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 AMX ₃ perovskites containing an active Jahn-Teller (J-T) ion on the octahedral site and quaternary A ₂ MM ' X ₆ perovskites containing a J-T ion on one-half of the octahedral sites have been examined. In AMX ₃ perovskites, the tendency is for the occupied 3 d _{3 x ² - r ²} and 3 d _{3 z ² - r ²} orbitals to order in the ac plane, as exemplified by the crystal structures of LaMnO ₃ and KCuF ₃ . This arrangement maintains a favorable coordination environment for the anion sites. In AMX ₃ perovskites, octahedral tilting tends to enhance the magnitude of the J-T distortions. In A ₂ MM ' X ₆ perovskites, the tendency is for the occupied 3 d _{3 z ² - r ²} 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 A ₂ MM ' X ₆ perovskites is compatible with octahedral rotations about the c axis (Glazer tilt system a ⁰ a ⁰ c ^- ) but appears to be incompatible with GdFeO ₃ -type octahedral tilting (tilt system a ^- b ⁺ a ^- ).