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

Abstract

In situ transmission electron microscopy observation revealed that BaGd2Mn2O7 undergoes several phase transitions upon heating. The orthorhombic Pnnm phase (I) at room temperature was transformed into a tetragonal P42/mnm (II) at around 420 K via an intermediate phase. On further heating, the other phase (III) nucleated in the phase II matrix at 623 K and grew until covering the whole inspected region by 723 K. Phase III has a 1.5% lattice mismatch along c compared with phase II, and presumably has a monoclinic symmetry. The transition between II and III was of the first order.

In-situ observation of phase transformations in layered perovskite BaEu2Mn2O7

“…12 Recently, however, we have found first-order phase transformations in the BaGd 2 Mn 2 O 7 and BaPr 2 Mn 2 O 7 by using a high-temperature transmission electron microscope (TEM). 13,14 These findings are contradictory to the conventional understanding of the successive phase transformation. We believe that the first-order phase transformation may uniquely occur in layered perovskite with double-block oxygen octahedra.…”

“…Above 277 • C, weak reflections started to appear, as indicated by the arrow in (b), and the intensities grew stronger at 714 • C. The split reflections indicate that the c length of phase III is 1.5% longer than that of the low-temperature phase, as with BaGd 2 Mn 2 O 7 . 13 With increasing temperature, the intensities of the reflections at 11 0 became weaker. Finally, the low-temperature phase disappeared above 750 • C. The repeated in-situ experiments revealed that the transformation speed is temperature-dependent.…”

Microstructural evolution by in-situ TEM observations of oxides

“…With increasing temperature, a new phase III nucleated in the phase II matrix at 623 K. The SAED pattern at 623 K can be interpreted as a mixture of phases II and III, and the coexistence of phases II and III at 623 K was also confirmed by the dark-field images. 43) The coherency between phases II and III was good in the [001] zone. Phase III is sheathshaped with dimensions of several tens of nanometers in width and much longer in length.…”

“…Phase III grew until covering the entire inspected region at around 1023 K. The III transition at around 420 K may accompany tilts of the MnO 6 octahedra because the symmetries suggest a change in the octahedral tilt system from (¯00) (0¯0) for P4 2 /mnm to (¯1¯20)(¯2¯10) for Pnnm. 43) On the other hand, the nucleation-growth process observed in the IIIII transition suggests that this is a first-order transition. Actually, the repeated in-situ experiments revealed that the transformation speed is temperature-dependent.…”

“…The orthorhombic phase of BaGd 2 Mn 2 O 7 was stable from room temperature up to approximately 450 K. The space group of phase I was assumed to be Pnnm by an electron diffraction method, and the atomic parameters of the structure were refined by an X-ray four-circle diffractometer, as described in our earlier paper. 43) Figure 5 shows the SAED patterns along the [001] zone axis at various temperatures. At around 333 K, the orthorhombic phase I started to transform into an intermediate phase with a rather complex diffraction pattern as shown in Fig.…”