In-situ observation of phase transformations in layered perovskite BaEu2Mn2O7

The layered perovskite BaLn2Mn2O7 (Ln = rare earth) has a Sr3Ti2O7-type structure with double-block oxygen octahedra belonging to the Ruddlesden–Popper-type homologous series AO(ABO3)2. The BaEu2Mn2O7 shows multiple phase transformations upon heating by high-temperature TEM, from tetragonal phase I (P42/mnm) to monoclinic phase III via phase II (I4/mmm). Weak reflections from phase III started to appear on the selected area diffraction pattern along the c-direction above 277 °C and became stronger gradually with increasing temperature. Finally, all of the reflections were from only phase III above 750 °C. This shows a first-order phase transformation, and the coexistence of low- and high-temperature phases was also observed clearly in a quenched specimen. The differences in those structures are the periodicity along the c-axis and the high-temperature phase's 1.5% longer c length than that of the low-temperature phase.