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Purification, growth and polytypism of single crystals of lead iodide
398
Journal of Crystal Growth 62 (1983) 398—400 North-Holland Publishing (ompanv
PURIFICATION, GROWTH AND POLYTYPISM OF SINGLE CRYSTALS OF LEAD IODIDE S.K. CHAUDHARY and G.C. TRIGUNAYAT Department of Physics and Astrophysics, University of Delhi, Delhi - 110007, India Received 21 July 1982; manuscript received in final form 19 January 1983
Single crystals of lead iodide have been purified and grown using the method of zone refining and their polytvpism 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.
1. Introduction While a satisfactory, comprehensive explanation of the phenomenon of polytypism is still awaited, recent theoretical and experimental studies on MX 2-type structures, including iodides of cadmium and lead, have shown that interlayer displacements in these structures can take place far more easily than intralayer displacements [1—4]. To identify the role played by such displacements it is necessary to study the nature of growth of polytypes. Work has been done in this direction on polytypes of Pb!2 grown from solution, vapour and gel [5,6], but no work has been done on crystals grown from the melt. Since the zone-refining technique of melt-growth is capable of purifying the material as well as yielding single crystals, it was proposed to grow single crystals of Pb12 by this technique and to study the nature of growth of polytypes in them. The technique also enables one to exercise control on the degree of purification of the material. Hence it was also decided to study the effect of impurities on the growth of Pb12 polytypes.
by us for Cd12 crystals [7] was employed. The starting material was analar grade Pb!2 kept in a glass boat. The zone-refinement was carried out in an atmosphere of argon. The speed of movements of the heaters was maintained at 2 cm/h. A good single crystal could be grown in about 16— 18 zone passes. In the final pass the height of the afterheater below the growth chamber was suitably reduced to permit slow cooling of the crystal. Fig. 1 shows the temperature profile of the chamber during the final zone pass. A crystal ingot measured nearly 10 cm in length and nearly 6 mm in thickness. The Pb12 crystals have a platy nature, rendering them amenable to easy cleavage along the basal 440
T7~~
rn P.
290 ~
ZONE MOVEMENT
240 190 140
2. Experimental
I
5
For the growth of single crystals and purification of Pb12, a zone-refining system used earlier 0022-0248/83/0000---0000/$03 .00
©
4
I
32
DISTANCE
I 1
I
0
FROM THE
I
2
3
4
5
FORE—HEATER (cm)
Fig. I. Temperature profile during final zone pass.
1983 North-Holland
—~
S. K. Chaudhary, G. C. Trigunayar
/ Purification,
planes. To facilitate mounting on the X-ray diffraction camera, crystals with thicknesses of 0.6—0.8 mm were cleaved from an ingot and finally cut into pieces of 5—8 mm length. For identification of polytypes, suitable 15° a-axis oscillation photographs of the crystals were obtained [8]. The direction of the a-axis, lying in the plane of the basal faces exposed by cleavage, was located by trial. The photographs were recorded on a cylindrical camera of radius 3 cm, employing Nifiltered Cu Ka radiation and a fine collimator of aperture 0.5 mm. The source of X-rays was a fine focus X-ray tube with a focal spot size of 0.4 X 0.8 mm.
3. Experimental results 3.1. Purification and growth
Single crystal formation, as judged from the homogeneity of the material in the ingot and later confirmed by X-ray diffraction, could be noticed after 6 to 8 zone passes. Some light-black specks were observed on the surface of the ingot. Some cracks were observed in the ingot perpendicular to the length of the boat. The crystals were soft, but harder than the earlier grown Cd!2 crystals. Up to 8 passes no transparency was observed in the ingot and it was found difficult to unstick the ingot from the walls of the boat. After 10 passes the initial portion of the ingot started shining and becoming transparent. A demarcation line could be seen between the transparent and the non-transparent regions, such that beyond the line the transparency gradually decreased along the ingot (fig. 2). After 20 passes, the light-black specks were found to be
growth and polytypism of PbI,
399
deposited at the far end of the boat and the mass of the material looked appreciably transparent and shining. Then the ingot could be easily removed from the boat, thus revealing that the impurities present in the material had been responsible for the observed sticking of the ingot with the walls of the boat. The extent of transparency could be taken as a reliable qualitative index for estimating the degree of purification and the single-crystal formation of the material. One of the impurities was believed to be silver, since the crystals grown after 6—8 zone passes often turned black when kept in daylight for 2 to 3 days. In general, the silver compounds are light sensitive. The crystals grown after 14 to 20 passes did not turn black. The initial transparent part of the ingot was quite soft and had to be handled carefully, whereas the far end (impure end) was found comparatively hard. Indeed, the softness of the crystal could be used as another index of its degree of purity and single crystal character. 3.2. X-ray studies It is well-known that polytypes residing on the two basal faces of a Pb!2 crystal often turn out to be different from one another. Therefore, oscillation photographs were separately obtained from the two faces. Further, because of the large size of the crystal and the fact that Pb!2 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 a crystal. For X-ray diffraction work, the crystals were classified into three batches: (i) Crystals grown after 6—8 zone passes (impure crystals). (iii) Crystals grown after 20 passes. A total number of 9 crystals from batch (i),
-
-
-
-
Fig. 2. The ingot after 10 zone passes, showing a demarcation line (AB) between the transparent and the non-tranparent region.
investigated Ffththhdth currence the rest polytypes + 12RR). Inof the of the l2Ro/12RR/(l2Ro cases, too, the same polytypes appeared, but in coalescence with the polytype 4H. Similarly 6 crystals chosen randomly from batch
400
S. K. Chaudhary, G.(~ Trigunavat
/
(ii), involving 46 X-ray diffraction photographs, were investigated. Of these, 36 exclusively showed 2RR/ the occurrence of in the8 cases polytypes l2R0/ofI these (l2R 12RR) and coalescence 0 + with 4H was observed. Two photopolytypes graphs showed coalescence of the polytypes I 2R and 2H. Another 5 crystals were randomly chosen from batch (iii). These involved 39 photographs, all of which the polytypes 2RR, exclusively except one showed that showed coalescence12R0/ with I 4H. The crystals were re-examined after storage at room temperature for periods extending over 7 months or more. It was found that (a) the structure of the crystals of batch (i), stored for nearly 12 months, did not change, (b) all but one crystal of batch (ii), stored for nearly 9 months, transformed into 2H, which is known to be the most commonly occurring polytype of lead iodide at room temperature, and (c) the structures of the crystals of batch (iii), stored for nearly 7 months, also did not change. The Pb12 crystals are often known to exhibit (i) streaking and (ii) arcing of reflections on their oscillation photographs, in which (i) diffraction spots on a layer line are seen to run into each other through streaks of various intensities and (ii) a diffraction spot is extended in the shape of an arc of a circle [9]. However, in the present investigation the X-ray photographs of the crystals did not show any streaking or arcing. Unlike the Pb12 crystals grown from gel, which frequently showed growth spirals of different shapes and step-heights on their basal faces, the present crystals did not exhibit any such spirals.
Purification, growth and polvtvpisni of Ph1~
of the small period polytype 4H of Pb17 at high temperatures. moreofpure the compound, the smaller are theThe chances formation of 4H. (iv) Occurrence of 12R polytypes in coalescence with 4H is a common feature of melt-grown Pb1 2 crystals. (v) The presence of impurities also influences solid state transformation of polytypes at room temperature. These features can be faults explained in terms dislocations and stacking produced due of to impurities during crystal growth. The observed features of streaking and arcing can also he cxplained similarly, which will be done in a separate publication elsewhere. The growth spirals have been commonly ohserved earlier on the basal faces of gel grown Pbl 2 crystals. They arise from the presence of screw dislocations in the structure. Their visibility depends upon their step-heights, such that ordinarily a spiral of step-height less than about 100 A may remain undetected when viewed with an optical microscope. At the prevalent high temperature of growth in the present work, any large screw dislocations are expected to dissociate into smaller ones. The Burgers vectors, and therefore the associated spiral step-heights, of the latter may he just a few molecular units, viz, nearly 14 A or a small multiple of that. Hence, although the growth spirals actually exist, they will remain undetected when viewed under a microscope.
References [1] J.L Verble and Ti. Wieiing. Solid State Commun.
II
[2] S. Nakashima, M. Daimon and A. Mitsuishi. J. Phys.
4 Discussion From the above observations, the following conclusions can be readily drawn: (i) At high temperatures the most common polytype of Pb12 is l2R. Thus 12R may be regarded as the stable high temperature modification and 2H as the stable room temperature modification of Pb12. (ii) Melt-grown Pb!2 crystals only contain small period polytypes. (iii) Impurities are responsible for the formation
Chem. Solids 40 (1979) 39. ]3[ RE. Wood and H.L. Ritter, J. Am. C’hem. Soc. 75 (1953)
471. [4] W.F. Knippenberg and G. Verspui, Mater. Res. Bull. 4
151
(1969) S33-S44. . G.C. Trigunayat and AR. Verma. in: Physics and C hemistry of Materials with Layered Structures, vol. 2. Ed. F. Levy (Reidel. Dordrecht, 1976) p. 269.
16] T, Minagawa, Ada Cryst. A31 (1975) 823. [7] 5K. Chaudhary and G.C. Trigunayat. J. Crystal Growth 57 (1982)Trigunayat 558. [8] G.C. and AR. Verma, Ada Cryst. 15 (1962) 499 [9[ G.C. Trigunayat. Nature 212 (1966) 808.
Journal of Crystal Growth 62 (1983) 398—400 North-Holland Publishing (ompanv
PURIFICATION, GROWTH AND POLYTYPISM OF SINGLE CRYSTALS OF LEAD IODIDE S.K. CHAUDHARY and G.C. TRIGUNAYAT Department of Physics and Astrophysics, University of Delhi, Delhi - 110007, India Received 21 July 1982; manuscript received in final form 19 January 1983
Single crystals of lead iodide have been purified and grown using the method of zone refining and their polytvpism 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.
1. Introduction While a satisfactory, comprehensive explanation of the phenomenon of polytypism is still awaited, recent theoretical and experimental studies on MX 2-type structures, including iodides of cadmium and lead, have shown that interlayer displacements in these structures can take place far more easily than intralayer displacements [1—4]. To identify the role played by such displacements it is necessary to study the nature of growth of polytypes. Work has been done in this direction on polytypes of Pb!2 grown from solution, vapour and gel [5,6], but no work has been done on crystals grown from the melt. Since the zone-refining technique of melt-growth is capable of purifying the material as well as yielding single crystals, it was proposed to grow single crystals of Pb12 by this technique and to study the nature of growth of polytypes in them. The technique also enables one to exercise control on the degree of purification of the material. Hence it was also decided to study the effect of impurities on the growth of Pb12 polytypes.
by us for Cd12 crystals [7] was employed. The starting material was analar grade Pb!2 kept in a glass boat. The zone-refinement was carried out in an atmosphere of argon. The speed of movements of the heaters was maintained at 2 cm/h. A good single crystal could be grown in about 16— 18 zone passes. In the final pass the height of the afterheater below the growth chamber was suitably reduced to permit slow cooling of the crystal. Fig. 1 shows the temperature profile of the chamber during the final zone pass. A crystal ingot measured nearly 10 cm in length and nearly 6 mm in thickness. The Pb12 crystals have a platy nature, rendering them amenable to easy cleavage along the basal 440
T7~~
rn P.
290 ~
ZONE MOVEMENT
240 190 140
2. Experimental
I
5
For the growth of single crystals and purification of Pb12, a zone-refining system used earlier 0022-0248/83/0000---0000/$03 .00
©
4
I
32
DISTANCE
I 1
I
0
FROM THE
I
2
3
4
5
FORE—HEATER (cm)
Fig. I. Temperature profile during final zone pass.
1983 North-Holland
—~
S. K. Chaudhary, G. C. Trigunayar
/ Purification,
planes. To facilitate mounting on the X-ray diffraction camera, crystals with thicknesses of 0.6—0.8 mm were cleaved from an ingot and finally cut into pieces of 5—8 mm length. For identification of polytypes, suitable 15° a-axis oscillation photographs of the crystals were obtained [8]. The direction of the a-axis, lying in the plane of the basal faces exposed by cleavage, was located by trial. The photographs were recorded on a cylindrical camera of radius 3 cm, employing Nifiltered Cu Ka radiation and a fine collimator of aperture 0.5 mm. The source of X-rays was a fine focus X-ray tube with a focal spot size of 0.4 X 0.8 mm.
3. Experimental results 3.1. Purification and growth
Single crystal formation, as judged from the homogeneity of the material in the ingot and later confirmed by X-ray diffraction, could be noticed after 6 to 8 zone passes. Some light-black specks were observed on the surface of the ingot. Some cracks were observed in the ingot perpendicular to the length of the boat. The crystals were soft, but harder than the earlier grown Cd!2 crystals. Up to 8 passes no transparency was observed in the ingot and it was found difficult to unstick the ingot from the walls of the boat. After 10 passes the initial portion of the ingot started shining and becoming transparent. A demarcation line could be seen between the transparent and the non-transparent regions, such that beyond the line the transparency gradually decreased along the ingot (fig. 2). After 20 passes, the light-black specks were found to be
growth and polytypism of PbI,
399
deposited at the far end of the boat and the mass of the material looked appreciably transparent and shining. Then the ingot could be easily removed from the boat, thus revealing that the impurities present in the material had been responsible for the observed sticking of the ingot with the walls of the boat. The extent of transparency could be taken as a reliable qualitative index for estimating the degree of purification and the single-crystal formation of the material. One of the impurities was believed to be silver, since the crystals grown after 6—8 zone passes often turned black when kept in daylight for 2 to 3 days. In general, the silver compounds are light sensitive. The crystals grown after 14 to 20 passes did not turn black. The initial transparent part of the ingot was quite soft and had to be handled carefully, whereas the far end (impure end) was found comparatively hard. Indeed, the softness of the crystal could be used as another index of its degree of purity and single crystal character. 3.2. X-ray studies It is well-known that polytypes residing on the two basal faces of a Pb!2 crystal often turn out to be different from one another. Therefore, oscillation photographs were separately obtained from the two faces. Further, because of the large size of the crystal and the fact that Pb!2 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 a crystal. For X-ray diffraction work, the crystals were classified into three batches: (i) Crystals grown after 6—8 zone passes (impure crystals). (iii) Crystals grown after 20 passes. A total number of 9 crystals from batch (i),
-
-
-
-
Fig. 2. The ingot after 10 zone passes, showing a demarcation line (AB) between the transparent and the non-tranparent region.
investigated Ffththhdth currence the rest polytypes + 12RR). Inof the of the l2Ro/12RR/(l2Ro cases, too, the same polytypes appeared, but in coalescence with the polytype 4H. Similarly 6 crystals chosen randomly from batch
400
S. K. Chaudhary, G.(~ Trigunavat
/
(ii), involving 46 X-ray diffraction photographs, were investigated. Of these, 36 exclusively showed 2RR/ the occurrence of in the8 cases polytypes l2R0/ofI these (l2R 12RR) and coalescence 0 + with 4H was observed. Two photopolytypes graphs showed coalescence of the polytypes I 2R and 2H. Another 5 crystals were randomly chosen from batch (iii). These involved 39 photographs, all of which the polytypes 2RR, exclusively except one showed that showed coalescence12R0/ with I 4H. The crystals were re-examined after storage at room temperature for periods extending over 7 months or more. It was found that (a) the structure of the crystals of batch (i), stored for nearly 12 months, did not change, (b) all but one crystal of batch (ii), stored for nearly 9 months, transformed into 2H, which is known to be the most commonly occurring polytype of lead iodide at room temperature, and (c) the structures of the crystals of batch (iii), stored for nearly 7 months, also did not change. The Pb12 crystals are often known to exhibit (i) streaking and (ii) arcing of reflections on their oscillation photographs, in which (i) diffraction spots on a layer line are seen to run into each other through streaks of various intensities and (ii) a diffraction spot is extended in the shape of an arc of a circle [9]. However, in the present investigation the X-ray photographs of the crystals did not show any streaking or arcing. Unlike the Pb12 crystals grown from gel, which frequently showed growth spirals of different shapes and step-heights on their basal faces, the present crystals did not exhibit any such spirals.
Purification, growth and polvtvpisni of Ph1~
of the small period polytype 4H of Pb17 at high temperatures. moreofpure the compound, the smaller are theThe chances formation of 4H. (iv) Occurrence of 12R polytypes in coalescence with 4H is a common feature of melt-grown Pb1 2 crystals. (v) The presence of impurities also influences solid state transformation of polytypes at room temperature. These features can be faults explained in terms dislocations and stacking produced due of to impurities during crystal growth. The observed features of streaking and arcing can also he cxplained similarly, which will be done in a separate publication elsewhere. The growth spirals have been commonly ohserved earlier on the basal faces of gel grown Pbl 2 crystals. They arise from the presence of screw dislocations in the structure. Their visibility depends upon their step-heights, such that ordinarily a spiral of step-height less than about 100 A may remain undetected when viewed with an optical microscope. At the prevalent high temperature of growth in the present work, any large screw dislocations are expected to dissociate into smaller ones. The Burgers vectors, and therefore the associated spiral step-heights, of the latter may he just a few molecular units, viz, nearly 14 A or a small multiple of that. Hence, although the growth spirals actually exist, they will remain undetected when viewed under a microscope.
References [1] J.L Verble and Ti. Wieiing. Solid State Commun.
II
[2] S. Nakashima, M. Daimon and A. Mitsuishi. J. Phys.
4 Discussion From the above observations, the following conclusions can be readily drawn: (i) At high temperatures the most common polytype of Pb12 is l2R. Thus 12R may be regarded as the stable high temperature modification and 2H as the stable room temperature modification of Pb12. (ii) Melt-grown Pb!2 crystals only contain small period polytypes. (iii) Impurities are responsible for the formation
Chem. Solids 40 (1979) 39. ]3[ RE. Wood and H.L. Ritter, J. Am. C’hem. Soc. 75 (1953)
471. [4] W.F. Knippenberg and G. Verspui, Mater. Res. Bull. 4
151
(1969) S33-S44. . G.C. Trigunayat and AR. Verma. in: Physics and C hemistry of Materials with Layered Structures, vol. 2. Ed. F. Levy (Reidel. Dordrecht, 1976) p. 269.
16] T, Minagawa, Ada Cryst. A31 (1975) 823. [7] 5K. Chaudhary and G.C. Trigunayat. J. Crystal Growth 57 (1982)Trigunayat 558. [8] G.C. and AR. Verma, Ada Cryst. 15 (1962) 499 [9[ G.C. Trigunayat. Nature 212 (1966) 808.