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The phase transition at −41°C in Cu4SnS4 as observed in electron microscopy
Solxd State Connnunxcatxons, Prxnted in Great Brltaln.
Vol.54,No.7,
pp.621-624,
1985.
0038-I098/85 $3 00 + .00 Pergamon Press Ltd.
THE PHASE TRANSITION AT - 4 1 " C IN CU~SnS~ AS OBSERVED IN ELECTRON MICROSCOPY J.Mahy, J.Van Landuyt and S.Amelinckx* University of Antwerp (RUCA), Groenenborgerlaan B 2020 Antwerpen,Belgium
(Received
7 February
171,
1985 by E BURSTEIN)
Evidence, by means of electron microscopy and diffraction, is presented for a phase transformation at around 232 K in Cu~SnS~. A superlattice is observed along ql,2 ± ~b* + ~c*, presumably associated with a displacive transformation. The first order transformation is associated with a space group change. Satellite dark field imaging revealed a domain structure composed of orientation and translation interfaces. TM
It was reported previously that Cu4SnS 4 exhibits a phase transition at around 232 K i. The Cu4SnS 4 phase occurs in the pseudo-binary system Cu2S_SnS 2. The Wurtzlte-derived structure consists of a deformed hexagonal close-packed arrangement of sulphur atoms, the metal atoms occupying roughly one half of the tetrahedral interstices (flg.1). An unusual feature of this transformation was noted by Khanafer et all: the lattice parameters were claimed to be the same above and below the transition temperature • a i 13.70 A ; b = 7.75 A ; c = 6.454 A. A number of thermal and electronic transport properties however, showed dramatic changes at the transition temperature, as was reported by Khanafer et al and Alley et al 1,3. Both groups report an important drop in the carrier density of Cu4SnS~, which was found to he an extrinsic semiconductor at room temperature (RT). In apparent contradiction w l t h 2 , Aliev et al observed also a corresponding drop in the carrier mobility 3 It was therefore considered worthwhile to verify these somewhat surprising results by means of electron microscopy and electron diffraction. Crystals were obtained in the manner described elsewhere ~. They could be identified as being the desired compound by comparison of the RT powder diffractograms with the results published in ref.|. On cooling the specimens to below -41°C it was found that the [001]* section of reciprocal space sharp and intense supplementary spots appear aligned along the a*-direction midway between the existing spots (fig. 2), and also other reflections arranged in planes midway the higher order Laue zones. Through a series of tilting experiments around the orthorhomblc axes at RT and low temperature (LT, well below 232K), the reciprocal lattice could be constructed (flg.3). The RT phase is primitive orthorhombic,
*Also at • qCK/CEN,
B2440-MOL
whilst the LT phase appears as A-face centered orthorhomblc. The RT reflection conditions were found to be in agreement with those published for the space group Pnma in ref.4. In applying a temperature gradient by means of the heat induced by the electron beam on a homogeneously cooled specimen, the propagation of a phase-front marking the transition could be followed "In-situ". The front can sweep the specimen reversibly as a function of temperature, after being nucleated at the foil edge (fig.4). Its appearance is characteristic for displaclve or shear transformations of first order. In the [i00]* section of reciprocal space, the presence of two orientation variants could be observed at LT. Under the appropriate imaging conditions (LT-satellite dark field electron microscopy) the corresponding domain structures were revealed (fig. 5). In a single domain, two
Cu
0
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@ l'"~ 0
Sn
•
e
°""; o
Fig. l. qchematlc representation of the Ideallsed Cu4qnS 4 structure, as projected along the c-axis. Large solid and dashed circles show the sulphur c.p.h, sublattice at z= 1/4 and 3/4, respectively, creating tetrahedral interstices in which part of the Cu and qn atoms are located.
(Belgium) 621
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TI~ PHASE TRANSIIION AT 4 1 ° C
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Fig. 2. Electron diffraction pattern of the [001]* section of Cu4SnS 4 (a) at room temperature and (b) at low temperature. Faint lines of diffuse intensity along a* at k = odd are observed in both (a) and (b).
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Vol
54, No
the diffraction pattern to lines of diffuse intensity directed along a* and located in the [001]* sections at k=odd. These lines could be observed at RT as well as LT (fig. 2). In some crystal regions, the (hko) spots with k=odd themselves become markedly streaked along a*, suggesting severe stacking disorder along (I00) planes. For .~he corresponding dis~lacement vector R the condition g.R~0 (mod. I) must be satisfied for all ~=(hko) with k=odd, which is in agreement with the visibility criterion for fringe contrast and with the observations. _The displacement vector is thus of the type R=I/2[Olw]. Nigh resolution images along this section also confirm the occurrence of such interfaces (flg.7); this suggests that at temperatures well above RT the metal atoms may become disordered within the sulphur sublattlce and on ordering at lower temperature give rise to stacking defects. Preliminary heating experiments indeed suggest that a defect structure results upon cooling from about 70OK. It is not yet clear however to what extent a possible change in the stoichiometry of the compound might play a role in the formation of this defect structure. In considering group to subgroup relationships for the transition from the RT to the LT phase, it is clear that the symmetry of the LT phase cannot he described by Pnma, because that would involve a tripling of one of the orthorhomblc axes 5. Also the observed presence of both orientation and translation variants can he accounted for most conveniently by assuming subsequent symmetry reductions from the RT spat•group Pnma to non-lsomorphic subgroups. This mechanism generally indicates that a firstorder transformation is involved, which is in agreement with the observed phase front. The material CudSnS ~ thus may contain two kinds of translation interfaces • in the room temperature phase stacking defects possibly related to a shear transformation and in the low temperature phase antl-phase boundaries due to
400
Fig.3. The reciprocal lattice of CudSnS 4. Circles refer to the room temperature phase, while triangles with top up and down represent the two variants of the low temperature phase respectively. Superposed triangles indicate LT reflections common to both variants.
dimensional interfaces could be distinguished. These interfaces moved in a temperature gradient and were absent in the RT phase (flg. 6). ~Ince there is no shade difference between the regions on either side of these interfaces, we conclude that they are translation interfaces. They appear at the ]unction of two crystal regions in which the presumed displacement of cations from their RT equilibrium position was nucleated at crystallographlcally different sites, thus creating antl-phase boundaries (APB). In a few cases these APB's were attached to planar interfaces parallel with the (I00) planes (fig.4). In practically all the observed regions, the latter interfaces were present and gave rise in
Fig.4. lection area.
Dark fleld showing a
image in (420) type RT refphase-front (F) across the
Vol. 54, No. 7
TIIL PHASE TRANSITION AT -41°C IN CU4SnS 4
Fig. 5. LT-sate111te dark field image of the area shown in flg.4 representing the two varlants In (3 512 I/2) LT satellite; translation interfaces, marked APB, can be observed. in (4 3/2 1/2) LT satellite. Indices refer to RT diffraction pattern, and are used only as a means of identification.
-
-
RT I" % ! i/
J / /
BF
LT
Fig.6. The same crystal area as observed in the bright fleld mode at room temperature (a) and low-temperature (b) and in satellite dark fleld (c). The LT translatlon interfaces (marked APB) can be seen to be attached to a stacking defect (SD) parallel to (lO0).
623
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11~ PHASE TRANSITION AT -4l°C IN CU4SnS 4
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Vol. 54, No. 7
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the increase in the unit cell size associated wlth the symmetry-reductlon as a result of a dlsplaclve transformation. A more detailed account on the nature and the occurrence of the two types of interfaces and the description of symmetry reduction at the transition Is in preparation.
Acknowledgement-The authors wlsh to thank Dr.G. Van Tendeloo and Drs.R.Deblleck for valuable discussions. Financial support was granted by the I.I.K.W. (Brussels).
REFERENCES
l,M.Khanafer, J.Rivet and J.Flahaut, Bulletin de la Socl~t& Chlmique France 12, 2670 (1974) 2.M.Khanafer, O.Gorochov and J.Blvet, Materials Research Bulletin ~, 1543 (1974) 3.M.I. Allev, D.G. Arasly and T.G.Dzhabrallov, Soviet Physics % l l d State, 23, 2009 (1983)
4.S.Jaulmes, J.Rlvet and P.Laruelle, Acta Crystallographlca, B 33, 540 (1977). 5.International Tables for Crystallography, ed.T.Rahn, Vol.A, (D.Reldel Publ.Company, Dordrecht-Boston, 1983).
Vol.54,No.7,
pp.621-624,
1985.
0038-I098/85 $3 00 + .00 Pergamon Press Ltd.
THE PHASE TRANSITION AT - 4 1 " C IN CU~SnS~ AS OBSERVED IN ELECTRON MICROSCOPY J.Mahy, J.Van Landuyt and S.Amelinckx* University of Antwerp (RUCA), Groenenborgerlaan B 2020 Antwerpen,Belgium
(Received
7 February
171,
1985 by E BURSTEIN)
Evidence, by means of electron microscopy and diffraction, is presented for a phase transformation at around 232 K in Cu~SnS~. A superlattice is observed along ql,2 ± ~b* + ~c*, presumably associated with a displacive transformation. The first order transformation is associated with a space group change. Satellite dark field imaging revealed a domain structure composed of orientation and translation interfaces. TM
It was reported previously that Cu4SnS 4 exhibits a phase transition at around 232 K i. The Cu4SnS 4 phase occurs in the pseudo-binary system Cu2S_SnS 2. The Wurtzlte-derived structure consists of a deformed hexagonal close-packed arrangement of sulphur atoms, the metal atoms occupying roughly one half of the tetrahedral interstices (flg.1). An unusual feature of this transformation was noted by Khanafer et all: the lattice parameters were claimed to be the same above and below the transition temperature • a i 13.70 A ; b = 7.75 A ; c = 6.454 A. A number of thermal and electronic transport properties however, showed dramatic changes at the transition temperature, as was reported by Khanafer et al and Alley et al 1,3. Both groups report an important drop in the carrier density of Cu4SnS~, which was found to he an extrinsic semiconductor at room temperature (RT). In apparent contradiction w l t h 2 , Aliev et al observed also a corresponding drop in the carrier mobility 3 It was therefore considered worthwhile to verify these somewhat surprising results by means of electron microscopy and electron diffraction. Crystals were obtained in the manner described elsewhere ~. They could be identified as being the desired compound by comparison of the RT powder diffractograms with the results published in ref.|. On cooling the specimens to below -41°C it was found that the [001]* section of reciprocal space sharp and intense supplementary spots appear aligned along the a*-direction midway between the existing spots (fig. 2), and also other reflections arranged in planes midway the higher order Laue zones. Through a series of tilting experiments around the orthorhomblc axes at RT and low temperature (LT, well below 232K), the reciprocal lattice could be constructed (flg.3). The RT phase is primitive orthorhombic,
*Also at • qCK/CEN,
B2440-MOL
whilst the LT phase appears as A-face centered orthorhomblc. The RT reflection conditions were found to be in agreement with those published for the space group Pnma in ref.4. In applying a temperature gradient by means of the heat induced by the electron beam on a homogeneously cooled specimen, the propagation of a phase-front marking the transition could be followed "In-situ". The front can sweep the specimen reversibly as a function of temperature, after being nucleated at the foil edge (fig.4). Its appearance is characteristic for displaclve or shear transformations of first order. In the [i00]* section of reciprocal space, the presence of two orientation variants could be observed at LT. Under the appropriate imaging conditions (LT-satellite dark field electron microscopy) the corresponding domain structures were revealed (fig. 5). In a single domain, two
Cu
0
;,z,,
@ l'"~ 0
Sn
•
e
°""; o
Fig. l. qchematlc representation of the Ideallsed Cu4qnS 4 structure, as projected along the c-axis. Large solid and dashed circles show the sulphur c.p.h, sublattice at z= 1/4 and 3/4, respectively, creating tetrahedral interstices in which part of the Cu and qn atoms are located.
(Belgium) 621
622
TI~ PHASE TRANSIIION AT 4 1 ° C
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Fig. 2. Electron diffraction pattern of the [001]* section of Cu4SnS 4 (a) at room temperature and (b) at low temperature. Faint lines of diffuse intensity along a* at k = odd are observed in both (a) and (b).
022
122
222
322
422
~
002
)421
0011 ~
IP'~-I
i ........~TF-~-~-;'~ 0
100
200
300
IN CU4SnS 4
Vol
54, No
the diffraction pattern to lines of diffuse intensity directed along a* and located in the [001]* sections at k=odd. These lines could be observed at RT as well as LT (fig. 2). In some crystal regions, the (hko) spots with k=odd themselves become markedly streaked along a*, suggesting severe stacking disorder along (I00) planes. For .~he corresponding dis~lacement vector R the condition g.R~0 (mod. I) must be satisfied for all ~=(hko) with k=odd, which is in agreement with the visibility criterion for fringe contrast and with the observations. _The displacement vector is thus of the type R=I/2[Olw]. Nigh resolution images along this section also confirm the occurrence of such interfaces (flg.7); this suggests that at temperatures well above RT the metal atoms may become disordered within the sulphur sublattlce and on ordering at lower temperature give rise to stacking defects. Preliminary heating experiments indeed suggest that a defect structure results upon cooling from about 70OK. It is not yet clear however to what extent a possible change in the stoichiometry of the compound might play a role in the formation of this defect structure. In considering group to subgroup relationships for the transition from the RT to the LT phase, it is clear that the symmetry of the LT phase cannot he described by Pnma, because that would involve a tripling of one of the orthorhomblc axes 5. Also the observed presence of both orientation and translation variants can he accounted for most conveniently by assuming subsequent symmetry reductions from the RT spat•group Pnma to non-lsomorphic subgroups. This mechanism generally indicates that a firstorder transformation is involved, which is in agreement with the observed phase front. The material CudSnS ~ thus may contain two kinds of translation interfaces • in the room temperature phase stacking defects possibly related to a shear transformation and in the low temperature phase antl-phase boundaries due to
400
Fig.3. The reciprocal lattice of CudSnS 4. Circles refer to the room temperature phase, while triangles with top up and down represent the two variants of the low temperature phase respectively. Superposed triangles indicate LT reflections common to both variants.
dimensional interfaces could be distinguished. These interfaces moved in a temperature gradient and were absent in the RT phase (flg. 6). ~Ince there is no shade difference between the regions on either side of these interfaces, we conclude that they are translation interfaces. They appear at the ]unction of two crystal regions in which the presumed displacement of cations from their RT equilibrium position was nucleated at crystallographlcally different sites, thus creating antl-phase boundaries (APB). In a few cases these APB's were attached to planar interfaces parallel with the (I00) planes (fig.4). In practically all the observed regions, the latter interfaces were present and gave rise in
Fig.4. lection area.
Dark fleld showing a
image in (420) type RT refphase-front (F) across the
Vol. 54, No. 7
TIIL PHASE TRANSITION AT -41°C IN CU4SnS 4
Fig. 5. LT-sate111te dark field image of the area shown in flg.4 representing the two varlants In (3 512 I/2) LT satellite; translation interfaces, marked APB, can be observed. in (4 3/2 1/2) LT satellite. Indices refer to RT diffraction pattern, and are used only as a means of identification.
-
-
RT I" % ! i/
J / /
BF
LT
Fig.6. The same crystal area as observed in the bright fleld mode at room temperature (a) and low-temperature (b) and in satellite dark fleld (c). The LT translatlon interfaces (marked APB) can be seen to be attached to a stacking defect (SD) parallel to (lO0).
623
624
11~ PHASE TRANSITION AT -4l°C IN CU4SnS 4
•
-
@
°
#
.
e
.
Vol. 54, No. 7
•
,~,
.
.
..
~--~
Flg.7. High resolution image of the [001]* zone. Several stacking defects can be distinguished. Inset shows the corresponding diffraction pattern, In which (hko) spots with k=odd are streaked along a*.
the increase in the unit cell size associated wlth the symmetry-reductlon as a result of a dlsplaclve transformation. A more detailed account on the nature and the occurrence of the two types of interfaces and the description of symmetry reduction at the transition Is in preparation.
Acknowledgement-The authors wlsh to thank Dr.G. Van Tendeloo and Drs.R.Deblleck for valuable discussions. Financial support was granted by the I.I.K.W. (Brussels).
REFERENCES
l,M.Khanafer, J.Rivet and J.Flahaut, Bulletin de la Socl~t& Chlmique France 12, 2670 (1974) 2.M.Khanafer, O.Gorochov and J.Blvet, Materials Research Bulletin ~, 1543 (1974) 3.M.I. Allev, D.G. Arasly and T.G.Dzhabrallov, Soviet Physics % l l d State, 23, 2009 (1983)
4.S.Jaulmes, J.Rlvet and P.Laruelle, Acta Crystallographlca, B 33, 540 (1977). 5.International Tables for Crystallography, ed.T.Rahn, Vol.A, (D.Reldel Publ.Company, Dordrecht-Boston, 1983).