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Lecture 6: Uranium Chemistry
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Lecture 6: Uranium Chemistry

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  1. Lecture 6: Uranium Chemistry • From: Chemistry of actinides • Nuclear properties • U purification • Free atom and ion property • Metallic state • Compounds • Chemical bonding • Structure and coordination chemistry • Solution chemistry • Organometallic and biochemistry • Analytical Chemistry

  2. Nuclear properties • Fission properties of uranium • Defined importance of element and future investigations • Identified by Hahn in 1937 • 200 MeV/fission • 2.5 neutrons • Natural isotopes • 234,235,238U • Ratios of isotopes established • 234: 0.005±0.001 • 235: 0.720±0.001 • 238: 99.275±0.002 • 233U from 232Th

  3. Uranium Minerals • 200 minerals contain uranium • Bulk are U(VI) minerals • U(IV) as oxides, phosphates, silicates • Classification based on polymerization of coordination polyhedra • Mineral deposits based on major anion • Secondary phases may be important for waste forms • Incorporation of higher actinides • Pyrochlore • A1-2B2O6X0-1 • A=Na, Ca, Mn, Fe2+, Sr,Sb, Cs, Ba, Ln, Bi, Th, U • B= Ti, Nb, Ta • U(V) may be present when synthesized under reducing conditions • XANES spectroscopy • Goes to B site

  4. Polyhedra classification U(VI) minerals • Linkage over equatorial position • Bipyramidalpolyhedra • Oxygens on uranyl forms peaks on pyramid • Different bond lengths for axial and equatorial O coordinated to U • Method for classification • Remove anions not bound by 2 cations, not equatorial anion on bipyramid • Associated cation removed • Connect anions to form polyhedra • Defines anion topology • Chains defined by shapes • P (pentagons), R (rhombs), H (hexagons), U (up arrowhead chain), D (down arrowhead chain)

  5. Uranium purification from ores • Leaching with acid or alkaline solutions • Acid solution methods • Addition of acid provides best results • Sulfuric or HCl (pH 1.5) • U(VI) soluble in sulfuric • Oxidizing conditions may be needed • MnO2 , chlorate, O2, chlorine • Generated in situ by bacteria • High pressure oxidation of sulfur, sulfides, and Fe(II) • sulfuric acid and Fe(III) • Carbonate leaching • Formation of soluble anionic carbonate species • Somewhat specific for uranium • Use of O2 as oxidant • Bicarbonate prevents precipitation of Na2U2O7 • OH-+HCO3-CO32- + H2O • Common steps • Preconcentration of ore • Based on density of ore • Leaching to extract uranium into aqueous phase • Calcination prior to leaching • Removal of carbonaceous or sulfur compounds • Destruction of hydrated species (clay minerals) • Removal or uranium from aqueous phase • Ion exchange • Solvent extraction • Precipitation

  6. Recovery of uranium from solutions • Ion exchange • U(VI) anions in sulfate and carbonate solution • UO2(CO3)34- • UO2(SO4)34- • Load onto anion exchange, elute with acid or NaCl • Solvent extraction • Continuous process • Not well suited for carbonate solutions • Extraction with alkyl phosphoric acid, secondary and tertiary alkylamines • Chemistry similar to ion exchange conditions • Chemical precipitation • Older method • Addition of base • Peroxide • Ultimate formation of (NH4)2U2O7 (ammonium diuranate), then heating to form U3O8 or UO3 • Contaminates depend upon mineral • V, Mo • TBP extraction • Based on formation of nitrate species • UO2(NO3)x2-x + (2-x)NO3- + 2TBP UO2(NO3)2(TBP)2

  7. Uranium atomic properties • Ground state electron configuration • [Rn]5f36d17s2 • Term symbol • 5L6

  8. cm-1

  9. Metallic Uranium • Three different phase • a, b, g phases • Dominate at different temperatures • Uranium is strongly electropositive • Cannot be prepared through H2 reduction • Metallic uranium preparation • UF4 or UCl4 with Ca or Mg • UO2 with Ca • Electrodeposition from molten salt baths

  10. Metallic Uranium phases • a-phase • Room temperature to 942 K • Orthorhombic • U-U distance 2.80 Å • Unique structure type • b-phase • Exists between 668 and 775 ºC • Tetragonal unit cell • g-phase • Formed above 775 ºC • bcc structure • Metal has plastic character • Gamma phase soft, difficult fabrication • Beta phase brittle and hard • Paramagnetic • Temperature dependence of resistivity a‐phase U-U distances in layer (2.80±0.05) Å and between layers 3.26 Å b-phase

  11. Resistivity–temperature curve for a-U along the [010] axis

  12. Intermetallic compounds • Wide range of intermetallic compounds and solid solutions in alpha and beta uranium • Hard and brittle transition metal compounds • U6X, X=Mn, Fe, Co, Ni • Noble metal compounds • Ru, Rh, Pd • Of interests for reprocessing • Solid solutions with: • Mo, Ti, Zr, Nb, and Pu

  13. Uranium-Titanium Phase Diagram. Uranium-Aluminum Phase Diagram.

  14. Chemical properties of uranium metal and alloys • Reacts with most elements on periodic table • Corrosion by O2, air, water vapor, CO, CO2 • Dissolves in HCl • Also forms hydrated UO2 during dissolution • Non-oxidizing acid results in slow dissolution • Sulfuric, phosphoric, HF • Exothermic reaction with powered U metal and nitric • Dissolves in base with addition of peroxide • peroxyuranates

  15. Uranium compounds • Uranium-hydrogen • b-UH3 from H2 at 250 ºC • a-UH3 prepared at -80 ºC from H2 at 250

  16. Uranium hydride compounds • Uranium borohydride • UF4 + 2Al(BH4)3U(BH4)4 + 2Al(BH4)F2 • U(BH)4 is tetragonal • U(BH4)3 forms during U(BH4)4 synthesis • Vapor pressure • log p (mmHg) =13.354-4265T-1 • UXAlHy compounds • UXAl absorbs hydrogen upon heating • X=Ni, Co, Mn • y = 2.5 to 2.74 • TGA analysis evaluates hydrogenation

  17. Uranium-oxygen • UO • Solid UO unstable, NaCl structure • From UO2 heated with U metal • Carbon promotes reaction, formation of UC • UO2 • Reduction of UO3 or U3O8 with H2 from 800 ºC to 1100 ºC • CO, C, CH4, or C2H5OH can be used as reductants • O2 presence responsible for UO2+x formation • Large scale preparation • UO4, (NH4)2U2O7, or (NH4)4UO2(CO3)3 • Calcination in air at 400-500 ºC • H2 at 650-800 ºC • UO2has high surface area

  18. Uranium-oxygen • U4O9 • UO2 and U3O8 • 5 UO2+ U3O82 U4O9 • Placed in evacuated ampoule • Heated to 1000 ºC for 2 weeks • Three phases • a-U4O9 up to 350 K • b-U4O9 350 K to 850 K • g-U4O9 above 850 K • Rearrangement of U4+ and U5+ forces disordering of O • U3O7 • Prepared by oxidizing UO2 below 160 ºC • 30 % of the oxygens change locations to new positions during oxidation • Three phases • b phase prepared by heating at 200 ºC • U2O5 • High pressure synthesis, three phases • a-phase • UO2 and U3O8 at 30 kbar and 400 ºC for 8 hours • Also prepared at 15 kbar and 500 ºC • b-phase forms at 40-50 kbar above 800 ºC • g-phase sometimes prepared above 800 ºC at 60 kbar

  19. Uranium-oxygen • U3O8 • From oxidation of UO2 in air at 800 ºC • a phase uranium coordinated to oxygen in pentagonal bipyrimid • b phase results from the heating of the a phase above 1350 ºC • Slow cooling

  20. Uranium-oxygen • UO3 • Seven phases can be prepared • A phase (amorphous) • Heating in air at 400 ºC • UO4.2H2O, UO2C2O4.3H2O, or (HN4)4UO2(CO3)3 • Prefer to use compounds without N or C • a-phase • Crystallization of A-phase at 485 ºC at 4 days • O-U-O-U-O chain with U surrounded by 6 O in a plane to the chain • Contains UO22+ • b-phase • Ammonium diuranate or uranyl nitrate heated rapidly in air at 400-500 ºC • g-phase prepared under O2 6-10 atmosphere at 400-500 ºC

  21. Uranium-oxygen • UO3 hydrates • 6 different hydrated UO3 compounds • UO3.2H2O • Anhydrous UO3 exposed to water from 25-70 ºC • Heating resulting compound in air to 100 ºC forms a-UO3.0.8 H2O • a-UO2(OH)2 [a-UO3.H2O] forms in hydrothermal experiments • b-UO3.H2O also forms

  22. Uranium-oxygen single crystals • UO2 to UO3system • Range of liquid and solid phases from O/U 1.2 to 3.5 • Hypostoichiometric UO2+x forms up to O/U 2.2 • Mixed with U3O8 at higher temperature • Large range of species from O/U 2.2 to 2.6 • UO2 from the melt of UO2 powder • Arc melter used • Vapor deposition • 2.0 ≤ U/O ≤ 2.375 • Fluorite structure • Uranium oxides show range of structures • Some variation due to existence of UO22+ in structure • Some layer structures

  23. UO2 Heat Capacity • High temperature heat capacity studied for nuclear fuel • Room temperature to 1000 K • Increase in heat capacity due to harmonic lattice vibrations • Small contribution to thermal excitation of U4+ localized electrons in crystal field • 1000-1500 K • Thermal expansion induces anharmonic lattice vibration • 1500-2670 K • Lattice and electronic defects

  24. Oxygen potential • Equilibrium oxygen partial pressure over uranium oxides • In 2 phase region of solid oxides • ΔG(O2)=RTln pO2 • Partial pressure related to O2 • Large increase above O/U = 2 • Increase in ΔG(O2) decreases with increasing ratio • Increase ΔG(O2) with increasing T • Entropy essentially independent of temperature • ΔS(O2)= -dΔG(O2)/dT • Enthalpy related to Gibbs and entropy through normal relationship • Large peak at UO2+x, x is very small

  25. Vaporization of UO2 • Above and below the melting point • Number of gaseous species observed • U, UO, UO2, UO3, O, and O2 • Use of mass spectrometer to determine partial pressure for each species • For hypostiochiometric UO2, partial pressure of UO increases to levels comparable to UO2 • O2 increases dramatically at O/U above 2

  26. Uranium-oxides: Oxygen diffusion • Vacancy based diffusion in hypostoichiometric UO2 • Based on diffusion into vacancy, vacancy concentration, migration enthalpy of vacancy • Enthalpy 52 kJ mol-1 • For stiochiometric UO2 diffusion temperature dependent • Thermal oxygen vacancies at lower T • Interstitial oxygen at higher T • Equal around 1400 ºC • For UO2+xdiffusion dominated by interstitial oxygen • Migration enthalpy 96 kJ mol-1

  27. Uranium-oxide: Electrical conductivity • UO2and UO2+x • Mobility of holes in lattice • 0.0015 to 0.021 cm2V-1s-1 • Semiconductor around 1 cm2V-1s-1 • Holes move in oxide structure along with local distortion within lattice • Holes and electrons localized on individual atoms • Holes U5+ and electrons form U3+ • From 500 to 1400 ºC for UO2+x • Decrease in conductivity with decrease in x when x<0.1 • U3O8-z • Similar to UO2+x • Phase transition at 723 K results in change of temperature dependence

  28. Uranium oxide chemical properties • Oxides dissolve in strong mineral acids • Valence does not change in HCl, H2SO4, and H3PO4 • Sintered pellets dissolve slowly in HNO3 • Rate increases with addition of NH4F, H2O2, or carbonates • H2O2 reaction • UO2+ at surface oxidized to UO22+

  29. Group 1 and 2 uranates • Wide series of compounds • M2UnO3n+1 for M+ • MUnO3n+1 for M2+ • Other compounds known • M4+UO5, M22+UO5, M32+UO6, and M22+U3O11 • Crystal structures • Layered structures and UO22+ in the crystals • Monouranates (n=1) • Layered planes, O atom coordinate to U on the plane • Some slight spacing around plane • Ba and Mg UO4 • Deformed ochahedron • Secondary O bridges adjacent U atoms • Shared corners • Shared edges • M4UO5 (M=Li, Na) • No uranyl group • 4 orthogonal planar U-O bonds • Preparation • Carbonates, nitrates or chlorides of group 1 or 2 elements mixed with U3O8 or UO3 • Heat in air 500-1000 ºC • Lower temperature for Cs and Rb • Different phases of some compounds

  30. Group 1 and 2 uranates • Physicochemical properties • Hydroscopic • Colored • Yellow to orange • Heavier group 1 species volatile • IR active • Asymmetric stretch of UO22+ • 600-900 cm-1 • Frequency varies based on other O coordinated to uranyl group • Diamagnetic compounds • Can be examined by U NMR • Some weak paramagnetism observed • Covalency in uranylgroup • Uranates (V) and (IV) • MUO3 (M=Li, Na, K, Rb) • M3UO4 (M=Li, Na) • MU2O6 (M=Mg, Ca, Sr, Ba) • MUO3 (M=Ca, Sr, Ba), tetravalent U • Synthesis • Pentavalenturanates • Tetravalent and hexavalent uranium species mixed in 1:1 ratio • Heated in evacuated sealed ampoule • UO2 + Li2UO42 LiUO3 • Hydrogen reduction of hexavalenturanates • at elevated temperatures tetravalent uranates form

  31. Group 1 and 2 uranates • Crystal structure • No uranyl present, lacks layered structure • Perovskite type structure is common • Physicochemical properties • Brown or black in color • Dissolves in mineral acids, nitric faster dissolution rates • Oxidize to hexavalent state when heated in air • Electronic spectra measured • Magnetic paramagnetic properties measured • 5f1 from U5+ • Oh crystal field • Some tetragonal distortions • Non-stoichiometry • Removal of oxide • Formation of xNa2O from Na2U2O7 forms Na2-2x+U2O7-x • Non-stoichiometric dissolution of metal in UO2 • NaxUO3(x≤0.14) • Oxygen non-stoichiometry • Na2U2O7-x (x≤0.5)

  32. Transition metal uranates • Wide range of compounds • Preparation method • heating oxides in air with UO3 or U3O8 • Changing stoichiometry can result in different compounds • U/M = 3, MU3O10(M=Mn, Co, Ni, Cu, Zn) • Uranyl nitrate as starting material • Metal nitrates, temperatures below 600 ºC • MxUO4 • Crystal structures • Chain of edge sharing of oxygen • Some influence of metal on uranyl oxygen bond length • Lanthanide oxides form solid solutions • Can form Ln6UO12

  33. Solid solutions with UO2 • Solid solutions formed with group 2 elements, lanthanides, actinides, and some transition elements (Mn, Zr, Nb, Cd) • Distribution of metals on UO2 fluorite-type cubic crystals based on stoichiometry • Prepared by heating oxide mixture under reducing conditions from 1000 ºC to 2000 ºC • Powders mixed by co-precipitation or mechanical mixing of powders • Written as MyU1-yO2+x • x is positive and negative

  34. Solid solutions with UO2 • Lattice parameter change in solid solution • Changes nearly linearly with increase in y and x • MyU1-yO2+x • Evaluate by change of lattice parameter with change in y • δa/δy • a is lattice parameter in Å • Can have both negative and positive values • δa/δy is large for metals with large ionic radii • δa/δx terms negative and between -0.11 to -0.3 • Varied if x is positive or negative

  35. Solid solutions of UO2 • Tetravalent MyU1-yO2+x • Zr solid solutions • Large range of systems • y=0.35 highest value • Metastable at lower temperature • Th solid solution • Continuous solid solutions for 0≤y≤1 and x=0 • For x>0, upper limit on solubility • y=0.45 at 1100 ºC to y=0.36 at 1500 ºC • Also has variation with O2 partial pressure • At 0.2 atm., y=0.383 at 700 ºC to y=0.068 at 1500 ºC

  36. Solid solutions of UO2 • Tri and tetravalent MyU1-yO2+x • Cerium solid solutions • Continuous for y=0 to y=1 • For x<0, solid solution restricted to y≤0.35 • Two phases (Ce,U)O2 and (Ce,U)O2-x • x<-0.04, y=0.1 to x<-0.24, y=0.7 • 0≤x≤0.18, solid solution y<0.5 • Air oxidized hyperstoichiometric • y 0.56 to 1 at 1100 ºC • y 0.26-1.0 1550 ºC • Tri and divalent • Reducing atmosphere • x is negative • fcc • Solid solution form when y is above 0 • Maximum values vary with metal ion • Oxidizing atmosphere • Solid solution can prevent formation of U3O8 • Some systematics in trends • For Nd, when y is between 0.3 and 0.5, x = 0.5-y

  37. Solid solution UO2 • Oxygen potential • Zr solid solution • Lower than the UO2+x system • x=0.05, y=0.3 • -270 kJ/mol for solid solution • -210 kJ/mol for UO2+x • Th solid solution • Increase in DG with increasing y • Compared to UO2 difference is small at y less than 0.1 • Ce solid solution • Wide changes over y range due to different oxidation states • Shape of the curve is similar to Pu system, but values differ • Higher DG for CeO2-x compared to PuO2-x

  38. Solid solution UO2 • Trivalent • Oxygen potential increases with increasing x • Inflection point at x=0 • For lanthanides La has highest DG due to larger ionic radius • Divalent • Higher oxygen potential than trivalent system • Configuration change • Formation of pentavalent U • At low O2 partial pressures cannot dissolve high levels of Mg

  39. Borides, carbides, silicides • UB2, UB4, UB12are known compounds • Prepared by mixing elements at high temperature • Other reactions • UCl4+2MgB2UB4 + 2MgCl2 • UB and UB4 form in gas phase • Inert species • Potential waste forms • UB12 more inert • Large amount of ternary systems • U5Mo10B24, UNi4B • Sheets with 6 and 8 member rings A view down the c‐axis of the structure of UB4

  40. Uranium carbides • Three known phases • UC, UC2, and U2C3 • UC and UC2 are completely miscible at higher temperature • At lower temperatures limited • Synthesized by mixture of elements at high temperature • U2C3 prepared by heating UC and UC2 in vacuo from 1250-1800 °C • Once formed stable at room temperature • Alkanes produced by arc-melting • Oxalic acid produced by carbide dissolution in nitric acid • Ternary carbides • Melting elements in carbon crucible • U2Al3C4 • UC2 reacts slowly in air • With N2 at 1100 °C to form UN

  41. Uranium-silicon • Compounds • U3Si, U3Si2, USi, U3Si5, USi1.88, and USi3 • Complicated phase diagram • Number of low temperature points • Forms ternary compounds with Al • U(Al, Si)3 • Formed in U in contact with Al • Cu, Nb, and Ru ternary phases • U2Nb3Si4 ferromagnetic below 35 K • URu2Si2 • Heavy fermion material • metallic materials having large electronic mass enhancement • antiferromagnetic interaction between conduction electrons and local magnetic moments (d- or f-electron)

  42. N, P, As, Sb, and Bi uranium • Monopnictides • UN, UP, UAs • Cubic NaCl structure • U-nitrides • UN, U2N3, UN2 • UN prepared by uranium metal with nitriding agents • N2, NH3 • Thermal decomposition of higher nitrides • Higher nitride unstable with respect to UN • Mixture of higher nitrides with uranium metal • Treat surface with HNO3 and washed with organics • Remove traces of oxides and carbides • UN easily oxidized by air, unstable in water

  43. P, As, Sb, Bi-uranium • UX, U3X4, and UX2 • X=P, As, Sb, Bi • UX is cubic except b-UBi • U3X4 is body centered cubic • UX2 is tetragonal • Preparation • Synthesis from the elements in an autoclave • 2U + P42UP2 • Uranium hydride with phosphine or arsine • UH3+PH3UP+3H2