“…The formula can also be written as AO(ABO 3 ) n , where the A site is usually occupied by a rare earth element, or an alkaline earth metal element, or by mixed rare earth and alkaline earth elements, while the B site is normally a transition metal element, i.e., Ni, Mn, Cu, Fe, etc., for most of the RP or RP-related structural materials that had been reported. These typical RP-structured materials with B site as transition metals have been extensively studied as potential electrode materials for SOFCs due to their excellent oxygen reduction reaction (ORR) activity, long-term stability, and chemical stability besides their high mixed oxide ion and electronic conductivities . However, for RP-structured materials with the B site being an indium element, cases have been reported only for NdBaInO 4 − and Ln 2 BaIn 2 O 7 (Ln = La and Nd), which have stoichiometric oxygen content, although there are also several special cases, e.g., (Ba, Sr) 3 In 2 O 6 , , which are isotypic to the La 2 SrCu 2 O 6 material and can be viewed as derivates of the RP family, forming double pyramidal but not octahedral indium layers on account of their intrinsic severe oxygen deficiency, intergrowth with single rock salt layers.…”