**3.2. Real part of the AC magnetic susceptibility and Tc**

The critical temperature Tc of the transition from the superconductor to the normal state depends strongly on the effect of [AO] heat treatment as seen in the real part of AC susceptibility χ′(T) in **Figure 4**. The imaginary part of AC susceptibility χ″(T) in **Figure 4** shows a single peak Tp. This defined clearly the value of Tc for all the samples. We can see in **Figure 5** that when x was increased from 0 to 1, Tc [O] decreased from 83 K to 79.3 K. Tc [AO] first decreases

Effects of Isovalent Substitutions and Heat Treatments on Tc, Orthorhombicity, Resistivity, AC… http://dx.doi.org/10.5772/intechopen.74354 133

**Figure 4.** χ′ and χ″ of (Y1−xSmx )SrBaCu<sup>3</sup> O6+z as a function of temperature. (a) Heat treatment [O], (b) heat treatment [AO].

**Figure 5.** Tc and Tp of (Y1−xSmx )SrBaCu<sup>3</sup> O6+z as a function of x(Sm) following the [O] and [AO] heat treatments.

The orthorhombicity depends strongly on the Sm content x. When x increases from 0 to 1, ε decreases quickly from 8.24 × 10−3 to 1.5 × 10−3 in the samples [O] in **Figure 3**. This indicated a structural phase transition from orthorhombic to tetragonal. ε decreases slowly from 9.9 × 10−3 to 5.24 × 10−3 with an orthorhombic symmetry in the samples [AO]. We found also that the orthorhombicity depends strongly on the heat treatment [AO]. For each x, the latter increased the orthorhombicity (for 0 ≤ x ≤1). The increase was maximum, from 1.5 × 10−3 to 5.24 × 10−3

)SrBaCu<sup>3</sup>

)SrBaCu<sup>3</sup>

depends strongly on the effect of [AO] heat treatment as seen in the real part of AC susceptibility χ′(T) in **Figure 4**. The imaginary part of AC susceptibility χ″(T) in **Figure 4** shows a sin-

of the transition from the superconductor to the normal state

O6+z as a function of x and heat treatment.

[O] decreased from 83 K to 79.3 K. Tc

for all the samples. We can see in **Figure 5** that

O6+z as a function of x and heat treatment in the left.

[AO] first decreases

for x = 1 in [12].

The critical temperature Tc

**3.2. Real part of the AC magnetic susceptibility and Tc**

gle peak Tp. This defined clearly the value of Tc

**Figure 2.** Variation of the parameters a, b and c of (Y1−xSmx

O6+z in the right.

)SrBaCu<sup>3</sup>

**Figure 3.** Variation of the orthorhombicity of (Y1−xSmx

The unit cell of (Y1−xSmx

132 Superfluids and Superconductors

when x was increased from 0 to 1, Tc

from 81.7 K (for x = 0) to 81.2 K (for x = 0.2) (like in the samples [O]) and then increases to 85 K for SmSrBaCu<sup>3</sup> O6+z. For each x, the [AO] heat treatment increases Tc for x ≥ 0.4 and decreases it for x < 0.4. A maximum of increase in Tc of 6 K was observed in SmSrBaCu<sup>3</sup> O6+z [AO] [8].

For each x, the [AO] heat treatment increases ε (for 0 ≤ x ≤ 1) in **Figure 3** and Tc (for x ≥ 0.4) in **Figure 5**. The [AO] heat treatment makes the coupling of the superconducting grains by Josephson junctions took place at higher temperature. This effect is revealed by the net displacement of Tp to higher temperature for x ≥ 0.4.


**Table 1.** Structural, superconducting and magnetic parameters of (Y1−xSmx)SrBaCu O3 6+z. **Table 1** shows the exact measured values of the structural parameters a, b, c, V and ε of each

Effects of Isovalent Substitutions and Heat Treatments on T

ment employed. 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 con

**Figure 6(a)**). **Table 1** shows the exact measured values of the superconducting parameters Tc

We can see in **Figure 7** the shielding effect S which is the amplitude of the real part of the AC

= 0 Oe.

= 126.5 Oe [13]. When Hdc increases, S[AO] decreases slowly than S[O]. For example, at

= 55 K, S[AO] decreases by 10% whereas S[O] decreases by 70%. This indicated an improve

–12]. S represents the exclusion of the magnetic flux by the sample in alterna

= 1), S[AO]

=

O6+z as a function of the temperature and heat treatment at four fields Hdc

2 S[O] at T

siderably compared to that of the [O] sample for each x (see, for example, the case x

<sup>p</sup> of each sample as a function of the heat treatment.

tive dynamic mode. S was set arbitrarily equal to 0.89, 0.97 and 1, respectively, for x

For each x > 0.5, the [AO] heat treatment increases the shielding effect at all T

ment of the quality of the grains and intergranular coupling in the samples [AO].

was remarkable. The temperature at which the dia

and it was found to be dependent on both x and the heat treat


135



,



= 0.8 in

= 0.5, 0.8,

and for

= 65 K and

< Tc

c, Orthorhombicity, Resistivity, AC… http://dx.doi.org/10.5772/intechopen.74354

**3.3. Real part of the AC magnetic susceptibility and the shielding effect**

sample as a function of the heat treatment.

The effect of [AO] heat treatment on Tc

and 1, for the sample [AO] at 55 K and for Hdc

**Figure 6.** (a) χ′ and (b) χ″ of (Y0.2Sm0.8)SrBaCu

< 126.5 Oe).

3

any applied Hdc. For example, in SmSrBaCu3O6+z (x

magnetism sets in is taken as Tc

and ΔT

susceptibility [10

T <sup>p</sup>, ΔTc

Hdc

T

(0 < Hdc **Table 1** shows the exact measured values of the structural parameters a, b, c, V and ε of each sample as a function of the heat treatment.

### **3.3. Real part of the AC magnetic susceptibility and the shielding effect**

The effect of [AO] heat treatment on Tc was remarkable. The temperature at which the diamagnetism sets in is taken as Tc and it was found to be dependent on both x and the heat treatment employed. 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 for each x (see, for example, the case x = 0.8 in **Figure 6(a)**). **Table 1** shows the exact measured values of the superconducting parameters Tc , Tp, ΔTc and ΔTp of each sample as a function of the heat treatment.

We can see in **Figure 7** the shielding effect S which is the amplitude of the real part of the AC susceptibility [10–12]. S represents the exclusion of the magnetic flux by the sample in alternative dynamic mode. S was set arbitrarily equal to 0.89, 0.97 and 1, respectively, for x = 0.5, 0.8, and 1, for the sample [AO] at 55 K and for Hdc = 0 Oe.

For each x > 0.5, the [AO] heat treatment increases the shielding effect at all T < Tc and for any applied Hdc. For example, in SmSrBaCu3O6+z (x = 1), S[AO] = 2 S[O] at T = 65 K and Hdc = 126.5 Oe [13]. When Hdc increases, S[AO] decreases slowly than S[O]. For example, at T = 55 K, S[AO] decreases by 10% whereas S[O] decreases by 70%. This indicated an improvement of the quality of the grains and intergranular coupling in the samples [AO].

**Figure 6.** (a) χ′ and (b) χ″ of (Y0.2Sm0.8)SrBaCu<sup>3</sup> O6+z as a function of the temperature and heat treatment at four fields Hdc (0 < Hdc < 126.5 Oe).

**x** **Heat treatment**

a (Å) b (Å) c (Å) V (Å) ε (10−3)

Tc (K) Tp (K)

ΔTc ΔTp K′ (Oe)

n **Table 1.**

Structural, superconducting and magnetic parameters of (Y1−xSmx)SrBaCu

O3 6+z.

—

—

1.41

1.50

1.33

1.53

1.13

1.88

1.14

1.63

0.91

1.54

1.31

1.33

—

—

2068

3390

1679

3848

1447

5177

1057

5910

936

8165

1677

11,741

0.3

1.2

0.89

1.3

0.7

0.9

0.65

0.56

1.2

2.2

1.05

0.56

0.96

0.51

0.4

0.41

1.42

2

1.1

1

0.92

0.8

1.7

2.9

1.58

0.72

1.5

0.8

82.9

80.4

82.13

80.3

81.02

81

80.77

81.24

80.07

82.1

79.7

83.86

79

84.4

83

81.7

82.24

81.2

81.3

81.5

81

82.02

80.7

82.5

80.1

84

79.3

84.6

8.24

9.90

4.24

8.32

3.99

7.46

4.02

7.16

2.93

7.02

1.56

5.52

1.50

5.24

168.0

168.4

168.8

169.0

169.6

169.7

170.1

170.1

170.5

170.4

171.3

171.1

172.3

172.0

11.55

11.56

11.55

11.56

11.57

11.57

11.58

11.58

11.59

11.59

11.60

11.60

11.62

11.62

3.784

3.780

3.807

3.793

3.814

3.801

3.817

3.805

3.825

3.808

3.837

3.820

3.844

3.827

3.846

3.855

3.839

3.856

3.845

3.858

3.848

3.860

3.848

3.862

3.849

3.862

3.856

3.867

134 Superfluids and Superconductors

**[O]**

**[AO]**

**[O]**

**[AO]**

**[O]**

**[AO]**

**[O]**

**[AO]**

**[O]**

**[AO]**

**[O]**

**[AO]**

**[O]**

**[AO]**

**0**

**0.2**

**0.4**

**0.5**

**0.6**

**0.8**

**1**

**Figure 7.** Shielding effect S of (Y1−xSmx )SrBaCu<sup>3</sup> O6+z as a function of the field Hdc and heat treatment at three different temperatures (55, 65 and 75 K).

and 11,741 Oe, respectively, for the samples [O] and [AO] in SmSrBaCu<sup>3</sup>

and heat treatment of (Y1−xSmx

line indicates the value n = 1.5 for the cuprites given by Miller et al. [15].

perature. For each temperature, ρ[AO] is superior to ρ[O]. For each x, Tc

This indicates a reduction of the interaction of carrier charges with phonons.

(χ′) is superior to Tc

part of ρ(T), in the normal state, follows the relationship ρ = ρ<sup>0</sup>

**3.5. Resistivity**

SmBaSrCu<sup>3</sup>

each heat treatment Tc

**Figure 8.** H as a function of t = Tp/Tc

be interpreted as the field necessary to reduce the intergranular critical current to zero in the limit of T<sup>p</sup> = 0 K. We note that the argon treatment considerably increases the value of K′ and n, in **Table 1** and **Figure 10**, indicating an improvement in the pinning properties. The dashed

)SrBaCu<sup>3</sup>

Effects of Isovalent Substitutions and Heat Treatments on Tc, Orthorhombicity, Resistivity, AC…

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137

O6+z.

**Figure 11** shows that the resistivity ρ(T) of the sample SmSrBaCu3O**6+z** increases with the tem-

ual resistivity extrapolated to T = 0 K and α is the slope dρ/dT. For example, the sample

treatment [AO] reduced considerably these parameters; in particular α[AO] = 0.9 (μΩ cm/K).

O6+z [O] has α = 1.8 (μΩ cm/K), ρ<sup>0</sup> = 242 (μΩ cm) and ρ297 K = 785 (μΩ cm). The

(ρ = 0) by 2–3 K with Tp (χ″) ≈ Tc

O6+z (x = 1). K′ may

(χ′) and for

is the resid-

(ρ = 0). The linear

(ρ = 0) ≈ Tc

+ α T, where ρ<sup>0</sup>

#### **3.4. Imaginary part of the AC magnetic susceptibility and irreversibility line**

Looking to the imaginary part of the AC susceptibility χ″, of the sample Y0.2Sm0.8SrBaCu<sup>3</sup> O6+z in **Figure 6(b)** for example, we can see that the width ΔTp at half maximum of the transition in χ″(T) (see **Table 1**) was smaller in the samples [AO] at all Hdc and the peak Tp shifted less than in the sample [O]. **Figure 8** shows the field Hdc as a function of t = Tp/Tc with an enhancement of the irreversibility line due to argon treatment for x ≥ 0.5 [14]. The data can be analyzed with the help of following relation H = K′ (1 − t)<sup>n</sup> [15]. Straight line plots were obtained when ln(H) was plotted against ln(1 – t) in **Figure 9**. For example, the value of K′ was estimated to be 1677

Effects of Isovalent Substitutions and Heat Treatments on Tc, Orthorhombicity, Resistivity, AC… http://dx.doi.org/10.5772/intechopen.74354 137

**Figure 8.** H as a function of t = Tp/Tc and heat treatment of (Y1−xSmx )SrBaCu<sup>3</sup> O6+z.

and 11,741 Oe, respectively, for the samples [O] and [AO] in SmSrBaCu<sup>3</sup> O6+z (x = 1). K′ may be interpreted as the field necessary to reduce the intergranular critical current to zero in the limit of T<sup>p</sup> = 0 K. We note that the argon treatment considerably increases the value of K′ and n, in **Table 1** and **Figure 10**, indicating an improvement in the pinning properties. The dashed line indicates the value n = 1.5 for the cuprites given by Miller et al. [15].

#### **3.5. Resistivity**

O6+z

with an enhancement

O6+z as a function of the field Hdc and heat treatment at three different

**3.4. Imaginary part of the AC magnetic susceptibility and irreversibility line**

in the sample [O]. **Figure 8** shows the field Hdc as a function of t = Tp/Tc

)SrBaCu<sup>3</sup>

**Figure 7.** Shielding effect S of (Y1−xSmx

temperatures (55, 65 and 75 K).

136 Superfluids and Superconductors

Looking to the imaginary part of the AC susceptibility χ″, of the sample Y0.2Sm0.8SrBaCu<sup>3</sup>

in **Figure 6(b)** for example, we can see that the width ΔTp at half maximum of the transition in χ″(T) (see **Table 1**) was smaller in the samples [AO] at all Hdc and the peak Tp shifted less than

of the irreversibility line due to argon treatment for x ≥ 0.5 [14]. The data can be analyzed with the help of following relation H = K′ (1 − t)<sup>n</sup> [15]. Straight line plots were obtained when ln(H) was plotted against ln(1 – t) in **Figure 9**. For example, the value of K′ was estimated to be 1677 **Figure 11** shows that the resistivity ρ(T) of the sample SmSrBaCu3O**6+z** increases with the temperature. For each temperature, ρ[AO] is superior to ρ[O]. For each x, Tc (ρ = 0) ≈ Tc (χ′) and for each heat treatment Tc (χ′) is superior to Tc (ρ = 0) by 2–3 K with Tp (χ″) ≈ Tc (ρ = 0). The linear part of ρ(T), in the normal state, follows the relationship ρ = ρ<sup>0</sup> + α T, where ρ<sup>0</sup> is the residual resistivity extrapolated to T = 0 K and α is the slope dρ/dT. For example, the sample SmBaSrCu<sup>3</sup> O6+z [O] has α = 1.8 (μΩ cm/K), ρ<sup>0</sup> = 242 (μΩ cm) and ρ297 K = 785 (μΩ cm). The treatment [AO] reduced considerably these parameters; in particular α[AO] = 0.9 (μΩ cm/K). This indicates a reduction of the interaction of carrier charges with phonons.

**4. Discussions**

**Figure 11.** Resistivity ρ(T) of SmBaSrCu<sup>3</sup>

But for each x, Tc

**Figure 12.** Variation of Tc

When x increases from 0 to 1, Tc

We saw that the [AO] heat treatment increases the orthorhombic cleaving and eliminated some weak unidentified impurity peaks in **Figure 1(b)**. This indicates a good crystallization

Our samples were prepared in 1 atm of oxygen. Our iodometry measurements show that the total oxygen constant was 6 + z = 6.94 ± 0.04 and do not change after the [AO] heat treatment.

[O] decreases with ε. Tc

shown in **Figure 12**. When x increases, the parameter b is constant but a (and c) increase

as a function of the orthorhombicity ε and heat treatments of (Y1−xSmx

bicity ε until x = 0.2 and afterward it increases from 79 to 85 K in SmSrBaCu<sup>3</sup>

[AO] increased for x ≥ 0.4. So this increase is not due to z but may lie in some

O6+z as a function of the temperature and heat treatment.

Effects of Isovalent Substitutions and Heat Treatments on Tc, Orthorhombicity, Resistivity, AC…

[AO] decreases with the orthorhom-

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139

)SrBaCu<sup>3</sup>

O6+z.

O6+z [AO], as

and an improvement of crystallographic quality of the samples [AO].

other factor which governs the superconductivity in these samples.

**Figure 9.** Ln(H) as a function of ln(1 − t) and heat treatment of (Y1−xSmx )SrBaCu<sup>3</sup> O6+z.

**Figure 10.** The field K′ and the exponent n as a function of x and heat treatment of (Y1−xSmx )SrBaCu<sup>3</sup> O6+z.

Effects of Isovalent Substitutions and Heat Treatments on Tc, Orthorhombicity, Resistivity, AC… http://dx.doi.org/10.5772/intechopen.74354 139

**Figure 11.** Resistivity ρ(T) of SmBaSrCu<sup>3</sup> O6+z as a function of the temperature and heat treatment.

## **4. Discussions**

**Figure 9.** Ln(H) as a function of ln(1 − t) and heat treatment of (Y1−xSmx

138 Superfluids and Superconductors

**Figure 10.** The field K′ and the exponent n as a function of x and heat treatment of (Y1−xSmx

)SrBaCu<sup>3</sup>

O6+z.

)SrBaCu<sup>3</sup>

O6+z.

We saw that the [AO] heat treatment increases the orthorhombic cleaving and eliminated some weak unidentified impurity peaks in **Figure 1(b)**. This indicates a good crystallization and an improvement of crystallographic quality of the samples [AO].

Our samples were prepared in 1 atm of oxygen. Our iodometry measurements show that the total oxygen constant was 6 + z = 6.94 ± 0.04 and do not change after the [AO] heat treatment. But for each x, Tc [AO] increased for x ≥ 0.4. So this increase is not due to z but may lie in some other factor which governs the superconductivity in these samples.

When x increases from 0 to 1, Tc [O] decreases with ε. Tc [AO] decreases with the orthorhombicity ε until x = 0.2 and afterward it increases from 79 to 85 K in SmSrBaCu<sup>3</sup> O6+z [AO], as shown in **Figure 12**. When x increases, the parameter b is constant but a (and c) increase

**Figure 12.** Variation of Tc as a function of the orthorhombicity ε and heat treatments of (Y1−xSmx )SrBaCu<sup>3</sup> O6+z.

indicating an increase of the number of oxygen atoms by chain (NOC) along a axis with a decrease of ε (Tc [O]) from orthorhombic toward tetragonal structure in SmSrBaCu<sup>3</sup> O6+z [O].

optimum superconducting properties and could account for the observed increase in Tc

number of oxygen atoms by chain (NOC) along a axis with a decrease of ε (Tc

rhombic toward tetragonal structure in **Figure 12**.

rhombicity ε for x ≥ 0.2 as seen in **Figure 12**.

able correlations observed between Tc

**Figure 14.** Correlation between Tc

SrBaCu<sup>3</sup>

O6+z.

were in good agreement.

atoms by chain (NOC) along b axis leading to an increase of Tc

in agreement with the model of transfer of charges. This is justified by the fact that, when x increases, the parameter b is constant but a (and c) increase leading to an increase of the

Effects of Isovalent Substitutions and Heat Treatments on Tc, Orthorhombicity, Resistivity, AC…

When Sm ion occupies Ba (or Sr) site, the same amount of Ba (or Sr) cation is pushed into Y site. Sm 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(5) along the a-axis in the basal plane have higher chance to be filled. On the other hand, Ba+2 (or Sr+2) in Y+3 (or Sm+3) site decrease the attractive force with oxygen anion in Cu(2) plane. This increases the buckling angle Cu(2)─O(3)─Cu(2) along the a axis. When x increased from 0 to 1, the two changes of cation sites increase the parameter a. For each x, the [AO] heat treatment decreases the parameter a and increases b as shown in **Figure 2**. This increases the number of oxygen

In the normal state, the heat treatment [AO] reduced considerably the linear resistivity parame-

any applied field indicating an enhancement of the quality of the grains and intergranular coupling in the samples [AO]. Also for x ≥ 0.5, an enhancement of the irreversibility line was noticed in the samples [AO] with an increase of the field K′ showing an improvement in the pinning properties. These results are justified by our XRD spectra, with Rietveld refinement, that showed an improvement of crystallographic quality of the samples [AO] in **Figure 1**.

The two arguments (cationic and anionic disorders) are justified here by the four remark-

(x), the volume of the unit cell V(x) in **Figure 14** and

and the volume V of the unit cell as a function of x and heat treatment of (Y1−xSmx

ters indicating a diminution of the interaction of carrier charges with phonons. Tc

For each x > 0.5, the [AO] heat treatment improved the shielding effect at all T < Tc

[AO]

141

[O]) from ortho-

with a decrease of the ortho-

http://dx.doi.org/10.5772/intechopen.74354

(χ′) and Tc

(ρ = 0)

and for

)

For each x, the [AO] treatment increases the orthorhombicity ε (for 0 ≤ x ≤ 1) and Tc (for x ≥ 0.4). For each x, the parameter a decreases and b increases after the [AO] heat treatment in the unit cell of **Figure 2**. Some oxygen atoms O(4) go to the vacant site O(5) along b axis. So the (NOC) and the anionic order in the basal plane increases leading to an increase of psh and Tc for x ≥ 0.4 in **Figure 15**.

For each x ≥ 0.4, the thermal parameter of the apical oxygen O(1) decreased from 2.02 to 0.27 Å<sup>2</sup> in the sample [AO] leading to a decrease of the cationic disorder; of Y (0.893 Å) (or Sm (0.965 Å) occupying some Ba (1.42 Å)/Sr (1.12 Å) sites along the c axis. Each sample [O] was heated in argon at 850°C. This action removes all the oxygen atoms from the structure and increases the atomic diffusion and the Y/Sm-Sr/Ba-Y/Sm order along c axis in the unit cell of **Figure 2**. In fact, the difference of bond valence (B.V.S.): V(Y)-V(Ba) = 0.77 in YBa<sup>2</sup> Cu<sup>3</sup> O6.7 and 1.00 in YBa<sup>2</sup> Cu<sup>3</sup> O6.32 indicate that the departure from reduced (6 + z) decreases the disorder of Y on the Ba site in YBa<sup>2</sup> Cu<sup>3</sup> O6+z [16]. So, the argon heat treatment decreases the disorder of Y/ Sm on the Ba/Sr site. This is justified by the fact that impurity peaks seen in the [O] samples in **Figure 1(a)** disappeared after the [AO] heat treatment in **Figure 1(b)**.

Our results can be explained by the disorder of the oxygen in the basal plane, on the 0(4) and 0(5) sites along b and a axis, respectively, in **Figure 2**. This order enhanced the orthorhombic symmetry and increased the ratio (b − a)/(b + a). As seen on **Figure 13** when x increases, the interatomic distance d[Cu(1)─(Sr/Ba)] increases for both heat treatments in agreement with the fact that the crystallographic parameter c and the volume of the unit cell increases with x. For each x, the [AO] heat treatment decreases this distance for x ≥ 0.5 (and increases it for x < 0.5). This decreases the distance d[Cu(1)─O(1)] and enhances the transfer of holes from the Cu(1)O chains to the superconducting planes Cu(2)O<sup>2</sup> via the apical oxygen O(1) resulting in an increase in the hole density psh and Tc for x ≥ 0.4 in **Figure 15**. Such an increase leads to

**Figure 13.** Interatomic distance d[Cu(1)─(Sr/Ba)] as a function of x and heat treatment in (Y1−xSmx )SrBaCu<sup>3</sup> O6+z.

optimum superconducting properties and could account for the observed increase in Tc [AO] in agreement with the model of transfer of charges. This is justified by the fact that, when x increases, the parameter b is constant but a (and c) increase leading to an increase of the number of oxygen atoms by chain (NOC) along a axis with a decrease of ε (Tc [O]) from orthorhombic toward tetragonal structure in **Figure 12**.

indicating an increase of the number of oxygen atoms by chain (NOC) along a axis with a

0.4). For each x, the parameter a decreases and b increases after the [AO] heat treatment in the unit cell of **Figure 2**. Some oxygen atoms O(4) go to the vacant site O(5) along b axis. So the (NOC) and the anionic order in the basal plane increases leading to an increase of psh and Tc

For each x ≥ 0.4, the thermal parameter of the apical oxygen O(1) decreased from 2.02 to

Sm on the Ba/Sr site. This is justified by the fact that impurity peaks seen in the [O] samples in

Our results can be explained by the disorder of the oxygen in the basal plane, on the 0(4) and 0(5) sites along b and a axis, respectively, in **Figure 2**. This order enhanced the orthorhombic symmetry and increased the ratio (b − a)/(b + a). As seen on **Figure 13** when x increases, the interatomic distance d[Cu(1)─(Sr/Ba)] increases for both heat treatments in agreement with the fact that the crystallographic parameter c and the volume of the unit cell increases with x. For each x, the [AO] heat treatment decreases this distance for x ≥ 0.5 (and increases it for x < 0.5). This decreases the distance d[Cu(1)─O(1)] and enhances the transfer of holes from

 in the sample [AO] leading to a decrease of the cationic disorder; of Y (0.893 Å) (or Sm (0.965 Å) occupying some Ba (1.42 Å)/Sr (1.12 Å) sites along the c axis. Each sample [O] was heated in argon at 850°C. This action removes all the oxygen atoms from the structure and increases the atomic diffusion and the Y/Sm-Sr/Ba-Y/Sm order along c axis in the unit cell of

O6.32 indicate that the departure from reduced (6 + z) decreases the disorder of

O6+z [16]. So, the argon heat treatment decreases the disorder of Y/

For each x, the [AO] treatment increases the orthorhombicity ε (for 0 ≤ x ≤ 1) and Tc

**Figure 2**. In fact, the difference of bond valence (B.V.S.): V(Y)-V(Ba) = 0.77 in YBa<sup>2</sup>

**Figure 1(a)** disappeared after the [AO] heat treatment in **Figure 1(b)**.

**Figure 13.** Interatomic distance d[Cu(1)─(Sr/Ba)] as a function of x and heat treatment in (Y1−xSmx

[O]) from orthorhombic toward tetragonal structure in SmSrBaCu<sup>3</sup>

O6+z [O].

Cu<sup>3</sup>

via the apical oxygen O(1) resulting

)SrBaCu<sup>3</sup>

O6+z.

for x ≥ 0.4 in **Figure 15**. Such an increase leads to

O6.7 and

(for x ≥

decrease of ε (Tc

140 Superfluids and Superconductors

0.27 Å<sup>2</sup>

1.00 in YBa<sup>2</sup>

for x ≥ 0.4 in **Figure 15**.

Cu<sup>3</sup>

Cu<sup>3</sup>

the Cu(1)O chains to the superconducting planes Cu(2)O<sup>2</sup>

in an increase in the hole density psh and Tc

Y on the Ba site in YBa<sup>2</sup>

When Sm ion occupies Ba (or Sr) site, the same amount of Ba (or Sr) cation is pushed into Y site. Sm 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(5) along the a-axis in the basal plane have higher chance to be filled. On the other hand, Ba+2 (or Sr+2) in Y+3 (or Sm+3) site decrease the attractive force with oxygen anion in Cu(2) plane. This increases the buckling angle Cu(2)─O(3)─Cu(2) along the a axis. When x increased from 0 to 1, the two changes of cation sites increase the parameter a. For each x, the [AO] heat treatment decreases the parameter a and increases b as shown in **Figure 2**. This increases the number of oxygen atoms by chain (NOC) along b axis leading to an increase of Tc with a decrease of the orthorhombicity ε for x ≥ 0.2 as seen in **Figure 12**.

In the normal state, the heat treatment [AO] reduced considerably the linear resistivity parameters indicating a diminution of the interaction of carrier charges with phonons. Tc (χ′) and Tc (ρ = 0) were in good agreement.

For each x > 0.5, the [AO] heat treatment improved the shielding effect at all T < Tc and for any applied field indicating an enhancement of the quality of the grains and intergranular coupling in the samples [AO]. Also for x ≥ 0.5, an enhancement of the irreversibility line was noticed in the samples [AO] with an increase of the field K′ showing an improvement in the pinning properties. These results are justified by our XRD spectra, with Rietveld refinement, that showed an improvement of crystallographic quality of the samples [AO] in **Figure 1**.

The two arguments (cationic and anionic disorders) are justified here by the four remarkable correlations observed between Tc (x), the volume of the unit cell V(x) in **Figure 14** and

**Figure 14.** Correlation between Tc and the volume V of the unit cell as a function of x and heat treatment of (Y1−xSmx ) SrBaCu<sup>3</sup> O6+z.

the number psh(x) of holes by Cu(2)─O2

(x) = Tc

effect of argon heat treatment.

The increase or decrease in Tc

between δTc

**5. Conclusions**

superconductors (Y1−xSmx

the effect of argon heat treatment.

Address all correspondence to: nafidi21@yahoo.fr

trons as the charge carriers. Nature. 1989;**337**:345-347

University Ibn Zohr, Agadir, Morocco

tors. Science. 1990;**247**:656-662

psh(x) by Cu(2)─O2

**Author details**

Abdelhakim Nafidi

**References**

the undersaturation zone of the universal relation Tc

)SrBaCu<sup>3</sup>

[AO] − Tc

superconducting planes in **Figure 15** (deduced from

must be related to the ionic size of the rare earth Sm, the varia-

O6+z by a simple argon heat treatment. These results are

[O] and δε(x) in **Figure 16** and between δTc

Effects of Isovalent Substitutions and Heat Treatments on Tc, Orthorhombicity, Resistivity, AC…

**Figure 17**. So the structural, electrical and superconducting properties are correlated with the

tion of the Cu(1)-apical oxygen distance, hole density, anionic and cationic disorders, etc.

These studies indicate the optimization of the superconducting properties of the high-Tc

a competition between oxygen disorder in basal plane and cationic disorder along c axis. In the samples [O], we are in the presence of a cationic disorder of Y/Sm on (Sr/Ba) sites that induced an anionic disorder of oxygen's chains in basal plane. Anionic order dominates in the samples [AO] in agreement with the previsions of [4, 5]. In the samples [AO], the remarkable improvement in the shielding effect (for x > 0.5) and the irreversibility line (for x ≥ 0.5) are explained, respectively, by the improvement of the quality of the grains and intergranular coupling, and to the improvement of the pinning properties and crystallographic quality of these samples. The structural, magnetic and superconducting properties are correlated with

These results were explained by the effect of the ionic size of the rare earth, the decrease in d[Cu(1)─(Sr/Ba)]; the increase in cationic and chain oxygen ordering; the number of holes

Laboratory of Condensed Matter Physics and Nanomaterials for Renewable Energy,

[1] Tokura Y, Takagi H, Uchida S. A superconducting copper oxide compound with elec-

[2] Cava RJ. Structural chemistry and the local charge picture of copper oxide superconduc-

superconducting plans and in phase purity for the [AO] samples.

/Tcmax (psh) [17]), and on the other hand,

http://dx.doi.org/10.5772/intechopen.74354

(x) and δK′(x) in

143

**Figure 15.** Correlation between psh and Tc as a function of x and heat treatment of (Y1−xSmx )SrBaCu<sup>3</sup> O6+z.

**Figure 16.** Correlation between δTc = Tc [AO] − Tc [O] and δε = ε[AO] − ε[O] as a function of x and heat treatment of (Y1−xSmx ) SrBaCu<sup>3</sup> O6+z.

**Figure 17.** Correlation between δTc and δK′ as a function of x and heat treatment of (Y1−xSmx )SrBaCu<sup>3</sup> O6+z.

the number psh(x) of holes by Cu(2)─O2 superconducting planes in **Figure 15** (deduced from the undersaturation zone of the universal relation Tc /Tcmax (psh) [17]), and on the other hand, between δTc (x) = Tc [AO] − Tc [O] and δε(x) in **Figure 16** and between δTc (x) and δK′(x) in **Figure 17**. So the structural, electrical and superconducting properties are correlated with the effect of argon heat treatment.

The increase or decrease in Tc must be related to the ionic size of the rare earth Sm, the variation of the Cu(1)-apical oxygen distance, hole density, anionic and cationic disorders, etc.
