Influence of Argon Heat Treatment on Structure, Critical Temperature, Magnetic Shielding and Irreversibility Line of Ln(SrBa)Cu₃O_6+z(Ln = Y, Eu)

Influence of Argon Heat Treatment on Structure, Critical Temperature, Magnetic Shielding and Irreversibility Line of Ln(SrBa)Cu₃O_6+z(Ln = Y, Eu)

Essediq Youssef El-Yakoubi, Abdelhakim Nafidi, Samir Melkoud, Merieme Benaadad, Ali Khalal.

Laboratory of Condensed Matter Physics and Nanomaterials for Renewable Energy, Faculty of sciences, University Ibn Zohr, 80000 Agadir, Morocco.

AbstractWe have studied the structural and superconducting properties of Ln(SrBa)Cu3O6+z(Ln= Y, Eu). Each of the two samples was submitted to two types of heat treatment: an annealing in oxygen [O] and a heat in argon followed by oxygen annealing [AO]. Our iodometry measurements indicate the same total oxygen constant 6 + z, which was around 6.95 ± 0.04 in each sample. The [AO] treatment increased the orthorhombicity (*10- 3)=(ba)/(b+a)from 8.23 to9.9 and from 3.1 to 6.97 in the samples with Ln= Y and Eu respectively., indicating a conservation of the orthorhombic structural phase. This was accompanied by an decrease of 1.3 K in critical temperature Tc to Tc[AO] = 81.7 K in the case of YSrBaCu3O6+z, while in the case of EuSrBaCu3O6+z, the Tc[O] = 81.1 K increase remarkably by 5.95 K to Tc[AO] = 86.7 K. Further, there was an enhancement of the irreversibility line whatever Ln. A combination of several factors such as the change of the ionic size of the rare earth Ln, its disorder on the (Sr, Ba) site, the chain oxygen ordering and increase in phase purity for the [AO] samples may qualitatively account for the observed data.

Mots-clés : LnSrBaCu3O6+z; Heat treatments; Substitutions; XR diffraction ; AC magnetics susceptibility.

1. INTRODUCTION

   Some of the technological applications of superconductivity include: the production of sensitive magnetometers based on SQUIDs, fast digital circuits (including those based on Josephson junctions and rapid single flux quantum technology), powerful superconducting electromagnets used in maglev trains, magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR) machines, magnetic confinement fusion reactors (tokamaks), the beam-steering and focusing magnets used in particle accelerators, electric motors and generators [1].

   Extensive research efforts have been directed toward the study of high-temperature superconducting cuprates ever since its discovery in early 1986 [2].The microscopic theory of this phenomenon has not yet been elaborated. X ray diffraction (XRD) and AC. susceptibility (ac = ‘ + i”) are very useful for characterizing the dynamics flux in high Tc superconductors (HTSC). A sharp decrease in the real part (‘), just below the critical temperature Tc, is a

   consequence of diamagnetic shielding whereas in the imaginary part (”), the peak Tp represents AC losses.

   It is well-know that the compound YBa2Cu3O6+z (with z

   1) is superconducting below 92 K and characterized by double Cu(2)O2 layers and Cu(1)O chains. The Cu(2)O2 layers are oriented along the a-b plane responsible for carrying the supercurrent, while the Cu(1)O chains along the b direction provide a charge reservoir for these planes [3, 4]. The majority of the most requested searches for the superconductors compounds are those of the LnBa2Cu3O6+z systems (Ln = rare earth) which is stipulated by several reasons. On one hand, these compounds have a relatively high critical temperature Tc 90 K above the temperature of liquid nitrogen [5, 6]. On the other hand, the electric transport characteristics of these compounds can rather easily be varied by doping of the compound with substituting elements [7, 8] or varying the oxygen content [9, 10, 11]. The samples preparation has been made by several technologies with a given fault structure [12, 13] that is very useful for basic research.

   Among the extensive work on the preparation and the study of the structural and superconducting properties of La1+xBa2-xCu3Oy (with 0 x 0.5). Wada et al. [14], Izumi et al. [15] concluded that in order to have Tc maximal, this structure must have an ordered arrangement of La and Ba along c axis with an occupation factor of 0 and 1 for the oxygen at (1/2, 0, 0) and (0, 1/2, 0) sites, respectively.

   We want to see if an isovalent substitution of Ba+2 by Sr+2 with smaller ionic radius can modify the results discussed above when Y+3 (r = 0.893 Å) is replaced by the rare earth Eu+3 (r = 0.95 Å) with bigger ionic radius. In the order to study the effect of the Y and Ba atomic plans on the superconductivity in LnSrBaCu3O6+z,we have studied the structural, superconducting and magnetic properties of the superconductor EuBaSrCu3O6+z [16]. This compound when annealed in oxygen at 450°C showed an orthorhombic structure and a Tc of 81.1 K. When the same sample was heated in argon followed by oxygen annealing; we observed an increase both of orthorhombicity and Tc by

   5.95 K. So Tc depends also on heat treatment.

   In the case of LnSrBaCu3O6+z (Ln = Y, Eu and z ~ 0.9) Wang et al. [17] and Badri et al. [18] obtained a quadratic structure and contradictory results (Table 1). To resolve these contradictions, we report here our results on sample

   preparation, X-ray diffraction and alternative magnetic susceptibility of LnSrBaCu3O6+z (Ln = Y, Eu). In addition, we will examine the effect of the [AO] treatment, which has had a considerable influence on the structural, superconducting and magnetic properties of our samples.

2. EXPERIMENTAL TECHNICS

   The polycrystalline samples have been prepared by solid-state sintering of the respective oxides and carbonates. The chemicals were of 99.999% purity except in the case of

   Table 1. Lattice parameters a, c and Tc of LnSrBaCu3O6+z (Ln

   =Y, Eu).

   Ln	ref	a(Ã…)	c(Ã…)	Tc(K)
   Y	[4]	3,789	11,563	83
   	[5]	3,780	11,558	81.7
   Eu	[4]	3,844	11,579	80
   	[5]	3,845	11,590	60

   BaCO3 which was 99.99% pure. Ln2O3(Ln = Y, Eu), SrCO3, BaCO3 and CuO were thoroughly mixed in required proportions and calcined at 950°C in air for a period of 12-18h. The resulting product was ground, pelletized and heated in air at 980°C for a period 16-24h. This was repeated twice. The pellets were annealing in oxygen at 450°C for a period of 60-72h and furnace cooled. This was denoted as sample [O]. XRD data of the sample ware collected with Philips diffractometer fitted with a secondary beam graphite monochromator and using CuK (40 kV/20 mA) radiation. The angle 2 was varied from 20° to 120° in steps of 0.025° and the courting time per step was 10 sec. The XRD specters were refined with Rietveld refinement [19].

3. RESULTS AND DISCUSSION Superconducting transitions were checked by

   measuring both the real and the imaginary parts of the AC susceptibility as a function of temperature in a field of 0.11 Oe and at a frequency of 1500 Hz.Then () and () were measured in a static field (0<Hdc<150 Oe) superimposed on the alternating field of 0.11 Oe.

   The same sample [O] was then heated in argon at 850°C for about 12h, cooled to 20°C and oxygen was allowed to flow instead of argon and the sample was annealed at 450°C for about 72h. This sample is denoted as [AO]. XRD and AC susceptibility measurements were performed on a part o this sample.

   As example, the measured XRD patterns and refined with Rietveld refinement in the case of LnSrBaCu3O6+z (Ln

   = Y, Eu) ([O] and [AO]) are shown in Figure 1. In general, the samples were well crystallized and the reflections were sharper after the [AO] heat treatment. The orthorhombic splitting was also influenced by the [AO] treatment. Some weak unidentified impurity peaks (at 2=31°) were seen in the [O] samples. They disappeared after the [AO] heat treatment. This indicates an improvement of crystallographic quality of the samples [AO].

   The [AO] treatment increased the orthorhombicity (*10-3)=(ba)/(b+a) from 8.23 to 9.9 and from 3.1 to 6.97

   in the samples with Ln= Y and Eu respectively., indicating a conservation of the orthorhombic structural phase. This was accompanied by an decrease of 1.3 K in critical temperature Tc to Tc[AO] = 81.7 K in the case of YSrBaCu3O6+z , while in the case of EuSrBaCu3O6+z, the Tc[O] = 81.1 K increase remarkably by 5.95 K to Tc[AO] = 86.7 K (Table 2).

   Further, there was an enhancement of their irreversibility line whatever Ln. The increase in Tc in LnBa2Cu3O6+z as a function of z is normally attributed to an increase in the orthorhombicity and to an increase in the oxygen ordering in the ab plane. The argon heat treatment did not sensibly change the total content of oxygen, which was around 6.95±0.04 (Table 2), from our iodometry measurements but increase Tc. Thus, the reason for the increase in Tc may lie in some other factor than z. The exact reason for the change in the symmetry from orthorhombic to tetragonal and vice versa depending on the heat treatment and when Y is replaced by Eu in

   Fig.1: XRD pattern of LnSrBaCu3O6+z, (Ln = Y, Eu) observed, calculated with Rietveld refinement and difference profiles for sample [O] and sample [AO].

   Table 2. Lattice and superconducting parameters of LnSrBaCu3O6+z (Ln= Y, Eu) as a function of heat treatment.

   Ln h Treat 6+z (0.4) a(Ã…) b(Ã…) c(Ã…) V(Ã…3) (*10-3) Tc(K) Tp(K) Tc(K) Tp(K) n K(Koe) K(Koe) [O] 6,96 3,789 3,852 11,563 168,73 8,23 83 82,9 0.41 0.3 3,22 27129,9

   Y

   [AO] 6,96 3,780 3,856 11,558 168,45 9,9 81.7 80,4 2.7 1.2 3,46 222,1
   [O] 6,946 3,831 3,855 11,607 171,2 3,1 81.1 80,1 2.71 2.29 1,41 1,6

   Eu

   [AO] 6,95 3,813 3,867 11,604 171,11 6,97 87.05 86,7 1.26 0.9 1,72 31,7

   -26907,8

   30,1

   ‘ ( a u)

   ‘ ( a u)

    

   0 [O]

   -1

   x=0 x=1

   (a)

   [AO]

   x=1 x=0

   0

   ‘ ( a u )

   ‘ ( a u )

    

   -1

   (a)

   b axis. The two changes of cation sites increase b and decrease a after the [AO] heat treatment (Table 2).This indicate the passage of some oxygen from O(5) sites to some vacant O(4) sites along b axis. So this increased ordering of chain oxygen along b direction (increase in the NOC). In addition c decreased by 0.013 Ã…. As result the Cu(1)-apical oxygen distance decreased. This could

   ” (a u )

   ” (a u )

    

   1 [O]

   x=0 x=1

   0

   70 75 80 85

   T (K)

   (b) (b)

   [AO] 1

   ” ( a u)

   ” ( a u)

    

   x=1 x=0

   0

   75 80 85 90

   T (K)

   improve the transfer of charge between the chains and the Cu(2)-planes resulting in an increase in the hole density. Such an increase would lead to optimum superconducting properties and could account for the observed increase in Tc and irreversibility line.

   In Y(SrBa)Cu3O6+z the conservation of the orthorhombic structure can be explained by that a decreases and b increases slowly. The decrease of Tc by

   Fig. 2. (a) and (b) of Y1-xEuxSrBaCu3O6+z (with: x = 0, 1) as a function of the temperature and heat treatment.

   YSrBaCu3O6+z is not known. We would like to suggest, however, that a tetragonal structure could be from an orthorhombic structure by rearranging the oxygen vacancies in the ab (basal) plane.

   In the present case, the argon treatment seems to favor the orthorhombic symmetry. We propose, hence, that a rearrangement of oxygen atoms in the basal plane following the [AO] treatment would have caused an increase in the NOC (Number of Oxygen atoms per Chain) and this in turn would increase Tc.

   When Eu ion occupies in Ba (or Sr) site, the same amount of Ba (or Sr) cation is pushed into Y site. Eu is a three-valence ion. It increases the positive charge density around Ba (or Sr) site and the attractive force with oxygen anion. As a result, oxygen vacancies O(4) along the b axis in the basal plane have higher chance to be filled. In the other hand, Ba2+ (or Sr2+) in Y3+ site make decrease the attractive force with oxygen anion in Cu(2) plane. This increased the buckling angle Cu(2)O(2)Cu(2) along the

   1.3 K, after the [AO] heat treatment, can be explained by an increase of cationic order (along c) and anionic order in the basal plane. All these arguments are in agreement with the reduction of impurity in the samples [AO].

   The real part ‘of the alternative susceptibility measured under Hac = 0.11 Oe as a function of temperature and heat treatment is shown on (Fig. 2); and on (Fig. 3) with 0 <Hdc<126.5Oe for EuSrBaCu3O6+z. Table 2 gives the critical temperature Tc of the transition from the superconducting state to the normal state. In the case EuSrBaCu3O6+z, Tc [O] = 87.05 K agrees with 80 K measured by Wang et al. [17] from resistivity but very high compared to Tc = 60 K observed by Badri et al. [18]. It is interesting to note that Tc increases remarkably by6 K to Tc [AO] = 87 K.

   The imaginary part ” (T, Hdc) of the alternative susceptibility as a function of the heat treatment for Ln = Eu is shown, as an example, on Fig. 3. They show that Tp move towards low temperatures, when Hdc increases, indicating the weakening of links between grains. This displacement is smaller in the sample [AO] than in the sample [O]. Table 2 shows that the Tp values of the ” (T)

   peaks follow those of Tc. These two effects are also observed in the case of Ln = Y. However, when H is plotted as a function of t = Tp/Tc in (Fig. 4 (a)), we observe an increase in the irreversibility line after the heat treatment [AO] in the case of EuSrBaCu3O6+z,

   whereas in the case of YSrBaCu3O6+z we observe a decrease in the irreversibility line due to the heat treatment [AO]. The plot of ln (H) as a function of ln (1 –

   t) (Fig. 4 (b)) leads to straight lines. Table 2 shows an increase of n (1.4 n 3.5) after the heat treatment [AO]. In the case of Ln = Eu, K[O] = 1573.41Oe increases

   0 Hdc = 0 Oe

   27.5 Oe

   56.65 Oe

   ‘ [u.a]

   ‘ [u.a]

    

   90.75 Oe

   126.5 Oe

   = 0 Oe

   H

   H

    

   dc

   dc

    

   27.5 Oe

   56.65 Oe

   90.75 Oe

   126.5 Oe

   0 remarkably to K[AO] = 31707.23Oe. K is interpreted as the field necessary to reduce the inter granular critical

   ‘ [u.a]

   ‘ [u.a]

    

   current at the limit of Tp = 0 K. Note that the argon heat treatment considerably increases the value of K indicating an improvement of the vortex shielding properties.

   It is only the aspect enhancement in the

   -1 (a) [O] (a) [AO] -1

   irreversibility line due to heat treatment that we wanted

   ” [u.a]

   ” [u.a]

    

   1 (b) [O]

   0

   40 50 60 70 80 90

   T [K]

   1. [AO] 1

      ” [u.a]

      ” [u.a]

       

      0

      60 70 80 90

      T [K]

      to emphasize. Vanacken et al. [20] have measured the field-cooled and zero-field-cooled DC magnetization of YBa2Cu3O6+z as a function of z to establish the irreversibility line and analyzed their data using the relation H = K(1 – t)n [21]. They showed that n was independent of z but Tc and K (and also the orthorhombicity) increased with z indicating possibly the role played by an increase ordering of chain oxygen. Slight differences in oxygenation might also induce

      Fig. 3. (a) and (b) of EuSrBaCu3O6+z as a function of

      temperature and the heat treatment t five different DC fields increasing form right to left: 0, 27.5, 56.7, 90.8 and 126.5 Oe.

      (a)

      x = 1

      x = 0

      x=1 [O]

      x=0 [O] x=1 [AO] x=0 [AO]

      (a)

      x = 1

      x = 0

      x=1 [O]

      x=0 [O] x=1 [AO] x=0 [AO]

       

      150

      H(Oe)

      H(Oe)

       

      100

      50

      0

      changes in Cu(1) oxygen apical distance and Tc as was discussed elsewhere [22,23]. The observed increase in the irreversibility line may also be explained qualitatively by the factors discussed above.

      Since the same sample was used for both heat treatments, one can compare the diamagnetic response and note that screening current of the [AO] sample increased considerably compared to that of the [O] sample(see for example the case Ln = Eu) in Fig. 2. (b).

      Let us now look at the amplitude of the real part of the AC susceptibility in Fig. 3 which is nothing but the shielding effect S [24]. S was set arbitrarily equal to 1 for Hdc = 0 Oe. This was measured at three temperatures 70 K, 75 K and 80 K in the presence of an externally applied DC field H in Fig. 3. S represents the exclusion of magnetic flux by the sample in alternative dynamic mode.

      0,8 0,9

      x = 0

      x = 1

      x=0 [O] x=0 [AO] x=1 [O] x=1 [AO]

      x = 0

      x = 1

      x=0 [O] x=0 [AO] x=1 [O] x=1 [AO]

       

      5

      Ln(H)

      Ln(H)

       

      4

      t=Tp/Tc

      1,0

      There was a remarkable Improvement in the shielding effect in the case of the samples [AO] at all T < Tc and for any applied field H. For example in EuSrBaCu3O6+z at T

      = 80 K , S at a field of 126.5 Oe was a factor of nearly nine higher in the case of the sample [AO] compared to that of the sample [0] (Fig.3). Further, the decrease in as a function of the field was much slower in the case of the sample [AO]. For example, at T = 80 K, the

      3

      -4 -3 -2

      Ln(1-t)

      Fig. 4. (a) t = Tp/Tc as a function of H, (b) ln(H) as a function of ln(1 – t): for the two samples Ln = Y and Eu as a function of heat treatment.

      1,0

      0,8

      S(a. u)

      S(a. u)

       

      0,6

      3

      3

       

      T=70K [O] T=75K [O]

      Y(SrBa)Cu O

      6+z

      with different rare earths, may be useful in discovering the role played by certain defects on the superconducting properties. The present studies indicate a simple heat treatment procedure to optimize superconducting properties.

4. CONCLUSIONS

We have shown that the high critical temperature superconductors Ln(SrBa)Cu3O6+z (Ln = Y, Eu) undergoing conventional oxygen heat treatment at

0,4 T=80K [O]

T=70K [AO]

T=75K [AO]

450C crystallize with orthorhombic symmetry in disaccord with the tetragonal structure reported earlier

0,2

T=80K [AO]

0 20 40 60 80 100

H(Oe)

by [17, 25]. The orthorhombicity, Tc and the irreversibility line increased the sample Ln = Y after [AO] heat treatment; whereas they decreases in Ln=Y. These results are due to interaction between the cationic disorder of Ln3+ at the (Sr/Ba) site and the anionic disorder of oxygen in the base plane. These disorders increases with the ionic radius of Ln and decrease after heat treatment [AO]. Our study indicates that a simple heat treatment process was able to optimize the structural and superconducting properties that should be investigated for other compounds of the LnSrBaCu3O6+z system (where Ln = rare earth). We believe that these results will be useful for testing or improving some theoretical models of electronic structure and atomic disorder in view of the technological applications of high temperature superconductors.

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Fig. 5. Shielding effect S of LnBaSrCu3O6+z (Ln = Y, Eu) as a function of the field Hdc and heat treatment at three different temperatures.

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