Antiphase modulated structure of Fe2O3(ZnO)15 studied by high-resolution electron microscopy

As one of our systematic studies of the modulated structures of a series of new homologous compounds RMO3(ZnO)(m) (R = Sc, In, Y, and La; M = In, Fe Ga, and AI; nl = integer), this paper reports the modulated structure of Fe2O3(ZnO)(15) (R=Fe; M= Fe) studied by high-resolution electron microscopy. It is found that the main and satellite spots in the diffraction patterns can be indexed as ha*+kb*+lc*+mq by considering a mono-clinic unit cell of a = 0.57 nm, b = 0.33 nm, c = 3.5 nm, and beta = 92 degrees and a modulation wave vector of q = b*/34.8+c*/2. The possible space groups for the basic structure are Ct, Cm, and C2/m, and the possible four-dimensional superspace groups for the modulated structure are C-1(C2), C-(1 over bar)(Cm), and C-1 (1 over bar)(C2/m). The crystal structure of Fe2O3(ZnO)(15) consists of Fe-O layers interleaved with 16 Fe/Zn-O layers. In the high-resolution image, the modulated structure appeared as zigzag-shaped contrasts among the Fe/Zn-O layers, where Fe is in high concentration. Compared with those of InMO3(ZnO)(m) (R = In; M= In, Fe and Ga) reported in our earlier paper, the special features of the present modulated structure are the half-periodicity relative shift of the neighboring zigzag-shaped contrasts and the appearance of modulation within the Fe-O layers. (C) 1999 Academic Press.