Xiaofeng Guo

Nitride-based materials present exceptional thermal and structural properties for advanced material applications, however accurate determination of their thermodynamic properties is critical to their deployment. Herein, the standard enthalpies of formation, Δ H _f ^° , for all Ln N compounds ( Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu; excluding Pm) are determined via high temperature oxide melt calorimetry. These measurements are supported by high temperature oxidation studies of La, Ce, Sm, Eu, Er, Yb bearing Ln N compounds using X-ray diffraction and thermogravimetric analysis, and are cross-examined against recent high temperature measurements on other Ln N compounds ¹ . It was determined that Δ H _f ^° systematically correlates with the ionic radius of Ln ³⁺ cations, with smaller Ln bearing nitrides having more negative values. A discontinuity in Δ H _f ^° was further observed between larger and smaller Ln bearing Ln N compounds. Using the determined Δ H _f ^° values with literature references, Gibbs energies of formation (Δ G _f ^° ), oxidation (Δ G _ox ^° ) and hydrolysis (Δ G _hyd ^° ) are calculated and compared against uranium nitride (UN) in the context of UN based nuclear fuel and related spent nuclear fuel (SNF) management. The calculations indicate that the occurrence of Ln N phases within UN based SNF should lead to stabilisation of the structure and inhibit its oxidation.

Brannerite (UTi2O6) is a mineral analogue that is both flexible to a wide range of actinide/lanthanide substitutions and is also highly refractory; thus, it has generated interest as a waste form for used nuclear fuel (UNF). Given the wide range of elements in UNF, systematically tracking how compositional changes map onto changes in stability will guide the rational design of waste forms like brannerite. Changes in valence state, not just cation substitutions, are particularly important for actinide-containing materials, and the brannerite structure is flexible to a variety of charge-balancing substitutions. To isolate the impact of valence contributions to (de)stabilization of brannerite materials, a series of Ce(III/IV)Ti2O6–x ceramics were synthesized for study. Their compositions and structures were rigorously characterized by X-ray absorption spectroscopy, X-ray diffraction, and thermogravimetric analysis. High temperature oxide melt drop solution calorimetry was used to investigate thermodynamic properties: 20% coupled Ce(III)-VO resulted in increased stability driven by enthalpic contributions. These findings highlight that valence-driven changes in stability are factors that must be considered when designing waste forms or other ceramic materials with longevity in mind.

Transition metal nitrides with rock-salt structure support significant electronic contributions to thermal transport, distinguishing their response to irradiation-induced defects from that of widely studied fluorite-structured oxides. Here, we report an experimental and modeling investigation of thermal conductivity in proton-irradiated uranium mononitride (UN) across irradiation temperatures of 25°C and 800°C and doses of 0.1, 0.3, and 1 dpa. Temperature-dependent thermal conductivity from 77 to 295 K is measured using spatial-domain thermoreflectance, with defect populations characterized by transmission electron microscopy. A semiclassical transport model is developed to account for electron and phonon contributions, electron-phonon coupling, magnetic scattering associated with 5f electrons, and defect scattering. A counterintuitive thermal conductivity increase of 25–30% is observed after 0.1 dpa irradiation at 25°C, with the measured conductivity at 295 K surpassing all previously reported pristine UN values prior to density correction. The model attributes this anomaly to suppression of crystal-field and low-temperature magnetic contributions to electrical resistivity, suggesting that irradiation-induced disorder modifies 5f electron correlations. At higher doses of 0.3 and 1 dpa, thermal conductivity saturates, with point defects identified as the dominant scatterers. Contrary to the conventional assumption that phonon transport is more severely degraded by atomic-scale defects, reductions in electronic and phonon contributions are comparable, both reaching approximately 30%. These results provide a physics-based, experimentally validated framework for understanding defect-transport coupling in irradiated rock-salt nitrides and provide broader insight into radiation-tolerant materials with mixed electron-phonon thermal transport.

A multiparametric study was developed to optimize the hydrothermal synthesis of chernobylite solid solutions (Zr_1-xU_xSiO₄), a technogenic mineral first discovered in the melted core of the Chernobyl nuclear power plant following the 1986 nuclear accident. The goal was to obtain phase-pure samples for thermodynamic investigation. Optimal synthesis conditions were determined starting from uranium(IV) and zirconium(IV) chloride precursors, using a hydrothermal treatment for seven days at 250°C with a reaction medium maintained within a pH range of 1.4 to 1.8, which minimized the formation of secondary phases Under these conditions, solid solutions of chernobylite were synthesized up to x = 0.60. However, the systematic presence of residual oxide phases in the samples required the development of a purification process involving alternating leaching steps in basic and nitric media. This process removed amorphous silica and selectively dissolved residual oxide phases, respectively. This protocol yielded purified Zr_1-xU_xSiO₄ samples over a wide range of compositions (x ≤ 0.6). The resulting powders consisted of spherical agglomerates of approximately 300 µm in diameter, with specific surface areas ranging from 19 to 22 m².g^-1. Finally, the thermal stability of the chernobylite solid solutions was evaluated using heat treatments between 1000°C and 1300°C. All samples, except for those most enriched in uranium, proved to be thermally stable even at the highest temperatures.

Nitride-based materials present exceptional thermal properties for advanced material applications, however accurate determination of their thermodynamic properties is critical for deployment. Herein, the standard enthalpies of formation, Δ H _f ^° , for all Ln N compounds ( Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu; excluding Pm) are determined via high temperature oxide melt calorimetry. These measurements are supported by high temperature oxidation studies of La, Ce, Sm, Eu, Er, Yb bearing Ln N compounds using a combination of X-ray diffraction and thermogravimetric analysis, and compared against similar measurements on other Ln N compounds. ¹ The Δ H _f ^° were found to systematically correlate with the ionic radius of Ln ^+ 3 cations, with smaller Ln bearing nitrides having more negative values. A discontinuity in Δ H _f ^° was further observed between larger and smaller Ln bearing Ln N compounds, consistent with the lanthanide tetrad effect. Using determined Δ H _f ^° with literature references, Gibbs energies of formation (Δ G _f ^° ), oxidation (Δ G _ox ^° ) and hydrolysis (Δ G _hyd ^° ) are calculated and compared against uranium nitride (UN) in the context of UN based nuclear fuel and spent nuclear fuel (SNF) management. The calculations indicate that the occurrence of Ln N phases within UN based SNF should lead to stabilisation of the structure and inhibit its oxidation.

Gadolinium oxide (Gd₂O₃) is widely used as a burnable neutron absorber in uranium dioxide (UO2) fuel, yet the atomic-scale mechanisms governing its impact on thermophysical performance remains elusive. Here, we establish a direct structure-property relationship in Gd-doped UO₂ (mass fractions of 6 % – 15 % Gd), by integrating X-ray diffraction (XRD), scanning electron microscopy (SEM/EDS), synchrotron-based Extended X-ray Absorption Fine Structure (EXAFS), resonant ultrasound spectroscopy (RUS), and thermophysical property measurements. XRD results confirm the formation of a single-phase fluorite solid solution with a linear lattice contraction following Vegard’s law, while EXAFS reveals local distortions, oxygen vacancies, and nanoscale bixbyite Gd₂O₃-like environments undetectable by bulk methods. Microstructural analysis shows homogeneous Gd distribution without secondary phases or microcracking with higher dopant levels reducing grain size and increasing porosity. Thermophysical measurements demonstrate that Gd incorporation significantly decreases thermal diffusivity and thermal conductivity, particularly at lower temperatures, due to enhanced phonon–defect scattering. Resonant ultrasound spectroscopy further reveals a progressive reduction in elastic stiffness with increasing Gd content, consistent with defect-induced lattice softening. Thermal resistivity analysis indicates that impurity scattering dominates in Gd-doped UO₂, while phonon–phonon Umklapp contributions are suppressed with increasing dopant concentration. These results demonstrate the Gd-doped UO2 is globally single-phase yet locally heterogeneous, and that this short-range structural complexity governs thermal conductivity behavior. The results provide critical experimental insight for atomistically informed fuel performance models and underscore the importance of resolving local atomic environments in the evaluation of doped nuclear fuels.

Selective Ln( iii )/An( vi ) separation is achieved via acidity-controlled crystallization of lanthanides with a Preyssler-type polyoxometalate, enabling record-high separation factors in a fully inorganic system.