XES studies of YBa2Cu3Oy and GdBa2Cu3Oy, single crystals

XES studies of YBa2Cu3Oy and GdBa2Cu3Oy, single crystals

Physica C 153-155 (1988) 137-138 North-Holland, Amsterdam XES STUDIES OF YBa~Cu~O v a ~. Gabriel JASIOLEK, AND GdBa~Cu~O v , ~, ~ SINGLE CRYSTALS ...

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Physica C 153-155 (1988) 137-138 North-Holland, Amsterdam

XES STUDIES OF YBa~Cu~O v a ~.

Gabriel JASIOLEK,

AND GdBa~Cu~O v , ~, ~

SINGLE CRYSTALS

Halina NICULESCU and Anna PAJACZKOWSKA

I n s t i t u t e of Physics.

Polish Academy of Sciences. 02-668 Warsaw. Poland

Mean valence of copper ions in the Y-Ba-Cu-O and Gd-Ba-Cu-O single crystals has been estimated bv using the X-ray emission spectroscopy to be +2.04 and +2.03.respectivelv. lt was confirmed that the binding energy of 3d electrons i s eoual to 3.5 eV in both cases. I t was found that the binding energy of a state close to the Fermi l e v e l , involving a s a t e l l i t e maximum in the CuL~ spectra of the c r y s t a l s at +2.6 eV and +2.8 eV. is equal t o 0.6 eV and 0.4 eV. respectively.

i.

INTRODUCTION Since the discovery of the superconducting properties of metallic oxides the valence of cations forming such compounds has been one of the main feature characterisino high temperature superconductors. In s p i t e of some d i f f e rences between authors describing c o p p e r valence .common observation has been repeated : valence of copper in new superconductors is close t o 2+ and the superconducting properties increase with an increase of Cu(3+) ion concentration (I). The main r o l e in the valence bands of the YBa~Cu,O v ~~ (YBCO) and GdBa2Cu30 v, (GBCO) crystals plays the overlap of 3d orbitals of Cu and 2p of oxvoen (2). To touch experimentally the valence bands of these compounds the electronic transitions connected with 3d shell of Cu have been studied (3). The present paper aims to present r e s u l t s of the X-ray emission epectroscoov (XES) of the YBCO and GBCO single cr,/stals. It focuses on experimental determination of the mean valence of copper and on comparison between XES r e s u l t s and the t h e o r e t i c a l description of Cu 3d dens i t v of states (2,4).

2. CRYSTALS AND METHOD Single crvstals of YBCO and GBCO were grown from the nonstoichiometric melt of composition with molar ratio Y(Gd):Ba:Cu = i:~:!0. We have found that the addition of 2% Bi203 leads to an increase of the crystal sizes. Crystals grew from melt heated up to 1 0 5 ~ C and then slowly cooled to room temperature in air. To improve the HTc properties the crystals were annealed in flowing oxvoen.

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The c r y s t a l s have been exposed on electron beam in the JXA-50A electron probe microanalvmer (EPMA).An accerelating voltage of 25 kV and a beam current of 0.3 HA have been used. These conditions allow to l i m i t the influence of the electron beams on the sample compositions. Measurements have been made on the CuLo l i n e of copper and GdL~2,15 l i n e of gadolinium.

The

single c r v s t a l spectrometers of the Johann type with a RAP analysing c r y s t a l and with a LIF one have been used, respectively. Energetic resolut i o n f o r the analvsis of c o p p e r spectrum was eoual to 0.06 eV and 0.6 eV f o r gadolinium one.

3. RESULTS AND DISCUSSION EPMA q u a n t i t a t i v e analysis confirmed the proportions expected between cations in the single c r y s t a l s and revealed 0.4 wt.% of Ai.The impurity was the main cause of rather low. 80K, transition temperature of these crystals. The morphology of the crvstals showed the characteristic features similar as reported in a few last reports. Main maxima in Fig.l correspond to the electronic transition between 3d shell of Cu(2+) configuration and 2p3/2 subshell. These maxima have been normalized and located at zero position on the energy scale. Lengths of the vertical lines called A,B and C correspond to the relative intensities of Lorentzian curves connetted with decomposition of the spectra for as grown single c r y s t a l s . The A maximum corresponds to the t r a n s i t i o n between 3d shell of Cu(3+) configuration and 2o3/2 subshell (3). I t s r e l a t i v e i n t e n s i t y was estimated to be 5.5 % f o r YBCO c r y s t a l s and 4 % f o r GBCO ones. These values can be treated also as the f a c t o r s of the Cu(T+)/Cu(2+) r a t i o . When

G. Jasiolek et al. / XES studies of single crystal YBaeCu3Q

138

When the areas under the Lorentzian curves are taken into consideration, then the values of this ratio change to 3% and 2%,respectively. The position of this maximum is +1.5 eV for both crystal types.

I.

A e

c

1

J

I. . . . . . -40

-2

0 2 4 6 eV RELATIVE ENERGY

FIGURE 1 CuL~ X-ray emission spectra of an YBCO c r y s t a l [ I ] and a GBCO one [2] (dashed l i n e s c o r r e s pond t o annealed c r y s t a l s ) . Considering the binding energy of 2p3/2 e l e c t r o n s (3) and the r e s u l t s obtained the binding energy of 3d e l e c t r o n s was found to be 3.5 eV. The B maximum corresponds to the t r a n s i t i o n between a s t a t e l y i n g close to the Fermi l e v e l of these compounds (2) and 2p3/2 subshell. It was confirmed t h a t i t s r e l a t i v e intensity is equal t o 7~5 % f o r YBCO and 9% f o r GBCO. I t was observed t h a t the B maximum s h i f t s from +2.6 eV t o +2.8 eV coming form YBCO to GBCO. The r e s u l t obtained gives f o r the binding energy of the s t a t e mentioned the values of 0.6 eV and 0.4 eV respectively. The C maximum i s connected with a Coster -Kronio t r a n i s t i o n (5) . I t was found t h a t i t s i n t e n s i t y i s equal t o 15.5 % of the main i n t e n s i t y f o r the both cases. For the annealed c r y s t a l s the C maxima change only. Thev increase by about 15 % of the r e l a t i v e i n t e n s i t i e s of as grown c r y s t a l s . Their p o s i t i o n correspond to +4.7 eV. Main maxima in Fig.2. are due to the t r a n s i t i o n 4d - 2p3/2 f o r gadolinium. Furthermore. the D maximum i s caused bv the m u l t i p l e t s p l i t t i n g of 4d s h e l l . This s p l i t t i n g depends s t r o n o l v on 4f e l e c t r o n s . I t was r e g i s t e r e d t h a t f o r a ceramic superconducting GBCO sample the D maximum i n t e n s i t y i s equal to 17 % and 12 % f o r the GBCO s i n g l e c r v s t a l s . The annealed GBCO c r y s t a l s are charmcterised bv the 2 eV s h i f t of the D maximum .

-20

0

20

40

R E L A T I V E ENERGY, eV GdL~32, 15 X-ray

FIGURE 2 emission spectra

of

a

GBCO

ceramic [I] and a GBCO single crystal [2] : dashed line corresponds to an annealed crystal. Taking i n t o account the change of the D maximum i n t e n s i t y observed i t can be estimated t h a t the e f f e c t i v e p a r t i c i p a t i o n of the 4f e l e c t r o n s of Gd in the valence band of the s i n g l e c r y s t a l s increases by the f a c t o r of about 30 % , in comparison with the ceramic sample.

4. CONCLUSIONS The results obtained allow the followino conclusions: i> mean valence of Cu ions in YBCO and GBCO s i n g l e c r y s t a l s do not change markedly a f t e r annealing of the c r y s t a l , i i ) the value of binding energy of 3d e l e c t r o n s ,3.5eV, is in a g o o d agreement with the theoretical reports (2,4) and iii> a state close to the Fermi level, having for YBCO theoretical and experimental value of energy equal to 0.6 eV. shifts to 0.4 eV for GBCO single crystals.

REFERENCES (I) (2) (3) (4) (5)

C.N.R. Rao, P. Gangulv et al. Mat. Res. Bull. 22 (1987) 849. L.F. Mattheiss , Phys. Rev. Lett. 58 (1987) 1028. N. Mori, y. Takano and H. Ozaki, Jpn. J. ADpl. Phys. 26 Suppl.26-3 (1987> 1017. W.Y. Ching, X. Yongnian et al. Phys. Rev. Lett. 59 (1987) 1335. T. Ishii, M. Taniguchi et al. Phys. Rev. B 33 (1986) 5664.