DyMnO3の過冷却融液からの急速凝固における核形成律速相 選択.

The crystal stmctuie of LnMnO₃ (Ln: lanthanide) has been reported to be orthorhombic from La to Dy aie used as Ln, and hexagonal from Ho to Lu. Whereas Kumai et al. reported that orthorhombic and hexagonal phases aie co-exist in samples of DyMnO₃ solidified from the undercooled melt under containerless state. We investigated whether this two-phases structuie was the result that the hexagonal phase formed as a metastable phase is frozen to room temperature or the result that the solid state phase transfonnation of the hexagonal # orthorhombic occurred at the cooling stage. The surface morphology of the samples, the nucleation of which were triggered by contacting with the Mo wire at given temperatures, indicates that hexagonal and orthorhombic phases are stable at high and low temperatures, respectively. Furthermore, in the experiment where the melt was dropped into water, it was found that if the temperatuie of the melt was below 1666 K, the XRD pattern of the as-solidified sample showed two phases, A-DyMnO₃ and o-DyMnC₃, whereas above 1666 K, it showed a single phase of A--DyMnC₃. This phenomenon can be quantified in terms of nucleation rate-determined phase selection. That is, the activation energy for nucleation calculated based on the model of the crystal-melt interface proposed by Turnbull and Spaepen suggests that the o-DyMnO₃ phase can be heterogeneously nucleated on the interface of the initially formed A-DyMnC₃ phase. [ABSTRACT FROM AUTHOR]