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.     

Growth and Dislocation Etching of InBi0.8Sb0.2 Single Crystal

InBi_0.8Sb_0.2 single crystals have been grown by zone melting method. The freezing interface temperature gradient of 3 °C/cm has been found to yield the best quality crystals obtainable at growth velocity 1.0 cm/hr. Traingular features have been obtained on the free surface of the as grown crystal. A new dislocation etchant based on nitric acid has been found to give reproducible etch-pitting on the cleavage surface. Standard tests for a dislocation etchant have been carried out and results are reported.     

Preparation and characterization of CdGeAs2 crystal by modified vertical Bridgman method

A large, crack-free CdGeAs₂ single crystal measuring 15 mm in diameter and 45 mm in length was grown in a vertical three-zone tubular furnace by a modified vertical Bridgman method, i.e. quasi-seed technique with small temperature gradient and descending quartz ampoule. High-purity, single phase CdGeAs₂ polycrystallite for crystal growth was synthesized using a rocking furnace with temperature oscillation techniques. Various measuring means, including X-ray diffractometer(XRD), Fourier transform infrared spectroscopy(FTIR), and Field emission scanning electron microscope(FE-SEM) were adopted to characterize the as-grown crystal. It is found that the cleavage plane of the as-grown crystal is {1 0 1} face; the crystal is integrated in structure and crystallized well; etch pits in the shape of pentagon on (1 1 2) face have been observed for the first time using the new preferential etchant we prepared. All these results encouragingly indicate that the modified vertical Bridgman method is a convenient and effective way for high quality CdGeAs₂ crystal growth.     

Comparison of Ba(B1-xAlx)2O4 Single Crystals on their Diameters

Small diameter crystals of Ba(B_1-xAl_x)₂O₄ were grown by Floating zone pulling down method and Micro-pulling down method to be compared with large diameter crystals grown by Czochralski method. While the Czochralski grown crystal was opaque, the crystal transparency was actually improved with the decrease of crystal diameter, i.e., the decrease of constitutional supercooling. The reason was the increase of temperature gradient caused by down-sizing of the crystal diameter deduced by down-sizing of the growth furnace. The adequate diameter for transparent crystal was determined depending on the Al content.     

Optimierung von Kristallisationsparametern beim Zonenschmelzen aus Temperatur-Feld-Berechnungen

The distribution of temperature in the crystal during the growth process decides the crystal perfection and first of all depends on the external conditions: on the nature and geometry of heater arrangement and on the enviroment of the crystal („atmosphere”︁, crucible, pull-respectivly hold rods a.s.o.). – A mathematical model is presented by which it's possible to calculate the distribution of temperature in the crystal on the basis of these external conditions. The efficiency of the theoretical model is discussed and evaluated by means of the special arrangement of vertical zone levelling. Important crystallization parameters are determined by use of the mathematical model for the growth of monocrystals of the Bi_1−xSb_x mixed crystal system through zone levelling.     

Sub-boundaries on (002) cleavages of Li2CO3 single crystals

Linear arrays of dislocations (straight, terminating and curved) are observed on etching with 2% citric acid (002) cleavages of lithium carbonate (Li₂CO₃) single crystals, grown by zone melting technique. As the dislocations are non-uniformly spaced and disappear on successive etching, they may not be revealing low-angle grain boundary, nor slip traces as they are terminating and curved meaning crystallographically non-oriented. On the observation of experiments on successive etching and etching of matched pairs, it is concluded that the arrays of etch pits reveal dislocation walls (dislocation density 1.1 × 10⁶ — 1.5 × 10⁷ pits cm⁻²) probably created due to the localized thermal stresses released as a result of high temperature annealing of the crystal during growth. The implications are discussed.     

Autoradiographic Investigations on the Incorporation of Ca Ions in KCl Crystals during Kyropoulos Growth

Ca doped KCl crystals were grown by the Kyropoulos method. 10⁻⁵ to 10⁻³ mole fractions of Ca (as CaCl₂ with a small amount of Ca-45 (β-emitter)) were added to the melt. By autoradiography of suitable cleavage planes the incorporation of the Ca ions was found to take place preferentially in a periodic manner parallel to the solid liquid interface. In the case of a rotating seed there is a segregation process in which only one zone of maximum Ca content is built up like a helix throughout the whole crystal, the axis being identical with the growth axis of the crystal and the slope of the helix corresponding to the growth rate per seed rotation, this leading to striations of long range periodicity on vertical cleavage planes. In the case of a non-rotating seed there is a different segregation process including a periodical incorporation too, but in discrete layers of relatively small separation parallel to the solid liquid interface in short range periodicity. Explanations for the segregation phenomena are given in terms of periodically varying effective growth rates related to periodic temperature fluctuations in the neighbourhood of the solid liquid interface.     

Etching of CaF2 single crystals in silica gels

Etching of {111} cleavage faces of CaF₂ crystals in aqueous solution of 50% HCl is carried out in citric acid set silica gel, and the kinetics of growth of etch pits at the sites of dislocations is investigated as a function of temperature, time of etching and height of gel column above the crystal surface. It is observed that the transient period required to initiate etch pit at the sites of a dislocation decreases (1) at particular temperature, with a decrease in gel height, and (2) for a particular gel height, with an increase in the temperature of etching. It is also observed that the morphology of dislocation etch pits remains triangular irrespective of the gel height and the temperature of etching. The results are compared with those of solution etching and discussed.     

Dislocation studies by chemical etching on solution-grown NaSbF4 and Na3Sb4F15 single crystals

An attempt has been made to study the etch pit dislocations of isolated as well as matched pairs of cleavage planes of solution grown NaSbF₄ and Na₃Sb₄F₁₅ single crystals when etched with analar grade methyl alcohol and ethyl alcohol. On etching of cleavage faces the pits nucleate at the intersection sites of dislocations with the cleavage face. On successive etching of a cleavage face, arrays of triangular shape etch pits are observed. After prolonged etching some of the etch pits disappear. These pits are curved and terminated indicating that they are crystallographically non-oriented. One-to-one correspondence with respect to number, shape, position and structure of the pits has been established by etching of a matched cleavage face. The etch pit densities of both NaSbF₄ and Na₃Sb₄F₁₅ crystals are found to be 10³ per cm².     

Growth and dissolution of BaFCl crystals

Single crystals of BaFCl have been growth by flux technique using BaF₂ and BaCl₂. Etching with formic acid revealed dislocation etch pits on (001) cleavage faces of the crystals, at room temperature. The influence of etching parameters such as undersaturation, temperature and concentration of poison in the etchant is studied. Decreasing the undersaturation of formic acid by reducing the percentage of water and increasing the temperature of the etchant were found not to have any effect on the morphology of etch pits. However, as the CdCl₂ poison concentration is gradually increased, the orientation of the pits change from 〈100〉 to 〈110〉 at high concentration.     

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.     

Growth,etching morphology and spectra of LiAlO2 crystal

γ‐LiAlO₂ single crystal was successfully grown by Czochralski method. The crystal quality was characterized by X‐ray rocking curve and chemical etching. The effects of air‐annealing and vapor transport equilibration (VTE) on the crystal quality, etch pits and absorption spectra of LiAlO₂ were also investigated in detail. The results show that the as‐grown crystal has very high quality with the full width at half maximum (FWHM) of 17.7‐22.6 arcsec. Dislocation density in the middle part of the crystal is as low as about 3.0×10³ cm^–2. The VTE‐treated slice has larger FWHM value, etch pits density and absorption coefficient as compared with those of untreated and air‐annealed slices, which indicates that the crystal quality became inferior after VTE treatment. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Epitaxial growth of α-SiC layers by chemical vapor deposition technique

Single crystals of α-SiC were grown on α-SiC substrates at a temperature between 1570 and 1630°C with the standard gas flow rate: H₂ ～ 1 liter/min, SiCl₄ ～ 1.7 ml/min and C₃H₈ ～ 0.1 ml/min. The grown layers were transparent greenish-blue, and surfaces were mirror-like. By an X-ray back-reflection Laue pattern and a reflection electron diffraction method, the grown layer was identified as 6H-SiC, one polytype of α-SiC. Crystal growth was influenced by substrate temperature, flow rates of reaction gases and the surface polarity of the substrate. The growth rate decreased with increase of the substrate temperature in the above temperature region. A lamellar structure was observed on the (0001) Si surface and a mosaic structure was observed on the (000$\text{1}$)C surface. The mole ratios of both SiCl₄ and C₃H₈ to H₂ and that of Si to C had some influence on crystal growth. Undoped layers were n-type due to nitrogen. P-type SiC was grown by doping Al during crystal growth. Doping effects were studied by photoluminescence and electrical measurements.     

Characterization of large‐sized Nd:YAG single crystals grown by horizontal directional solidification

Nd³⁺‐doped Y₃Al₅O₁₂ single crystals have been grown by the horizontal directional solidification (HDS) method in different thermal zone. The Grashof (G_r), Prandtl (P_r), Marangoni (M_a) and Rayleigh (R_a) numbers of melt in HDS system have been discussed for our experimental system to understand the mechanism of melt flow patterns and concentration gradient of dopant. The concentration gradient of Nd³⁺ ions was explained with melt flow processes during crystal growth in different thermal zone, and results indicated that high growth temperature will be helpful for uniformity of dopant in HDS‐grown single crystal. The main microscopic growth defects such as bubbles and irregular inclusions in HDS‐grown Nd:YAG crystals were observed, and the causes were discussed as well. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The solid state controlled growth of sulphides and selenides of Ag and Cu using crystal rotation

An improved crystal growth method for ionic-electronic mixed conductors, such as the high temperature cubic modifications of Ag₂X and Cu₂X (where X = S or Se), is presented. Ag or Cu atoms are mobile in these compounds. If the respective sulphur or selenium vapour pressure was sufficiently high at the growth surface, the growth rate was limited by the solid state chemical diffusion of Ag or Cu atoms which was controlled by the rate of the injection of the metal atoms into the compounds. The crystals were grown in a silica ampoule, held in a vertical two zone furnace and rotated at 1 rpm about the vertical axis to achieve a uniform axial temperature distribution. Nearly cylindrical single crystals of size about 3–10 mm diameter, 10–35 mm long were grown by this method.     

Crystal growth of the intermetallic compound Nd2PdSi3

Nd₂PdSi₃ single crystals were grown by a vertical floating zone method with radiation heating at a zone traveling rate of 3 mm/h. The compound exhibits congruent melting behavior at a liquidus temperature of about 1790 °C. The actual crystal composition (35.3 ± 0.5) at.% Nd, (16.2 ± 0.5) at.% Pd, and (48.5 ± 0.5) at.% Si is slightly depleted in Pd and Si with respect to the nominal stoichiometry. Therefore, the gradual accumulation of these elements in the traveling zone led to a decrease of the operating temperature during the growth process. Single crystalline samples exhibit a large anisotropy due to the crystal electric field effect and order ferromagnetically below the Curie temperature T_C = 15.1 K. The [001] orientation was identified as the magnetic easy axis at low temperatures. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Etching studies of tungstenite (WS2) crystals in acids

Single crystals of tungstenite (WS₂) have been grown by direct vapour transport or sublimation method. An etchant capable of revealing the sites of dislocations has been developed. The effects of etching time, etchant concentration and temperature on the selective etch rate have been worked out. It is seen that the etch rate is independent on time, but is influenced by the temperature and concentration of the etchant. The values of activation energy determined from the etch rate against temperature plots established that the process of etching in chromic acid is chemical reaction rate controlled. The effect of addition of HF to the above etchant has led to some further interesting results.     

Growth of corundum single crystals by the top-seeded solution growth technique (TSSG)

Single crystals of corundum were grown by the top-seeded solution growth technique from a cryolite, Na₃AlF₆, solvent. The relationship between the growth rate (mg/h) of a crystal and the temperature difference (= supersaturation) or the rotation rate of a seed crystal was investigated, and optimum growth conditions for obtaining single crystals with good quality are discussed.     

Etching Figures in Neodymium Gallate and Yttrium Aluminate Crystals

The dislocations existing in single crystals of neodymium gallate and yttrium aluminate grown by the Czochralski technique have been studied by means of etch pits. The data concerning their solubility in cases of different directions of a face orientation, various treatment temperatures and several enchant types are reported. The investigation of etch pits in the twinned YAlO₃ and NdGaO₃ crystals showed that twins are formed during a growth process. In the [110]‐pulled NdGaO₃ crystals the discrepancy between the twin and matrix parts of a crystal is accommodated by the dislocation congestion and the dislocation low‐angle boundaries whereas in [010]‐pulled YAlO₃ crystals the microcracks perform this function.     

Hydrothermal etching of flux grown phenakite (Be2SiO4) single crystals

The etching studies were made on flux grown phenakite (Be₂SiO₄) single crystals to elucidate the twinning phenomenon and the growth mechanism. The sharp and distinct etch pits and the twin boundaries were successfully obtained after the hydrothermal etching. From the etching and the X-ray precession experiments, it was conjectured that the phenakite had an inversion twin along the c axis. The twinning should be originated during the earlier stage of growth where the dendritic and acicular crystals were grown and thereafter the twinned crystals grew to the prismatic crystals during the latter stage with decreasing the temperature.