Abstract
The physical and chemical properties at low temperatures of hexavalent disodium neptunate α-Na2NpO4 are investigated for the first time in this work using Mössbauer spectroscopy, magnetization, magnetic susceptibility, and heat capacity measurements. The Np(VI) valence state is confirmed by the isomer shift value of the Mössbauer spectra, and the local structural environment around the neptunium cation is related to the fitted quadrupole coupling constant and asymmetry parameters. Moreover, magnetic hyperfine splitting is reported below 12.5 K, which could indicate magnetic ordering at this temperature. This interpretation is further substantiated by the existence of a λ-peak at 12.5 K in the heat capacity curve, which is shifted to lower temperatures with the application of a magnetic field, suggesting antiferromagnetic ordering. However, the absence of any anomaly in the magnetization and magnetic susceptibility data shows that the observed transition is more intricate. In addition, the heat capacity measurements suggest the existence of a Schottky-type anomaly above 15 K associated with a low-lying electronic doublet found about 60 cm−1 above the ground state doublet. The possibility of a quadrupolar transition associated with a ground state pseudoquartet is thereafter discussed. The present results finally bring new insights into the complex magnetic and electronic peculiarities of α-Na2NpO4.
Highlights
The ternary oxides of uranium, neptunium, and plutonium formed with sodium metal have been studied extensively since the 1960s because of their technological relevance for Sodium-cooled Fast Reactors (SFRs)
The local structural environment around the Np cation has been related to the fitted Mössbauer parameters: η = 0 due to the axial symmetry in the NpO6 octahedra, while the large value of the quadrupole coupling constant, e2qQ = − 170.6(3) mm·s−1, is related to the (NpO2)2+ neptunyl type of ion
The Mössbauer spectra collected below 12.5 K have shown magnetic hyperfine splitting, which could be related to a magnetic ordering transition at this temperature
Summary
The ternary oxides of uranium, neptunium, and plutonium formed with sodium metal have been studied extensively since the 1960s because of their technological relevance for Sodium-cooled Fast Reactors (SFRs). In case of a clad breach of the stainless steel cladding, extremely rare during normal operation, the sodium metallic coolant can come into contact with the (U,Np,Pu)O2 fast reactor fuel, leading to the formation of sodium actinide oxide phases at the fuel-cladding interface [1, 2]. The latter phases show a lower density and lower thermal conductivity than the fuel [3,4,5], and can induce local swelling and temperature increase in the fuel pin. In the case of the [Rn]5f 1 electronic configuration, corresponding to pentavalent uranium, hexavalent neptunium, or heptavalent plutonium, the contribution from electronic repulsion is removed, which simplifies greatly the interpretation
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