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

Abstract

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.

 

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 155–161. The abstract and captions of Figs. 1–10 are modified.

Fig. 5 Typical LSM images for the surface of DyMnO₃ samples nucleated from different ΔT by triggering the system with a Mo needle.