Effect of fluorination on the structure and superconducting properties

Physica C 301 Ž1998. 155–164

Effect of fluorination on the structure and superconducting properties of Y2 Ba 4Cu 7 O 14qd phases A.M. Abakumov a , M.G. Rozova a , R.V. Shpanchenko a , M.L. Kovba a , S.N. Putilin a , E.V. Antipov a , O.I. Lebedev b, G. Van Tendeloo b,) , E.M. Kopnin c , J. Karpinski c a b

Department of Chemistry, Moscow State UniÕersity, 119899 Moscow, Russian Federation EMAT, UniÕersity of Antwerp (RUCA), Groenenborgerlaan 171, B-2020 Antwerp, Belgium c Laboratorium fur ETH-Honggerberg, CH-8093 Zurich, Switzerland ¨ Festkorperphysik ¨ ¨ ¨ Received 29 January 1998; accepted 16 February 1998

Abstract Strongly reduced Y2 Ba 4Cu 7 O14.09 has been successfully fluorinated using XeF2 as a fluorination agent. The structure of the fluorinated material has been studied by a combination of X-ray diffraction, electron diffraction, high resolution electron microscopy ŽHREM. and X-ray microanalysis ŽEDX.. For a high level of fluorination, the intercalation of fluorine is accompanied by a significant structural rearrangement and leads to an increase of Tc from 30 to 62 K. HREM reveals the ˚ Fluorine enters into anion-defipresence of a partially fluorinated, well ordered Y2 Ba 4Cu 7 O14 Fd phase with c s 51.96 A. cient Cu1 layers, resulting in the formation of an octahedral arrangement of part of the Cu1 atoms with a significant elongation of the apical Cu–O distances and the c-parameter. Local ordering of fluorine atoms leads to the formation of defects with a limited extension where all Cu1 atoms have an octahedral coordination; the c-parameter in these areas is as ˚ q 1998 Elsevier Science B.V. All rights reserved. high as 53.5 A. Keywords: Y2 Ba 4 Cu 7 O14q d ; Fluorination; Electron microscopy

1. Introduction Recently, we demonstrated that fluorination of reduced non-superconducting YBa 2 Cu 3 O6.11 using XeF2 as a fluorinating agent is accompanied by the appearance of bulk superconductivity with Tc up to 94 K w1x. At a high level of fluorination, a significant structural transformation occurs due to the formation of a fully fluorinated YBa 2 Cu 3 O6 F2 compound. Full occupation of the 1r2,0,0 and 0,1r2,0 positions in

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Corresponding author.

the Cu1 layers by fluorine results in an octahedral coordination of the Cu1 atoms. Such an arrangement leads to a remarkable distortion of the Cu1ŽO,F. 6 octahedra due to a Jahn–Teller effect, followed by an increase of the apical Cu1–O separation by 0.5– ˚ and a c-parameter increase up to 13.2 A. ˚ This 0.7 A Ž fully fluorinated YBa 2 Cu 3 O6 F2 phase called F2. was revealed by high-resolution electron microscopy as noticeable areas included within a matrix of partially fluorinated YBa 2 Cu 3 O6 Fd Ž0 - d - 1. ŽF1. or as isolated defects with limited extension w2x. Another high-Tc superconductor with a structure closely related to that of the 123 compound is

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 8 . 0 0 1 0 8 - 7

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Y2 Ba 4 Cu 7 O 14q d Žabbreviated as 247.. Its ideal crystal structure can be described as an ordered alternation of blocks comprising single CuO chains Ž123units. and double CuO chains Ž124 or YBa 2 Cu 4 O 8 units. along the c-axis. The 247 phase exhibits a superconducting transition with Tc ranging from 30 to 95 K, depending on the oxygen content w3,4x. By analogy to the stoichiometric 124 compound, the double CuO chains remain fully oxidized and oxygen non-stoichiometry of the 247 compound is connected to the presence of 123 blocks w5–8x. In contrast to the non-superconducting reduced Y123, the reduced Y247 phase exhibits a superconducting transition with Tc about 30 K, probably due to hole transfer from the fully oxidized 124 to the 123 block. According to the structural resemblance, one may expect that 123 and 247 compounds under fluorination should exhibit a similar behavior. Taking into account the results previously obtained by the fluorination of Y123, a strongly reduced anion deficient starting material looks more promising than an oxidized one, since anion exchange in the latter case requires higher reaction temperatures leading to the formation of very stable admixtures ŽBaF2 , YOF, etc... The aim of the present work is to prepare superconducting fluorinated 247 compounds using a softchemistry technique with XeF2 as a fluorinating agent and to characterise the fluorinated materials structurally by X-ray diffraction, electron diffraction and high-resolution electron microscopy.

All subsequent operations were carried out in a glove box in dried N2 atmosphere excluding the presence of O 2 . 0.4 g of Y2 Ba 4 Cu 7 O 14.09 was mixed with 0.03–0.08 g of XeF2 and intimately ground in an agate mortar. Syntheses were carried out in Nicrucibles placed into N2-filled and sealed copper tubes at 3008C for 30 h. Phase compositions of the samples and lattice parameters of the compounds were determined by X-ray powder diffraction using a focusing Guinier-camera FR-552 ŽCuK a 1-radiation, germanium internal standard. and a STADIrP diffractometer ŽCuK a 1-radiation, transmission mode, scintillation counter.. The formal copper valence Ž VCu . in the initial and fluorinated samples was determined by iodometric titration. AC susceptibility measurements were carried out in the temperature range 12–100 K at an external field amplitude of 1 Oe and a frequency of 27 Hz. Electron diffraction ŽED. and high-resolution electron microscopy ŽHREM. were performed using a Jeol 4000 EX microscope operating at 400 kV. The Scherzer resolution of the microscope is of the order ˚ EDX analysis and electron diffraction was of 1.7 A. carried out with a Philips CM20 microscope equipped with a LINK-2000 attachment. Image simulations were carried out with MacTempas software.

3. Results 3.1. X-ray study and magnetic measurements

2. Experimental CuO, single phase YBa 2 Cu 3 O6q d and XeF2 were taken as starting materials. First Y2 Ba 4 Cu 7 O 14q d was obtained by annealing a stoichiometric mixture of YBa 2 Cu 3 O6q d and CuO at 9508C under 20 kbar oxygen pressure. The resulting samples had a composition Y2 Ba 4 Cu 7 O 14.92 . The reduced material Y2 Ba 4 Cu 7 O 14.09 was prepared by annealing Y2 Ba 4 Cu 7 O 14.92 pellets in an Ar flow under controlled partial oxygen pressure Ž PO 2 f 10y3 atm. at 6258C for 10 h. After annealing, the samples were quenched into a massive metal vessel. X-ray measurements indicated that the initial Y2 Ba 4 Cu 7 O 14.09 compound, used for further fluorination, was single phase.

The treatment conditions, lattice constants and superconducting properties of the oxidized Y2 Ba 4 Cu 7 O 14.92 , the reduced Y2 Ba 4 Cu 7 O 14.09 and some of the fluorinated samples are presented in Table 1. The temperature and time of the reaction were inspired by the results previously obtained for YBa 2 Cu 3 O6.11 w1x. The variation of the fluorine amount to be introduced into the sample was achieved by a change of the 247:XeF2 molar ratio from 1:0.6 to 1:1.7. The X-ray patterns of fluorinated samples exhibit significant changes with respect to the amount of XeF2 used. Sample 3, obtained with a deficiency of XeF2 Ž1:0.6., contained two distinct phases with a 247-type structure. The first one Žcalled F1. exhibits

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Table 1 Fluorination conditions and results of treatment for Y2 Ba 4 Cu 7 O14q d Sample

˚. a ŽA

˚. b ŽA

˚. c ŽA

˚ 3. V ŽA

VCu

Tc ŽK.

1 2 3

Y2 Ba 4 Cu 7 O14.92 Y2 Ba 4 Cu 7 O14.09 247:XeF2 s 1:0.6

50.363Ž7. 50.794Ž4. 50.664Ž8. 51.78Ž3. 51.775Ž9. 51.962Ž8.

749.49 756.47 756.19 773.70 774.40 777.34

80 30 40

247:XeF2 s 1:1.2 247:XeF2 s 1:1.7

3.8791Ž6. 3.8693Ž4. 3.8769Ž5. 3.876Ž2. 3.8777Ž9. 3.878Ž1.

q2.26 q2.02

4 5

3.8364Ž8. 3.8490Ž4. 3.8499Ž7. 3.855Ž1. 3.8572Ž9. 3.8576Ž9.

q2.10 q2.11

56 62

a

F1 F2 F2 F2

slightly increased a- and b-parameters but decreased c-parameter in comparison with the initial 247 phase. The F1 phase is probably formed in the first step of the fluorination reaction, we will discuss this further below. The second phase Žcalled F2. exhibits a strong enlargement of the c-parameter Žfrom 50.794 ˚ . as well as the cell volume Žfrom 756.5 to to 51.78 A ˚ 3 .. An increase of the amount of XeF2 in the 773.7 A reacting mixture allows to suppress the formation of the F1 phase; the X-ray pattern of sample 4 only reveals the presence of the F2 phase. The formation of the F2 phase is accompanied by a drastic change of the intensities and a broadening of the reflections in the X-ray pattern. The hkl reflections with large values of l, exhibit significant broadening and a loss of intensity Žsee, e.g., the reflections 011 and 0.1.13, 111 and 1.1.13 for samples 2 and 4 in Fig. 1.. The reflections with l s 0,1 however, remain sharp and intense. Such X-ray patterns suggest a well established order in the a–b plane and a strong disorder along the c-axis or the presence of numerous defects along Ž001.. Further fluorination leads to a progres˚ The sive increase of the c-parameter up to 51.962 A. iodometric titration reveals that fluorine incorporation leads to an increase of the formal copper valence from q2.02 for the initial reduced compound to q2.11 for sample 5. An increase of the fluorination reaction temperature to 3508C leads to decomposition of the 247 compound. One observes the formation of an ill crystallized cubic ŽBa,Y.-mixed oxyfluoride with a ˚ The oxygen released during the des 5.986Ž5. A. composition is absorbed by the inner surface of the copper tube and results in the formation of traces of Cu 2 O. Both initial reduced Y2 Ba 4 Cu 7 O 14.09 and oxidized Y2 Ba 4 Cu 7 O 14.92 samples exhibit supercon-

ducting transitions at 30 and 80 K, respectively. The temperature dependence of the magnetic susceptibility for the fluorinated samples shows a successive increase of Tc Žonset. with fluorination ŽFig. 2.. Starting from 30 K for the non-fluorinated Y2 Ba 4 Cu 7 O 14.09 sample, Tc arises to 62 K for sample 5. The estimated superconducting volume fraction is about 15%.

Fig. 1. X-ray diffraction patterns of Y2 Ba 4 Cu 7 O14.09 before Ža2. and after Ža4. fluorination. Both patterns contain only peaks of the 247 phase.

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appearance of these forbidden reflections can be attributed to double diffraction and the presence of 908 rotation twins along w001x; i.e., parallel to the

Fig. 2. Temperature dependence of the magnetic susceptibility for initial Y2 Ba 4 Cu 7 O14.09 Ža2. and fluorinated samples Žaa3, 4, 5..

3.2. Electron microscopy study Since the reflections on the X-ray pattern exhibit a significant broadening and decreased intensities, the Rietveld refinement has limited possibilities to reveal the structure of the fluorinated compounds. Therefore, ED and HREM investigations were performed to clarify the structural transformations which occur during the fluorination. The best fluorinated sample Ž5., was chosen for an electron microscopy study. The a and b unit cell parameters determined by X-ray diffraction were used for internal calibration of the electron diffraction patterns and the HREM images. ED patterns along the w001x ) , w010x ) and w100x ) zones are shown in Fig. 3. All diffraction patterns can be indexed in an A-centered orthorhombic unit ˚ b s 3.88 A˚ and c s 51.9 A. ˚ cell with a s 3.86 A, ) w x The 001 pattern shows the intensity and spot distribution typical for the usual 247 compound. The presence of hk 0 reflections with k / 2 n in the w001x ) ED pattern violates the extinction conditions imposed by the Ammm space group. The intensity of these reflections is systematically lower than the intensity of the hk 0 reflections with k s even. The

Fig. 3. Electron diffraction patterns along w001x ) , w010x ) and w100x ) for sample a5.

A.M. AbakumoÕ et al.r Physica C 301 (1998) 155–164

Fig. 4. Part of the EDX spectra containing OŽK a . and FŽK a . peaks for two different crystallites from sample a5; note varying F-content.

incident beam. The formation of such composite diffraction patterns was observed previously for 124 material w9x. This twinning should be accompanied by a splitting of spots which, however, is hardly visible on the ED pattern even for reflections with large ™ g-vectors since the difference between the a and b parameters for this compound is very small. The additional reflections h0 l, l / 2 n, related to the presence of rotation twins, were also found in the rows with h s odd in the w010x ) pattern. Direct space confirmation follows from the HREM images. Both w010x ) and w100x ) patterns exhibit diffuse streaking along the c) axis. Such streaking may arise from the presence of planar Ž001. defects. Some ED patterns along the w100x ) axis also show additional weak streaks parallel to the c) direction at the position 0, k q 1r2, l. The w001x ) pattern for such areas also exhibits 0,k q 1r2,0 superstructure reflections, resulting in a doubling of the b-parameter. The fluorine content in the crystallites of sample 5 was evaluated by EDX analysis performed inside the electron microscope. All studied crystallites do contain fluorine, however, a quantitative analysis is difficult because of the low atomic number of fluorine. The O:F intensity peak ratio was used for an estimation of the fluorine amount. This ratio signifi-

159

cantly varies for different crystallites and reveals the inhomogeneous fluorine distribution in the sample ŽFig. 4.. The HREM images along the w010x zone reveal that two different areas, perfect and defected ones, are present in the sample. The perfect one shows a typical 247-type structure ŽFig. 5.. Direct measurement of cra ratio by optical diffraction ŽOD. yields ˚ which is in a good agreea c-parameter of 51.9 A, ment with the X-ray diffraction data. The defect, arrowed in Fig. 5, presents a single w100x-oriented block in the w010x matrix. Within a well ordered 247 matrix, containing a high F-content, numerous pancake type defects were observed; some of them are indicated by arrows in Fig. 6. The bright dots in this image can be shown to correspond to the heavy atom configuration Žsee image simulations, to be discussed below.. It is visually clear that the contrast inside the defects

Fig. 5. w010x HREM image of a well ordered, partially fluorinated 247 phase Žsample 5.. The arrow marks the 908 rotation twin of half a unit cell wide in the 124 part of the unit cell. The optical diffraction pattern is shown as an inset.

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Fig. 6. w010x HREM image showing the appearance of a fully fluorinated 247 compound as defects Žmarked by arrows. in the Cu1 layers. The optical diffraction pattern is shown as an inset.

significantly differs from the matrix contrast: the dots imaging Ba atoms become particularly bright. This contrast variation is accompanied by a significant increase of the distance between the Cu1-layer and the neighboring Ba–O layers in the block with the 123-type structure. As a result the local value of ˚ At the same the c-parameter increases to 53.5 A. time, the brightness of the Cu1 dots becomes similar to that of the copper atoms neighbouring the Y-layer. This can be understood by the formation of a similar equatorial environment for these copper atoms. The presence of numerous defect areas produces OD patterns with intense streaks along the c)-direction. The extension of these defects is mostly limited to 15–20 repeat units along the a-direction. The alternations of limited areas with different c-parameter imply that the Ž00 l . layers cannot remain straight and become wavy resulting in a local ‘loss’ of periodicity along the c-axis. The variation of the c-parameter along the a-axis appears to be related to the full occupation of anion positions within the Cu1 layers by fluorine atoms. We assume that this leads to an elongation of the apical Cu1–O distances followed by a shift of the Ba atoms away from the Cu1 plane. To verify this assumption, image simulations were performed for

two structural models which have a different occupation of the anion positions in the Cu1 layers. The ˚ first model was based on a unit cell with c s 51.9 A, with full occupation of the 0,1r2,0 site and a random placement of fluorine and anion vacancies on the 1r2,0,0 site. The number of fluorine atoms per formula unit was varied in the range 1 - d - 2. The ˚ second model is based on a unit cell with c s 53.5 A and full occupation of the anion sites in the Cu1 layers by fluorine atoms; it corresponds to a composition Y2 Ba 4 Cu 7 O 14 F2 . The atomic coordinates for both models were calculated by taking into account the increased unit cell with elongated Cu1–O apical distances Ždue to the Jahn–Teller effect. and a shift of the neighboring Ba atoms. In order to maintain the interatomic distances calculated from the structure of fully oxidized Y2 Ba 4 Cu 7 O 14.94 w5x, the positions of the atoms in the other layers were adapted. The comparison of the theoretical images calculated for different thickness and defocus values ŽFig. 7. with the experimental ones shows that the model with partial occupation of the anion sites in the Cu1 layers corresponds to the perfect structure as in Fig. 5, while the model with a full occupation is in good agreement with the specific contrast observed in the local defects of Fig. 6.

A.M. AbakumoÕ et al.r Physica C 301 (1998) 155–164

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Fig. 7. Calculated matrix of w010x HREM images for partially Ža. and fully fluorinated Žb. 247 phases for different thicknesses and defocus values.

It should be noted that the insertion of fluorine into the Y-layer should also result in the formation of an octahedral arrangement for Cu atoms in the neighboring layers, followed by an increase of the distance between adjacent Cu–O layers and result in a significant enlargement of the c-parameter. However, our HREM observations never show the variation of interlayer distances which could be connected to the insertion of fluorine into the Y-layer. A number of other defects, such as stacking faults, microtwins due to 908 rotation about the w001x axis, etc., also were found in the sample. However, these defects have already been studied before for non-flu-

orinated 247 and 124 compounds w9,10x; they are formed during the preparation of the initial samples. 4. Discussion The schematic crystal structure of fully reduced Y2 Ba 4 Cu 7 O 14 is shown on Fig. 8a. Assuming a constant anion stoichiometry for the 124 block, there are two possible positions for the insertion of fluorine atoms into the 123 blocks. Both positions are placed in the xy0 plane ŽCu1-layers.. A small amount of fluorine Žup to a composition Y2 Ba 4 Cu 7 O 14 F. may either occupy one of the two positions and form

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Fig. 8. Schematic representation of the structural changes in the 247 phase induced by fluorine insertion: fully reduced Ža., partially Žb. and fully fluorinated Žc. compounds.

a square Cu1 coordination polyhedron or may be randomly distributed among the available 1r2,0,0 and 0,1r2,0 sites. By analogy with the oxygenated samples, we may expect that fluorine will preferentially occupy the 0,1r2,0 site, leading to a square planar coordination of the Cu1 atoms. Such an arrangement creates single chains of copper–oxygen– fluorine squares running along the b-direction in the 123 block, together with double CuO chains in the 124 block ŽFig. 8b.. The insertion of fluorine in this position should enhance the orthorhombic distortion, as it was shown in Ref. w4x for oxygenated 247 compounds. If the fluorine content is more than one atom per 123 block, the extra anions will also occupy the 1r2,0,0 site leading to a reduction of orthorhombic distortion. Full occupation of both positions in the Cu1 plane results in the formation of an octahedral coordination for the Cu1 atoms. It will

introduce a distortion of the octahedra due to a Jahn–Teller effect and to a significant increase of the apical Cu1–O1 separation. As it was shown by fluorination of the 123 phase w1,2x, this enlargement ˚ and, consequently, the can be as large as 0.5–0.7 A c-parameter for the 247 phase should increase to ˚ ŽFig. 8c.. Such fluorination will lead to about 53.5 A an increase of the formal valence for the Cu atoms and keep the conducting CuO 2 layers unaltered. Indeed, the fluorine incorporation increases the formal Cu valence from q2.03 to q2.11 and Tc from 30 to 62 K. However, this VCu s q2.11 is not enough to obtain the maximum Tc s 80 K, which w as observed for the fully oxygenated Y2 Ba 4 Cu 7 O 14.92 sample. The insufficient formal copper valence in the fluorinated samples may be explained by the presence of a competing anion exchange reaction with a replacement of oxygen

A.M. AbakumoÕ et al.r Physica C 301 (1998) 155–164

atoms by fluorine ones. When one oxygen atom is replaced by one fluorine atom and the released oxygen is absorbed by the walls of the copper tube Žtraces of Cu 2 O were observed on the inner surface of the Cu tube. this reaction should decrease the formal copper valence. It was shown by fluorination of fully oxidized Y123 by XeF2 in a closed vessel that anion exchange really takes place and leads to a reduction of VCu from q2.33 to q2.22 w1x. At 3508C the anion exchange becomes the predominant reaction and the 247 phase decomposes, producing mixed ŽBa,Y.-oxofluorides. The presence of Cu 2 O on the inner walls of the copper tube allows us to propose that the partial oxygen pressure during the fluorination should be determined by equilibrium oxygen pressure under the CurCu 2 O mixture. We estimate that ln PO 2 f y22 at the temperature of fluorination Ž3008C.. This partial oxygen pressure corresponds in order of magnitude to the partial PO 2 for reduced YBa 2 Cu 3 O6.1 Žln PO f y21.1.. Moreover, the 247 compound loses 2 oxygen from Y2 Ba 4 Cu 7 O 14.91 to Y2 Ba 4 Cu 7 O 14.21 when annealed in the temperature range 250–5508C at significantly higher oxygen pressure ln PO 2 f y12.4 w3x. This allows us to suppose that, under our particular reaction conditions, oxygen intercalation cannot be the origin of the changes in phase composition of the fluorinated samples or be responsible for the structural transformations of the resulting compounds and the increase of Tc . The formation of two phases F1 and F2 on the first stage of fluorination confirms the assumption that intercalation of fluorine occurs in different steps. The first one: Y2 Ba 4 Cu 7 O 14 q XeF2 ™ Y2 Ba 4 Cu 7 O 14 Fd q Xe

Ž 0 F d F 1.

Ž 1.

yields the F1-phase with a reduced c-parameter Ž c s ˚ in our experiment.. The intercalation of a 50.66 A small amount of fluorine is accompanied by an increase of the orthorhombic distortion 2Ž b y a.rŽ b q a. from 0.53% for the initial Y2 Ba 4 Cu 7 O 14.09 compound to 0.70% for the F1 phase in the sample a3. This result allows to suppose that fluorine preferentially occupies the 0,1r2,0 site. At the second stage:

Ž 1 . q XeF2 ™ Y2 Ba 4 Cu 7 O14 Fd q Xe Ž 1 - d F 2 . ,

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the F2-phase is formed. This phase exhibits a significant enlargement of the c-parameter in comparison with the initial non-fluorinated compound and the F1 phase. The changes in c-parameter for fluorinated and oxygenated samples have an opposite behavior. For instance, the increase of oxygen content from Y2 Ba 4 Cu 7 O 14.09 to Y2 Ba 4 Cu 7 O 14.92 leads to a decrease of the c-parameter by about 0.9%. The orthorhombic distortion for the F2 phase decreases to 0.52% in comparison with the F1 phase Ž0.70%. and oxygenated Y2 Ba 4 Cu 7 O 14.92 phase Ž1.11%.. The structural information extracted from ED and HREM data allows us to propose a possible model for the anion distribution in the Cu1 layers. At a high level of fluorination, when the amount of fluorine exceeds 1 atom per formula unit, the extra anions occupy the 1r2,0,0 site. As a result of a random placement of fluorine in this position, only part of the Cu1 atoms will have an octahedral environment and these octahedrally coordinated atoms are statistically distributed in the xy0 plane resulting in the presence of large areas with the well ordered 247 structure and an enlarged cell parameter. X-ray diffraction showed ˚ an increase of the c-parameter up to 51.8–51.96 A. The variation of this parameter, probably, reflects different levels of fluorination of the 247 phase. High resolution electron microscopy revealed that an inhomogeneous fluorine intercalation leads to a local ordering of the fluorine atoms and defects with a limited extension are formed. In such defected areas, all Cu1 atoms are octahedrally arranged and the ˚ From our c-parameter is locally increased to 53.5 A. point of view, this value corresponds to the maximal level of fluorine insertion. The areas with partially and fully occupied anion sites in the xy0 planes alternate along the a-axis resulting in a distortion of the other atomic layers and a violation of periodicity along the c-direction. The inhomogeneity of the fluorine distribution and the non-negligible distortion of the structure are probably the reasons of the broad superconducting transition and the relatively small superconducting volume fraction. The present investigation shows that the results of fluorination of strongly reduced 123 and 247 phases by XeF2 are quite similar. For both phases, the insertion of a fluorine-oxidizer leads to an increase of the formal copper valence, followed by an appearance of superconductivity in the 123 phase and an

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increase of Tc for 247 phase. For both structures, the fluorine enters into the anion-deficient Cu1 layers, resulting in the formation of an octahedral arrangement of Cu atoms with a significant elongation of the apical Cu–O distances. However, in contrast to the fully fluorinated tetragonal YBa 2 Cu 3 O6 F2 phase, the 247 phase remains orthorhombic, even for a high fluorination degree since the double CuO chains are unaffected by the fluorination. The intercalation of fluorine into the 123 compound occurs more inhomogeneous, which allows us to observe relatively large areas of the fully fluorinated YBa 2 Cu 3 O6 F2 phase inside a matrix of partially fluorinated phase. The areas of the 247 compound with fully occupied anion positions in the Cu1 layers were detected only as a local defects.

Acknowledgements This work has been performed in the framework of NATO grant HTECH.LG 960325 and partially supported by the Russian Scientific Council on Superconductivity ŽPoisk., Swiss National Science Foundation Ž7SUPJ048713. and RFBR-INTAS

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