Magnetic studies on (U,La)O2+x solid solutions

Magnetic susceptibilities of La_yU_1−yO_2+x solid solutions with fluorite structure were measured from 4.2 K to room temperature. An antiferromagnetic transition was observed for the solid solutions with lanthanum concentration0 ≤ y ≤ 0.3 in both stoichiometric (x = 0) and hypostoichiometric oxygen range (x < 0). From comparison of the magnetic properties of solid solutions with different oxygen amounts (x < 0, x = 0, x > 0), both oxygen vacancies and interstitial oxygens were found to weaken the magnetic exchange interactions between uranium ions. The Ne´el temperature decreased with decreasing uranium concentration. The variation of the Ne´el temperature of (U, La)O₂ solid solutions with uranium concentration was different from that of (U, Th)O₂ solid solutions. The magnetic moment decreased with decreasing uranium concentration. Its rate was larger than that of (U, Th)O₂ solid solutions and was comparable with that of (U, Y)O₂ solid solutions. The oxidation state of uranium in the solid solutions was examined from the magnetic susceptibility data. The uranium ions were found to be in either the tetravalent or the pentavalent state. The effect of magnetic dilution was larger with La₂O₃ than with Y₂O₃ and/or Sc₂O₃.