Sr4+nMn43+Mnn4+O10+3n: a new homologous series of oxygen-vacancy-ordered perovskites built from Mn3+O5 pyramids and Mn4+O6 octahedra

A new homologous series of oxygen-vacancy-ordered perovskites with the formula Sr4+nMn43+Mnn4+O10+3n is proposed based on the structural trends found for the recently described Sr4Mn4O10, Sr5Mn5O13 and Sr7Mn7O19 compounds. These compounds correspond to n=0 (Sr4Mn43+O10), n=1 (Sr5Mn43+Mn4+O13) and n=3 (Sr7Mn43+Mn34+O19) members of the series. A linear set of four Mn3+O5 pyramids placed on the ab plane and pointing along the +x, -y, +y, -x directions defines the n=0 building block for the series. The nth members can be constructed from blocks containing four pyramids and n Mn4+O6 octahedra with 2/m symmetry. Compounds in the related systems CaMnOx and LaCuOx, containing Mn3+ and Cu2+ pyramids and Mn4+ and Cu3+ octahedra have also been found to be members of the series. The size and charge of the A-site cation and the apical distortion of the pyramidally coordinated B-site cation are shown to be important factors in the stabilization of certain members of the series. A qualitative explanation for the absence of some of the possible members of the series is presented based on these factors. (orig.)