Phase selection in Ln(Fe, Mn)O ₃ : Insights from undercooling experiments, tolerance factor, and crystal field splitting

Abstract The formation of metastable hexagonal LnFeO 3 ( h ‐LnFeO 3 ) during solidification from an undercooled melt was systematically investigated for a series of lanthanide elements (Ln = Lu–La) using containerless aerodynamic levitation. The nucleation and liquidus temperatures were determined from high‐speed optical imaging and two‐color pyrometry, enabling quantitative evaluation of phase selection between the hexagonal ( h ‐LnFeO 3 ) and orthorhombic ( o ‐LnFeO 3 ) structures. The results reveal that h ‐LnFeO 3 forms for 0.83 < t h < 0.866, where t h denotes the tolerance factor, whereas only o ‐LnFeO 3 crystallizes for larger Ln 3+ ionic radii. The critical threshold of t h ≈ 0.866 coincides with the limit at which the trigonal bipyramidal FeO 5 unit can no longer accommodate Ln 3+ without structural distortion. Comparison with the LnMnO 3 system indicates that crystal field splitting of the Fe 3+ 3d orbitals destabilizes the FeO 5 complex relative to MnO 5 , restricting the formation range of the hexagonal phase. These findings establish a unified framework linking geometric tolerance, electronic structure, and metastable phase selection in rare‐earth ferrite melts.