- Email: [email protected]
Pressure-induced phase transition in Sb2Te3
~Solid State Communications, Vol.40, pp. I045-1047. ~'Pergamon Press Ltd. 1981. Printed in Great Britain.
0038-I098/81/481045-3502.00/0
PRESSURE-INDUCED PHASE TRANSITION IN SbzTe3 N.Sakai and T.Kajiwara Department of Chemistry, Toho University, Miyama 2-2-I Funabashi-shi, Chiba-ken 274, Japan K.Takemura* and S.Minomura Institute for Solid State Physics, The University of Tokyo Roppongi, Minato-ku, Tokyo 106, Japan and Y.Fujii Department of Physics, Brookhaven National Laboratory, Upton New York 11973, U.S.A. ( Received 8 September 1981 by H. Kawamura )
An anomalous change in electrical resistance with pressure was observed in crystalline Sb2Te3 around 80 kbar at room temperature. This anomaly was found to be closely related with a pressure-induced structural phase transition which is discovered by the present x-ray diffraction experiment.
The VB-VIB compounds such as Sb2Te3, BizTe~, and Bi2Se3 are narrow-band-gap semiconductors with the homologous layeredcrystal structure. Their electrical and optical properties have been studied extensively because of the potential applicability to efficient thermoelectric devices. I-7 Studies of these properties under pressure offer particulary important information on their characteristic electronic structure. In fact, Bi2Se3 is known to undergo the pressure-induced semiconductor-metal transition accompanied with a sharp decrease in the electrical resistance at about 100 kbar. a'9 In the present paper we report the pressure-induced phase transition in Sb2Te3 found by measurements of the electrical resistance and the powder x-ray diffraction pattern. The polycrystalline SbzTe3 with the 99.999 % purity was obtained from Furuuchi Chemical Co., Ltd. The pressure dependence of electrical resistance was measured by the four-terminal method using the Drickamer type high pressure cell modified to introduce more than four leads, l° X-ray diffraction patterns of the powdered sample at high pressure were measured by a diamondanvil cell combined with a positionsensitive detector (PSD) for the MoK a radiation, ll, 12 Figure I shows the electrical resistance measured in SbeTe3 as a function of pressure at room temperature. The resistance decreases gradually with increasing pressure to about 80 kbar, where a
10 2
,
i
1
Sb2Te3
g W ~_) Z
W Cr
10-10
I
I
50
100
150
PRESSURE (kbar) Fig.1
Electrical resistance versus pressure for SbzTe3 at room temperature.
resistance minimum occurs. A broad maximum is around 90 kbar, and then the resistance decreases again at higher pressures. The electrical conductivity is about 2xi03 ~-l'cm-1 at 107 kbar and room temperature. This hump in the resistance-pressure curve was observed at almost the same pressure reproducibly. The temperature dependence of electrical resistance was measured at three different pressures and found to obey R=R0exp(AE/kT) over the temperature range
* Present address: Experimental Physics III, Dusseldorf University, D-4000 Dusseldorf I, Federal Republic of Germany. ]045
1046
PRESSURE-INDUCED
investigated from 20°C to 85°C. The activation energy &E was obtained as 0.029 eV at 46 kbar, 0.024 eV at 77 kbar, and 0.000 eV at 107 kbar. The profile of resistance anomaly with pressure in SbzTe3 is considerably different from that in BizSe3, in which a sharp decrease in the electrical resistance has been reported at about 100 kbar.
PHASE TRANSITION
IN Sb2Te 3
!
12
s%r%
2oL0! I
~"
_J
,5
t
I
I
~
The crystal structure of Sb2Te 3 at atmospheric pressure belongs to the space group D3~-R]m. z3'I~ Figure 2 shows the hexagonal pillar made by three of a hexagonal unit cell together with the corresponding
&
o
C a
B C
A
r
!lii
~"
60 ~bar
z_ o
I
I
'
t$ 20 25 2g (~-g ~ FOR ~k~-Ka
Fig.3
X-ray diffraction 60 and 107 kbar.
patterns
3Lo
of Sb2Te 3 at
:
~:'
~
A
DScf~]m Fig.2
Vol. 40, No.
Crystal structure of Sb2Te3 (Ref.13). o:Te, e:Sb. Letters at the left side of the drawing indicate the stacking repetition.
rhombohedral unit cell represented by dashed lines. The lattice constants of the hexagonal unit cell are as follows: a=4.262 A, c=30.450 ~, and z=3. The layered structure of Sb~Te3 consists of three Te(1)-Sb-Te(2)~Sb-Te(1~ sheets whose stacking along the c-axis is a repetition of AbCaB-CaBcA'BcAbC. Here the capital letters represent tellurium layers and small ones antimony layers. The Sb atoms and the Te (2) atoms are almost octahedrally bonded to six tellurium atoms and to six antimony atoms, respectively. The Te (I) atoms are bonded to three antimony atoms and are adjacent to three Te(I) atoms in the next sheet. The bond length between Te (1) atoms in adjacent sheets is 3.65 A, and is longer than that between tellurium atoms in adjacent spiral chains in the crystal of tellurium. The bonding character between Te (1) atoms in adjacent sheets seems to be of the van der Waals type. Sb2Te3 has a characteristic easy cleavage perpendicular to the c-axis. The x-ray diffraction experiment was carried out up to 150 kbar at room temperature. The diffraction patterns at 60 and 107 kbar are shown in Fig. 3. The pattern at 60 kbar can be explained by the same space group as that at I bar, whose main indices are represented in the figure. The lattice constants and atomic parameters were
obtained as a=4.098 ~, c=29.14 A, ZSb=0.395, and ZTe=0.794, both in 6(c). The parameters at 60 kbar were obtained from analyzing the observed intensities in Fig. 3 to minimize the reliability factor defined by R=EIIFcall-IFobsl[/ljFobsl. When the values are the above-mentioned ones, the R factor is 0.19. The pattern at 107 kbar, on the other hand, can not be assigned to the same crystal structure as that of the low-pzessure phase. Sb2Te3 must have a new crystal modification at 107 kbar. On release of the pressure from 150 kbar to I bar, the original pattern was reproduced. The relative changes of the lattice constants, a/a0, c/c0, and V/V0, are plotted against pressure up to 60 kbar in Fig. 4. The c/c0 decreases more rapidly than a/a0 does with increasing pressure till 40 kbar. Above 40 kbar, the rate of decrease of c/c0
1.00
~
. . . . . . .
0.95
8o P ( kbar Fig.4
)
Relative changes of the lattice constants versus pressure for Sb2Te3 in the low pressure phase.
35
PRESSURE-INDUCE~D PHASE TRANSITION IN Sb2Te 3
Vol. 40, No. [2
approaches to that of a/a0. If the compression curves are extrapolated to higher pressure, c/c0 becomes approximately equal to a/a0 at about 85 kbar. The atomic positions in hexagonal unit cell at 60 kbar projected onto the a H plane are shown in Fig. 5 compared with those at I bar. The distance between Te (I) atoms in adjacent sheets is most compressible among all bondings and is 3.29 ~ at 60 kbar. The anisotropy of bondings becomes smaller at 60 kbar.
a=4.262~, c =30.45 ~, ~ ZSb=0.400 ~ / " Zre=O"789
~
a=4.098~, c=29.14~, I
1 bar ~ ~ , ~ c
ZTe= 0"794 60 kbar
o
a Fig.5
Zsb=0395
t:Sb o:Te
Atomic positions in hexagonal unit cell at I bar (Ref. T4) and 60 kbar projected onto the a. plane with the crystal H parameters of Sb2Te3.
Drabble and Goodman ~s proposed a model for the chemical bonding in BizTe3, which has a structure homologous with Sb2Te3. If we assume that their model can be applied to the present system, the chemical bonding scheme of Sb2Te3 is as follows. The bonding orbitals of the Sb atoms and the Te(~) atoms with nearly octahedral coordination are sp3d 2 hybrids, which are occupied by both s and p electrons. For the Te (I) atoms, on the other
1047
hand, only the p electrons and the p orbitals take part in the chemical bonding. The two 5s electrons in Te (I) atoms are left as a lonepair~ The twenty four valence electrons, {(Te~Z):5p~)X2+(Sb:5sZSp3)X2+(Te(Z):5s25p4)}, are shared in the twelve bonds per unit sheet. This model suggests that the lone-pair orbitals form the upper valence band in semiconducting SbzTe3. The compression of SbzTe3 increases the overlap of the lone-pair states and thus the width of the bands. The decrease of the electrical resistance with increasing pressure up to 80 kbar can be explained by the decrease of the band gap resulting from this increase of the band width. However, the fact that the rate of decrease of c/c0 diminishes above 40 kbar suggests an increase of repulsion between lone-pair electrons at higher pressures. The conductivity independent of temperature observed at 107 kbar, 2xi03 ~-Z.cm-Z, is close to the minimum metallic conductivity of 103-104 ~-Z'cm -z given by Mort. 16 Therefore, the resistance anomaly at 80 kbar is regarded due to the semiconductor-to-metal transition accompanying the structural change. It is probable that the transformation of crystal structure takes place at about 80 kbar where c/c0 almost equals a/a0 in order to release the electronic repulsion, which inhibits continuous metallization due to the bandoverlap without a structural change. The comparison of diffraction patterns in both phases (Fig. 3) suggests that the atomic arrangement does not change so much at the transition point although the crystal structure of the high-pressure phase has not been successfully determined yet. The measurements of pressure effect on electrical resistance of Bi2Te3 are in progress. The comparison among SbzTe3, Bi2Te3, and Bi2Se3 concerning the pressure effect on their physical properties will be very interesting in the aspect of the difference in the nature of chemical bonding. Acknowledgments - The authors are thankful to Y. Oda and S. Kumagai of Toho University for their assistance with electrical resistance measurements. Work at Brookhaven was supported by the Division of Basic Energy Sciences, U.S. Department of Energy, under contract No. DE-AC02-76CHO0016.
REFERENCES I. 2. 3.
4. 5. 6. 7.
8.
N.B.Hannay, "Semiconductors" p.418 (Reinhold Pub., New York, 1959). H.J.Goldsmid, J. Appl. Phys. Suppl. 32, 2198(1961). J.R.Drabble, "Progress in Semiconductors" Vol. 7, p.45, edited by A.F.Gibson and R.F.Burgess (Wiley, New York, 1963). H.J.Goldsmid, Advan. Phys. 14,273(1965). D.R.Lovett, "Semimetals & Narrow-bandgap Semiconductors" p.181(Pion, London, 1977). L.P.Caywood,Jr., and G.R.Miller, Phys. Rev. B2,3209(1970). B.Roy, B.R.Chakraborty, R.Bhattacharya, and A.K.Dutta, Solid State Cormun. 25,617 (1978). E.S.Itskevich, E.Ya.Atabaeva, and S.V. Popova, Soy. Phys.- Solid State 6,1385 (1964).
9.
10. 11.
12.
13. 14. 15. 16.
L.F.Vereshchagin, E.S.Itskevich, E.Ya. Atabaeva, and S.V.Popova, Sov. Phys.Solid State 6,1763(1965). N.Sakai and H. Fritzsche, Phys. Rev. B15, 973(1977). K.Takemura, O.Shimomura, K.Tsuji, and S.Minomura, High Temp.- High Pressures 11, 311(1979). Y.Fujii, O.Shimomura, K.Takemura, S.Hoshino, and S.Minomura, J. Appl. Cryst. 13,284(1980). R.---W.G.Wyckoff, "Crystal Structures" 2nd ed. Vol.2, p.30(Interscience Pub., 1967). ASTM cards J.R.Drabble and C.H.L.Goodman, J. Phys. Chem. Solids 5,142(1958). N.F.Mott, PhiTos. Mag. 22,7(1970).
0038-I098/81/481045-3502.00/0
PRESSURE-INDUCED PHASE TRANSITION IN SbzTe3 N.Sakai and T.Kajiwara Department of Chemistry, Toho University, Miyama 2-2-I Funabashi-shi, Chiba-ken 274, Japan K.Takemura* and S.Minomura Institute for Solid State Physics, The University of Tokyo Roppongi, Minato-ku, Tokyo 106, Japan and Y.Fujii Department of Physics, Brookhaven National Laboratory, Upton New York 11973, U.S.A. ( Received 8 September 1981 by H. Kawamura )
An anomalous change in electrical resistance with pressure was observed in crystalline Sb2Te3 around 80 kbar at room temperature. This anomaly was found to be closely related with a pressure-induced structural phase transition which is discovered by the present x-ray diffraction experiment.
The VB-VIB compounds such as Sb2Te3, BizTe~, and Bi2Se3 are narrow-band-gap semiconductors with the homologous layeredcrystal structure. Their electrical and optical properties have been studied extensively because of the potential applicability to efficient thermoelectric devices. I-7 Studies of these properties under pressure offer particulary important information on their characteristic electronic structure. In fact, Bi2Se3 is known to undergo the pressure-induced semiconductor-metal transition accompanied with a sharp decrease in the electrical resistance at about 100 kbar. a'9 In the present paper we report the pressure-induced phase transition in Sb2Te3 found by measurements of the electrical resistance and the powder x-ray diffraction pattern. The polycrystalline SbzTe3 with the 99.999 % purity was obtained from Furuuchi Chemical Co., Ltd. The pressure dependence of electrical resistance was measured by the four-terminal method using the Drickamer type high pressure cell modified to introduce more than four leads, l° X-ray diffraction patterns of the powdered sample at high pressure were measured by a diamondanvil cell combined with a positionsensitive detector (PSD) for the MoK a radiation, ll, 12 Figure I shows the electrical resistance measured in SbeTe3 as a function of pressure at room temperature. The resistance decreases gradually with increasing pressure to about 80 kbar, where a
10 2
,
i
1
Sb2Te3
g W ~_) Z
W Cr
10-10
I
I
50
100
150
PRESSURE (kbar) Fig.1
Electrical resistance versus pressure for SbzTe3 at room temperature.
resistance minimum occurs. A broad maximum is around 90 kbar, and then the resistance decreases again at higher pressures. The electrical conductivity is about 2xi03 ~-l'cm-1 at 107 kbar and room temperature. This hump in the resistance-pressure curve was observed at almost the same pressure reproducibly. The temperature dependence of electrical resistance was measured at three different pressures and found to obey R=R0exp(AE/kT) over the temperature range
* Present address: Experimental Physics III, Dusseldorf University, D-4000 Dusseldorf I, Federal Republic of Germany. ]045
1046
PRESSURE-INDUCED
investigated from 20°C to 85°C. The activation energy &E was obtained as 0.029 eV at 46 kbar, 0.024 eV at 77 kbar, and 0.000 eV at 107 kbar. The profile of resistance anomaly with pressure in SbzTe3 is considerably different from that in BizSe3, in which a sharp decrease in the electrical resistance has been reported at about 100 kbar.
PHASE TRANSITION
IN Sb2Te 3
!
12
s%r%
2oL0! I
~"
_J
,5
t
I
I
~
The crystal structure of Sb2Te 3 at atmospheric pressure belongs to the space group D3~-R]m. z3'I~ Figure 2 shows the hexagonal pillar made by three of a hexagonal unit cell together with the corresponding
&
o
C a
B C
A
r
!lii
~"
60 ~bar
z_ o
I
I
'
t$ 20 25 2g (~-g ~ FOR ~k~-Ka
Fig.3
X-ray diffraction 60 and 107 kbar.
patterns
3Lo
of Sb2Te 3 at
:
~:'
~
A
DScf~]m Fig.2
Vol. 40, No.
Crystal structure of Sb2Te3 (Ref.13). o:Te, e:Sb. Letters at the left side of the drawing indicate the stacking repetition.
rhombohedral unit cell represented by dashed lines. The lattice constants of the hexagonal unit cell are as follows: a=4.262 A, c=30.450 ~, and z=3. The layered structure of Sb~Te3 consists of three Te(1)-Sb-Te(2)~Sb-Te(1~ sheets whose stacking along the c-axis is a repetition of AbCaB-CaBcA'BcAbC. Here the capital letters represent tellurium layers and small ones antimony layers. The Sb atoms and the Te (2) atoms are almost octahedrally bonded to six tellurium atoms and to six antimony atoms, respectively. The Te (I) atoms are bonded to three antimony atoms and are adjacent to three Te(I) atoms in the next sheet. The bond length between Te (1) atoms in adjacent sheets is 3.65 A, and is longer than that between tellurium atoms in adjacent spiral chains in the crystal of tellurium. The bonding character between Te (1) atoms in adjacent sheets seems to be of the van der Waals type. Sb2Te3 has a characteristic easy cleavage perpendicular to the c-axis. The x-ray diffraction experiment was carried out up to 150 kbar at room temperature. The diffraction patterns at 60 and 107 kbar are shown in Fig. 3. The pattern at 60 kbar can be explained by the same space group as that at I bar, whose main indices are represented in the figure. The lattice constants and atomic parameters were
obtained as a=4.098 ~, c=29.14 A, ZSb=0.395, and ZTe=0.794, both in 6(c). The parameters at 60 kbar were obtained from analyzing the observed intensities in Fig. 3 to minimize the reliability factor defined by R=EIIFcall-IFobsl[/ljFobsl. When the values are the above-mentioned ones, the R factor is 0.19. The pattern at 107 kbar, on the other hand, can not be assigned to the same crystal structure as that of the low-pzessure phase. Sb2Te3 must have a new crystal modification at 107 kbar. On release of the pressure from 150 kbar to I bar, the original pattern was reproduced. The relative changes of the lattice constants, a/a0, c/c0, and V/V0, are plotted against pressure up to 60 kbar in Fig. 4. The c/c0 decreases more rapidly than a/a0 does with increasing pressure till 40 kbar. Above 40 kbar, the rate of decrease of c/c0
1.00
~
. . . . . . .
0.95
8o P ( kbar Fig.4
)
Relative changes of the lattice constants versus pressure for Sb2Te3 in the low pressure phase.
35
PRESSURE-INDUCE~D PHASE TRANSITION IN Sb2Te 3
Vol. 40, No. [2
approaches to that of a/a0. If the compression curves are extrapolated to higher pressure, c/c0 becomes approximately equal to a/a0 at about 85 kbar. The atomic positions in hexagonal unit cell at 60 kbar projected onto the a H plane are shown in Fig. 5 compared with those at I bar. The distance between Te (I) atoms in adjacent sheets is most compressible among all bondings and is 3.29 ~ at 60 kbar. The anisotropy of bondings becomes smaller at 60 kbar.
a=4.262~, c =30.45 ~, ~ ZSb=0.400 ~ / " Zre=O"789
~
a=4.098~, c=29.14~, I
1 bar ~ ~ , ~ c
ZTe= 0"794 60 kbar
o
a Fig.5
Zsb=0395
t:Sb o:Te
Atomic positions in hexagonal unit cell at I bar (Ref. T4) and 60 kbar projected onto the a. plane with the crystal H parameters of Sb2Te3.
Drabble and Goodman ~s proposed a model for the chemical bonding in BizTe3, which has a structure homologous with Sb2Te3. If we assume that their model can be applied to the present system, the chemical bonding scheme of Sb2Te3 is as follows. The bonding orbitals of the Sb atoms and the Te(~) atoms with nearly octahedral coordination are sp3d 2 hybrids, which are occupied by both s and p electrons. For the Te (I) atoms, on the other
1047
hand, only the p electrons and the p orbitals take part in the chemical bonding. The two 5s electrons in Te (I) atoms are left as a lonepair~ The twenty four valence electrons, {(Te~Z):5p~)X2+(Sb:5sZSp3)X2+(Te(Z):5s25p4)}, are shared in the twelve bonds per unit sheet. This model suggests that the lone-pair orbitals form the upper valence band in semiconducting SbzTe3. The compression of SbzTe3 increases the overlap of the lone-pair states and thus the width of the bands. The decrease of the electrical resistance with increasing pressure up to 80 kbar can be explained by the decrease of the band gap resulting from this increase of the band width. However, the fact that the rate of decrease of c/c0 diminishes above 40 kbar suggests an increase of repulsion between lone-pair electrons at higher pressures. The conductivity independent of temperature observed at 107 kbar, 2xi03 ~-Z.cm-Z, is close to the minimum metallic conductivity of 103-104 ~-Z'cm -z given by Mort. 16 Therefore, the resistance anomaly at 80 kbar is regarded due to the semiconductor-to-metal transition accompanying the structural change. It is probable that the transformation of crystal structure takes place at about 80 kbar where c/c0 almost equals a/a0 in order to release the electronic repulsion, which inhibits continuous metallization due to the bandoverlap without a structural change. The comparison of diffraction patterns in both phases (Fig. 3) suggests that the atomic arrangement does not change so much at the transition point although the crystal structure of the high-pressure phase has not been successfully determined yet. The measurements of pressure effect on electrical resistance of Bi2Te3 are in progress. The comparison among SbzTe3, Bi2Te3, and Bi2Se3 concerning the pressure effect on their physical properties will be very interesting in the aspect of the difference in the nature of chemical bonding. Acknowledgments - The authors are thankful to Y. Oda and S. Kumagai of Toho University for their assistance with electrical resistance measurements. Work at Brookhaven was supported by the Division of Basic Energy Sciences, U.S. Department of Energy, under contract No. DE-AC02-76CHO0016.
REFERENCES I. 2. 3.
4. 5. 6. 7.
8.
N.B.Hannay, "Semiconductors" p.418 (Reinhold Pub., New York, 1959). H.J.Goldsmid, J. Appl. Phys. Suppl. 32, 2198(1961). J.R.Drabble, "Progress in Semiconductors" Vol. 7, p.45, edited by A.F.Gibson and R.F.Burgess (Wiley, New York, 1963). H.J.Goldsmid, Advan. Phys. 14,273(1965). D.R.Lovett, "Semimetals & Narrow-bandgap Semiconductors" p.181(Pion, London, 1977). L.P.Caywood,Jr., and G.R.Miller, Phys. Rev. B2,3209(1970). B.Roy, B.R.Chakraborty, R.Bhattacharya, and A.K.Dutta, Solid State Cormun. 25,617 (1978). E.S.Itskevich, E.Ya.Atabaeva, and S.V. Popova, Soy. Phys.- Solid State 6,1385 (1964).
9.
10. 11.
12.
13. 14. 15. 16.
L.F.Vereshchagin, E.S.Itskevich, E.Ya. Atabaeva, and S.V.Popova, Sov. Phys.Solid State 6,1763(1965). N.Sakai and H. Fritzsche, Phys. Rev. B15, 973(1977). K.Takemura, O.Shimomura, K.Tsuji, and S.Minomura, High Temp.- High Pressures 11, 311(1979). Y.Fujii, O.Shimomura, K.Takemura, S.Hoshino, and S.Minomura, J. Appl. Cryst. 13,284(1980). R.---W.G.Wyckoff, "Crystal Structures" 2nd ed. Vol.2, p.30(Interscience Pub., 1967). ASTM cards J.R.Drabble and C.H.L.Goodman, J. Phys. Chem. Solids 5,142(1958). N.F.Mott, PhiTos. Mag. 22,7(1970).