Browsing by Author "M.T. Sebastian"

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An X-ray diffraction study of faulting in single crystals of cubic ZnS grown from the vapour phase
(1984) M.T. Sebastian; P. Krishna
An X-ray diffraction study is made in order to determine the nature of stacking faults present in vapour-grown cubic ZnS crystals, as well as in cubic crystals obtained by solid-state transformation from the 2H phase by thermal annealing. For this the point intensity distribution along the 10.L reciprocal lattice row of both kinds of disordered 3C crystals was recorded on a single-crystal diffractometer. The observed intensity profiles are found to be asymmetrically broadened and do not show any peak shifts, indicating that stacking faults present in both as-grown and annealed crystals are predominantly twin faults distributed randomly. The experimentally obtained intensity profiles are compared with those calculated theoretically for a random distribution of twin faults. The experimental results indicate (i) that the disordered 3C structures result by solid-state transformation of the 2H phase during the cooling of the growth furnace (ii) that they contain a random distribution of twin or growth faults and (iii) that the 2H-3C transformation in ZnS occurs by the non-random nucleation of deformation faults, occurring preferentially at two layer separations. © 1984 Taylor & Francis Group, LLC.
Anomalous photovoltaic effect and disorder in ZnS crystals
(Springer India, 1983) M.T. Sebastian; P. Krishna
Some crystals of ZnS are known to produce an anomalously high photovoltage, up to several hundred volts per cm, when illuminated by uv light in the absorption edge region. This has been attributed to the presence of alternate regions of hexagonal and cubic packing with charged dislocations at the interfaces producing built-in electric fields. Differential absorption of the incident light in the hexagonal and cubic regions is believed to create the necessary asymmetry in the built-in fields, causing an addition of tiny photovoltages at a series of interfaces which finally results in the abnormally high photovoltages observed. This paper investigates the possible mechanism by which disordered ZnS crystals containing alternating regions of cubic and hexagonal packing can result. X-ray diffraction studies show that such a disordered configuration results during the 2 H to 3 C phase transformation in ZnS. It is suggested that the transformation occurs by the non-random nucleation of deformation faults wherein the probability (α) of random nucleation of the faults is much less than the probability (β) for the faults to occur at two-layer separations. © 1983 Indian Academy of Sciences.
Diffraction effects from h.c.p. crystals with random growth faults undergoing transformation to the f.c.c. phase by the deformation mechanism
(1987) M.T. Sebastian; P. Krishna
Hexagonally close‐packed cobalt crystals often contain a random distribution of growth faults. The h.c.p. to f.c.c. transformation is effected by a non‐random insertion of deformation faults at two layer separations. The kinematical theory is developed of diffraction from h.c.p. crystals containing randomly distributed growth faults undergoing a transformation to the f.c.c. phase. The presence of a small amount of random growth faults give rise to an additional broadening of some of the reflections. Copyright © 1987 WILEY‐VCH Verlag GmbH & Co. KGaA
Mechanism of phase transformation in polytypes.
(1986) P. Krishna; M.T. Sebastian
Several materials with a close-packed structure, like ZnS and SiC, are known to undergo solid-state transformation from one polytype structure to another by the insertion of stacking faults. During the course of the phase transformation the stacking faults occur preferentially at such layer spacings as to statistically create the new structure, which is usually faulted. In such cases the classical theories of X-ray scattering from randomly faulted close-packed structures break down and a probability distribution of faults has to be assumed to compute the diffraction effects. This probability distribution depends on the mechanism of the transformation in the material being studied. The nature of the stacking faults involved in the transformation and their distribution can be determined by arresting the transformation at an intermediate stage and studying the disordered partially transformed crystal by XRD techniques. A comparison of the diffraction effects recorded on a single-crystal diffractometer with those computed theoretically for different models of the transformation enable both the type of faults involved and the probability distribution of their occurrence during the transformation to be determined. The application of this method to investigate the mechanism of transformations in ZnS, SiC, ZnxCd1-xS and ZnxMn1-xS is reviewed and the results obtained are discussed.-J.M.H.
Mechanism of phase transformations in ZnS
(Springer India, 1984) M.T. Sebastian; P. Krishna
X-ray diffraction of the 2H-3C transformation in ZnS crystals has been studied to determine the mechanism of the phase transformation. Single crystals of 2H ZnS were annealed in vacuum at different temperatures to induce the phase transformation and then quenched to arrest it at different intermediate stages. The transformation is found to occur by the non-random nucleation of stacking faults in the 2H structure which produce characteristic diffuse steaks along reciprocal lattice rows parallel to c* for which H-K ≠ 0 (mod 3). All the crystals finally transform to a disordered twinned 3C structure. A study of the broadening of the x-ray diffraction maxima reveals that the stacking faults involved in the transformation are basal plane deformation faults. Initially these nucleate at random producing a random distribution of cubic nuclei within the 2H structure. As the transformation proceeds these 3C nuclei grow into thick 3C regions by a preferential nucleation of the faults at 2-layer separations. Since the 3C nuclei can have twin orientations the resulting 3C structure invariably contains a random distribution of twin faults. This is confirmed by comparing the experimentally observed intensity profile of the 10. L reflections as recorded on a single crystal diffractometer, with those calculated theoretically for a randomly twinned cubic structure. © 1984 Indian Academy of Sciences.
Mechanism of Solid State Transformations in Single Crystals of ZnxCd1−xS
(1983) M.T. Sebastian; P. Krishna
The mechanism of solid state transformations in single crystals of ZnxCd1−x is investigated by annealing metastable 2H crystals at different temperatures, arresting the solid state transformation by quenching in cold water, and examining the structure of the intermediate state by X‐ray diffraction. In the range x = 0.93 to 0.95, the 2H structure is found to transform to a disordered 6H structure at temperatures around 600 °C. This 6H structure does not transform further to the 3C structure on annealing at higher temperatures. It is observed that the mechanism of the 2H to 6H transformation in these crystals is very different from that proposed by Pandey, Lele, and Krishna for a similar transformation in Sic. A model of transformation involving the non‐random insertion of stacking faulhs is developed using a fault probability α for random nucleation of the 6H phase and a fault probability β for the growth of the 6H nuclei. The diffraction effects calculeted from such a model for both, deformation faults and layer‐displacement faults are compared with those recorded on a single crystal diffractometer using partially transformed crystals. It is concluded that the model of transformation by a non‐random insertion of deformation faults with β ≫ α, closely approximates the actual mode of transformation in these crystals. Copyright © 1983 WILEY‐VCH Verlag GmbH & Co. KGaA
Single crystal diffraction studies of stacking faults in close-packed structures
(1987) M.T. Sebastian; P. Krishna
[No abstract available]
Stacking faults and structural transformations in ZnxMn1-xS single crystals grown from vapour
(1984) M.T. Sebastian; P. Krishna
This paper reports an X-ray diffraction study of the stacking faults and solid state transformations observed in ZnxMn1-xS (0.9 < x < 1) single crystals grown from the vapour phase at 1100°C in the presence of H2S. The crystals were found to contain 2H, 3C, 2H + 3C and polytypes as well as structures with considerable disorder due to random stacking faults. Needle shaped 2H crystals were annealed in vacuum in the temperature range from 300 to 1100°C to induce structural transformations. Nearly 75% of the crystals transformed to a disordered twinned 3C structure on annealing around 600°C. The rest of the crystals transformed first to a disordered 6H structure and then, on further annealing at higher temperatures, to the disordered twinned 3C structure. Most of the transformed 3C structures showed an intensity enhancement near the positions of the 6H reflections on X-ray diffraction photographs. All the 3C crystals transform back to a disordered 2H structure on annealing above 1050°C. To determine the nature of stacking faults involved in the 2H-6H transformation we have investigated the broadening of X-ray diffraction maxima along reciprocal lattice rows with H - K ≠ 3n produced by crystals undergoing the transformation. The point intensity distribution along the 10.L reciprocal lattice row of several partially transformed crystals were recorded on a four circle single crystal diffractometer in steps of ΔL = 0.01. A study of the experimental profiles obtained by plotting the diffractometer record of intensity versus L in reciprocal space shows that the transformation occurs by the non-random insertion of deformation faults in the 2H structure. It is shown that the one-parameter model proposed by Pandey, Lele and Krishna [Proc. Roy. Soc. (London) A369 (1980) 451 is not applicable to 2H-6H solid state transformation in ZnxMn1-xS crystals. A two-parameter model involving different fault probabilities for the nucleation (α) and the growth (β) of the 6H phase in the 2H structure is suggested. © 1984.
Structural disorder and solid state transformations in single crystals of Zn x Cd1-x S and Zn x Mn1-x S
(Springer India, 1984) M.T. Sebastian; P. Krishna
Single crystals of Zn x Cd1-x S and Zn x Mn1-x S were grown from the vapour phase at 1100°C in the range x=0·9 to 1. X-ray characterization shows that polytypes and disordered structures occur in Zn x Cd1-x S for x ≥ 0·94, whereas Zn x Mn1-x S displays disordered and polytype structures in the entire range x=0·9 to 1. It is observed that Zn x Cd1-x S and Zn x Mn1-x S undergo a 2H-6H solid state transformation on annealing in vacuum around 600°C. Experimental analysis of the intensity distribution along the 10·L reciprocal lattice row as recorded on a single crystal diffractometer from partially transformed crystals shows that the mechanism of the transformation cannot be explained in terms of the one-parameter models of non-random faulting reported earlier. A two-parameter theoretical model with α representing the probability of random insertion of a fault in the 2H structure and β representing the probability of the growth of the 6H nucleus, is developed both for a deformation mechanism and a layer displacement mechanism. It is found that the theoretical model of non-random deformation faulting with β ≫ α approximates the actual mechanism of transformation in these crystals. © 1984 The Indian Academy of Sciences.
The discovery of a 2H-6H solid state transformation in ZnxCd1-xS single crystals
(1983) M.T. Sebastian; P. Krishna
Single crystals of ZnxCd1-xS have been grown from the vapour phase at 1100°C in the presence of H2S gas. X-ray diffraction studies of the as-grown crystals show that polytypism and stscking faults occur in ZnxCd1-xS crystals for x ≥ 0.94. It is observed that for 0.92 < x < 0.98 the 2H structure of ZnxCd1-xS crystals transforms to a disordered 6H structure on annealing in vacuum around 600°C. For 0.95 < x < 0.98 this 6H structure finally transforms to a disordered 3C structure on annealing further at higher temperatures around 800°C. The structural transformations occur through a non-random insertion of stacking faults, as revealed by the diffuse streak joining the X-ray diffraction maxima along the 10.L reciprocal lattice row. Experimental investigation of the diffuse intensity distribution, as recorded on a single crystal diffractometer from partially transformed single crystals, reveals that the mechanism of the transformation is very different from that reported for the same transformation in silicon carbide and cannot be described in terms of a single-parameter model of non-random deformation faulting. © 1983.
X‐Ray Diffraction Effects from 2H Crystals Undergoing Transformation to the 3C Structure by the Layer Displacement Mechanism
(1987) M.T. Sebastian; K. Narayanan; P. Krishna
On annealing at elevated temperatures 2H silicon carbide crystals transform to a disordered twinned 3C structure. It is suggested that the transformation proceeds by nucleation and propagation of stacking faults causing layer‐displacement in the solid state. The theory of X‐ray diffraction for 2H crystals is developed undergoing transformation to the 3C phase by such a mechanism. The different diffraction effects such as peak breadth and peak broadening are predicted. Copyright © 1987 WILEY‐VCH Verlag GmbH & Co. KGaA
X‐ray diffraction study of the 2H to 3C solid state transformation in vapour grown single crystals of ZnS
(1982) M.T. Sebastian; D. Pandey; P. Krishna
An X‐ray diffraction study is made of the 2H → 3C solid‐state structural transformation in ZnS. Single crystals of 2H ZnS (wurtzite), grown from the vapour phase above 1100 °C in the presence of H2S2, are annealed in vacuum at different temperatures ranging from 300 to 650 °C. The transformation is found to commence with a statistical insertion of stacking faults as revealed by the intensification of the diffuse streaks along reciprocal lattice rows parallel to c*. Diffraction spots characteristic of the 3C structure appear on the streaks at a later stage and the end product is invariably a disordered, twinned 3C structure. The rate of transformation is found to depend on the annealing temperature as well as the perfection of the initial crystal. No change in the external shape of the crystals is observed. To determine the nature of stacking faults involved in the transformation the broadening of the X‐ray diffraction maxima are investigated produced by annealing 2H ZnS crystals. The point intensity distribution along the 10.L reciprocal lattice row of a slightly faulted 2H crystal is recorded on a four‐circle single‐crystal diffractometer in steps of ΔL = 0.01. It is found that the half widths of the L even and L odd reflections are equal, indicating that the stacking faults introduced during annealing are predominantly deformation faults. The observed and calculated intensity profiles of different individual reflections are found to be in good agreement. A slight discrepancy observed is attributed to the non‐random insertion of stacking faults during transformation. Copyright © 1982 WILEY‐VCH Verlag GmbH & Co. KGaA
X‐ray investigation of the mechanism of phase transformation in single crystals of ZnS, ZnxCd1−xS and ZnxMn1−xS (I). Calculation of diffraction effects by a three parameter model
(1987) M.T. Sebastian; P. Krishna
Hexagonal close‐packed (2H) single crystals of ZnS, ZnxCd1−xS and ZnxMn1−xS are known to undergo solid state transformation to the cubic close‐packed (3C) and 6H‐structures on annealing at elevated temperatures. The transformations occur by the non‐random nucleation of stacking faults on individual close‐packed layers parallel to (0001). The nature of the faults involved and their probability distribution during transformation determine the diffraction effects produced along the 10. L reciprocal lattice row by a crystal quenched in an intermediate state of transformation. We have investigated the mechanism of the transformation by comparing the diffraction effects recorded from such crystals on a single crystal diffractometer, with those calculated for an assumed model of the transformation. It is known that in these materials the faults involved in the transformation are deformation faults. To explain the observed diffraction effects we develop a three parameter theoretical model employing a fult probability α for the radom insertion of a deformation fault in the 2H structure, a fault probability β for the deformation faults to occur at three layer separations and a fault probability γ of their occcurrence at 2‐layer separations. The probability α corresponds to the development of a fresh nucleus, the probability β to the growth of the 6H nucleus and the probability γ to the growth of a 3C nucleus. This paper develops the necessary theory of X‐ray scattering for such a model of the transformation and predicts the diffraction effects for different values of α, β, and γ. The next paper compares these results with experimental observations. Copyright © 1987 WILEY‐VCH Verlag GmbH & Co. KGaA
X‐ray investigation of the mechanisms of phase transformations in single crystals of ZnS, ZnxCd1−xS, and ZnxMn1−xS (II). Comparison of observed and calculated diffraction effects
(1987) M.T. Sebastian; P. Krishna
The experimentally observed intensity profiles recorded from ZnS, ZnxCd1−xS and ZnxMn1−xS crystals at different stages of the 2H‐3C and 2H‐6H transformations are compared with those computed theoretically from the three parameter model in the previous paper. It is found that the 2H‐3C and 2H‐6H transformations occur by the nonrandom nucleation of deformation faults. The probability of the growth of a nucleus is much greater than that of the creation of a fresh nucleus. At least two different fault probabilities have therefore to be employed in computing the diffraction effects in each case. The transformation behaviour of ZnxCd1−xS and ZnxMn1−xS crystals is strongly influenced by variations in stoichiometry (x). Large values of x (x > 0.95) favour the 2H‐3C transformation, whereas smaller values (x ≦ 0.94) favour the formation of the 6H phase. A comparison of the calculated and observed intensity distributions indicates that in some crystals the 2H‐3C and 2H‐6H transformations occur simultaneously in different regions of the same single crystal. Copyright © 1987 WILEY‐VCH Verlag GmbH & Co. KGaA