Phase transformation of mixed lanthanide oxides in an aqueous solution

Abstract

Binary and ternary mixed lanthanide oxides including (La,Nd)₂O₃, (La,Eu)₂O₃, (Eu,Tm)₂O₃, (La,Nd,Eu)₂O₃, and (La,Eu,Tm)₂O₃ were synthesized at 1000 °C for 4 h, followed by characterization using X-ray diffraction (XRD) techniques before and after contact with an aqueous solution at neutral pH and 60 °C for 4 weeks. The XRD patterns of the mixed oxides confirmed a tendency to form oxide or hydroxide solid solutions depending on their mixing ratios. A complete solid solution of (La,Nd)₂O₃ underwent transformation to (La,Nd)(OH)₃, while the separated La₂O₃ + Nd₂O₃ were converted to La(OH)₃ + Nd(OH)₃. A similar trend was observed for (La,Eu)₂O₃, while (Eu,Tm)₂O₃ resulted in separate Eu(OH)₃ and Tm₂O₃ after contact with an aqueous solution because of the resistance of Tm₂O₃ to the hydration reaction. The formation of ternary oxide solid solutions and their transformation in an aqueous solution could be satisfactorily explained by the trends observed for binary mixed lanthanides.