Phase relation investigation of U-La-O system under oxidizing conditions and observation of novel meta-stable and mixed-valent uranium phase- Ln3U11O36 (Ln=La, Nd, Sm, Gd)

Abstract

Total of twelve samples in the U-La-O system with the compositions U1-yLayO2+x (y=0.025, 0.05, 0.1, up to 0.3) were synthesized by gel combustion synthesis method followed by appropriate heat treatment in air atmosphere. Comprehensive experimental analysis using various techniques like X-ray diffraction, thermogravimetry and oxygen to uranium ratio (O/U) are used to establish the phase relation in U1-yLayO2+x system at 1173 K and 1523 K heated in air. A novel meta-stable phase with stoichiometry La3U11O36 having mixed-valent uranium is reported for the first time in U-La-O system. The thermal property of this new phase is reported along with the updated phase relations at and above 1173 K temperature for the compositions U1-yLayO2+x (y < 0.3). Further, formation and stability of this new Ln3U11O36 phase was also investigated with smaller cations like Nd3+, Sm3+, Gd3+ and the smallest Y3+ cation.