Nickel substitution in Ln2Ba4Cu7−xNixO14+δ

Physica C 297 Ž1998. 95–102

Nickel substitution in Ln 2 Ba 4Cu 7yx Ni xO 14qd ž Ln s lanthanide, yttrium/ D.B. Currie, M.T. Weller

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Department of Chemistry, UniÕersity of Southampton, Southampton SO17 1BJ, UK Received 3 January 1997; revised 1 October 1997; accepted 10 November 1997

Abstract The system Ln 2 Ba 4Cu 7yx Ni x O14q d Ž247., Ln s lanthanide, yttrium, has been investigated by powder X-ray and neutron diffraction, thermogravimetric analysis and magnetic susceptibility measurements. Extended X-ray absorption fine structure ŽEXAFS. experiments at the Ni K-edge have been carried out on a number of the compounds. Results have shown that, as with the related material YBa 2 Cu 3 O 7y d Ž123., nickel doping takes place predominantly at the square pyramidal copper sites, with some occupation at the square planar sites at low dopant levels. A deterioration of the superconducting properties following introduction of nickel content is observed. q 1998 Elsevier Science B.V. Keywords: Nickel doping; Neutron diffraction; EXAFS

1. Introduction The superconducting compound Ln 2 Ba 4 Cu 7O 14q d Ž247. w1–3x, where Ln s most lanthanides and also yttrium, has a structure related to both YBa 2 Cu 3 O 7y d Ž123. w4,5x and YBa 2 Cu 4 O 8 Ž124. w6,7,5x and may be regarded as an ordered bulk phase intergrowth of these two compounds. As such, the structure exhibits regions of oxygen non-stoichiometry in one-dimensional chains of Cu–O square planes Žfrom the 123 part of the intergrowth. and complete oxygenation in one-dimensional double chains of Cu–O square planes Žfrom the 124 part of the intergrowth.. This ordering also gives rise to the existence of two distinct square pyramidal copper

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Corresponding author. Tel.: q44 170 359 5000; Fax: q44 170 359 3781.

sites, though the immediate environment of these sites is very similar. Fig. 1 shows part of the 247 structure and in particular the different copper coordination geometries in the three different types of site. It is noteworthy that the effect of transition metal doping on the structure of any 247 compound, which is a way of interpreting the roles of the different copper sites, has not been previously studied to the same degree as has doping in 123 compounds w8x. We have previously presented preliminary results on these 247 materials w9x, but here we present full analysis with powder neutron diffraction studies and additional EXAFS measurements. We have prepared a range of nickel-doped rareearth 247 materials and characterised them by a number of experimental techniques, particularly with the aim of speciation of the nickel. Attempts were also made to incorporate other transition metals

0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 8 5 4 - 6

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D.B. Currie, M.T. Wellerr Physica C 297 (1998) 95–102

samples were reground periodically during a reaction time of four days. Products were examined using powder XRD and found to consist of a mixture of two phases: CuO and a nickel-doped Ln123-type phase. There appeared to be no sign of unreacted nickel oxide by XRD indicating that the nickel was present in the Ln123-type phase. This intimate mixture was then subjected to a high pressure oxygen annealing treatment. The sample was loaded into a gold crucible and heated at 10208C in 70 atm of flowing oxygen gas w10x for a period of 12 h. A cooling rate of 38Crmin was used and this process was repeated to achieve full reaction. Powder X-ray diffraction confirmed the presence of a Ln247-type phase and the apparent absence of either nickel or copper oxide.

3. Other synthesis experiments A range of further synthetic experiments of the general description: 2LnBa 2 Cu 3 O 7y d q MO x ´ Ln 2 Ba 4 Cu 6 MO15 " d ,

Fig. 1. The structure of the 247 materials showing the different copper environments. Small shaded spheres represent Ln and larger shaded spheres represent barium.

within the 247 structure and the results of these studies are presented here.

where M s Fe, Co, Ti, Ga, Pd. Both large and small lanthanide system were investigated, e.g. Nd and Er, and high pressures from 50 to 500 atm, with temperatures up to 10208C, were used. Despite the facile formation of the nickelate phase, all these experiments proved unsuccessful. With iron and cobalt, the reaction proceeded as: 2LnBa 2 Cu 3 O 7y d q MO x ´ 2LnBa 2 Cu 2 MO 7 " d q CuO.

2. Synthesis Samples can be prepared using two synthetic routes: either by high pressure annealing of the reactant oxides, or from the reaction of precursors with copper oxide. The former route has the apparent advantage of avoiding potential carbonate contamination, but the obvious disadvantage of the necessity of use of protective atmospheres. In the latter procedure, a stoichiometry representative of the required phase was prepared from high purity Ln 2 O 3 , BaCO 3 , CuO and NiO. This mixture was then fired in air at temperatures ranging between 900 and 9308C and

That is, copper in the 123 structure was displaced from the square planar site by octahedral Fe or Co with a subsequent increase to tetragonal symmetry. This was confirmed by XRD which showed the presence of CuO and YBa 2 Cu 2 ŽFerCo.O 8 . With titanium a doped 123-type phase was formed, together with other impurities. With the attempted palladium incorporation into a 247 structure it was expected that the dopant would localise on either, or both, of the square planar sites as is found with the compound, YBa 2 Cu 2.5 Pd 0.5 O6.8 w17x. However, even at temperatures as low as 8008C, and irrespective of oxygen pressure, barium was extracted from the 123

D.B. Currie, M.T. Wellerr Physica C 297 (1998) 95–102

structure to form BaPdO 2 w18x there was also a commensurate growth of impurity phases such as Y2 BaCuO5 . Gallium has a strong preference for tetrahedral coordination and though octahedral w19x is known, it was thought that if gallium incorporation into a 247-type structure was possible it would take place with tetrahedra replacing the SCSP copper site; this has an obvious analogue in YSr2 Cu 2 GaO 7 w20x. Unfortunately, the 247 structure did not accommodate the gallium and, as with iron and cobalt, copper oxide was ejected from the 123 structure and the phase YBa 2 Cu 2 GaO 7 was formed.

4. Experimental analysis 4.1. X-ray diffraction Powder X-ray diffraction data were collected on all samples using a Siemens D5000 diffractometer using Cu K a1 radiation at a wavelength of 1.5406 ˚ Sample purity was ascertained and cell parameA. ters calculated for all samples. Rietveld refinements of the XRD data were attempted for the samples Y2 Ba 4 Cu 7yx Ni xO 14q d x s 0.2, 0.4, 0.6, 0.8, 1.0, but despite reasonable fits to the observed data, discrimination of the nickel in the structure was not possible, due, of course, to the similarity in scattering of copper and nickel. The variation of cell parameters for the above series is shown in Fig. 2, clearly showing a marked reduction in the c-parame-

Fig. 2. Variation of cell parameters vs. composition for Y2 Ba 4 Cu 7yx Ni x O14q d .

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ter, accompanied by a slight reduction in the orthorhombic distortion. 4.2. ThermograÕimetric measurements The oxygen contents of the samples were investigated by reduction in 5% H 2rN2 at 10008C in a Stanton Redcroft TGA1500 thermal balance. As with the 123 materials, the degree of orthorhombic distortion of 247 materials is related to the oxygen content w3x. In agreement with the cell parameters shown in Fig. 2, the total oxygen content of the Y2 Ba 4Cu 7y x Ni xO1 4q d samples was found to increase slightly from 15.0 to 15.1 as the nickel content was increases across the range. For samples containing other lanthanides, the oxygen content was found to increase with larger lanthanide ion size, reaching approximately 15.3 for Ln s Nd; this is in good agreement with the behaviour displayed by undoped samples w3x. 4.3. Neutron diffraction The neutron scattering lengths of copper and nickel are significantly different Ž7.718 and 10.3 fm respectively. w11x, thus neutron diffraction experiments should yield the location of the dopant in the crystal structure. To this end, data from samples of nominal compositions Y2 Ba 4 Cu 6.8 Ni 0.2 O 14q d , Y2 Ba 4 Cu 6.5 Ni 0.5 O 14q d , Y2 Ba 4 Cu 6.0 Ni 1.0 O 14q d , Dy 2 Ba 4 Cu 6.0 Ni 1.0 O 14q d , Ho 2 Ba 4 Cu 6.0 Ni 1.0 O 14q d and Nd 2 Ba 4 Cu 6.0 Ni 1.0 O 14q d were collected on the D2B powder neutron diffractometer at InstitutLaue-Langevin in Grenoble, France. Samples were mounted in thin-walled vanadium cans and data were ˚ and at obtained using a wavelength of 1.5943 A room temperature; a step-size of 0.058 2 u was utilised. Rietveld refinement of the data using the program ‘GSAS’ w12x proceeded with the refinement of cell parameters, atomic positions and isotropic temperature factors. Site occupancies of nickel and copper, distributed over the four different B cation type sites within the Ln247 structure as shown in Fig. 1, and constrained to fit the compound stoichiometry, were refined in order ascertain the location of the nickel dopant within the structure. Final fits to the data

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D.B. Currie, M.T. Wellerr Physica C 297 (1998) 95–102

were, in the most part, excellent, although it became apparent that, in contrast with the observations of the XRD data, there existed, in some samples, small amounts of unreacted Ln123 and NiO impurities; these were included in the refinements where necessary. An example observedrcalculatedrdifference plot for the Rietveld refinement of Ho 2 Ba 4Cu 6 NiO14q d is shown in Fig. 3. The main purpose of the data analysis was to determine unequivocally the location of the nickel dopant, and after exhaustive manipulation and refinement of the occupanciesrcompositions of the four transition metal sites, this was achieved for the highly doped compositions, the dopant being localised on the SPy sites, Fig. 1. At low nickel concentrations Ž x - 0.4. refinement of the proportion on the SPy site proved unreliable, but the presence of some nickel at that site improved the value of the isotropic temperature factor. Refinement of nickel occupancy relative to copper on the DCSP site resulted in convergence at a copper occupancy very close to unity, strongly rejecting any presence of the strongly scattering nickel at that site. Refinement of the occupancy of the SCSP site showed no evidence as to the existence of nickel on that site. Interestingly the site also showed a tendency to maintain a copper occupancy close to unity, thus implying the absence of carbonate groups at that site which had previously been observed for single crystals w13x.

Free refinement of the basal plane oxygen sites, that is, untied variation of site occupancies and temperature factors for the two sites, introduced instability to the refinement. Thus constrained temperature factors andror oxygen occupancies were employed in most cases. Overall oxygen occupancies showed good agreement with the values determined by thermogravimetric measurements and subsequently were removed from the refinement as a variable. Final refined parameters are summarised in Table 1. 4.4. Extended X-ray absorption fine structure measurements EXAFS experiments were carried out on a variety of samples from the prepared range with data collected at the Ni K-edge. The overlap of this edge with the copper K-edge results in a shorter data set than would be normally preferred; similarly interference by the L III absorption edges of some of the lanthanide ions restricts the number of suitable samples. The experiments were carried out in transmission mode on Station 7.1 on the SRS at Daresbury, UK. Background subtraction was performed using the PAXAS software w14x and the resultant EXAFS data files transferred to the XRSSERV1 compute server at Daresbury for analysis using EXCURV92 w15x.

Fig. 3. Observed Žq., calculated Žsolid line. and difference Žlower line. plot for PND Rietveld refinement of Ho 2 Ba 4 Cu 6.0 Ni 1.0 O14q d .

Table 1 Refined structural parameters for Ln 2 Ba 4 Cu 7yx Ni x O14q d Ž300 K. Dy2 Ba 4 Cu 6 NiO14q d

Ho 2 Ba 4 Cu 6 NiO14q d

Y2 Ba 4 Cu 6 NiO14q d

Y2 Ba 4 Cu 6.5 Ni 0.5 O14q d

Y2 Ba 4 Cu 6.8 Ni 0.2 O14q d

Nd 2 Ba 4 Cu 6 NiO14q d

0.1151Ž1. 0.0422Ž2. 0.1883Ž2. 0.0821Ž1. 0.1489Ž1. 0.2302Ž1. 0.0356Ž3. 0.0869Ž2. 0.0865Ž2. 0.1446Ž2. 0.1441Ž2. 0.1934Ž2. 0.2324Ž2. 0.58Ž2. 0.42

0.1151Ž1. 0.0423Ž1. 0.1884Ž1. 0.0821Ž1. 0.1488Ž1. 0.2304Ž1. 0.0359Ž2. 0.0872Ž1. 0.0869Ž1. 0.1443Ž1. 0.1440Ž1. 0.1930Ž1. 0.2321Ž1. 0.55Ž2. 0.45

0.1153Ž1. 0.0421Ž1. 0.1885Ž1. 0.0817Ž1. 0.1487Ž1. 0.2303Ž1. 0.0363Ž1. 0.0875Ž1. 0.0870Ž1. 0.1443Ž1. 0.1438Ž1. 0.1933Ž1. 0.2326Ž1. 0.77Ž2. 0.23

0.1147Ž1. 0.0432Ž2. 0.1877Ž2. 0.0819Ž1. 0.1486Ž1. 0.2301Ž1. 0.0357Ž2. 0.0877Ž1. 0.0871Ž1. 0.1439Ž1. 0.1441Ž1. 0.1938Ž2. 0.2323Ž2. 0.81Ž2. 0.19

0.1151Ž1. 0.0429Ž2. 0.1884Ž3. 0.0821Ž1. 0.1484Ž1. 0.2299Ž1. 0.0357Ž2. 0.0876Ž1. 0.0867Ž1. 0.1442Ž1. 0.1436Ž1. 0.1936Ž1. 0.2323Ž1. 0.83Ž2. 0.17

0.1162Ž3. 0.0414Ž3. 0.1884Ž3. 0.0794Ž2. 0.1511Ž2. 0.2312Ž2. 0.0372Ž4. 0.0858Ž3. 0.0855Ž2. 0.1483Ž3. 0.1461Ž2. 0.1937Ž3. 0.2319Ž2. 0.41Ž4. 0.59

˚. a 0 ŽA ˚. Ž b0 A

3.8634Ž1. 3.8818Ž1.

3.8589Ž1. 3.8796Ž1.

3.8460Ž1. 3.8810Ž1.

3.8423Ž1. 3.8815Ž1.

3.8433Ž1. 3.8803Ž1.

3.8945Ž2. 3.9014Ž3.

˚. c 0 ŽA

50.344Ž2.

50.315Ž1.

50.3824Ž8.

50.482Ž2.

50.511Ž2.

50.677Ž3.

R wp Ž%. x2

3.0 1.4

3.7 3.5

6.2 5.8

6.7 11.8

5.8 12.2

6.8 19.5

D.B. Currie, M.T. Wellerr Physica C 297 (1998) 95–102

Position Ln 1r2,1r2, z BaŽ1. 1r2,1r2, z BaŽ2. 1r2,1r2, z CuŽ2. 0,0, z CuŽ3. 0,0, z CuŽ4. 0,0, z OŽ1. 0,0, z OŽ2. 1r2,0, z OŽ3. 0,1r2, z OŽ4. 1r2,0, z OŽ5. 0,1r2, z OŽ6. 0,0, z OŽ7. 0,1r2, z OŽ8. 0,1r2,0 OŽ9. 1r2,0,0

Estimated standard deviations are given in brackets.

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D.B. Currie, M.T. Wellerr Physica C 297 (1998) 95–102

As with the powder neutron diffraction data analysis, different structural models were set up within the EXCURV92 EXAFS refinement software on the basis of nickel occupying the three different types of copper sites: single chain square planar ŽSCSP., double chain square planar ŽDCSP., and square pyramidal ŽSPy.. These three geometries exhibit very similar first shell coordination, all having essentially ˚ the SPy site four oxygens at around 1.7–1.9 A, ˚ Given exhibiting a single apical oxygen at ca. 2.4 A. the normal limitations of the EXAFS technique and the truncation of the data by the interference of the Cu-K absorption edge, differentiation and resolution of the three different types of site on the basis of the first oxygen shells, would not be expected. However,

it is in the modelling of subsequent shells, to cations such as copper, yttrium and barium, that distinction between the sites becomes possible. For example, DCSP coordination is distinguished by the existence of a shell of two copper atoms, corresponding to the distance between copper atoms in adjacent edgesharing square planes. SCSP and SPy geometries are ˚ to 8 discriminated by the former’s shell at ; 3.3 A barium atoms, which in the latter geometry is split into two shells of 4 barium atoms and 4 yttrium ˚ The disparate backscatatoms, again around 3.3 A. tering of Ba and Y allows this discrimination. Modelling of the EXAFS data for the compound Y2 Ba 4 Cu 6 NiO14q d was attempted in the three coordination geometries, and it was soon possible to

Fig. 4. EXAFS refinement fit for Y2 Ba 4 Cu 6.0 Ni 1.0 O14q d .

D.B. Currie, M.T. Wellerr Physica C 297 (1998) 95–102

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Table 2 EXAFS data table for Y2 Ba 4 Cu 6.0 Ni 1.0 O14q d Ž300 K.

Table 4 EXAFS data table for Y2 Ba 4 Cu 6.4 Ni 0.6 O14q d Ž300 K.

˚ . Debye–Waller factor ŽA˚ 2 . No. of atoms in shell Distance ŽA

˚ . Debye–Waller factor ŽA˚ 2 . No. of atoms in shell Distance ŽA

4ŽO. IŽO. 4ŽY. 4ŽBa. 4ŽCu. 14ŽO. 4ŽCu. 4ŽCu. 4ŽCu. 4ŽBa. 8ŽY. 8ŽBa.

4ŽO. 1ŽO. 4ŽY. 4ŽBa. 4ŽCu. 14ŽO. 4ŽCu.

1.968Ž4. 2.466Ž8. 3.224Ž5. 3.249Ž8. 3.91Ž1. 4.320Ž7. 5.087Ž4. 5.188Ž5. 5.746Ž8. 5.93Ž2. 6.24Ž1. 6.34Ž1.

0.0185Ž7. 0.005Ž2. 0.010Ž1. ) 0.014Ž2. ) ) 0.036Ž3. 0.003Ž1. 0.001 0.001 0.001 0.014 ) ) 0.010 ) 0.014 ) )

Y2 Ba 4 Cu 6.0 Ni 1.0 O14q d ; R s w HŽ x T y x E . k 3 d k rHx E k 3 d k x = 100%s16.53%; fit index sÝ i wŽ x t " x e . k i3 x2 s 2.3. Estimated standard deviations are given in brackets. Sets of parameters constrained to identical values are shown by ) or ) ) .

discount the presence of Ni in the DCSP geometry, mainly through the absence of the Cu–Cu shell described above. Fitting of the EXAFS data using EXCURV92 to the SCSP and SPy models very quickly gave highly improved fits to the latter even allowing the presence of the apical single oxygen. This provided strong agreement with the PND data in indicating at least a high degree of localisation of the nickel atoms on the SPy site. Full refinement of the EXAFS data sets was then attempted. In order to limit the number of variables, the temperature factors of like shells were con˚ strained to be equal. Beyond approximately 5 A backscattering from light atoms such as oxygen was Table 3 EXAFS data table for Y2 Ba 4 Cu 6.2 Ni 0.8 O14q d Ž300 K.

˚ . Debye–Waller factor ŽA˚ 2 . No. of atoms in shell Distance ŽA 4ŽO. 1ŽO. 4ŽY. 4ŽBa. 4ŽCu. 14ŽO. 4ŽCu.

1.936Ž4. 2.41Ž2. 3.24Ž1. 3.28Ž1. 3.809Ž8. 4.29Ž1. 5.04Ž1.

0.0118Ž7. ) 0.0118 ) 0.015Ž2. 0.016Ž4. 0.027Ž3. 0.013Ž3. 0.012Ž2.

Y2 Ba 4 Cu 6.2 Ni 0.8 O14q d ; R s w HŽ x T y x E . k 3 d k rHx E k 3 d k x = 100%s 26.46%; fit index sÝ i wŽ x t " x e . k i3 x 2 s 5.4. Estimated standard deviations are given in brackets.

1.933Ž6. 2.41Ž2. 3.19Ž2. 3.30Ž3. 3.845Ž7. 4.28Ž1. 5.12Ž2.

0.0058Ž9. 0.0058 ) 0.020Ž3. 0.03Ž1. 0.017Ž3. ) ) 0.003Ž3. 0.017 ) )

Y2 Ba 4 Cu 6.4 Ni 0.6 O14q d ; R s w HŽ x T y x E . k 3 d k rHx E k 3 d k x = 100%s 26.12%; fit index sÝ i wŽ x t " x e . k i3 x 2 s 7.2. Estimated standard deviations are given in brackets.

not observed and only shells of cations were included in the refinement. Analysis proceeded smoothly and Fig. 4 shows the fit for the Y2 Ba 4 Cu 6 NiO14q d compound; statistics and shell distances for Y2 Ba 4 Cu 7yx Ni x O 14q d , x s 1.0, 0.8, 0.6 are shown in Tables 2–4 respectively. Examination of the data for samples with x s 0.2 and x s 0.4 revealed a more complicated situation. The Fourier transform of the background-subtracted ˚ and data showed the existence of a shell at ; 2.7 A ˚ showed a broadening the oxygen shell at ; 1.9 A and shift to lower k. The former shell, as previously stated, may be taken as an indicator of DCSP coordination, and the changes induced in the latter shell are in agreement with the presence of nickel in square planar coordination. Refinement of the data to any one of the three structural models proved impossible, and this was taken as evidence that the nickel dopant was present on more than one site. Similar evidence was given on examination of the copper K-edge EXAFS data from Y2 Ba 4 Cu 6 NiO14q d . The Fourier transform of the data was very similar to the FT of the low-nickel content data and proved similarly unrefinable. Some work has been carried out by Bridges et al. w16x on nickel K-edge EXAFS in doped yttrium 123. In agreement with the work presented herein, they show that at low concentrations, nickel dopant exist on both square planar and square pyramidal sites. Similarly, they note a preference for SPy coordination at higher concentrations, though with the added caveat of the presence of NiO impurity. The 247 system actually appears to be more accommodating

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D.B. Currie, M.T. Wellerr Physica C 297 (1998) 95–102

to incorporation of Ni within the structure in comparison with 123, with concentrations above 20% being possible on the SPy sites.

5. Conclusion C om pounds of stoichiom etry Ln 2 B a 4 Cu 7y x Ni x O 14q d , Ln s lanthanide, yttrium, 0 F x F 1.0, have been prepared by solid state synthesis under high pressure oxygen. The materials have been shown to retain the 247 structure even at high levels of doping in contrast with comparable Fe- and Cocontaining compositions. The combination of powder X-ray and neutron diffraction and extended X-ray absorption spectroscopy measurements reveal marked localisation of the nickel dopant on the square pyramidal sites, especially at high levels of doping. As with the parent 123-type materials, the superconducting properties of these materials quickly deteriorate on incorporation of the nickel; measurements on Ln 2 Ba 4 Cu 6 NiO14q d , Ln s Ho, Dy both showed very weak superconducting transitions at 45 K superimposed on semiconductor type behaviour. Apical bond distances of the NiO5 square pyramids determined by EXAFS are significantly longer than those refined from the PND data, but this merely reflects the difference between the ‘averaged’ and ‘local’ nature of the two techniques. It does, however, indicate significant disorder of the apical oxygen site, which is of key importance in the transfer of charge into and out of the superconducting planes.

Acknowledgements The authors would like to acknowledge the EPSRC for funding in connection with this work, and the Institut-Laue-Langevin for neutron beam time and technical help.

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