Ultraviolet photoemission spectroscopy of (TaSe4)2I

~

Solid State Communications, Vol.55,No.12, pp. I049-1052, Printed in Great Britain.

ULTRAVIOLET

PHOTOEMISSION

1985.

SPECTROSCOPY

0038-1098/85 $3.00 + .00 Pergamon Press Ltd.

OF ( T a S e 4 ) 2 1

E. Sato ~, K. Ohtake, R. Yamamoto and M. Doyama Department of Metallurgy and Materials Science, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan T. Mori, K. Soda and S. Suga Synchrotron Radiation Laboratory, Institute for Solid State Physics, The University of Tokyo, Tanashi-shi, Tokyo 188, Japan K. Endo Electrotechnical

Laboratory, Umezono, Sakura-mura, Niihari-gun, Ibaraki 305, Japan (Received 24 June 1985 by T. Tsuzuki )

The vacuum ultraviolet photoemission spectra of quasi-one-dimensional charge density wave ( CDW ) system, (TaSe4)21 , were measured for photon energies between 32 and i00 eV at room temperature ( in the normal phase ) and at about I00 K ( in the CDW phase ). The spectrum of Ta 4f core-levels has shown no additional splitting due to the two different Ta sites. The spectra of the valence and conduction bands have revealed the resonant enhancement for the excitation of the Ta 5p core states, which demonstrates the remarkable hybridization of Ta 5d orbitals with Se 4p orbitals with binding energies smaller than 4 eV. In the CDW phase, the partial cross section decreases for both Ta 5d bands and Se 4p bands with Ta 5d components.

Recently, much attention was paid to a new family of chain-like compounds, (MX4)nY ( M = Nb, Ta ; X = S, Se ; Y = Br, I ) [1]. (TaSe4)2I belongs to this family and is known to be a quasi-one-dimensional conductor which undergoes the charge density wave ( CDW ) transition at 263 K. In the CDW phase below this transition temperature, the nonlinear electrical conduction, current oscillation [2,3] and the periodic lattice distortion [4,5] were observed. These properties are typical of one dimensional CDW systems. (TaSe4)21 crystal consists of (-TaSe4- ) chains, which are parallel and well separated from one another by the inter-chain iodine atoms [6]. These iodine atoms induce slight modifications of the Ta-Se bond length, providing two kinds of Ta sites. According to the purely ionic crystal model, (TaSe4)2I unit cell has been thought to be formulated as 4( Ta 4+ Ta 5+ 4(Se2)2- I- ). So we expect that the valence and conduction bands are constructed of 4Ta(5d) l, 32Se(4s) 2 and 32Se(4p) 5. From the calculations of the band structure [7], the bottom of the conduction bands, which are partly occupied by 4 electrons, has the character of Ta 5dz2, and the top of valence bands lies 0.6 eV below the bottom of the conduction bands. The angular correlation measurement of positron annihilation showed that the Fermi surface exists preferentially for the conducting direction, which is the origin of one dimensional

conductivity of this material [8]. The aim of the present study is to investigate the band structure of (TaSe4)2I from the view point of electron density of states by means of vacuum ultraviolet photoemission spectroscopy ( UPS ). Single crystals used in this experiment were prepared by direct reaction of constituent materials, i.e. tantalum, selenium and iodine. An evacuated quartz tube with nearly stoichiometric mixture of constitutions was set in a furnace with a temperature gradient from 920 to 660 K for a week. Black single crystals were obtained at the place with a temperature of about 770 K with the maximum size of 2 x 2 x i0 mm 3. The cleavage plane was found to be (llO) plane by the X-ray Laue diffraction method. UPS spectra were measured at the second beam line of SOR-RING, the 400 MeV electron s t o r a g e ring of S y n c h r o t r o n Radiation Laboratory, Institute for Solid State Physics, The University of Tokyo. The samples were cleaved in situ ( in the ultra high vacuum b e t t e r than 3 x 10 -I0 Torr ), and the photoemission spectra were measured by use of a double stage cylindrical mirror analyzer and a modified Rowland-type monochromator. The resolution of the monochromator was set to 0.2 eV at the photon energy ( h~ ) of 50 eV, and that of the analyzer to 0.2 or 0.3 eV. First, angle integrated UPS spectra were measured at room temperature for different excitation photon energies between 32 and i00 eV. The sample was so mounted as the crystallographic <001> direction of the sample was parallel to the photon polarization. The energy distribution curves

Present address : The Institute of Space and Astronautical Science, Komaba, Meguro-ku, Tokyo 153, Japan. 1049

050

( EDC's ) were normalized by the photon flux. Then, angle integrated UPS spectra were measured at about i00 K as w e l l as at room temperature for two different photon energies of 32 and i00 eV. In this measurement, the photons were polarized in direction. Figure 1 shows the EDC of angle integrated UPS of (TaSe4)21 for photon energy of I00 eV measured at room temperature. The structures [

[

!

I

i

|

I

et al. [i0] assert that they have observed two different Ta sites in the EDC of Ta 4f corel e v e l s by XPS experiment ( one has the binding energies of 23 and 25 eV, and the other has those of 26 and 28 eV ), though the r e l a t i v e intensity of the two doublets in Fig. 2 of I

I

hP=

-

0 r~

100 eV

I

Ta4f w2 -

,.%

~, -

a

I

Ta 4f 5/2

h~= IOOeV

-j

I

(TaSe4)21

;

(Ta Se4)21 p

v

Vol. 55, No. 12

ULTRAVIOLET PHOTOEMISSION SPE CTRO S COPY OF (TaSe 4) 21

,

"

~ .....,~..,m:.~~

w

hO xo

28

,

I

I

60

I

40

I

20

Binding Energy

0

(eV)

Fig. i The angle integrated UPS spectrum of (TaSe4)21 for photon energy of i00 eV measured at room temperature. The structures above E B = 18 eV are related to the valence and conduction bands, and four sets of doublet peaks below E B = 20 eV, which are indicated by arrows, are assigned to individual core states as shown in Table I. above E B ( binding energy from the Fermi level ) = 18 eV are r e l a t e d to the v a l e n c e and conduction bands and will be discussed later. B e l o w E B = 20 eV, four sets of doublet peaks, which originate from individual core states, are indicated by arrows. The binding energies of these core-levels of (TaSe4)21 are compared with those of corresponding elements [9] in Table I. In the EDC o b t a i n e d by a p r e c i s e r measurement for photon energy of i00 eV at room temperature around Ta 4f core-levels ( Fig. 2 ), we cannot recognize any trace of additional structure besides the spin-orbit splitting, due to the two different Ta sites. The total resolution in this measurement was 0.7 eV of full width at half maximum ( FWHM ). Kikkawa Table I C o r e - l e v e l binding energies of (TaSe4)21 and corresponding elements. (TaSe4)21 (eV) Ta 4f7/2 4f5/2 Ta 5P3/2 5Pi/2 I 4d5/2 4d3/2

Se 3d5/2 3d3/2

23.8 25.7 35.2 45.2 48.2

elements [9] (eV) 23.0 24.8 33.9, 34.1 42.4, 42.1 )50

49.9

55.0 55.5

54.64 55.47

I

I

I

26 Binding

I

24 Energy

I

22

(eV)

Fig. 2 The angle integrated UPS spectrum of (TaSe4)2I for photon energy of I00 eV measured at room temperature around Ta 4f core-levels. By the resolution of this measurement ( FWHM = 0.7 eV ), only one set of doublet peaks is observed. ref. i0 is much deviated from one. The present result, however, suggests that the 4f corel e v e l s of two different Ta sites are observed within one doublet peaks by its resolution. In the purely ionic crystal model, we can rather represent (TaSe4)21 unit cell as 4( 2Ta 4.5+ 4(Se2)2- I- ), than 4( Ta 4+ Ta 5+ 4(Se2)2- I- ). Figures 3 (a) to (e) show a part of the EDC's of angle integrated UPS of (TaSe4)2I in the region of valence and conduction bands at room temperature, which are measured for different excitation photon energies between 32 and I00 eV. From the band calculation, peak I is assigned to the occupied conduction bands, which are dominated by Ta 5dz2 orbitals. Besides, the other structures in the region from E B = 1 to 8 eV are ascribed to the valence bands Se 4p states. In Figure 4, the intensities of peaks i, 4 and 5 in Fig, 3 are plotted as functions of the excitation photon energy, where the intensities are evaluated from the peak height of the EDC measured from the zero line. Thus we get the constant initial state ( CIS ) spectra, which represent the partial cross sections of the Ta 5d and Se 4p states. As for peak i, we observe two m a x i m a at hv = 39 and 48 eV. We can interpret them as the resonance effect between the directly excited states of Ta 5d valence electron and the final states realized through the Auger decay of the Ta 5p core excited states. The s i m i l a r r e s o n a n c e e f f e c t was observed in tantalum di-selenides ( IT- and 2HTaSe 2 ) [ii]. For the resonant behavior of peak i, the following two processes are considered to occur in the unit cell of (TaSe4)21 :

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I

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I

~s

(TaSe4)2

../~ .% - %,'; ;

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( Ta Se4)21

..".: .... ":""... L)

h~'= 32 eV

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tO

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1051

ULTRAVIOLET PHOTOEMISSION SPECTROSCOPY OF (TaSe4)21

Vol. 55, No. 12

% t.

v

oO :

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32 eV

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Binding

2

Energy

..

ov

0

(eV)

Fig. 3 Photon energy dependence of the angle integrated UPS spectra of (TaSe4)2I at room temperature. The energy position of the peak 1 ( E B : 0.6 eV ) is assigned to the occupied conduction bands, which are dominated by Ta 5dz= orbitals. Besides, the other structures in the region from E B = 1 to 8 eV are ascribed to the valence bands Se 4p states. I t-

=.5

"~

I

I

I

m

b 100 K

%@

;..,

IJ) C

c.

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Cb

•~ 06

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~ 1.4

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I

e-

"~ "'w. d. 45 eV

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(Ta Se4)2 I

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I I 2 0 Energy

(eV)

Fig. 5 (a) and (b) : The angle integrated UPS spectra of (TaSe4)2I for photon energy of 32 eV measured at room temperature and at about I00 K, respectively. (c) : The relative intensity of (a) and (b), where (a) is divided by (b). The partial cross sections of Ta 5d bands decrease in the CDW phase, and in addition, those of Se 4p bands with Ta 5d components decrease a little. 5p485d4-->5p485d3+e

(direct excitation)

5p485d4->5P 475d5 ->5p485d3+e

(02 3 excitation) (super Coster-Kronig decay).

=4 0

~

\,\°

~3 u}

82 0

p~k4 L_ n$

a- 0

30

40 P h o t on

50 Energy

60 (eV)

Fig. 4 Photon energy dependence on the photoemission intensity of the three dominant peaks (1, 4 and 5) of (TaSe4)2I. We can clearly observe the resonant enhancement of peaks 1 and 4 corresponding to the Ta 5p core excitation in these CIS spectra.

The strength of this interference may strongly depend on the localized character of the Ta 5d states. We can also observe the resonant enhancement corresponding to the Ta 5p core excitation in the CIS spectrum of peak 4, but not in the case of peak 5. This type of Auger decay of the 02 3 excited states for Se 4p bands I will be effectlve only when Ta 5d orbitals are strongly hybridized with the Se 4p orbitals. Accordingly, we conclude that the hybridization of Ta 5d orbitals with Se 4p orbitals is remarkable above E B = 4 eV. Figures 5 (a) and (b) show the EDC's of angle integrated UPS of (TaSe4)2I for photon energy of 32 eV at room temperature and at about i00 K. (TaSe4)21 is in the normal phase at room temperature and in the CDW phase at lO0 K. Here, the EDC's were so normalized that the peak heights of Ta 4f c o r e - l e v e l s are the same at

I052

ULTRAVIOLET PHOTOEMISSION SPECTROSCOPY OF (TaSe4)2I

both temperatures, since the structures of the EDC in the core-level regions seem to be not affected by the CDW transition. The difference between Fig. 3 (a) and Fig. 5 (a) is ascribed to the difference of the direction of the excitation photon polarization. The intensities of peaks l, 2, 3, and 4 in the CDW phase ( Fig. (b)) are smaller than those in the normal phase ( Fig. 5 (a)). This can be clearly seen from Fig. 5 (c), where the spectrum in the normal phase is divided by that in the CDW phase. The partial cross sections of Ta 5d bands are found to decrease in the CDW phase, and in addition, those of Se 4p bands with Ta 5d components decrease a little. This suggests that the p-d hybridization is influenced by the lattice distortion caused by the CDW transition, although further studies are required to understand this phenomenon. In c o n c l u s i o n , we h a v e o b s e r v e d no additional structure of Ta 4f core-levels besides the spin-orbit splitting, ascribable to

Vol. 55, No. 12

the two different Ta sites in (TaSe4)2I. The CIS spectra in the region of the conduction and valence bands have shown the resonant enhancement corresponding to the Ta 5p core excitation. From the strength of the enhancement, we have found that the hybridization of Ta 5d orbitals with Se 4p orbitals is noticeable above E B = 4 eV. Finally the EDC in the CDW phase has been compared with that in the normal phase, and it has been o b s e r v e d that the p a r t i a l cross sections of both Ta 5d bands and Se 4p bands with Ta 5d component decrease in the CDW phase, suggesting that the p-d hybridization is influenced by the lattice distortion caused by the CDW transition. Acknowledgement - The authors are very grateful to Dr. S. Yoshida for encouragement, to Mr. S. Koshiba for preparing the samples, and to all the members of Synchrotron Radiation Laboratory of Institute for Solid State Physics for their help with the UPS experiments.

REFERENCES [i]

[2]

[3] [4] [5] [6] [7]

P. Gressier, A. Meerschaut, L. Guemas, J. Rouxel and P. Monceau : J. Solid State Chem. 51 (1984) 141. Z. Z. Wang, M. C. Saint-Lager, P Monceau, M. Renard, P. G r e s s i e r , A. M e e r s c h a u t , L. G u e m a s and J. R o u x e l : S o l i d State Commun. 46 (1983) 325. M. Maki, K. Kaiser, Z. Zettl and G. Gruner : Solid State Commun. 46 (1983) 497. H. Fujishita, M. Sato and S. Hoshino : Solid State Commun. 49 (1984) 313. C. Roucau, R. Ayroles, P. Gressier and A. Meerschaut : J. Phys. C17 (1984) 2993. P. Gressier, L. Guemas and A. Meerschaut : Acts Cryst. B38 (1982) 2877. P. Gressier, M. -H. Wangbo, A. Meerschaut and J. Rouxel : Inorg. Chem. 23 (1984) 1221.

[8] K. Ohtake, E. Sato, S. Koshiba, R. Yamamoto, M. Doyama, and K. Endo : to be published in Proc. 7th Int. Conf. Positron Annihilation, New Delhi, 1985. [9] M. Cardona and L. Ley : Photoemission in Solid I, ed. M. C a r d o n a and L. Ley, (Springer, Berlin, 1978) p. 265. [i0] S. Kikkawa, S. Uenosono and M. Koizumi : Proc. Int. Symp. Nonlinear Transport and Related Phenomina in fnorEanic Quasi One Dimensional Conductors, Sapporo, 1983, p. 245. [II] H. Sakamoto, S. Suga, M. Taniguchi, H. Kanzaki, M. Yamamoto, M Seki, M. Naito and S. Tanaka : Solid State Commun. 52 (1984) 721.