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Structural study of Cr3S4 crystals
Journal of the Less-Common
STRUCTURAL
Met&,
80 (1981)
165
165 - 170
STUDY OF Cr3S4 CRYSTALS
Y. NODA Department of Materials Science, 980 (Japan)
Faculty of Engineering,
Tohoku
University, Sendai
(Received January 2,198l)
Summary Crystals with the nominal composition C&as were investigated by X-ray diffraction. A series of precession photographs is explained on the basis of the superposition of monoclinic and triclinic reciprocal lattices. The monoclinic unit cell coincides with that of the 2c-type FesSed structure. The triclinic supercell is explained on the basis that the sample is a twinned crystal consisting of domains mutually oriented at 120” and each domain has the Bc-type FesSe, structure. Two distinct kinds of superposition of the reciprocal lattices correspond to the cases of twinning around the C axis and the C* axis respectively. The domain structure was revealed by electron diff~ction and microscopy.
1. Introduction The crystal structures of chromium sulphides have been studied by several researchers. By using powder X-ray diffraction techniques at room temperature, Haraldsen [l] has divided the region CrS - CrS1,a into three ranges. From more detailed X-ray powder diffraction ~vestigations, Jellinek [Z] has establish~ the existence of six phases-in the range CrS0.s6 - CrS1.50, The structure of these chromium sulphides, except monoclinic CrS, are intermediate between the NiAs- and the Cd(OH)%-type structures. Monoclinic Cra& crystallizes in a pseudo-NiAs-type structure with ordered metal vacancies corresponding to the 2c-type FesSe, structure. Neutron diffraction data for a powdered sample of CT&& provide confirmation of this structure [ 31. Igaki et al. [ 41 have succeeded in preparing single crystals of Cr3S4 and have reported that the X-ray diffraction patterns suggested a superstructure with double-hexagonal axes, whilst Lutz and Bertram [ 51 have proposed several modifications for the twinning of the supercell. The present study is aimed to give a further account of the supercell observed for crystals of the Cr& phase.
0 Elsevier Sequoia/Printed in The Netherlands
166
2. Experimental
procedure
Chromium sulphide Cr& was prepared according to the usual method by a direct reaction between accurately weighed amounts of the constituent elements. The chromium had a nominal purity of 99.99%. Sulphur with a nominal purity of 99.5% was purified by distillation in a vacuum-sealed Pyrex tube. A total amount of 10 g was weighed with a specified ratio of chromium to sulphur in the CrsS4 phase region. This was then vacuum sealed (to a pressure of about 0.1 Pa) in a quartz tube after repeated evacuations and admissions of argon. The tube was placed horizontally in a two-zone furnace in which a reaction chamber was kept at 900 “C at one end and a sulphur condensation chamber was kept at a temperature between 300 and 500 “C at the other end. Heating for 1 week accomplished the reaction. In order to prepare the CrsS4 crystals by a chemical transport reaction, a synthesized product with the nominal composition CrS1.sO was vacuum sealed with iodine as a transport agent, at a concentration of 2 kg mv3, and was placed at one end of the evacuated quartz ampoule (length, 100 mm; internal diameter, 8 mm). The ampoule was then placed horizontally in the tubular two-zone furnace in which the evaporation chamber was kept at 850 “C and the crystallization chamber at 1830 “C. After 1 week, welldeveloped crystals were obtained in a hexagonal platelet form ranging up to 0.05 mm in thickness and 1 mm in side length. The crystals used for X-ray diffraction measurements had the approximate dimensions of 0.03 mm in thickness and 0.2 mm in side length. Oscillation photographs were taken with nickel-filtered Cu Ka radiation. A series of precession photographs was taken with zirconium-filtered MO Ka radiation. The sample used for the transmission electron microscopy investigation was cleaved from a grown crystal. Electron diffraction patterns and electron micrographs were taken on a JEM-200B electron microscope at 200 kV. In the following discussion, diffraction patterns will be considered in terms of the orthohexagonal cell (lattice constants A, B, C and p) which is related to the hexagonal cell (a0 = Iail= la,I;c,)byA=2a,+a,,B=a,,C=c0and p = 90”.
3. Results and discussion Oscillation photographs taken around the axes perpendicular and parallel to a side of a sample are shown in Figs. l(a) and l(b) respectively. The photographs show splitting of the strong reflections and are essentially the same patterns as those for the Cr3S4 single crystals grown by the sublimation method [4] ; this means that no effect of iodine as a transport agent was found in the diffraction patterns. The full lines indicate layer lines of the NiAs-type structure; these give the identity periods of the A axis (Fig. l(a)) and of the B axis (Fig. l(b)) which are the respective axes of rotation. The weak reflections observed (indicated by broken lines) are accounted for if the lengths of the A and the B axes are doubled.
167
Fig. 1. Oscillation photographs taken with rotations of 30” around (a) the A axis and (b) the B axis: -, layer lines of the NiAs-type structure; ---, layer lines of a superstructure.
Fig. 2. (a) Zero-level precession photograph taken around the B axis with a precession angle of 30”; (b) the (hOf) precession diagram indicating intense spots on the superposed reciprocal lattices with a common C* axis.
Figure Z(a) represents a zero-level precession photo~aph taken around the B axis. The photograph shows a splitting of reflections into two. A composite reciprocal lattice representation is given in Fig. 2(b). A splitting of fundamen~ reflection spots from the NiAs-type structure can be seen except for the OOEreflections. The splitting can be explained on the basis of the superposed reciprocal lattices I and II, which are mutually oriented with the C* axis in common. The A* axes were chosen to lie in opposite directions since the interaxial angle fi is greater than 90”. The axial angles A: A C: and A,; A C,: are 88” 30’ and 89” 10’ respectively. Judged from the reciprocal axial angles, all the faint spots belong to the reciprocal lattice I and give a unit cell with the A axis and the doubled C axis. Figure 3(a) exhibits the - $ level precession pattern showing a number of weak reflections, although no complete set was obtained because of insufficient exposure. These faint spots belong to the reciprocal lattice II, as shown in a composite reciprocal lattice representation in Fig. 3(b), and give a supercell with the doubled A and C axes. Similarly, it was found that the reflection spots in the (Okl) and (-$-kl) patterns also lie on the superposed reciprocal lattice I and II with the common C” axis. Con~quently, the unit cells can be described
168
(4
(b)
Fig. 3. (a) x f level precession photograph taken around the B axis with a precession angle of 20 ; (b) the (h - $ I) precession diagram indicating the faint spots from the reciprocal lattice II.
(4
tb)
Fig. 4. (a) Supercells defined by As’ and Bar and by Asi and Bg’ in threefold orthohexagonal cells of the %-type FeaSeb structure (P, vacant metal sites at z = 0; 0, vacant metal sites at t = 12 ); (b) the axial angles ar and fi for the supercells shown in relation to those (ar and 61 of the o~hohex~o~l cell.
by a monoclinic cell with AI = 311200,Br = ao, Cr = 2c0 and PI = 91” 30’ and a triclinie cell with A,, = 2 X 31/2a,, Bn = 2a,-,,Cn = 2c0, aII = 91” 20’, pn = 90” 50’ and rII = 90”. The lattice constants of the monoclinic cell coincide with those of the !&-type FeaSel structure 12] , which is closely related to the N&-type lattice by cell dimensions A = 3f’2ae, B = ao, C = 2c, and p = 90”. Figure 4(a) shows only vacancy sites of the 2c-type structure projected onto the basal plane along the C direction. Three equivalent orthohexagonal cells can be chosen by successive rotations of 120” about the axis perpendicular to the basal plane. When one of the cells is taken as a reference with AI = A 1 and Br = B, , the identity periods of the other cells along these two axes are twice as large as those of the reference cell, as shown by the bold lines. Thus let the edges of new supercells be chosen in such a way that AI1 = A,’ = A; and Bn = B,’ = Bs) where the new edges (Ai and A;, and Ba’ and I?;) are almost ~tip~el to the respective reference edges. On taking cy= 90” and 0 = fir, the new angles o’ and p’ for the supercell are shown in Fig. 4(b) in relation to the old angles (a and p) and coincide
169
Fig. 5. Tradition
electron diffraction pattern from a region at the domain boundary.
Fig. 6, Dark field image at the domain boundary.
with the observed values for en and &i respectively. The intensity distributions in the (OiO) reciprocal plane calculated for these two supercells were mutually complementary, whilst the d~tribu~ons in the (010) reciprocal plane were nearly common and corresponded to that in the reciprocal lattice II. The detailed crystallographic descriptions have been reported for results on cobalt selenide [ 61. The (h - $ I) precession pattern in Fig. 3 can be explained with the intensity distribution from one of these two supercells. From these relations it can be concluded that the triclinic supercell obtained from the reciprocal lattice II arises from this multiple tw~n~g. The basal plane of the reciprocal lattice was examined with the aid of a tilting device and the electron diffraction pattern is shown in F’ig. 5. The reflection spots are fundamental spots. The intense spots lie approximately on two circles, the peripheries of which pass through the centre of the pattern. The diffraction patterns can be deduced from the intersection between the reflection sphere and the superposed basal planes of the reciprocal lattice. By tilting the spot (600) (which lies on one of the circles) into the objective aperture of the electron microscope, a domain of the sample that contributes to the diffraction beam was revealed as shown in Fig. 6. The domain extends to a width of more than 1 ,Ctm. Figure 7(a) shows a different precession pattern obtained from a crystal randomly chosen from the growth ampoule. The splitting of strong reflections also occurred. The composite representation is given in Fig. 7(b). The pattern is explained on the basis of the superposed reciprocal lattices, which are composed of the same reciprocal lattices as I and II with a common A* axis. This is the case where multiple twinning occurred around the C axis. The supers~uct~e reported for Cr& [4] can be explained in terms of twinning of the Zc-type Fe&%* structure and now the possibility of a new phase is definitely ruled out. The growth conditions which control the occurrence of these twins is not known. It is possible to form a homologous series of twins by varying the orientation between the domains by
170
(a)
(b)
Fig. 7, (a) Zero-level precession pattern showing a different type of splitting of the strong reflections from that shown in Fig. 2; (b) the (hOI) precession diagram indicating the superposed reciprocal lattices with a common A* axis.
60”, 120”, 180” and 240’ successively [5] but the different patterns, except those of the 120” orientation, have not been observed yet.
Acknowledgment The author wishes to thank to Professor S. Nagakura, Tokyo Institute of Technology, for his helpful criticism and impost comments concerning the manuscript.
References 1 H. Haraldsen, Z. Anorg. Chem., 234 (1937) 372. 2 F. Jellinek, Acta Crystallogr., 10 (1957) 620. 3 E. F. Bertaut, G. Roult, R. Aleonald, R. Pathenet, M. Chevreton and R. Jansen, J. Phys. (Paris), 25 (1964) 582. 4 K. Igaki, N. Ohashi and M. Mikami, J. Phys. SOC. Jjm, 31 (1971) 1424. 5 H. D. Lutz and K. H. Bertram, 2. Anorg. Allg. Chem., 401 (1973) 185. 6 Y. Nodaand K. Igaki, Trans. Jpn. Inst. Met., 19 (1978) 217.
STRUCTURAL
Met&,
80 (1981)
165
165 - 170
STUDY OF Cr3S4 CRYSTALS
Y. NODA Department of Materials Science, 980 (Japan)
Faculty of Engineering,
Tohoku
University, Sendai
(Received January 2,198l)
Summary Crystals with the nominal composition C&as were investigated by X-ray diffraction. A series of precession photographs is explained on the basis of the superposition of monoclinic and triclinic reciprocal lattices. The monoclinic unit cell coincides with that of the 2c-type FesSed structure. The triclinic supercell is explained on the basis that the sample is a twinned crystal consisting of domains mutually oriented at 120” and each domain has the Bc-type FesSe, structure. Two distinct kinds of superposition of the reciprocal lattices correspond to the cases of twinning around the C axis and the C* axis respectively. The domain structure was revealed by electron diff~ction and microscopy.
1. Introduction The crystal structures of chromium sulphides have been studied by several researchers. By using powder X-ray diffraction techniques at room temperature, Haraldsen [l] has divided the region CrS - CrS1,a into three ranges. From more detailed X-ray powder diffraction ~vestigations, Jellinek [Z] has establish~ the existence of six phases-in the range CrS0.s6 - CrS1.50, The structure of these chromium sulphides, except monoclinic CrS, are intermediate between the NiAs- and the Cd(OH)%-type structures. Monoclinic Cra& crystallizes in a pseudo-NiAs-type structure with ordered metal vacancies corresponding to the 2c-type FesSe, structure. Neutron diffraction data for a powdered sample of CT&& provide confirmation of this structure [ 31. Igaki et al. [ 41 have succeeded in preparing single crystals of Cr3S4 and have reported that the X-ray diffraction patterns suggested a superstructure with double-hexagonal axes, whilst Lutz and Bertram [ 51 have proposed several modifications for the twinning of the supercell. The present study is aimed to give a further account of the supercell observed for crystals of the Cr& phase.
0 Elsevier Sequoia/Printed in The Netherlands
166
2. Experimental
procedure
Chromium sulphide Cr& was prepared according to the usual method by a direct reaction between accurately weighed amounts of the constituent elements. The chromium had a nominal purity of 99.99%. Sulphur with a nominal purity of 99.5% was purified by distillation in a vacuum-sealed Pyrex tube. A total amount of 10 g was weighed with a specified ratio of chromium to sulphur in the CrsS4 phase region. This was then vacuum sealed (to a pressure of about 0.1 Pa) in a quartz tube after repeated evacuations and admissions of argon. The tube was placed horizontally in a two-zone furnace in which a reaction chamber was kept at 900 “C at one end and a sulphur condensation chamber was kept at a temperature between 300 and 500 “C at the other end. Heating for 1 week accomplished the reaction. In order to prepare the CrsS4 crystals by a chemical transport reaction, a synthesized product with the nominal composition CrS1.sO was vacuum sealed with iodine as a transport agent, at a concentration of 2 kg mv3, and was placed at one end of the evacuated quartz ampoule (length, 100 mm; internal diameter, 8 mm). The ampoule was then placed horizontally in the tubular two-zone furnace in which the evaporation chamber was kept at 850 “C and the crystallization chamber at 1830 “C. After 1 week, welldeveloped crystals were obtained in a hexagonal platelet form ranging up to 0.05 mm in thickness and 1 mm in side length. The crystals used for X-ray diffraction measurements had the approximate dimensions of 0.03 mm in thickness and 0.2 mm in side length. Oscillation photographs were taken with nickel-filtered Cu Ka radiation. A series of precession photographs was taken with zirconium-filtered MO Ka radiation. The sample used for the transmission electron microscopy investigation was cleaved from a grown crystal. Electron diffraction patterns and electron micrographs were taken on a JEM-200B electron microscope at 200 kV. In the following discussion, diffraction patterns will be considered in terms of the orthohexagonal cell (lattice constants A, B, C and p) which is related to the hexagonal cell (a0 = Iail= la,I;c,)byA=2a,+a,,B=a,,C=c0and p = 90”.
3. Results and discussion Oscillation photographs taken around the axes perpendicular and parallel to a side of a sample are shown in Figs. l(a) and l(b) respectively. The photographs show splitting of the strong reflections and are essentially the same patterns as those for the Cr3S4 single crystals grown by the sublimation method [4] ; this means that no effect of iodine as a transport agent was found in the diffraction patterns. The full lines indicate layer lines of the NiAs-type structure; these give the identity periods of the A axis (Fig. l(a)) and of the B axis (Fig. l(b)) which are the respective axes of rotation. The weak reflections observed (indicated by broken lines) are accounted for if the lengths of the A and the B axes are doubled.
167
Fig. 1. Oscillation photographs taken with rotations of 30” around (a) the A axis and (b) the B axis: -, layer lines of the NiAs-type structure; ---, layer lines of a superstructure.
Fig. 2. (a) Zero-level precession photograph taken around the B axis with a precession angle of 30”; (b) the (hOf) precession diagram indicating intense spots on the superposed reciprocal lattices with a common C* axis.
Figure Z(a) represents a zero-level precession photo~aph taken around the B axis. The photograph shows a splitting of reflections into two. A composite reciprocal lattice representation is given in Fig. 2(b). A splitting of fundamen~ reflection spots from the NiAs-type structure can be seen except for the OOEreflections. The splitting can be explained on the basis of the superposed reciprocal lattices I and II, which are mutually oriented with the C* axis in common. The A* axes were chosen to lie in opposite directions since the interaxial angle fi is greater than 90”. The axial angles A: A C: and A,; A C,: are 88” 30’ and 89” 10’ respectively. Judged from the reciprocal axial angles, all the faint spots belong to the reciprocal lattice I and give a unit cell with the A axis and the doubled C axis. Figure 3(a) exhibits the - $ level precession pattern showing a number of weak reflections, although no complete set was obtained because of insufficient exposure. These faint spots belong to the reciprocal lattice II, as shown in a composite reciprocal lattice representation in Fig. 3(b), and give a supercell with the doubled A and C axes. Similarly, it was found that the reflection spots in the (Okl) and (-$-kl) patterns also lie on the superposed reciprocal lattice I and II with the common C” axis. Con~quently, the unit cells can be described
168
(4
(b)
Fig. 3. (a) x f level precession photograph taken around the B axis with a precession angle of 20 ; (b) the (h - $ I) precession diagram indicating the faint spots from the reciprocal lattice II.
(4
tb)
Fig. 4. (a) Supercells defined by As’ and Bar and by Asi and Bg’ in threefold orthohexagonal cells of the %-type FeaSeb structure (P, vacant metal sites at z = 0; 0, vacant metal sites at t = 12 ); (b) the axial angles ar and fi for the supercells shown in relation to those (ar and 61 of the o~hohex~o~l cell.
by a monoclinic cell with AI = 311200,Br = ao, Cr = 2c0 and PI = 91” 30’ and a triclinie cell with A,, = 2 X 31/2a,, Bn = 2a,-,,Cn = 2c0, aII = 91” 20’, pn = 90” 50’ and rII = 90”. The lattice constants of the monoclinic cell coincide with those of the !&-type FeaSel structure 12] , which is closely related to the N&-type lattice by cell dimensions A = 3f’2ae, B = ao, C = 2c, and p = 90”. Figure 4(a) shows only vacancy sites of the 2c-type structure projected onto the basal plane along the C direction. Three equivalent orthohexagonal cells can be chosen by successive rotations of 120” about the axis perpendicular to the basal plane. When one of the cells is taken as a reference with AI = A 1 and Br = B, , the identity periods of the other cells along these two axes are twice as large as those of the reference cell, as shown by the bold lines. Thus let the edges of new supercells be chosen in such a way that AI1 = A,’ = A; and Bn = B,’ = Bs) where the new edges (Ai and A;, and Ba’ and I?;) are almost ~tip~el to the respective reference edges. On taking cy= 90” and 0 = fir, the new angles o’ and p’ for the supercell are shown in Fig. 4(b) in relation to the old angles (a and p) and coincide
169
Fig. 5. Tradition
electron diffraction pattern from a region at the domain boundary.
Fig. 6, Dark field image at the domain boundary.
with the observed values for en and &i respectively. The intensity distributions in the (OiO) reciprocal plane calculated for these two supercells were mutually complementary, whilst the d~tribu~ons in the (010) reciprocal plane were nearly common and corresponded to that in the reciprocal lattice II. The detailed crystallographic descriptions have been reported for results on cobalt selenide [ 61. The (h - $ I) precession pattern in Fig. 3 can be explained with the intensity distribution from one of these two supercells. From these relations it can be concluded that the triclinic supercell obtained from the reciprocal lattice II arises from this multiple tw~n~g. The basal plane of the reciprocal lattice was examined with the aid of a tilting device and the electron diffraction pattern is shown in F’ig. 5. The reflection spots are fundamental spots. The intense spots lie approximately on two circles, the peripheries of which pass through the centre of the pattern. The diffraction patterns can be deduced from the intersection between the reflection sphere and the superposed basal planes of the reciprocal lattice. By tilting the spot (600) (which lies on one of the circles) into the objective aperture of the electron microscope, a domain of the sample that contributes to the diffraction beam was revealed as shown in Fig. 6. The domain extends to a width of more than 1 ,Ctm. Figure 7(a) shows a different precession pattern obtained from a crystal randomly chosen from the growth ampoule. The splitting of strong reflections also occurred. The composite representation is given in Fig. 7(b). The pattern is explained on the basis of the superposed reciprocal lattices, which are composed of the same reciprocal lattices as I and II with a common A* axis. This is the case where multiple twinning occurred around the C axis. The supers~uct~e reported for Cr& [4] can be explained in terms of twinning of the Zc-type Fe&%* structure and now the possibility of a new phase is definitely ruled out. The growth conditions which control the occurrence of these twins is not known. It is possible to form a homologous series of twins by varying the orientation between the domains by
170
(a)
(b)
Fig. 7, (a) Zero-level precession pattern showing a different type of splitting of the strong reflections from that shown in Fig. 2; (b) the (hOI) precession diagram indicating the superposed reciprocal lattices with a common A* axis.
60”, 120”, 180” and 240’ successively [5] but the different patterns, except those of the 120” orientation, have not been observed yet.
Acknowledgment The author wishes to thank to Professor S. Nagakura, Tokyo Institute of Technology, for his helpful criticism and impost comments concerning the manuscript.
References 1 H. Haraldsen, Z. Anorg. Chem., 234 (1937) 372. 2 F. Jellinek, Acta Crystallogr., 10 (1957) 620. 3 E. F. Bertaut, G. Roult, R. Aleonald, R. Pathenet, M. Chevreton and R. Jansen, J. Phys. (Paris), 25 (1964) 582. 4 K. Igaki, N. Ohashi and M. Mikami, J. Phys. SOC. Jjm, 31 (1971) 1424. 5 H. D. Lutz and K. H. Bertram, 2. Anorg. Allg. Chem., 401 (1973) 185. 6 Y. Nodaand K. Igaki, Trans. Jpn. Inst. Met., 19 (1978) 217.