Low-temperature heat capacity and magnetism in (U_1-yLn_y)O₂ and (U_1-yAm_y)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 C-p 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 lambda 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 Am(3+ )are attributed to the f-f exchange interaction. Am-241 decay causes in addition self-irradiation effects (point defects), and reduces Neel temperature, peak intensity, and magnetic entropy as a function of decay (accumulated alpha dose). The observed effects are similar to Pu-238-doped UO2, the slight differences being attributed to a higher degree of disorder in the Am material. (C) 2022 Author(s).