Effects of Isovalents Substitutions and Argon Heat Treatment on the Structural and Superconducting Properties of Y_0.5Ln_0.5SrBaCu₃O_6+z

Effects of Isovalents Substitutions and Argon Heat Treatment on the Structural and Superconducting Properties of Y_0.5Ln_0.5SrBaCu₃O_6+z

Keltoum Khallouq, Abdelhakim Nafidi, Abdeljabar Aboulkassim, Mohammed Bellioua, Essediq Youssef El Yakoubi, Ali Khalal

Laboratory of Condensed Matter Physics and Nanomaterials for Renewable Energy, University Ibn Zohr

Agadir, Morocco

Abstract We report here our investigations on the preparation, X-ray powder diffraction with Rietveld refinement, AC magnetic susceptibility measurements, effect of heat treatments and substitution of Y by the rare earth Ln = Eu, Sm and Nd in Y0.5Ln0.5SrBaCu3O6+z. Each sample prepared by solid-state sintering at a high temperature under reducing atmosphere, was subject to two type of heat treatment: oxygen annealing [O] and argon annealing followed by oxygen annealing [AO]. In each sample, the [AO] heat treatment increase the orthorhombicity

= (b a)/(b + a) (where a and b are the parameters of the unit cell in the basal plane) and the critical temperature Tc. An enhancement of the irreversibility line H(t=Tp/Tc) due to the argon heat treatment was observed. Different correlations were observed between structural, electrical and magnetic properties. Several factors, such as the ionic size r of the rare earth Ln, an increase in cationic and chain oxygen ordering; the number psh(r) of holes by Cu(2)O2 superconducting planes and in-phase purity of the [AO] samples may account for the observed data.

KeywordsHigh-Tc superconductors, Heat treatments, Tc, X- ray diffraction, Orthorhombicity, AC magnetic susceptibility.

1. INTRODUCTION

   Most of the extensive research efforts in solid state physics have been directed towards the study of high Tc superconducting cuprates since 1986. It is well known that the physical properties of these compounds are closely related to the conditions of their preparation. The high Tc superconductivity is carried by positive charges (holes) on the highly correlated CuO2 superconducting planes. The electronic states of the later are sensitive and influenced by structural, electrical and magnetic characteristics, such as the size of the ionic radius, the applied magnetic field, the valence, the distribution of surrounding ions and the oxygen content. Furthermore, all these characteristics are strongly influenced by heat treatments applied during the preparation of the samples. In particular, when Ln is a rare earth element, such as Eu, Sm and Nd, various effects are expected because of large ionic radius of the Ln ions to Y.

   Among the most studied compounds are LnBa2Cu3O6+z (Ln = Y or Lanthanides) that is stipulated by several reasons: On the one hand, the electric transport characteristics can rather easily be varied by doping of the compound with substituting elements [1,2] or varying the oxygen content. For example when 0 < z < 0.5, the compound YBa2Cu3O6+z is

   tetragonal and antiferromagnetic insulator and for 0.5 < z < 1, the compound is orthorhombic, p-type metal and becomes superconducting at low temperatures [3,4,5,6]. For each oxygen content 6+z, Tc increased with the size r of the Ln ions [7,8]. On the other hand, these compounds have a rather high critical temperature Tc 90 K above the nitrogen liquefaction temperature [9,10,11].

   Many studies have been done on Sr substitution in the compound Ln(Ba1-xSrx)2Cu3O6+z [12-15]. These authors conclude that: for each Ln, the critical temperature and the orthorhombicity decreases with increasing the concentration x of Sr [12,13,16]. For each x, the crystalline structure and Tc depend on the ionic size r of the Ln ions [17,18,19].

   In this paper, the samples have been synthesized at high temperature under effect of two heat treatments ([O] and [AO]). In order to obtain higher solubility of Ln in Y0.5Ln0.5SrBaCu3O6+z (where Ln = Eu, Sm and Nd). The powder XRD and AC magnetic susceptibility measurements have been carried out. It is interesting to check if an isovalent substitution of Ba2+ by Sr2+ with smaller ionic radius would modify some of the results discussed above when Y3+ is replaced by Ln3+ with bigger ionic radius.

   In order to study the role played by the Yttrium and Barium atomic plans and find out the factors conditions which govern the superconductivity in these compounds, we have investigated the structural and superconducting properties of Y0.5Ln0.5SrBaCu3O6+z. Indeed, the correlations between these properties, with the influence of argon heat treatment and the ionic size of Ln will be discussed.

2. EXPERIMENTAL TECHNIQUES

   The polycrystalline samples Y0.5Ln0.5SrBaCu3O6+z with Ln = Eu, Sm and Nd were synthesized by solid-state reaction of the respective oxides and carbonates. The chemicals were of 99.999% purity except in the case of BaCO3 which was 99.99% pure. Y2O3, Ln2O3, SrCO3, BaCO3 and CuO were weighted in composition ratio and well mixed and calcined at 950 °C in air for a period of 12 18 h. The resulting product was ground, mixed, pelletized and heated in air at 980 °C for a period of 16 -24 h. This was repeated twice. The pellets were annealed in oxygen at 450 °C for a period of 60 -72 h and furnace cooled. This was denoted as sample [O] for each Ln.

   Powder XRD data were collected at room temperature by using a diffractometer fitted with a secondary beam graphite monochromator with a CuK (40 kV/20mA) radiation source. The angle 2 was varied from 20° to 120° in steps of 0.025° and the counting time per step was 10 sec. The crystalline structure has been refined by Rietveld analysis of powder XRD data. For each sample, the oxygen content value was close to 6.94 ± 0.04, it was determined by the iodometric method.

   The samples magnetic response is sensed by a pick-up coil surrounding the sample. The AC magnetic susceptibility measurements (ac = + i) as function of temperature in a field of 0.11 Oe were carried out at 1500 Hz. In addition, and were measured in a static field (0 < H = Hdc < 150 Oe) superimposed on the AC field of Hac = 0.11 Oe.

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

3. RESULTS

   1. Crystalline Structure and real part of the AC magnetic susceptibility

      Fig. 1(a) and (b) shows the powder XRD patterns of the Eu- Sm- and Nd- systems, respectively. The samples were well crystallized and the reflections were sharper and well

      Fig. 1 XRD patterns for Y0.5Ln0.5SrBaCu3O6+z as a function of ionic radius r(Ln3+). (a) Samples [O], (b) samples [AO].

      Whereas the volume V of the unit cell remain constant indicating a rearrangement of the unit cell (see Table. I). Also the [AO] heat treatment increase , for Y0.5Eu0.5SrBaCu3O6+z from 5.872 10-3 to 8.355 10-3, Y0.5Sm0.5SrBaCu3O6+z from

      4.033 10-3 to 6.638 10-3 and Y0.5Nd0.5SrBaCu3O6+z from 3.913

      10-3 to 7.174 10-3.

      resolved after the [AO] heat treatment. The lattice parameters determined from the XRD patterns by Rietveld refinement are collected in table. I. These diffraction patterns confirmed that all samples have the orthorhombic perovskite structure, and no impurity phase was observed after [AO] heat treatment (indicated by cross in fig. 1 (a) disappeared after the [AO] heat treatment). This indicates an improvement of crystallographic quality of the samples [AO]. As seen in fig. 2, foreach heat treatment in Y0.5Ln0.5SrBaCu3O6+z, the parameter c decrease, b is nearly constant for all systems but a increase with the ionic radius r(Ln3+) from Ln = Eu to Sm then decrease for Nd leading to a decrease of the

      c(Ã…)

      c(Ã…)

      11,6

      11,5

      b(Ã…)

      b(Ã…)

      3,84

      a(Ã…)

      a(Ã…)

      3,80

      Eu Sm Nd

      [O]

      orthorhombicity = (b a)/(b + a). This is due to the fact that the ionic radius of Eu3+(0.95 Ã…), Sm3+(0.965 Ã…) and Nd3+(0.995 Ã…) ions are much larger than that of Y3+(0.893 Ã…). For each Ln, the [AO] heat treatment increases b and c, but decreases a.

      [AO]

      0,94 0,96 0,98 1,00

      r(Ln3+)

      Fig. 2 Crystalline parameters as a function of the ionic radius r(Ln3+). In the right the unit cell of Y0.5Ln0.5SrBaCu3O6+z.

      In order to have an insight in the internal strains induced by the Ln substitution, we have calculated the interatomic distances d[Cu(1)(Sr/Ba)]. An increase of this distance is observed with r(Ln3+) indicating an increase of the stress introduced by the larger ionic radius replacing the smaller Y atom in the unit cell. For each r(Ln3+), the [AO] heat treatment decreases the distance d[Cu(1) (Sr/Ba)] (see Table. I).

      Table. I : Tc and crystalline parameters of the Y0.5Ln0.5BaSrCu3O6+z as function of heat treatment.

      Ln	r(Ln)(Ã…)	h. Treat	a(Ã…)	b(Ã…)	c(Ã…)	V(Ã…3)	*10-3	Tc(K)	d[Cu(1)-(Sr/Ba)]
      Eu	0.950	[O]	3.809	3.854	11.582	170.02	5.872	82.1	3.43811
      		[AO]	3.798	3.862	11.586	169.94	8.355	82.5	3.43759
      Sm	0.965	[O]	3.828	3.859	11.580	171.06	4.033	81.0	3.43582
      		[AO]	3.816	3.867	11.580	170.88	6.638	82.0	3.43491
      Nd	0.995	[O]	3.818	3.848	11.559	169.82	3.913	78.9	3.43502
      		[AO]	3.806	3.861	11.572	170.05	7.174	80.5	3.43173

      ' (a u)

      ' (a u)

      0 [O]

      	Eu
      Nd Sm

      -1

      '' (a u)

      '' (a u)

      1 [O]

      0

      (a)

      (c)

      [AO]

      Eu

      Nd

      Sm

      [AO]

      0

      ' (a u)

      ' (a u)

      (b) -1

      " (a u)

      " (a u)

      (d) 1

      0

      in Tc of Tc = Tc[AO] – Tc[O] = 1.68 K in sample Ln = Nd, and a minimum of Tc = Tc[AO] – Tc[O] = 0.37 K in sample Ln = Eu were observed. So Tc depends in heat treatment and the ionic radius of the rare earth.

   2. Imaginary part of the ac magnetic susceptibility and irreversibility line

   According to the results of the imaginary part of the susceptibility measurements () of our samples in (Fig. 3 (c,d)), we can see that is very sensitive to heat treatment and the ionic radius size of Ln. The peak at Tp in the reflects the intergranular critical current [20]. When the ionic

   76 78 80 82

   T (K)

   76 78 80 82

   T (K)

   radius of Ln increased, Tp shifted to low temperatures in the case of the samples [O]. For each Ln, Tp increased after [AO] heat treatment fallowing the increase of Tc.

   Fig. 3 et of Y0.5Ln0.5SrBaCu3O6+z as a function of the temperature and heat treatment; (a) Samples [O] (b) Samples [AO].

   The temperature dependence of the real part of the susceptibility measurements (), is plotted in Fig. 3 (a,b) for

   [O] and [AO] heat treatment. Since the same sample was used for both heat treatments, one can compare the diamagnetic shielding response (amplitude of (T)) and note that screening current of the [AO] sample increased considerably compared to that of the [O] sample. The superconducting transition temperatures Tc, defined as the onset of diamagnetic transition, increases from 82.1 K [O], 80.98 K [O], and 78.86 K [O] to 82.47 K [AO], 82.02 K

   120

   H (Oe)

   H (Oe)

   dc

   dc

   60

   0

   1. [AO]

      Eu[O]

      Eu[AO]

      Nd[O]

      Nd[AO]

      Sm[O]

      Sm[AO] [AO] and 80.54 K [AO] for Eu-, Sm- and Nd- samples, respectively.

      0,9 t = Tp/Tc

      1,0

      In Fig. 4 we shown the dependence of Tc versus r(Ln3+) of Y0.5Ln0.5SrBaCu3O6+z systems. For each ionic size of Ln, the [AO] heat treatment increases Tc. For each heat treatment, when the ionic size r of the rare earth ion decreases Tc increases to a maximum for Ln = Eu. A maximum of increase

      Fig. 5 Hdc as a function of t = Tp /Tc and heat treatment of Y0.5Ln0.5SrBaCu3O6+z with Ln = Eu, Sm and Nd.

      Eu[O]

      Eu[AO]

      Nd[O]

      Nd[AO]

      Sm[O]

      Sm[AO]

      Eu[O]

      Eu[AO]

      Nd[O]

      Nd[AO]

      Sm[O]

      Sm[AO]

      5

      Eu Sm

      83

      82

      T (K)

      T (K)

      81

      c

      c

      80

      79

      Nd

      c

      c

      T [O]

      4

      ln H

      ln H

      dc

      dc

      3

      -4 ln (1-t)

      -3 -2

      c

      c

      T [AO]

      78

      Fig. 6 ln(Hdc) as a function of ln(1 t) and heat treatment for Y0.5Ln0.5SrBaCu3O6+z with Ln = Eu, Sm and Nd.

      0,94 0,96 0,98 1,00

      r(Ln) (Ã…)

      Fig. 4 Tc as a function of r(Ln3+) and heat of Y0.5Ln0.5SrBaCu3O6+z with Ln = Eu, Sm and Nd.

      Table. II Superconducting and magnetic parameters of Y0.5Ln0.5SrBaCu3O6+z as a function of r(Ln3+) and heat treatment.

      It is well-known that YBa2Cu3O6+z materials are orthorhombic superconducting and Tc is close to 92 K [22].

      Ln	r(Ln)(Ã…)	h. Treat	Tp (K)	K(Oe)	n
      Eu	0.95	[O]	81.6	1463.8	1.19
      		[AO]	81.9	5860.0	1.55
      Sm	0.965	[O]	80.8	2423.1	1.22
      		[AO]	81.2	9011.6	1.86
      Nd	0.995	[O]	78.6	6982.0	1.90
      		[AO]	80.0	366553.6	2.76

      Ln	r(Ln)(Ã…)	h. Treat	Tp (K)	K(Oe)	n
      Eu	0.95	[O]	81.6	1463.8	1.19
      		[AO]	81.9	5860.0	1.55
      Sm	0.965	[O]	80.8	2423.1	1.22
      		[AO]	81.2	9011.6	1.86
      Nd	0.995	[O]	78.6	6982.0	1.90
      	[AO]	80.0	366553.6	2.76

      These materials are characterized by double Cu(2)O2 layers (oriented along the a-b plane) responsible for carrying the

      supercurrent and Cu(1)O chains (along the b direction) which provide a charge reservoir for these planes [23] [24].

      Eu Sm Nd

      The magnetic field Hdc(t) sets an upper limit to the irreversibility line (I.L.) marking the onset of dissipation and the region in which a superconductor can remain useful. In fact, when a static field Hdc is plotted as a function of t = Tp/Tc (with Tp = Tirr) for the six samples in (fig. 5), an enhancement (an increase in the slope) of the irreversibility line was observed due to Argon heat treatment.

      These results can be analyzed using the following equation Hdc = K(1-t)n [21]. Straight line plots were obtained

      0,125

      p

      p

      0,120

      sh

      0,115

      psh(r)[O]

      psh(r)[AO]

      Tc(r)[O]

      Tc(r)[AO]

      80

      Tc(K)

      75

      when Ln(Hdc) was plotted against Ln(1 t) in (fig. 6). For each heat treatment, K increases remarkably with r(ln3+) in

      0,96

      r(Ln3+

      0,98 1,00

      ) (Ã…)

      (fig. 7).

      400

      8

      6

      4

      2

      8

      6

      4

      2

      K' (KOe)

      K' (KOe)

      9

      K' [O]

      K' [AO] n [O]

      n [AO]

      K' [O]

      K' [AO] n [O]

      n [AO]

      K' (KOe)

      K' (KOe)

      200

      0,950 0,955 0,960 0,965

      r(Ln) (Ã…)

      0,950 0,955 0,960 0,965

      r(Ln) (Ã…)

      6

      n

      0 3

      0,94 0,96 0,98 1,00

      r(Ln3+) (Ã…)

      Fig. 7 The field Kas a function of the ionic size r(Ln3+) and heat treatment of Y0.5Ln0.5SrBaCu3O6+z with Ln = Eu, Sm and Nd.

      For each r(Ln3+), the [AO] heat treatment increases the field K and n indicating an improvement in the pinning properties. For example, the value of K was estimated to be 6.982 KOe and 366.55 KOe respectively for the sample [O] and [AO] in Y0.5Nd0.5SrBaCu3O6+z (Table. II). K may be interpreted as the field necessary to reduce the intergranular critical current to zero in the limit of Tp = 0, and n define the type of the Josephson junction between the grains and intergrains in the superconductor.

4. DISCUSSION

   Our samples were prepared in 1 atm of oxygen. Further, the t

   Fig. 8 Correlation between psh and Tc as a function of the ionic radius r(Ln3+) and heat treatment of Y0.5Ln0.5SrBaCu3O6+z with Ln = Eu, Sm and Nd.

   As seen in table. I, the rare earths (Ln) substitution causes changes in the crystalline parameters a, b, and c, but keeps the orthorhombic character although there are minor changes in the orthorhombicity. This indicates that the ionic size r(Ln3+) does not change the orthorhombic structure of Y0.5Ln0.5SrBaCu3O6+z systems. It was found that the argon heat treatment considerably increases the orthorhombicity, indicating an improvement of the structural and superconducting properties. G. Uimin et al. indicates that this increase of the orthorhombicity due to increase the order of oxygen in chain [25]. These authors indicated that the transfer of holes in the planes Cu(2)O2 is strongly connected with the oxygen filling mechanism in basal plane on the O(4) and O(5) sites along b and a-axis respectively, as seen in the unit cell in the right of (fig. 2). This would lead to optimum superconducting properties and could account for the observed increase in Tc and the irreversibility line.

   As seen in the left of (fig. 2), for each Ln, the [AO] heat treatment decrease the crystalline parameter a and increase b and c. This increase in parameter b leads to an increase of the number of oxygen atoms by chain (NOC) along the b-axis (increase of the anionic order in the basal plane). Indicating an improvement of transfer of holes from the Cu(1)-O chains (along the b direction) to the superconducting planes, via the apical oxygen O(1) (Fig. 2). This argument is justified by the decrease of d[Cu(1)-(Sr/Ba)] in (Fig. 9).

   Therefore, the number of hole psh (Fig. 8) by Cu(2)O2

   [AO] heat treatment did not sensibly change he total

   oxygen content 6+z, which was around 6.94±0.04 from our iodometry measurements but increased Tc. Thus the reason for this increase may lie in some other factor than z.

   Tc(r) [O]

   Tc(r) [AO]

   d[Cu(1)-(Sr/Ba)](r) [O]

   d[Cu(1)-(Sr/Ba)](r) [AO]

   Tc(r) [O]

   Tc(r) [AO]

   d[Cu(1)-(Sr/Ba)](r) [O]

   d[Cu(1)-(Sr/Ba)](r) [AO]

   Eu Sm

   d[Cu(1)-(Sr/Ba)] (Ã…)

   d[Cu(1)-(Sr/Ba)] (Ã…)

   3,440

   3,435

   Nd

   80

   Tc(K)

   75

   The two arguments (cationic and anionic disorders) are justified here by the three remarkable correlations observed between Tc(r), the number psh(r) of holes by Cu(2)O2 superconducting planes and d[Cu(1)-(Sr/Ba)] (r) in (fig. 8 and

   9) respectively and on the other hand, between Tc(r) = Tc[AO] – Tc[O] and (r) = [AO] – [O] in (fig. 10). So the structural and superconducting properties are correlated with the effect of Argon heat treatment.

   Hence we are tempted to believe that the changes (increase or decrease) observed in Tc, need not be related only to the ionic size of the rare earth but rather to a

   3,430

   0,96 0,98 1,00

   r(Ln3+ (Ã…)

   r(Ln3+ (Ã…)

   )

   combination of several factors such as changes in the Cu(1)- O(1) distances, cationic and oxygen disorders, hole density

   Fig. 9 Correlation between d[Cu(1)-(Sr/Ba)] and Tc as a function of the ionic radius r(Ln3+) and heat treatment of Y0.5Ln0.5SrBaCu3O6+z

   with Ln = Eu, Sm and Nd

   etc.

5. CONCLUSIONS

Eu

3,2

3,0

2,8

Sm

Tc(r)

Nd

1,5

Tc(K)

Tc(K)

1,0

To summarize, the effect of Ln substitution on the structural and superconducting properties of Y0.5Ln0.5SrBaCu3O6+z system is analyzed. To determine the Structural properties, the results of the XRD measurements are analyzed using Rietveld refinement. The superconducting properties are analyzed using the AC magnetic susceptibility measurement approach. The following results are obtained:

• No obvious impurity phase after [AO] heat treatment in

  2,6

  2,4

  0,5

  (r)

  0,96 r(Ln3+) 0,98 1,00

  the XRD results. This result shows that there is an improvement of crystallographic quality of the samples [AO].

• For each heat treatment, Tc increases when the ionic size r(Ln3+) decreases.

Fig. 10 Correlation between Tc = Tc[AO] – Tc[O] and = [AO] – [O] as a function of the ionic radius r(Ln3+) and heat treatment of Y0.5Ln0.5SrBaCu3O6+z with Ln = Eu, Sm and Nd.

superconducting planes increases (deduced from the under saturation zone of the universal relation c as a function of number psh of holes (with c = Tc/Tcmax) [26]) and Tc. For each r(Ln3+), the [AO] heat treatment increases , Tc and decrease d[Cu(1)-(Sr/Ba)]. These results indicate a rearrangement of the same volume V of the unit cell and the enhancement of the charge transfer from chains to planes yielding to the increase of Tc.

When the Ln ion occupies Ba/Sr sites, the same amount of Ba/Sr cation is pushed in the Y sites (with Ln a trivalent ion). This increases the positive charge density around the Ba/Sr site and the attractive force with the oxygens. Therefore, the vacant oxygen sites O(4) have a greater chance

• For each ionic size of Ln, the [AO] heat treatment increases Tc. Regardless r(Ln3+) and heat treatment, Tc increases when the ionic size r(Ln3+) decreases and take a maximum for Ln = Eu. A maximum of increase in Tc of

  Tc = 1.68 K in sample Ln = Nd, and the minimum of Tc

  = 0.37 K in sample Ln = Eu were observed. So c depends in heat treatment and the ionic radius of the rare earth.

• The observed enhancement of the irreversibility lines in Y0.5Ln0.5SrBaCu3O6+z after the Argon heat treatment were explained by the improvement of the quality of the grains, the intergranular coupling and the crystallographic quality of the [AO] samples. Three remarkable correlations were observed. So the structural and superconducting properties are correlated with the effect of Argon heat treatment.

  of being filled.

  2+ 2+

  3+ 3+

• The [AO] heat treatment leads to an increase of number of

On the other hand, Ba /Sr occupying the Y /Ln site reduces the intensity of the attractive force applied to the oxygen in the plane Cu(2)O2. This increases the distortion angle of Cu(2)-O(2)-Cu(2) along the b-axis. Both changes in cation sites, increases b and decrease a after the heat treatment [AO]. This indicates the passage of oxygens in O(5) sites to the vacant sites O(4) along the b-axis. This increases the NOC and the oxygen chain order along b direction. This increase of NOC is responsible for transporting the negative charges of planes Cu(2)O2 to chains Cu(1)-O via O(1), this increases the number of hole psh in superconductors plans. For each r(Ln3+), the [AO] heat treatment increase psh indicating an improvement of superconducting properties in our systems.

oxygen per chain and decreases the distance between chains and the superconducting planes Cu(2)O2 which increases the number of holes psh. This improve the transfer of charges via O(1), respectively, indicating that there is an improvement of superconducting properties in our systems.

All these results are the product of an interaction between the cationic disorder of Ln3+ on the site Sr/Ba along c axis and oxygen anion disorder in the basal plane. A combination of several factors such as the ionic size of the rare earth; decrease in d[Cu(1)-(Sr/Ba)]; increase in cationic and chain oxygen ordering; in-phase purity for the [AO] samples may account for the observed data.

REFERENCES

1. H.A. Borges, M.A. Continentino, Solid State Commun. 80, 197 1991

2. R. Vovk, N. Vovk, G. Khadzhai, I. Goulatis, A. Chroneos, Solid State Commun. 190, 18 2014

3. S. Sadewasser, J.S. Schilling, A.P. Paulikas, B.W. Veal, Phys. Rev. B

   61, 741 2000

4. R. Beyers, B. T. Ahn, G. Gorman, V. Y. Lee, S. S. P. Parkin, M. L. Ramirez, K. P. Poche, J. E. Vazquez, T. M. Gut and R. A. Huggins, Oxygen ordering, phase separation and the 60-K and 90-K plateaus in YBa2Cu3Ox, Nature 340, 619, 1989.

5. R.V. Vovk, N.R. Vovk, O.V. Dobrovolskiy, J. Low Temp. Phys. 175, 614 2014

6. R. J. Cava, B. Batlogg, C. H. Chen, E. A. Rietman, S. M. Zahurak and

   D. J. Werder, Oxygen stoichiometry, superconductivity and normal- state properties of YBa2Cu3O7-, Nature 329, 423, 1987.

7. G. V. M. Williams and J. L. Tallon, Ion size effects on Tc and interplanar coupling in RBa2Cu3O7-, Physica C 258, 41 1996.

8. J. G. Lin, C. Y. Huang, Y. Y. Xue, C. W. Chu, X. W. Gao and J. C.

   Ho, Origin of the R-ion effect on Tc in RBa2Cu3O7, Phys. Rev. B 51, 12900 1995.

9. M.K.Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J.

   Huang, Y.Q.Wang, C.W. Chu, Phys. Rev. Lett. 58, 908 1987

10. B. Raveau, C. Michel, M. Hervieu, and D. Groult, Crystal Chemistry of High T Superconducting Copper Oxides, Springer Series in Materials Science, Vol. 15, edited by H. K. V. Lotsch (Springer- Verlag, Berlin), 1992.

11. R.V. Vovk, N.R. Vovk, G.Y. Khadzhai, I.L. Goulatis, A. Chroneos,

    Physica B 422, 33 2013

12. R.A. Gunasekaran, B. Hellebrand, P.L. Steger, Physica C 270 1996 25.

13. X.Z. Wang, D. Bo¨ uerle, Physica C 176 1991 507.

14. Y.G. Zhao, S.Y. Xiong, Y.P. Li, B. Zhang, S.S. Fan, B. Yin, J.W. Li,

    S.Q. Guo, W.H. Tang, G.H. Rao, D.J. Dong, B.S. Cao, B.L. Gu, Phys. Rev. B 56 1997 9153.

15. T. Wuernisha, Y. Takahashi, K. Takase, Y. Takano, K. Sekizawa, J. Alloys Compds. 377 2004 216.

16. P. Karen, H. Fjellvag, A. Kjekshus, A.F. Andresen, J. Solid State Chem. 92 1991 57.

17. X.Z. Wang, B. Hellebrand, D. Böuerle, Physica C 200 1992 12.

18. X.Z. Wang, B. Hellebrand, D. Böuerle,. G. Wortmann, M. Strecer, W. Lang, Physica C 242 1995 55.

19. X.Z. Wang, P.L. Steger, M. Reissner, W. Steiner, Oxygen ordering and superconductivity in DyBaSrCu3Oy, Physica C 196 247, 1992.

20. H. Kiipfer, I. Apfelstedt, R. FliJkiger, C. Keller, R. Meier- Hirmer, B. Runtsch, A. Turowski, U. Wiech and T. Wolf, Intragrain junctions in YBa2Cu3O7-x ceramics and single crystals, Cryogenics 29 268, 1989.

21. K. A. Muller, M. Takashige, and J. G. Bednorz, Flux Trapping and Superconductive Glass State in La2CuO4-y, Phys. Rev Lett. 58 1143, 1987.

22. M. B. Maple, Y. Dahichaouch, J. M. Ferreira, R. R. Hake, B. W. Lee,

    J. J. Neumeier, M. S. Torikachvili, K. N. Yang, H. Zhou, R. P. Guartin and M. V. Kuric, RBa2Cu3O7 (R=rare earth) high-Tc magnetic superconductors, Physica B 148, 155 1987.

23. Y. Tokura, H. Takagi, and S. Uchida, A superconducting copper oxide compound with electrons as the charge carriers, Nature, vol. 337, pp. 345-347, 1989.

24. R.J. Cava, Structural chemistry and the local charge picture of copper oxide superconductors, Science, vol. 247, p. 656-662, 1990.

25. G. Uimin, J. Rossat-Mignod, Role of Cu-O chains in the charge transfer mechanism in YBa2Cu3O6+x , Physica C 199 251; S. Senoussi,

    J. Physique III 2 1041, 1992.

26. H. Zhang and H. Sato, Universal relationship between and the hole content in p-type cuprate superconductors, Phys. Rev. Lett., vol. 70, no. 11, pp. 16971699, March 15, 1993.