**1. Introduction**

A citrate pyrolysis technique is similar to nitrate combustion synthesis methods [1] and a unique route to prepare reactive precursor mixtures through an ignition process of concentrated aqueous solution including stoichiometric amounts of metal ions. For 80 K-class high- *T*<sup>c</sup> cuprate superconductors such as YBa2Cu4O8, highquality single-phase polycrystalline samples have been successfully prepared at ambient oxygen pressure [2, 3]. We believe that the present technique is a powerful tool to fabricate highly homogeneous and fine crystalline grains for functional materials, in comparison to conventional solid-state reaction methods.

High- *T*<sup>c</sup> copper-oxide superconductors discovered as far have close relationship with two-dimensional CuO2 planes. In quasi one-dimensional (1D) Cu2O3 ladder system without CuO2 planes, it has been reported in [4] that the application of external pressure under 3 GPa causes bulk superconductivity at *T*<sup>c</sup> = 12 K. For Pr-based copper oxides including insulating CuO2 planes, we have reported that Pr2Ba4Cu7O15�*<sup>δ</sup>* (Pr247) with metallic CuO double-chain structure achieves a superconducting-state with a higher *T*<sup>c</sup> (15 K) after a reduction treatment [5].

In the subsequent microscopic research on Pr247 [6], nuclear quadrupole resonance observations have resolved that the newly discovered superconductivity occurs along the CuO double-chain block. The Pr-based cuprates, PrBa2Cu3O7�*<sup>δ</sup>* (Pr123) and PrBa2Cu4O8 (Pr124), have identical crystal structures as Y-based high-*T*<sup>c</sup> superconductors, YBa2Cu3O7�*<sup>δ</sup>* (Y123) and YBa2Cu4O8 (Y124), respectively. Both the Pr123 and Pr124 compounds share insulating CuO2 planes and exhibit no superconductivity [7, 8]. The CuO single chains in Pr123 follow semiconducting property but Pr124 has a metallic conduction along the CuO double chain block [9]. For Pr124, it is hard to control the carrier density of doped double chains, because it is thermally stable against high temperature heat treatment.

For Pr247 intermediate compound existing between Pr123 and Pr124 phases, there are alternate stacks of CuO single-chain and double-chain blocks along the *c*-axis such as {-S-D-S-D-} sequence [10, 11] (see **Figure 1**). Here, CuO single-chain and double-chain blocks along the *b*-axis are abbreviated as S and D, respectively. Under thermal control of the oxygen content along the semiconducting CuO single chains in Pr247, it is possible to investigate the physical characters of the metallic CuO double chains. In oxygen defect polycrystalline sample, we succeeded in the appearance of superconductivity at an onset temperature *T*c,on of �15 K [5].

The B-site substituted perovskite oxides A2B<sup>0</sup> B0 0 O6 have been widely studied because of their attractive physical properties and potential applications [13] (see **Figure 2a**). Some of the double perovskite compounds such as A2FeMoO6 exhibiting negative tunneling magnetoresistance effect at room temperature are of great interest with a wide range of applications in magnetic devices [14]. Furthermore, multiferroic double perovskite oxides have an effective coupling between spontaneous ferroelectric polarization and ferromagnetic ordering, which is considered to be promising materials from view points of physics and its applications [15]. Recently, a series of double perovskite oxides Ba2Ln<sup>0</sup> Bi<sup>0</sup> 0 O6 (Ln:lanthanides) has been examined on the view point of photocatalytic semiconductors for hydrogen generation by water splitting and are taken as alternative materials for TiO2 oxide [16, 17]. In particular, Ba2PrBiO6 is found to possess highly photocatalytic performance, which is probably close to the valence mixing state [18, 19]. Furthermore, a previous study on the magnetic properties of the Ba2PrBiO6 compound revealed that the average valence of Pr ions is an intermediate state between trivalent and tetravalent [20].

As for the key factors to fabricate visible light driven photocatalysts, it is desirable to control the energy band gap between the valence and conduction bands of their semiconductors to utilize a wide range of visible light [21, 22]. Accordingly, we think that the Sb substitution at the B site of the parent material Ba2PrBiO6 is an effective approach to adjusting the band gaps. For high photocatalytic activity, we avoid charge recombination between electron and hole and try to promote the photogenerated charge separation in the photocatalystic materials [21, 23]. In our research [24], it has been demonstrated that the valence mixing states between Pr<sup>3</sup><sup>þ</sup> and Pr<sup>4</sup><sup>þ</sup> are closely related to the phenomena of charge separation.

For our further understanding of the enhanced effect of the mixed valence states at B-site ions of the double perovskite compound on the photocatalytic

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

### **Figure 1.**

*(a) Typical crystal structure of Pr*2*Ba*4*Cu*7*O*<sup>15</sup>*<sup>δ</sup> (Pr247) with CuO single-chain and double-chain blocks stacked along the c-axis. Here, S and D denote CuO single-chain and double-chain blocks along the b-axis. For comparison, the crystal structure of Pr124 is shown on the right hand side. (b) TEM image of superconducting Pr247. CuO single-chain and double-chain blocks are alternately stacked along the c-axis such as {-S-D-S-D-} sequence [12]*.

performance, we demonstrate the 2-propanol decomposition and methylene blue (MB) degradation under the irradiation of visible right for Ba2Tb(Bi,Sb)O6 samples prepared by the citrate pyrolysis technique. MB aqueous solution is adopted as the model pollutant [25].
