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

The formation of metastable phases from an undercooled LuFeO₃ melt was investigated under reduced Po₂ since the iron ion has the tendency to change its valence state from Fe³⁺ to Fe²⁺ in an ambient atmosphere with low Po₂. The nucleation and the post-recalescence temperatures of the phases were decreased with decreasing process Po₂. Phase equilibrium was established in the Lu–Fe–O system at 1473 K by varying the oxygen partial pressure from 10⁵ to 10⁻¹ Pa. A possible ternary metastable phase diagram depending on the oxygen composition in the bulk sample was also constructed. The formation of the LuFe₂O₄ phase where the Fe³⁺ and Fe²⁺ ratio is 1:1 clearly indicated that the formation of metastable phases is related to the presence of Fe²⁺ ions. Thermogravimetric analysis revealed that the increase in sample mass with decreasing process Po₂, down to 10⁻¹ Pa, is relatively dependent on the amount of Fe²⁺ ions.