Continuous recovery of uranium oxides by electrolysis of M2MoO4-M2Mo2O7-UO2MoO4 melts (M = Li, Na, K, Cs)

Effect of the electrolyte composition and of the solvent-salt cation on the oxygen coefficient of the cathodic product (O/U atomic ratio) and basic characteristics of the potentiostatic electrodeposition of uranium dioxide in prolonged recovery of uranium oxides from electrolytes of the system M₂MoO₄-M₂Mo₂O₇-UO₂MoO₄ Melts (M = Li, Na, K, Cs) in air was analyzed. A decrease in the UO₂MoO₄ concentration and accumulation of M₂Mo₂O₇ in the electrolyte in the course of a prolonged electrolysis suppress the solvolysis of uranyl ions and make lower the oxygen coefficient of the cathodic product. Li₂MoO₄-based melts possessing pronounced oxygenacceptor properties exhibit an anomalous behavior in these experiments. The current efficiency, initial current density, and deposition rate of the product decrease as electrolytes are depleted of uranium. In discussions of numerical data, it is necessary to take into account the formation of lower valence forms of uranium due to the chemical corrosion of the cathodic product, and in the case of melts of the lithium system, the additional cathodic process in which the solvent is reduced.     

Chemical and electrochemical behavior of uranium(VI) in oxide salt melts: The effect of the nature of the salt-solvent’s cation and anion

The effect of the electrolyte composition, deposition potential, temperature, and the salt-solvent’s cation on the oxygen coefficient (atomic ratio [O]/[U]) of uranium oxides is studied. The oxides are obtained by potentiostatic electrolysis of the M₂EO₄-M₂E₂O₇-UO₂EO₄ melts (M = Li, Na, K, Cs; E = W, Mo, S). The oxygen coefficient of the cathodic product is found to increase with the melt temperature. Shifting the deposition potential toward more electronegative values, raising the M₂E₂O₇ concentration, and lowering the concentration of UO₂EO₄ in the oxide salt electrolytes promotes the formation of uranium oxides at the cathode with smaller values of the oxygen coefficient. All other conditions being equal, the latter diminishes with increasing radius of the salt-solvent’s cation in a series of alkali metals. The electrolytes on the basis of Li₂WO₄ and Li₂MoO₄ are discovered to behave abnormally. Replacing the tungstate electrolytes by the molybdate ones with the same cationic composition (at constant \(c_{M_2 E_2 O_7 } \), \(c_{UO_2 EO_4 } \), and T) amplifies the oxygen coefficient of the cathodic product. The results of the study are satisfactorily explained in the framework of a model for the ionic composition of uranyl-containing oxide salt electrolytes, which is based on the notions concerning the complexing and stepwise solvolysis of the uranyl ions.     

Densities of Melts of the System KF?K2MoO4?B2O3

The density of melts of the system KF? K₂MoO₄? B₂O₃ was measured. The molar volume in the binary system KF? K₂MoO₄ deviates only little from the ideal course, which indicates the extended thermal dissociation of the congruently melting additive compound K₃FMoO₄. In the KF? B₂O₃ binary system the formation of KBF₄ and K₂B₄O₇ leads to the volume expansion, like in the K₂MoO₄? B₂O₃ system, where the volume expansion may be described by the formation of the heteropolyanions [BMo₆O₂₄]^9?. The significant deviation from the ideal behaviour in the ternary system KF? K₂MoO₄? B₂O₃ refers to the pronounced interaction, most probably due to the substitution of oxygen atoms in the coordination sphere of the heteropolyanion with the fluorine ones.     

Effect of cationic composition of melts M2WO4-M2W2O7-UO2WO4 (M = Li, Na, K, Cs) and parameters of electrolysis to uranium dioxide electrodeposition characteristics

The effect of solvent salt cation, temperature of electrolysis, and cathodic deposition potential to basic characteristics of uranium dioxide potentiostatic electrodeposition (cathodic current efficiency, current density, and product deposition rate) in electrolyte system M₂WO₄-M₂W₂O₇-UO₂WO₄ (M = Li, Na, K, Cs) was studied in air. A conclusion was made that optimum electrodeposition and production of deposits not contaminated with supporting electrolyte decomposition products need higher cathodic polarization close to the solvent decomposition potentials and higher temperatures (up to 900°C). A melt based on K₂WO₄ was selected as the electrolyte providing satisfactory electrodeposition parameters of uranium dioxide with the composition close to stoichiometric.     

Phase equilibrium in the Cs2MoO4-Bi2(MoO4)3-Zn(MoO4)2 system and the crystal structure of new triple molybdate Cs5BiZr(MoO4)6

The subsolidus region of the Cs₂MoO₄-Bi₂(MoO₄)₃-Zr(MoO₄) system was studied by X-ray powder diffraction. Quasi-binary sections were elucidated, and triangulation performed. Triple molybdates with the component ratios 5: 1: 2 (S ₁) and 2: 1: 4 (S ₂) were prepared for the first time. Crystals of cesium bismuth zirconium molybdate of the 5: 1: 2 stoichiometry (Cs₅BiZr(MoO₄)₆) were grown from fluxed melts with spontaneous nucleation. The composition and crystal structure of this triple molybdate were refined using X-ray diffraction data (collected on X8 APEX automated diffractometer, MoK _α radiation, 2348 F(hkl), R = 0.0226). The trigonal unit cell parameters were as follows: a = b = 10.9569(2), c = 39.804(4) Å, V = 4138.4(4) ų, Z = 6, space group R $ \bar 3 The subsolidus region of the Cs₂MoO₄-Bi₂(MoO₄)₃-Zr(MoO₄) system was studied by X-ray powder diffraction. Quasi-binary sections were elucidated, and triangulation performed. Triple molybdates with the component ratios 5: 1: 2 (S ₁) and 2: 1: 4 (S ₂) were prepared for the first time. Crystals of cesium bismuth zirconium molybdate of the 5: 1: 2 stoichiometry (Cs₅BiZr(MoO₄)₆) were grown from fluxed melts with spontaneous nucleation. The composition and crystal structure of this triple molybdate were refined using X-ray diffraction data (collected on X8 APEX automated diffractometer, MoK _α radiation, 2348 F(hkl), R = 0.0226). The trigonal unit cell parameters were as follows: a = b = 10.9569(2), c = 39.804(4) ?, V = 4138.4(4) ?³, Z = 6, space group R c. The mixed-metal three-dimensional framework in this structure is built of Mo tetrahedra and two sorts of (Bi,Zr)O₆ octahedra. Large interstices accommodate two sorts of cesium atoms. The Bi³⁺ and Zr⁴⁺ cation distributions over two positions were refined during structure solution. Original Russian Text ? B.G. Bazarov, T.V. Namsaraeva, R.F. Klevtsova, A.G. Anshits, T.A. Vereshchagina, R.V. Kurbatov, L.A. Glinskaya, K.N. Fedorov, Zh.G. Bazarova, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 9, pp. 1585–1589.     

New triple molybdates Cs3LiCo2(MoO4)4 and Rb3LiZn2(MoO4)4, filled derivatives of the Cs6Zn5(MoO4)8 type

Two new isotypic triple molybdates, namely tri­cesium lithium dicobalt tetra­kis­(tetra­oxo­molybdate), Cs₃LiCo₂(MoO₄)₄, and tri­rubidium lithium dizinc tetra­kis­(tetra­oxo­molybdate), Rb₃LiZn₂(MoO₄)₄, crystallize in the non‐centrosymmetric cubic space group I3d and adopt the Cs₆Zn₅(MoO₄)₈ structure type. In the parent structure, the Zn positions have 5/6 occupancy, while they are fully occupied by statistically distributed M²⁺ and Li⁺ cations in the title compounds. In both structures, all corners of the (M_2/3Li_1/3)O₄ tetra­hedra (M = Co and Zn), having point symmetry , are shared with the MoO₄ tetra­hedra, which lie on threefold axes and share corners with three (M,Li)O₄ tetra­hedra to form open mixed frameworks. Large alkaline cations occupy distorted cubocta­hedral cavities with symmetry. The mixed tetra­hedral frameworks in the structures are close to those of mayenite (12CaO·7Al₂O₃) and the related compounds 11CaO·7Al₂O₃·CaF₂, wadalite (Ca₆Al₅Si₂O₁₆Cl₃) and Na₆Zn₃(AsO₄)₄·3H₂O, but the terminal vertices of the MoO₄ tetra­hedra are directed in opposite directions along the threefold axes compared with the configurations of Al(Si)O₄ or AsO₄ tetra­hedra. The cation arrangements in Cs₃LiCo₂(MoO₄)₄, Rb₃LiZn₂(MoO₄)₄ and Cs₆Zn₅(MoO₄)₈ repeat the structure of Y₃Au₃Sb₄, being stuffed derivatives of the Th₃P₄ type.     

Li3Al(MoO4)3, a lyonsite molybdate

Trilithium aluminium trimolybdate(VI), Li₃Al(MoO₄)₃, has been grown as single crystals from α‐Al₂O₃ and MoO₃ in an Li₂MoO₄ flux at 998 K. This compound is an example of the well known lyonsite structure type, the general formula of which can be written as A₁₆B₁₂O₄₈. Because this structure can accomodate cationic mixing as well as cationic vacancies, a wide range of chemical compositions can adopt this structure type. This has led to instances in the literature where membership in the lyonsite family has been overlooked when assigning the structure type to novel compounds. In the title compound, there are two octahedral sites with substitutional disorder between Li⁺ and Al³⁺, as well as a trigonal prismatic site fully occupied by Li⁺. The (Li,Al)O₆ octahedra and LiO₆ trigonal prisms are linked to form hexagonal tunnels along the [100] axis. These polyhedra are connected by isolated MoO₄ tetrahedra. Infinite chains of face‐sharing (Li,Al)O₆ octahedra extend through the centers of the tunnels. A mixed Li/Al site, an Li, an Mo, and two O atoms are located on mirror planes.     

Beiträge zur Chemie der Oxidhalogenide von Molybdän und Wolfram. IV. Der Transport von MoO2 mit J2 und die Existenz von MoO2J2

The possibility to transport MoO₂ with J₂ in a temperature gradient T₂/T₁ suggests the existence of MoO₂J₂. Starting from the reaction MoO₂ + J₂ ? MoO₂J₂ in the consideration of the function of temperature for the rates of chemical transport, the values ΔH^O_R ? 28.8 (±2) kcal/mole and ΔS^O_R ? 9.0 (±2) cl are deduced. From this the values ΔH^O(MoO₂J₂, g, 298) ? ?99.5 (±3.5) kcal/mole and S^O(MoO₂J₂, g, 298) ? 86 (±3) cl are derived. The comparison of the thermodynamic data for MoO₂X₂ and WO₂X₂ (X = Cl, Br, J) leads to the conclusion, that the existence of MoO₂J₂ in the vapour phase is very probable indeed.     

Systems Tl2MoO4-E(MoO4)2, where E = Zr or Hf, and the crystal structure of Tl8Hf(MoO4)6

Systems Tl₂MoO₄-E(MoO₄)₂ (E = Zr, Hf) are studied using X-ray powder diffraction, DTA, and IR spectroscopy. Compounds Tl₈E(MoO₄)₆ and Tl₂E(MoO₄)₂ are found in these systems. T-x diagrams for the Tl₂MoO₄-Zr(MoO₄)₂ system are designed. Single crystals are grown and structure is solved for Tl₈Hf(MoO₄)₆. The compound crystallizes in a monoclinic structure with the unit cell parameters a = 9.9688(6) Å, b = 18.830(1) Å, c = 7.8488(5) Å, β = 108.538(1)°, Z = 2, space group C2/m. The main structural fragment is a [HfMo₆O₂₄]^8? isle group. Three crystallographically independent types of Tl polyhedra uniformly fill spaces between [HfMo₆O₂₄]^8? fragments to form a three-dimensional framework.     

LiFe(MoO4)2, LiGa(MoO4)2 and Li3Ga(MoO4)3

The structures of lithium iron dimolybdate, LiFe(MoO₄)₂, and lithium gallium dimolybdate, LiGa(MoO₄)₂, are shown to be isomorphous with each other. Their structures consist of segregated layers of LiO₆ bicapped trigonal bipyramids and Fe(Ga)O₆ octahedra separated and linked by layers of isolated MoO₄ tetrahedra. The redetermined structure of trilithium gallium trimolybdate, Li₃Ga(MoO₄)₃, shows substitional disorder on the Li/Ga site and consists of perpendicular chains of LiO₆ trigonal prisms and two types of differently linked Li/GaO₆ octahedra.     

Synthesis and crystal structure of the binary molybdate Li8Bi2(MoO4)7

Single crystals of Li₈Bi₂(MoO₄)₇ were synthesized; the composition and crystal structure of this compound were determined from X-ray diffraction data (CAD-4 automatic diffractometer, MoK_a radiation, 1767 reflections, R = 0.031). The parameters of the tetragonal unit cell are as follows: a = 21.130, c = 5.287 Å, Z = 4, space group -14. The structure of the binary molybdate is a three-dimensional mixed framework of MoO₄ tetrahedra of four varieties, Bi eight-vertex potyhedra, and Li(l)O₆ and Li(2)O₆ octahedra. The large channels of the framework along the c axis contain MoO₄ tetrahedra of the fifth variety with Li(3)O₄ and Li(4)O₄ tetrahedra attached to them via common vertices and forming four symmetrically related chains of pyroxene type. The structure of Li₈Bi₂(MoO₄)₇ involves structural fragments of Li₃Fe(MoO₄)₃ and a-RbPr(MoO₄)₂ and is a new structural type in the class of binary molybdates and tungstates of uniand trivalent metals.     

The LiF-LiCl-Li2SO4-Li2MoO4 quaternary system

Phase equilibria in the LiF-LiCl-Li₂SO₄-Li₂MoO₄ quaternary system have been investigated by differential thermal analysis. The eutectic composition (in mol %) has been determined as LiF, 16.2; LiCl, 51.5; Li₂SO₄, 16.2; and Li₂MoO₄, 16.2. The melting point of the eutectic is 402°C, and the enthalpy of melting is 291 J/g.     

LiF-LiCl-LiBr-Li2MoO4 quaternary system

Phase equilibria in the LiF-LiCl-LiBr-Li₂MoO₄ quaternary system were studied by differential thermal analysis. The composition of an eutectic was determined to be 20.0 mol % LiF, 27.3 mol % LiCl, 43.1 mol % LiBr, and 9.6 mol % Li₂MoO₄, with the melting point being 424°C.     

Cs3LiZn2(WO4)4 and Rb3Li2Ga(MoO4)4: different filled derivatives of the cation‐deficient Cs6Zn5(MoO4)8 structure

Two new compounds, namely cubic tricaesium lithium dizinc tetrakis(tetraoxotungstate), Cs₃LiZn₂(WO₄)₄, and tetragonal trirubidium dilithium gallium tetrakis(tetraoxomolybdate), Rb₃Li₂Ga(MoO₄)₄, belong to the structural family of Cs₆Zn₅(MoO₄)₈ (space group I 3d , Z = 4), with a partially incomplete (Zn_5/6□_1/6) position. In Cs₃LiZn₂(WO₄)₄, this position is fully statistically occupied by (Zn_2/3Li_1/3), and in Rb₃Li₂Ga(MoO₄)₄, the 2Li + Ga atoms are completely ordered in two distinct sites of the space group I 2d (Z = 4). In the same way, the crystallographically equivalent A ⁺ cations (A = Cs, Rb) in Cs₆Zn₅(MoO₄)₈, Cs₃LiZn₂(WO₄)₄ and isostructural A ₃LiZn₂(MoO₄)₄ and Cs₃LiCo₂(MoO₄)₄ are divided into two sites in Rb₃Li₂Ga(MoO₄)₄, as in other isostructural A ₃Li₂R (MoO₄)₄ compounds (AR = TlAl, RbAl, CsAl, CsGa, CsFe). In the title structures, the WO₄ and (Zn,Li)O₄ or LiO₄, GaO₄ and MoO₄ tetrahedra share corners to form open three‐dimensional frameworks with the caesium or rubidium ions occupying cuboctahedral cavities. The tetrahedral frameworks are related to that of mayenite 12CaO·7Al₂O₃ and isotypic compounds. Comparison of isostructural Cs₃M Zn₂(MoO₄)₄ (M = Li, Na, Ag) and Cs₆Zn₅(MoO₄)₈ shows a decrease of the cubic lattice parameter and an increase in thermal stability with the filling of the vacancies by Li⁺ in the Zn position of the Cs₆Zn₅(MoO₄)₈ structure, while filling of the cation vacancies by larger Na⁺ or Ag⁺ ions plays a destabilizing role. The series A ₃Li₂R (MoO₄)₄ shows second harmonic generation effects compatible with that of β′‐Gd₂(MoO₄)₃ and may be considered as nonlinear optical materials with a modest nonlinearity.     

LiF-LiVO3-Li2SO4-Li2MoO4 four-component system

The LiF-LiVO₃-Li₂SO₄-Li₂MoO₄ four-component system was studied using differential thermal analysis. The eutectic composition was determined (mol %): LiF, 25.0; LiVO₃, 43.8; Li₂SO₄, 14.8; Li₂MoO₄, 16.5. The eutectic melting point is 428°C; the enthalpy of melting is 260 J/g.     

Calorimetric study of high-pressure polymorphs of Li2WO4 and Li2MoO4

Heats of transition among the Li₂WO₄ polymorphs, Li₂WO₄I (phenacite-type structure), Li₂WO₄II, Li₂ WO₄III, and Li₂WO₄IV, and that between Li₂MoO₄ (phenacite) and Li₂MoO₄(spinel) were measured by transposed temperature drop calorimetry. The heats of fusion of Li₂WO₄I and Li₂MoO₄(ph) were also obtained. Using these data, the phase boundaries among the polymorphs of Li₂WO₄ and of Li₂MoO₄ were calculated. The calculated phase diagrams were compared with those reported previously. They agree well for Li₂WO₄ but show significant discrepancies, perhaps related to problems in attaining equilibrium at lower temperature, for Li₂MoO₄.     

LiF-LiBr-LiVO3-Li2MoO4-Li2SO4 quinary system

Phase equilibria in the LiF-LiBr-LiVO₃-Li₂MoO₄-Li₂SO₄ quinary system were studied by differential thermal analysis. A eutectic composition was determined to be 4.0 mol % LiF, 38.4 mol % LiBr, 30.8 mol % LiVO₃, 19.2 mol % Li₂MoO₄, and 7.6 mol % Li₂SO₄ with a melting point of 372°C and an enthalpy of melting of 164 ± 7 kJ/kg.     

Electronic structures and X-ray photoelectron spectra of MoO2 and Li2 MoO4

Electronic structures of MoO₂ (4d²) and molybdatc (4d^o) are calculated by the discrete-variational Xα method employing [Mo₂O₁₀^12? and [MoO₄]^2? clusters. The calculations indicate that the Mo—O bond is more covalent in the molybdatc than in MoO₂. Level structures for the valence band region arc in agreement with XPS spectra of MoO₂ and Li₂MoO₄.     

Phase formation in the Rb2MoO4-Er2(MoO4)3-Hf(MoO4)2 system and the crystal structure of new triple molybdate Rb5ErHf(MoO4)6

The subsolidus region of the Rb₂MoO₄-Er₂(MoO₄)₃-Hf(MoO₄)₂ ternary salt system is studied using X-ray powder diffraction. A novel 5: 1: 2 triple molybdate, Rb₅ErHf(MoO₄)₆, is found to form in the system. Crystals of Rb₅ErHf(MoO₄)₆ are flux-grown under spontaneous nucleation conditions. The composition and crystal structure of Rb₅ErHf(MoO₄)₆ are refined in a single-crystal X-ray diffraction experiment (X8 APEX diffractometer, MoK _α radiation, 1753 reflections, R = 0.0183). The crystals are trigonal; a = 10.7511(1) Å, c = 38.6543(7) Å, V = 3869.31(9) ų, d _calc = 4.462 g/cm³, Z = 6, space group $R\bar 3c$ . The mixed three-dimensional framework of the structure is formed of MoO₄ tetrahedra, each sharing corners with two ErO₆ and HfO₆ octahedra. Two types of Rb atoms occupy large cavities of the framework. The distribution of the Er³⁺ and Hf⁴⁺ cation over two positions is refined in the course of structure solution.     

New NASICON type oxyanion high capacity anode, Li2Co2(MoO4)3, for lithium-ion batteries: preliminary studies

We describe in this paper the lithium insertion/extraction behavior of a new NASICON type Li₂Co₂(MoO₄)₃ at a low potential and explored the possibility of considering this new oxyanion material as anode for lithium-ion batteries for the first time. Li₂Co₂(MoO₄)₃ was synthesized by a soft-combustion glycine-nitrate low temperature protocol. Test cells were assembled using composite Li₂Co₂(MoO₄)₃ as the negative electrode material and a thin lithium foil as the positive electrode material separated by a microporous polypropylene (Celgard® membrane) soaked in aprotic organic electrolyte (1 M LiPF₆ in EC/DMC). Electrochemical discharge down to 0.001 V from OCV (～3.5 V) revealed that about 35 Li⁺ could ^possibly be inserted into Li₂Co₂(MoO₄)₃ during the first discharge (reduction) corresponding to a specific capacity amounting to 1,500 mAh g^?1. This is roughly fourfold higher compared to that of frequently used graphite electrodes. However, about 24 Li⁺ could be extracted during the first charge. It is interesting to note that the same amount of Li⁺ could be inserted during the second Li⁺ insertion process (second cycle discharge) giving rise to a second discharge capacity of 1,070 mAh g^?1. It was also observed that a major portion of lithium intake occurs below 1.0 V vs Li/Li⁺, which is typical of anodes being used in lithium-ion batteries.