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Local structure of YBa2Cu3Oy with varying oxygen content and substitutional dopants on the Cu sites
Physica C 153-155 (1988) 852-853 North-Holland, Amsterdam
LOCAL STRUCTURE OF YBa2Cu30y WITH VARYING OXYGEN CONTENT AND SUBSTITUTIONALDOPANTS ON THE Cu SITES J.B. BOYCE Xerox Paio Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304, USA F. BRIDGES Department of Physics, University of California, Santa Cruz, CA 95064, USA T. CLAESON Physics Department, Chalmers University of Technology, S-412 96 Gothenburg, Sweden R.S. HOWLAND and T.H. GEBALLE Department of Applied Physics, Stanford University, Stanford, CA 94305, USA M. NYGREN Department Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden The local structure of oxygen rich and deficient YBa2Cu307_fi and of the same compound doped withFe, Co, Ni, and Zn has been studied with extended x-ray absorption fine structure (EXAFS) and near-edge structure. The environmentof the Cu and Y agree well with the structure determined from diffraction. The low-oxygen content sample .displays features in the absorption edge characteristic of O-Cu-O bonding. The EXAFS data indicate that Fe and Co impurities are surrounded by four oxygen atoms at about 1.8/~ and one at 2.3/~. The temperature dependent EXAFS gives an Einstein temperature for Cu-O vibrations of about 600K in the high-oxygen content compounds.
1. INTRODUCTION Limited dimensionality is important in the new superconductors. In YBa2Cu307_ 6 (YBCO),there are buckled planes of Cu-O separated by Cu-O chains. In the former, Cu has four oxygen neighbors in a square arrangement in the plane and a fifth one at a considerably larger distance. In the chains, Cu has a four-fold planar O environment with two somewhat shorter Cu-O distances perpendicular to the chain direction. The importance of chains vs planes is discussed lively. The oxygen content is a crucial factor. For fi>0.5, the structure transforms tetragonally and superconductivity is lost. Another way to affect Tc is to substitute atoms. Doping the Cu sites leads to a reduction in T c. There seems to be no drastic difference between magnetic and non-magnetic substitutes. To elucidate the problem of planes vs. chains, it is important to know if the dopants go into the chain[Cu(1)] or plane [Cu(2)] positions. Previous measurements and arguments indicate that Co, A1, and Ga go into chain sites, Ni and Zn into plane sites, and Fe into both (1-4). We have used x-ray absorption to study YBCO, doped and with varying oxygen content. 2. EXPERIMENTS YBCO samples were made to give O contents of 6.87, 6.98, and 6.15. The preparation of YBa2Cu3_xMxO7_g alloys has been described elsewhere (1). We used M = Fe (x = 0.05, 0.1, 0.3, 0.5), Co(0.02, 0.05, 0.1, 0.2, 0.5, 0921-4534/88/$03.50 @ElsevierSciencePublishersB.V. (North-Ho//andPhysicsPublishingDivision)
0.9), Ni(0.1, 0.2, 0.3), and Zn (0.1, 0.2, 0.3). Data were taken with a Si (400) monochromator giving a resolution of approximately 1 eV. The EXAFS data were reduced in the usual way (5). Data in real space were fitted to structural standards in order to determine the number of neighbors at specific distances and spreads in distance with temperature. 3. RESULTS 3.1 Near edge structure The absorption edges for three of the standards, Cu20, CuO and KNa4[Cu(HIO6)2]. 12H20 (with Cu +3), two YBCO samples and a doped sample are shown in Fig. 1. The edge is shifted to higher energies, as expected, when the valence is increased. There definitely are differences in the edges of the +1, +2 and +3 Cu standards. The pronounced low energy structure is characteristic of the ligands to the absorbing atoms (6) : e.g., in Cu20, the peak at (E-E0)=0eV is due to the linear O-Cu-O configuration. The absorption edge of the superconductor is broad with the main rise occurring near the formal valence +2 region. The lower part of the absorption edge is shifted towards lower energy, and a pronounced structure occurs at the energy of the CuO ligand peak in the oxygen-depleted superconductor. This agrees with the assumption that O is lost from the linear Cu-O chains, leaving O-Cu-O structures. The Cu-K edge in the alloys is almost identical to that of
J.B. Boyce et al. / Local structure of YBa2Cu30~ pure YBCO. Only one Co (x=0.9) sample showed a somewhat sharper edge. The Ni- and Zn-K edges in all alloys were very similar to the corresponding NiO and ZnO standards. The Co-K edges at all compositions were practically the same, rather similar to Co304. The low energy part of the Fe edge matched the Fe3+ edge in Fe203 but at higher energy there is a shift towards higher energy. A "ligand" peak similar to the one in low oxygen YBCO was pronounced in the Fe doped samples and present in the Co ones. 3.2 Environment by EXAFS The number of O neighbors to Cu and their distances were extracted from fits to a Cu-O standard obtained from Cu20. These were made using fixed ratios of the number of neighbors according to diffraction results. The distances are in remarkable agreement with diffraction results (7). There is only a small discrepancy of a few percent for the long distance Cu(2)-0(4) bond in the oxygen-rich YBCO. Fitting Y data to Y203 gave the expected Y-O coordination number (within about 10%) and the correct distance. The EXAFS of the Fe-K and Co-K edges were very simi- lar. The best fits were obtained for five oxygen neighbors to each impurity atom - four at a shorter distance of about 1.8/~ and a fifth at 2.3/~. They yielded a ratio of 4:1 for the two amplitudes consistent with the YBCO environment for plane atoms. Comparisons using an amplitude ratio of 2:2 for the two distances (as on an undisturbed chain) did not fit the data. 3.3 Temperature dependence No pronounced shift with temperature was seen in the absorption edge for the YBCO samples between 6 and 600 K. Neither was the character of the EXAFS changed. The changes could be well accounted for by a smoooth
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temperature dependent Debye-Waller-type broadening of the absorber- to-neighbor distances. The latter could be fit to an Einstein oscillator model (5), and we can extract values of the Einstein temperature characteristic of the pair vibrations. We obtain values of O E = 593 + 20 K for the two oxygen-rich samples and O E = 563 + 20 K for the oxygen-deficient sample. The small difference agrees with the expectation that the more disordered oxygen-deficient compound would be somewhat softer for the average Cu-O bond. (Also for the Cu-Cu pair, we extract a similar difference for the rich and poor oxygen samples). 4. CONCLUSIONS We find that the EXAFS-deterrnined local structure for YBCO agrees well with the long-range order given by diffraction results. This is true for both the oxygen-rich and -deficient compounds. The linear O-Cu-O bonds characteristic of YBa2Cu306 are evident in the absorption edges. The formal "valence" of Cu in the superconductor agrees with a mixture of +2 and +3 with a growing +1 contribution in the low O insulator. The local structure surrounding the impurity Co and Fe atoms looks more like most of these substituting Cu(2) (on the planes) rather than Cu(1) (on the chains). However, the Co environment may be distorted on the chain site in order to give four short and one long distances. The local distortion has to be substantial in order to explain the large long distance. Several facts and arguments speak in favor of Co on chain sites. We conclude that Co atoms do not go into chain sites solely unless the local structure is strongly distorted. The Cu-O vibration frequencies seem to be lower than required to obtain a sufficiently high T c using a conventional electron-phonon coupling theory. ACKNOWLEDGEMENTS We thank J.M. Tarascon for the doped samples. The experiments were performed at Stanford Synchrotron Radiation Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Sciences, and the National Institute of Health, Biotechnology Resource Divison. This research was supported in part by NSF, AFOSR and the Swedish NFR.
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E - E0 (eV3 FIGURE 1 Near edge x-ray absorption of (a) Cu20, (b) CuO, (c) KNa4[Cu(HIO6)2].12H2 O, (d) YBa2Cu306.15, (e) YBa2Cuo6.98, (f) YBa2Cu2.1Co0.907_&The data were taken at the Cu-K edge (Eo=8980.3 eV) and at 83 K.
REFERENCES (1) J.M. Tarascon, P. Barboux, P.F. Miceli, L.H. Greene, G.W. Hull, M. Eibschutz, and S.A. Sunshine, to be published. (2) P.F. Miceli, J.M. Tarascon, L.M. Greene, P. Barboux, F.J. Rotella, and J.D. Jorgensen, to be published. (3) Gang Xiao, M.Z. Cieplak, D. Musser, A. Gaurin, F.H. Streitz, C.L. Chien, J.J. Rhyne, and J.A. Gotaas, to be published. (4) T. Tamahi, T. Komai, A. Ito, Y. Maeno, and T. Fujita, Sol. State Commun. 65 (1988) 43. (5) J.B. Boyce, F. Bridges, T. Claeson, R.S. Howland, and T.H. Geballe, Phys. Rev. B36 (1987) 5251. (6) T.A. Smith, J.E. Penner-Hahn, M.A. Berding, S. Doniach, and K.O. Hodgson, J. Am. Chem. Soc.107 (1985) 5945. (7) C.C. Torardi, E.M. McCarron, P.E. Bierstedt, A.W. Sleight, and D.E. Cox, Sol.State Comm. 64 (1987) 497.
LOCAL STRUCTURE OF YBa2Cu30y WITH VARYING OXYGEN CONTENT AND SUBSTITUTIONALDOPANTS ON THE Cu SITES J.B. BOYCE Xerox Paio Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304, USA F. BRIDGES Department of Physics, University of California, Santa Cruz, CA 95064, USA T. CLAESON Physics Department, Chalmers University of Technology, S-412 96 Gothenburg, Sweden R.S. HOWLAND and T.H. GEBALLE Department of Applied Physics, Stanford University, Stanford, CA 94305, USA M. NYGREN Department Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden The local structure of oxygen rich and deficient YBa2Cu307_fi and of the same compound doped withFe, Co, Ni, and Zn has been studied with extended x-ray absorption fine structure (EXAFS) and near-edge structure. The environmentof the Cu and Y agree well with the structure determined from diffraction. The low-oxygen content sample .displays features in the absorption edge characteristic of O-Cu-O bonding. The EXAFS data indicate that Fe and Co impurities are surrounded by four oxygen atoms at about 1.8/~ and one at 2.3/~. The temperature dependent EXAFS gives an Einstein temperature for Cu-O vibrations of about 600K in the high-oxygen content compounds.
1. INTRODUCTION Limited dimensionality is important in the new superconductors. In YBa2Cu307_ 6 (YBCO),there are buckled planes of Cu-O separated by Cu-O chains. In the former, Cu has four oxygen neighbors in a square arrangement in the plane and a fifth one at a considerably larger distance. In the chains, Cu has a four-fold planar O environment with two somewhat shorter Cu-O distances perpendicular to the chain direction. The importance of chains vs planes is discussed lively. The oxygen content is a crucial factor. For fi>0.5, the structure transforms tetragonally and superconductivity is lost. Another way to affect Tc is to substitute atoms. Doping the Cu sites leads to a reduction in T c. There seems to be no drastic difference between magnetic and non-magnetic substitutes. To elucidate the problem of planes vs. chains, it is important to know if the dopants go into the chain[Cu(1)] or plane [Cu(2)] positions. Previous measurements and arguments indicate that Co, A1, and Ga go into chain sites, Ni and Zn into plane sites, and Fe into both (1-4). We have used x-ray absorption to study YBCO, doped and with varying oxygen content. 2. EXPERIMENTS YBCO samples were made to give O contents of 6.87, 6.98, and 6.15. The preparation of YBa2Cu3_xMxO7_g alloys has been described elsewhere (1). We used M = Fe (x = 0.05, 0.1, 0.3, 0.5), Co(0.02, 0.05, 0.1, 0.2, 0.5, 0921-4534/88/$03.50 @ElsevierSciencePublishersB.V. (North-Ho//andPhysicsPublishingDivision)
0.9), Ni(0.1, 0.2, 0.3), and Zn (0.1, 0.2, 0.3). Data were taken with a Si (400) monochromator giving a resolution of approximately 1 eV. The EXAFS data were reduced in the usual way (5). Data in real space were fitted to structural standards in order to determine the number of neighbors at specific distances and spreads in distance with temperature. 3. RESULTS 3.1 Near edge structure The absorption edges for three of the standards, Cu20, CuO and KNa4[Cu(HIO6)2]. 12H20 (with Cu +3), two YBCO samples and a doped sample are shown in Fig. 1. The edge is shifted to higher energies, as expected, when the valence is increased. There definitely are differences in the edges of the +1, +2 and +3 Cu standards. The pronounced low energy structure is characteristic of the ligands to the absorbing atoms (6) : e.g., in Cu20, the peak at (E-E0)=0eV is due to the linear O-Cu-O configuration. The absorption edge of the superconductor is broad with the main rise occurring near the formal valence +2 region. The lower part of the absorption edge is shifted towards lower energy, and a pronounced structure occurs at the energy of the CuO ligand peak in the oxygen-depleted superconductor. This agrees with the assumption that O is lost from the linear Cu-O chains, leaving O-Cu-O structures. The Cu-K edge in the alloys is almost identical to that of
J.B. Boyce et al. / Local structure of YBa2Cu30~ pure YBCO. Only one Co (x=0.9) sample showed a somewhat sharper edge. The Ni- and Zn-K edges in all alloys were very similar to the corresponding NiO and ZnO standards. The Co-K edges at all compositions were practically the same, rather similar to Co304. The low energy part of the Fe edge matched the Fe3+ edge in Fe203 but at higher energy there is a shift towards higher energy. A "ligand" peak similar to the one in low oxygen YBCO was pronounced in the Fe doped samples and present in the Co ones. 3.2 Environment by EXAFS The number of O neighbors to Cu and their distances were extracted from fits to a Cu-O standard obtained from Cu20. These were made using fixed ratios of the number of neighbors according to diffraction results. The distances are in remarkable agreement with diffraction results (7). There is only a small discrepancy of a few percent for the long distance Cu(2)-0(4) bond in the oxygen-rich YBCO. Fitting Y data to Y203 gave the expected Y-O coordination number (within about 10%) and the correct distance. The EXAFS of the Fe-K and Co-K edges were very simi- lar. The best fits were obtained for five oxygen neighbors to each impurity atom - four at a shorter distance of about 1.8/~ and a fifth at 2.3/~. They yielded a ratio of 4:1 for the two amplitudes consistent with the YBCO environment for plane atoms. Comparisons using an amplitude ratio of 2:2 for the two distances (as on an undisturbed chain) did not fit the data. 3.3 Temperature dependence No pronounced shift with temperature was seen in the absorption edge for the YBCO samples between 6 and 600 K. Neither was the character of the EXAFS changed. The changes could be well accounted for by a smoooth
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temperature dependent Debye-Waller-type broadening of the absorber- to-neighbor distances. The latter could be fit to an Einstein oscillator model (5), and we can extract values of the Einstein temperature characteristic of the pair vibrations. We obtain values of O E = 593 + 20 K for the two oxygen-rich samples and O E = 563 + 20 K for the oxygen-deficient sample. The small difference agrees with the expectation that the more disordered oxygen-deficient compound would be somewhat softer for the average Cu-O bond. (Also for the Cu-Cu pair, we extract a similar difference for the rich and poor oxygen samples). 4. CONCLUSIONS We find that the EXAFS-deterrnined local structure for YBCO agrees well with the long-range order given by diffraction results. This is true for both the oxygen-rich and -deficient compounds. The linear O-Cu-O bonds characteristic of YBa2Cu306 are evident in the absorption edges. The formal "valence" of Cu in the superconductor agrees with a mixture of +2 and +3 with a growing +1 contribution in the low O insulator. The local structure surrounding the impurity Co and Fe atoms looks more like most of these substituting Cu(2) (on the planes) rather than Cu(1) (on the chains). However, the Co environment may be distorted on the chain site in order to give four short and one long distances. The local distortion has to be substantial in order to explain the large long distance. Several facts and arguments speak in favor of Co on chain sites. We conclude that Co atoms do not go into chain sites solely unless the local structure is strongly distorted. The Cu-O vibration frequencies seem to be lower than required to obtain a sufficiently high T c using a conventional electron-phonon coupling theory. ACKNOWLEDGEMENTS We thank J.M. Tarascon for the doped samples. The experiments were performed at Stanford Synchrotron Radiation Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Sciences, and the National Institute of Health, Biotechnology Resource Divison. This research was supported in part by NSF, AFOSR and the Swedish NFR.
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E - E0 (eV3 FIGURE 1 Near edge x-ray absorption of (a) Cu20, (b) CuO, (c) KNa4[Cu(HIO6)2].12H2 O, (d) YBa2Cu306.15, (e) YBa2Cuo6.98, (f) YBa2Cu2.1Co0.907_&The data were taken at the Cu-K edge (Eo=8980.3 eV) and at 83 K.
REFERENCES (1) J.M. Tarascon, P. Barboux, P.F. Miceli, L.H. Greene, G.W. Hull, M. Eibschutz, and S.A. Sunshine, to be published. (2) P.F. Miceli, J.M. Tarascon, L.M. Greene, P. Barboux, F.J. Rotella, and J.D. Jorgensen, to be published. (3) Gang Xiao, M.Z. Cieplak, D. Musser, A. Gaurin, F.H. Streitz, C.L. Chien, J.J. Rhyne, and J.A. Gotaas, to be published. (4) T. Tamahi, T. Komai, A. Ito, Y. Maeno, and T. Fujita, Sol. State Commun. 65 (1988) 43. (5) J.B. Boyce, F. Bridges, T. Claeson, R.S. Howland, and T.H. Geballe, Phys. Rev. B36 (1987) 5251. (6) T.A. Smith, J.E. Penner-Hahn, M.A. Berding, S. Doniach, and K.O. Hodgson, J. Am. Chem. Soc.107 (1985) 5945. (7) C.C. Torardi, E.M. McCarron, P.E. Bierstedt, A.W. Sleight, and D.E. Cox, Sol.State Comm. 64 (1987) 497.