High-temperature orbital, charge, and structural phase transitions in the cation-ordered manganites TbBaMn2O6 and YBaMn2O6

The high-temperature structures and phase transitions of the cation-ordered manganite perovskites TbBaMn2O6 and YBaMn2O6 have been studied by powder neutron diffraction and differential scanning calorimetry. TbBaMn2O6 undergoes an unprecedented transition from a charge and antiferro-orbitally ordered phase to a charge and ferro-orbitally ordered structure at 94.5°C, and the charge and orbital ordering melts at 200°C. An orthorhombic Cmmm phase with rotational order of MnO6 octahedra is observed above the transition, and at 514°C this transforms to the aristotype tetragonal P4 mmm arrangement. YBaMn2O6 is charge and ferro-orbitally ordered up to 225°C. Above this charge ordering transition, a C2 m phase with two rotational orders is found. This transforms to the Cmmm structure at 437°C. Discontinuities in volume and volume expansion, and a limited phase coexistence region are observed at the charge ordering transitions, whereas the other transitions are continuous.