Cianchi, L.

Del Giallo, F.

Mancini, M.

Pieralli, F.

Sciortino, Silvio

Spina, Gabriele

Florence Research

11 July 2022151-Eu Mossbauer studies on Eu(l+x)Ba(2-x)Cu(3)O(7+delta) oxide systems / L. Cianchi; F. Del Giallo; F. Pieralli; M.Mancini; S. Sciortino; G. Spina. - In: HYPERFINE INTERACTIONS. - ISSN 0304-3843. - STAMPA. - 66(1991), pp. 387-391.Original Citation:151-Eu Mossbauer studies on Eu(l+x)Ba(2-x)Cu(3)O(7+delta) oxide systemsTerms of use:Publisher copyright claim:(Article begins on next page)La pubblicazione è resa disponibile sotto le norme e i termini della licenza di deposito, secondo quanto stabilito dallaPolicy per l'accesso aperto dell'Università degli Studi di Firenze (https://www.sba.unifi.it/upload/policy-oa-2016-1.pdf)Availability:This version is available at: 2158/772234 since:Questa è la Versione finale referata (Post print/Accepted manuscript) della seguente pubblicazione:FLORERepository istituzionale dell'Università degli Studi diFirenzeOpen AccessHyperfine Interactions 66 (1991) 387-392 387 lSlEu MOSSBAUER STUDIES ON EUl+xBa2_xCu307+ ~ OXIDE SYSTEMS L. CIANCHI,  F. DEL GIALLO,  F. P I E R A L L I  lstituto di Ricerca sulle Onde Elettrornagnetiche Via Panciatichi 64, 50127 Firenze, Italia M. MANCINI,  S. SCIORTINO and G. SPINA Dipartimento di Fisica, Unil'ersith Via S. Marta 3, 50139 Firenze, Italia M6ssbauer parameters of ~5~Eu in Eu l+xBa2 ,Cu307+ a have been studied as a function of temperature, for x = 0.15 and x = 0.25. The areas of the absorption spectra for the two samples are almost equal. This could mean that the f-M6ssbauer factor for ~51Eu at the barium sites is much smaller than that for ~5~Eu at the rare earth sites, i.e. we only observe the spectra of the latter sites. According to this, the fits of the spectra with a single quadrupolar pattern and with two quadrupolar patterns have given equivalent X 2 values. 1. Introduction From the early days of the high-T c superconductivity, the M6ssbauer spec- troscopy has been applied in order to study high-T c superconducting oxides [1]. In these experiments the material has to be doped with a given concentration of M6ssbauer isotopes, which must occupy the lattice sites that we want to study. In many papers, substitutions of LSlEu for Y in the 90 K superconductors have been presented [2-6]. These experiments show that the isomer shift of Eu looks like trivalent Eu [2]. Moreover, in more recent high resolution ~51Eu-M6ssbauer measurements  on EuBazCu307+ ~, no anomalies around T c have been observed in the isomer shift and in the f-factor  (recoil-free fraction) [3]. The reported values at room temperature  of the quadrupole splitting eQgVzz = - 5  m m / s  and asymmetry parameter  r / - -0 .8  are in agreement  between the various authors [2-6]. Our aim in this paper is to investigate the Ba site, by substituting 15~Eu for Ba in Eu~+xBa2_,Cu307+ ~ oxide superconductors [7-11]. For this we note the following. First, in order to obtain a single-phase superconducting compound, the concentrat ion of substituted Ba must be not greater than 20% [12]. Second, we have to substitute Eu for both Ba (partially) and Y (wholly), so that each M6ssbauer spectrum is the superposition of two spectra. By assuming that the f-factor  values are the same for the two sites, the two lines have a relative intensity about equal to x. Since the Eu valence at both sites is almost the same, it can be very cumbersome to detect and characterize the line corresponding to 9 J.C. Baltzer A.G. Scientific Publishing Company 388 L. Cianchi et al. / M S  of Eu~ + xBa2_ xCu307 + 8 thin and homogeneous samples, so as to reduce, as far as possible, the broadening of the spectrum lines. Following ref. [11], where Mt~ssbauer spectra of Eu1.25Bal.75Cu3OT+6 at 90 K and 300 K have been reported, we have taken into account the quadrupole structure of the source: eQgV~ = - 3 . 6  mm,/s  and t / =  0 [13]. 2. Experiments and results Eul+xBa2_xCu307+8 samples were prepared by an oxalate coprecipitation method which allows one to obtain a more precise stoichiometry than the ordinary solid state reaction method [14]. The powders have been characterized by SEM, TEM and XRD analyses. The SEM and TEM analyses have shown that the grains have a well defined shape, are well linked between each other and homogeneous in size: the average dimension is about 7 ~m. The Ba and Eu stoichiometry has been checked by energy dispersion spectrometry, connected with TEM. The XRD analyses have shown that the materials consist of one orthorombic single phase with a trend to a tetragonal transition as x increases. Resistivity measurements have shown a transition temperature decreasing as x increases. It changes from 92 K for x = 0 to 60 K for x = 0.15 and 40 K for x = 0.25. Absorbers were prepared, with a small thickness ( =  3.5 mg 151Eu/cmZ ), so as to avoid line distortions due to absorption saturation. M~3ssbauer spectra between 25 K and room temperature for each sample were collected, using a 50 mCi ~51SmF3 source at 300 K. All of them exhibit only a single line (fig. 1), around 0.6 m m / s ,  since europium has nominal oxidation number  + 3 in both sites [10,11]. First we fitted the spectra with a single quadrupolar  pattern. Then we made another fit with two quadrupolar  patterns, whose relative intensity was assumed equal to x. In the latter case our values at 90 K and 300 K are in agreement with those reported in ref. [11] for Eul.25Bal.vsCu307. However, the two fits gave equivalent values of X 2. Moreover, we found the area of the absorption spectrum almost equal for the two samples, at all temperatures (fig. 2). Thus, it seems that the contr ibution of Eu at the Ba site may be somehow negligible. On the other hand, in ref. [11] Bauminger et al. report M~ssbauer parameters of ~5~Eu at the bar ium site, obtained from a double quadrupole fit on Eu~.25Bal.75Cu30 7. These values are very close to the values obtained from a fit on EuCaBaCu30  7. In this latter compound it is assumed that Ca occupies the rare earth sites and Eu is located in the Ba sites, since it was proved [15], that the same substitution occurs with La in LaCaBaCu30 7. However, this assumption is not entirely convincing. In fact, by considering the ionic radii of Eu 3+ (98 pm), Ca 2+ (106 pm), y3§  (106 pm), La 3§ (122 pm) and Ba 2§ (143 pm) [16], one can unders tand why La 3§ instead of Ca 2§ can wholly substitute for Ba 2+, but it does not seem that this should be the same L. Cianchi et al. / M S  of  Eu~ + .~Ba e _ xCus07+ 8 389 z 9 z < > 100 96 92 88 100 96 92 88 100 96 92 88 1 J , " ~ 9 , ~ x = 0 25 .t T = 300 K I t i i -8 -6 -4 -2 0 ',r% / r I I 2 4 6 8 RELATIVE VELOCITY (ram/s) Fig. 1. Spectra for the x = 0.15 a nd  x = 0.25 samples at the indicated temperatures. The dotted line is drawn at 0.65 m m / s .  I t  can be seen, from the two spectra at T =  25 K, that the m a x i m u m  absorption depth is almost the same for the two samples.  for Eu. In fact, in ref. [12] it is shown that the ability of a rare earth ion to substitute onto the Ba site, to form a single-phase compound, decreases with the ionic radius. Moreover, in ref. [17] it is shown that the solubility of Eu with Ba is practically absent in YBa2_xEuxCu307 and that these compounds are not single-phase. Thus, it is possible that we do not observe the spectrum of Eu at the 390 L. Cianch i  et  al. / M S  o f  E u  I + x B a :  _ xCu307 + a 0.6 c-1 0.5 o II < 0.4 < 0.3 / /  I ~ i j j  1 . ~  ...... '" [ ~ .  . , '  ..'" [ ]  40K ,. []~ 60 K 0.3 0.4 0.5 0.6 A R E A  ( x  = 0 . 1 6 )  Fig. 2. Area of the absorp t ion  line of the more doped sample vs. the area of the absorp t ion  line of the less doped one. The full straight line corresponds  to a rat io  of the areas equal to the rat io of the actual concentra t ions  of Eu in the two samples (1 .24/1 .16 =1.07).  The exper imenta l  points  are shown with their  error bars. Ba site. This could be due to the fact that the corresponding f-factor is much smaller than that for Eu at the rare earth site. In fact the Eu 3+ ion, substituting for Ba 2+, occupies a region appreciably larger than its ionic size, so that it could be loosely held in a potential which is flattened in its central region. Such a Table  1 Isomer shift (IS) and quadrupole  spli t t ing (eQgV..) for the samples studied. The  value of the asymmetry parameter  7/, de termined  by the fits, is about  equal  to zero at all temperatures .  Compounds  T Isomer shift Quadrupo le  in terac t ion (K) IS ( m m / s )  eQvV:z ( m m / s )  EuHsBal.s~Cu3OT+8 25 0.65(1) - 5.1(2) 40 0.66(1) - 5.2(2) 60 0.66(1) - 5.3(2) 90 0.64(1) - 4.7(2) 200 0.63(1) - 5.2(2) 300 0.61(1) - 5.0(2) Eul.25Bal.75Cu307 + a 25 0.64(1) - 5.0(2) 40 0.64(1) - 4.7(2) 60 0.64(1) - 4.8(2) 90 0.64(1) - 5.0(2) 200 0.62(1) - 5.0(2) 300 0.61(1) - 5.1(2) L. Cianchi  et al. , / M S  o f  E u  j + x Ba  : _ xCu ~07 + 8 391 phenomenon is not new, i.e.: the occurence of very low f-factors in some compounds has been explained in this way [18]. From the above discussion, it is our opinion that the spectra are only due to the europium located at the rare earth sites. The single quadrupole fit has given values of eQgV== and IS almost equal for the two samples: x = 0.15 and x = 0.25. The differences are within the errors (see table 1). The values are in agreement with those reported in literature for EuBa2Cu 3Or. On the contrary, our value of Tt ( = 0) is closer to that reported by Bauminger et al. (0.3) than the ones reported by the other authors (0.8) [3-6]. Most likely this difference is due to the fact that the hyperfine structure of the source has been taken into account in the fitting procedure. This work has been partially supported by the National Research Council of Italy, CNR, under the Progetto Finalizzato "Superconductive and Cryogenic Technologies". References [1] B.D. Dunlap and C.W. Kimball, Hyp. Int. 49 (1989) 187. [2] M. Eibschutz, D.W. Murphy, S. Sunshine, L.G. Van Uitert, S.M. Zahurak and W.H. Grod- kiewicz, Phys. Rev. B 35 (1987) 8714. [3] G. Wortmann, S. Blumenr6der, A. Freimuth and D. Riegel, Phys. Lett. A 126 (1988) 434. [4] Z.M. Stadnik, G. Stroink and R.A Dunlap, Phys. Rev. B 39 (1989) 9108. [5] M. Lippmaa, E. Realo and K. Realo, Phys. Lett. A 139 (1989) 353. [6] M.Z. Jin, X.W. Liu, M.L. Liu and Y.Q Jia, Phys. kett. A 148 (1990) 217. [7] T. Iwata, M. Hikita, Y. Tajima and S. Tsurumi, Jap. J. Appl. Phys. 26 (1987) L2049. [8] H. Akinaga, H. Katoh, K. Takita, H. Asano and K. Masuda, Jap. J. Appl. Phys. 27 (1988) L610. [9] F. Munukata, K. Shinohara and M. Yamanaka, Jap. J. Appl. Phys. 27 (1988) L1507. [10] C.V. Tomy, R. Nagarajan, S.K. Malik, D.T. Adrojia, N.C. Soni and R. Prasad, Sol. State Commun. 68 (1988) 531. [11] E.R. Bauminger, I. Felner, Y. Lehavi and I. Nowik, Hyp. Int. 55 (1990) 1199. [12] K. Zhang, B. Dabrowski, C.U. Segre, D.G. Hinks, I.K. Schuller, J.D. Jorgensen and M. Slaski, J. Phys. C 20 (1987) 935. [13] I. Nowik and I. Felner, Hyp. Int. 28 (1986) 959. [14] N. Ammannati, G. Barani, R. Garr6 and S. Manganelli, Vuoto 19 (1989) 24. [15] A.H. Carim, A.F. de Jong and D.M. de Leeuw, Phys. Rev. B38 (1988) 7009. [16] J. Emsley, The Elements (Oxford University Press, New York, 1989). [17] A. Suzuki, E.V. Sampathkumaran, K. Kohn, T. Shibuya, A. Tohdake and M. Ishikawa, Jap. J. Appl. Phys. 27 (1988) 792. [18] B. Kolk, Studies of dynamical properties of solids with the M/Sssbauer effect, in: Dynamical Properties of Solids, eds. G.K. Horton and A.A. Maradudin (Elsevier, Amsterdam, 1984) p. 106. 

151-Eu Mossbauer studies on Eu(l+x)Ba(2-x)Cu(3)O(7+delta) oxide systems

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