Hidden order and low-energy excitations in NpO2

Thermal Energy Transport in Oxide Nuclear Fuel

To efficiently capture the energy of the nuclear bond, advanced nuclear reactor concepts seek solid fuels that must withstand unprecedented temperature and radiation extremes. In these advanced fuels, thermal energy transport under irradiation is directly related to reactor performance as well as reactor safety. The science of thermal transport in nuclear fuel is a grand challenge as a result of both computational and experimental complexities. Here we provide a comprehensive review of thermal transport research on two actinide oxides: one currently in use in commercial nuclear reactors, uranium dioxide (UO₂), and one advanced fuel candidate material, thorium dioxide (ThO₂). In both materials, heat is carried by lattice waves or phonons. Crystalline defects caused by fission events effectively scatter phonons and lead to a degradation in fuel performance over time. Bolstered by new computational and experimental tools, researchers are now developing the foundational work necessary to accurately model and ultimately control thermal transport in advanced nuclear fuels. We begin by reviewing research aimed at understanding thermal transport in perfect single crystals. The absence of defects enables studies that focus on the fundamental aspects of phonon transport. Next, we review research that targets defect generation and evolution. Here the focus is on ion irradiation studies used as surrogates for damage caused by fission products. We end this review with a discussion of modeling and experimental efforts directed at predicting and validating mesoscale thermal transport in the presence of irradiation defects. While efforts in these research areas have been robust, challenging work remains in developing holistic tools to capture and predict thermal energy transport across widely varying environmental conditions.

X-ray synchrotron radiation studies of actinide materials

By reviewing a selection of X-ray diffraction (XRD), resonant X-ray scattering (RXS), X-ray magnetic circular dichroism (XMCD), resonant and non-resonant inelastic scattering (RIXS, NIXS), and dispersive inelastic scattering (IXS) experiments, the potential of synchrotron radiation techniques in studying lattice and electronic structure, hybridization effects, multipolar order and lattice dynamics in actinide materials is demonstrated.

Hidden order and multipolar exchange striction in a correlated f-electron system

Second-order phase transitions in solids occur due to spontaneous symmetry breaking with an order parameter continuously emerging from the disordered high-temperature phase. In some materials, the phase transitions are clearly detected in thermodynamic functions (e.g., specific heat), but the microscopic order parameters remain “hidden” from researchers, in some cases for decades. Here, we show how such hidden-order parameters can be unambiguously identified and the corresponding ordered phase fully described using a first-principles many-body linear response theory. Considering the canonical “hidden-order” system neptunium dioxide, we also identify an unconventional mechanism of spontaneous multipolar exchange striction that induces an anomalous volume contraction of the hidden-order phase in NpO₂.

The nature of order in low-temperature phases of some materials is not directly seen by experiment. Such “hidden orders” (HOs) may inspire decades of research to identify the mechanism underlying those exotic states of matter. In insulators, HO phases originate in degenerate many-electron states on localized f or d shells that may harbor high-rank multipole moments. Coupled by intersite exchange, those moments form a vast space of competing order parameters. Here, we show how the ground-state order and magnetic excitations of a prototypical HO system, neptunium dioxide NpO₂, can be fully described by a low-energy Hamiltonian derived by a many-body ab initio force theorem method. Superexchange interactions between the lowest crystal-field quadruplet of Np⁴⁺ ions induce a primary noncollinear order of time-odd rank 5 (triakontadipolar) moments with a secondary quadrupole order preserving the cubic symmetry of NpO₂. Our study also reveals an unconventional multipolar exchange striction mechanism behind the anomalous volume contraction of the NpO₂ HO phase.

A New Look at the Structural and Magnetic Properties of Potassium Neptunate K2NpO4 Combining XRD, XANES Spectroscopy, and Low-Temperature Heat Capacity

The physicochemical properties of the potassium neptunate K₂NpO₄ have been investigated in this work using X-ray diffraction, X-ray absorption near edge structure (XANES) spectroscopy at the Np-L₃ edge, and low-temperature heat capacity measurements. A Rietveld refinement of the crystal structure is reported for the first time. The Np(VI) valence state has been confirmed by the XANES data, and the absorption edge threshold of the XANES spectrum has been correlated to the Mössbauer isomer shift value reported in the literature. The standard entropy and heat capacity of K₂NpO₄ have been derived at 298.15 K from the low-temperature heat capacity data. The latter suggest the existence of a magnetic ordering transition around 25.9 K, most probably of the ferromagnetic type.

The structure of K₂NpO₄ has been refined using the Rietveld method, and the hexavalence of neptunium has been confirmed using XANES spectroscopy. The measured edge absorption threshold has been correlated to the Mössbauer isomer shift reported in the literature. In addition, low-temperature heat capacity measurements have revealed a magnetic transition around 25.9 K, most probably of the ferromagnetic type.