Crystallographic shear structures as a route to anion-deficient perovskites

Because of the technological importance of perovskite-based oxides, numerous pathways towards new structures have been developed, including ordering at the A and/or B cation sites of the perovskite (ABO3) structure, vacancy ordering at the anion site, the intergrowth of perovskite and related structures, and the formation of hexagonal perovskite polytypes. However, it has been generally accepted that crystallographic shear (CS) structures cannot be realized in perovskites. CS planes commonly occur in anion-deficient oxides derived from the ReO3 or rutile (TiO2) structures, which are based on metal-centered octahedra. Arrangement of the CS planes at different periodicities generates a homologous series. Some of the well-known examples of CS structures are MnO3n 1, MnO3n 2 (M=Mo, W), TinO2n 1, and the block structures of Nb2O5 and its Ti-containing derivatives. [2–6] The shear operation is such that along the CS plane the octahedra share edges or faces, instead of corners or edges as in the basic structure. The ReO3 structure is based on the same threedimensional framework of corner-sharing BO6 octahedra found in the perovskite structure, but the 12-coordinate A cation sites of the perovskite structure are empty in ReO3. Usually, the presence of A cations in perovskites favors point defects in the anion sites (in a random or ordered arrangement), leading to a lower coordination number for the B cations, and prevents the elimination of anion point defects by a shear operation. Herein, we report the occurrence of CS structures in perovskite-based “Pb2Fe2O5” for the first time. We also describe the general crystallographic mechanism of CS-plane formation in a perovskite framework with the A sites filled by cations bearing a lone 6s electron pair, which involves the elimination of O vacancies by the shear operation and the relaxation of the structure at the CS plane. We discuss the role of the lone electron pair in directing structure formation, as well as the relationship between the crystallographic orientation of the CS plane and the chemical composition. The discovery of this mechanism opens up new possibilities for the design of novel perovskite-based compounds containing A cations with a lone 6s electron pair. “Pb2Fe2O5” (or “PbFeO2.5”) is expected to be an aniondeficient perovskite of the brownmillerite (Ca2AlFeO5) type; however, its structure has not yet been determined. Because of the complexity of the structure and the prevalence of domain fragmentation, the only road to solving the structure is by transmission electron microscopy (TEM). The brighter reflections in the electron diffraction (ED) patterns of “Pb2Fe2O5” (Figure 1a) reveal a perovskite sublattice with a parameter ap 3.9 < (p denotes the perovskite subcell). Linear arrays of satellite reflections associated with each basic reciprocal lattice node are typical for periodic CS planes. The displacement vector for the CS planes was deduced from the ED patterns as R= 1/2 [110]p+ 1/ 3 [001]p by measuring the fractional shifts hR for the satellite reflections (Figure 1b). 11][*] Closer inspection of the [010]* ED pattern reveals the incommensurate character of the modulation. The monoclinic unit-cell parameters (denoted by