**4. Conclusions**

A citrate pyrolysis technique is a unique route to prepare reactive precursor mixtures through an ignition process of concentrated aqueous solution including metallic ions of stoichiometric composition. This procedure enables to synthesize highly homogeneous and fine powders for functional materials. The double-chain based superconductor Pr2Ba4Cu7O15�*<sup>δ</sup>* and double perovskite photocatalytic semiconductor Ba2Tb(Bi,Sb)O6 were synthesized by using the citrate pyrolysis technique.

The TEM image of the 48-h-reduced Pr247 revealed that CuO single-chain and double-chain blocks are alternately stacked along the *c*-axis such as {-D-S-D-S-} sequence. For the present Pr247 sample with a reduction treatment for 72 h, a sharp superconducting transition appeared at an onset temperature *T*c,on=26 K accompanied by a zero-resistance state at *T*c,zero=22 K. The superconducting volume fraction estimated from the magnetization measurement reached an excellent value of � 58%. Both reduction treatment in a vacuum and subsequent quenching procedure are needed to realize higher superconductivity due to further oxygen defects. For the 48-h-reduced sample, the *RH* data exhibited negative values in the limited temperature range between 30 and 100 K, accompanied by electron doping due to the reduced heat treatment in a vacuum. The re-entrant superconducting behavior observed at 2.0 GPa is an open question to be resolved in future through detailed structure analysis under applied pressures.

The polycrystalline samples for Ba2Tb(Bi1�*<sup>x</sup>*,Sb*x*)O6 (*x* ¼ 0 and 0.5) were formed in the monoclinic and cubic crystal structures. The magnetic data suggest that about half of Tb ions are oxidized to the tetravalent state over the wide range of Sb substitution, which is a common trend in the case of Ba2Pr(Bi,Sb)O6. For Ba2Tb

(Bi1*<sup>x</sup>*,Sb*x*)O6, we estimated *Eg* = 0.92 eV at *x*=0 and 2.45 eV at *x*=0.5, assuming indirect and direct photon transitions, respectively.

We conducted the gaseous 2-propanol (IPA) and methylene blue (MB) degradation experiments under a visible light irradiation, to evaluate photocatalytic activities of the powder samples. The band gap opening due to the heavy Sb substitution suppresses the formation of electron–hole pairs, causing a decrease of the photocatalytic reaction processes in the IPA decomposition. The MB degradation rates under visible light irradiation show rapid increases due to Sb-substitution. For the Sb50% substituted Ba2Tb(Bi0*:*5,Sb0*:*5)O6 sample, the highest performance of MB degradation was observed. In the Ba2(Pr,Tb)(Bi,Sb)O6 system, the effect of Sbsubstitution on the photocatalytic degradation of MB is in direct contrast to that on the IPA decomposition under visible light irradiation. We conclude that the citrate pyrolysis samples of Ba2Tb(Bi,Sb)O6 exhibit excellent performances in comparison with the data of the solid-state samples with the identical composition. The enhanced photocatalytic properties in the citrate samples are attributed to their morphology, where fine particles are homogeneously distributed with a sub micron order.
