DEPENDENCE OF THE ELECTRONIC STRUCTURE OF YBa2Cu3O7-∂ CERAMICS ON OXYGEN STOICHIOMETRY : A PHOTOEMISSION AND PHOTOABSORPTION STUDY

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DEPENDENCE OF THE ELECTRONIC

STRUCTURE OF YBa2Cu3O7-∂ CERAMICS ON

OXYGEN STOICHIOMETRY : A PHOTOEMISSION

AND PHOTOABSORPTION STUDY

M. Maurer, T. Gourieux, G. Krill, M. Ravet, H. Tolentino, A. Fontaine

To cite this version:

M. Maurer, T. Gourieux, G. Krill, M. Ravet, H. Tolentino, et al..

DEPENDENCE OF THE

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JOURNAL DE PHYSIQUE

Colloque C8, Supplement au no 12, Tome 49; dkcembre 1988

DEPENDENCE OF THE ELECTRONIC STRUCTURE OF YBa2Cu307-, CERAMICS ON OXYGEN STOICHIOMETRY: A PHOTOEMISSION A N D PHOTOABSORPTION STUDY

M. Maurer (I), T. Gourieux (2), G . Krill (2), M. F. Ravet (I), H. Tolentino (3) and A. Fontaine (3)

(I) Laboratoire CNRS-St Gobain (UM 37 CNRS) BP 109, 54704 Pont-&-Mowson, France

(2) Laboratoire de Physique du Solide (UA 155 CNRS) Universite' de Nancy I, B P 239, 54506 Vandoeuvre-les- Nancy, France

(3) LURE ( L P CNRS) Bait 209 d, Universite' de Paris-Sud, 91405 Orsay, France

A b s t r a c t . - X-ray photoemission (XPS) and X-ray absorption (XAS) have been performed on YBa2Cu307-8 ceram- ics as a function of a(0.1

<

8

<

0.7). The results show the modifications of the Cu electronic structure around the superconducting-semiconducting transition. XPS and XAS experiments give complementary informations allowing the identification of CU+, c u 2 + and the so-called c u 3 + configurations as a function of

a.

The nature of the ground state, in the high

T,

ceramics compounds, is still a n open question. the study of YBazCusO;.-a compounds presents the advantage that the ground state can be changed from a superconducting t o a semiconducting state by varying the oxygen stoichiometry [I]. The oxygen content is de- termined, at equilibrium, by the oxygen partial pressure and by the temperature. Thus XAS experiments can be carried out in-situ by taking advantage of the dispersive station a t LURE (Orsay). A Cu K edge spectrum [2] is recorded in about 1 s, thus opening also the possibility t o study the kinetics of the transition. High energy spectroscopies, like XPS and XAS, are very powerful techniques for the investigations of the ground state, even if their interpretation may be seriously complicated by final states effects. How- ever these techniques have clearly indicated that the ground state of high T, compounds is highly correlated

ressembling the ground state of Cerium intermetallics to some extent [3-41. Under some interpretations the spectroscopic experiments can provide the percentage of the several electronic configurations which built the ground state wave function [6]. For instance, in the presence case, we aim t o determine the signature of Cu atoms in the [3d1°), 13d9)

,

[3d8) etc

...

which are the so-called mono, di- and trivalent configurations of copper ions.

The XPS experiments were performed in a conven- tional apparatus using the Mg K, radiation [5]. We essentially studied the Cuz, and 01, core level spectra. In figure 1, the Cu2,(3/2) spectra are reported for several values of 6. The important fact is the strong variation in the asymmetry of the low binding energy structure. This structure is the unambiguous signature of the presence of 13d1°) configurations in the fi-

nal state; such structures are identified as shake-down from a 13dg) initial state. Their intensities are di- rectly proportional t o the strenght of hybridization.

Fig. 1. - Cu 2p (3/2) XPS spectra of YBazCu3O7-g for different oxygen stoichiometry 8 .

The higher binding energy structure reflects the contributions of those configurations which remain 13d9) in the final state. Finally we argue that it is impossible, from XPS experiments only, t o decide if /3d1°) configurations are present in the ground state; nevertheless we can exclude any significant trivalent configuration (in the sense of 13ds)) in the ground state. The asymmetry of the shake-down structures decreases with 6; the simplest way t o explain the modification is t o as-

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C8

-

2132 JOURNAL DE PHYSIQUE

sume that these lines are built from, a t least, two contributions separated by about 2 eV. The occurrence of the high

Tc

state can be thus correlated with the increase in the higher energy component. From simple arguments [5], we ascribed such component to a shake- down from a 3d

I

L configuration where L represents

9-) -

a hole in the Cu-0 ligand [3]. This configuration can be formally considered as trivalent copper. As

a

goes to zero, the intensity of this component increases up t o 40 %

,

which is not far away from the value expected from neutrality arguments (i.e. 33 % ).

XAS K edges of copper of YBaCusO7-a were recorded upon changing the oxygen pressure from 150 to torr, and vice versa, at T = 600 OC (Fig. 2a). Using previous results [I], we deduce that 8 varies between 0.2 and 0.6 in such a cycle. The absolute dif-

ferences are reported (Fig. 2b), since XAS spectra are measured in-situ from the same sample. First we ob- serve perfect reversibility upon cycling 8. Secondly several signatures are evident on the difference spectra. The most resolved one, which is located at lower photon energy, is assigned to CU' (i.e. 13dl0)) (feature A) configurations in the ground state. As

a

goes to 0.6, this feature significantly increases. This structure r e v d s the formation of Cu+ configuration, most probably located on the Cu(1) sites, as the oxygen concentration decreases [I]. Such contributions can be numerically wipped out from the spectra, allowing to observe subtle variations at the remaining part which are associated t o the other copper configurations. The dashed lines in figures 2e, f, g represents the results after this substraction. The spectra which result here from an admixture of cu2+ and

"

cu3+

'

compare with K edge of copper in either divalent or "trivalent" compounds. Depending on a , the shift of the reduced spectra is about 2 eV, resulting from a change in the respective weight of the unresolved c u 2 + (/3d9)) and

"

cu3+

"

(13dgL)) spectra [7] (fea- tures B and C respectively). Thus XAS experiments confirm and complete the informations we obtain from XPS: the ground state of high

Tc

superconductors is made of both 13d1°)

,

13d9) and 3 d 9 ~

I

_-)

coafigurations, with much larger hybridization between the latters; the appearance of the superconducting state can be correlated with the increase of the 13dg4) configurations at the expense of the 13d1°) cohfiguitions. We finally want to underline that the method we use to made the XAS experiments avoids the use of extra references for the K edge analysis.

0 10 20 30 40 50 ENERGY (eV)

Fig. 2. - a) Cu K edge XAS spectra at 600 OC and

torr 0 2 ; b) Idem as a) at 600 Torr 0 2 ; c) difference of XAS spectra at T = 600 OC for

a

changing from 0.32 to 0.55 (deacreasing pressure); d) idem as c) d changing from 0.55 t o 0.32 (increasing pressure) e, f, g) difference of XAS spectra at 450 O C corrected for the 3d1° contribution at sev-

eral stages of the oxygen uptake: d decreases from e) to g), the apparent shift of the edge corresponds t o an increasing fraction of 3 d 9 ~

_-

as compared to 3d9 species. The inflection points of the absorption are approximately located at the arrows A (3d1°)

,

B (3d9) and C

fl] Yamaguchi, S. et al., J p n J. Appf. Phys. 27

(1988) L179;

Kishio, K. et al., J p n J. Appl. Phys. 26 (1988) L1228.

[2] Dartyge, E. et al., Nucl. Instrum. Methods A 246

(1986) 1482.

[3] Bianconi, A. et al., Solid State C o m m u n . 63

(1987) 1135.

[4] Baudelet, F. et al., 2. Phys. B 69 (1987) 141. [5] Gourieux, T. et al., Phys. Rev. B 37 (1988) 7516. [6] Gunnarson, 0. et al., Phys. Rev. B 28 (1987)

4315.