Formation of Hexagonal Metastable Phases from an Undercooled LuFeO₃ Melt in an Atmosphere with Low Oxygen Partial Pressure

Abstract

Controlling the oxygen partial pressure (PO2) in the ambient atmosphere is an important parameter for material processing because the transition metal ion changes its valence depending on PO2. In the present study, containerless solidification of the LuFeO3 melt, where the undercooling level can be treated as another experimental parameter, was carried out in order to explore the unidentified metastable phases under the controlled PO2 using an aerodynamic levitator. Decreasing PO2 down to 1 × 103 Pa, the unidentified phase was solidified from the undercooled melt. The X‐ray diffractometry results after annealing at 1 × 103 Pa showed the peak profile of the stable perovskite LuFeO3 phase, suggesting that this unidentified phase was thermodynamically metastable. Thermogravimetric analysis showed that the mass of the sample solidified at PO2= 1 × 103 Pa significantly increased, suggesting that the formation of the metastable phases might be related to the presence of Fe2+ ions.

Microstructure and Magnetic Properties of Metastable RFeO3 (R: Rare‐earth element) Formed from Undercooled Melt

“…(c) and (d) was identified as a hexagonal structure with a space group of P 6 3 / mmc . The peak patterns that were identified in the sample solidified at various were also compared with those of the other compounds reported in the R–Fe–O system . These results clearly indicate that would play an important role in phase formation from an undercooled melt under controlled oxygen partial pressure.…”

“…In the case of oxide material, oxygen partial pressure () is another important parameter which affects the phase equilibrium of the constituents. In fact, we already reported that in the R–Fe–O system with heavy rare‐earth elements such as Yb and Lu, metastable h ‐RFeO 3 can be formed from the undercooled melt under controlled oxygen partial pressure () . These results clearly indicate that not only undercooling but also may play an important role on the phase selection between stable and metastable phases.…”

Formation of LuFe2O4 phase from an undercooled LuFeO3 melt in reduced oxygen partial pressure

“…1 The spontaneously solidified samples under various Po 2 were subjected to systematic characterization. To deeply undercool the LuFeO 3 melt under a precisely controlled oxygen partial pressure, an aerodynamic levitator (ADL) combined with a ZrO 2 oxygen sensor was designed.…”

“…1 The Po 2 was decreased to 10 3 Pa, and new metastable phases were found to form in an ambient atmosphere with low Po 2 . 1 The Po 2 was decreased to 10 3 Pa, and new metastable phases were found to form in an ambient atmosphere with low Po 2 .…”

“…1 By contrast, containerless processing provides a large undercooling prior to nucleation, which often leads to the formation of metastable phases. 1 By contrast, containerless processing provides a large undercooling prior to nucleation, which often leads to the formation of metastable phases.…”