DIRECT OBSERVATION ON THE FORMATION OF ANTIPHASE DOMAIN BOUNDARIES IN PIGEONITE

The formation of the antiphase domain structure in pigeonite was observed in situ using a heating stage in a transmission electron microscope. The phase transition between high and low pigeonite occurred athermal-martensitically over a range of about 100-degrees-C. The observation showed that the APBs parallel to the c axis (L type) form at an earlier stage and the APBs transverse to the c axis (H type) form at the final stage of the C2/c to P2(1)/c pigeonite transition interval during cooling. The observation suggests that the H APBs are formed as the traces of C2/c pigeonite rather than as the original APBs. The formation of the H APBs is controlled by the movement of the interfaces between P2(1)/c) and C2/c pigeonite in the transition interval. C2/c pigeonite grew from the H APBs, where there is expected to be Ca enrichment. The Ca enrichment seems to facilitate the transition to C2/c pigeonite. Specifically, the Ca concentration locally lowers the transition temperature. Based on the difference between the two types of APBs with regard to the formation stage and the Ca enrichment, the fact that the H APBs are commonly observed in slowly cooled pigeonite, whereas the L APBs are commonly found in rapidly cooled pigeonite, can be explained qualitatively.