Actinide Nitrides Research Articles

Multiconfigurational actinide nitrides assisted by double Möbius aromaticity.

Understanding the bonding nature between actinides and main-group elements remains a key challenge in actinide chemistry due to the involvement of f orbitals. Herein, we propose a unique "aromaticity-assisted multiconfiguration" (AAM) model to elucidate the bonding nature in actinide nitrides (An2N2, An = Ac, Th, Pa, U). Each planar four-membered An2N2 with equivalent An-N bonds possesses four delocalized π electrons and four delocalized σ electrons, forming a new family of double Möbius aromaticity that contributes to the molecular stability. The unprecedented aromaticity further supports actinide nitrides to exhibit multiconfigurational characters, where the unpaired electrons (2, 4 or 6 in naked Th2N2, Pa2N2 or U2N2, respectively) either are spin-free and localized on metal centres or form metal-ligand bonds. High-level multiconfigurational computations confirm an open-shell singlet ground state for actinide nitrides, with small energy gaps to high spin states. This is consistent with the antiferromagnetic nature observed experimentally in uranium nitrides. The novel AAM bonding model can be authenticated in both experimentally identified compounds containing a U2N2 motif and other theoretically modelled An2N2 clusters and is thus expected to be a general chemical bonding pattern between actinides and main-group elements.

Incorporation and migration of xenon in uranium-plutonium mixed nitride; A density functional theory study

Actinide nitride materials are promising candidates for advanced nuclear fuels. In this work, we investigate the bulk properties of the mixed nitrides U0.75Pu0.25N and U0.5Pu0.5N, and study the incorporation and migration behaviour of the fission gas Xe. The disordered U0.75Pu0.25N and U0.5Pu0.5N structures are constructed using the special quasi-random structure method. Their lattice parameters are closer to the experimentally determined values than the corresponding ordered structures. The density of states show that Pu f states are located at lower energy than U f, consistent with the trend of increasing f orbital stability across the actinide series. The actinide vacancy formation energy (Ef) in disordered and ordered U0.5Pu0.5N is highly dependent on the chemical environment around the vacancy: it increases as the number of U atoms in the first nearest-neighbour shell (NU(1NN)) increases, but decreases as the number of U atoms in the second nearest-neighbour shell (NU(2NN)) increases. The Xe incorporation energy (Ei) is found to be independent of vacancy species, depending only on the chemical environment of the vacancy. As does Ef, Ei increases with increasing NU(1NN), while decreases with increasing NU(2NN), because the smaller the NU(1NN) and the larger the NU(2NN), the larger the vacancy steric space. The Ei of Xe and Kr are calculated to be within the ranges 4.47–6.01 eV and 3.30–4.64 eV, respectively. The Xe migration energy barrier in ordered U0.5Pu0.5N allows us to set the energy range for Xe diffusion in disordered U0.5Pu0.5N as 0.50–1.75 eV. A lower range of 0.30–1.25 eV is found for Kr diffusion.

Synthesis of a heterobimetallic actinide nitride and an analysis of its bonding.

Reaction of [K(DME)][Th{N(R)(SiMe2CH2)}2(NR2)] (R = SiMe3) with 1 equiv. of [U(NR2)3(NH2)] (1) in THF, in the presence of 18-crown-6, results in formation of a bridged uranium–thorium nitride complex, [K(18-crown-6)(THF)2][(NR2)3UIV(μ-N)ThIV(NR2)3] (2), which can be isolated in 48% yield after work-up. Complex 2 is the first isolable molecular mixed-actinide nitride complex. Also formed in the reaction is the methylene-bridged mixed-actinide nitride, [K(18-crown-6)][K(18-crown-6)(Et2O)2][(NR2)2U(μ-N)(μ–κ2-C,N–CH2SiMe2NR)Th(NR2)2]2 (3), which can be isolated in 34% yield after work-up. Complex 3 is likely generated by deprotonation of a methyl group in 2 by [NR2]−, yielding the new μ-CH2 moiety and HNR2. Reaction of 2 with 0.5 equiv. of I2 results in formation of a UV/ThIV bridged nitride, [(NR2)3UV(μ-N)ThIV(NR2)3] (4), which can be isolated in 42% yield after work-up. The electronic structure of 4 was analyzed with EPR spectroscopy, SQUID magnetometry, and NIR-visible spectroscopy. This analysis demonstrated that the energies of 5f orbitals of 4 are largely determined by the strong ligand field exerted by the nitride ligand.

Theoretical study on stability, mechanical and thermodynamic properties of (Pu, Zr)N

Actinide nitrides possessing the advantages of high burn-up, superior thermophysical properties and easy reprocessing in nitric acid, are currently considered as promising fuels for advanced nuclear reactors. In this work, a systemic study on the stability, mechanical and thermodynamic properties of (Pu, Zr)N was conducted employing the first-principles density functional theory approaches. We found that the ternary compound (Pu, Zr)N tends to exist in the form of single cubic NaCl-type solid solution, confirmed by incorporation and solution energies of Zr in PuN crystals. The calculated elastic constants of the (Pu, Zr)N crystals satisfy the stability criteria of tetragonal structures, proving their mechanical stability. It is revealed that the elastic constants and mechanical properties such as mechanic moduli and Poisson's ratio monotonically change with the concentration of Zr in (Pu, Zr)N crystals. Additionally, the specific heat also changes monotonically with the concentration of Zr derived from phonon-dispersion spectrums. The obtained results can shed light on the understanding of plutonium nitride materials in the nuclear energy cycles.

Actinide Nitride Clusterfullerene: Synthesis, Isolation and Spectroscopic Characterization of U2N@C80

All the nitride clusterfullerenes discovered so far adopted a trimetallic nitride template. Here, we report that an unprecedented nitride cluster endohedral fullerene with only two metal atoms in the cluster, U2N@C80, has been successfully synthesized by a modified arc-discharge method and isolated by a multistage HPLC process. Mass spectroscopy, UV-vis-NIR absorption spectrometry and Raman spectroscopy were employed to investigate its structure and some intriguing properties have been revealed. The UV-vis-NIR absorption spectrum of U2N@C80 in CS2 is featured with two weak absorption peaks at 446nm and 680 nm. Compared with previously reported M3N@I h(7)-C80 (M= Tm, Gd, Y, Er, Dy) fullerenes, their overall absorption spectra, characteristic absorption peaks and optical band gaps are almost identical, providing evidence for a probable six-electron transfer from the interior cluster to the exterior fullerene cage. In addition, the features of the low energy Raman spectrum of U2N@C80 also show similarities to those of fullerenes with I h-C80 cage, which agree well with the UV-vis-NIR absorption study. However, in the range of 100-200 cm‒1, the peaks of the stretching metal-cage mode is observed at 119,135 cm‒1, which shows great difference from those of lanthanide endohedral fullerenes, M3N@I h(7)-C80 (M=Tm, Gd, Y, Er, Dy), indicating a quite different metal-to-cage interaction between the inner cluster and the I h(7)-C80 cage from that of the lanthanide nitride endohedral fullerenes.

Theoretical investigation on electronic and mechanical properties of ternary actinide (U, Np, Pu) nitrides

Actinide mononitrides as a promising advanced nuclear fuel have recently earned much attention. We herein studied the electronic and mechanical properties of the ternary actinide mixed mononitrides A0.5B0.5 N (A, B = U, Np, and Pu) using the density functional theory +U method. It is found that in the studied ternary mixed mononitrides, the 5f electronic states of all actinide atoms maintain the local electronic character and do not overlap with each other. Compared with their corresponding binary mononitrides, the U-N bond becomes more ionic, where the Np-N and Pu-N bonds become more covalent in ternary actinide mixed mononitrides. The mechanical properties (such as bulk and shear moduli, Young's modulus, and Poisson's ratio) of three ternary actinide (U-Pu) mononitrides are found to be similar to that of their corresponding binary actinide mononitrides and thus are expected not to misbehave with actinide mononitrides in respect of mechanics. In addition, all the three ternary actinide mononitrides have no imaginary frequencies in their vibration curves and correspondingly satisfy the stability criteria for elastic constants of tetragonal structures.

Recent progress of actinide nitride fuel properties through first-principles simulation

Recent progress of actinide nitride fuel properties through first-principles simulation

First principles study of structural, electronic, mechanical and magnetic properties of actinide nitrides AnN (An = U, Np and Pu)

The electronic, structural, mechanical and magnetic properties of Actinide nitrides AnN (An = U, Np and Pu) are investigated in three cubic phases, namely, NaCl (B1), CsCl (B2) and zinc blende (B3). At normal pressure, UN is stable in antiferromagnetic state while the other two nitrides are stable in the ferromagnetic state with NaCl (B1) structure. A pressure induced structural phase transition from B1 to B3 phase is predicted in these nitrides. The electronic structure reveals that these nitrides are metallic in nature. The magnetic phase transition from antiferromagnetic to non-magnetic state is observed in UN at a pressure of 127 GPa while ferromagnetic to non-magnetic state is observed in NpN and PuN at the pressures of 67 GPa and 102.3 GPa respectively. The computed structural parameters, bulk modulus density of states and charge density distributions are compared with experimental and other theoretical calculations.

Thorium Mono- and Bis(imido) Complexes Made by Reprotonation of cyclo-Metalated Amides

Molecules containing actinide-nitrogen multiple bonds are of current interest as simple models for new actinide nitride nuclear fuels, and for their potential for the catalytic activation of inert hydrocarbon C-H bonds. Complexes with up to three uranium-nitrogen double bonds are now being widely studied, yet those with one thorium-nitrogen double bond are rare, and those with two are unknown. A new, simple mono(imido) thorium complex and the first bis(imido) thorium complex, K[Th(═NAr)N″3] and K2[Th(═NAr)2N″2], are readily made from insertion reactions (Ar = aryl, N″ = N(SiMe3)2) into the Th-C bond of the cyclometalated thorium amides [ThN″2(N(SiMe3)(SiMe2CH2))] and K[ThN″(N(SiMe3)(SiMe2CH2))2]. X-ray and computational structural analyses show a "transition-metal-like" cis-bis(imido) geometry and polarized Th═N bonds with twice the Wiberg bond order of the formally single Th-N bond in the same molecule.

Theoretical investigation on multiple bonds in terminal actinide nitride complexes.

A series of actinide (An) species of L-An-N compounds [An = Pa-Pu, L = [N(CH2CH2NSiPr(i)3)3](3-), Pr(i) = CH(CH3)2] have been investigated using scalar relativistic density functional theory (DFT) without considering spin-orbit coupling effects. The ground state geometric and electronic structures and natural bond orbital (NBO) analysis of actinide compounds were studied systematically in neutral and anionic forms. It was found that with increasing actinide atomic number, the bond length of terminal multiple An-N1 bond decreases, in accordance with the actinide contraction. The Mayer bond order of An-N1 decreases gradually from An = Pa to Pu, which indicates a decrease in bond strength. The terminal multiple bond for L-An-N compounds contains one σ and two π molecular orbitals, and the contributions of the 6d orbital to covalency are larger in magnitude than the 5f orbital based on NBO analysis and topological analysis of electron density. This work may help in understanding of the bonding nature of An-N multiple bonds and elucidating the trends and electronic structure changes across the actinide series. It can also shed light on the construction of novel An-N multiple bonds.

An investigation on the structural transition and stability of some actinide nitrides under pressure

A theoretical study of the structural and related properties of some actinide nitrides under high pressure has been carried out using three-body interaction (TBI) potential model. The present TBI model constitutes the long range Coulombic, three body interaction forces with inclusion of covalency effect, van der Waals’ interaction and short range overlap repulsive forces operative up-to next nearest neighbor ions. The covalency effect is introduced to explain the partial covalent nature of the present compounds. Using this static simulation technique we have investigated the phase transition pressures, volume collapses and elastic behaviour of ThN, NpN and CmN at high-pressures. A detailed discussion of stability criteria using various theories has also been done. The results found are well agreed with available experimental and other data.

Thermal expansion and self-irradiation damage in curium nitride lattice

A powder sample of curium nitride (CmN) containing 0.35%-Pu and 3.59%-Am as actinide impurities was prepared by carbothermic nitridation of the oxide. The lattice expansion induced by self-irradiation damage at room temperature was measured as a function of time. The saturated Δa/ao value was 0.43%, which is greater than those for transuranium dioxides available in literature. The undamaged lattice parameter at 297±1K was determined to be 0.50261±0.00006nm. Temperature dependence of the lattice parameter was measured by a high temperature X-ray diffractometer in the temperature range up to 1375K. The linear thermal expansion from 293 to 1273K was 0.964% and the corresponding thermal expansion coefficient was 9.84×10−6K−1. Comparing with the other actinide nitrides, it was found that CmN lies between the higher expansion nitrides (PuN and AmN) and the lower expansion nitrides (UN and NpN).

GGA + U studies of the early actinide mononitrides and dinitrides

We present a detailed comparative study of the electronic and mechanical properties of the early actinide mononitrides and dinitrides within the framework of the Perdew–Burke–Ernzerhof generalized gradient approximation (GGA [PBE]) and GGA+U implementations of density functional theory with the inclusion of spin–orbit coupling. The dependence of selected observables of these materials on the effective U-parameter is investigated in detail. The properties include the lattice constant, bulk modulus, charge density distribution, hybridization of the atomic orbitals, energy of formation and the lattice dynamics. The inclusion of the Hubbard U parameter results in a proper description of the 5f electrons, and is subsequently used in the determination of the structural and electronic properties of these compounds. The mononitrides and dinitrides of the early actinides are metallic except for UN2, which is a semiconductor. These actinide nitrides are non-magnetic with the exception of UN, NpN, PuN, NpN2 and PuN2 that are magnetic systems with orbital-dependent magnetic moments oriented in the z-axis. We observed that ThN2 is elastically unstable to isotropic pressure. We discovered that UN2 is thermodynamically unstable, but may be stabilized by N vacancy formation.

A Screened Hybrid DFT Study of Actinide Oxides, Nitrides, and Carbides

A systematic study of the structural, electronic, and magnetic properties of actinide oxides, nitrides, and carbides (AnX1–2 with X = C, N, O) is performed using the Heyd–Scuseria–Ernzerhof (HSE) hybrid functional. Our computed results show that the screened hybrid HSE functional gives a good description of the electronic and structural properties of actinide dioxides (strongly correlated insulators) when compared with available experimental data. However, there are still some problems reproducing the electronic properties of actinide nitrides and carbides (strongly correlated metals). In addition, in order to compare with the results by HSE, the structures, electronic, and magnetic properties of these actinide compounds are also investigated in the PBE and PBE+U approximation. Interestingly, the density of states of UN obtained with PBE compares well with the experimental photoemission spectra, in contrast to the hybrid approximation. This is presumably related to the need of additional screening in the Hartree–Fock exchange term of the metallic phases.

First-principles study of the magnetic properties of NpN, PuN and AmN

The electronic structure, magnetic, dynamical and thermodynamic properties of the actinide nitrides, NpN, PuN, and AmN are explored using ab initio pseudopotential and density functional perturbation theories. The results show that PuN exhibits half-metallic properties, while AmN and NpN are found to be metallic; the calculated total magnetic moments are 3.11μB for NpN, 4.85μB for PuN and 6.00μB for AmN. The electronic structure calculation indicates, for the minority-spin, that the states close to Fermi-level are mainly due to the N-p states with a small admixture of Np, Pu and Am d and f states, while for the majority spin, the states close to Fermi-level exhibit mainly N-p character with a strong admixture of huge Np, Pu and Am f states, and a small admixture of Np, Pu and Am d orbitals. Moreover, the phonon band structure of NpN, PuN and AmN shows that the longitudinal and transverse acoustic phonon modes have flatness along a large part of the Brillouin zone, which gives–consequently–two sharp peaks for the acoustic modes. Finally we used quasiharmonic approximation to determine the temperature dependence of mean-squared displacements, entropy, and heat capacity. We found that for high temperatures, N atoms show higher vibrational amplitudes as compared to Am, Pu and Np atoms. In addition, we found that the entropy and heat capacity originate principally from actinides (Am, Pu and Np)-acoustic modes and from N-optical modes.

Pressure-induced structural phase transition and electronic properties of actinide nitrides: Ab initio calculations

We report ab initio calculations of pressure-induced structural phase transition and electronic properties of the NaCl-type actinide nitrides namely AnN (An=Np and Cm). The total energy as a function of volume is obtained using the self-consistent tight-binding linear muffin tin orbital (TB-LMTO) method by performing spin and non-spin-polarized calculations. Magnetically the AnN compounds are stable in ferromagnetic (FM) state at ambient condition. From the present study, we predict a structural phase transition from ferromagnetic (FM) NaCl-type (B 1 phase) structure to ferromagnetic CsCl-type (B 2 phase) structure for these in the pressure range of 80.0–50.0 GPa. The pressure-induced transition is found to be first order. The band structure and density of states (DOS) are reported in both FM-B 1 phase and FM-B 2 phase. The ground-state properties such as magnetic moment, lattice parameter and bulk modulus are calculated and compared with the available theoretical and experimental results.

Electrolysis of Burnup-Simulated Uranium Nitride Fuels in LiCl-KCl Eutectic Melts

The electrochemical behavior of burnup-simulated uranium nitride fuels containing representative solid fission product elements, UN+Mo (Mo = 2.84 wt%), UN+Pd (Pd = 4.6 wt%) and (U, Nd)N (NdN = 8.0 wt%), was investigated in the molten LiCl-KCl eutectic salt with 0.54 wt% UCl3 in order to clarify the effects of fission products on the dissolution of actinide nitrides and the behavior of FPs in the electrorefining of spent nitride fuel. The rest potentials of burnup-simulated UN pellets were similar to that of pure UN. The electrochemical dissolution of UN began at about _0:75V vs Ag/AgCl reference electrode in all samples as well as that of pure UN. After the electrolyses at the constant anodic potential of −0:65––0:60V vs Ag/AgCl, most of UN was dissolved into LiCl-KCl as UCl3 at the anode, and U was recovered in the liquid Cd cathode in all samples. Furthermore, Nd was dissolved at the anode and accumulated into LiCl-KCl as NdCl3, while Mo and Pd were not dissolved but remained at the anode.

Thermochemical and thermophysical properties of minor actinide compounds

Burning or transmutation of minor actinides (MA: Np, Am, Cm) that are classified as the high-level radioactive waste in the current nuclear fuel cycle is an option for the advanced nuclear fuel cycle. Although the thermochemical and thermophysical properties of minor actinide compounds are essential for the design of MA-bearing fuels and analysis of their behavior, the experimental data on minor actinide compounds are limited. To support the research and development of the MA-bearing fuels, the property measurements were carried out on minor actinide nitrides and oxides. The lattice parameters and their thermal expansions were measured by high-temperature X-ray diffractometry. The specific heat capacities were measured by drop calorimetry and the thermal diffusivities by laser-flash method. The thermal conductivities were determined by the specific heat capacities, thermal diffusivities and densities. The oxygen potentials were measured by electromotive force method.

Electrolysis of Burnup-Simulated Uranium Nitride Fuels in LiCl-KCl Eutectic Melts

The electrochemical behavior of burnup-simulated uranium nitride fuels containing representative solid fission product elements, UN+Mo (Mo = 2.84 wt%), UN+Pd (Pd = 4.6 wt%) and (U, Nd)N (NdN = 8.0 wt%), was investigated in the molten LiCl-KCl eutectic salt with 0.54 wt% UCl3 in order to clarify the effects of fission products on the dissolution of actinide nitrides and the behavior of FPs in the electrorefining of spent nitride fuel. The rest potentials of burnup-simulated UN pellets were similar to that of pure UN. The electrochemical dissolution of UN began at about _0:75V vs Ag/AgCl reference electrode in all samples as well as that of pure UN. After the electrolyses at the constant anodic potential of −0:65––0:60V vs Ag/AgCl, most of UN was dissolved into LiCl-KCl as UCl3 at the anode, and U was recovered in the liquid Cd cathode in all samples. Furthermore, Nd was dissolved at the anode and accumulated into LiCl-KCl as NdCl3, while Mo and Pd were not dissolved but remained at the anode.

Synthesis of Actinide Nitrides in Molten Cadmium

Pyrochemical processing has several advantages over conventional wet processing for the treatment of spent nitride fuel. Besides the molten salt electrorefining of nitride fuel, actinide nitride formation in and recovery from the liquid cadmium cathode is a key technology in the nitride fuel cycle with pyrochemical processing. It was found that heating a cadmium alloy containing plutonium and uranium at 973 K under a nitrogen gas stream resulted in the simultaneous nitridation of the actinides and the distillation of cadmium. Furthermore, nitride pellets were prepared from the powder recovered without any milling or granulating process. The results obtained in the present study suggested that the nitride fuel cycle with pyrochemical processing is technologically feasible for transmutation of minor actinides and advanced fast reactors.