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Photoelectron spectroscopic study of NbSe3 and TaSe3
Physica 105B (1981) 159-162 North-Holland Publishing Company
PHOTOELECTRON SPECTROSCOPIC STUDY OF NbSe3 AND TaSe3 K. ENDO, H. IHARA, S. G O N D A and K. W A T A N A B E Electrotechnical Laboratory, Sakura-mura, Ibaraki, Japan
The X-ray photoelectron spectroscopic (XPS) measurements were carried out to elucidate the valence band pictures and bonding characters of NbSe3 and TaSe3 with quasi-one-dimensional structures. The valence band spectra agree well with the theoretical densities of states (DOS) after Bullet and reflect the two-dimensional (2-D) character in contrast with those of NbS3 and TaS3. The results are discussed in relation to their CDW formation temperatures.
1. Introduction In recent years the trichalcogenides of the group IV, V and VI transition metals have received considerable attention because of their unique properties such as charge density wave (CDW) instabilities or superconductivities. Common to their lattice structures is a onedimensional feature that each metal atom lies at the centre of a trigonal prism of chalcogen atoms; successive prisms are arranged in infinite chains along the b-axis. The structures of NbSe3 [1] and TaSe3 [2] are shown in fig. 1. These structures can be regarded as two-dimensional (2-D) layers (see fig. lb) of one-dimensional (lD) chains (see fig. la). Although these structures resemble each other, the transport properties are markedly different. NbSe3 shows two major resistivity anomalies at 145 and 59 K suggestive of CDW formation [3--6], although no such anomalies are observed in the temperature dependence of the resistivity of TaSe3 [7-10]. In order to explain the difference in such transport properties, Bullet [11, 12] calculated the band structures and densities of states (DOS) of NbSe3 and TaSe3, considering the small difference in the arrangement of these chains. However, there is no direct experimental evidence for these band structures. The present study aims to elucidate the electronic band structures of NbSe3 and TaSe3 by X-ray photoelectron spectroscopic (XPS) measurements.
a)
b
T OSe o M
b) ,0......0.....0.....0.0..... ..0....
NbSe3
>-¢
Fig. 1. Structures of NbSe3 and TaSe3. (a) Stacking along the b-axis; (b) projection of the structures on the ac plane. Open circles are at 0 and full circles at 1/2b.
0378-4363/81/0000-0000/$2.50 © North-Holland Publishing Company and Yamada Science Foundation
160
g. Endo et al/XPS study of NbSe3 and TaSe3
2. Experimental The crystals used in this study were grown in a temperature range of 650-900°C by heating a mixture of selenium and metal powder in an evacuated quartz tube. The resistivity measurements of the crystals confirm that NbSe3 shows C D W anomalies similar to those described in the literature [3-6], but no such anomalies appear in TaSe3. The crystals were taken out from a sealed reaction tube, arrayed on the sample holder in an atmosphere of argon and then introduced into the XPS analyzing chamber without exposing to air. The XPS spectra were obtained using an H P 5950A E S C A spectrometer with a resolution of 0 . 6 e V under a pressure of 10-9Torr. The core-level spectra reveal that the samples are free from oxygen. Measurements were repeated on samples from three batches which were synthesized by the same method. Little differences between batches were observed.
3. Results and discussion Fig. 2 shows the valence band spectra of NbSe3 and TaSe3 (solid line) obtained by subtracting the background due to inelastically scattered electrons. The accumulation times for these spectra were 18 h. Also in the figure the theoretical DOS after Bullet [12] are given for comparison (dashed line). The characteristic features of these experimental spectra are in good agreement with those of the theoretical DOS in the valence band width ( - 8 eV), the binding energy and the gross features. Thus, our XPS results give evidence that the band calculations of NbSe3 and TaSe3 after Bullet are valid. In particular, the band structure and DOS of NbSe3 are calculated for such extreme cases as a single 1-D chain (see fig. la) and a 2-D layer (see fig. lb) [11]. The difference of theoretical DOS pictures between 1-D and 2-D cases are quite marked. In the 1-D case the Fermi level falls in the middle of an isolated band which does not overlap with other bands. Such a situation is favourable for a Peierls' distortion into an insulating state. No such isolated band occurs in the 2-D layer. Probably, such an isolated band
NbSe3
!~
V t
I aII1
18
16
i a
~
II ,
14
aii
~.,L~;"
\,,~ i ,,
12
10
8
6
4
2
EF
12
10
8
6
4
2
EF
4 U v
TaSe3 m
•*In ~
18
16
14
Binding energy ( eV ) Fig. 2. XPS valence band spectra (solid lines) of NbSe3 and TaSe3 c o m p a r e d with the theoretical D O S (dashed lines).
might be observed as a shoulder at the top of the valence band in the XPS spectrum. Our XPS measurements for NbS3 and TaS3 reveal that the shoulder clearly appears at the top of valence band [13]. In contrast, no such shoulders are clearly observed in the XPS spectra of N b S e 3 and TaSe3. This fact suggests that NbS3 and TaS3 reflect the 1-D character against that of NbSe3 and TaSe3. Here, the degree of one-dimensionality might be related to the C D W transition temperature. TaS3 shows the C D W transition at 218 K [14-16] and NbSe3 at 145 and 59 K [3--6]. NbS3 is semiconductor below 6 0 0 K [17-19], which will be inferred to have the C D W transition temperature above room temperature from the electron diffraction [20] and XPS measurements [13]. TaSe3 shows the metallic behaviour above the superconducting transition temperature
K. Endo et al/XPS study of NbSe3 and TaSe3
(2.1 K) [7-10]. These facts imply that the degree of one-dimensionality increases in the following order, TaSe3, NbSe3, TaS3 and NbS3. Such a tendency to 1-D character in transition metal trichalcogenides corresponds well to the XPS results, although it is difficult to determine the hierarchy of 1-D character of TaSe3 and NbSe3 from the present study. Fig. 3 shows the core-level spectra of NbSe3 and TaSe3 such as Nb 3d, Ta 4f and Se 3p levels. The binding energies and the full widths at halfmaximum (FWHM) are listed in table I together with those of Nb, Ta and Se. From the peak intensity the atomic ratios of Se to Nb or Ta are determined as 2.98 or 3.03, respectively. These values agree well with those from the X-ray fluorescence analysis.
210
205 Ta/~
TaSe3
161
Table I Binding energies and full widths at half maximum (FWHM) of core-levels of NbSe3, Nb, Ta and Se Binding energy
Full widths at half-maximum
Sample
Core-level
(eV)
(eV)
NbSe3
Nb 3dsn Se 3p3/2 Ta 4b/2 Se43p3/2 Nb 3d5/2 Ta 4fTn Se 3p3/2
204.2 160.5 23.7 160.1 202.3 21.8 161.4
1.4 2.8 1.2 2.5 0.9 0.9 2.1
TaSe3 Nb Ta Se
As shown in table I the FWHM of Nb 3d, Ta 4f and Se 3p peaks of NbSe3 and TaSe3 are wider than those of their elements. In addition, the double splitting of Se 3d peak are clearly obser-
200 a4fT/2
? c D
3b
2'5
2b
eD
U v
O
2"
o
_c
m eel
1~5 TaSe3
1~,o /~Se3P~
60
Binding energy ( eV ) Fig. 3. Core-level spectra of NbSe3 and TaSe3.
5'5
Binding energy (eV) Fig. 4. Se 3d spectra of NbSe3, TaSe3 and pure Se.
162
K. Endo et al/XPS study of NbSe3 and TaSe3
ved in the pure Se spectrum but not in NbSe3 and V a S e 3 spectra (see fig. 4). This means the presence of several inequivalent bonds of Nb, Ta and Se atoms in NbSe3 and TaSe3, as is expected from their crystal structure (see fig. 1). One can guess such inequivalent bondings between Se atoms are the origin of the broadening of Se s-band located between 11 and 17 eV below the Fermi level, EF, in the NbSe3 and TaSe3 valence band spectra (see fig. 2). Charge transfers of Nb and Ta for NbSe3 and TaSe3 are estimated at about +0.2 electrons. A detailed description of the method for estimation is given elsewhere [21]. These values are smaller than those of NbS3 and TaS3 [22] which suggests that the ionic bondings of selenides are weaker than those of sulfides. In conclusion, the electronic structures of NbSe3 and TaSe3 with quasi-one-dimensional structures were characterized through the use of XPS. The valence band spectra were in agreement with the theoretical DOS after Bullet. In particular, the XPS spectra of NbSe3 and TaSe3 reflect the 2-D character in contrast with those of NbS3 and TaS3. This is consistent with the order of dimensionality expected from their CDW formation temperatures. References [1] A. Meerschaut and J. Rouxel, J. Less-Common Metals 39 (1975) 197.
[2] A. Bjerkelund, J.H. Fermor and A. Kjekshus, Acta Chem. Scand. 20 (1966) 1836. [3] J. Chaussy, P. Haen, J.C. Lasiaunias, P. Monceau, G. Waysand, A. Waintal, A. Meerschaut, P. Molinie and J. Rouxel, Solid State Commun. 20 (1976) 759. [4] P. Monceau, N.P. Ong, A.M. Portis, A. Meerschaut and J. Rouxel, Phys. Rev. Lett. 37 (1976) 602. [5] N.P. Ong and P. Monceau, Phys. Rev. B16 (1977) 3443. [6] N.P. Ong, Phys. Rev. B17 (1978) 3243. [7] T. Sambongi, M. Yamamoto, K. Tsutsumi, Y. Shiozaki, K. Yamaya and Y. Abe, J. Phys. Soc. Japan 42 (1977) 1421. [8] P. Haen, P. Monceau, B. Tissier, G. Waysand, A. Meerschaut, P. Molinie and J. Rouxel, Ferroelectrics 17 (1977) 447. [9] M. Yamamoto, J. Phys. Soc. Japan 45 (1978) 431. [10] P. Haen, F. Lapierre, P. Monceau, M.N. Regueiro and J. Richard, Solid State Commun. 26 (1978) 725. [11] D.W. Bullet, Solid State Commun. 26 (1978) 563. [12] D.W. Bullet, J. Phys. C: Solid State Phys. 12 (1979) 277. [13] K. Endo, H. Ihara, K. Watanabe and S. Gonda, in press, J. Solid State Chem. [14] T. Sambongi, K. Tsutsumi, Y. Shiozaki, M. Yamamoto, K. Yamaya and Y. Abe, Solid State Commun. 22 (1977) 729. [15] K. Tsutsumi, T. Sambongi, S. Kagoshima and T. Ishiguro, J. Phys. Soc. Japan 44 (1978) 1735. [16] G. van Tendeloo, J. van Landuyt and S. Amelinckx, Phys. Stat. Sol. (a) 43 (1977) K137. [17] L.A. Grigoryan and A.V. Novoselova, Dokl. Akad. Nauk SSR 144 (1962) 795. [18] F. Kadijk and F. Jellinek, J. Less-Common Metals 19 (1969) 421. [19] K. Endo, M. Hirabayashi and S. Gonda, ACS/CSJ Chemical Congress (Hawaii) (1--6 Apr. 1979) Abstr. of Papers, Part II, Phys. 220. [20] F.W. Boswell and A. Prodan, Physica 99B (1980) 361. [21] H. Ihara, Res. Electrotechn. Lab. 775 (1977)98. [22] K. Endo, H. Ihara, K. Watanabe and S. Gonda, to be published.
PHOTOELECTRON SPECTROSCOPIC STUDY OF NbSe3 AND TaSe3 K. ENDO, H. IHARA, S. G O N D A and K. W A T A N A B E Electrotechnical Laboratory, Sakura-mura, Ibaraki, Japan
The X-ray photoelectron spectroscopic (XPS) measurements were carried out to elucidate the valence band pictures and bonding characters of NbSe3 and TaSe3 with quasi-one-dimensional structures. The valence band spectra agree well with the theoretical densities of states (DOS) after Bullet and reflect the two-dimensional (2-D) character in contrast with those of NbS3 and TaS3. The results are discussed in relation to their CDW formation temperatures.
1. Introduction In recent years the trichalcogenides of the group IV, V and VI transition metals have received considerable attention because of their unique properties such as charge density wave (CDW) instabilities or superconductivities. Common to their lattice structures is a onedimensional feature that each metal atom lies at the centre of a trigonal prism of chalcogen atoms; successive prisms are arranged in infinite chains along the b-axis. The structures of NbSe3 [1] and TaSe3 [2] are shown in fig. 1. These structures can be regarded as two-dimensional (2-D) layers (see fig. lb) of one-dimensional (lD) chains (see fig. la). Although these structures resemble each other, the transport properties are markedly different. NbSe3 shows two major resistivity anomalies at 145 and 59 K suggestive of CDW formation [3--6], although no such anomalies are observed in the temperature dependence of the resistivity of TaSe3 [7-10]. In order to explain the difference in such transport properties, Bullet [11, 12] calculated the band structures and densities of states (DOS) of NbSe3 and TaSe3, considering the small difference in the arrangement of these chains. However, there is no direct experimental evidence for these band structures. The present study aims to elucidate the electronic band structures of NbSe3 and TaSe3 by X-ray photoelectron spectroscopic (XPS) measurements.
a)
b
T OSe o M
b) ,0......0.....0.....0.0..... ..0....
NbSe3
>-¢
Fig. 1. Structures of NbSe3 and TaSe3. (a) Stacking along the b-axis; (b) projection of the structures on the ac plane. Open circles are at 0 and full circles at 1/2b.
0378-4363/81/0000-0000/$2.50 © North-Holland Publishing Company and Yamada Science Foundation
160
g. Endo et al/XPS study of NbSe3 and TaSe3
2. Experimental The crystals used in this study were grown in a temperature range of 650-900°C by heating a mixture of selenium and metal powder in an evacuated quartz tube. The resistivity measurements of the crystals confirm that NbSe3 shows C D W anomalies similar to those described in the literature [3-6], but no such anomalies appear in TaSe3. The crystals were taken out from a sealed reaction tube, arrayed on the sample holder in an atmosphere of argon and then introduced into the XPS analyzing chamber without exposing to air. The XPS spectra were obtained using an H P 5950A E S C A spectrometer with a resolution of 0 . 6 e V under a pressure of 10-9Torr. The core-level spectra reveal that the samples are free from oxygen. Measurements were repeated on samples from three batches which were synthesized by the same method. Little differences between batches were observed.
3. Results and discussion Fig. 2 shows the valence band spectra of NbSe3 and TaSe3 (solid line) obtained by subtracting the background due to inelastically scattered electrons. The accumulation times for these spectra were 18 h. Also in the figure the theoretical DOS after Bullet [12] are given for comparison (dashed line). The characteristic features of these experimental spectra are in good agreement with those of the theoretical DOS in the valence band width ( - 8 eV), the binding energy and the gross features. Thus, our XPS results give evidence that the band calculations of NbSe3 and TaSe3 after Bullet are valid. In particular, the band structure and DOS of NbSe3 are calculated for such extreme cases as a single 1-D chain (see fig. la) and a 2-D layer (see fig. lb) [11]. The difference of theoretical DOS pictures between 1-D and 2-D cases are quite marked. In the 1-D case the Fermi level falls in the middle of an isolated band which does not overlap with other bands. Such a situation is favourable for a Peierls' distortion into an insulating state. No such isolated band occurs in the 2-D layer. Probably, such an isolated band
NbSe3
!~
V t
I aII1
18
16
i a
~
II ,
14
aii
~.,L~;"
\,,~ i ,,
12
10
8
6
4
2
EF
12
10
8
6
4
2
EF
4 U v
TaSe3 m
•*In ~
18
16
14
Binding energy ( eV ) Fig. 2. XPS valence band spectra (solid lines) of NbSe3 and TaSe3 c o m p a r e d with the theoretical D O S (dashed lines).
might be observed as a shoulder at the top of the valence band in the XPS spectrum. Our XPS measurements for NbS3 and TaS3 reveal that the shoulder clearly appears at the top of valence band [13]. In contrast, no such shoulders are clearly observed in the XPS spectra of N b S e 3 and TaSe3. This fact suggests that NbS3 and TaS3 reflect the 1-D character against that of NbSe3 and TaSe3. Here, the degree of one-dimensionality might be related to the C D W transition temperature. TaS3 shows the C D W transition at 218 K [14-16] and NbSe3 at 145 and 59 K [3--6]. NbS3 is semiconductor below 6 0 0 K [17-19], which will be inferred to have the C D W transition temperature above room temperature from the electron diffraction [20] and XPS measurements [13]. TaSe3 shows the metallic behaviour above the superconducting transition temperature
K. Endo et al/XPS study of NbSe3 and TaSe3
(2.1 K) [7-10]. These facts imply that the degree of one-dimensionality increases in the following order, TaSe3, NbSe3, TaS3 and NbS3. Such a tendency to 1-D character in transition metal trichalcogenides corresponds well to the XPS results, although it is difficult to determine the hierarchy of 1-D character of TaSe3 and NbSe3 from the present study. Fig. 3 shows the core-level spectra of NbSe3 and TaSe3 such as Nb 3d, Ta 4f and Se 3p levels. The binding energies and the full widths at halfmaximum (FWHM) are listed in table I together with those of Nb, Ta and Se. From the peak intensity the atomic ratios of Se to Nb or Ta are determined as 2.98 or 3.03, respectively. These values agree well with those from the X-ray fluorescence analysis.
210
205 Ta/~
TaSe3
161
Table I Binding energies and full widths at half maximum (FWHM) of core-levels of NbSe3, Nb, Ta and Se Binding energy
Full widths at half-maximum
Sample
Core-level
(eV)
(eV)
NbSe3
Nb 3dsn Se 3p3/2 Ta 4b/2 Se43p3/2 Nb 3d5/2 Ta 4fTn Se 3p3/2
204.2 160.5 23.7 160.1 202.3 21.8 161.4
1.4 2.8 1.2 2.5 0.9 0.9 2.1
TaSe3 Nb Ta Se
As shown in table I the FWHM of Nb 3d, Ta 4f and Se 3p peaks of NbSe3 and TaSe3 are wider than those of their elements. In addition, the double splitting of Se 3d peak are clearly obser-
200 a4fT/2
? c D
3b
2'5
2b
eD
U v
O
2"
o
_c
m eel
1~5 TaSe3
1~,o /~Se3P~
60
Binding energy ( eV ) Fig. 3. Core-level spectra of NbSe3 and TaSe3.
5'5
Binding energy (eV) Fig. 4. Se 3d spectra of NbSe3, TaSe3 and pure Se.
162
K. Endo et al/XPS study of NbSe3 and TaSe3
ved in the pure Se spectrum but not in NbSe3 and V a S e 3 spectra (see fig. 4). This means the presence of several inequivalent bonds of Nb, Ta and Se atoms in NbSe3 and TaSe3, as is expected from their crystal structure (see fig. 1). One can guess such inequivalent bondings between Se atoms are the origin of the broadening of Se s-band located between 11 and 17 eV below the Fermi level, EF, in the NbSe3 and TaSe3 valence band spectra (see fig. 2). Charge transfers of Nb and Ta for NbSe3 and TaSe3 are estimated at about +0.2 electrons. A detailed description of the method for estimation is given elsewhere [21]. These values are smaller than those of NbS3 and TaS3 [22] which suggests that the ionic bondings of selenides are weaker than those of sulfides. In conclusion, the electronic structures of NbSe3 and TaSe3 with quasi-one-dimensional structures were characterized through the use of XPS. The valence band spectra were in agreement with the theoretical DOS after Bullet. In particular, the XPS spectra of NbSe3 and TaSe3 reflect the 2-D character in contrast with those of NbS3 and TaS3. This is consistent with the order of dimensionality expected from their CDW formation temperatures. References [1] A. Meerschaut and J. Rouxel, J. Less-Common Metals 39 (1975) 197.
[2] A. Bjerkelund, J.H. Fermor and A. Kjekshus, Acta Chem. Scand. 20 (1966) 1836. [3] J. Chaussy, P. Haen, J.C. Lasiaunias, P. Monceau, G. Waysand, A. Waintal, A. Meerschaut, P. Molinie and J. Rouxel, Solid State Commun. 20 (1976) 759. [4] P. Monceau, N.P. Ong, A.M. Portis, A. Meerschaut and J. Rouxel, Phys. Rev. Lett. 37 (1976) 602. [5] N.P. Ong and P. Monceau, Phys. Rev. B16 (1977) 3443. [6] N.P. Ong, Phys. Rev. B17 (1978) 3243. [7] T. Sambongi, M. Yamamoto, K. Tsutsumi, Y. Shiozaki, K. Yamaya and Y. Abe, J. Phys. Soc. Japan 42 (1977) 1421. [8] P. Haen, P. Monceau, B. Tissier, G. Waysand, A. Meerschaut, P. Molinie and J. Rouxel, Ferroelectrics 17 (1977) 447. [9] M. Yamamoto, J. Phys. Soc. Japan 45 (1978) 431. [10] P. Haen, F. Lapierre, P. Monceau, M.N. Regueiro and J. Richard, Solid State Commun. 26 (1978) 725. [11] D.W. Bullet, Solid State Commun. 26 (1978) 563. [12] D.W. Bullet, J. Phys. C: Solid State Phys. 12 (1979) 277. [13] K. Endo, H. Ihara, K. Watanabe and S. Gonda, in press, J. Solid State Chem. [14] T. Sambongi, K. Tsutsumi, Y. Shiozaki, M. Yamamoto, K. Yamaya and Y. Abe, Solid State Commun. 22 (1977) 729. [15] K. Tsutsumi, T. Sambongi, S. Kagoshima and T. Ishiguro, J. Phys. Soc. Japan 44 (1978) 1735. [16] G. van Tendeloo, J. van Landuyt and S. Amelinckx, Phys. Stat. Sol. (a) 43 (1977) K137. [17] L.A. Grigoryan and A.V. Novoselova, Dokl. Akad. Nauk SSR 144 (1962) 795. [18] F. Kadijk and F. Jellinek, J. Less-Common Metals 19 (1969) 421. [19] K. Endo, M. Hirabayashi and S. Gonda, ACS/CSJ Chemical Congress (Hawaii) (1--6 Apr. 1979) Abstr. of Papers, Part II, Phys. 220. [20] F.W. Boswell and A. Prodan, Physica 99B (1980) 361. [21] H. Ihara, Res. Electrotechn. Lab. 775 (1977)98. [22] K. Endo, H. Ihara, K. Watanabe and S. Gonda, to be published.