High quality YBa$_{2}$Cu$_{3}$O$_{6+x}$ (YBCO) superconducting thin films were implanted with the radioactive $^{197m}$Hg (T$_{1/2}$ = 24 h) isotope to low fluences of 10$^{13}$ atoms/cm$^{2}$ and 60 keV energy. The lattice location and stability of the implanted Hg were studied combining the Perturbed Angular Correlation (PAC) and Emission Channeling (EC) techniques. We show that Hg can be introduced into the YBCO lattice by ion implantation into unique regular sites. The EC data show that Hg is located on a highly symmetric site on the YBCO lattice, while the PAC data suggests that Hg occupies the Cu(1) site. Annealing studies were performed under vacuum and O$_{2}$ atmosphere and show that Hg starts to diffuse only above 653 K

Alves, E

Amaral, V S

Araújo, J P

Correia, J G

Da Silva, M F A

Galindo, V

Langouche, G

Lourenço, A A

Marques, J G

Melo, A A

Senateur, J P

Soares, J C

Sousa, J B

Vantomme, A

Von Papen, T

Wahl, U

Weiss, F

CERN Document Server

Stability studies of Hg implanted YBa2Cu3O6xJ.P. Araujo a,b,*, J.G. Correia b,c, U. Wahl d, J.G. Marques c,e, E. Alves e, V.S. Amaral a,A.A. Lourencßo f, V. Galindo g, T. von Papen g, J.P. Senateur g, F. Weiss g,A. Vantomme d, G. Langouche d, A.A. Melo c, M.F. da Silva e, J.C. Soares c,J.B. Sousa a, ISOLDE Collaboration ba IFIMUP, Dep. Fısica, Faculdade de Cie^ncias, Rua do Campo Alegre 687, P-4150 Porto, Portugalb CERN, CH-1211 Geneva 23, Switzerlandc CFNUL, Av. Prof. Gama Pinto 2, P-1699 Lisboa Codex, Portugald Instituut voor Kern- en Stralingsfysica, Celestijnenlaan 200 D, B-3001 Leuven, Belgiume Instituto Tecnologico e Nuclear, Estrada Nacional 10, P-2685 Sacavem, Portugalf INESC, Rua Campo Alegre 687, P-4150 Porto, and Dep. Fısica, University Aveiro, P-3800 Aveiro, Portugalg UMR CNRS 5628, INPG-ENSPG, BP 46-38402 St. Martin DÕHeres, Cedex, FranceAbstractHigh quality YBa2Cu3O6x (YBCO) superconducting thin films were implanted with the radioactive 197mHg(T1=2 24 h) isotope to low fluences of 1013 atoms/cm2 and 60 keV energy. The lattice location and stability of theimplanted Hg were studied combining the Perturbed Angular Correlation (PAC) and Emission Channeling (EC)techniques. We show that Hg can be introduced into the YBCO lattice by ion implantation into a unique regular site.The EC data show that Hg is located on a highly symmetric site on the YBCO lattice, while the PAC data suggests thatHg occupies the Cu(1) site. Annealing studies were performed under vacuum and O2 atmosphere and show that Hgstarts to diuse only above 653 K. Ó 1999 Elsevier Science B.V. All rights reserved.PACS: 68.55.L; 74.62.D; 74.76Keywords: Ion implantation; Thin films; High-Tc superconductors; Doping; Nuclear techniques1. IntroductionThe YBa2Cu3O6x (YBCO) compound is one ofthe most studied and interesting high-TC super-conductor materials. Doping studies have shownthat the superconducting properties of YBCO arealways degraded, except when doping with Au orHg, which increase TC by 2 K [1] and 10 K [2]. Inthese studies Hg was introduced to several percentduring sintering by changing the composition ofthe reactants. Therefore the Hg site was notknown, and the presence of dierent crystallinephases cannot be excluded.In the present work radioactive 197mHg was in-troduced into YBCO thin films by ion implanta-tion to very low fluences. The atomic environmentNuclear Instruments and Methods in Physics Research B 147 (1999) 244–248* Corresponding author. Tel.: 41 22 767 6491/9709; fax: 4122 767 8990; e-mail: joao.pedro.esteves.de.araujo@cern.ch0168-583X/98/$ – see front matter Ó 1999 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 5 4 4 - 8open-2001-01103/03/1998of Hg and its lattice site position were studied withthe eÿ)c Perturbed Angular Correlation (PAC),and Emission Channeling (EC) nuclear techniques[3]. For control, the critical temperature (Tc) wasmeasured with the Alternate Susceptibility (vac)technique in the as-grown state and after the im-plantation and annealing treatments.2. Experimental detailsPoly- and [001] single-crystalline high qualityYBCO thin films (100–350 nm) were deposited byPulsed Laser Ablation or MOCVD on [100]SrTiO3 substrates. The crystalline quality waschecked by Rutherford Backscattering/Channelingand X-ray diraction. The 197mHg radioactiveisotope was implanted into the films at roomtemperature (RT) and 60 keV energy at theISOLDE facility [4]. To avoid the production ofunknown crystalline phases the beam was swept toget homogeneously low fluences of 1.0´1013 Hg/cm2. The Hg projected range is RP 16 nm, and itspeak concentration 7´1018 at/cm3 [5], i.e. about0.01 at.%. The implanted samples were firstly an-nealed in vacuum (3´10ÿ6 mbar), increasing thetemperature from RT up to 757 K in 100 K stepsof 10 h each. PAC measurements were made afterimplantation and during vacuum annealing. Sub-sequently the samples were annealed at 723 Kunder O2 flow (1 bar). The temperature was in-creased from RT to 723 K in 30 min and after 6 hannealing, slowly decreased (1 K/min) back to RT.The PAC measurements were performed usingthe well-known 165–134 keV cascade from thedecay of the 24 h half-life 197mHg isomeric state [6].Details of the experimental setup and procedurescan be found in Ref. [7]. The PAC technique isparticularly suitable to probe the vicinity of theimplanted Hg nuclei at an atomic level [8]. Itssensitivity is given by the electric field gradient(EFG) which is caused by the deviation from cubicsymmetry of the charge distribution around theprobes. This results in a modulation of the PACspectrum, R(t). For a cascade with an intermediatelevel with spin I 5/2, three frequencies are ob-servable per EFG. From these frequencies thecoupling constant of the interaction, mQ  eQVzz=h,and the asymmetry parameter g  Vxx ÿ Vyy=Vzzare extracted. Vzz is the principal component of theEFG tensor, which is produced from the extranuclear charge distribution. Q)0.057  0.007 bis the quadrupole moment of the 134 keV excitedstate. The interaction with randomly distributeddefects does not lead to the measurement of asharp frequency, but rather to a distribution offrequencies. Here we assume such distributions tobe Lorentzian-like, with average value ámQñ andstandard deviation rQ, depending on the densityand variety of the lattice defects.The EC experiments were performed after im-plantation by measuring the yield of the L +Mconversion electrons from both the 165 and 134keV transitions, as a function of angle towards the[001] crystallographic direction. For this purposewe used 30´ 30 mm2 position sensitive detectorsdeveloped at CERN for high-energy physics ex-periments and recently adapted for the present use[9].3. ResultsFig. 1 shows R(t) spectra measured at severaltemperature steps up to 757 K. All spectra werefitted assuming one frequency distribution cen-tered at ámQ1ñ 122 1 MHz. In Fig. 2 rQ and g areplotted as a function of annealing/measuringtemperature. rQ decreases strongly from rQ292 MHz (as-implanted) to 6.6 0.6 MHz(TM 511 K), enhancing the amplitude of the R(t)spectra (Fig.1). The strong attenuation of the as-implanted spectrum is due to Hg nuclei that in-teract with randomly distributed defects. Withinour range of sensitivity, a few nanometers aroundthe Hg probe nuclei, we identify one annealingstep at 511 K where the lattice recovers and nopoint defects are in the neighborhood of theprobes. The g parameter vanishes for annealingtemperatures above 511 K.The Hg stability was studied measuring the ccount rate of the 134 keV transition and the PACcoincidence rate. No losses were observed up to653 K. Above this temperature Hg starts to diuse.During annealing at 757 K both the c and the PACcoincidence rates had the same relative decrease,J.P. Araujo et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 244–248 245indicating a strong out-diusion of Hg. If Hgwould only diuse within the film, the coincidencerate would decrease at a higher rate than the c rate,due to a higher absorption and scattering of theconversion electrons through the film. At 757 Kthe diusion was still slow and the Hg losses smallenough to measure an 8 h long R(t) spectrum(Fig.1), which displays a dierent pattern, char-acterized by ámQ2ñ 233 MHz and rQ2 25 9MHz. This change was shown to be irreversiblewhen returning to measurements at temperatureslower than 653 K. We further checked that all theremaining activity was still on the film. Thus theR(t) spectrum taken at 757 K is due to Hg atomswhich are no more in the initial lattice sites but arestill in the YBCO lattice.Fig. 3 shows the normalized conversion elec-tron yields measured after implantation, as aFig. 2. Plot of the standard deviation rQ, and the asymmetryparameter g as a function of annealing/measuring temperaturein vacuum.Fig. 3. Normalised conversion electron intensities measured othe [001] direction, after implantation.Fig. 1. PAC spectra measured after implantation and duringannealing in vacuum up to 757 K.246 J.P. Araujo et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 244–248function of the angle o the [001] axis. Theincrease of the electron yield along the [00 1] axisreveals that Hg lies along the c-axis either on theY/Ba or the Cu rows, since EC eects along pure Orows are expected to be much less pronounced.This agrees with the high symmetry of the 197Hg(YBCO) EFG, whose principal component (Vzz)was found to be aligned with the [001] axis [10].New EC experiments investigating dierent crys-talline axes are in progress.Fig. 4 shows the in-phase component of thealternate susceptibility (v0) measured as a functionof temperature in the as-grown film, after im-plantation and subsequent annealing in vacuumup to 757 K, and after subsequent annealing at 723K for 6 h under flowing O2. As expected, thesample didnÕt show the superconducting phasetransition after annealing in vacuum. The oxygenlosses should have caused the orthorhombic–te-tragonal structural phase transition, thus trans-forming the superconductor compound into anantiferromagnetic insulator. Superconductivitywas recovered after 6 h annealing at 723 K underO2 flow. The structure that is seen in the v0 curveafter annealing in oxygen, is most likely due toinhomogeneous O doping.4. Discussion and conclusionsThe results presented here were shown to be fullyreproducible for all thin films we studied, inde-pendent of their production method. After im-plantation the Hg probes are already in uniquelattice sites but are still interacting with randomlydistributed defects. When annealing up to 653 K,100% of the implanted Hg atoms remain on thesame site, with no more point defects in its neigh-borhood. In the same temperature range the fun-damental frequency mQ1 didnÕt show any relevantchange. This behavior can be understood by thefact that the lattice parameters should not showappreciable changes, although the orthorhombic–tetragonal phase transition can occur due to theannealing in vacuum [11]. Then g should be moresensitive to the phase transition. We have shownthat when annealing under O2 flow at 723 K [10], orby annealing in vacuum at temperatures below 511K [this work], Hg occupies a regular lattice site withcharge symmetry lower than tetragonal (g 0.20).When annealing in vacuum at temperatures above511 K g vanishes to zero, revealing that the Hg sitesymmetry changed to tetragonal. In previous workwhere the oxygen diusion coecient (DO) wasstudied for thin films in O2 atmosphere [12], it wasfound that oxygen starts to migrate, being ex-changed with the atmosphere, above 523 K. Byusing DO(588 K)  1.5´10ÿ13 cm2/s we estimatedthe diusion length for a 10 h long annealing,L (DO t)1=2 735 nm, which is much larger thanthe film thickness. The vac measurements performedafter annealing in vacuum have shown that thefilms were no more superconducting. Then the su-perconducting transition was reestablished afterannealing in O2 flow, reinforcing the interpretationthat oxygen was released during annealing in vac-uum to a relative content x <0.4 on the YBa2Cu3O6x structure. Thus we interpret the vanishingof g as being due to the orthorhombic–tetragonalYBCO structural phase transition, which occurredin our thin films between 511 and 587 K.The EC results show that Hg lies along theCu or Y/Ba c-axis rows. It is known that Hg has abig anity to form linear-like coordinated bonds,e.g., in the Hg1Ba2Canÿ1CunO2n2d high-Tcsuperconductor family and in HgO. In both casesFig. 4. In-phase component of alternate susceptibility (vac)measured: (a) as deposited; (b) after Hg implantation andsubsequent annealing in vacuum (no superconductor transitionwas found down to 20 K), and (c) after subsequent annealingunder flowing O2.J.P. Araujo et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 244–248 247Hg has two linearly coordinated (apical) oxygenatoms as its first neighbors. Moreover, the ratio|mQ/Q| |eVzz/h| 2140263 MHz bÿ1 found inthis work is very close to the value |eVzz/h| 1829210 MHz bÿ1, found for 199Hg : HgO[13]. Then the lattice contribution for the EFG isin first order proportional to the ratio RHgVzz/(1)c1) 14.6 V/A2, where (1)c1) is the Stern-heimer antishielding coecient that accounts forthe EFG enhancement due to the polarization ofthe probeÕs (Hg) electronic core [14,15]. Takingequivalent ratios from the EFGs calculated fordierent lattice sites in YBCO6x0 [16], we findRCu1  13.5 V/A2, RCu2  4.2 V/A2, RBa 1.4 V/A2, and RY 1.1 V/A2. Thus the Cu(1) YBCOlattice site appears as the most favorable to beoccupied by the Hg ions.When annealing above 653 K Hg starts to dif-fuse out slowly and the R(t) spectra changes irre-versibly. The Hg diusion mechanism is notknown. However, the very low value ofámQ2ñ233 MHz suggests that Hg is no morebound to oxygen and occupies low EFG latticesites in the YBCO lattice. Finally, the abundanceof oxygen seems to delay this diusion, since in aprevious work we have shown that after annealingunder O2 flow a small amount of Hg was lost at723 K and mQ did not change [10].In conclusion, in this work we have applied thePAC and EC nuclear techniques to study the sta-bility and lattice site of implanted Hg into YBCOthin films. We found that Hg occupies one uniquelattice site, which is stable up to 653 K. Both the ECand the PAC results suggest that Hg occupies ahighly symmetric lattice site, most likely the Cu(1)site, bonded to two apical oxygen atoms. A firstannealing step of the implantation defects wasfound at 511 K. The orthorhombic–tetragonalphase transition occurred between 511 and 587 Kunder vacuum annealing. Hg diusion starts onlyabove 653 K. The PAC results at 757 K show thatHg is no more bound to oxygen and is now occu-pying higher symmetry sites in the YBCO lattice.AcknowledgementsThis work was partially funded by FCT,Portugal, through projects CERN/S/FIS/1048/95,PBICT/C/CTM/1891/9 and grants under thePRAXIS XXI Programme (J.P.A., J.G.C. andJ.G.M.).References[1] G. Ganakas, M.J. Morgan, G. Jakovidis, J. Mater. Sci.:Mater. Electr. 6 (1995) 244.[2] A.K. Chakraborty, K. Bose, G. Som, B.K. Chaudhuri, J.Mater. Sci.: Mater. Electr. 5 (1994) 22.[3] J.G. Correia, Nucl. Instr. and Meth. B 136–138 (1998)736.[4] E. Kugler, D. Fiander, B. Jonson, H. Haas, A. Przewloka,H.L. Ravn, D.J. Simon, K. Zimmer, ISOLDE Collabora-tion, Nucl. Instr. and Meth. B 70 (1992) 41.[5] J.F. Ziegler, J.P. Biersack, U. Littmark, in: The Stop-ping and Range of Ions in Solids, Pergamon, NewYork, 1985.[6] W. Troger, T. Butz, P. Blaha, K. Schwarz, Hyp. Int. 80(1993) 1109.[7] J.G. Marques, J.G. Correia, A.A. Melo, M.F. da Silva,J.C. Soares, ISOLDE Collaboration, Nucl. Instr. andMeth. B 99 (1995) 645.[8] Th. Wichert, N. Achziger, H. Metzner, R. Sielemann,Perturbed angular correlation, in: G. Langouche (Ed.),Hyperfine Interactions of Defects in Semiconductors,Elsevier, Amsterdam, 1992, p. 1.[9] U. Wahl, J.G. Correia, S. Cardoso, J.G. Marques, A.Vantomme, G. Langouche, ISOLDE Collaboration, Nucl.Instr. and Meth. B 136–138 (1998) 744.[10] V.S. Amaral et al., J. Magn. Mag. Mater. 177–181 (1998)511.[11] J.D. Jorgensen et al., Phys. Rev. B 36 (1987) 3608.[12] Y. Li, J.A. Kilner, T.J. Tate, M.J. Lee, R.J. Chater, H.Fox, R.A. De Souza, P.G. Quincey, Phys. Rev. B 51(1995) 8498.[13] T. Butz, Th. Volkel, O. Nuyken, Chem. Phys. 149 (1991)437.[14] R.S. Raghavan, E.N. Kaufmann, P. Raghavan, Phys. Rev.Lett. 34 (1975) 1280.[15] R.P. Gupta, S.K. Sen, Phys. Rev. A 8 (1973) 1169; F.D.Feiock, W.R. Johnson, Phys. Rev. 187 (1969) 39.[16] K. Schwarz, C. Ambrosch-Draxl, P. Blaha, Phys. Rev. B42 (1990) 2051.248 J.P. 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Stability studies of Hg implanted YBa$_{2}$Cu$_{3}$O$_{6+x}$

Stability studies of Hg implanted YBa $_{2}$ Cu $_{3}$ O $_{6+x}$

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Stability studies of Hg implanted YBa $_{2}$ Cu $_{3}$ O $_{6+x}$