**1. Introduction**

YBa<sup>2</sup> Cu<sup>3</sup> O6.95 is superconducting below 92 K and characterized by double Cu(2)O<sup>2</sup> layers (oriented along the a-b plane) responsible for carrying the supercurrent and Cu(1)O chains (along the b direction) that provide a charge reservoir for these planes [1, 2].

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The four distinct crystallographic sites Y, Ba, Cu plane, and Cu chain can be substituted with different elements. Single-phase LnBa<sup>2</sup> Cu<sup>3</sup> O6+z (Ln = rare earth) can be synthesized with Tc = 92 K. All these compounds show an orthorhombically distorted oxygen-deficient tripleperovskite structure and both the orthorhombic distortion and Tc depend sensitively on the oxygen content (6 + z) [3]. Wada et al. [4], Izumi et al. [5] studied the structural and superconducting properties of La1+xBa2−xCu<sup>3</sup> Oy (with 0 ≤ x ≤ 0.5). They 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), respectively.

for about 72 h. This sample is denoted as [AO]. XRD, resistivity and AC susceptibility measurements were done on a part of this sample. We measured 6 + z by iodometry technique on

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

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The X-ray diffraction spectra of all the samples are shown in **Figure 1** [8]. After the [AO] heat treatment, the reflections were sharper so the samples were well crystallized. The [AO] heat treatment increases the orthorhombic cleaving. For example, the (123) and (213) peaks at 2θ ≈ 58.5° (and (200) and (006) reflections at 2θ ≈ 47°) which were ill-resolved for the [O] samples were clearly identified after the [AO] heat treatment, as shown in **Figure 1**. Some weak unidentified impurity peaks (marked by crosses in **Figure 1(a)** were seen in the [O] samples and their amplitudes increase with x. They disappeared after the [AO] treatment shown in **Figure 1(b)**. This indicates an improvement of crystallographic quality of the samples [AO]. In **Figure 2** we show, respectively, the variation of the parameters a, b, c and the volume V of the unit cell obtained with Rietveld refinement [9] as a function of x and heat treatment. When x increases, the lattice parameter a (c and the volume V of the unit cell) increased but b is constant leading to a decrease of the orthorhombicity (ε = (b − a)/(b + a)) ε [O] in **Figure 3**. The substitution of Y+3 (0.893 Å) by the rare earth Sm+3 (0.965 Å), with a superior ionic radius,

a part of each sample.

**3.1. Crystalline structure**

leads to a linear increase of c and V.

**Figure 1.** XRD (Cu Kα) patterns of (Y1−xSmx

)SrBaCu<sup>3</sup>

(b) samples [AO] heated in argon at 850°C followed by annealing in oxygen at 450°C (x = impurity peaks).

O6+z as a function of x. (a) Samples [O] annealed in oxygen at 450°C,

**3. Results**

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 is replaced by the rare earth Sm+3 with bigger ionic radius. Understanding the effect of the Y and Ba atomic plans on the superconductivity in these compounds, we have studied the structural, superconducting and magnetic properties of (Y1−xSmx )SrBaCu<sup>3</sup> O6+z. We found that the effect of heat treatments on these properties depended on the content of Sm.
