Dislocation etching of tin iodide single crystals

Single crystals of tin-iodide (SnI₂) have been grown using the controlled reaction between SnCl₂ and KI by diffusion process in gel medium. As grown (010) surfaces of the crystals have been optically studied. Characteristic etch pits have been observed on them. This suggests that SnI₂ crystals might go into dissolution in the acid-set gel. By successively etching (010) surfaces in a mixture of ammonia, acetic acid, and CdCl₂ solution, it is established that the pits indicate the site of dislocations in the crystals. This is further confirmed by comparing the etch patterns before and after chemically polishing (010) surfaces. The average dislocation density in the crystals have been evaluated and found to be 3.2 × 10³ cm⁻² and the implications are discussed.     

Microstructures on {111} faces of stannic iodide single crystals

Single crystals of stannic iodide (SnI₄) havebeen grown using the controlled reaction between SnCl₂ and KI by diffusion process in silica gel medium. Orange to reddish octahedral stannic iodide crystals up to 3–4 mm in size have been grown at room temperature. Optical studies have been made on the various surface structures of {111} faces of the asgrown crystals. On octahedral faces of these crystals, triangular-shaped hillocks with growth layers in the 〈110〉 directions have been observed. Occasionally, growth spirals on octahedral faces have also been reported. Close loops of growth fronts have been investigated and have been interpreted. It has been suggested that two-diemensional nucleation, spreading and pilling up of triangular growth layers is mainly responsible for the growth and occasionally the growth is due to screw dislocations. The implications are discussed.     

Microstructures of Tin Iodide Single Crystals Grown in Silica Gels

Single crystals of tin iodide (SnI₂) have been grown in silica gels. A detailed microtopographical study of {100} faces have been described. Horizontal striations are predominant on these faces for most of the crystals, while few of them show vertical striations. The horizontal striations are associated with the two-dimensional nucleation theory whereas the vertical striations relate to the growth fronts. Growth layers modified by the presence of misaligned microcystals have been illustrated. The natural etch pits on {100} faces of the crystal are attributed to the dissolution of crystals in the acid set gel. In the light of these observations, the mechanism of the development and growth of these faces have been assessed and the implications are discussed.     

Growth and etching of calcite crystals

Single crystals of calcite (CaCO₃) have been grown by the method employed by GRUZENSKY , using an aquoeus solution of CaCl₂ and a solid (NH₄)₂SO₃, The chemical reaction takes place according to the following equation: CaCl₂ + (NH₄)₂SO₃ CaCO₃ + 2 NH₄Cl The crystals grown by this method are about 0.2 to 0.8 mm in edge dimensions. Synthetic calcite crystals have been cleaved along (100) planes and the cleavage surfaces have been studied by multiple beam interferometry. The interferograms have revealed that the cleavages are quite flat. The cleavage faces have also been chemically etched and the etch patterns studied optically. By etching a cleavage successively for three different periods it was found that the bottoms of the point-bottomed pits follow a linear etch path. By etching a cleavage pair, one face in one etchant and the other face in a different etchant and by comparing the etch patterns produced, before and after polishing a cleavage face it has been shown that the etch pits nucleate at the sites of dislocations in the crystal. The etch patterns have also been compared with those produced on the cleavage faces of natural crystals. The density of dislocations in the syntheitc calctie crystals was generally less than the density of dislocations in the natural calcite crystals. The implications have been discussed.     

On the Morphological Changes and Twinning of ZnS (Sphalerite) Crystallites under Hydrothermal Conditions

Morphological characteristics and twinning mechanism of ZnS crystals under hydrothermal conditions have been investigated in this paper. It was shown that under hydrothermal conditions the morphology of ZnS crystallites changes along the four‐fold axis directions, and the crystals are observed in a positive or negative tetrahedron, or in a combination of positive and negative tetrahedra depending on the growth conditions. The positive tetrahedral areas on the crystallites get larger with increase of the concentrations of OH^‐ and S^2‐ in solutions, whereas the twinned crystallites of ZnS taking an elliptic shape with (111) as composition plane are easily formed in weak basic solutions. It can be found that the morphologies of ZnS crystals are in accordance with the crystallization orientations of positive or negative coordination tetrahedra ([S‐Zn₄]⁶⁺, [Zn‐S₄]^6‐) in the crystal although, in some cases, the practical morphology could be greatly affected by growth conditions, and the twinning mechansim can be suggested based on the linkage of growth units of positive and negative coordination tetrahedra, which were formed in the solution. The present investigations further indicated that the crystal chemistry approach based on the linkage/incorporation of growth units previously proposed by us can be sucessfully applied to interpret the growth mechanisms of the crystals and to control a desirable morphology.     

Atomic force microscopy studies on surface morphologies of CdHg(SCN)4 crystals grown in solutions containing excessive amount of Cd(II) cations

Surface morphologies of CdHg(SCN)₄ (CMTC) crystals grown from solutions with excessive amount of Cd(II) cations (5%, 20% and 50% in molar ratio) have been investigated by atomic force microscopy (AFM). [Cd(SCN)_n]^2‐n (n ≤ 4) complex anions formed by addition of excessive Cd(II) cations in the solutions have been found to act either as growth units or impurities during CMTC crystal growth. On the prismatic faces, incorporation of [Cd(SCN)_n]^2‐n (n ≤ 4) complex anions as growth units leads to the formation of well‐oriented protuberance trains at the step fronts, named as “locally anisotropic crystal growth”. These protuberances become fewer, less distinct and nearly disappeared with the increase of excessive Cd(II) cations in the solutions. The pyramidal face, however, varies from regular 2D nucleation growth at a low concentration of Cd(II) to much rougher growth surfaces at high concentrations, exhibiting typical surface morphologies where crystal growth is completely inhibited by impurities. Observations in this experiment provide a new picture of crystal growth.     

Effect of neutron irradiation on stratigraphy in calcium fluoride single crystals

Natural single crystals of calcium fluoride have been cleaved along {111} planes and a number of matched pairs have been obtained. One piece from etch pairs have been irradiated with thermal and pile neutrons, the flux varying from 1.3 × 10⁴ to 3.0 × 10¹³ neutrons cm⁻². After irradiation, the irradiated and their non-irradiated counterparts have been matched. They were then etched simultaneously in 0.2 N hydrochlorid acid for the same period. The stratigraphical etch patterns produced have been studied optically. There is no exact correspondence of the stratigraphical etch patterns have been observed on the irradiated and non-irradiated matched pairs. The disappearing in the stratigraphical pattern of the irradiated crystals increases with increase in the dose of irradiation. The implications are dicussed.     

Crystal growth and dislocation structure of gallium antimonide

Single crystals of GaSb have been grown by the Czochralski method under reducing conditions. Crystals were grown in the 〈100〉, 〈111〉, and 〈112〉 directions. The 〈111〉 growth direction was found to be the most suitable for the successful and reliable crystal growth. Dislocation densities in 〈111〉 oriented crystals were examined by chemical etching. The etch pits density in these crystals didn't exceed the value of 1 × 10² cm⁻².     

Untersuchung epitaktischer Molybdänkarbidschichten an Molybdän-(110)-Oberflächen

The initial phases of high temperature (1700 K) carburization of (110) molybdenum surfaces by C₆H₆ at low pressure are examined with low energy electron diffraction (LEED) and emission electron microscopy. After an exposition of about 8 · 10⁻⁵ Torr · sec a (4 × 4) superstructure was observed by means of LEED, interpreted as coincidence lattices with the Mo₂C (10.0) plane parallel to the Mo (110) surface. Heating a sample without further exposition resulted in the appearance of a complex and of a (2 × 2) superstructures. After an exposition of about 2 … 9 · 10⁻³ Torr · sec needle shaped molybdenum carbide crystallites grew on the surface into the [111] and [111] directions. The orientation Mo₂C (10.1) parallel to Mo (110) was concluded from the LEED patterns.     

Controlled synthesis of Cu2O cubic and octahedral nano‐ and microcrystals

Cubic and octahedral Cu₂O nano‐ and microcrystals were selectively synthesized via a simple wet chemical reduction route at room temperature, with CuCl₂ and NaOH as starting reactants, and ascorbic acid or hydrazine hydrate as the reducer. Hydrazine hydrate could be preferentially adsorbed on different crystal faces of Cu₂O, affecting the growth rate along the 〈100〉 to that along the 〈111〉 direction, which resulted in the formation of octahedral Cu₂O crystals. When ascorbic acid was used as the reducer, the growth rate along the 〈100〉 to that along the 〈111〉 direction was different, which resulted in the formation of cubic Cu₂O crystals. The size of cubic and octahedral Cu₂O crystals could be varied by adjusting the molar ratio of OH^– to Cu²⁺. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Direct Growth of Aggregates on {110} Faces of Cadmium Mercury Thiocyanate Crystals

New growth phenomena ‐ direct incorporation of aggregates have been observed on the {110} faces of cadmium mercury thiocyanate CdHg(SCN)₄ crystals by atomic force microscopy. These aggregates grow in two forms: some directly cover up the steps and forms new growth layers; while others are just incorporated at the step edges. These aggregates, which are mostly oriented along [111] direction, are formed by small columnar structural units. The aggregates have the similar structure of CdHg(SCN)₄ crystals and greatly vary in nature with the variation of solution supersaturation σ and growth time t. With the increase of σ the aggregates become larger, consistent with the variation of growth units dimension with the supersaturation; and with the increase of growth time the aggregates become more structurally substantial. These observations have led to a new understanding about the crystal growth.     

Investigation of the crystallization of the system KAl(Cr)(SO4)212 H2O under various conditions

The crystallization of the isomorphous system KAl(Cr)(SO₄)₂ · 12 H₂O under various conditions has been studied. It has been found that the rate of linear crystallization of the face (100) passes through a maximum on the interval of Cr³⁺ concentrations 0.02–0.08 g ions/l which is shifted to the right with increasing temperature. Under the same conditions, the equilibrium coefficient of distribution of the microcomponent D has been determined in single crystals. D decreases with the increase of the Cr³⁺ concentration and the crystallization temperature. These results can be explained with the formation of complexes of Cr³⁺ in the solution which decreases the active concentration of the ion and are selectively adsorbed on the face (100). At the same Cr³⁺ concentration (0.058 g ion/l), the temperature dependence of the rate of growth of the face (100), as well as the influence of supersaturation on the rate of growth of the faces (100) and (111), have been studied. The apparent activation energy of crystallization of the face (100) has been determined to be E = 11.2 Kcal. · mol⁻¹. The data obtained are compared with analogical results for pure KAl(SO₄)₂ 12 H₂O.     

Crystal and molecular structures of potassium cobalt hydrogen phthalate K2[Co(H2O)6](C8H5O4)4 ⋅ 4H2O and mechanism of Co2+ impurity trapping in KAP crystals

The single-crystal X-ray diffraction analysis of K₂[Co(H₂O)₆](C₈H₅O₄)₄ ? 4H₂O has been carried out. The K₂[Co(H₂O)₆](C₈H₅O₄)₄ ? 4H₂O single crystals are obtained in attempting to grow the KAP crystals with the maximum possible content of Co²⁺ impurity cations. The crystals are isostructural to the earlier-studied similar crystals with Ni(II). The structure is formed by double layers of biphthalate anions and the Co²⁺ and K⁺ cations in between. The Co²⁺ cations are coordinated only by water molecules, whereas the coordination of the K⁺ cations involves both the biphthalate anions and water molecules. A detailed crystal chemical analysis, together with the data on the growth kinetics of KAP crystals in the presence of Co²⁺ and the mass-spectrometric data obtained earlier for the KAP crystals, leads to the conclusion that the Co²⁺ impurity cations should be located in the form of the [Co(H₂O)₆]²⁺ cationic complexes in the interblock layers of the KAP crystals.     

On the orientation dependence of effective distribution coefficients of impurities

Germanium crystals doped by antimony are grown by the Czochralski technique along 12 different crystallographic directions. The effective distribution coefficients (K_ef) are determined here. It is shown that orientation dependence of k_ef could be explained by supercooling at growth surfaces correlating with number of layer systems {111} crossing that surfaces and with angles between that systems and direction of shift of the crystallization front.     

Microhardness studies on as‐grown (111) faces of some alkaline earth nitrates

Single crystals of Sr(NO₃)₂, Ba(NO₃)₂ and Pb(NO₃)₂ are grown from their aqueous solutions at a constant temperature of 35 °C by slow evaporation technique. Crystals of size 8 to 10 mm along one edge are obtained in a period of 10 days. Chemical etching technique has been employed to study the dislocations in these crystals. The dislocations are randomly distributed and the dislocation density is about 10⁴ to 10⁵ /cm². Microhardness studies are made on as–grown (111) faces of these crystals upto a load of 100 g. The hardness of the crystals increases with an increase in load and thereafter it becomes independent of the applied load. These results are discussed on the basis of reverse indentation size effect. Meyer index number n for these crystals is estimated at both low and high load regions. An analysis of hardness data of these crystals as well as some other cubic crystals like alums and alkali halates are discussed using Gilman–Chin parameter H_v/C₄₄, where H_v is the microhardness and C₄₄ is the shear constant. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dislocations in magnetic garnet crystals

1. Dislocations in magnetic flux-grown garnet crystals (Y₃Fe₅O₁₂ and others) have been observed. As a rule, {110} growth pyramids have more defects than {211} ones. 2. The highest content of defects (dislocation density 10³–10⁴ cm⁻²) is observed in Y₃Fe₅O₁₂ that grows on crucible walls adjacent to the free surface of the solution where the flow of heat is not uniform to the greatest degree. Bottom grown crystals usually have less dislocations. Far fewer dislocations are in wall grown crystals, least of all dislocations are contained in crystals that grow inside the solution. The solution pouring off at the end of the crystallization period increases dislocation density by some dozens. 3. Heat treatment decreases dislocation density. The less dislocation content and the lower ordering are in the initial crystal, the higher heat treatment effectiveness.     

Growth and characterization of InxBi2−xTe3 single crystals

The V–VI group narrow band gap compounds are known to have important photoconductivity and thermoelectric properties. Among these, Bi₂Te₃ is the most potential material for thermoelectric devices having a direct band gap of 0.16 eV. There has been ample study reported on crystal growth and polycrystalline thin films of both pure and indium doped Bi₂Te₃ pertaining to its basic semiconducting, optoelectronic and thermoelectric properties. It has been shown that on exceeding certain limiting concentration of indium in Bi₂Te₃, the conductivity changes from p-type to n-type. However, there is hardly any work reported in literature on crystal growth, dislocation etching and optical band gap of In_xBi_2?xTe₃ (x=0.1, 0.2, 0.5) single crystals. The authors have grown their single crystals using the zone melting method. The freezing interface temperature gradient of 70 °C/ cm^?1 has been found to yield the best quality crystals obtainable at the growth rate of 0.4 cm/h. The as-grown crystals have been observed to exhibit certain typical features on their top free surfaces. The crystals have been characterized using XRD technique. A chemical dislocation etchant has been used for estimating perfection in terms of dislocation density in the crystals. The optical absorption was measured in the wave number range 500 to 4000 cm^?1. The transitions in all the cases were observed to be allowed direct type. The detailed results are reported in the paper.     

Raman Study of Lattice Vibration Modes and Growth Mechanism of KDP Single Crystals

The space group theoretical analyses and assignment of the lattice modes of the KDP crystal have been made, and the Raman spectra of their growth solution have been observed in different growth regions. The attention is focused on the analysis of the 912 cm^‐1 band arising from the H₂PO₄^‐ anions in the interface between the KDP crystals and their growth solution. This has been assigned to the asymmetrical stretching mode of the deformation P(OH)₂ . From these results, the growth units of KDP crystal has been concluded to be the dimers of H₂PO₄^‐ anions. We consider that the result presented here is an important step towards the development of more complete crystal growth theories.     

Growth parameters for large diameter float zone silicon crystals

Factors that directly affect the ability to grow dislocation free float zone silicon crystals up to 80 mm in diameter have been experimentally determined. The highest yield is obtained for 80 mm diameter crystals by starting with 68 mm to 74 mm diameter poly crystal rod stock. Lower transport speeds for crystal growth of (111) orientation crystals were 3 to 4 mm/min and for (100), 2 to 3 mm/min. Rotation rates of both upper and lower shafts were found to have an effect on growth at the solid-liquid interface. Rates established for lower shaft were 6 to 8 rpm for the (111) crystals and 3 to 4 rpm for (100), counter-clock-wise. Upper rotation rates were 2 rpm on (111) crystals and 3 to 5 rpm on (100), clockwise. Seed orientation, which is critical, was held to within plus or minus $\text{1}\text{2}\text{°}$ of perfect orientation. The minimum seed growth length was 50 to 70 mm. To assist in reducing the side lobes on (111) dislocation free crystals, a cooling ring with a flow or argon was used. For best (100) growth the shape of the lower side of a one turn copper rf work coil was made conical. Six to ten dislocation free crystals in each orientation group were produced using these parameters.     

Single crystals growth and absorption spectra of Cr-doped Al2−xInx(WO4)3 solid solutions

The growth conditions of pure and Cr³⁺-doped Al_2−xIn_x(WO₄)₃ single crystals, using top-seeded solution growth (TSSG) technique, have been studied. A series of experiments have been performed at different In concentrations, x=0.0, 0.3, 0.6 and 1.0, as well as at different concentrations of Cr³⁺ (0.0, 0.1, 0.2, 0.5 and 1.0) in at% with respect to the initial total concentration of Al and In in the starting solutions. The basic parameters of the crystal growth are varied over a wide range: seed orientation, speed of rotation, axial and radial temperature differences in the solution and the solution cooling rate. The investigated relations between the basic defects in the crystals and these parameters result in determination of the optimal conditions for growth of defect-free crystals. Distribution coefficients of Al, In and Cr have been determined, so the growth of crystals with given compositions is possible. Values of Dq/B (crystal field strength) for the various crystal compositions are calculated from the optical absorption spectra. The calculated values show that the discussed solid solutions have weak crystal field and are suitable for media with broadband emission spectra.