- Email: [email protected]
Structural stability of tin clathrates under high pressure
ARTICLE IN PRESS Journal of Physics and Chemistry of Solids 71 (2010) 587–589
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Structural stability of tin clathrates under high pressure b ¨ T. Imai a, T. Kume a,n, S. Sasaki a, H. Shimizu a, A. Kaltzoglou b, T.F. Fassler a b
Department of Materials Science and Technology, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan Department of Chemistry, Technical University Munich, D-85747 Garching, Germany
a r t i c l e in fo
abstract
Article history: Received 2 June 2009 Received in revised form 16 October 2009 Accepted 20 October 2009
High pressure Raman scattering experiments have been performed for Rb8Sn44&2 in order to investigate the pressure induced phase transition. At pressures of 6.0 and 7.5 GPa, Raman spectrum was drastically changed, indicating the phase transitions. The irreversibility of the spectral change and the disappearance of Raman peak observed at 7.5 GPa strongly suggest the occurrence of irreversible amorphization. & 2009 Elsevier Ltd. All rights reserved.
Keywords: C. High pressure D. Phase transition
1. Introduction The group-14 elements Si, Ge, and Sn can form expanded volume phases known as clathrates, which have open-framework structures [1]. The host cages encapsulate guest atoms, and the choice of the guest can tune the material’s properties, including the vibrational modes. These clathrates continue to be of considerable interest due to their potential thermoelectric [2,3], superconducting [4–6], and electro-optic applications [7]. The largest framework cavities in Sn clathrates, compared with their Si and Ge homologues, favor the ‘‘rattling’’ of the guest atoms [8–11], and therefore decrease the lattice contribution to the thermal conductivity [12]. It is well established that the type-I Sn clathrates deviate from the ideal formula A8Sn46 (A= K, Rb, Cs) by having two framework vacancies (&) per unit cell, i.e., A8Sn44&2. The cage structure with vacancies is presented in Fig. 1. Recently, the order–disorder phase transition related with the vacancies has been reported, at ambient pressure, for Rb8Sn44&2 at 80 1C and Cs8Sn44&2 at 90 1C [13,14]. It was found that the vacancies on the 6c site show the spiral ordering at room temperature (Fig. 1), whereas at higher temperatures, indicate disordering. In this paper, in order to clarify the pressure induced phase transition for the tin clathrates, we have made Raman experiments for the clathrate Rb8Sn44&2 under high pressures and room temperature. As a result, two Raman spectral changes have been observed at 6.0 and 7.5 GPa. Since no Raman peak was observed above 7.5 GPa and the peak was not recovered on
n
Corresponding author. Tel./fax: + 81 582932681. E-mail address: [email protected] (T. Kume).
0022-3697/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2009.12.043
decompression, the latter change can be interpreted with amorphization.
2. Experimental The sample of Rb8Sn44&2 was synthesized by the method established elsewhere [15]. For high pressure experiments, we used a diamond anvil cell (DAC). A small peace of the sample ( 40 mm in size) placed into the sample chamber, which was made by making a hole ( 100 mm in diameter) on the tungsten gasket. As the pressure medium, the dense Ar was used. The pressure calibration has been done by the ruby method. Raman measurements were performed with an apparatus, which was improved through detecting the low-wavenumber signals for various clathrate compounds [16–18]. Radiation of 532 nm from a solid-state laser (Verdi2W) was incident on the sample with a power of 10 mW. The backscattered Raman spectrum was recorded with a spectrometer (JASCO NR1800) equipped with a triple polychromator and a charge coupled device detector [18]. The illuminated spot was less than 5 mm in size and the resolution of the spectra 1 cm 1 with the slit width of 50 mm.
3. Results and discussion Fig. 2 shows Raman spectra of various Sn-based clathrates, Rb8Sn44&2, Cs8Sn44&2 with vacancies, and I8Sb8Sn38, Rb8Hg4Sn42 without vacancies. As has been reported elsewhere [19], the peaks around 70 cm 1 can be observed only for the clathrate with vacancy. Therefore, the 70 cm 1 peaks are interpreted with the vacancy-induced modes. A localized vibration of Sn atoms around
ARTICLE IN PRESS
167 cm-1
2
Rb8Sn44
2
165
150
105
40
196 I8Sb8Sn38
106
170
38 50
185
175
110
85
30
77
40
29 35
Intensity (arb. units) Fig. 1. View of the A8Sn44&2 (A =Rb, Cs) structure containing partially occupied Sn sites (boxes), which are surrounded by four framework Sn atoms that are only 3-bonded and indicated by dotted lines.
the vacancy can be considered as the vacancy mode. The guest vibrations of Sn clathrates are identified by comparing the spectra of Rb8Sn44&2 and Cs8Sn44&2. These two spectra strongly resemble each other in feature on the region above 50 cm 1. However, the spectral feature is different in the low-frequency region below 50 cm 1. Therefore, the guest vibrations for Sn clathrates are observed in this frequency region. Fig. 3 shows Raman spectra measured for Rb8Sn44&2 at high pressures. Two spectral changes were found at 6.0 and 7.3 GPa. At 6.0 GPa, the vacancy related peaks (two solid circles at 70 cm 1 and at 1 bar) almost disappeared, and simultaneously a new peak (solid triangle) appeared at around 35 cm 1, demonstrating that the phase transition occurs at 6.0 GPa. When the pressure is further increased, all the Raman peaks disappeared above 7 GPa. Since the spectrum was not changed back to that before compression when the pressure was released, the spectral change found at 7 GPa is irreversible. The irreversible spectral change has been observed for other clathrate compounds, e.g., I8Sb8Ge38[20], Ba8Ge43&3[21], and so on, and the amorphization was proposed for most cases. Since no Raman peak was observed in the recovered sample, the amorphization probably occurs at 7 GPa. We now discuss the phase transition at 6.0 GPa. As mentioned above, the vacancy related mode around 70 cm 1 disappeared at 6.0 GPa, which may be interpreted with disordering of the vacancies. According to the previous work on the order–disorder transition for the vacancy, the spiral distribution of the vacancies on 6c site changes to the random distribution on the 6c site at high temperature [15]. Since, however, the order–disorder transition did less affect on the vacancy modes [19], the present spectral change of the vacancy mode is not the case. A possible interpretation is displacements of the vacancies from the 6c site to the other sites (to 16i and/or 24k site). To confirm this, it is highly required to make the X-ray study under high pressure. We note that the phase transition at 6.0 GPa causes an enhancement of a low-frequency peak located at 35 cm 1. From its sufficiently low-frequency range, this peak is likely to originate
Cs8Sn44 69 73
34
46 51
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T. Imai et al. / Journal of Physics and Chemistry of Solids 71 (2010) 587–589
47
588
Rb8Hg4Sn42
0
50
100 150 Wavenumber (cm-1)
200
250
Fig. 2. Raman spectra of defect clathrates Rb8Sn44&2 and Cs8Sn44&2, and defect free Rb8Hg4Sn42 and I8Sb8Sn38 at ambient conditions are shown with values of Raman peaks and shoulders.
from guest Rb vibration. Although the reason why the peak intensity is enhanced is not clear at present, one can suppose the change in the electronic distribution around the guests, as suggested for the case of Ba8Si46[22]. The electronic distribution can change the Raman intensity. As the other explanation, the low-frequency jump of the vacancy mode can be considered. If the 6.0 GPa phase transition is due to disordering of 6c site vacancy, it can be considered that the Sn–Sn bonding is easy to become broken so as to move the vacancy. Thus, the Sn–Sn bonding is expected to be weak, giving rise to the lower-frequency shift of the vacancy-induced localized mode.
4. Conclusion High pressure Raman experiments for Rb8Sn44&2 have been performed at room temperature. Spectral changes were observed at 6.0 and 7.5 GPa. The spectral change at 7.5 GPa is considered as an irreversible amorphization. The phase transition at 6.0 GPa is likely to be related with the behavior of vacancies.
ARTICLE IN PRESS T. Imai et al. / Journal of Physics and Chemistry of Solids 71 (2010) 587–589
Rb8Sn44
589
Science Using Regulated Nano Spaces-Strategy in Ubiquitous Elements’ from the Ministry of Education, Culture, Sports, Science and Technology of Japan and by the European Union’s RTN program (Eu-project Nr. HPRN-CT 2002-00193) of Nanocage Materials.
3
9.4 GPa
8.7 References
7.9
[1] [2] [3] [4]
7.3
[5] [6]
6.4
[7]
6.0 Intensity (arb. units)
[8] [9] [10]
5.4
[11]
5.0
[12] [13] [14] [15] [16]
4.3
[17]
3.7
[18]
3.2
[19] [20]
2.6
[21] [22]
1.3
1 atm
0
50
100 150 -1 Wavenumber (cm )
200
250
Fig. 3. Pressure dependence of Raman spectra of Rb8Sn44&2 measured up to 9.4 GPa at room temperature.
Acknowledgements This work was partially supported by KAKENHI (Grant-in-Aid for Scientific Research) on the Priority Areas ‘New Materials
K.A. Kovnir, A.V. Shevelkov, Russ. Chem. Rev. 73 (2004) 923. G.S. Nolas, J.L. Cohn, G.A. Slack, S.B. Schujman, Appl. Phys. Lett. 73 (1998) 178. J. Martin, G.S. Nolas, H. Wang, J. Yang, J. Appl. Phys. 102 (2007) 103719. H. Kawaji, H. Horie, S. Yamanaka, M. Ishikawa, Phys. Rev. Lett. 74 (1995) 1427. S. Yamanaka, E. Enishi, H. Fukuoka, M. Yasukawa, Inorg. Chem. 39 (2000) 56. K. Tanigaki, T. Shimizu, K.M. Itoh, J. Teraoka, Y. Moritomo, S. Yamanaka, Nat. Mater. 2 (2003) 653. J. Gryko, P.F. McMillan, R.F. Marzke, G.K. Ramachandran, D. Patton, S.K. Deb, O.F. Sankey, Phys. Rev. B 62 (2000) R7707. J. Dong, O.F. Sankey, G.K. Ramachandran, P.F. McMillan, J. Appl. Phys. 87 (2000) 7726. G.S. Nolas, C.A. Kendziora, Phys. Rev. B 62 (2000) 7157. C.W. Myles, J. Dong, O.F. Sankey, C.A. Kendziora, G.S. Nolas, Phys. Rev. B 65 (2002) 235208. R.P. Hermann, W. Schweika, O. Leupold, R. Ruffer, G.S. Nolas, F. Grandjean, G.J. Long, Phys. Rev. B 72 (2005) 174301. J.S. Tse, Z. Li, K. Uehara, Europhys. Lett. 56 (2001) 261. ¨ F. Dubois, T.F. Fassler, J. Am. Chem. Soc. 127 (2005) 3264. ¨ A. Kaltzoglou, S.D. Hoffmann, T.F. Fassler, Eur. J. Inorg. Chem. (2007) 4162. A. Kaltzoglou, et al., J. Mater. Chem. 18 (2008) 5630. T. Kume, H. Fukuoka, T. Koda, S. Sasaki, H. Shimizu, S. Yamanaka, Phys. Rev. Lett. 90 (2003) 155503. J.S. Tse, T. Iitaka, T. Kume, H. Shimizu, K. Parlinski, H. Fukuoka, S. Yamanaka, Phys. Rev. B 72 (2005) 155441. H. Shimizu, T. Iitaka, T. Fukushima, T. Kume, S. Sasaki, N. Sata, Y. Ohishi, H. Fukuoka, S. Yamanaka, J. Appl. Phys. 101 (2007) 063549. ¨ H. Shimizu, T. Imai, T. Kume, S. Sasaki, A. Kaltzoglou, T.F. Fassler, Chem. Phys. Lett. 464 (2008) 54. H. Shimizu, R. Oe, S. Ohno, T. Kume, S. Sasaki, K. Kishimoto, T. Koyanagi, Y. Ohishi, J. Appl. Phys. 105 (2009) 043522. H. Shimizu, T. Iitaka, T. Fukushima, T. Kume, S. Sasaki, N. Sata, Y. Ohishi, H. Fukuoka, S. Yamanaka, J. Appl. Phys. 101 (2007) 063549. J.S. Tse, R. Flacau, S. Desgreniers, T. Iitaka, J.Z. Jiang, Phys. Rev. B 76 (2007) 174109.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Structural stability of tin clathrates under high pressure b ¨ T. Imai a, T. Kume a,n, S. Sasaki a, H. Shimizu a, A. Kaltzoglou b, T.F. Fassler a b
Department of Materials Science and Technology, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan Department of Chemistry, Technical University Munich, D-85747 Garching, Germany
a r t i c l e in fo
abstract
Article history: Received 2 June 2009 Received in revised form 16 October 2009 Accepted 20 October 2009
High pressure Raman scattering experiments have been performed for Rb8Sn44&2 in order to investigate the pressure induced phase transition. At pressures of 6.0 and 7.5 GPa, Raman spectrum was drastically changed, indicating the phase transitions. The irreversibility of the spectral change and the disappearance of Raman peak observed at 7.5 GPa strongly suggest the occurrence of irreversible amorphization. & 2009 Elsevier Ltd. All rights reserved.
Keywords: C. High pressure D. Phase transition
1. Introduction The group-14 elements Si, Ge, and Sn can form expanded volume phases known as clathrates, which have open-framework structures [1]. The host cages encapsulate guest atoms, and the choice of the guest can tune the material’s properties, including the vibrational modes. These clathrates continue to be of considerable interest due to their potential thermoelectric [2,3], superconducting [4–6], and electro-optic applications [7]. The largest framework cavities in Sn clathrates, compared with their Si and Ge homologues, favor the ‘‘rattling’’ of the guest atoms [8–11], and therefore decrease the lattice contribution to the thermal conductivity [12]. It is well established that the type-I Sn clathrates deviate from the ideal formula A8Sn46 (A= K, Rb, Cs) by having two framework vacancies (&) per unit cell, i.e., A8Sn44&2. The cage structure with vacancies is presented in Fig. 1. Recently, the order–disorder phase transition related with the vacancies has been reported, at ambient pressure, for Rb8Sn44&2 at 80 1C and Cs8Sn44&2 at 90 1C [13,14]. It was found that the vacancies on the 6c site show the spiral ordering at room temperature (Fig. 1), whereas at higher temperatures, indicate disordering. In this paper, in order to clarify the pressure induced phase transition for the tin clathrates, we have made Raman experiments for the clathrate Rb8Sn44&2 under high pressures and room temperature. As a result, two Raman spectral changes have been observed at 6.0 and 7.5 GPa. Since no Raman peak was observed above 7.5 GPa and the peak was not recovered on
n
Corresponding author. Tel./fax: + 81 582932681. E-mail address: [email protected] (T. Kume).
0022-3697/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2009.12.043
decompression, the latter change can be interpreted with amorphization.
2. Experimental The sample of Rb8Sn44&2 was synthesized by the method established elsewhere [15]. For high pressure experiments, we used a diamond anvil cell (DAC). A small peace of the sample ( 40 mm in size) placed into the sample chamber, which was made by making a hole ( 100 mm in diameter) on the tungsten gasket. As the pressure medium, the dense Ar was used. The pressure calibration has been done by the ruby method. Raman measurements were performed with an apparatus, which was improved through detecting the low-wavenumber signals for various clathrate compounds [16–18]. Radiation of 532 nm from a solid-state laser (Verdi2W) was incident on the sample with a power of 10 mW. The backscattered Raman spectrum was recorded with a spectrometer (JASCO NR1800) equipped with a triple polychromator and a charge coupled device detector [18]. The illuminated spot was less than 5 mm in size and the resolution of the spectra 1 cm 1 with the slit width of 50 mm.
3. Results and discussion Fig. 2 shows Raman spectra of various Sn-based clathrates, Rb8Sn44&2, Cs8Sn44&2 with vacancies, and I8Sb8Sn38, Rb8Hg4Sn42 without vacancies. As has been reported elsewhere [19], the peaks around 70 cm 1 can be observed only for the clathrate with vacancy. Therefore, the 70 cm 1 peaks are interpreted with the vacancy-induced modes. A localized vibration of Sn atoms around
ARTICLE IN PRESS
167 cm-1
2
Rb8Sn44
2
165
150
105
40
196 I8Sb8Sn38
106
170
38 50
185
175
110
85
30
77
40
29 35
Intensity (arb. units) Fig. 1. View of the A8Sn44&2 (A =Rb, Cs) structure containing partially occupied Sn sites (boxes), which are surrounded by four framework Sn atoms that are only 3-bonded and indicated by dotted lines.
the vacancy can be considered as the vacancy mode. The guest vibrations of Sn clathrates are identified by comparing the spectra of Rb8Sn44&2 and Cs8Sn44&2. These two spectra strongly resemble each other in feature on the region above 50 cm 1. However, the spectral feature is different in the low-frequency region below 50 cm 1. Therefore, the guest vibrations for Sn clathrates are observed in this frequency region. Fig. 3 shows Raman spectra measured for Rb8Sn44&2 at high pressures. Two spectral changes were found at 6.0 and 7.3 GPa. At 6.0 GPa, the vacancy related peaks (two solid circles at 70 cm 1 and at 1 bar) almost disappeared, and simultaneously a new peak (solid triangle) appeared at around 35 cm 1, demonstrating that the phase transition occurs at 6.0 GPa. When the pressure is further increased, all the Raman peaks disappeared above 7 GPa. Since the spectrum was not changed back to that before compression when the pressure was released, the spectral change found at 7 GPa is irreversible. The irreversible spectral change has been observed for other clathrate compounds, e.g., I8Sb8Ge38[20], Ba8Ge43&3[21], and so on, and the amorphization was proposed for most cases. Since no Raman peak was observed in the recovered sample, the amorphization probably occurs at 7 GPa. We now discuss the phase transition at 6.0 GPa. As mentioned above, the vacancy related mode around 70 cm 1 disappeared at 6.0 GPa, which may be interpreted with disordering of the vacancies. According to the previous work on the order–disorder transition for the vacancy, the spiral distribution of the vacancies on 6c site changes to the random distribution on the 6c site at high temperature [15]. Since, however, the order–disorder transition did less affect on the vacancy modes [19], the present spectral change of the vacancy mode is not the case. A possible interpretation is displacements of the vacancies from the 6c site to the other sites (to 16i and/or 24k site). To confirm this, it is highly required to make the X-ray study under high pressure. We note that the phase transition at 6.0 GPa causes an enhancement of a low-frequency peak located at 35 cm 1. From its sufficiently low-frequency range, this peak is likely to originate
Cs8Sn44 69 73
34
46 51
145 cm-1
70
25 32
73
53
T. Imai et al. / Journal of Physics and Chemistry of Solids 71 (2010) 587–589
47
588
Rb8Hg4Sn42
0
50
100 150 Wavenumber (cm-1)
200
250
Fig. 2. Raman spectra of defect clathrates Rb8Sn44&2 and Cs8Sn44&2, and defect free Rb8Hg4Sn42 and I8Sb8Sn38 at ambient conditions are shown with values of Raman peaks and shoulders.
from guest Rb vibration. Although the reason why the peak intensity is enhanced is not clear at present, one can suppose the change in the electronic distribution around the guests, as suggested for the case of Ba8Si46[22]. The electronic distribution can change the Raman intensity. As the other explanation, the low-frequency jump of the vacancy mode can be considered. If the 6.0 GPa phase transition is due to disordering of 6c site vacancy, it can be considered that the Sn–Sn bonding is easy to become broken so as to move the vacancy. Thus, the Sn–Sn bonding is expected to be weak, giving rise to the lower-frequency shift of the vacancy-induced localized mode.
4. Conclusion High pressure Raman experiments for Rb8Sn44&2 have been performed at room temperature. Spectral changes were observed at 6.0 and 7.5 GPa. The spectral change at 7.5 GPa is considered as an irreversible amorphization. The phase transition at 6.0 GPa is likely to be related with the behavior of vacancies.
ARTICLE IN PRESS T. Imai et al. / Journal of Physics and Chemistry of Solids 71 (2010) 587–589
Rb8Sn44
589
Science Using Regulated Nano Spaces-Strategy in Ubiquitous Elements’ from the Ministry of Education, Culture, Sports, Science and Technology of Japan and by the European Union’s RTN program (Eu-project Nr. HPRN-CT 2002-00193) of Nanocage Materials.
3
9.4 GPa
8.7 References
7.9
[1] [2] [3] [4]
7.3
[5] [6]
6.4
[7]
6.0 Intensity (arb. units)
[8] [9] [10]
5.4
[11]
5.0
[12] [13] [14] [15] [16]
4.3
[17]
3.7
[18]
3.2
[19] [20]
2.6
[21] [22]
1.3
1 atm
0
50
100 150 -1 Wavenumber (cm )
200
250
Fig. 3. Pressure dependence of Raman spectra of Rb8Sn44&2 measured up to 9.4 GPa at room temperature.
Acknowledgements This work was partially supported by KAKENHI (Grant-in-Aid for Scientific Research) on the Priority Areas ‘New Materials
K.A. Kovnir, A.V. Shevelkov, Russ. Chem. Rev. 73 (2004) 923. G.S. Nolas, J.L. Cohn, G.A. Slack, S.B. Schujman, Appl. Phys. Lett. 73 (1998) 178. J. Martin, G.S. Nolas, H. Wang, J. Yang, J. Appl. Phys. 102 (2007) 103719. H. Kawaji, H. Horie, S. Yamanaka, M. Ishikawa, Phys. Rev. Lett. 74 (1995) 1427. S. Yamanaka, E. Enishi, H. Fukuoka, M. Yasukawa, Inorg. Chem. 39 (2000) 56. K. Tanigaki, T. Shimizu, K.M. Itoh, J. Teraoka, Y. Moritomo, S. Yamanaka, Nat. Mater. 2 (2003) 653. J. Gryko, P.F. McMillan, R.F. Marzke, G.K. Ramachandran, D. Patton, S.K. Deb, O.F. Sankey, Phys. Rev. B 62 (2000) R7707. J. Dong, O.F. Sankey, G.K. Ramachandran, P.F. McMillan, J. Appl. Phys. 87 (2000) 7726. G.S. Nolas, C.A. Kendziora, Phys. Rev. B 62 (2000) 7157. C.W. Myles, J. Dong, O.F. Sankey, C.A. Kendziora, G.S. Nolas, Phys. Rev. B 65 (2002) 235208. R.P. Hermann, W. Schweika, O. Leupold, R. Ruffer, G.S. Nolas, F. Grandjean, G.J. Long, Phys. Rev. B 72 (2005) 174301. J.S. Tse, Z. Li, K. Uehara, Europhys. Lett. 56 (2001) 261. ¨ F. Dubois, T.F. Fassler, J. Am. Chem. Soc. 127 (2005) 3264. ¨ A. Kaltzoglou, S.D. Hoffmann, T.F. Fassler, Eur. J. Inorg. Chem. (2007) 4162. A. Kaltzoglou, et al., J. Mater. Chem. 18 (2008) 5630. T. Kume, H. Fukuoka, T. Koda, S. Sasaki, H. Shimizu, S. Yamanaka, Phys. Rev. Lett. 90 (2003) 155503. J.S. Tse, T. Iitaka, T. Kume, H. Shimizu, K. Parlinski, H. Fukuoka, S. Yamanaka, Phys. Rev. B 72 (2005) 155441. H. Shimizu, T. Iitaka, T. Fukushima, T. Kume, S. Sasaki, N. Sata, Y. Ohishi, H. Fukuoka, S. Yamanaka, J. Appl. Phys. 101 (2007) 063549. ¨ H. Shimizu, T. Imai, T. Kume, S. Sasaki, A. Kaltzoglou, T.F. Fassler, Chem. Phys. Lett. 464 (2008) 54. H. Shimizu, R. Oe, S. Ohno, T. Kume, S. Sasaki, K. Kishimoto, T. Koyanagi, Y. Ohishi, J. Appl. Phys. 105 (2009) 043522. H. Shimizu, T. Iitaka, T. Fukushima, T. Kume, S. Sasaki, N. Sata, Y. Ohishi, H. Fukuoka, S. Yamanaka, J. Appl. Phys. 101 (2007) 063549. J.S. Tse, R. Flacau, S. Desgreniers, T. Iitaka, J.Z. Jiang, Phys. Rev. B 76 (2007) 174109.