Phase behavior of layered manganites BaLn ₂ Mn ₂ O ₇ (Ln = rare earth)

Abstract

We have found that many phases appear in the layered manganites BaLn 2 Mn 2 O 7 , depending on the synthesis and annealing conditions for each rare earth metal. The BaLn 2 Mn 2 O 7 phase, with a stoichiometric composition synthesized under low oxygen pressure, transforms into another structure upon heating. The transition temperature varies according to the rare earth ion. In the present paper, we summarized the relationship between the transition temperature and rare earth ions. We successfully observed in-situ multiplephase transition of the manganite by thermal TEM. We found a first-order phase transition in Gdmanganite at a relatively high temperature, presumably caused by a change in the behavior of 3d electrons in the Mn 3+ ion. We discuss here the structural transition in BaLn 2 Mn 2 O 7 .

In situ Transmission Electron Microscopy Observation of Multiple Phase Transition in BaGd2Mn2O7

“…Kamegashira has suggested that the tetragonal I 4/ mmm structure is stable at high temperatures 15 . The transition temperatures tend to increase with decreasing ionic radii of the Ln cations 14 . In the Gd–manganite, the tetragonal phase is reported to transform into another orthorhombic form at 773 K on heating 16 .…”

In-situ observation of phase transformations in layered perovskite BaEu2Mn2O7

“…Then, a new phase III started to nucleate in the low-temperature phase matrix at 277 • C, and the temperature agreed with the DSC and electrical conductivity results. 10,11 The complex pattern (e) can be interpreted as a mixture of the low-temperature phase (d) and the high-temperature phase (f). At around 500 • C, the intensity of 1 1 0 reflection became very weak, which indicated that the low-temperature phase nearly disappeared in (f).…”

“…We believe that the first-order phase transformation may uniquely occur in layered perovskite with double-block oxygen octahedra. 10,14 The multiple transformations are related to the distortion of the structure. In the repeated trial, the transformation occurred slowly via the I4/mmm phase as a meta-stable phase.…”

“…The phase transformations of the oxides have been investigated by high-temperature X-ray diffraction (XRD), electrical conductivity, and thermal analyses. 3,10,11 The transformation temperatures tend to increase with decreasing ionic radii of the Ln cations. 10 These results showed that the transformation occurring is the successive phase transition series common in perovskite-related families, whose symmetries are expected to change from lower to higher with increasing temperature.…”

Microstructural evolution by in-situ TEM observations of oxides

“…The changes in the cell dimensions indicate that the transition is irreversible by comparison of measurements on cooling and heating in XRD. 45) In general, phase transition is accompanied by tilting of the MnO 6 octahedra in the perovskite, and the symmetries change in the octahedral tilt system. In this case, however, the first-order phase transition occurred even in BaPr 2 Mn 2 O 7 (I4/mmm), which had no tilting of the octahedra.…”