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

Abstract

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 −).