High-temperature structural phase transition in Ca2Fe2O5Ca2Fe2O5 studied by in-situ X-ray diffraction and transmission electron microscopy

Ca2Fe2O5Ca2Fe2O5 shows a first-order phase transition from space group Pnma   to a modulated structure described in superspace group Imma(00γ)s00Imma(00γ)s00. The transition starts at 947 K during heating and the reverse transition in cooling conditions starts at 923 K, as determined from differential thermal analysis. In-situ high-temperature single-crystal X-ray diffraction experiments revealed a phase coexistence over a range of ca. 25 K. Data collected in this range show a good fit to a combined model of the two simultaneously occurring phases, which was refined using the software Jana2006. In-situ high-temperature transmission electron microscopy (selected area diffraction, dark field and high resolution images) was utilized to prove the existence of Pnma   and Imma(00γ)s00Imma(00γ)s00 domains within the temperature range of the phase coexistence. Remaining planar defects were observed within the Pnma   phase at temperatures below the phase transition. From the analysis of high-resolution electron micrographs, these defects were proven to be antiphase boundaries with a displacement vector of R=12[111].

High-temperature HRTEM (zone axis [010]) of a phase boundary between domains of the Pnma (lower left) and Imma(00g)s00 (upper right) structure. Micrograph recorded at 970 K. Corresponding FFT patterns are shown on the right.Figure optionsDownload as PowerPoint slide