- Email: [email protected]
Occurrence of polytypism in SnSe2
Journal of Crystal Growth 55 (1981) 395—397 North-Holland Publishing Company
395
LETTER TO THE EDITORS OCCURRENCE OF POLYTYPISM IN SnSe2 Santa ACHARYA and O.N. SRIVASTAVA Department of Physics, Banaras Hindu University, Varanasi 221005, India Received 25 May 1981
X-ray diffraction studies of single crystals of SnSe2 grown by closed tube vapour transport technique reveal the existence of the following polytypes: 611, 18R, 1011, 18H, 22H, 3011, 36H, 4211 and 42R in addition to the basic 2H Structure. Cases of syntactic coalescence and disordered structures have also been encountered in the present investigation.
Tin di-selenide (SnSe2) is one of the Cd12-type layer where basic structural consists compounds of two sheets of the hexagonally closeunit packed selenium atoms with the tin atoms occupying the octahedral voids sandwiched between the selenium layers. Although recent work [1—4] has indicated the existence of more than one polytypic variant in SnSe 2, the magnitude of polytypism and a series of polytypes embodying both the low and high period variants have not been revealed so far. Single crystals of SnSe2 were grown from the stoichionietric proportions of the pure elements by iodine vapour transport technique. The charge in the
I ig. 1. 15°a-axis oscillation photograph for a 18H crystal.
0022-0248/81 /0000—0000/$02 .50 © 1981 North-Holland
powdered form 3 together a small amount of of tube with volume) was placed in an iodine (5 mg/cm evacuated and sealed quartz ampoule of length 20 cm and diameter 16 mm. The transport was carried out in a horizontal two-zone gradient furnace [5] from a 530°C reaction zone to a 430°C growth zone measured by chromel—alumel thermocouples placed 20 cm apart for 50 h. The SnSe2 crystals obtained were shining black hexagonal platelets, with lateral dimensions ranging from one to a few mm and thicknesses from 10 to 200 jim. The end temperatures were varied for different ainpoules and variations in growth temperatures were found to favour the
~96
S 1 cliarva. 0. V Srha~taia / Occorrcnci of /)O1lt)~piS?fl0)
I ig. 2. 15° a-axis oo~illationphotograph for a cr~~tal in ~ luch 2H and 42R polytvpes are ~yntactical1y coalesced. Sonic of the / valuc~of the 1(1 / ~pohs for both 211 and 42R con’~titoent~ arc indicated.
growth of polytypic crystals. Polytypes weie identilied through 15°a-axis oscillation technique from the 10 1 row of the diffraction photograph by superposilion on a 211 photograph and the c-parameter was calculated from the s-values as read on the Bernal charts. The diffraction photographs of as many as fifty single crystals of SnSe2 revealed the existence ni based as ~vell as non-based polytypes. Sonic of the
4
long period polytypes are based on ÔH as exhibited by the intense diffraction spots in the 6H positions. Many disordered structures were encounte red in our crystals as manifested by continuous diffuse streak along the c*_dircction of the diffraction photograph. The occurrence of polytypism in SnSe 2. the existence of ordered high period polytypes as well as syntactically coalesced polytypes and one-dinien-
4
I
— C
Fi~.3. 10 structure.
/ row of reflections for various polvtypcs
iif
SnSe2 : (a) 211, (b) 611, (c) 2211. (d) 211 and 1 8R, (c) 4211. (1) disordered
S. .4charya, ON. Srii’astava / Occurrence of polytypism in SnSe
2
sional disordered structures are not accountable satisfactorily on the basis of any single theory of polytypism presently prevalent. The as-grown crystals do not exhibit any growth spiral on the basal plane under the projection microscope. Since the basic phase for SnSe2 is 2H, Frank’s screw dislocation theory cannot predict the formation of polytypes because any screw dislocation created in the parent 2H phase will lead to the same structure [6]. However, the most severe limitation of Frank’s theory comes from the fact that it cannot predict the formation of rhombohedral polytypes in SnSe2 type sandwich structures [6,7]. Also one-dimensional disordered structures cannot be accounted for by Frank’s theory. Moreover, Lang topographs [8] reveal the existence of large areas free from dislcications in the SnSe2 crystals. Jagodzinski’s layer transposition disorder theory, however, offers a plausible explanation for the existence of syntactically coalesced and disordered polytypes. An adequate theory to account for the occurrence of polytypism and all the structural features observed in the diffraction investigations is yet to be propounded. Investigations are carried out to correlate polytypism with growth conditions and topographical features. Like the other isostructural compound SnS2 [9,10], the polytypes of SnSe2 are found to represent varying band gap materials. For example, the band gap for the basic 2H structure is found to be 1.21 eV and for the high period polytype 22H band gap is found to be 0.66 eV.
397
A detailed investigation of the electronic properties of the SnSe2 crystals and also of the solid solution SnS~Se2_5(0
References [1] A.J. Smith, P.E. Meek and WY. Liang.J. Phys. C (Solid State Phys.) 10(1977)1321.
121 T. Minagawa, J. Phys. Soc. Japan. 49 (1980) 2317. [31 Ml. Karakhanova, A.S. Pashinkin and A.V. Novoselova, Izv. Akad. Nauk SSSR 2(1966)1186. [4] G. Busch and C. Frbhlich, I-Ielv. Phys. Ada 34 (1961)
15] A.A. Balchin, in: Crystallography and Crystal Chemistry of Materials with Layered Structures, Ed. F. Levy (Reidel, Dordrecht, Netherlands, 1976) pp. 1—SO.
[61 AR. and P. Krishna, typisniVerma in Crystals (Wiley, New Polymorphism York, 1966). and Poly[7] C.R. Whitehouse and A.A. Balchin, J. Crystal Growth 47 (1979) 203. [8] H.P.B. Rimmington, A.A. Balchin and B.K. Tanner, J. Crystal Growth 15 (1972) 51. [91 5. Acharya and ON. Srivastava, Phys. Status Solidi (a) 56(1979) Ki. 1101 ~. Acli~ya and ON. Srivastava, Phys. Status Solidi (a) 65(1981).
395
LETTER TO THE EDITORS OCCURRENCE OF POLYTYPISM IN SnSe2 Santa ACHARYA and O.N. SRIVASTAVA Department of Physics, Banaras Hindu University, Varanasi 221005, India Received 25 May 1981
X-ray diffraction studies of single crystals of SnSe2 grown by closed tube vapour transport technique reveal the existence of the following polytypes: 611, 18R, 1011, 18H, 22H, 3011, 36H, 4211 and 42R in addition to the basic 2H Structure. Cases of syntactic coalescence and disordered structures have also been encountered in the present investigation.
Tin di-selenide (SnSe2) is one of the Cd12-type layer where basic structural consists compounds of two sheets of the hexagonally closeunit packed selenium atoms with the tin atoms occupying the octahedral voids sandwiched between the selenium layers. Although recent work [1—4] has indicated the existence of more than one polytypic variant in SnSe 2, the magnitude of polytypism and a series of polytypes embodying both the low and high period variants have not been revealed so far. Single crystals of SnSe2 were grown from the stoichionietric proportions of the pure elements by iodine vapour transport technique. The charge in the
I ig. 1. 15°a-axis oscillation photograph for a 18H crystal.
0022-0248/81 /0000—0000/$02 .50 © 1981 North-Holland
powdered form 3 together a small amount of of tube with volume) was placed in an iodine (5 mg/cm evacuated and sealed quartz ampoule of length 20 cm and diameter 16 mm. The transport was carried out in a horizontal two-zone gradient furnace [5] from a 530°C reaction zone to a 430°C growth zone measured by chromel—alumel thermocouples placed 20 cm apart for 50 h. The SnSe2 crystals obtained were shining black hexagonal platelets, with lateral dimensions ranging from one to a few mm and thicknesses from 10 to 200 jim. The end temperatures were varied for different ainpoules and variations in growth temperatures were found to favour the
~96
S 1 cliarva. 0. V Srha~taia / Occorrcnci of /)O1lt)~piS?fl0)
I ig. 2. 15° a-axis oo~illationphotograph for a cr~~tal in ~ luch 2H and 42R polytvpes are ~yntactical1y coalesced. Sonic of the / valuc~of the 1(1 / ~pohs for both 211 and 42R con’~titoent~ arc indicated.
growth of polytypic crystals. Polytypes weie identilied through 15°a-axis oscillation technique from the 10 1 row of the diffraction photograph by superposilion on a 211 photograph and the c-parameter was calculated from the s-values as read on the Bernal charts. The diffraction photographs of as many as fifty single crystals of SnSe2 revealed the existence ni based as ~vell as non-based polytypes. Sonic of the
4
long period polytypes are based on ÔH as exhibited by the intense diffraction spots in the 6H positions. Many disordered structures were encounte red in our crystals as manifested by continuous diffuse streak along the c*_dircction of the diffraction photograph. The occurrence of polytypism in SnSe 2. the existence of ordered high period polytypes as well as syntactically coalesced polytypes and one-dinien-
4
I
— C
Fi~.3. 10 structure.
/ row of reflections for various polvtypcs
iif
SnSe2 : (a) 211, (b) 611, (c) 2211. (d) 211 and 1 8R, (c) 4211. (1) disordered
S. .4charya, ON. Srii’astava / Occurrence of polytypism in SnSe
2
sional disordered structures are not accountable satisfactorily on the basis of any single theory of polytypism presently prevalent. The as-grown crystals do not exhibit any growth spiral on the basal plane under the projection microscope. Since the basic phase for SnSe2 is 2H, Frank’s screw dislocation theory cannot predict the formation of polytypes because any screw dislocation created in the parent 2H phase will lead to the same structure [6]. However, the most severe limitation of Frank’s theory comes from the fact that it cannot predict the formation of rhombohedral polytypes in SnSe2 type sandwich structures [6,7]. Also one-dimensional disordered structures cannot be accounted for by Frank’s theory. Moreover, Lang topographs [8] reveal the existence of large areas free from dislcications in the SnSe2 crystals. Jagodzinski’s layer transposition disorder theory, however, offers a plausible explanation for the existence of syntactically coalesced and disordered polytypes. An adequate theory to account for the occurrence of polytypism and all the structural features observed in the diffraction investigations is yet to be propounded. Investigations are carried out to correlate polytypism with growth conditions and topographical features. Like the other isostructural compound SnS2 [9,10], the polytypes of SnSe2 are found to represent varying band gap materials. For example, the band gap for the basic 2H structure is found to be 1.21 eV and for the high period polytype 22H band gap is found to be 0.66 eV.
397
A detailed investigation of the electronic properties of the SnSe2 crystals and also of the solid solution SnS~Se2_5(0
References [1] A.J. Smith, P.E. Meek and WY. Liang.J. Phys. C (Solid State Phys.) 10(1977)1321.
121 T. Minagawa, J. Phys. Soc. Japan. 49 (1980) 2317. [31 Ml. Karakhanova, A.S. Pashinkin and A.V. Novoselova, Izv. Akad. Nauk SSSR 2(1966)1186. [4] G. Busch and C. Frbhlich, I-Ielv. Phys. Ada 34 (1961)
15] A.A. Balchin, in: Crystallography and Crystal Chemistry of Materials with Layered Structures, Ed. F. Levy (Reidel, Dordrecht, Netherlands, 1976) pp. 1—SO.
[61 AR. and P. Krishna, typisniVerma in Crystals (Wiley, New Polymorphism York, 1966). and Poly[7] C.R. Whitehouse and A.A. Balchin, J. Crystal Growth 47 (1979) 203. [8] H.P.B. Rimmington, A.A. Balchin and B.K. Tanner, J. Crystal Growth 15 (1972) 51. [91 5. Acharya and ON. Srivastava, Phys. Status Solidi (a) 56(1979) Ki. 1101 ~. Acli~ya and ON. Srivastava, Phys. Status Solidi (a) 65(1981).