Phase relations and lattice parameter trends in Nd3+−substituted UO2 system under reducing and oxidizing conditions

Abstract

While Am-substituted UO2 solid solutions have been considered as advanced fuels for heterogeneous transmutation of americium, U-substituted Am2O3 with cubic structure has been envisaged as a promising candidate for radioisotope heat unit and radioisotope thermoelectric generator. Considering Nd2O3 as a surrogate for Am2O3, a series of Nd3+-substituted UO2 samples with the general formula U1-xNdxO2±δ (0 ≤ x ≤ 1, δ ≥ 0) have been investigated for phase relations under reducing and oxidizing conditions. In this direction, samples have been synthesized by a solid-state route and thoroughly characterized by X-ray diffraction and Raman spectroscopy. XRD studies revealed the formation of fluorite-type solid solution at x ≤ 0.7 and a biphasic phase field consisting of C-type (U,Nd)O2−δ phase and A-type Nd2O3 at x ≥ 0.8 in reducing conditions. In the fluorite-type regime, due to the aliovalent Nd3+-substitution in UO2, the charge neutrality was achieved primarily through oxygen vacancy creation as evident from lattice parameter trends as well as Raman spectroscopic investigation that exhibit Raman band due to oxygen vacancies. The reduced samples have a high tendency to get oxidized even during storage under ambient conditions leading to a decrease in lattice parameters, which can be attributed to the formation of the higher oxidation state of uranium (U5+/U6+) following a charge compensation mechanism. Synthesis in the oxidizing conditions yielded a biphasic phase field at x ≤ 0.2, a single-phase fluorite-type phase field at 0.3 ≤ x ≤ 0.7, a biphasic mixture of fluorite-type (U,Nd)O2−δ and trigonal Nd6UO12 phases at x = 0.8 and another biphasic mixture of Nd6UO12 and Nd2O3 phases at x = 0.9. The transition of reduced single-phase fluorite-type solid solution at x ≤ 0.2 to a biphasic mixture upon oxidation has been probed by a high-temperature XRD.