Nucleation-Controlled Phase Selection in Rapid Solidification from Undercooled Melt of DyMnO3

The equilibrium crystal structure of LnMnO3 (Ln: lanthanide) has been reported to be orthorhombic when La3+ to Dy3+ are used as Ln3+, and hexagonal when Ho3+ to Lu3+ are used. Whereas Kumar et al. reported a two-phase structure of orthorhombic and hexagonal phases is formed in DyMnO3 when it was solidified from the undercooled melt under containerless state. The reason for the formation of the two-phase structure was not thoroughly addressed and discussed. We investigated the formation mechanism for the two-phase structure from the undercooled melt of DyMnO3 in detail. As a result, the surface morphology, microstructure, and crystal structure of the samples, in which the nucleation was forced at a predetermined temperature with a Mo needle, indicated that the hexagonal and orthorhombic phases are dominant at high and low temperatures, respectively. When the sample was quenched from below 1670 K in a water bath, as-solidified sample consisted of h-DyMnO3 and o-DyMnO3. Whereas a single phase of h-DyMnO3 was obtained in the sample quenched from above 1670 K. This phenomenon can be quantified in terms of nucleation-rate determined phase selection. That is, the activation energy for forming a critical nucleus calculated based on the model of the crystal-melt interface proposed by Turnbull and Speapen suggests that the o-DyMnO3 phase can be heterogeneously nucleated on the interface of the initially formed h-DyMnO3 phase.