Layered Ruddlesden−Popper Manganese Oxides:  Synthesis and Cation Ordering

The preparation and crystal structures of the n = 2 Ruddlesden−Popper phases Sr2 - x Ln1+ x Mn2O7 (0 ≤ x ≤ 0.5, Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, and Er) are described. The crystal chemistry and stability of this structure is governed by the size of the lanthanide cation. Partial ordering of the Sr2+ and Ln3+ cations occurs between the two available A cation (A = Ln3+, Sr2+) sites, with the smaller lanthanides preferring the site in the rock-salt layer over that in the perovskite block. This ordering is almost complete for the small lanthanides (Tb−Er), and these ordered compounds can be prepared as single phases. Cation disorder in compounds 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 phase model with strain broadening included. For a particular lanthanide, both the ease of synthesis of single phases and the extent of cation ordering depend on the manganese oxidation state.