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Growth and polytypism of single crystals of cadmium bromide
Journal of Crystal Growth 73 (~985)543—545 North-Holland, Amsterdam
543
GROWTH AND POLYTYPISM OF SINGLE CRYSTALS OF CADMIUM BROMIDE S.K. CHAUDHARY Department of Physics, University College, M.D. University, Rohtak, India
and G.C. TRIGUNAYAT Department of Physics and Astrophysics, University of Delhi, Delhi, India
Received 7 November 1984; manuscripts received in final form 6 June 1985
Single crystals of cadmium bromide have been grown by Bridgman—Stockbarger technique and their polytypism studied by X-ray diffraction. Oscillation photographs obtained from several different regions of the crystals have revealed the occurrence of only the small-period polytype 6R in the crystals. The results have been compared with those reported earlier for the isostructural compounds Cd1 2 and Pb12.
1. Introduction The polytypism of Cd12 and Pb12 crystals has been extensively investigated and reviewed from time to time [1]. By comparison, the polytypism of the isostructural crystals of CdBr2 has received far less attention. So far, the CdBr2 crystals have been grown either from solution or by sublimation and a few polytypes have been discovered in them [2—5].No results have been reported on crystals grown from the melt. Indeed, no successful attempts appear to have been made to grow crystals from the melt. In view of some significant results having been reported on the polytypism of meltgrown Cd12 and Pb!2 crystals [6,7], we proposed to grow CdBr2 crystals from the melt and examine the nature of their polytype growth. The results have been reported.
2. Experimental
mic acid, nitric acid and distilled water and then thoroughly dried. Reagent grade CdBr2 4H20, dehydrated by heating it at 150°Cfor about 24 h, was used as the starting material. The ampoule was vacuum sealed using a mercury diffusion pump and subsequently introduced in the upper zone of a two-zone furnace, the details of which have been earlier described elsewhere [8]. The best rate of lowering the ampoule for growing a good single crystal was found to be 0.5 cm/h. The correspond-
-
600500
-
400
-
A
1
~30O200
The melting point of CdBr2 is 567°C.A tapered quartz ampoule was employed to grow the crystals. The ampoule was successively cleaned with chro-
m.p.
5
I
I
6 7 8 9 10 11 DISTANCE FROM TOP OF ELEMENT
I
12 13 INCHES)
I
14
Fig. 1. Temperature profile of the furnace for the growth of CdBr2 crystals.
0022-0248/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
544
S. K. Chaudharv, G. C. Trigunavat
ing temperature profile is shown in fig. 1. As soon as the ampoule reached the region AB, its movement was stopped and to minimize any thermal stresses, the furnace was slowly cooled by reducing the voltage fed to the furnace such that room temperature was reached in nearly 12 h. The finally grown crystal ingot measured nearly 1.2 cm in diameter and nearly 3 cm in length. The CdBr2 crystals have a platy nature, rendering them amenable to easy cleavage along the basal planes. For the sake of convenience in mounting and adjustment of the crystals on an X-ray diffraction camera, long pieces of thickness ranging between 0.6 and 0.8 mm were cleaved from the ingot and finally cut into pieces of length ranging between 5 and 8 mm. For a quick identification of the polytypes, suitable 15°a-axis oscillation photographs of the crystals were obtained to record a large sequence of 10./ diffraction spots in the region of back reflection [9]. All oscillation photographs were recorded on a cylindrical camera of radius 3 cm, employing Ni-filtered Cu Ka radiation and a collimator of aperture 0.5 mm.
3. Results and discussion Earlier, cadmium iodide and lead iodide have been purified and their single crystals grown by using the zone-refining technique [6,7]. Initially we proposed to follow the same course for CdBr2 crystals, which are isostructural with the crystals of Cd!2 and Pb12. However, on account of high vapour pressure of CdBr2 at its melting point (about 7 times the corresponding values for Cd!2 or Pb12), most of the material evaporated and got deposited on the walls of the growth chamber. Since an important factor for controlling the evaporation rate is the external pressure just above the melt (nearly 1 atm in our case), we first tried to zone-refine the material in a sealed evacuated ampoule instead of a boat, in the hope that the pressure built up by the evaporation of CdBr2 would suppress the undesirable evaporation. Then, the evaporation was partly suppressed, but after 2—3 zone passes most of the material evaporated off. Hence we planned to grow the crystals by employing the Bridgman—Stockbarger system in
/ Single crystals of CcjBr, which the vertical position of the ampoule restricts the exposed melt surface to just the area of crosssection of the ampoule and thus effectively suppresses the evaporation. The first crystal ingot thus grown was fairly transparent, except for its uppermost portion and a negligible lower portion near the tip. A yellow impurity was found suspended near the tip and some black specks appeared in the uppermost portion of the ingot, thus indicating that the lowest and the uppermost portions were relatively impure. To obtain purer crystals, the tip and the top portion of the ingot were removed, the remainder was resealed in the quartz ampoule, and the growth procedure was repeated. After three such repetitions, the grown crystal ingot did not show any impurity at the top or at the tip. It stuck only a bit to the walls of the ampoule, such that a slight tapping was found enough to separate the ingot from the ampoule. Indeed the sticking was found to decrease steadily in the successive crystallizations, thus indicating increasing purity of the material. It was noticed that when the grown crystal was taken out of the furnace, sometimes the top of the ingot was not horizontal, in spite of the axis of the ampoule being vertical. The X-ray reflections of such a crystal were found to be diffuse and considerably elongated normal to the layer lines, thus revealing that the crystal had a high density of dislocations. For a good crystal growth it was found necessary to position the ampoule axially in the centre of the furnace during crystal growth, so that no radial temperature gradients existed in the ampoule. When this condition was achieved, the top surface of the ingot was found to be horizontal, i.e. perpendicular to the vertical direction of transition of the ampoule. In most of the ingots, a crack was found to be running across their lengths. Similar cracks have also been observed in the Cd17 crystals and have been conjectured as arising from the differential contraction of the materials of the ingot and the ampoule during cooling [7].A greater possibility is that the crack is produced by internal stresses inside the ingot, resulting from uneven distribution of impurities. Although the material was purified by repetitively rejecting the tip and the upper portion of the ingot, the middle portion
S. K. Chaudhary. G. C. Trigunayat
/ Single crystals of CdBr,
545
cause slip can occur in them both along the closepacked basal planes (0001) and along the equally close-packed non-basal planes (1012). Thus the observation of heavy disorder in the solution-grown CdBr2 crystals could be justified [9]. In sharp
Fig. 2. A 15° a-axis oscillation 6RR and photograph 6R of a CdBr2 crystal showing reflections of 0. Cu Ka radiation; 6RR and 6R 3 cm camera. The positions of the reflections, each of 0, have been indicated by arrow marks, pointing upwards and downwards, respectively.
of the finally grown ingot showed better transparency than the other portions. Hence, the intermediate portion of the ingot was chosen for the X-ray diffraction studies. It is well known that CdBr2 crystals frequently exhibit syntactic coalescence of polytypes. Therefore, oscillation photographs were separately obtamed from the two faces (parallel to the basal plane) of a crystal. Further, because of the large size of the crystal and the fact that CdBr2 crystals also sometimes exhibit parallel growth of two or more polytypes on the same face, oscillation photographs were taken from at least four different parts of each face of the crystal. A total number of 8 crystals, involving nearly 60 X-ray photographs, exclusively showed diffraction spots of the common type 6R (observe and/or reverse). One such photograph is reproduced in fig. 2. In an earlier study of polytypism of CdBr2 crystals [9], the polytypes were found to be heavily disordered, showing strong features of both streaking and arcing of the reflections on their oscillation photographs. These crystals had been grown from solution. The phenomenon of streaking arises from the existence of random stacking faults in the structure and arcing from the movement of edge dislocations into small angle tilt boundaries [9]. In comparison to the Cd!2 and Pb!2 crystals, the chances of creation of stacking faults and edge dislocations are greater in the CdBr2 crystals be-
contrast, in the present crystals, the streaking and arcing have been observed just in a few cases and that, too, only to a small extent. Further, it is to be noted that well purified melt-grown crystals of Cd12 and Pb!2 do not show any arcing [6,7] (the solution-grown Cd!2 and Pb12 crystals do). Thus the small extent of arcing exhibited by the present crystals might be attributed to the presence of some residual impurities in the material. The presence of impurities may lead to constitutional supercooling, resulting in irregular crystal growth and subsequent production of edge dislocations [10], which may align themselves into small-angle tilt boundaries. From the above studies it can be concluded that (i) at high temperatures the most common polytype of CdBr2 is 6R and (ii) melt-grown CdBr2 crystals contain only the small period common polytype 6R.
References [1] G.C. Trigunayat and AR. Verma, in: Physics and Chemistry of Materials with Layered Structures, Vol. 2, Ed. F. Levy (Reidel. Dordrecht, 1976). [2] Z.G. Pinsker, Electron Diffraction (Butterworths. London, 1953). [3] R.S. Mitchell, Z. Krist. 117 (1962) 309. [4] V.K. Agrawal and G.C. Trigunayat, Acta Cryst. A26 (1970) 426. [5] K. Mehrotra, J. Crystal Growth 44 (1978) 45. [6] 5K. Chaudhary and G.C. Trigunayat, J. Crystal Growth 62 (1983) 398. [7] S.K. Chaudhary and G.C. Trigunayat, J. Crystal Growth 57 (1982) 558. [8] S.K. Chaudhary and G.C. Trigunayat, Crystal Res. Technol. 17 (1982) 465. [9] V.K. Agrawal and G.C. Trigunayat. Acta Cryst. A25 (1969) 401. [10] W.D. Lawson and S. Nielson. Preparation of Single Crystals (Butterworths. London, 1958).
543
GROWTH AND POLYTYPISM OF SINGLE CRYSTALS OF CADMIUM BROMIDE S.K. CHAUDHARY Department of Physics, University College, M.D. University, Rohtak, India
and G.C. TRIGUNAYAT Department of Physics and Astrophysics, University of Delhi, Delhi, India
Received 7 November 1984; manuscripts received in final form 6 June 1985
Single crystals of cadmium bromide have been grown by Bridgman—Stockbarger technique and their polytypism studied by X-ray diffraction. Oscillation photographs obtained from several different regions of the crystals have revealed the occurrence of only the small-period polytype 6R in the crystals. The results have been compared with those reported earlier for the isostructural compounds Cd1 2 and Pb12.
1. Introduction The polytypism of Cd12 and Pb12 crystals has been extensively investigated and reviewed from time to time [1]. By comparison, the polytypism of the isostructural crystals of CdBr2 has received far less attention. So far, the CdBr2 crystals have been grown either from solution or by sublimation and a few polytypes have been discovered in them [2—5].No results have been reported on crystals grown from the melt. Indeed, no successful attempts appear to have been made to grow crystals from the melt. In view of some significant results having been reported on the polytypism of meltgrown Cd12 and Pb!2 crystals [6,7], we proposed to grow CdBr2 crystals from the melt and examine the nature of their polytype growth. The results have been reported.
2. Experimental
mic acid, nitric acid and distilled water and then thoroughly dried. Reagent grade CdBr2 4H20, dehydrated by heating it at 150°Cfor about 24 h, was used as the starting material. The ampoule was vacuum sealed using a mercury diffusion pump and subsequently introduced in the upper zone of a two-zone furnace, the details of which have been earlier described elsewhere [8]. The best rate of lowering the ampoule for growing a good single crystal was found to be 0.5 cm/h. The correspond-
-
600500
-
400
-
A
1
~30O200
The melting point of CdBr2 is 567°C.A tapered quartz ampoule was employed to grow the crystals. The ampoule was successively cleaned with chro-
m.p.
5
I
I
6 7 8 9 10 11 DISTANCE FROM TOP OF ELEMENT
I
12 13 INCHES)
I
14
Fig. 1. Temperature profile of the furnace for the growth of CdBr2 crystals.
0022-0248/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
544
S. K. Chaudharv, G. C. Trigunavat
ing temperature profile is shown in fig. 1. As soon as the ampoule reached the region AB, its movement was stopped and to minimize any thermal stresses, the furnace was slowly cooled by reducing the voltage fed to the furnace such that room temperature was reached in nearly 12 h. The finally grown crystal ingot measured nearly 1.2 cm in diameter and nearly 3 cm in length. The CdBr2 crystals have a platy nature, rendering them amenable to easy cleavage along the basal planes. For the sake of convenience in mounting and adjustment of the crystals on an X-ray diffraction camera, long pieces of thickness ranging between 0.6 and 0.8 mm were cleaved from the ingot and finally cut into pieces of length ranging between 5 and 8 mm. For a quick identification of the polytypes, suitable 15°a-axis oscillation photographs of the crystals were obtained to record a large sequence of 10./ diffraction spots in the region of back reflection [9]. All oscillation photographs were recorded on a cylindrical camera of radius 3 cm, employing Ni-filtered Cu Ka radiation and a collimator of aperture 0.5 mm.
3. Results and discussion Earlier, cadmium iodide and lead iodide have been purified and their single crystals grown by using the zone-refining technique [6,7]. Initially we proposed to follow the same course for CdBr2 crystals, which are isostructural with the crystals of Cd!2 and Pb12. However, on account of high vapour pressure of CdBr2 at its melting point (about 7 times the corresponding values for Cd!2 or Pb12), most of the material evaporated and got deposited on the walls of the growth chamber. Since an important factor for controlling the evaporation rate is the external pressure just above the melt (nearly 1 atm in our case), we first tried to zone-refine the material in a sealed evacuated ampoule instead of a boat, in the hope that the pressure built up by the evaporation of CdBr2 would suppress the undesirable evaporation. Then, the evaporation was partly suppressed, but after 2—3 zone passes most of the material evaporated off. Hence we planned to grow the crystals by employing the Bridgman—Stockbarger system in
/ Single crystals of CcjBr, which the vertical position of the ampoule restricts the exposed melt surface to just the area of crosssection of the ampoule and thus effectively suppresses the evaporation. The first crystal ingot thus grown was fairly transparent, except for its uppermost portion and a negligible lower portion near the tip. A yellow impurity was found suspended near the tip and some black specks appeared in the uppermost portion of the ingot, thus indicating that the lowest and the uppermost portions were relatively impure. To obtain purer crystals, the tip and the top portion of the ingot were removed, the remainder was resealed in the quartz ampoule, and the growth procedure was repeated. After three such repetitions, the grown crystal ingot did not show any impurity at the top or at the tip. It stuck only a bit to the walls of the ampoule, such that a slight tapping was found enough to separate the ingot from the ampoule. Indeed the sticking was found to decrease steadily in the successive crystallizations, thus indicating increasing purity of the material. It was noticed that when the grown crystal was taken out of the furnace, sometimes the top of the ingot was not horizontal, in spite of the axis of the ampoule being vertical. The X-ray reflections of such a crystal were found to be diffuse and considerably elongated normal to the layer lines, thus revealing that the crystal had a high density of dislocations. For a good crystal growth it was found necessary to position the ampoule axially in the centre of the furnace during crystal growth, so that no radial temperature gradients existed in the ampoule. When this condition was achieved, the top surface of the ingot was found to be horizontal, i.e. perpendicular to the vertical direction of transition of the ampoule. In most of the ingots, a crack was found to be running across their lengths. Similar cracks have also been observed in the Cd17 crystals and have been conjectured as arising from the differential contraction of the materials of the ingot and the ampoule during cooling [7].A greater possibility is that the crack is produced by internal stresses inside the ingot, resulting from uneven distribution of impurities. Although the material was purified by repetitively rejecting the tip and the upper portion of the ingot, the middle portion
S. K. Chaudhary. G. C. Trigunayat
/ Single crystals of CdBr,
545
cause slip can occur in them both along the closepacked basal planes (0001) and along the equally close-packed non-basal planes (1012). Thus the observation of heavy disorder in the solution-grown CdBr2 crystals could be justified [9]. In sharp
Fig. 2. A 15° a-axis oscillation 6RR and photograph 6R of a CdBr2 crystal showing reflections of 0. Cu Ka radiation; 6RR and 6R 3 cm camera. The positions of the reflections, each of 0, have been indicated by arrow marks, pointing upwards and downwards, respectively.
of the finally grown ingot showed better transparency than the other portions. Hence, the intermediate portion of the ingot was chosen for the X-ray diffraction studies. It is well known that CdBr2 crystals frequently exhibit syntactic coalescence of polytypes. Therefore, oscillation photographs were separately obtamed from the two faces (parallel to the basal plane) of a crystal. Further, because of the large size of the crystal and the fact that CdBr2 crystals also sometimes exhibit parallel growth of two or more polytypes on the same face, oscillation photographs were taken from at least four different parts of each face of the crystal. A total number of 8 crystals, involving nearly 60 X-ray photographs, exclusively showed diffraction spots of the common type 6R (observe and/or reverse). One such photograph is reproduced in fig. 2. In an earlier study of polytypism of CdBr2 crystals [9], the polytypes were found to be heavily disordered, showing strong features of both streaking and arcing of the reflections on their oscillation photographs. These crystals had been grown from solution. The phenomenon of streaking arises from the existence of random stacking faults in the structure and arcing from the movement of edge dislocations into small angle tilt boundaries [9]. In comparison to the Cd!2 and Pb!2 crystals, the chances of creation of stacking faults and edge dislocations are greater in the CdBr2 crystals be-
contrast, in the present crystals, the streaking and arcing have been observed just in a few cases and that, too, only to a small extent. Further, it is to be noted that well purified melt-grown crystals of Cd12 and Pb!2 do not show any arcing [6,7] (the solution-grown Cd!2 and Pb12 crystals do). Thus the small extent of arcing exhibited by the present crystals might be attributed to the presence of some residual impurities in the material. The presence of impurities may lead to constitutional supercooling, resulting in irregular crystal growth and subsequent production of edge dislocations [10], which may align themselves into small-angle tilt boundaries. From the above studies it can be concluded that (i) at high temperatures the most common polytype of CdBr2 is 6R and (ii) melt-grown CdBr2 crystals contain only the small period common polytype 6R.
References [1] G.C. Trigunayat and AR. Verma, in: Physics and Chemistry of Materials with Layered Structures, Vol. 2, Ed. F. Levy (Reidel. Dordrecht, 1976). [2] Z.G. Pinsker, Electron Diffraction (Butterworths. London, 1953). [3] R.S. Mitchell, Z. Krist. 117 (1962) 309. [4] V.K. Agrawal and G.C. Trigunayat, Acta Cryst. A26 (1970) 426. [5] K. Mehrotra, J. Crystal Growth 44 (1978) 45. [6] 5K. Chaudhary and G.C. Trigunayat, J. Crystal Growth 62 (1983) 398. [7] S.K. Chaudhary and G.C. Trigunayat, J. Crystal Growth 57 (1982) 558. [8] S.K. Chaudhary and G.C. Trigunayat, Crystal Res. Technol. 17 (1982) 465. [9] V.K. Agrawal and G.C. Trigunayat. Acta Cryst. A25 (1969) 401. [10] W.D. Lawson and S. Nielson. Preparation of Single Crystals (Butterworths. London, 1958).