Phase selection of peritectic phase in undercooled Nd-based superconducting oxides Original Research Article

The congruent growth of a peritectic phase, NdBa2Cu3O7−δ (Nd123), from undercooled melt below a peritectic temperature (TP) is discussed from the viewpoint of phase selection theory. This is based on competitive growth in the peritectic system under a free growth condition. Although the results of calculations based on the phase selection theory indicate the achievement of congruent growth, they also suggest that the Nd123 phase grows preferentially over the Nd4Ba2Cu2O10 phase (Nd422) even above TP. A seeding experiment to clarify the temperature dependence of the phase appearance, however, revealed that the volume fraction of Nd123 drastically increased when seeding was carried out at a temperature below TP. This suggests that the steady growth of the Nd123 phase is unstable above TP. The measured growth velocity and volume fraction of the Nd123 phase agreed well with the theoretical calculation below the critical temperature, where the interface temperature of the Nd123 phase during steady growth remains below TP. Achievement of congruent growth of the Nd123 phase results in the steady and preferential growth of the Nd123 phase over the Nd422 phase with lower growth velocity accompanied by the rejection of solute atoms.