*3.1.1 Structural and superconducting properties of Pr*2*Ba*4*Cu*7*O*<sup>15</sup>�*<sup>δ</sup>*

X-ray diffraction data revealed that the as-sintered polycrystalline samples are an almost single phase with an orthorhombic structure (*Ammm*), as shown in **Figure 3a**. The lattice parameters of the as-sintered sample prepared using the three-zone controlled furnace are *a* ¼ 3*:*8919 Å, *b* ¼ 3*:*9143 Å, and *c* ¼ 50*:*7927 Å.

These values fairly agree with the lattice parameters of Pr247 estimated by a previous study [30]. It was from gravimetric analysis estimated that the oxygen deficiencies in the 48 and 72 h reduced samples in a vacuum are *δ* ¼� 0*:*56 and *δ* ¼ � 0*:*72, corresponding to samples #2–1, #2–2 and sample #1, respectively (**Table 1**). According to a variation in *T*<sup>c</sup> as a function of the oxygen deficiency [31], the *T*c,on shows a rapid increase at *δ*≥ � 0*:*2, then follows a stable increase with *δ*, and finally remains saturation around 26–27 K at *δ*≥ � 0*:*6. Therefore, we conclude that the doped carrier concentrations in the present samples are located near the optimally doped region.

First of all, the temperature dependences of electric resistivities of the Pr123, Pr124, and as-sintered Pr247 compounds are shown in **Figure 5a**. The Pr123 and Pr124 samples exhibit semiconducting and metallic behaviors in *ρ*. The as-sintered Pr247 sample shows a weakly semiconducting property at high temperatures, it then reaches a maximum peak at intermediate tempertures around � 150 K, and finally it follows a metallic behavior with lowering *T*. It is well known that CuO single chains in Pr123 and CuO double chains in Pr124 show semiconducting and metallic behaviors, respectively. The reduction heat treatment on the as-sintered sample in a vacuum results in the appearance of superconducting state with *T*c=22-27 K, accompanied by the strongly metallic properties over a wide range of temperature (**Figure 6a**). For the present Pr247 sample with a reduction treatment for 72 h, a sharp superconducting transition appears at an onset temperature *T*c,on=26 K and then it achieves a zero resistance state at *T*c,zero=22 K.

Furthermore, to check bulk superconductivity, we performed to measure lowtemperature dependence of magnetic susceptibility *χ* of the 72-h-reduced superconducting sample measured at 5 mT under ZFC scan. **Figure 6b** exhibits diamagnetic signals below *T*c,on = 26 K for the 72-h-reduced sample. In addition, the superconducting volume fraction due to the shielding effect is estimated to be � 58% from the ZFC values at 5 K. In the inset of **Figure 6**, the magnified data are plotted to clarify the definition of *T*c,on. We note that the quenching procedure in

### **Figure 5.**

*(a) Temperature dependences of electric resistivities of the Pr123, Pr124, and as-sintered Pr247 compounds. (b) Schematic view of Cu(3dx*<sup>2</sup>�*y*<sup>2</sup> *) orbitals and O(2p<sup>σ</sup> ) orbitals in a CuO double chain of Pr*2*Ba*4*Cu*7*O*<sup>15</sup>�*<sup>δ</sup>. Here, tpp and tdd denote the hoping term between 2p<sup>σ</sup> orbitals at the nearest neighbor oxygen sites and that between 3dx*<sup>2</sup>�*y*<sup>2</sup> *orbitals at the nearest neighbor copper sites, respectively [32].*

*Functional Materials Synthesis and Physical Properties DOI: http://dx.doi.org/10.5772/intechopen.100241*

### **Figure 6.**

*(a) Temperature dependence of electric resistivity of the 72-h-reduced superconducting Pr247 compound. (b) Low-temperature dependence of magnetic susceptibility χ of the 72-h-reduced superconducting sample measured at 5 mT under ZFC scan. In the insets, the magnified data are plotted to clarify the definition of Tc*,*on.*

air promotes a sharp superconducting transition appearing at temperatures between *T*c,on=26 K and *T*c,zero=22 K.
