Cadmium Iodide Crystals Research Articles

Effect of variation in PbI2doping on the polytypism of dendritic CdI2single crystals

Highly pure dendritic single crystals of cadmium iodide, variously (2 to 8%) doped with lead iodide, have been grown from the vapour phase in a vacuum. The study of polytypism and related phenomena in the crystals has been carried out by X-ray diffraction. An uneven high rate of increase in the occurrence of the 12-layered rhombohedral polytype 12R has been found to exist when the amount of PbI2 in the crystals rises from 2 to 8%. Simultaneously, the relative intensity of the 12R reflections has been found to rise rapidly. Another curious feature has been an unusually high frequency of occurrence of a particular high polytype 48H in the heavily (8%) doped crystals. The percentage occurrences of streaking and arcing have been found to be low and to vary slowly with the PbI2 content of the crystals. The observed polytypism has been explained in terms of the nucleation of two different thermodynamically stable end-member polytypes, 4H and 12R, of CdI2 and PbI2, respectively.

Polytypism in PbI2-doped dendritic crystals of cadmium iodide

Cadmium iodide and lead iodide have been highly purified by a horizontal zone refining technique. Dendritic single crystals of PbI2-doped cadmium iodide have been grown from vapour phase in vacuum. The effect of doping on polytype formation and other structural characteristics has been studied by X-ray diffraction. The crystals have shown the formation of rhombohedral polytype 12R in about 50% of cases, along with the common polytype 4H of cadmium iodide. This is in sharp contrast to earlier findings for undoped dendritic crystals which exclusively contained the polytype 4H. Further, unlike the undoped crystals, some of the doped crystals also show streaking and arcing on their X-ray photographs. The observed structural changes may be governed by both thermodynamical considerations and the kinetics of crystal growth and the observed streaking and arcing result from internal stresses built up around the relatively large Pb2+ ions in the host CdI2 structure.

Study of cracks in melt‐grown cadmium iodide crystals

Study of cracks in melt‐grown cadmium iodide crystals

Some observations on melt‐grown cadmium iodide crystals

AbstractSingle crystals of richly polytypic compound cadmium iodide have been grown using a zone‐refining system. The frequent appearance of cracks in the grown crystals has been attributed to the thermal stresses and the presence of impurities. Structure of all the crystals has been found to be of the type 4H.

Effect of tin-doping on polytypism of melt-grown crystals of cadmium iodide

Abstract The effect of doping of metallic tin in cadmium iodide crystals has been systematically studied. The method of zone melting has been employed both for purification and for the growth of single crystals of cadmium iodide. The doping has been carried out by zone levelling technique. The as-grown crystals have been characterized by X-ray diffraction and physical methods. All the crystals have been found to consist of the most common polytype 4H. Unlike the case of pure undoped crystals of cadmium iodide grown from melt, all the X-ray photographs showed the presence of arcing. None of the photographs showed the presence of streaking. Besides, the doped crystal were found to be harder than the crystals of undoped cadmium iodide. Unlike the undoped crystals, cleavage in the doped crystals was found to be difficult and highly localized. The results have been discussed.

Low temperature X‐ray diffraction study of CdI2 crystals

AbstractSolid state structural transformations in polytypic crystals of cadmium iodide have been investigated at low temperatures using X‐ray diffraction techniques. The crystals were cooled to liquid nitrogen and liquid helium temperatures employing three different methods. The results obtained are discussed on the basis of bond dissociation energies of interlayer and intralayer bonds and the possible mode of layer displacements in MX2‐type polytypic crystals.

The mechanism of the 2H→4H solid-state transformation in cadmium iodide crystals

Abstract The mechanism of the 2H→4H transformation in CdI2 has been investigated. Such a solid-state transformation can occur by the non-random insertion of deformation, layer displacement or extrinsic stacking faults. The theory of the X-ray diffraction by 2H crystals undergoing transformation by the layer displacement mechanism has been developed and it is shown that the observed diffraction effects are in good agreement with the layer displacement mechanism but not with the mechanism of deformation on extrinsic faults.

Melt growth and characterization of lead-doped crystals of cadmium iodide

Etude systematique de l'effet du dopage par Pb ou PbI 2 de l'iodure de cadmium, par la methode de fusion de zone. Le dopage modifie la durete et le clivage des cristaux

Effect of purification on the polytypism in solution‐grown cadmium iodide crystals

AbstractThe effect of purification on the polytypism in cadmium iodide crystals has been studied by X‐ray diffraction in three hundred and sixty polytypes. Formation of small period polytype 2H is governed by both temperature and impurities contained in the starting material. 4H is the most stable polytype and higher occurrence of unidentified polytypes in crystals of purified material has been attributed to free movement of edge dislocations during growth. The results have been examined against empirical conclusions of earlier investigations.

Study of substitutional solid solutions in melt-grown crystals of cadmium iodide

Study of substitutional solid solutions in melt-grown crystals of cadmium iodide

Polytypism of Pb‐doped Single Crystals of Cadmium Iodide

AbstractThe zone refining technique has been used both for the purification of cadmium iodide and for the growth of lead‐doped single crystals of cadmium iodide. The as‐grown crystals when studied by X‐ray diffraction show exclusive presence of the most common polytype 4H. The hexagonal plate‐shaped crystals of the doped material, grown from aqueous solution, show polytypism, but of a different nature than the crystals of pure cadmium iodide. The results have been discussed.

Purification and growth of single crystals of cadmium iodide by zone refining and study of their polytypism by x-ray diffraction

Single crystals of cadmium iodide have been purified and grown using the method of zone refining, and their polytypism has been studied by X-ray diffraction. Oscillation photographs obtained from several different regions of the crystals have revealed a total absence of any streaking or arcing of reflections and the occurence of only the most common polytype 4H in the crystals. The results have been examined against known theoretical and empirical ideas.

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

AbstractSingle 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.

X-ray and electron diffraction studies of cadmium iodide crystals in relation to the hexagonal–rhombohedral polytypic transformation

X-ray and electron diffraction studies of as-grown CdI2 crystals from solution show that in a number of them hexagonal and rhombohedral polytypes coexist on the same basal face. Analyses of selected area electron diffraction patterns taken from different regions on the edges of various crystals reveal that the transformation of a hexagonal polytype into a rhombohedral along the basal face takes place as a result of formation of twinning in the orientation (11.n) [11.6/n] in the intermediate regions between them. Röntgen- und Elektronenbeugungsuntersuchungen von frisch gewachsenen CdJ2-Kristallen aus der Lösung zeigen, daß in einer Anzahl von ihnen hexagonale und rhomboedrische Polytypen gleichzeitg auf derselben Basalfläche vorhanden sind. Analysen der Elektronenbeugungsdiagramme ausgewählter Flächen von verschiedenen Bereichen auf den Kanten verschiedener Kristalle ergeben, daß der Übergang von einem hexagonalen zu einem rhomboedrischen Polytyp auf der Basalfläche als Ereignis einer Zwillingsbildung der Orientierung (11.n) [11.6/n] in den Zwischenbereichen stattfindet.

Effect of growth conditions on polytypism in cadmium iodide crystals

The effect of temperature, rate of crystallization and impurities on polytypism in cadmium iodide crystals has been studied by X-ray diffraction in nearly two and a half thousand polytypes. The frequency of occurrence of the small period polytypes 2 H and 6 H decreases as the temperature of growth increases, thus implying that these polytypes may be regarded as low temperature modifications of cadmium iodide. A slower rate of crystallization promotes the growth of the common polytype 4 H. The frequency of occurrence of the unidentified polytypes rises with increase in both the temperature and the rate of crystallization. The presence of dust particles promotes the incidence of polytypism as well as the growth of the total number of crystals in a particular batch. The results have helped to explain the mutually contradictory empirical conclusions of the earlier investigations.

Hexagonal - rhombohedral phase transformation in crystals

The crystals of cadmium iodide and lead iodide when subjected to X-ray examination have exhibited the rhombohedral polytypes occurring quite frequently in coexistence with their respective basic hexagonal types 4H and 2H, respectively. This hexagonal - rhombohedral phase transformation taking place during crystal growth can be easily understood in terms of stacking faults occurring periodically either during growth or due to creation and movement of Shockley partials, generated either independently or as a result of dissociation of unit edge dislocations. It has been found that the basic hexagonal types are transformed more frequently into those rhombohedral structures for which (i) the hexagonality is almost the same as that of their parents, (ii) the top and bottom layers of two successive sandwiches, respectively, around the boundary of rotation are not in the same orientation, and (iii) the stacking fault energy is minimum. Non-occurrence of the types 6R and 12R, (13) 3, in CdI 2 and PbI 2 as a result of transformation of their basic types have been explained. The possibility of occurrence of polytypes with Zhdanov number 4 is pointed out.

Electron microscopic study of decomposition in vapour grown cadmium halide crystals

Vapour grown single crystals of cadmium bromide and cadmium iodide have been examined in transmission electron microscope to study the electron radiation effect on them. Initially a large number of crystallites of a new phase, identified as cadmium in diffraction patterns, appear which most probably precipitate down on dislocations. Decomposition proceeds by their merger into large platelets by the formation of cavities along crystallographic directions. Complete multiple double diffraction patterns of CdBr 2 and CdI 2 have been observed and this confirms the decomposition mechanism in which cadmium is formed by nucleation and bromine/iodine is liberated.

On Growth and Structure of Vapor‐Grown Cadmium Iodide Crystals

Cadmium iodide crystals have been grown by the vapor phase technique. Under different thermal and supersaturation conditions, cadmium iodide crystals of the same morphological form and shape, but of different size and thickness, have been obtained. No spirals are observed on the crystals which grow with their basal planes normal to the wall of the growth tube, whereas, spirals have been observed on crystals grown on a glass substrate at high super‐saturation. The observations indicate that a screw dislocation mechanism is not operative in the growth of crystals in the present case. The growth starts with a dendritic skeleton and then changes to flat platelets by the filling in process and by two dimensional nucleation. Formation of spirals on crystals grown on glass slides has been attributed to adsorbed impurities or debris and irregularities on the substrate surface. An x‐ray study of the crystals has revealed that nearly all vapor‐grown crystals show the most stable basic polytype (4H) of . Observations indicate that conditions of growth play an important role in deciding the structure.

Present state of polytypism in cadmium iodide crystals

physica status solidi (a)Volume 38, Issue 1 p. 11-44 Review Article Present state of polytypism in cadmium iodide crystals R. Prasad, R. Prasad Department of Physics, Banaras Hindu University, Varanasi Search for more papers by this author R. Prasad, R. Prasad Department of Physics, Banaras Hindu University, Varanasi Search for more papers by this author First published: 16 November 1976 https://doi.org/10.1002/pssa.2210380102Citations: 11 Varanasi-5, India. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Citing Literature Volume38, Issue116 November 1976Pages 11-44 RelatedInformation

Structure and phase transformation of 20 layered hexagonal polytypic crystals of cadmium iodide

Studies of transformations of 20H polytypic crystal of cadmium iodide have been described. Two curious intrapolytypic transformations 20H 7→20H 1 and 20H 8→20H 1 have been observed. The structural sequences of these polytypes have been determined and the transformations have been analysed in terms of the determined sequences. It has been shown that the two independent transformations lead to the same common transformed structure since this structure is the most stable out of the various structures.