G. C. Trigunayat

Purification, growth and polytypism of single crystals of lead iodide

Abstract Single crystals of lead 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 that the polytype 12R has its maximum occurrence in melt-grown crystals, followed by an occasional occurrence of the polytype 4H. An examination of crystals with different degrees of purification has revealed that impurities present in the starting material are responsible for an occasional transformation of 12R into 4H.

Effect of impurities on solid state structure transformations in gel-grown PbI2 crystals

Abstract The mode of growth of polytypic crystals of lead iodide, grown from silica gel containing AgI as impurity, has been studied by repeatedly cleaving the crystals and examining their crystal structures by X-ray diffraction after each cleavage. Characteristic structural changes during growth, including changes from lower to higher polytypes and vice versa as well as changes in the degree of disorder of the crystals and the arrangement of dislocations inside them, have been observed. The investigations have yielded useful information regarding role played by stacking faults in the growth and transformation of PbI 2 polytypes.

Effect of growth conditions on polytypism in cadmium iodide crystals

Abstract 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.

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

Abstract 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.

Phase transformations at high temperatures in polytypic crystals of cadmium iodide

Abstract Solid state phase transformations of various kinds have been observed to occur in polytypic crystals of cadmium iodide grown from solution, upon heating them to temperatures well below their melting points. These include the transformation of lower polytypes into higher polytypes and vice versa. All polytypes have been found to transform eventually into the common CdI 2 type 4H. Characteristic changes in “streaking” and “arcing” on the oscillation photographs have been observed. The results are explicable in terms of the movement of edge dislocations, either already existing inside the crystal or subsequently generated upon heating, and cast light on the kinetics of stacking faults and the role played by them in the formation of polytypes.

Solid state structural changes at high temperatures in cadmium iodide

Abstract Further experiments on the heating of polytypic crystals of cadmium iodide at high temperatures have yielded more information on the kinetics of stacking faults existing in the crystals and their role in structural transformations and polytype formation. The streaking of X-ray reflections progressively decreases in successive heating runs, showing that random stacking faults resulting from partial dislocations are gradually eliminated and that no fresh dislocations are created by heating. On the other hand, the extent of arcing of the reflections always increases, implying that more dislocations, both unit and partial, which had been earlier held up against some obstacles, move into existing tilt boundaries. The arc stays practically unchanged during further heating runs, thus indicating that the process of heating does not produce fresh dislocations and that no further significant movement of the dislocations takes place. The common type 4H has been established as the phase of maximum thermodynamic stability. The successive phase transformations in a crystal have revealed a systematic elimination of the stacking faults. The long period polytypes have been found to be thermally more stable than the short period polytypes.

Study of structural transformations during growth of lead iodine polytypes

Abstract The growth history of the polytypic crystals of lead iodide, grown by silica gel method, has been revealed by successively cleaving them and examining the crystal structure by X-ray diffraction, after each cleavage. Characteristic structural changes have been observed. The results have been discussed in terms of the stacking faults produced during crystal growth.

Effect of growth conditions on the genesis of CdI2 polytypes

Abstract The growth history of solution-grown cadmium iodide crystals has been unfolded by successive cleavage. It has been revealed that all the crystals involve structural transformations during growth, including changes from lower to higher polytypes and vice-versa, as well as changes in the degree of disorder and the arrangement of the dislocations. The results can be explained in terms of the creation and ordering of stacking faults during growth.

Growth in U-tubes and crystal structure of a 24-layered hexagonal polytype of lead iodide

Thick and optically perfect crystals of lead iodide have been grown in U-tubes by silica gel method. As compared to the growth in test tubes, the number of nucleation centres is considerably reduced. It has been discovered that the diffusion of lead ions through the gel is quicker than the diffusion of iodine ions. The complete crystal structure of a 24-layered hexagonal polytype grown by this method has been worked out. The structure is represented as (11)52112112112 in Zhdanov symbol, with space group P3m1. The complete growth history of the crystal of which this polytype formed a part has also been studied by successive cleavages followed by X-ray examination. They have revealed curious structural transformation of crystal structure during growth.

A combined optical, SEM and STM study of growth spirals on the polytypic cadmium iodide crystal

Some novel results of a combined sequential study of growth spirals on the basal surface of the richly polytypic CdI2 crystals by optical microscopy, scanning electron microscopy (SEM) and scanning tunneling microscopy (STM) are presented and discussed. In confirmation of the known structural data, the STM pictures clearly reveal the value of unit cella(= b) dimension to be equal to 0.428 nm a and the value of the interaxial angle γ to be equal to 120°. Under the high resolution and magnification achieved in the scanning electron microscope, the growth steps of large heights seen in the optical micrographs are found to have a large number of additional steps of smaller heights existing between any two adjacent large height growth steps. When further seen by a scanning tunneling microscope, which provides still higher resolution, several unit substeps are seen to exist between two consecutive additional steps. The height of each substep is found to be equal to the unit cell height of the underlying polytype. The height of the larger additional steps are found to be integral multiple of the unit cell height. The horizontal distance between two consecutive growth steps is found to be an integral multiple of the lattice parametera. The interspacing of the growth steps of same step height is found to vary at different positions on the same crystal face.