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Anomalies in the CuO structure of EuBa2Cu3O7−δ at 110 K and 250 K
P h y s i c a C 153 North-Holland,
155 ( 1 9 8 8 ) 2 7 0 - 2 7 1 Amsterdam
ANOMALIES IN THE Cu-O S T R U C T U R E OF EuBazCu307_ ~ AT 110 K AND 250 K Udo Murek, Katharina Keulerz and Jtirgen Rtihler II. Physikalisches lnstitut, Universit~t z u KSln, Ziilpicherstr. 77, 5000 K S l n 41 , Fed. Rep. Germany
The temperature dependence (77 - 300 K) of the Cu - O bonds in EuBa2Cu3OT_ s has been investigated through the extended x-ray absorption fine structures (EXAFS) of Cu. The absorption spectra were recorded from a compact sample in the fluorescence mode simultaneously with the electrical resistance. Inereasiug the temperature the effective Cu-O Debye-Waller factors &oz (T) exhibit an anomalous kink at 110 K and a strong singularity at 253 K. The anomalies are interpreted through a characteristic temperature dependence of the oxygen defect structure.
1. INTRODUCTION The oxygen stoichiometry and oxygen defect structure are crucial quantities determining whether the RE-Ba-Cu-O systems become superconductors or not. Superconducting RE Ba2 Cu307_ s crystals have ordered vacancies along the c axis andalong the a-b directions The latter are responsible for the formation of the orthorhombically distorted unit cell and the resulting formation of twin lamellae in the realstructure of the crystals. Here we report on significant temperature dependences of the oxygen defect structure at 253 K and 110 K as extracted from the photoelectron interference pattern occuring beyond the photoabsorption threshold (extended x-ray absorption fine structure, EXAFS) of the Cu Is (K) level in EuBa2Cu307-s. 2. EXPERIMENT We have measured the temperature dependence (83 - 300 K) of the Cu K- EXAFS in the fluorescence mode from compact (8 mm diameter, 2 mm thick), sintered EuBazCu3 O7-s. The sample was prepared through the usual solid state reaction and showed the 1-2-3 structure with the following lattice parameters (at 300 K) : a= 3.82 ~, b = 3.87 ~, c = II. 58 ~. The x-ray absorption spectra were recorded in the Hamburger Synchrotronstrahlungslabor. We used the fluorescence mode for being able to trace ,simultaneously the x-ray absorption and the electrical resistance. A cylindrically shaped ionisation chamber served for the detection of 0921-4534/88/$03.50 ©ElsevierSciencePublishers B.V. (North-Holland Physics Publishing Division)
the fluorescence signal, and a 4-point DC technique for the measurement of the electrical resistance. The effect of selfabsorption on the EXAFS fluorescence signal from this "thick" sample was found to be ~ 0.5g from comparison with spectra traced in the transmission mode from finely ground powder. The sample i
I
I
i
1
I
i
I
0L 83 K "-.~//r'~ 0.0/.,-
110K~/
r-
2 5 6 K ~
E
253K
EU
Ba, Cu
# 0,02
,,o
,
0
1
,
,
,
2 3 Reduced Distance [~]
,
,
4
Fig.h Magnitudes of the Fourier transforms of k3)~(10 at the Cu K edge for EuBazCu307_ ~ at 83, 110, 253 and 256 K. Note that the Eu peak increases as the oxygen peak decreases. Between 2 and 4 .~--1 the labels indicate only the dominant contributions.
U. Murek et al. / Anomalies in the Cu-O structure
was slowly cooled down to 77 K in 800 mbar He exchange gas. At the first cooling the superconducting transition temperature was observed at 101 K. It decreased to 93.5 K after repeated thermal cycling up to 300 K. The data shown in figs. 1 and 2 were recorded after a 36 h thermal treatment in He gas in a warming up cycle at a heating rate of 0.1 K/min. 3. RESULTS The raw data were reduced in the usual manner. Fig. 1 displays the magnitude of the Fourier transform FT [k3×ck)] of the EXAFS at 83 K, 110 K, 253 and 256 K. The transform range is k = 2.5 - 12.5 A, weightened with a i
i
i
Tc
10
253K
rn'id
•E5 rt-
-5"
cosine - type window centered at 7.5 A , optimizing the isolation of the nearest neighbor oxygen peak (0.8 - 2.1 A-l). The peaks between 2 and 4 A-l come from a combination of Eu, Ba, Cu and O atoms. The Fourier transform shows a strong dependence on temperature. In this communication we focus on a brief discussion on the temperature dependence of the Cu-O peak. The striking temperature dependence of the peaks between 2 and 4 •-1 will be subject of a full paper. Using other transformation windows and weights of ×ck) we can isolate some of these peaks including the shoulder at the reduced distance of 2.5 A (labelled X). Such a bondlength (not found from neutron and x-ray diffraction) is inferred from the K absorption edge.l)We analyze the magnitude of the filtered Cu-O EXAFS through the "log-ratio" method and obtain the temperature dependence of the effective relative Debye-Waller factor Aa2 -- ~2
._J
0 7" 6 5
271
I
Cu- O
!i 110K
EuBa2Cu3OT'~
4/" 'i
~'2 0 -1 t
50
I
100
o
I
t
150 200 Temperature [K]
I
250
3(30
Fig.2 (bottom) : The temperature dependence of effective Cu-O Debye-Waller factor Aa2 (relative to 83 K) in EuBa2Cu3OT_ ~ at increasing temperature (closed circles) after thermal treatment (see text). The point at 300 K was measured before the thermal treatment. Note at 253 K the coincidence of the resistivity anomaly (top) with the strong singularity of Ao2 and at Te (midpoint) a weak singularity (within 0.01K). The dashed line indicates the slope of o2 as obtained from an Einstein model with @5 = 600 K.
4. DISCUSSION The relative mean square displacements A~z(T) compromise thermal a n d structural disorder. The change of the thermal disorder as obtained from an Einstein model (ee = 600 K) is indicated through the dashed line in fig. 2. The weak singularity at Tc m a y be due to a critical phenomenon, the singularity at 253 K, however, clearly must be attributed to a phase transformation in the Cu-O structure. The kink of A02 at 110 K probably is due to the onset of an ordering of the oxygen defect structure2) and the distortion of the a-b planes observed through diffraction3). We thank A. Freimuth for the preparation o f the sample.
REFERENCES 1) F. W. Lytle, R.B. Greegor, A. J. Panson. Phys. Rev. B (1988) in press. 2) A.Robledo and C.Varea, Phys. Rev. B 37, 631 (1988). 3) P.M.Horn, D.T. Keane, G.A. Held, J.L.Jordan-Sweet, D.L. Kaiser, and F. Holtzberg, Phys. Rev. Lett. 59, 2772 (1987).
155 ( 1 9 8 8 ) 2 7 0 - 2 7 1 Amsterdam
ANOMALIES IN THE Cu-O S T R U C T U R E OF EuBazCu307_ ~ AT 110 K AND 250 K Udo Murek, Katharina Keulerz and Jtirgen Rtihler II. Physikalisches lnstitut, Universit~t z u KSln, Ziilpicherstr. 77, 5000 K S l n 41 , Fed. Rep. Germany
The temperature dependence (77 - 300 K) of the Cu - O bonds in EuBa2Cu3OT_ s has been investigated through the extended x-ray absorption fine structures (EXAFS) of Cu. The absorption spectra were recorded from a compact sample in the fluorescence mode simultaneously with the electrical resistance. Inereasiug the temperature the effective Cu-O Debye-Waller factors &oz (T) exhibit an anomalous kink at 110 K and a strong singularity at 253 K. The anomalies are interpreted through a characteristic temperature dependence of the oxygen defect structure.
1. INTRODUCTION The oxygen stoichiometry and oxygen defect structure are crucial quantities determining whether the RE-Ba-Cu-O systems become superconductors or not. Superconducting RE Ba2 Cu307_ s crystals have ordered vacancies along the c axis andalong the a-b directions The latter are responsible for the formation of the orthorhombically distorted unit cell and the resulting formation of twin lamellae in the realstructure of the crystals. Here we report on significant temperature dependences of the oxygen defect structure at 253 K and 110 K as extracted from the photoelectron interference pattern occuring beyond the photoabsorption threshold (extended x-ray absorption fine structure, EXAFS) of the Cu Is (K) level in EuBa2Cu307-s. 2. EXPERIMENT We have measured the temperature dependence (83 - 300 K) of the Cu K- EXAFS in the fluorescence mode from compact (8 mm diameter, 2 mm thick), sintered EuBazCu3 O7-s. The sample was prepared through the usual solid state reaction and showed the 1-2-3 structure with the following lattice parameters (at 300 K) : a= 3.82 ~, b = 3.87 ~, c = II. 58 ~. The x-ray absorption spectra were recorded in the Hamburger Synchrotronstrahlungslabor. We used the fluorescence mode for being able to trace ,simultaneously the x-ray absorption and the electrical resistance. A cylindrically shaped ionisation chamber served for the detection of 0921-4534/88/$03.50 ©ElsevierSciencePublishers B.V. (North-Holland Physics Publishing Division)
the fluorescence signal, and a 4-point DC technique for the measurement of the electrical resistance. The effect of selfabsorption on the EXAFS fluorescence signal from this "thick" sample was found to be ~ 0.5g from comparison with spectra traced in the transmission mode from finely ground powder. The sample i
I
I
i
1
I
i
I
0L 83 K "-.~//r'~ 0.0/.,-
110K~/
r-
2 5 6 K ~
E
253K
EU
Ba, Cu
# 0,02
,,o
,
0
1
,
,
,
2 3 Reduced Distance [~]
,
,
4
Fig.h Magnitudes of the Fourier transforms of k3)~(10 at the Cu K edge for EuBazCu307_ ~ at 83, 110, 253 and 256 K. Note that the Eu peak increases as the oxygen peak decreases. Between 2 and 4 .~--1 the labels indicate only the dominant contributions.
U. Murek et al. / Anomalies in the Cu-O structure
was slowly cooled down to 77 K in 800 mbar He exchange gas. At the first cooling the superconducting transition temperature was observed at 101 K. It decreased to 93.5 K after repeated thermal cycling up to 300 K. The data shown in figs. 1 and 2 were recorded after a 36 h thermal treatment in He gas in a warming up cycle at a heating rate of 0.1 K/min. 3. RESULTS The raw data were reduced in the usual manner. Fig. 1 displays the magnitude of the Fourier transform FT [k3×ck)] of the EXAFS at 83 K, 110 K, 253 and 256 K. The transform range is k = 2.5 - 12.5 A, weightened with a i
i
i
Tc
10
253K
rn'id
•E5 rt-
-5"
cosine - type window centered at 7.5 A , optimizing the isolation of the nearest neighbor oxygen peak (0.8 - 2.1 A-l). The peaks between 2 and 4 A-l come from a combination of Eu, Ba, Cu and O atoms. The Fourier transform shows a strong dependence on temperature. In this communication we focus on a brief discussion on the temperature dependence of the Cu-O peak. The striking temperature dependence of the peaks between 2 and 4 •-1 will be subject of a full paper. Using other transformation windows and weights of ×ck) we can isolate some of these peaks including the shoulder at the reduced distance of 2.5 A (labelled X). Such a bondlength (not found from neutron and x-ray diffraction) is inferred from the K absorption edge.l)We analyze the magnitude of the filtered Cu-O EXAFS through the "log-ratio" method and obtain the temperature dependence of the effective relative Debye-Waller factor Aa2 -- ~2
._J
0 7" 6 5
271
I
Cu- O
!i 110K
EuBa2Cu3OT'~
4/" 'i
~'2 0 -1 t
50
I
100
o
I
t
150 200 Temperature [K]
I
250
3(30
Fig.2 (bottom) : The temperature dependence of effective Cu-O Debye-Waller factor Aa2 (relative to 83 K) in EuBa2Cu3OT_ ~ at increasing temperature (closed circles) after thermal treatment (see text). The point at 300 K was measured before the thermal treatment. Note at 253 K the coincidence of the resistivity anomaly (top) with the strong singularity of Ao2 and at Te (midpoint) a weak singularity (within 0.01K). The dashed line indicates the slope of o2 as obtained from an Einstein model with @5 = 600 K.
4. DISCUSSION The relative mean square displacements A~z(T) compromise thermal a n d structural disorder. The change of the thermal disorder as obtained from an Einstein model (ee = 600 K) is indicated through the dashed line in fig. 2. The weak singularity at Tc m a y be due to a critical phenomenon, the singularity at 253 K, however, clearly must be attributed to a phase transformation in the Cu-O structure. The kink of A02 at 110 K probably is due to the onset of an ordering of the oxygen defect structure2) and the distortion of the a-b planes observed through diffraction3). We thank A. Freimuth for the preparation o f the sample.
REFERENCES 1) F. W. Lytle, R.B. Greegor, A. J. Panson. Phys. Rev. B (1988) in press. 2) A.Robledo and C.Varea, Phys. Rev. B 37, 631 (1988). 3) P.M.Horn, D.T. Keane, G.A. Held, J.L.Jordan-Sweet, D.L. Kaiser, and F. Holtzberg, Phys. Rev. Lett. 59, 2772 (1987).