Curium Ions Research Articles

Dynamic dissolution of Cm3+ ions incorporated at the calcite-water interface: an ab initio molecular dynamics simulation study.

The stability of actinide-mineral solid solution in a water environment is critical for assessing the safety of nuclear-waste geological repositories and studying actinide migration in natural systems. However, the dissolution behavior of actinide ions incorporated at the mineral-water interface is still unclear at the atomic level. Herein, we present metadynamics simulations of the reaction pathways, thermodynamics and kinetics of trivalent curium ions (Cm3+) dissolving from calcite surfaces. Cm3+ ions incorporated in different calcite surfaces (i.e., terrace and stepped surfaces) with distinct coordination environments have different reaction pathways, free energy barriers and free energy changes. We found that Cm dissolution from a stepped surface is more favorable than that from a terrace surface, both thermodynamically and kinetically. In addition, water molecules seem to promote the detachment of curium ions from the surface by exerting a pulling force via water coordination with Cm3+ and a pushing force via proton migration to the surface layer and water diffusion in the vacant Cm site. Thus, the findings from this work prove to be a milestone in revealing the dynamic dissolution mechanism of trivalent actinides from minerals and would also help predict the dissolution behaviors of other metal ions at the solid-water interface in chemical and environmental sciences.

Americium(III)/Curium(III) Complete Separation and Sensitive Fluorescence Detection by Capillary and Gel Electrophoresis Using Emissive Hexadentate/Octadentate Polyaminocarboxylate Ligands

Abstract Highly efficient and effective separation between americium (Am3+) and curium ion (Cm3+) was achieved by two simple electrophoresis-based techniques. Am3+ and Cm3+ ions were complexed with fluorophore-modified acyclic hexadentate and octadentate polyaminocarboxylates and then were electrophoretically separated and fluorescently detected in free solution with ternary complexation or in gel medium.

Revisiting the Complexation of Cm(III) with Aqueous Phosphates: What Can We Learn from the Complex Structures Using Luminescence Spectroscopy and Ab Initio Simulations?

The coordination chemistry of Cm(III) with aqueous phosphates was investigated by means of laser-induced luminescence spectroscopy and ab initio simulations. For the first time, in addition to the presence of Cm(H2PO4)2+, the formation of Cm(H2PO4)2+ was unambiguously established from the luminescence spectroscopic data collected at various H+ concentrations (-log10 [H+] = 2.52, 3.44, and 3.65), ionic strengths (0.5-3.0 mol·L-1 NaClO4), and temperatures (25-90 °C). Complexation constants for both species were derived and extrapolated to standard conditions using the specific ion interaction theory. The molal enthalpy ΔRHm0 and molal entropy ΔRSm0 of both complexation reactions were derived using the integrated van't Hoff equation and indicated an endothermic and entropy-driven complexation. For the Cm(H2PO4)2+ complex, a more satisfactory description could be obtained when including the molal heat capacity term. While monodentate binding of the H2PO4- ligand(s) to the central curium ion was found to be the most stable configuration for both complexes in our ab initio simulations and luminescence lifetime analyses, a different temperature-dependent coordination to hydration water molecules could be deduced from the electronic structure of the Cm(III)-phosphate complexes. More precisely, where the Cm(H2PO4)2+ complex could be shown to retain an overall coordination number of 9 over the entire investigated temperature range, a coordination change from 9 to 8 was established for the Cm(H2PO4)2+ species with increasing temperature.

Importance of Counteranions on the Hydration Structure of the Curium Ion

Using density functional theory based ab initio molecular dynamics and metadynamics, we show that counterions can trigger noticeable changes in the hydration shell structure of the curium ion. On the basis of the free energies of curium–water coordination, the eight-fold coordination state is dominant by at least 98% in the absence of counteranions and in the presence of chloride and bromide counteranions. In addition, the solvent hydrogen bond (HB) lifetimes are relatively longer. In contrast, the solvent hydrogen bond (HB) lifetimes are relatively shorter in the presence of perchlorate counteranions, with the nine-fold and eight-fold states existing in an 8/2 ratio, which is in good agreement with the reported ratio measured by X-ray scattering experiments. To our knowledge, this is the first time that molecular simulations have shown that counteranions can directly affect the first hydration shell structure of a cation.

Electrochemical behaviour of curium(III) ions in fused 3LiCl–2KCl eutectic

Abstract This work presents the electrochemical study of Cm(III) in fused 3LiCl–2KCl eutectic in the temperature range 723–923 K. Transient electrochemical techniques such as cyclic, square wave voltammetries, and chronopotentiometry have been used in order to investigate the reduction mechanism of curium ions up to metal. The results obtained show that the reduction reaction takes place in a single step Cm ( III ) + 3 e ¯ ⇒ Cm ( 0 ) . The diffusion coefficient of Cm 3+ ions was determined by cyclic voltammetry at different temperatures by applying Berzins–Delahay equation. The validity of the Arrhenius law was also verified and the activation energy for diffusion was found to be 28.21 kJ mol −1 . The apparent standard electrode potential of the redox couple Cm ( III ) / Cm ( 0 ) was found by chronopotentiometry at several temperatures. The thermodynamic properties of curium trichloride have also been calculated.

Electrochemistry of oxygen-free curium compounds in fused NaCl–2CsCl eutectic

This work presents the electrochemical study of Cm(III) in fused NaCl–2CsCl eutectic in the temperature range 823–1023 K. Transient electrochemical techniques such as cyclic, differential pulse and square wave voltammetry, and chronopotentiometry have been used in order to investigate the reduction mechanism of curium ions up to the metal. The results obtained show that the reduction reaction takes place in a single step Cm(III) + 3 e ¯ ⇒ Cm(0) . The diffusion coefficient of [ CmCl 6 ] 3 - complex ions was determined by cyclic voltammetry at different temperatures by applying the Berzins–Delahay equation. The validity of the Arrhenius law was also verified and the activation energy for diffusion was found to be 44.46 kJ/mol. The apparent standard electrode potential of the redox couple Cm(III) / Cm(0) was found by chronopotentiometry at several temperatures. The thermodynamic properties of curium trichloride have also been calculated.

Hydrothermal syntheses, structural, Raman, and luminescence studies of Cm[ M(CN) 2] 3·3H 2O and Pr[ M(CN) 2] 3·3H 2O ( M=Ag, Au) : 2. Hetero-bimetallic coordination polymers consisting of trans-plutonium and transition metal elements

We have prepared Cm[Au(CN) 2] 3·3H 2O and Cm[Ag(CN) 2] 3·3H 2O as a part of our continuing investigations into the chemistry of the 5 f-elements’ dicyanometallates. Single crystals of Cm[Au(CN) 2] 3·3H 2O were obtained from the reaction of CmCl 3 and KAu(CN) 2 under mild hydrothermal conditions. Due to similarities in size, the related praseodymium compounds were also synthesized and characterized for comparison with the actinide systems. The compounds crystallize in the hexagonal space group P6 3/ mcm, where the curium and the transition metals interconnect through cyanide bridging. Crystallographic data (Mo K α, λ=0.71073 Å): Cm[Au(CN) 2] 3·3H 2O ( 1), a=6.6614(5) Å, c=18.3135(13) Å, V=703.77(9), Z=2; Pr[Au(CN) 2] 3·3H 2O ( 3), a=6.6662(8) Å, c=18.497(3) Å, V=711.83(17), Z=2; Pr[Ag(CN) 2] 3·3H 2O ( 4), a=6.7186(8) Å, c=18.678(2) Å, V=730.18(14), Z=2. The Cm 3+ and/or Pr 3+ ions are coordinated to six N-bound CN − groups resulting in a trigonal prismatic arrangement. Three oxygen atoms of coordinated water molecules tricap the trigonal prismatic arrangement providing a coordination number of nine for the f-elements. The curium ions in both compounds exhibit a strong red emission corresponding to the 6D 7/2→ 8S 7/2 transition. This transition is observed at 16,780 cm −1, with shoulders at 17,080 and 16,840 cm −1 for the Ag complex, while the emission is red shifted by ∼100 cm −1 in the corresponding gold complex. The Pr systems also provide well-resolved emissions upon f– f excitation.

Ab initio MO studies on the hydration of trivalent curium ion

We have performed a series of ab initio relativistic molecular-orbital calculations to model the hydration of trivalent curium ion. Up to the hexa-hydrate model, Dirac-Hartree-Fock calculations as a four-component recipe were used to investigate the interaction between the ion and water molecules. The fluorescence transition energy was estimated by complete open-shell configuration interaction calculations. Hydrates having more than six water molecules were treated using the B3LYP hybrid density-functional theory with effective-core potential.

A Time-Resolved Laser Fluorescence Spectroscopy (TRLFS) Study of the Interaction of Trivalent Actinides (Cm(III)) with Calcite

Cm(III) interaction with calcite was investigated by time-resolved laser fluorescence spectroscopy (TRLFS) in the trace concentration range. Two different Cm(III)/calcite sorption species were found. The first Cm(III) sorption species consists of a curium ion bonded onto the calcite surface. The second Cm(III) sorption species has lost its complete hydration sphere and is incorporated into the calcite bulk structure. The Cm(III)/calcite complexes are characterized by their emission spectra (peak maxima at 607.5 and 618.0 nm) and their fluorescence emission lifetimes (314±6 and 1302±75 μs).

Crystal field analysis and Monte Carlo simulation of lattice disordering for Cm3+ in YPO4 and LuPO4

A method of analyzing inhomogeneous line broadening with crystal-field theory is presented. For actinide ions with unfilled f-shells in solids, the most important contribution to line broadening of f-electron transitions is from short-range interactions with distorted lattice. Optically active curium ions are used to measure the degree of radiation damage in the vicinity of their lattice site. The radiation damage was produced by decay of distant 244Cm3+ ions. Fluorescence line narrowing (FLN) spectra show that inhomogeneous line broadening induced by α-decay of the actinide ion 244Cm3+ in single crystals of LuPO4 and YPO4 has a microscopic nature. Namely, correlation is absent between energy levels of the Cm3+ ions at different local environments. Monte Carlo simulation of lattice distortion and crystal-field calculation of Cm3+ energy levels have been conducted to provide a quantitative interpretation of the experimental results. This method should be applicable to 4f- and 3d-ions in crystalline materials with lattice distortion.

High-Energy Organometallic Chemistry of Curium: Laser Ablation of Cm7O12 Dispersed in Polyimide

Gas-phase curium organometallic ions were synthesized by metal/polymer co-ablation. Vacuum laser ablation of a dilute dispersion of curium oxide in polyimide resulted in several organocurium ions, evidently formed by nucleation of Cm+ and various neutral polymer fragment radicals in the ablation plume. The compositions and abundances of simple species, such as CmC2+, CmC2H+, and CmCN+, can be rationalized on the basis of the electronic structure and energetics of the curium ion, Cm+. The results for curium are in accord with a general thermochemical model previously employed to understand the organometallic speciation of preceding actinides. The identification of the above-specified species, along with several larger organocurium ions, extends the realm of organoactinide chemistry; specifically, it represents identification of a series of unique curium complexes which presumably incorporate substantially covalent (σ) Cm−C bonding. Also identified in the ablation plume were small clusters incorporating two or three curium atoms. Simple clusters, such as Cm3O4+, reveal a trivalent, lanthanide-like character for curium and reflect the tendency toward bulk, solid-state chemical behavior upon coalescence of only a few atoms. Finally, a notable ancillary observation was the formation of several clusters in which C2 evidently substituted sequentially for an O atom, revealing the pseudo-oxygen character of the dicarbide moiety.

Recoil escape efficiencies and ranges of actinide recoils from actinide targets

Escape efficiencies of recoiling Cm and Am nuclei with energies of 50–400 keV from Pu and Am targets have been measured. Recoil ranges for curium ions in plutonium and americium oxide are deduced. The experimental results are compared with predictions of the Lindhard, Scharff and Schiøtt theory. The calculated ranges are in reasonable agreement with the experimental values.

Paramagnetic Resonance of the Tripositive Curium Ion in Ethylsulfate and Trichloride Crystals

The paramagnetic resonance of the ion ${\mathrm{Cm}}^{3+}$ has been studied by incorporating various amounts of curium in crystals of lanthanum ethylsulfate and anhydrous lanthanum trichloride. For the ethylsulfate crystals, a single line was observed at helium temperatures, characterized by ${g}_{\mathrm{II}}=1.925\ifmmode\pm\else\textpm\fi{}0.002$ and ${g}_{\ensuremath{\perp}}=7.73\ifmmode\pm\else\textpm\fi{}0.02$. A similar line, for which ${g}_{\mathrm{II}}=1.925\ifmmode\pm\else\textpm\fi{}0.002$ and ${g}_{\ensuremath{\perp}}=7.67\ifmmode\pm\else\textpm\fi{}0.02$, occurs for the trichloride crystals. These lines are interpreted as corresponding to transitions within the lowest doublet of the ground level $5{f}^{7}^{8}S_{\frac{7}{2}}$, which, owing to the high degree of intermediate coupling, does not possess the extremely small splitting typical of $S$ states. The Land\'e $g$ value of the ground level is found to be 1.925. Previous experimental results on the paramagnetic resonance of ${\mathrm{Cm}}^{3+}$, which were difficult to account for theoretically, are shown to be spurious; evidently the reported spectra should have been ascribed to ${\mathrm{Gd}}^{3+}$.