularly.     

Impurity adsorption mechanism of borax for a suspension growth condition: A comparison of models and experimental data

A fluidized bed crystallizer is employed to investigate the growth and dissolution rates of MgSO₄·7H₂O from aqueous solutions in the presence of borax as impurity at 25°C. By adding 0.5, 1, 2 and 5 wt % of impurity the pH value changes from 6.7 to 7.11, while the saturation temperature shifts to 24.8, 24.4, 24 and 23.1°C, respectively. The data on crystal growth rates from aqueous solutions as a function of impurity concentration are discussed from the standpoint of Cabrera and Vermileya, and Kubota and Mullin. The value of the impurity effect, αθ_eq, determined from analysis of the data on growth kinetics was found to be in good agreement with the value obtained from direct adsorption experiments. The estimated value of the average spacing between the adjacent adsorption active sites and the average distance between the neighbouring impurity‐adsorbed sites are also reported. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Die Molekül- und Kristallstruktur von 1.1′-Bis-[phenyl-glyoxyloyl]-ferrocen

The 1.1′-Bis-[phenylglyoxyloyl]-ferrocen (PGF) crystallizes within the monoclinic space group P₁/c with the lattice constants a = 7,338 Å, b = 16,872 Å, c = 8,046 Å, β = 87,0°. The PGF belongs to metal-π-complexes. With the iron atom in the center of symmetry, both the heavy atom ligands are centrosymmetric-to each other. The two cyclopentadinyl rings linked to the iron atom are arranged in the sandwichtype with staggered conformation. The mean planes are separated by 3,32 Å. In a single phenyl-glyoxyloyl molecule exists an expanded Csp²–Csp² bondlength (1,528 Å). The two plane parts of the molecule are twisted around this bond. These two plane parts consist on the one hand of the carbon-5-ring and a C  O group and on the other hand of the carbon-6-ring and a C  O group. The dihedral angle is 62,5°. The expanded bond length mentioned is explained by Coulomb repelling forces of partially charged atoms. The packing of the molecules in the crystal lattice is discussed in detail.