LAYERED RUDDLESDEN-POPPER MANGANESE OXIDES - SYNTHESIS AND CATION ORDERING

The preparation and crystal structures of the n = 2 Ruddlesden-Popper phases Sr(2-x)Ln(1+x)Mn(2)O(7) (0 less than or equal to x less than or equal to 0.5, Ln La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, and Er) are d escribed. The crystal. chemistry and stability of this structure is go verned by the size of the lanthanide cation. Partial ordering of the S r2+ and Ln(3+) cations occurs between the two available A cation (A = Ln(3+), Sr2+) sites, with the smaller lanthanides preferring the site in the rock-salt layer over that in the perovskite block. This orderin g is almost complete for the small lanthanides (Tb-Er), and these orde red compounds can be prepared as single phases. Cation disorder in com pounds of the larger lanthanides is accompanied by a subtle separation into two n = 2 Ruddlesden-Popper phases, which is apparent only upon detailed inspection of Rietveld refinements of the X-ray profiles. In these cases, the two-phase model is found to be superior to a single p hase model with strain broadening included. For a particular lanthanid e, both the ease of synthesis of single phases and the extent of catio n ordering depend on the manganese oxidation state.