CHARACTERISATION OF MERCURY-SUBSTITUTED (Bi,Pb)&Ca2Cu30x (2223) HIGH-T, PHASE USING SYNCHROTRON RADIATION AND NEUTRON DIFFRACTION DATA

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1 Pnnwinht 6.If?PnS-intnrndinnal Centre for Diffraction Data 1997 CHARACTERISATION OF MERCURY-SUBSTITUTED (Bi,Pb)&Ca2Cu30x (2223) HIGH-T, PHASE USING SYNCHROTRON RADIATION AND NEUTRON DIFFRACTION DATA D.Y. Li, B.H. O Connor, A. van Riessen and C.W. MacKinnon* Materials Research Group, Department of Applied Physics Cur-tin University of Technology GPO Box U1987, Perth, WA, Australia 6001 D. J. Cookson, R.F. Garrett and B.A. Hunter, Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW, Australia 2234 ABSTRACT Hg-substituted (Bi,Pb)-(2223) high-t, superconductor material, synthesised by a process involving solid state reaction of nitrate starting materials, has been characterised using synchrotron radiation diffi-action (SRD) and neutron diffraction (ND) data. The absence of SRD pattern lines from prospective contaminant phases, notably from low-t, (2212), indicated that the material could be described as single-phase. The SRD data provided indexing results for satellite reflections which are clearly superior to those for Bragg-Brentano laboratory XRD data. Indexing gave a one-dimensional modulated structure, described by a two-component wave vector, which is very similar to that reported earlier for the unsubstituted (Bi,Pb)-(2223) phase. A structure for (Bi,Pb)-(2223) with Hg substitution has been proposed based on models reported in the literature and ND Rietveld refinement calculations with a commensurate structure model. The study has highlighted the value of using SRD data, rather than Bragg-Brentano laboratory-source x-ray data, to test for single-phase character in high-t, research. INTRODUCTION The discovery of high temperature superconducting behaviour in the (Bi,Pb)-Sr-Ca-Cu-0 system, represented by the formula Bi$r$a,,_iCun04+2,, (where n = 1, 2 and 3) has attracted considerable attention due to the occurrence of bulk superconductivity for the (Bi,Pb)zSr$a2Cu30, (2223) phase at T, > 110 K [l-7]. The crystal structure of the phase is still not fully described due to the difficulty of preparing single-phase material as well as considerable complexity of the structure. Li et al. [8] commented on inconsistencies in the indexing of four (Bi,Pb)-(2223) XRD powder patterns reported in the International Centre for Diffraction Data PDF-43 data base for the stated unit cells if conventional commensurate crystallography principles are employed. Analysis of an XRD pattern for apparently single-phase (Bi,Pb)-(2223), which had been synthesised by Li et al. by a conventional solid state reaction/sintering procedure, also showed inconsistencies which were attributed to either incommensurate character and/or the presence of unidentified contaminants. *Now at Julius Kruttschnitt Mineral Research Centre, University of Queensland, Queensland, Australia 4068

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Indexing of (2223) selected area electron diffraction patterns using incommensurate modulation was described by (i) Namgung et al. [9] for Pb-fi-ee (2223) and by (ii) MO et al. [lo] for (Bi,Pb)-(2223) material. Li and Mackinnon [l l] reported successful incommensurate modulation indexing of Bragg-Brentano XRD data for the Pb-doped phase. A comparison of indexing results for one- and two-component modulation wave vectors [l l] showed both to be plausible for indexing satellite reflections but that the latter provides clearly superior results. That conclusion is re-examined in this paper using flat plate synchrotron radiation diffraction (SRD) data for which indexing of higher quality is expected due to the enhanced resolution of the diffraction patterns. Preceding crystallographic work on (Bi,Pb)-(2223) has been extended in the present study by investigating the value of using Hg-doping and solid state reaction/sintering to prepare singlephase material. SRD characterisation has confirmed the high quality of the synthesised samples which may be produced by Hg doping, there being no unassigned lines, i.e. all features could be indexed using incommensurate modulation. Addition of 1-3 molar Hg has resulted in single-phase material for which T, is within the range K. The present study has demonstrated the clear superiority of flat-plate SRD analysis over Bragg-Brentano XRD data in testing for contamination and/or for incommensurate modulation indexing. Re-analysis of the (Bi,Pb)-(2223) material described in [S] using SRD has revealed the presence of (2212) features, even though these were not detected previously in the XRD patterns, thus underlining the power of SRD for characterising high-t, superconductor materials. Rietveld ND analysis has been employed to investigate the likely locations of Hg atoms in the doped (Bi,Pb)-(2223) material. A structural model has been developed starting with models proposed in the literature for (Bi, Pb)-(2223) not doped with Hg [12,13]. While the atomic positions for the cations are similar for the two reported models, there are substantial differences in the site occupancies. There are also major differences between the two models for some oxygen atoms in terms of both positions and site occupancies. The present study has provided reasonable agreement with the model in [ 121. EXPERIMENTAL Synthesis of (Bi,Pb)-(2223) The difficulty of preparing pure single-phase (2223) material may be attributed to (i) the slow transformation of (2212) to (2223) and (ii) the instability of the (2223) phase at high temperature, the development of processing methods being hindered by the relatively poor level of knowledge of the transformation [l]. The superconductor specimens studied previously in this system have mainly been synthesised by solid state reactions involving the mixing of metal oxides and carbonates followed by thermal processing. Because such reactions proceed by solid state difl%sion and via the (2212) to (2223) phase transformation, temperatures as high as 870 C and reaction times longer than 100 h are required if the material is to contain substantial levels of the (2223) phase. A novel processing method, involving the use of nitrate starting materials, was developed in the present study for preparing single-phase Hg-doped (Bi,Pb)-(2223). The method draws in part on results reported in the literature [4,16]. Further details will be given in a patent specification. The sintered specimens were subsequently powdered for diffraction analysis - see following sub-sections.

4 Cowriaht 0 JCPDS-International Centre for Diffraction Data 1997 Diffvaction Data Acquisition SRD data were measured using the BIGDIFF instrument at the Australian National Beamline Facility, Photon Factory, Tsukuba, Japan [ 171. Powder patterns were collected with flat-plate specimen and parallel-beam optics, according to following settings: h = A, aperture for incident/diffracted beam = 0.6 mm, beam width = 2 mm, specimen rotation, 20 range = (step = 2 ) NaI(T1) detector, monitor counting employed (time/step = 3.5 s). Details for the corresponding CuKa Bragg- Brentano XRD measurements cited in this paper are given in [ 111. ND measurements were conducted with the HRPD instrument operated by ANSTO at Lucas Heights, Australia [18]. Powder data were collected using Debye-Scherrer optics (h = A) with material being packed into V cans of diameter 12 mm and height 50 mm. An assembly of 24 detectors was used to collect data over the range 28 = (step = 5 ). Counting was performed against a monitor which resulted in counting times = 80 s/step. Indexing Using Incommensurate Modulation SRD pattern peak positions were determined with program EVA (supplier Socabim-Sarl a bis, villa du Bel-Air, Paris). Starting with the reported commensurate unit cell parameters for unsubstituted (Bi,Pb)-(2223) the SRD pattern of the Hg-doped phase was first indexed with program PIRUM [ 191. Indexing was then extended to include incommensurate modulation using program ISRIMS [20] as described in [l 11. Finally, the cell parameters and incommensurate modulation vector values were refined with program RICICLE [2 I]. RESULTS AND DISCUSSION Synthesis of Single-Phase Material / Value of SRLI for Characterisation Figure 1 demonstrates the improved quality of the materials obtained with the nitrate processing route developed for the study. Figure 1 (top) shows the presence of various lines not associated with the (2223) phase when the oxide/carbonate route was employed. The other two patterns, for materials produced by the nitrate route, are very similar. Indexing indicated that both may be described as singlephase materials. The difference between the two nitrate-route materials is that the processing time was considerably shorter when Hg-doping was employed. Figure 2 demonstrates the considerable advantage offered by SRD over Bragg-Brentano XRD for line identification. Three SRD peaks, indicated by markers Al, A2 and A3 (d-spacings 2.675, and A, respectively), are not evident in the XRD pattern. Peak Al is a principal line and Al and A2 are satellite lines. Another two satellite peaks, marked by arrows Bl and B2 (2.218 and A), can be identified in the XRD pattern, but with considerably less certainty. Paper [S] reported an XRD pattern from a high-quality (2223) material, synthesised by solid state reaction (Figure 1 for which the characteristic lines of the (2212) phase at 28 =5.8 (I& = 15 in paper]141 and I/I0 = 8 in paper [15]) and 20 = 27.5 (I/I0 = 100 in [14, 151)) cannot be seen in the nitrateroute patterns. This sample was considered at that time to be a pure (2223) specimen in the sense of XRD identification. However, the existence of trace phase (2212) in the sample has now be established using SRD (Figure 1, this paper). These observations have clearly demonstrated the superiority of SRD over XRD for phase identification particularly in relation to the need to obtain single-phase material. Incommensurate Modulation Indexing The crystallographic data and final indexing results for the Hg-substituted specimen, obtained using program RICICLE, are given in Table 1 and the Appendix, respectively. The standard deviations for the parameters obtained with SRD data are generally superior to those for XRD data. As for the XRD analysis of unsubstituted (Bi,Pb)-(2223) reported in [ 111, the two-component wave modulation vector gave a superior indexing result - see Table 1. All satellite reflections listed in the Appendix for

5 c * SRD patterns for: (top) oxide/carbonate solid state reaction route; (centre) nitrate-route without Hg addition; and (below) nitrate route with Hg addition. Wavelength = A.

6 the two-component wave vector can also be indexed using a one-component modulation wave vector by increasing the error window 1 Asin 1 from 006 and 008 (sin28 I 5 and > 5, respectively) compared with values of 01 and 02, correspondingly. Rietveld analysis of crystal structure with ND data Rietveld refinements were conducted using the structure models in [ 12,131. The model in [12], due to Carrilo-Cabrera and Gopel, provided considerably superior refinements. The best refinement results are listed in Table 2. Attempts to refine all site occupancies simultaneously were unsuccessful. In order to obtain an indication of the placement of the Pb and Hg atoms, constrained refinements were conducted in which the known amounts of these atom tvpes were set according to the comnosition of the starting materials. First, refinements were conducted with-the level of Pb set at 0.40 L Boo 0 30 a aoooo[ theta Figure 2. Comparison of (above) part of SRD pattern for wavelength A with (below) corresponding XRD pattern for CuKa radiation, A - Hg-su5bstituted (2223).

7 Table 1. Crystallographic Data for Hg-Substituted (Bi,Pb)-(2223) Phase Compared with Unsubstituted Phase Data Hg-Substituted [SRD- Present Study] Unsubstituted [XRD- Ref 1 I] System Orthorhombic Cell Parameters a = (1) A b = (1) c = (1) Modulation wave vector, q= (1) b* (2) C* (1) A (2) (2) (4) b (6) c Table 2. Rietveld Refinement of Atom Positions and Occupancy Parameters for Hg-substituted (Bi,Pb)- (2223) Compared with Data for Unsubstituted Phase. First line of data for each atom is for Hg-substituted unsubstituted material material (present study); second line for Atom Site Occupancy x/a Yb Z/C (Bi,Pb) Sr (WW Ca Cul cu (44 (W (W (W 1.02(2) (2) 0.86(2) 0.92(2) (l) (l) ( 1) (l) 0.70(5) 0.85(2) 0.71(3) 1.09(2) 0.71(3) 0.83(l) (2) (3) 0.238(2) (l) 0.773(4) (4) (2) 0.758(4) (4) (3) 0.25 l(3) (3) (3) (3) 0.876(7) 0.206(4) (5) 0.461(l) (7) (l) (7) 0.337(l) (6) (l) (6) Refinement Space group: Amaa. Z = 4. Details: Refined unit cell : a = (7), b = (8), c = (4)A. Rp = 10.90%; Rwp = 13.48; Rexp = 6.87%, Rb = 6.70%, GOF = Formula by refinement: (Bi~.64Pb.~)Sr~.~~(Ca~.83Hg.~~)Cu2.~208.58

8 and the atom placed, in turn, at the sites for Bi, Sr and Ca all of these being considered as possible sites. The results obtained with Pb on the Bi site clearly indicated that Pb is most likely placed at this position. Similarly, the placement of the Hg atoms was studied by conducting refinements in which the atom was placed on the three cation positions at the level 1. The Rietveld trials for the prospective Hg sites clearly pointed to the Ca site for Hg substitution. ACKNOWLEDGEMENT The authors wish to acknowledge (i) the Australian Institute of Nuclear Science and Engineering for AINSE NS grant 94/139 and (ii) a grant in 1995 from the Australian National Beamline Facility which is funded by a consortium comprising the Australian Research Council: the Department of Industry, Technology and Regional Development; the Australian Nuclear Science and Technology Organisation; the Australian National University and the University of New South Wales. REFERENCES 1. Maeda, H., Tanaka, Y., Fukutomi, M. and Asano, T., 1988, Jpn. J. Appl. Phvs., 27: Koyama, S., Endo, U. and Kawai, T.,1988, Jnn. J. Appl. Phys., 27:L Maeda, A., Noda, K., Uchinokura, K. and Tanaka, S., 1989, Jpn. J. Appl. Phys., 28:L Song, K.H., Liu, H.K., Dou, S.X. and Sorrell, C.C., 1990, J. Am. Ceram.Soc.,73: Wang, N., Tan, M., Wang, J., Xu, Z., Sha, J., Xing, S. and Zhang, Q., 1991, J. Mater. Sci. m., 10: Chen, Y.L. and Stevens, R., 1992, J. Am. Ceram. Sot., 75: Primo, V., Sapina, F., Sanchis, M.J., Ibranez, R., Beltran, A.and Beltran, D., 1992, Materials I&. 15: 149. Li, D.Y., Wang, H., MacKinnon, C.W., Low, I.M. and Davis, R.L., 1994, Proc. Int. Ceramics Conf., Austceram 94, Sydney, Australia. Edited by C.C. Sorrel1 and A.J. Ruys, Namgung, C., Lachowski, E.E., Irvine, J.T.C. and West, A.R. (1992) Powd. Diff., 7:49. MO, Y.D., Chen, T.Z., Fan,H.F., Li, J.Q., Sha, B.D., Zheng, C.D., Li, F.H. and Zhoa, Z.X., 1992, Supercond. Sci. Technol., 5:69. Li, D.W. and Mackinnon, C.W., 1996, Indexing of incommensurate satellite reflections in x-ray powder diffraction pattern of (Bi,Pb)2Sr2Ca&,t30x (2223) phase. Submitted to Powd. Diff. Carrilo-Cabrera, W. and Gopel, W., 1989, Phvsica C, 161:373. Sequeira, A., Yakhmi, J.V., Iyer, R.M., Rajagopal, H. and Sastry, P.V.P.S.S., 1990, Physica C, 167:291. Onoda, M., Yamamoto, A., Takayama-Muromachi, E. and Takekawa, S., 1988, Jap. J. Appl. Phys., 27:L833. Matheis, D.P. and Snyder, R.L., 1994, Powd. Diff., 9:28. Rao, C.N.R., Nagarajam, R. and Vijayaraghavan, R., 1993, Supercond. Sci.Technol., 6: 1. Foran, G., Cookson, D. and Garrett, R., 1974, Procs. Asian Forum on Synchrotron Radiation, Japanese Society of Synchrotron Radiation Research and the Japan Synchrotron Radiation Research Institute, Kobe Howard, C.J., Ball, C.J., Davis, R.L. and Elcombe, M.M.,1983, Aust J. Phys., 36:507. Werner, P-E., 1969, Arkiv. Kemi.. 31:513. Li, D.Y., 1995, ISRIMS: a program for indexing satellite reflections of incommensurate modulated structures, SPS lAP58, School of Physical Sciences, Curtin University of Technology, Perth, Western Australia. Copyright (C) 21. JCPDS-International Smith, R.I., 1993, Centre Powd. for Diff., Diffraction 8:168. Data 1997

9 Appendix. Observed and calculated d-spacings for Hg-substituted (2223) using the two-component wave vector defined in Table 1. d-obs Ire1 d-cal h k 1 m d-obs Ire1 d-cal h k 1 m l (Cont d)

10 Appendix (Cont d) d-obs Ire1 d-cal h k 1 m d-obs Ire1 d-cal h k 1 m l l