Ferro-orbital order in the charge- and cation-ordered manganite YBaMn₂O₆

Powder neutron diffraction reveals a subtle triclinic distortion in the 300 K structure of the Y³⁺∕Ba²⁺ cation-ordered and Mn³⁺∕Mn⁴⁺ charge-ordered perovskite YBaMn₂O₆(Y_0.5Ba_0.5MnO₃). The refined structure shows a ferro-orbital ordering (FOO) in which d_z²-type orbitals associated with the Jahn-Teller Mn³⁺ states are oriented in a parallel fashion. This ground state has not previously been found in AMnO₃ manganites, although the alternative antiferro-orbital ordering (AFOO) is commonly observed, for example, in the analog TbBaMn₂O₆. Lattice distortions that determine the balance between FOO and AFOO states result from the size mismatch between Y or Tb and Ba, and a high-temperature AFOO to FOO transition is evidenced in TbBaMn₂O₆.