Novel rare-earth-containing manganites Ba 4REMn 3O 12 ( RE=Ce, Pr) with 12 R structure

Abstract

Novel rare-earth-containing manganites, Ba 4 REMn 3O 12 ( RE=Ce, Pr), with 12 R structure, have been prepared by solid-state reaction. Although the phases are formed at 950°C, to obtain single-phase samples high temperatures (up to 1300°C) and long synthesis periods are needed. Their structure is built up from chains of BO 6 face-sharing and corner-sharing octahedra running along the c-axis giving a quasi-one-dimensional oxide. Every polyhedral column consists of (Mn 3O 12) units of three face-sharing octahedra, both ends connected by the three terminal oxygen atoms to three different ( REO 6) octahedra. Mixed occupation of the three octahedral positions in the structure, (Mn(1), Mn(2) and Re), was not found. Vacancies are not observed, neither in the cationic sublattice nor in the oxygen one. Thus, as in all the other 1-D manganites, the oxidation state of manganese ions seems to be four, as the rare-earth valence is. High-resolution electron microscopy suggests the eventual existence of ordered polytypes for different compositions, which could be stabilized by adjusting the thermodynamic conditions.