Synthesis and characterization of Th2N2(NH) isomorphous to Th2N3.

Using a new, low-temperature, fluoride-based process, thorium nitride imide of the chemical formula Th(2)N(2)(NH) was synthesized from thorium dioxide via an ammonium thorium fluoride intermediate. The resulting product phase was characterized by powder X-ray diffraction (XRD) analysis and was found to be crystallographically similar to Th(2)N(3). Its unit cell was hexagonal with a space group of P3m1 and lattice parameters of a = b = 3.886(1) and c = 6.185(2) Å. The presence of -NH in the nitride phase was verified by Fourier transform infrared spectroscopy (FTIR). Total energy calculations performed using all-electron scalar relativistic density functional theory (DFT) showed that the hydrogen atom in the Th(2)N(2)(NH) prefers to bond with nitrogen atoms occupying 1a Wyckoff positions of the unit cell. Lattice fringe disruptions observed in nanoparticle areas of the nitride species by high-resolution transmission electron microscopic (HRTEM) images also displayed some evidence for the presence of -NH group. As ThO(2) was identified as an impurity, possible reaction mechanisms involving its formation are discussed.

Crystal and electronic structures of neptunium nitrides synthesized using a fluoride route.

A low-temperature fluoride route was utilized to synthesize neptunium mononitride, NpN. Through the development of this process, two new neptunium nitride species, NpN(2) and Np(2)N(3), were identified. The NpN(2) and Np(2)N(3) have crystal structures isomorphous to those of UN(2) and U(2)N(3), respectively. NpN(2) crystallizes in a face-centered cubic CaF(2)-type structure with a space group of Fm3m and a refined lattice parameter of 5.3236(1) Å. The Np(2)N(3) adopts the body-centered cubic Mn(2)O(3)-type structure with a space group of Ia3. Its refined lattice parameter is 10.6513(4) Å. The NpN synthesis at temperatures ≤900 °C using the fluoride route discussed here was also demonstrated. Previous computational studies of the neptunium nitride system have focused exclusively on the NpN phase because no evidence was reported experimentally on the presence of NpN(x) systems. Here, the crystal structures of NpN(2) and Np(2)N(3) are discussed for the first time, confirming the experimental results by density functional calculations (DFT). These DFT calculations were performed within the local-density approximation (LDA+U) and the generalized-gradient approximation (GGA+U) corrected with an effective Hubbard parameter to account for the strong on-site Coulomb repulsion between Np 5f electrons. The effects of the spin-orbit coupling in the GGA+U calculations have also been investigated for NpN(2) and NpN.

Fluoride-conversion synthesis of homogeneous actinide oxide solid solutions.

The synthesis of (U,Th)O(2) solid solutions at a relatively low temperature of 1100 °C using a new technique is described. First, separate actinide oxides were reacted with ammonium hydrogen fluoride to form ammonium actinide fluorides at room temperature. Subsequently, this fluoride was converted to an actinide oxide solid solution using a two-phase reaction process, which involved heating of the fluoride first at 610 °C in static air followed by heating at 1100 °C in flowing argon. Oxide solid solutions of UO(2) and ThO(2) were synthesized for a ThO(2) content from 10 to 90 wt %. Microscopic investigation showed that the (U,Th)O(2) solid solutions synthesized using this method had high crystallinity and homogeneity up to nanoscale.

Reaction sequence and kinetics of uranium nitride decomposition.

The reaction mechanism and kinetics of the thermal decomposition of uranium dinitride/uranium sesquinitride to uranium mononitride under inert atmosphere at elevated temperature were studied. An increase in the lattice parameter of the UN(2)/alpha-U(2)N(3) phase was observed as the reaction temperature increased, corresponding to a continuous removal of nitrogen. Electron density calculations for these two compounds using XRD powder patterns of the samples utilizing charge-flipping technique were performed for the first time to visualize the decrease in nitrogen level as a function of temperature. Complete decomposition of UN(2) into alpha-U(2)N(3) at 675 degrees C and the UN formation after a partial decomposition of alpha-U(2)N(3) at 975 degrees C were also identified in this study. The activation energy for the decomposition of the UN(2)/alpha-U(2)N(3) phase into UN, 423.8 +/- 0.3 kJ/mol (101.3 kcal/mol), was determined under an inert argon atmosphere and is reported here experimentally for the first time.

Synthesis and nanoscale characterization of (NH(4))(4)ThF(8) and ThNF.

Synthesis of (NH(4))(4)ThF(8) by a solid state reaction of ThO(2) and NH(4)HF(2) and the formation of ThNF by ammonolysis of (NH(4))(4)ThF(8) and ThF(4) under different experimental conditions were investigated. The solid state reaction of ThO(2) with NH(4)HF(2) led to the terminal product (NH(4))(4)ThF(8) through a known intermediate (NH(4))(3)ThF(7) and most likely two other unknown chemical phases as determined by X-ray powder diffraction. Conversion of (NH(4))(4)ThF(8) into ThNF occurs through a ThF(4) intermediate phase. Studies on the ammonolysis of ThF(4) revealed it converted into ThNF through a continuous formation of low-stoichiometric thorium-nitride-fluorides such as ThN(0.79)F(1.63) and ThN(0.9)F(1.3). Thermal behavior of ThNF was also examined under different atmospheres and temperatures, with evaluation of formation kinetics. The ThNF decomposed to low-stoichiometric thorium nitride fluorides (ThN(x/3)F(4-x)) under different environments up to 1100 degrees C. Significant morphological changes in the products compared to that of the precursors confirmed the reaction steps involved. Microstructural characterization of (NH(4))(4)ThF(8) and ThNF were performed by HRTEM and are presented in this work for the first time. The (NH(4))(4)ThF(8) product was shown to contain polycrystalline characteristics in the majority of its nanostructure. On the other hand, ThNF has a high order of nanostructure, which explains the high thermal stability of the compound up to 1100 degrees C and the difficulty of making ThN(x), in initial target product, from the described experimental conditions.