YBa2Cu3O7 − δ superconducting composites

0 Elsevier, Paris FelYBa@1~0~-~ superconducting

LOW TEMPERATURE

composites

VARIATION

IN Fe NBa,Cu,O,_s Carole

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ALFRED-DUPLANa, Harald

Ann. Chim. Sci. Mat, 2000,25,

OF YBa,Cu30,-5

SUPERCONDUCTING Gilbert RITTER’,

LATTICE

pp. 281-292

PARAMETERS

COMPOSITES

VACQUIERa, Jannie Joerg IHRINGERC

MARFAINGb,

Laboratoire de Physico-Chimie des Matkriaux, Universite de Provence, 3 Place V. Hugo, 13331 Marseille Cedex 03, France. Laboratoire MATOP associe CNRS, Case 151, Facultk des. Sciences et Techniques de St-Jerome, 13397 Marseille cedex 20, France. Institut tiir Kristallografie, Universitit Tubingen, Charlottenstrasse 33, D-72070 Ttibingen, Germany.

Abstract - In order to complement a previous study of the electrical properties of xFel(I-x) YBa&!u307-s composites, the thermal dependence of lattice constants and orthorhombic strain (b-a) / (a+b) in the range 15 K < T < 300 K was investigated by high resolution X-ray diffraction. For low Fe contents (x = 2%), the main differences with the pure YBa.$&O,~~ (YBCO) parameters are found at temperatures lower than 100 K only; moreover, for higher concentrations (x = 5%) the orthorhombic strain is found to be about 10 % smaller than in the pure material in the whole temperature range. The quantitative differences of the a and b lattice constants with Fe content, compared with the pure YBCO value, and their temperature dependence suggest that the Cu(1) and Cu(2) positions are preferred for substitution. The substitution, however, is small enough to retain the orthorhombic structure, as the transition to the insulating tetragonal structure is not reached. At T < 100 K, the composite with x = 2% Fe exhibits an anomalous thermal behavior of lattice constants which may be caused by a magnetostrictive response to a magnetic ordering of Fe in the lattice.

R&urn6 - Variation i basse tempkrature des paramktres rhticulaires de YBa&u,O,sdans des composites supraconducteurs xFe/(l-x) YBa2Cu307-s. Pour completer une etude sur les propriettts tlectriques de composites xFel(l-x) YBa,Cu,O,-s la dkpendance en temperature de leurs parametres cristallographiques a ettc examinde ainsi que la contrainte orthorhombique dtfinie par (b-a) / (a+b) par diffraction de rayons X a haute r&solution, dans le domaine de temperature 15 K < T < 300K. Pour de faibles concentrations de Fe (x = 2%), les principales differences avec les param&res de YBa,Cu,O,-6 pur (YBCO) apparaissent pour des temperatures inferieures a 100 K Tires-a-mu-t: J. Marfaing St-Jerome,

F-13397

Marseille

Laboratoire MATOP cedex 20, France.

Associt

CNRS,

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settlement; de plus, pour des concentrations plus Clevees, la contrainte orthorhombique estd’environ 10% plus faible que pour le ma&au pur dans tout le domaine de temperatures. Les differences quantitatives des paramBtres a et b, qui varient avec la concentration en Fe, et leur dependance en temperature suggerent que la substitution se fait sur les sites Cu(1) et Cu(2). Elle est cependant suffisamment faible pour que la structure orthorhombique reste stable et que la transition tetragonale, isolante du point de vue tlectrique, ne soit pas atteinte. Lorsque T < 100 K, les parametres du reseau du composite contenant x = 2% Fe presentent un comportement en temperature anormal qui peut Ctre attribue a une reponse magnetostrictive induite par un ordre magnetique du fer dans le reseau. 1. INTRODUCTION Composites offer a number of benefits over more sophisticated configurations such as thin films or doped bulk material. The main ones are their easy fabrication and a confortable control of their performances using convenient parameters. For example, the inclusion concentration, x, is extremely fine to detect any modification in the matrix and the sintering temperature very pertinent, when combined with x , for optimization and adjustement of the composite properties. Moreover, among the composites, the high temperature superconductor composites (HTSC) emerge as an important class of materials of both academic and technological interest with regard to different applications. Several studies addressed electrical transport in metal-HTSC composites by considering their percolative behaviour [l-7] and some works examined the insulating-HTSC transition [S-9]. However, it is known that electric and magnetic behavior of these materials depend not only on the type of inclusion in the matrix but also on the processing methods [lo]. Due to the weak-link problem associated with grain boundaries, tremendous efforts have been expanded towards the improvement of these materials. In this domain, some recent results have been reported on the role of inclusions, either metallic [ 1 l-121 or insulating [ 13-141 which can improve their magnetic as well as their transport properties [ 151. Clarification of some interaction phenomena which occur during the sintering of compounds can be obtained by comparing the dopant role either in the composites or in the corresponding doped materials. In the case of superconducting composites, the YBa2Cu307-8 -based ones (XYBCO) are very competitive for electrical applications and literature is rich in results concerning the YBCO system either doped or not. Numerous studies were devoted to the YBCO structure and to the electric properties at room-, low- and high-temperatures; some of them are reported in [ 1624]. A large number of investigations dealt with substitution of several elements, mainly in the 3dmetal family, in which Fe appears as the favorite [25-341. One of the reasons is that its magnetic character greatly influences the superconductivity. More specifically, a very low Fe content in the YBCO matrix changes the critical temperature of the superconducting transition and drastically modifies the electrical properties of the pure material. In parallel, the substitution stimulates the orthorhombic-to-tetragonal structural transition in YBCO. By considering these results and to complete a previous electric study revealing electric moditications induced by Fe, a study was carried out in order to investigate the structural properties of these composites [35-371. In this paper, are reported the thermal dependence of electric response and lattice parameters of xFe/(l-x) YBCO samples as a function of the Fe concentration. The results exhibit modifications of the electric response and changes in the lattice constants values in the range 20 K - 300 K. Moreover, an anomalous behavior in the lattice constants is detected when x is low. Interpretation is given in terms of position of the dopant in the matrix and of atomic difmsion when Fe and YBCO grains are in contact. To our knowledge, there is no comparable study in the literature.

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2. SAMPLE PREPARATION AND CHARACTERIZATION Initial YBCO powders with nominal composition YBa2Cu307-8 were prepared using a method of atomisation of nitrate precursors at high temperature, previously described elsewhere [38], with successive deagglomeration sequences between each thermal treatment. The accomplishment of the reaction between the constituents, yielding nearly single phased ~a2cU306.9, was controlled by X-ray powder difI?action (Siemens D5000 diffractometer, Cu-K, radiation operated with 35 kV and 30 mA.) Gur composites were fabricated by pressing a mixture of YBCO and Fe powders (Aldrich 99,9 %) under 4 x lo* Pa at room temperature during l/2 hour. Different concentrations were obtained by varying the Fe weight from x = 2 % to x = 5 % and sintering was achieved under argon flow (at a rate of 18 Wmin) up to 820°C < T < 850°C. The highest temperature was kept for 10 min, followed by cooling at a rate of 7”Clmin in oxygen atmosphere until 450°C. After two hours at 45O”C, the samples were finally cooled down to 200°C at the rate of 1.5Wmin and air quenched. Different methods were used to characterize the samples: X-ray diffraction, scanning electron microscopy (SEM) with energy dispersive spectroscopy analysis (EDS) and ac-resistance measurements, R(T), in a classical PAR closed-cycle helium cryostat in the range 20 K < T < 300 K. Differential thermal analysis controlled the interactions during annealing in argon atmosphere with a rate of 5Wmin. All the powders, YBCO and Fe as well as YBCO blend with 2 % Fe, were studied in argon and in air. The X-ray experiments were performed with a low temperature Guinier camera and difiactometer [39-401. By moving the film perpendicular to the plane of reflection behind a 2 mm horizontal slit, the diffraction pattern is recorded continuously at 15 K < T < 300 K. Evaluation of the films with an automated densitometer yields 50 data files, each file with about 3000 data belongs to a temperature interpolated from the position of the diffraction pattern on the film. Lattice constants were refined with a special profile refinement program SIMREF 2.4 [41] for automatic processing of multiple data sets belonging to different temperatures. The program performes a Rietveld refinement for each data file, a special feature is the automated scaling of the 20-scale to the temperature dependent line positions of a Si-standard, mixed into the sample material. The complete electrical study, carried out on the Fe-YBCO composites, has revealed the influence of Fe concentration on the sample resistance, changing for example the resistance slope in the normal state or broadening the superconducting transition. Four samples, with typical behavior representative of the Fe-concentration induced modifications, have been selected for a detailed analysis of their difractograms: first, pure YBCO sintered at 910 “C (# YBCO) for reference, then two samples with 2 % of iron, sintered at 840°C and 850°C (respectively # 2/840 and # 2/850), and finally a composite with 5 % of iron, sintered at 840°C (# 5/840). From these specimens, influences of the iron concentration (2 % and 5 %) and of the sintering temperature (840°C and 850°C) can be easily separated and compared with the pure superconducting material.

3. RESULTS AND DISCUSSION 3.1. Electrical Qualities of Fe-YBCO comnosites The previous results concerning the electrical properties of these xFel(l-x) YBCO composites [34-351 have evidenced some points: - compared with pure YBCO ceramics, the sintering temperature has to be decreased by about 100 “C to obtain superconductivity in Fe/YBCO composites ; - with Fe contents of 2 % and 5 %, the composites are superconducting and present a zeroresistance critical temperature 77 K < Tot < 89 K only when the sintering is achieved at a

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temperature between 800 “C! and 840 “C; the resistance versus temperature curves, R(T), are given infigure I; - secondary phases due to Fe reaction are detected by X-ray diffraction; the SEM studies show Fe-rich phases at the grain boundaries; - these phases broaden the transition or cause a shoulder in R(T) below the onset of the superconducting transition (called double phased transition), as it may be seen in figure 1 for sample #2/850; - DTA experiments performed under argon atmosphere show that Fe oxidation is due to the oxygen present in YEKO; it should be noted that, during annealing under argon, Fe oxidation is observed at temperatures as low as 300 “C; therefore the superconducting grains in the composite are under-oxygenated, leading to a semiconductor behavior of the electric response in the normal state (&w-e I).

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Figure 1. Resistance versus temperature for pure YEKO (# YBCO) and Fe/YBCO composites with 2% Fe sintered at 840°C (# 2/840) and 850°C (# 2/850) and with 5 % Fe sintered at 840°C (# 51840). The main result is that depending on both thermal annealing and Fe composition, the electrical behaviour of the composites evolves and the superconducting character is damaged at high temperature and Fe contents. As the sintering is achieved by a solid-phase process, the chemical reactions, mainly attributed to Fe diffusion and reactivity, are probably responsible for the change in the electrical properties. This complementary study is intentionally focused on the structural behaviour and essentially concerns samples presenting a superconducting transition. 3.2. Thermal exnansion as a function of Fe content When pure YBCO is considered, the lattice constants determined at room temperature are in good agreement with literature [23-27, 30, 32, 341 and are very consistent with Sharma et al. ‘s results [23], obtained from a combination of ion channeling and neutron diftktion experiments in therange 15K
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The thermal dependence of each lattice constant was fitted by polynomials up to the fifth order, the coefficients are given in Table I.

Figure 2. a, b, c latticeparameter variations versus temperature for the four samples # YBCO (o), # 2/840 (II), # 2/850 (0) and # 5/840 (x ).. exp erimental results (points) and polynomial fit (line). In a general manner and over the whole temperature range, when increasing the Fe content or the sintering temperature, the initial orthorhombic structure of the lattice is maintained in the composites: the a-values have a tendency to increase, the c-values are roughly constant while the b-values display more dispersion and an inclination to systematically decrease. This is in agreement with results obtained at room temperature [42] concerning the lattice parameter measurements of YF%a2(Cul-xFex)307-6:samples annealed under inert atmosphere and careful low

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temperature annealing in 02 have an orthorhombic structure, while they present a final tetragonal structure. when they are conventionally sintered under oxygen atmosphere Table I : Coefficients of the polynomials for the thermal dependence of the lattice constants y = Co + Cl T + C2 T* + Cs T3 + Ch T4 + Cs T5 with y = a, b and c [A] for the different samples, in the range 15K < T < 250 K for # YBCO, # 2/840, # 2/850 and # 5/840 (see text). The Cr and C2 coefficients are zero. Standard deviations: 2~1 on the last digit.

More specifically, in the composite with 2% Fe content sintered at 840°C the thermal expansion abruptly changes below 110 K, so the coefficients for this material hold for 110 K < T < 250 K only. Moreover, compared with pure YBCO, the thermal dependence of the cell volume v;gUre 3) exhibits that the presence of Fe induces a volume contraction. This result is significative as the pure YBCO cell volume, determined in our experiments, is in very good agreement with Sharma’s results, even the a, b, c values are slightly different.

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4#2/850

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Figure 3. Evolution of the YBCO lattice cell volume versus temperature for the four samples # YBCO, # 21840, # 21850 and # 51840.

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Most interesting is the anomalous contraction of the lattice constants and volume below 100 K for the sample with 2% Fe sintered at 840°C. The variation of a and b with temperature affects the orthorhombicity (&we 4) defined as normalized orthorhombic strain (b-u) I (a+b). The orthorhombicity in pure YBCO, found in this work, is somewhat lower than this detected by neutron diffraction [23]; however, the thermal dependence is very similar. Compared with #YBCO, the orthorhombicity of the two composites sintered at 840°C is smaller. The smallest one, that is the most tetragonal like cell, is found for # 2/850. In contrast to the other samples it is nearly constant and does not increase with decreasing temperature.

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Figure 4. Evolution of the YBCO orthorhombic strain (b-u) / (a+b) versus temperature for # YBCO, # 2/840, # 2/850 and # 5/840. (+) are values from Sharma’s results [23] . In none of the samples a complete transition to a tetragonal lattice is observed. This is surprising because in YBa2(Cul-xFe,)307-6, such a transition from orthorhombic to tetragonal symmmetry could easily be induced either by heat treatment [42-431 (at high temperature and oxygen atmosphere) or by a critical Fe-substitution in the YBCO lattice [29, 31, 44-451. Investigations made by several techniques such as X-ray [26-28, 30, 331 and neutron diffraction [29] or MGssbauer spectrometry [25, 31, 331 allow to state that the orthorhombic to tetragonal transition occurs in samples with an Fe content beyond 2.3% [26]. Clearly, samples with x SO.03 are found to have an orthorhombic structure similar to the undoped material, while samples having x 20.05 are found to be tetragonal. So, we can have a rough estimate of the Fe content which could substitute in the YBCO lattice when preparing the composites (less than 0.03). The most critical question is the assignment of the location of the Fe impurities. 3.3. Models for the Fe inclusion For the interpretation of the data found in this work, let us review the models for the Fe insertion described in the literature using the most common techniques. There are different suggestions. Miissbauer spectroscopy indicates that Fe mainly substitutes at the Cu( 1) sites of the CuO chains [33]. However, thermal treatments modify the Fe distribution and for example under inert atmospheres at high temperature Fe will migrate from the Cu(1) site into the Cu(2) site in pure YBCO [42] and Ca-YBaCu-Fe0 material [46]. This random redistribution of Fe atoms is often accompagnied with a decrease of the critical temperature and a change of the initial

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structure; the final symmetry of the lattice is correlated not only with oxygen content but with the iron distribution between copper sites, and hence with the thermal treatment applied before reoxygenation [46]. Neutron diffraction shows also the complexity of Fe substitution in YBa2(Cur-xFe,J307-6 [29] either on the Cu(1) chain- or on the Cu(2) sites; location of Fe on the pyramidal Cu(2) sites has also been evidenced in Y~&axBa2(Cut-,,Fey)~07 [46-471. To compare the results reviewed above with the findings of the present study, it should be mentioned that the former have been determined from studies performed at room temperature. Furthermore, element doping obtained by synthesis from precursor powders (BaC03 and YzO3, CuO and Fe203 oxides) yields materials different from composites fabricated with a time dependent sintering. In the composites, a low Fe concentration of 2% changes the lattice parameters figure 2) as well as the electric response in the normal and in the superconducting state figure I). At room temperature, the b- and c-values are more altered by the Fe content and the sintering temperature than the u-values which, however, significantly change at 850°C. From this evolution, we may assume that some Fe atoms enter the orthorhombic YBCO lattice. Yet, it is not sufficient for a transition to tetragonal symmetry, as reported for Fe doped material [25-271. The most important changes appear on the results obtained at high temperature, and we will examine in detail the sample with 2% Fe, sintered at 85O”C, (#2/850). In this case, the reduction of the b and c lattice constants and expansion of the a values, over the whole temperature range exhibit that most of the Fe atoms probably substitute the Cu atoms. This may be seen by calculation of the constants using the radii rcu = 0.072 mn for Cu+2 and rFe = 0.064 nm for Fe+3. Assuming n the number of equal Cu ions on each axis in pure YBCO (on the c-axis, n =3), hence c is given by : cpure=2nrc,,fA The constant A is the difference between the actual value of c- and the sum of the radii of the metals atoms. When Cu on the c-axis is substituted by Fe in the ration (I-X)X, the length c becomes : cse=2n [(1-x)rc,+xm,]+A From these equations x follows to be : X= (CxFe - Cpure )/2n he-m4 1 The values for c pure = 1.16785 mn, c xFe = 1.16724 mn, found in this work, yield x = 0.013 f 0.006. The equivalent calculation using the a- and b-axis of this material, with n =l, yields x = 0.018* 0.003 and 0.033 f 0.007 using respectively apure = 0.3825~1, bpure = 0.38858 m and a,Fe = 0.3822 nm, bxFe = 0.38805 nm. The comparison between the x values, which are shown to be direction dependent, suggests that Fe substitution occurs in the chains and in the ab plane of the YBCO lattice. A credible scenario is that the Fe atoms enter the YBCO lattice preferentially along the b-axis, on the Cu(1) sites as the b-axis parameter is significantly affected by a low Fe content and a small temperature gradient. When the temperature increases, they are moving into the Cu(2) sites of the ab plane as the u-parameter variations attest. In any case, the substitution is lower than 0.03. Another explanation for the change from c PUre to c ~~ would be a change in oxygen contents 0 7-s from 6 = 0.066 to 6 = 0.012 [24]. However, this minor change in stoichiometry would not change the a- and b- axes in the observed magnitude. So we suggest that the difference in lattice constants, calculated at 296K but existing over the temperature range, mainly reflects the different proportion of Fe substitution in the samples on the Cu( 1) and Cu(2) sites. Besides, the lattice parameters are very sensitive to the sintering temperature. Figure 5 presents patterns of #YBCO, #2/840 and #2/850 obtained from the SEM study showing the influence of the solid-sintering process. From the structural results, a difference of 10 “C between 840 “C and 850 “C, increases a and decreases b while c remains constant. With decreasing difference between a and b, the lattice becomes more tetragonal.

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Figure 5. SEM micrographs showing the results of the solid-sintering process for #YBCO, #2/840 and #2/850. The change in the lattice may be caused by a deoxygenation induced by the higher temperature during sintering (07 to 06 transition) or by Fe insertion in the 07 cell. In the first case, c is expected to increase t?om 1.16 run to 1.184 mn [24], as was discussed in the paragraph above. In the present work, at all temperatures, c remains identical or slightly lower than in the superconducting state. This behavionr is consistent with the fact that the scenario involved in the composites is due to Fe substitution rather than to a deoxygenation of the material. These results could illustrate the first stage of the Fe-Cu substitution process induced in the xFe/(l-x) YEXO samples during the sintering. 3.4. Anomalous exvansion in the comvosite with 2% Fe sintered at 840 “C Of special interest is the contraction of the lattice in the #2/840 composite below 110 K. A similar anomaly in the thermal contraction is observed in H~,,~C!a&lrrO~ [48]. The latter is correlated with the magnetic order of Mn, as it may be seen from the temperature dependent intensities of magnetic reflections in neutron dimaction pattern. Mn ordering was also evidenced in Mn/YBCO composites [49] by EPR experiments. In this case the EPR signal exhibits a quasi one dimensional low temperature behavior, attributed to magnetic interactions of a one dimensional system of Mn ions. Interpretation is that Mir probably enters the CuO chains in the YBCO lattice, as the b-component is affected. This assumption is supported by a theoretical investigation that yields the symmetry of the most stable model, based on the method of static concentration waves [50]. It shows a superstrucure of long range ordered doped atoms, substituted in the CuO chains. So, by analogy with the magnetostrictive lattice deformation in Hoa,iCa,,.@n03 and with the magnetic ordering found in the related MnM3CO composites, it might be speculated that the anomalous lattice contraction in Fe/YBCO is the magnetostrictive response to the magnetic ordering of the Fe atoms. It is evident that the magnetic order has to be confirmed by a neutron diffraction pattern below 110 K or EPR experiments for example. 4. CONCLUSION This study, focused on the low-temperature behaviour of superconducting FeM3CO composites, shows in what manner the Fe content influences the electrical response as well as the lattice parameters of the pure superconducting material. The electrical response displays a

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semiconducting-like character in the normal state or a shoulder feature at low temperature when the Fe content or the sintering temperature are increased. Moreover, incorrelation with the Fe concentration, the lattice constants of the composites are affected. A minor increase in the sintering temperature of 10 “C results in an increase of a and a decrease of b in the range 15K c T < 300 K. This evolution leads to an incomplete orthorhombictetragonal transition. Compared with pure YBCO, the contraction of the cell volume in the composites, indicates that the lattice deformation is merely caused by Fe-insertion or Fesubstitution in the YBCO lattice and not by a deoxygenation of the superconducting material. The analysis of the a- and b-axis deformations suggests that the substitution mainly affects the Cu( 1) sites of the chains but also the Cu(2) sites in the ab plane of the YBCO lattice. However, the substitution remains low, estimated from lattice parameter measurements to 0.03, as the tetragonal structure, known for highly substituted material, is not reached. Of special interest is the lattice deformation in the composite with 2% Fe, sintered at 84O”C, as the unusual contraction below 110 K could suggest a magnetic ordering of the Fe ions. 5. REFERENCES [I] [2] [3] [4] [5] [6] [7] [S] [9] [IO] [l l] [12] [ 131

[14] [ 151

G. Xiao, F.H. Streitz, A. Gavrin, Y.W. Du, CL. Chien, Effect of transition-metal elements on the superconductvity of YBaCuO, Phys. Rev. B 35 (1987) 8782. G. Xiao, F.H. Streitz, M.Z. Cieplack, A. Bakhshai, A. Gavrin, C.L. Chien, Electrical transport and superconductivity in Au-YBaCuO percolation system, Phys. Rev. B 38 (1988) 776. B. Dwir, M. Affronte, D. Pavuna, Evidence for enhancement of critical current by intergrain Ag in YBaCuO-Ag ceramics, Appl. Phys. Lett. 55 (1989) 399. B. Ropers, F. Carmona, S. Flandrois, Phenomenological approach to the resistive transition of YBaCuO-Ag superconducting random composites, Physica C 204 (1992) 7 1. J.J. Calabrese, M.A. Dubson, J.C. Garland, The critical current of the AgM3aCuO random bulk composites, J. Appl. Phys. 72 (1992) 2958. K. Yoshida, Y. Sano, Y. Tomii, Percolative behavior of the Ag-phase clusters in superconducting Bi-Pb-Sr-Ca-Cu-0 ceramics, Physica C 206 (1993) 127. S. Dubois, F. Carmona, S. Flandrois, Evidence for eddy currents and skin effects in 123/Ag random composites, Physica C 2 16 (1993) 111. J. Koshy,K.V. Paulose, M.K. Jayaraj, A.D. Damodaran, Transport properties of the percolation system YBaCu0,M3a,Sn0,,s, Phys. Rev. B 47 (1993) 15304. J.K. Thomas, J. Koshy, J. Kurian, Y.P. Yadava, A.D. Damoran, Electrical transport and superconductivity in YBaCuO,/YBa,HfO,, percolation system, J. Appl. Phys. 76 (1994) 2376. K.Salama and D.F. Lee, Progress in melt-texturing of YBCO superconductors, Applied Superconductivity, ed. H.C. Freyhardt (1994) p. 261. M. T. Lanagan, R. B. Poeppel, J. P. Singh, D. I. DOSSantos, J. K. Lumpp, U.Balachandran, J. T. Dusek, K. C. Goretta, Superconducting wires, J. Less Common Metals 149 (1989) 305. J.Langhom, Y.J. Bi, J.S. Abell, Platinium group metals as flux pinning additions in screen printed superconducting YBazCusO,-a thick films, Physica C 271 (1996) 164. F. Sandiumenge, N. Vilalta, S. Pinol, B. Martinez, X. Obradors, Aging of the microstructure of melt-textured Y13a2Cu307-a composites and implications on their superconducting properties, Phys. Rev. B 5 1 (1995) 6645. T. S. Sampathkumar, S. Srinivasan, T. Nagarajan, U. Balachandran, Properties of YBa2Cu307-a minus BaBiO composite superconductors, Appl. Supercond. 2 (1994) 29. J. Marfaing, S. Regnier, J. M. Debierre, C. Caranoni, Insulating-(super)conducting transitions in new ferroelectic-superconductor composites: Pbr(ScTa)Oc-YBarCu30T-a, Physica C 280 (1997) 21.

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(Article recu le 24/03/99, sous forme definitive le 03/12/99)