| For phase selection at rapid solidification into undercooled melts, the entropy-undercooling regime criterion in which a phase with a lower entropy of fusion preferentially nucleates has been proposed. To verify this criterion, real-time X-ray diffraction during rapid solidification of a deeply undercooled melt was performed on REFeO3 (RE: rare-earth element) using synchrotron radiation. The result shows that for YFeO3 the crystal structure of the phase formed at the first recalescence is hexagonal, and that at the second recalescence is orthorhombic. Thermogravimetry analysis shows that the hexagonal phase contains high deficit amounts of oxygen. Since the oxygen vacancies on the oxygen sub-lattice increase configurational entropy as well as vibrational entropy due to weighed phonon density of states in the acoustic region, the entropy of the hexagonal phase is concluded to be higher than that of the orthorhombic phase. This result is consistent with the prediction that the hexagonal phase with a lower entropy of fusion preferentially nucleates as a metastable phase at high undercooling. |
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