Physica C I56 ( 1988) 303-306 North-Holland. Amsterdam

SOME DOUBTS ABOUT Fe SUBSTITUTION IN REBa2Cu30,s6

M-W. DIRKEN, R.C. THIEL and H.H.A. SMIT Kamerlingh Onnes Laboratory, Leiden Vnive+v, P.O. Box 9506. 2300 RA Leiden, The Netherlands

H.W. ZANDBERGEN Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands

Received 15 April 1988

Mbssbauer spectra have been obtained for YBaZCu2.94Fe0.0607-6 and GdBa2Cu2.94Feo.060,-~ at various temperatures down to 0.6 K. The low temperature measurements show four magnetic hyperfine split spectra, two of which are not compatible with Fe’+; combined with IS. data, these spectra are indicative of the Fed’ state. A comparisoc is made with the Miissbauer parameters for BaFeO- ,. The transition from paramagnetic to magnetically split spectra indicates that much of the “Fe substituted into these high-T, materials is inhomogeneously distributed over the Cu sites. Because of the likelyhood that the high oxidation state of the iron will effect the local oxyge-x surroundings, the usefulness of “Fe substitution into these materials is at best questionable.

b

1. Introduction

Many recent publications utilizing Miissbauer ef- fect spectroscopy (MES) have been concerned with the substitution of 57Fe into REBaZCu307--6 (here- after the “l-2-3” systems) [ l-8 ] . Three subspectra have been reported and all authors have explained their results in terms of high spin Fe3+ substitution into the two Cu-si?es, with the third sub-spectrum being caused by extra local oxygen at one of the two Cu-sites.

In the following, we will attempt to show that ( 1) there are at least four distinct Fe-sites present in iron- doped, oxygen-poor l-2-3 material, (2) two of the four sites are in the 4+ state, (3) there is reason to suspect the possible presence of small clusters of BaFe03-, in the samples, (4) the distribution of iron in the l-2-3 system is probably not homogeneous, and (5) the usefulness of 5’Fe substitution into the l-2-3 system is at best questionable.

to 0.6 K. In both samples, noniaally 2W of the Cu has been replaced by s7Fe during the preparative step [ 91, and both showed relatively sharp supercon- ducting transitions, the Y-sample at 65 K and the Gd- sample at 55 K. From the depth of the outer room temperature doublet shown in fig. 1, it is clear that our samples are somewhat oxygen deficient [ 8,10]. However, this will not seriously effect the conclu- sions we will attempt to draw. The MES parameters, shown in table I, are much the same as those in the literature. The hypefine splittings (hfs), with sub- stantial relaxation broadening, found at 0.6 K (fig.

2. Experimental results

Spectra of Y-l-2-3 have been measured at several temperatures down to 1.6 K and of Gd-I-2-3 down

-6 -3 0 3 6

Velocity (mm/s)

Fig. 1. GdBazCu2.94Feo.0607 -h at 293 K.

0921-4534/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )

304 hf. W Dkw et al. /Sum doubts about Fesubmturron rn REBa2Ch307-6

Table I MES data on the l-2-3 system at 0.6 K; I.S. values are relative to iron metai at RT (For the site indicated by an asterisk we could only ocrive a value for the magnetic tifper!i~-ns splitting, since only the two outer peaks of the sextet can be fitted.)

l-Z-3: 5’Fe

Fe”+ site Fe’+ * Fe’+, site A Fe’+, site B

hfs Q.S. IS. (T) (mm/s) (mm/s)

50(2) 0.6( I ) +0.38(2) 46(2) - 30(Z) 2.0( 1) +0.16(2) 30(2) 0.9(l) +0.01(2)

-I2 -8 -4 0 4 8 12 Velocity (mm/s)

Fig. 2. GdBa2Cu2.94Fe3,0607-6r at 0.6 K. The tit shown is for re- laxation at all four sites,

2) are also given in table I. The quadrupole splittings (Q.S.‘s) of two of the sites were slightly temperature dependent; site A in table I increased by about 4% between 300 K and 4 K, siti- B decreased by about 15%. From the small change of the hfs’s on going from 1.6 K to 0.6 K for the Gd-sample, we can con- clude that we are seeing close to the fully saturated hfs’s for all the sites. Note also the fourth site which has become clearly visible in the spectrum of the Gd- sample at low temperature. This site is also visible in our low temperature spectra of the Y-sample, as well as in the low temperature spectra of others [ 4,7]. At room temperature, the absorption lines from this fourth site apparently lie under the doublet from the other Fe3+ site.

It has proven impossible for us to obtain satisfac- tory fits to our low temperature spectra, excellent statistics, considerable experience, and specialized tools notwithstanding [ 11,121. Attempts to fit with two relaxing sextets were less than adequate; making use of a Gaussian distribution cr-f hype&me param-

zters was also unsatisfactory. The onset of the mag- netlc iijlperfine splitting does not seem to depend on whether the Y is substituted by the Gd. This can be understood from the consideration that ihc copper sites do not experience a magnetic field from the an- tiferromagnetically coupled R-E. planes [ 13 ] =

While X-ray powder diffraction measurements in- dicated these materials to be single phase, a micro- probe analysis of the Gd sample (performed with a spot diameter of about SO mm) showed 6Oh BaCuOz and 4Oh CuO to be present.

3. Analysis of results

The sites with the large hfs, shown in table I, arc both clearly in the high-spin 3+ state. Examination of the ranges of isomer shift (I.S. ) data for the var- ious valence states of iron [ 141 shows that there are a number of valence states possible for the two sites with the very small I.S. data: low spin Fe*+, with S=O, low spin Fe3+, with S= f, high spin Fe4+, with S=2, and diamagnetic covalent iron. All the other iron valencies give I.S. values either much too large or much too small. Because of the magnetic hfs ob- served for these two sites, we can exclude the S=O and the diamagnetic state. Making use of the - 22T( S,) rule, we can also exclude the Fe3+, S= f state, which cannot give a hfs as large as observed. This leaves only Fe4+ as an alternative. This high va- lence state will probably lead to local changes in the oxygen distribution relative to the unsubstituted copper compound. A similar relation between the lo- cal oxygen concentration and oxidation state has been observed in the BaFeO,-, system [ 15,161.

Note that all the normal iron oxides give I.S. data of at least 0.27 mm/s (relative to metallic iron) for Fe’+ sites. Altho@ the values of the Q.S.‘s for the two sites in question are much larger than normal for a lattice contribution, this does not provide a proof that the ions are not in the 3+ state, since point charge lattice sum calculations, based on various as- sumptions for the charges used (7, p.i. ) indicate that the lattice contribution to the EFG tensor can be very large in the pure l-2-3 system.

Moreover, the spin reduction of about 5096, needed to obtain the low values of the hfs’s for these two sites from a high spin Fe3+ state, would be very difficult

i

to explain. Enough covalency to explain the huge spin reduction is very improbable, since the iron ions are in an oxygen environment. To the authors knowl- edge zero-point spin reduction of the order of 50% for Fe3 + Pias noi been rcp:,rted OG in the Werature.

It has been recently pointed out [ 171 that to ob- tain satisfactory refinements in a neutron diffraction study of iron doping in Y-l-2-3 it was necessary to use a 4f charge state for part of the iron.

For comparison, in table II we have given the MES parameters for the hexagonal form of BaFeOJWx (with O<x<O.S) [ 15,161, in which both Fe3+ and Fe4+ are present. It should be mentioned that the possible presence of BaFe03-x in the l-2-3 system, while not unlikely considering the proven presence of BaCuOz, is not sufficient to explain the MES re- sults, because of the mismatch of Q.S.‘s and number of observed subspectra. However, comparison of the values of the magnetic hfs’s given in table I for the l-2-3 system and in table II for BaFe03-, does clearly show the differences between the Fe’+ and the Fe4+ sites in both systems.

The apparent relaxation effects exhibited by t3e two Fe4+ sites could be the result of super-paramag- In$c relaxation of small clusters of iron ions located on copper sites near grain boundaries. If this is the case, the average cluster sizes would have to be well below 10 nm in order to show the range of onset tem- peratures (blocking temperatures) observed. Note that this postulated cluster size is much smaller than the microprobe spot used, and such clusters could therefore not be seen individually. Such local “con- centrations” of iron would also go undetected by X- ray and/or neutron, diffraction. The relative inten- sities of the outer room temperature doublets are not only strongly sensitive to iron substitution, but also to the oxygen content [ 5,8, IO]. These relative in- tensities can be drastically changed by exposing the sample to a vacuum for a few seconds at a temper-

Table II MESdataon Baf;et&-., (~~0.15) from refs. [ 16) and [ 171. I.S. values are relative to iron metal at RT.

BaFe03-,, hfs Q.S. IS. 0) (mm/s) (mm/s)

Fe3+ site Fe4+ site

48.2 0.20 +0.37 24.0 0.00 +0.03

ature of about 200°C [ 81, or a few minutes at 350°C 5 lo]. Even with a relatively high diffusion rate for oxygc n at these temperatures, this suggests that a ‘arge portion of the substituted iron might be located very close to the surface and/or grain boundaries of the particles forming the bulk of the l-2-3 samples.

HREM photos [ 171 indicated that iron substi- tuted into Y-l-2-3 apparently forms chains of about 90 A length along twinning boundaries. This would give yet another possibility for understanding the ap- parent relaxation observed at low temperature. The magrrtic exchange interaction between Fe4+ ions in these chain segments would most probably have strong Ising-like anisotropy and could lead to the fointation of moving domain walls [ 111. MES, which probes the spin autocorrelation function, will regis- ter full magnetic hfs when the average time between passages of a domain wall becomes substantially longer than 1 /oL (Larmor frequency), even at tem- peratures far above the magnetic ordering temper- ature. If this supposition is true, the long range magnetic ordering temperature of the chain seg- ments will certainly be well down into the low mil- likelvin range. This is due to the relatively large distances between twin boundaries, which would lead to extremely weak inter-chain exchange interactions. The inclusion of a number of segments of magnetic chains at the twin boundaries might also help to ex- plain the sharp upturn in the specific heat of the Fe- substituted samples at low temperatures [ 181.

4. Conclusions

There is clear evidence that there are four distin- guishable sites in the s7Fe substituted l-2-3 systems. Two of these are in the high-spin 4+ valence state, indicating a local increase in the oxygen distribution in the neighborhood of the Fe4+ ions. There seems to be no proof for the general assumption that iron substitution into the l-2-3 system produces a rea- sonably homogeneous distribution of iron through- out the system. Clustering of the iron on copper sites, possibly as chains along the twin boundaries, may provide an explanation for the relaxation behaviour observed for the Fe4+ sites observed at temperatures below about 15 K. A small fraction of the iron may be present in the form of BaFe03-,.

306 M. W. Dirken et al. /Some doubts about Fe substitution in REBa2CuJ07--6

Drawing conclusions about the Cu-sites from 57Fe MES resul% other than the general observation that the environment is highly oxidating, does not seem to be justified.

Acknowledgements

Our samples were prepared by Mr. C. Snel and characterized by Mr. T.J. Gortermuller Stimulating discussions with Dr. D.J.W. Ijdo are acknowledged. The present work is part of the research program of the “Stichting voor Fundamenteel Onderzoek der Materie” (Foundation for Fundamental research on Matter) and was made possible by financial support from the Nederlandse Organisatie voor Zuiver-We- tenschappelijk Onderzoek (Netherlands Organiza- tion for the Advancement of Pure Research). The investigations are sponsored by the L&den Materials Science Centre ( Werkgroep Fundamenteel Materi- alen Onderzoek ) .

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