Structure for perovskites with layered ordering of A-site cations

The structure of La_1/3NbO₃ is that of a perovskite with La³⁺ cations ordered into alternate layers of perovskite A-sites. This is the description of a tetragonal structure and yet the room-temperature structure shows an orthorhombic distortion. The structure of La_2/3TiO₃ shows similar features. It has been recognized only very recently that the ortho­rhombic distortion in both these compounds is due to octahedral tilting. It seems clear from the literature that Ce_2/3TiO₃, Pr_2/3TiO₃, Nd_2/3TiO₃ and Ce_1/3NbO₃ adopt the same structure. Structures of other perovskites, such as Ln_2/3TiO₃, Ln_1/3NbO₃ and Ln_1/3TaO₃ (Ln = lanthanoid), when orthorhombically distorted, may be similar.