Molten Salt Synthesis of a Mixed-Valent Lanthanide(III/IV) Oxychloride with an Unprecedented Sillen X₂⁴ Structure: Ce_1.3Nd_0.7O₃Cl

A new cerium neodymium oxychloride, Ce_1.3Nd_0.7O₃Cl, has been synthesized by precipitation in a LiCl–CaCl₂ molten salt by humid argon sparging. Chemical and structure characterization have been undertaken by powder X-ray diffraction, scanning electron microscopy, high-temperature X-ray diffraction, thermogravimetric analysis, and X-ray photoelectron scattering. This oxychloride crystallizes in space group <i>P</i>4/<i>nmm</i>, <i>a</i> = 3.9848(3) Å and <i>c</i> = 12.467(2) Å, in a new Sillen-type phase represented by the symbol X₂⁴ where “quadruple” fluorite-type layers [M₄O₆], containing Ce^IV in “inner” sublayers and both Ce^III and Nd^III in “outer” sublayers, alternate with double-halide ion sheets. The structure is also described as a stacking of LnOCl and fluorite-type blocks and constitutes the term <i>n</i> = 2 of a possible series (MO₂)_<i>n</i>(NdOCl)₂

Single Crystal Synthesis Methods Dedicated to Structural Investigations of Very Low Solubility Mixed-Actinide Oxalate Coordination Polymers

Two crystal growth methods dedicated to very low solubility actinide coordination polymers have been developed and applied to the synthesis of mixed actinide­(IV)–actinide­(IV) or actinide­(IV)–actinide­(III) oxalate single crystals of a size (typically 100–300 μm) suitable for isolating them and examining their crystal structure. These methods have been optimized on mixed systems composed of U­(IV) and lanthanide (surrogate of trivalent actinides) and then assessed on U­(IV)–Am­(III), Pu­(IV)–Am­(III), and U­(IV)–Pu­(IV) mixtures. Three types of single crystals characterized by different structures have been obtained according to the synthesis and the chemical conditions. This is the first time that these well-known or recently discovered key compounds are formed by crystal growth methods specifically developed for actinide crystal handling (i.e., in glove boxes), thus enabling direct structural studies on transuranium element systems and acquisition of basic data. Characterization by X-ray diffraction, UV–visible solid spectroscopy, thermal ionization mass spectroscopy (TIMS), energy-dispersive X-ray spectroscopy (EDS), and inductively coupled plasma atomic emission spectroscopy (ICP-AES) demonstrates the potentialities and complementarity of the two crystal growth methods for obtaining the targeted mixed oxalates (actinide oxidation state and presence of both metallic ions in the crystal). More generally, this development opens broad prospects for single crystal synthesis of novel actinide organic frameworks and their structural description

Crystal Growth and First Crystallographic Characterization of Mixed Uranium(IV)–Plutonium(III) Oxalates

The mixed-actinide uranium­(IV)–plutonium­(III) oxalate single crystals (NH₄)_0.5[Pu^III_0.5U^IV_0.5(C₂O₄)₂·H₂O]·<i>n</i>H₂O (<b>1</b>) and (NH₄)_2.7Pu^III_0.7U^IV_1.3(C₂O₄)₅·<i>n</i>H₂O (<b>2</b>) have been prepared by the diffusion of different ions through membranes separating compartments of a triple cell. UV–vis, Raman, and thermal ionization mass spectrometry analyses demonstrate the presence of both uranium and plutonium metal cations with conservation of the initial oxidation state, U^IV and Pu^III, and the formation of mixed-valence, mixed-actinide oxalate compounds. The structure of <b>1</b> and an average structure of <b>2</b> were determined by single-crystal X-ray diffraction and were solved by direct methods and Fourier difference techniques. Compounds <b>1</b> and <b>2</b> are the first mixed uranium­(IV)–plutonium­(III) compounds to be structurally characterized by single-crystal X-ray diffraction. The structure of <b>1</b>, space group <i>P</i>4/<i>n</i>, <i>a</i> = 8.8558(3) Å, <i>b</i> = 7.8963(2) Å, <i>Z</i> = 2, consists of layers formed by four-membered rings of the two actinide metals occupying the same crystallographic site connected through oxalate ions. The actinide atoms are nine-coordinated by oxygen atoms from four bidentate oxalate ligands and one water molecule, which alternates up and down the layer. The single-charged cations and nonbonded water molecules are disordered in the same crystallographic site. For compound <b>2</b>, an average structure has been determined in space group <i>P</i>6/<i>mmm</i> with <i>a</i> = 11.158(2) Å and <i>c</i> = 6.400(1) Å. The honeycomb-like framework [Pu^III_0.7U^IV_1.3(C₂O₄)₅]^2.7‑ results from a three-dimensional arrangement of mixed (U_0.65Pu_0.35)­O₁₀ polyhedra connected by five bis-bidentate μ²-oxalate ions in a trigonal-bipyramidal configuration

Crystal Growth and First Crystallographic Characterization of Mixed Uranium(IV)–Plutonium(III) Oxalates

The mixed-actinide uranium­(IV)–plutonium­(III) oxalate single crystals (NH₄)_0.5[Pu^III_0.5U^IV_0.5(C₂O₄)₂·H₂O]·<i>n</i>H₂O (<b>1</b>) and (NH₄)_2.7Pu^III_0.7U^IV_1.3(C₂O₄)₅·<i>n</i>H₂O (<b>2</b>) have been prepared by the diffusion of different ions through membranes separating compartments of a triple cell. UV–vis, Raman, and thermal ionization mass spectrometry analyses demonstrate the presence of both uranium and plutonium metal cations with conservation of the initial oxidation state, U^IV and Pu^III, and the formation of mixed-valence, mixed-actinide oxalate compounds. The structure of <b>1</b> and an average structure of <b>2</b> were determined by single-crystal X-ray diffraction and were solved by direct methods and Fourier difference techniques. Compounds <b>1</b> and <b>2</b> are the first mixed uranium­(IV)–plutonium­(III) compounds to be structurally characterized by single-crystal X-ray diffraction. The structure of <b>1</b>, space group <i>P</i>4/<i>n</i>, <i>a</i> = 8.8558(3) Å, <i>b</i> = 7.8963(2) Å, <i>Z</i> = 2, consists of layers formed by four-membered rings of the two actinide metals occupying the same crystallographic site connected through oxalate ions. The actinide atoms are nine-coordinated by oxygen atoms from four bidentate oxalate ligands and one water molecule, which alternates up and down the layer. The single-charged cations and nonbonded water molecules are disordered in the same crystallographic site. For compound <b>2</b>, an average structure has been determined in space group <i>P</i>6/<i>mmm</i> with <i>a</i> = 11.158(2) Å and <i>c</i> = 6.400(1) Å. The honeycomb-like framework [Pu^III_0.7U^IV_1.3(C₂O₄)₅]^2.7‑ results from a three-dimensional arrangement of mixed (U_0.65Pu_0.35)­O₁₀ polyhedra connected by five bis-bidentate μ²-oxalate ions in a trigonal-bipyramidal configuration

Crystal Growth and First Crystallographic Characterization of Mixed Uranium(IV)–Plutonium(III) Oxalates

The mixed-actinide uranium­(IV)–plutonium­(III) oxalate single crystals (NH₄)_0.5[Pu^III_0.5U^IV_0.5(C₂O₄)₂·H₂O]·<i>n</i>H₂O (<b>1</b>) and (NH₄)_2.7Pu^III_0.7U^IV_1.3(C₂O₄)₅·<i>n</i>H₂O (<b>2</b>) have been prepared by the diffusion of different ions through membranes separating compartments of a triple cell. UV–vis, Raman, and thermal ionization mass spectrometry analyses demonstrate the presence of both uranium and plutonium metal cations with conservation of the initial oxidation state, U^IV and Pu^III, and the formation of mixed-valence, mixed-actinide oxalate compounds. The structure of <b>1</b> and an average structure of <b>2</b> were determined by single-crystal X-ray diffraction and were solved by direct methods and Fourier difference techniques. Compounds <b>1</b> and <b>2</b> are the first mixed uranium­(IV)–plutonium­(III) compounds to be structurally characterized by single-crystal X-ray diffraction. The structure of <b>1</b>, space group <i>P</i>4/<i>n</i>, <i>a</i> = 8.8558(3) Å, <i>b</i> = 7.8963(2) Å, <i>Z</i> = 2, consists of layers formed by four-membered rings of the two actinide metals occupying the same crystallographic site connected through oxalate ions. The actinide atoms are nine-coordinated by oxygen atoms from four bidentate oxalate ligands and one water molecule, which alternates up and down the layer. The single-charged cations and nonbonded water molecules are disordered in the same crystallographic site. For compound <b>2</b>, an average structure has been determined in space group <i>P</i>6/<i>mmm</i> with <i>a</i> = 11.158(2) Å and <i>c</i> = 6.400(1) Å. The honeycomb-like framework [Pu^III_0.7U^IV_1.3(C₂O₄)₅]^2.7‑ results from a three-dimensional arrangement of mixed (U_0.65Pu_0.35)­O₁₀ polyhedra connected by five bis-bidentate μ²-oxalate ions in a trigonal-bipyramidal configuration