PAC- and channeling experiments in YBa2Cu3O7 − x

Journal of the Less-Common Met&,

PAC- AND C~ELIN~

164 & 16.5 (1990) 1121-1128

~PERIM~S

1121

IN YBaoCmfh-x

A. Bartosi H. Plank2 D. Forkel2, S. Jahn3, J. Marke12, R. Polewk#,'M. Uhrmache;' , S. Winter", W. Witthuhn2 and the ISOLDE Collaboration 1 11. Physikalisches Institut der Universitst Bunsenstr. 7-9, D-3400 Gijttingen, FRG

Giittingen

2 Physikalisches Institut der Universitgt Erlangen-Ntirnberg Erwin Rommel Str.1, D-8520 Erlangen, FRG 3 Fakult;it f. Physik, supported

Universitst

Konstanz,

D-7750 Konstanz

by the BMFT

performed with l'lCd PAC experiments in YBa2Cu307-x were lllAg and populated from different parent nuclei, i.e. "'In, I"mCd. lriIn can substitute at the Cu(l)-, the Ba- or the Ysite, depending on the annealing temperature. On the other side, lliAg substitutes at the Cu(2)-site, whereas the measurements at tllaCd did not allow assignement. any site Channeling experiments performed with electrons and positrons emitted from '12*In and IllmCd support this interpretation.

I.. INTRODUCTION Since the discovery of the high Tc superconductivity by Miiller 1 and Bednorz many efforts have been made to elucidate the mechanism leading to the various properties of these oxides. Nuclear methods like NMR, ME or perturbated angular correlations (PAC) allow to look into a bulk superconductor at a microscopic scale. Here we present our results on electric field gradients (EFG) reflecting the local charge distribution around lilCd probe atoms in YBa2Cu307-X. Different parent nuclei, i.e. "'In, l'*Ag and t"mCd allow to place the probe atoms at different lattice sites depending on their valency and size. The channeling and blocking effects of charged particles emitted in the nuclear decay from 112aIn and *ltsCd give further evidence on the lattice site occupied by the In and Cd atoms. 2. EXPERIMENT The PAC measurements were performed with the isomeric 5/2'which was populated either by the EC state of lttCd (T1/~=84 ns), of 1llIn (T112=2.8 d) or by P--decay of ltlAg (Ti/2=7.5 d) decay or by the 151 keV-x-decay of "smCd (T1/2=49 min), respectively. A conventional four detector set-up with BaFz crystals combined with 0022"5088/90/$3.50

0 Elsevier Sequoia, Printed in The Netherlands

1122

used. Further timing electronics was slow-fast coincidence Combining in Ref 2. eight experimental details are given were intensity ratios coincidence time spectra properly, R(t) obtained, which were fitted for polycrystalline samples by the function F(t):

F(t)

= A22

Z fi E s~nfni)

i

coslan (ni)v9itt expi-ao(nif6pitl

n

fi corresponds to the relative number of probe The fraction atoms, which are exposed to an EFG characterized by the quadrupole the asymmetry parameter coupling COnStant up i =eQVP z /h, ni=(VuxIllIn Vyv l/V,, and frequency distribution b~i. The parent nuclei were introduced into the YBazCus07-x samples either by diffusion or by implantation. The diffusion was performed in an oxygen atmosphere of lO=Pa at various temperatures. After implantation of -1013 Univ. "lIn+-ions with an energy of 400 keV at IONAS, the samples were annealed at 975 E in order to anneal Gettingen, were radiation damage. For IllIn, procedures all annealing followed by a slow cooling down process to restore the oxygen stoichiometry of the samples. tt*Ag and lllfn were implanted simultanously into YBanCua0~-s (Geneva). with an energy of 60 keV at the ISOLDE facility at CERN spectra were obtained at room temperature after succesive R(t) The lilnCd isotopes were annealing steps in an oxygen atmosphere. implanted an energy of 60 keV at ISOLDE by with too, followed short annealing procedures. Localisation measurements were done with the emission and blocking methodl'. Therefore we measured the channeling anisotropic countrate of conversion-electrons and positrons from the decay of the isotopes tt*mfn and lltrCd, which were implanted (60keV, 7*10'2 cm-2) into YBaCuG-films deposited on SrTiOa. 3. RESULTS lliIn/lllCd, PAC data at room temperature obtained with diffused into YBazC~a07-~ at T=1223 K, are shown in fig. 1. About fl=60% of the Cd atoms are exposed to an EFG characterized by an asymmetry parameter nt=O.2912) and a coupling constant vQl=39.4(9) MHz (fig. l).Another fp=lO % of the probe atoms are located at a site with nz=0.39(21 and vgz=138.8(9) MHz. Further fractions with small from probe amplitude were observed: probably they result atoms diffused into nonstoichiometric parts of the sample and will not be considered in the following. annealing at After implantation of lliIn and subsequent T-=975 K two fractions (f3zfda30%) with q3=na=l.O, vQ3=143(1) MHz and ~~4=156(1) MHz are observed (see fig. 2). The influence of the on a PolYannealing temperature was checked by PAC measurements

1123

0

250

500

FRE~ENCY

TIME hsf PAC spectra

500 IMHzl

250

0

FIGURE 1 after diffusion of IllIn and annealing

at 1223 K

(after doping with 'ilIn by diffusion) at crystalline sample different fig. 3. The Fourier transform of the temperatures, the reveals spectrum obtained after annealing at 973 K clearly implantapeaks corresponding to fraction f3 und f4, seen after are fractions tion. After annealing at 1073 K and 1173 K these replaced by fl and fz with a decreasing amplitude of fz when Similarly, the fraction fl appears in increasing the temperature. implanted samples after heating up to 1200 K.

-0.1 0

E("'In') =LOO keV 200

0

100

TIME [nsl FIGURE

PAC spectra

500

1000

FREQUENCY [MHz1

after implantation

2

of lllIn and annealing

at 975 K

temperature obtained from lilAg/lllCd at room The spectrum after annealing at 1223 K in an oxygen atmosphere followed by a slow cooling down process is shown in fig. 4. About fa=60% of the site with Vp!~=767(2) MHz and lllAg occupy a lattice atoms annealing at about ns=O.O813) * This fraction appears after

1124

Ta=lOOO E, at the same time, the **'In/"*Cd spectra of the sample show the well known fractions f3 and fs. The measurements on the l*lmCd/l~lCd showed only heavily damped PAC spectra system indicating large EFG distributions. After room temperature implantation of Il*=In, the conversion electrons show a channeling effect in the direction of the c-axis, whereas the positrons perform blocking minima (fig.5). The data reveal, that at least 40% of the In atoms occupy lattice sites on the c-axis. At an implantation temperature of 50 K the effects are slightly reduced, but still at least 30% of the In isotopes are placed on one of the two rows along the c-axis, i,e. either the

500

250 FREQUENCY [MHz FIGURE 3 Fouriertransforms

1

Cull)-0-Cu(2)-row or the Y-Ba-row. When the implantation temperature is increased again, the channeling and blocking effects recover-After implantation of lll=Cd at 5OK, 150K and 3OOK only a vanishing channeling effect could be After annealing up to observed. 6OOR for one minute the effect grew slightly.

after various

annealings

of one sample

4. DISCUSSION atoms in The EFG parameters of fraction fi of the lllCd YBazCu307 (fig.11 agree with those already published4, the small originate from including fraction f2 in the fitting differences that the In atoms most 4 it was argued, procedure. In Ref. the Y atoms due to the same number of valence probably replace This picture was supported electrons and the similar ionic radii. the by calculations of the EFG at the metal lattice sites within has charge model (PCMj4 (see table 1). Often this model point difficulties to estimate the strength of the EFG, but the symmetry of the EFG is predicted more reliable. For the Ba site the PCM These values are in calculations yield q = 0.46 and up=110 MHz. reasonable the measured data of fraction fz: agreement with nz= 0.39(2) and UQZ= 138.8(g) MHz.

1125

From NMR experiments with b3Cu and 6sCu the asymmetry parameters of the EFG at the Cu-sites are known to be n(NMR,Cu(l)) L 0.92 and n(NMR,Cu(2)) < 0.14 6.7 . If the charges value the high of the Cu and 0 ions are adjusted to reproduce for n(NMR,Cu(l) )=l then the PCM calculates very high frequencies lllCd at the Cu-sites (for details see table 1). Although the oxygen coordination may alter the antishielding factor used' all predict much higher EFGs for IllCd at the cuPCM calculations sites compared to the other sites. Consequently a dublett of high

100

50

0

TIME hsl

PAC spectra

1000 FREQUENCY

FIGURE 4 after implantation of lllAg and annealing

2000 [MHz] at 1223 K

VQ~= 156(l) MHz both having vQ3= 143(l) MHz and frequencies at the Cu(l)-site'-". n3*4= 1 had been attributed to lllCd frequencies were observed ;fter implantation of lllIn into These polycrystalline and into a single crystal of as welll;fntered ,::lets A similar EFG was found after diffusion of YBaCuO-1 5 lllIn into sintered samples and into YBaCuOT-films too .

Table 1: Point charge model calculations Site Y

Ba Cu(l) Cu(2)

VP (MHz) 74 110 694 672

for YBaZCu307-X n 0.23 0.46 0.99 0.11

The following charges were assumed: Y= + , Ba2+, Cu(l)'+, Cu(2)2+, O(l)'-,0(2,3,4)*-; the EFG was calculated for lllCd with Q=O.SO b and g-=-31.9.

1126

observer of the lllCd at the Cu(l)-site should be an ideal phase transition between the tetragonal and orthorombic phase of YBaCuO. The measured PAC parameters in the tetragonal phase (qr=O, coordination". vpr=134 MHz) are typical for a linear oxygen These results can be taken as an additional proof for a Cull)-site occupation by "*In. known14'16, that trivalent ions attract an It is well additional oxygen atom at the O(5)-site, if they substitute the Cu(l)-site in YBaCu07. The same is expected to happen for In=+ but demonstrate wether UP to now the experiments could not clearly five or six oxygen l*lIn at this site is coordinated by four, The "blue phase" (YzCUZOS) was investigated to solve atomsll. In this compound the Cu-sites are Q-fold oxygenthis question. coordinated, but less planar than the Cu(lf-site of the superthe natural site occupation for an conductor. On the other hand, Y-site. In atom should be the octahedral coordinated In both and in cases one expects different PAC paramters in p3Cu20s . The reason YBanCu307, but surprisingly they are quite similar is not yet and new similarity understood for this striking experiments are in progress to solve this question. Now we are able to build a model of the dependence of the EKG parameters on the annealing temperature: Below lOOOK the In atoms diffuse along the Cu-O-chains and substitute for Cu atoms at the This picture is supported by experiments indicating a Cu(lf-site. great of Cu(l)-vacancies (4-8*10-a even in number per atom), single At higher annealing temperatures the probe crystals15. atoms may leave the W(l)-sites and diffuse to the Y-sites via the Ba-sites. This explains the decrease of the amplitude of fraction (fig. 3). fz after annealing at Ta= 1173K compared to Ta= 1073K Such a partial occupation of the Y- and the Ba-sites by In is also in agreement with predictions based on ;Elculations of solution energies for various dopants by Baetzold . Thus, by variation three of four sites of temperature, cation of the annealing YBazCu307-x can be occupied by lllIn probe atoms. The main risk in this attempt is the high process temperature needed (up to 12OOK) which may cause phase transitions or segregation of the superconductor. The channeling experiments performed with X*a*In show a considirectly fraction occupying lattice sites on the c-axis derable In order to get the PAC spectra of fig. 2 the after implantation. samples had to be annealed at least at 900 K after the implantaCombining both results it seems that the tion at IONAS or ISOLDE. radiation damage from the In implantation mainly concerns the Oatoms in the Cu(l)-0-Cu(l) chains perpendicular to the c-axis; if

1127

1.6

T=300K

t++t+ /t+t +t

1

L

-2 0 2 -2 0 ; TILT ANGLE t DEG I

Channeling

FIGURE 5 in c-axis with electrons and positrons

from ll*=In

around In In substitutes at the Cu(l)-site then oxygen disorder but hardly influence the would strongly disturb the PAC spectra, channeling effect along the c-axis, as observed. have been given which Up to now no convincing arguments correlate one oilthe frequencies observed in YBaCuO with illIn at the the CuiZl-site . Following the ideas of Baetzold17 implantation of twovalent Cd-atoms may lead to a probe location at the Cu(2)-site. Our channeling and PAC experiments performed with indicating a lil.Cd both revealed only spectra of poor quality, strongly perturbated surrounding of the Cd atoms. the co-implantation of Promising results are obtained from show the litAg and illIn : The PAC data from the IllIn probes The known PAC parameters which were attributed to the Cu(l)-site. probes reveal completely new EFG PAC spectra obtained with 1ilAg parameters, different to any results obtained with lliIn (fig. 4). Because of the chemical equivalence of Cu and Ag one would expect A fit to the data yields the that Ag atoms replace Cu atoms. This small asymmetry frequency vgg= 767(2) MHz and IJS= 0.08(3). rules out the Cu(l)-site. So we conclude, that IllAg parameter resides at the Cu(2)-site. PCM calculations (table 1) and NQR with this site measurements at 63Cu and @"Cu are in agreement assignement6. In particular, the Cut21 site is the only one in as 0.1. PAC YBanCu307 with an aysmmetry parameter as small experiments at lt*Ag in YBaaC~z.7Ago.306~~ were already performed

1128

Shen and coworkers17, but they observed only very low vp values of 67.9(10) MHz and 59.0(8) MHz. The main difference to our experiments is the substitution of a macroscopic amount of Cu by lead to a clustering of the Ag atoms or to the Ag< which may formation of nonstoichiometric compounds. Furthermore, the poor time resolution the of 2 3.5 ns quoted in Ref. 17 prevented detection of frequencies as high as 1 GHz seen in the present experiment based on BaFz-detectors (time resolution better than 1 ns for the detected low x-energies). bY

5. CONCLUSION In these experiments we populated the sensitive 5/2+ level of which were 'l'Cd at 245 keV from three different mother nuclei, implanted into YBaCu07 samples. As always the same 'sensing-state' measures observed PAC-parameters show the EFG, the different clearly, that the valency of the implanted probes governs the site 1llIn3+ is found on three of four cation sites, occupation: IlinCd2’ doesn't reside sites at all on lattice (PAC and channeling experiment) and lllAg prefers the Cu(2)-site. ACKNOWLEDGEMENTS We thank J. Geerk and G. Linker (KfK/IFNP) for supplying YBaCuO-films and Mrs. H. Thomas (Gottinger HTSL-Verbund) preparing the bulk samples.

the for

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