Anomalous octahedral distortion and multiple phase transitions in the metal-ordered manganite YBaMn₂O₆

By means of powder X-ray diffraction, powder neutron diffraction and transmission electron microscopy (TEM), we determined the crystal structures of a metal-ordered manganite YBaMn₂O₆ which undergoes successive phase transitions. A high-temperature metallic phase (T _c1=520K<T) crystallizes in a triclinic P1 with the following unit cell: Z=2, a=5.4948(15)Å, b=5.4920(14)Å, c=7.7174(4)Å, α=89.804(20)°, β=90.173(20)°, γ=91.160(4)°. The MnO₆ octahedral tilting is approximately written as a ⁰b^-c^-, leading to a significant structural anisotropy within the ab plane. The structure for T_c2<T<T _c1 is a monoclinic P2 (Z=2, a=5.5181(4)Å, b=5.5142(4)Å, c=7.6443(3)Å, β=90.267(4)°) with an a^-b ^-c^- tilting. The structural features suggest a d _x2-y2 orbital ordering (OO). Below T_c2=480K, crystallographically inequivalent two octahedra show distinct volume difference, due to the Mn³⁺/Mn⁴⁺ charge ordering. The TEM study furthermore revealed a unique d_3x2-r2/d_3y2-r2 OO with a modified CE structure. It was found that the obtained crystal structures are strongly correlated to the unusual physical properties. In particular, the extremely high temperature at which charge degree of freedom freezes, T _c2, should be caused by the absence of the structural disorder and by heavily distorted MnO₆ octahedra.