Low-temperature heat capacity and magnetism in (U1−yLny)O2 and (U1−yAmy)O2 (y = 0.01 − 0.05) solid solutions: Effects of substitution and self-irradiation

The low-temperature heat capacity of lanthanide-doped (La, Nd) and americium-doped UO2 samples was measured by calorimetry. The results showed a strong effect on the antiferromagnetic Cp anomaly as a result of crystal lattice substitution, defects resulting from self-irradiation, and electron interaction. It was found that the substitution of trivalent ions (La3+, Nd3+, Am3+) leads to a split of the λ peak, for which we hypothesize that the clustering of the trivalent ions with the charge compensator pentavalent U5+ can play a role. The overall behavior is confirmed by magnetization measurements using SQUID. The observed differences between La3+, Nd3+, and Am3+ are attributed to the f–f exchange interaction. 241Am decay causes in addition self-irradiation effects (point defects), and reduces Néel temperature, peak intensity, and magnetic entropy as a function of decay (accumulated α dose). The observed effects are similar to 238Pu-doped UO2, the slight differences being attributed to a higher degree of disorder in the Am material.