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Substitution on the Cu(1) sites in Y1Ba2Cu3O7−δ; evidence for distorted chains of Co and Fe atoms
Physica C 162-164 (1989) 969-970 North-Holland
SUBSTITUTION ON THE Cu(1) SITESIN Y1Ba2Cu307-6, EVIDENCEFOR DISTORTEDCHAINS OF Co AND Fe ATOMS F. BRIDGESL2, J. B. BOYCE2, T. CLAESON3, T. H. GEBALLE4 and J. M. TARASCONs ~Departrnent of Physics,University of California, Santa Cruz, CA 95064 2Xerox Palo Alto Research Center, Palo Alto, CA 94304 3PhyslcsDepartment, Chalmers Univ. of Techn., S-41296 Gothenburg, Sweden ~Department of Applied Physics,Stanford University, Stanford, CA 94305 SBell Communications Research Laboratory Red Bank, New Jersey07701 X-ray absorption studies indicate that Co replaces Cu(1) in Y1Ba2Cu307-6at low concentrations, but the CoO) sites are severely distorted. A zig-zag chain model for the Co is consistent with the observed first and second neighbor distances. A similar behavior is observed for Fe; however, Ni-doped samples exhibit little distortion and a uniform occupation of Cu(1) and Cu(2) sites. 1. INTRODUCTION
metal substitution, and the orthorhombic distortion
Small perturbations of the structure and composition
vanishes at very low concentrations of the dopant2.3.
of superconductors have profound effects on the
Determination of which Cu site(s) the dopants occupy
superconducting properties. To understand the
and the local environment is of importance for
interaction between the structure of Y1Ba2Cu307-8 (YBCO) and its superconducting properties, we have
understanding the role defects and structural changes play in these high Tc materials.
made an extensive investigation of YBCO doped with Co and other metals, using x-ray absorption spectroscopy. The structure of YBCO is a distorted, trilayer
2. EXPERIMENTALDETAILS Samples of YBCO doped with Co, Fe and Ni were
orthorhombic perovskiteL with two distinct Cu sites: the
prepared by a solid-state reaction. Resistive and
Cu(2) site in the Cu-O planes and the Cu(1) site in the 1-
magnetic measurements of the superconducting
D Cu-O chains. Tc is strongly suppressed with transition I
I
I
properties are reported in Ref. 3. Other Co doped samples were prepared by M. Kakihana using a sol-gel route. The samples were single-phase within a few
l
Fig.1
A
percent.
comparison of the Cu Kedge data, Co K-edge
S i m u l a t i o n of 2nd neighbor p e a k for undistorted Co(l) sites I t
I
In Fig. 1 we compare the real space Co K-edge EXAFS
with the Co
data [FT kx(k)] for the Coo.17sample with the Cu K-edge
K-edge data. Note the
data for normal YBCO, and with a simulated spectra assuming Co is on undistorted Cu(1) sites. Both the
reduced amplitude of
amplitude and the real component of the complex fourier transform are plotted. The k-space transform
the Co second
range is 3.6-11.7 A-1 with a gaussian broade,,ing o1 U.5
neighbor
A-1. The main features to note are: 1) the first r~e,ghbor CoO peak is shifted to lower r by roughly 0.1A relative
peak.
I 2
3. Co (and Cu) FIRSTNEIGHBOR ENVIRONMENT
I 4
r [A/ 0921--4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
to Cu-O, 2) there is additional weight near 2.0~ for the Co edge, and 3) the Co second neighbor peak, in the range 2.5 - 4.5~, is considerably reduced in amplitude.
F. Bridges et al. /Substitution on the Cu(1) sites in YtBaeCu~Ov_6
970
The data for all the Co samples are similar, including
neighbor peak that is rather similar to normal YBCO. We
samples prepared with each technique. The first
obtain a good fit with only a small broadening and
neighbor results are qualitatively and quantitatively
11% of the Co on Cu(2) sites. No significant Ba disorder
inconsistent w i t h the predictions f o r Co on an
is present. To achieve the large reduction in the
undistorted Cu(1) site, (and also for Co on the Cu(2) sites
observed Co-Ba peak amplitude therefore requires that
or a uniform distribution.) Our fits converged to an
some of the Co must be displaced a large distance
unexpected result: the first neighbor shell about Co
(~0.3A) from the normal Cu(1) sites. We propose a
contains ~ 5 0 atoms, 3.50 at a short distance (-1.8~)
distorted chain model, similar to the chain models
and about ~ 1 . 3 0 at a much larger distance (2.4~).
proposed by Border et. al.4, but with alternate Co atoms having a < 110> off-center displacement (Fig. 2). With
4. SECONDNEIGHBOR ENVIRONMENTS The second neighbor environment for a direct substitution of Co at the Cu(1) sites should be simpler than the average Cu second neighbor environment in YBCO. The total amplitude for the undistorted Cu(1) site alone is 30~ larger than the total weighted amplitude in YBCO (see simulation in Fig.l) The experimental reality is quite different; the Co K-edge second neighbor peak is surprisingly small. We maintain that the reason for the small amplitude of the second neighbor peak is an off-center displacement of some of the Co on the Cu(1) sites. The reduced amplitude is not caused by a
a Co off-center displacement of ~0.4~, we obtain Co-O distances and Co-Ba distances (at 3. 56~ and 3.56 + 0.32A) that are self-consistent and give a good fit. The point here is not that we have found a specific model, but that displacements of the Co atoms can accoufit for both the nearest neighbor O peak as well as the reduced amplitude second neighbor multi-peak. The EXAFS results for the Fe substituted samples are qualitatively similar to the Co results discussed above. The Fe second neighbor peak is also small, suggesting a distorted environment for the Fe atoms in YBCO. By contrast, the Ni second neighbor peak is comparable to
large disorder of the neighboring atoms; the Cu K-edge
the Cu data s with little broadening of the peaks and a
EXAFS in the highly Co doped sample has a second
uniform distribution on the two Cu sites.
Fig. 2 The zig-zac~chain model for YBCO:Co.
5. CONCLUSIONS The proposed a zigzag chain model accounts for the different C o O bond lengths in the Cu(1) plane, provides
oI
a good fit to the Co second neighbor peak for the low concentration data, and suggests t w i n n i n g on a microscopic scale as an explanation for the apparent tetragonal structure of the Co doped samples.
v
• O
w
Cu atom 0 atom
[] Co atom L a n d S are long and short b o n d s
REFERENCES 1. See, for example,T. Siegrist, S. Sunshine, D. W. Murphy, R. J. Cava, and S. M. Zahurak, Phys. Rev. B 35, 7137(1987). 2. Y. Maeno, T. Tomita, M. Kyogoku, S. Awaji, Y. Aokl, K. Hoshino, A. MInami, and T Fujita, Nature 328, 512 (1987). 3. J. M. Tarascon, P. Barboux, P. F. Miceli, L. H Greene, G. W. Hull, M. Eibschutz and S. A. Sunshine, Phys. Rev B 37, 7458 (1988). 4. P. Bordet, J. L. Hodeau, P. Strobel, M. Marezio, and A. Santoro, Solid State Comm. 66, 435 (1988) 5. J. B. Boyce, F. Bridges, T. Claeson, and M. Nygren, Phys. Rev. B 39, 6555 (1989)
SUBSTITUTION ON THE Cu(1) SITESIN Y1Ba2Cu307-6, EVIDENCEFOR DISTORTEDCHAINS OF Co AND Fe ATOMS F. BRIDGESL2, J. B. BOYCE2, T. CLAESON3, T. H. GEBALLE4 and J. M. TARASCONs ~Departrnent of Physics,University of California, Santa Cruz, CA 95064 2Xerox Palo Alto Research Center, Palo Alto, CA 94304 3PhyslcsDepartment, Chalmers Univ. of Techn., S-41296 Gothenburg, Sweden ~Department of Applied Physics,Stanford University, Stanford, CA 94305 SBell Communications Research Laboratory Red Bank, New Jersey07701 X-ray absorption studies indicate that Co replaces Cu(1) in Y1Ba2Cu307-6at low concentrations, but the CoO) sites are severely distorted. A zig-zag chain model for the Co is consistent with the observed first and second neighbor distances. A similar behavior is observed for Fe; however, Ni-doped samples exhibit little distortion and a uniform occupation of Cu(1) and Cu(2) sites. 1. INTRODUCTION
metal substitution, and the orthorhombic distortion
Small perturbations of the structure and composition
vanishes at very low concentrations of the dopant2.3.
of superconductors have profound effects on the
Determination of which Cu site(s) the dopants occupy
superconducting properties. To understand the
and the local environment is of importance for
interaction between the structure of Y1Ba2Cu307-8 (YBCO) and its superconducting properties, we have
understanding the role defects and structural changes play in these high Tc materials.
made an extensive investigation of YBCO doped with Co and other metals, using x-ray absorption spectroscopy. The structure of YBCO is a distorted, trilayer
2. EXPERIMENTALDETAILS Samples of YBCO doped with Co, Fe and Ni were
orthorhombic perovskiteL with two distinct Cu sites: the
prepared by a solid-state reaction. Resistive and
Cu(2) site in the Cu-O planes and the Cu(1) site in the 1-
magnetic measurements of the superconducting
D Cu-O chains. Tc is strongly suppressed with transition I
I
I
properties are reported in Ref. 3. Other Co doped samples were prepared by M. Kakihana using a sol-gel route. The samples were single-phase within a few
l
Fig.1
A
percent.
comparison of the Cu Kedge data, Co K-edge
S i m u l a t i o n of 2nd neighbor p e a k for undistorted Co(l) sites I t
I
In Fig. 1 we compare the real space Co K-edge EXAFS
with the Co
data [FT kx(k)] for the Coo.17sample with the Cu K-edge
K-edge data. Note the
data for normal YBCO, and with a simulated spectra assuming Co is on undistorted Cu(1) sites. Both the
reduced amplitude of
amplitude and the real component of the complex fourier transform are plotted. The k-space transform
the Co second
range is 3.6-11.7 A-1 with a gaussian broade,,ing o1 U.5
neighbor
A-1. The main features to note are: 1) the first r~e,ghbor CoO peak is shifted to lower r by roughly 0.1A relative
peak.
I 2
3. Co (and Cu) FIRSTNEIGHBOR ENVIRONMENT
I 4
r [A/ 0921--4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
to Cu-O, 2) there is additional weight near 2.0~ for the Co edge, and 3) the Co second neighbor peak, in the range 2.5 - 4.5~, is considerably reduced in amplitude.
F. Bridges et al. /Substitution on the Cu(1) sites in YtBaeCu~Ov_6
970
The data for all the Co samples are similar, including
neighbor peak that is rather similar to normal YBCO. We
samples prepared with each technique. The first
obtain a good fit with only a small broadening and
neighbor results are qualitatively and quantitatively
11% of the Co on Cu(2) sites. No significant Ba disorder
inconsistent w i t h the predictions f o r Co on an
is present. To achieve the large reduction in the
undistorted Cu(1) site, (and also for Co on the Cu(2) sites
observed Co-Ba peak amplitude therefore requires that
or a uniform distribution.) Our fits converged to an
some of the Co must be displaced a large distance
unexpected result: the first neighbor shell about Co
(~0.3A) from the normal Cu(1) sites. We propose a
contains ~ 5 0 atoms, 3.50 at a short distance (-1.8~)
distorted chain model, similar to the chain models
and about ~ 1 . 3 0 at a much larger distance (2.4~).
proposed by Border et. al.4, but with alternate Co atoms having a < 110> off-center displacement (Fig. 2). With
4. SECONDNEIGHBOR ENVIRONMENTS The second neighbor environment for a direct substitution of Co at the Cu(1) sites should be simpler than the average Cu second neighbor environment in YBCO. The total amplitude for the undistorted Cu(1) site alone is 30~ larger than the total weighted amplitude in YBCO (see simulation in Fig.l) The experimental reality is quite different; the Co K-edge second neighbor peak is surprisingly small. We maintain that the reason for the small amplitude of the second neighbor peak is an off-center displacement of some of the Co on the Cu(1) sites. The reduced amplitude is not caused by a
a Co off-center displacement of ~0.4~, we obtain Co-O distances and Co-Ba distances (at 3. 56~ and 3.56 + 0.32A) that are self-consistent and give a good fit. The point here is not that we have found a specific model, but that displacements of the Co atoms can accoufit for both the nearest neighbor O peak as well as the reduced amplitude second neighbor multi-peak. The EXAFS results for the Fe substituted samples are qualitatively similar to the Co results discussed above. The Fe second neighbor peak is also small, suggesting a distorted environment for the Fe atoms in YBCO. By contrast, the Ni second neighbor peak is comparable to
large disorder of the neighboring atoms; the Cu K-edge
the Cu data s with little broadening of the peaks and a
EXAFS in the highly Co doped sample has a second
uniform distribution on the two Cu sites.
Fig. 2 The zig-zac~chain model for YBCO:Co.
5. CONCLUSIONS The proposed a zigzag chain model accounts for the different C o O bond lengths in the Cu(1) plane, provides
oI
a good fit to the Co second neighbor peak for the low concentration data, and suggests t w i n n i n g on a microscopic scale as an explanation for the apparent tetragonal structure of the Co doped samples.
v
• O
w
Cu atom 0 atom
[] Co atom L a n d S are long and short b o n d s
REFERENCES 1. See, for example,T. Siegrist, S. Sunshine, D. W. Murphy, R. J. Cava, and S. M. Zahurak, Phys. Rev. B 35, 7137(1987). 2. Y. Maeno, T. Tomita, M. Kyogoku, S. Awaji, Y. Aokl, K. Hoshino, A. MInami, and T Fujita, Nature 328, 512 (1987). 3. J. M. Tarascon, P. Barboux, P. F. Miceli, L. H Greene, G. W. Hull, M. Eibschutz and S. A. Sunshine, Phys. Rev B 37, 7458 (1988). 4. P. Bordet, J. L. Hodeau, P. Strobel, M. Marezio, and A. Santoro, Solid State Comm. 66, 435 (1988) 5. J. B. Boyce, F. Bridges, T. Claeson, and M. Nygren, Phys. Rev. B 39, 6555 (1989)