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.

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.

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.

Structural studies of technetium-zirconium alloys by X-ray diffraction, high-resolution electron microscopy, and first-principles calculations.

The structural properties of Tc-Zr binary alloys were investigated using combined experimental and computational approaches. The Tc(2)Zr and Tc(6)Zr samples were characterized by X-ray diffraction analysis, scanning electron microscopy, electron probe microanalysis, and transmission electron microscopy. Our XRD results show that Tc(6)Zr crystallizes in the cubic alpha-Mn-type structure (I43m space group) with a variable stoichiometry of Tc(6.25-x)Zr (0 < x < 1.45), and Tc(2)Zr has a hexagonal crystal lattice with a MgZn(2)-type structure (P6(3)/mmc space group). Rietveld analysis of the powder XRD patterns and density functional calculations of the "Tc(6)Zr" phase show a linear increase of the lattice parameter when moving from Tc(6.25)Zr to Tc(4..80)Zr compositions, similar to previous observations in the Re-Zr system. This variation of the composition of "Tc(6)Zr" is explained by the substitution of Zr for Tc atoms in the 2a site of the alpha-Mn-type structure. These results suggest that the width of the "Tc(6)Zr" phase needs to be included when constructing the Tc-Zr phase diagram. The bonding character and stability of the various Tc-Zr phases were also investigated from first principles. Calculations indicate that valence and conduction bands near the Fermi level are dominated by electrons occupying the 4d orbital. In particular, the highest-lying molecular orbitals of the valence band of Tc(2)Zr are composed of d-d sigma bonds, oriented along the normal axis of the (110) plane and linking the Zr network to the Tc framework. Strong d-d bonds stabilizing the Tc framework in the hexagonal unit cell are also in the valence band. In the cubic structures of Tc-Zr phases, only Tc 4d orbitals are found to significantly contribute near the Fermi level.

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.

Application of electron microscopy in the observation of technetium and technetium dioxide nanostructures.

Transmission electron microscopy (TEM) was applied to characterize the microstructure of Tc metal and technetium dioxide synthesized by the decomposition of NH(4)TcO(4). The morphology of the Tc metal and well-resolved TcO(2) particles were characterized using bright-field TEM and scanning TEM modes. Structural characterization of these two samples using high-resolution (HR) TEM was successfully performed for the first time on the nanoscale. The morphology of Tc metal showed significant differences when compared to TcO(2). The crystal structure of Tc metal on the nanoscale was shown to contain well-resolved lattice fringes without any defects. Because of the deficiency in point resolution, however, the two-dimensional structure details of Tc metal could not be observed as expected. On the other hand, structural details of TcO(2) were prominent at high resolution. With a 2-fold multiplicity in both directions, TcO(2) showed a unique atomic distribution corresponding to a monoclinic unit cell. Furthermore, the lattice parameters of the samples were refined by the Rietveld analysis of the powder X-ray diffraction patterns and were estimated by HRTEM. In the case of technetium dioxide, the stoichiometry was approximated to be TcO(2.3) using quantitative analysis of X-ray energy dispersive spectrometry. Electron energy-loss spectrometry verified the chemical phase of the two samples by their different chemical environments based on an energy shift of 2.0 eV of the N(23) edge between TcO(2) and Tc metal.

Investigation of nanostructure and thermal behavior of zinc-substituted fluorapatite.

The incorporation of various cations such as Zn (2+) into the structure of fluorapatite, Ca 5(PO 4) 3F, is governed by the effectiveness of the cations to substitute for Ca (2+) ions. In this work different concentrations of zinc were used to substitute for calcium. Microscopic characterization was done by observing the nanostructural variations induced by these zinc substitutions and by relating these observations to thermal behavior of the zinc-substituted fluorapatite. Random incorporation of zinc into the fluorapatite structure and the zinc induced amorphization effects oriented in the nanostructure of fluorapatite led to phase impurities of zinc-substituted fluorapatites when the amount of zinc used was greater than 25 mol % of the total cation concentration. It also was observed that the thermal stability of the samples decreased with increasing zinc concentration. Among the zinc-incorporated samples, the most similar chemical and physical properties to the single-phased fluorapatite were identified in the sample containing the lowest amount of zinc (25 mol %). After calcination, however, this sample showed to contain some defects in the atomic arrangement of the nanostructure which led to a thermal instability of the fluorapatite.