Shift-Twin Option in Eight-Layer Hexagonal Perovskite Niobates BaMNbO.

Two new eight-layer hexagonal perovskites with the composition BaMNbO (M = Fe and Cu) are synthesized by solid-state reaction at 1350-1400 °C. Their crystal structures have been investigated using X-ray and electron diffractions as well as high-resolution transmission electron microscopy. Although both compounds have similar M size, BaFeNbO and BaCuNbO adopt shifted and twinned structures, respectively. Through comparison with the reported shifted BaMNbO (M = Mn, Co, and Zn) and twinned BaNiNbO as well as inexistent BaMg(Nb/Ta)O, we elucidate that the twin-shift competition of BaMNbO family could be related with multiple chemical factors including tolerance factors, B-cationic size difference, entropy variation with B-cation and vacancy disorder, Jahn-Teller distortion, and FSO B-B d orbit interactions.