Jahn-Teller distortions, cation ordering and octahedral tilting in perovskites

In transition metal oxides, preferential occupation of specific d orbitals on the transition metal ion can lead to the development of a long-range ordered pattern of occupied orbitals. This phenomenon, referred to as orbital ordering, is usually observed indirectly from the cooperative Jahn-Teller distortions (CJTDs) that result as a consequence of the orbital ordering. This paper examines the interplay between orbital ordering, octahedral tilting and cation ordering in perovskites. Both ternary AMX₃ perovskites containing an active Jahn-Teller (J-T) ion on the octahedral site and quaternary A₂MM'X₆ perovskites containing a J-T ion on one-half of the octahedral sites have been examined. In AMX₃ perov­skites, the tendency is for the occupied 3d_3x²-r² and 3d_3z²-r² orbitals to order in the ac plane, as exemplified by the crystal structures of LaMnO₃ and KCuF₃. This arrangement maintains a favorable coordination environment for the anion sites. In AMX₃ perovskites, octahedral tilting tends to enhance the magnitude of the J-T distortions. In A₂MM'X₆ perovskites, the tendency is for the occupied 3d_3z²-r² orbitals to align parallel to the c axis. This pattern maintains a favorable coordination environment about the symmetric M'-cation site. The orbital ordering found in rock-salt ordered A₂MM'X₆ perovskites is compatible with octahedral rotations about the c axis (Glazer tilt system a⁰a⁰c^-) but appears to be incompatible with GdFeO₃-type octahedral tilting (tilt system a^-b⁺a^-).