•

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

6-1

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



234,235,238 U



Ratios of isotopes established



234: 0.005±0.001

 

235: 0.720±0.001

238: 99.275±0.002

233 U from 232 Th

6-2

• •

Uranium Minerals

200 minerals contain uranium



Bulk are U(VI) minerals



U(IV) as oxides, phosphates, silicates

  

polyhedra Mineral deposits based on major anion Secondary phases may be important for waste forms



Incorporation of higher Pyrochlore



A 1-2 B 2 O 6 X 0-1



A=Na, Ca, Mn, Fe 2+ , Sr,Sb,

 

B= Ti, Nb, Ta U(V) may be present when synthesized under reducing conditions

*

XANES spectroscopy

*

Goes to B site

6-3

• • •

Polyhedra classification U(VI) minerals

Linkage over equatorial position



Bipyramidal polyhedra



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)

6-4

•

Uranium purification from ores

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

•

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 MnO 2 , chlorate, O 2 , 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 O 2

*

as oxidant Bicarbonate prevents precipitation of Na 2 U 2 O 7

6-5

OH +HCO 3 -



CO 3 2 + H 2 O

• • • •

Recovery of uranium from solutions

Ion exchange



U(VI) anions in sulfate and carbonate solution



UO 2 (CO 3 ) 3 4-

 

UO 2 (SO 4 ) 3 4 Solvent extraction



Continuous process



Load onto anion exchange,



carbonate solutions Extraction with alkyl secondary and tertiary



Chemistry similar to ion exchange conditions Chemical precipitation



Older method



Addition of base



Peroxide

*

Ultimate (NH 4 ) 2 U 2 O 7 diuranate), then heating to form 3 8 Contaminates depend upon mineral



V, Mo 3

•

TBP extraction

 

Based on formation of nitrate species UO 2 (NO 3 ) x 2-x + (2-x)NO



UO 2 (NO 3 ) 2 (TBP) 2 3 + 2TBP

6-6

• •

Uranium atomic properties

Ground state electron configuration



[Rn]5f 3 6d 1 7s 2 Term symbol



5 L 6

6-7

cm -1 6-8

6-9

Metallic Uranium

• • •

Three different phase

 a, b, g

phases



Dominate at different temperatures Uranium is strongly electropositive



Cannot be prepared through H 2 reduction Metallic uranium preparation



UF 4 or UCl 4 with Ca or Mg

 

UO 2 with Ca Electrodeposition from molten salt baths

6-10

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

 a ‐phase U-U distances in layer (2.80±0.05) Å and between layers

Gamma phase soft, difficult fabrication

3.26 Å 

Beta phase brittle and hard Paramagnetic Temperature dependence of resistivity

6-11 b -phase

Resistivity–temperature curve for a -U along the [010] axis 6-12

•

Intermetallic compounds

Wide range of intermetallic compounds and solid solutions in alpha and beta uranium



Hard and brittle transition metal compounds



U 6 X, X=Mn, Fe, Co, Ni



Noble metal compounds



Ru, Rh, Pd

*

Of interests for reprocessing



Solid solutions with:



Mo, Ti, Zr, Nb, and Pu

6-13

Uranium-Titanium Phase Diagram.

Uranium-Aluminum Phase Diagram.

6-14

• • • • •

Chemical properties of uranium metal and alloys

Reacts with most elements on periodic table



Corrosion by O 2 , air, 2 Dissolves in HCl



Also forms hydrated UO 2 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

6-15

•

Uranium compounds

Uranium-hydrogen

 b

-UH 3 from



H 2 at 250 ºC

a

-UH 3 prepared at 80 ºC from H 2 at 250

6-16

• • •

Uranium hydride compounds

Uranium borohydride UF 2Al(BH 2Al(BH

 

+ 4 4 ) 3



2 U(BH 4 ) 4 + U(BH) is tetragonal



U(BH 4 ) 3 synthesis forms 4 ) 4 Vapor pressure



log p (mmHg) =13.354-4265T 1 UXAlH



y compounds UXAl absorbs hydrogen upon heating



X=Ni, Co, Mn

 

y = 2.5 to 2.74

TGA analysis evaluates hydrogenation

6-17

• •

Uranium-oxygen

UO

 

Solid UO unstable, NaCl structure From UO 2



heated with U metal Carbon promotes reaction, formation of UC UO 2

  

Reduction of UO 3



CO, C, CH 4 or U , or C 2 3 H O 5 8 with H 2 from 800 ºC to 1100 ºC OH can be used as reductants O 2 presence responsible for UO 2+x formation Large scale preparation

   

UO 4 , (NH 4 ) 2 U 2 O 7 , or (NH 4 ) 4 UO 2 (CO 3 ) 3 Calcination in air at 400-500 ºC H 2 at 650-800 ºC UO 2 has high surface area

6-18

• • •

Uranium-oxygen

U 4 O 9



UO 2

  

and U 3 O 8 5 UO 2 + U 3 O 8



2 U 4 O 9 Placed in evacuated ampoule Heated to 1000 ºC for 2 weeks

*

Three phases

 a-

U 4 O 9 up to 350 K

   b-

U 4 O 9 350 K to 850 K

g-

U 4 O 9 above 850 K Rearrangement of U 4+ and U 5+ forces disordering of O U 3 O 7

 

Prepared by oxidizing UO 2



below 160 ºC 30 % of the oxygens change locations to new positions during oxidation Three phases

 b

phase prepared by heating at 200 ºC U 2 O 5

   

High pressure synthesis, three phases

a

-phase

 

UO 2 and U 3 O 8 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

6-19

•

Uranium-oxygen

U 3 O 8

 

From oxidation of UO

b 

phase results from the heating of the

a  a

phase uranium coordinated to oxygen in pentagonal bipyrimid Slow cooling 2 in air at 800 ºC phase above

6-20

Uranium-oxygen

•

UO 3



Seven phases can be prepared

•

A phase (amorphous)



Heating in air at 400 ºC

*

UO 4 .

2H 2 O, UO 2 C 2 O 4 .

3H 2 O, or (HN 4 ) 4 UO 2 (CO 3 ) 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 UO 2 2+

 b

-phase



Ammonium diuranate or uranyl nitrate heated rapidly in air at 400-500 ºC

 g

-phase prepared under O 2 6-10 atmosphere at 400-500 ºC

6-21

Uranium-oxygen

• •

UO 3

 

hydrates 6 different hydrated UO 3 compounds UO 3 .

2H 2 O Anhydrous UO 25-70 ºC 3 exposed to water from

 

Heating resulting compound in air to 100 ºC forms

a

-UO 3 .

0.8 H 2 O

a

-UO 2 (OH) 2 [

a

UO



3 .

b

H 2 O] forms in hydrothermal experiments -UO 3 .

H 2 O also forms

6-22

6-23

• • •

Uranium-oxygen single crystals

UO 2



from the melt of UO 2 Arc melter used powder



Vapor deposition 2.0 ≤ U/O ≤ 2.375



Fluorite structure

•

Uranium oxides show range of structures



Some variation due to existence of UO 2 2+ in structure



Some layer structures UO 2

  

to UO 3 system Range of liquid and solid phases from O/U 1.2 to 3.5

Hypostoichiometric UO 2+x 2.2



Mixed with U 3 O 8 forms up to O/U at higher temperature Large range of species from O/U 2.2 to 2.6

6-24

•

UO

2 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 U 4+ localized electrons in crystal field

Heat Capacity

 

1000-1500 K



Thermal expansion induces anharmonic lattice vibration 1500-2670 K



Lattice and electronic defects

6-25

Oxygen potential

• • • •

Equilibrium oxygen partial pressure over uranium oxides



In 2 phase region of solid oxides



ΔG(O 2 )=RTln pO 2

*

Partial pressure related to O 2 Large increase above O/U = 2



Increase in ΔG(O 2 )



decreases with increasing ratio Increase ΔG(O 2 ) with increasing T Entropy essentially independent of temperature



ΔS(O 2 )= -dΔG(O 2 )/dT Enthalpy related to Gibbs and entropy through normal relationship



Large peak at UO very small 2+x , x is

6-26

6-27

• •

Vaporization of UO

2 Above and below the melting point Number of gaseous species observed



U, UO, UO 2 , UO 3 , O, and O 2



Use of mass spectrometer to determine partial pressure for each species

 

For hypostiochiometric UO UO 2 , partial pressure of UO increases to levels comparable to 2 O 2 increases dramatically at O/U above 2

6-28

Uranium-oxides: Oxygen diffusion

• • •

Vacancy based diffusion in hypostoichiometric UO 2



Based on diffusion into vacancy, vacancy concentration, migration enthalpy of vacancy



Enthalpy 52 kJ mol -1 For stiochiometric UO 2



diffusion temperature dependent Thermal oxygen vacancies at lower T



Interstitial oxygen at higher T



Equal around 1400 ºC For UO 2+x



diffusion dominated by interstitial oxygen Migration enthalpy 96 kJ mol -1

6-29

Uranium-oxide: Electrical conductivity

• •

UO

 

2 and UO 2+x Mobility of holes in lattice



0.0015 to 0.021 cm 2 V -1 s -1

*

Semiconductor around 1 cm 2 V -1 s -1 Holes move in oxide structure along with local distortion within lattice

 

Holes and electrons localized on individual atoms



Holes U 5+ and electrons form U 3+ From 500 to 1400 ºC for UO 2+x



Decrease in conductivity with decrease in x when x<0.1

U 3 O 8-z

 

Similar to UO 2+x Phase transition at 723 K results in change of temperature dependence

6-30

•

Uranium oxide chemical properties

Oxides dissolve in strong mineral acids



Valence does not change in HCl, H 2 SO 4 , and H 3 PO 4



Sintered pellets dissolve slowly in HNO 3



Rate increases with addition of NH 4 F, H 2 O 2 , or carbonates

*

H 2 O 2 reaction



UO 2 + at surface oxidized to UO 2 2+

6-31

Group 1 and 2 uranates

• • •

Wide series of compounds



M 2 U n O 3n+1 for M +



MU n O 3n+1 for M 2+



Other compounds known

*

M 4 + UO 5 , M 2 2+ UO 5 , M 3 2+ UO 6 , and M 2 2+ U 3 O 11 Crystal structures



Layered structures and UO 2 2+ in the crystals

 

Monouranates (n=1)



Layered planes, O atom coordinate to U on the plane

*

Some slight spacing around plane Ba and Mg UO 4



Deformed ochahedron

*

Secondary O bridges adjacent U atoms



Shared corners



Shared edges



M 4 UO

 

5 (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 U 3 O 8 or UO 3



Heat in air 500-1000 ºC



Lower temperature for Cs and Rb



Different phases of some compounds

6-32

Group 1 and 2 uranates

• • •

Physicochemical properties



Hydroscopic

  

Colored



Yellow to orange Heavier group 1 species volatile IR active

 

Asymmetric stretch of UO 600-900 cm -1

*

2 2+



Frequency varies based on other O coordinated to uranyl group Diamagnetic compounds



Can be examined by U NMR

*

Some weak paramagnetism observed



Covalency in uranyl group Uranates (V) and (IV)



MUO 3 (M=Li, Na, K, Rb)

  

M 3 UO 4 (M=Li, Na) MU 2 O 6 (M=Mg, Ca, Sr, Ba) MUO 3 (M=Ca, Sr, Ba), tetravalent U Synthesis



Pentavalent uranates



Tetravalent and hexavalent uranium species mixed in 1:1 ratio

*

Heated in evacuated sealed ampoule



UO 2 + Li 2 UO 4



2 LiUO 3

 

Hydrogen reduction of hexavalent uranates at elevated temperatures tetravalent uranates form

6-33

• • •

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



5f 1 from U 5+



O

*

h crystal field Some tetragonal distortions Non-stoichiometry

  

Removal of oxide



Non-stoichiometric dissolution of metal in UO 2



Formation of xNa Na x UO 3 (x≤0.14) 2 O from Na Oxygen non-stoichiometry



Na 2 U 2 O 7-x (x≤0.5) 2 U 2 O 7 forms Na 2-2x+ U 2 O 7-x

6-34

Transition metal uranates

• • •

Wide range of compounds Preparation method

 

heating oxides in air with UO



Changing stoichiometry can result in different compounds 3 or U 3 O 8

*

U/M = 3, MU 3 O 10 (M=Mn, Co, Ni, Cu, Zn) Uranyl nitrate as starting material



M x UO 4 Crystal structures

  

Metal nitrates, temperatures below 600 ºC Chain of edge sharing of oxygen Some influence of metal on uranyl oxygen bond length



Lanthanide oxides form solid solutions



Can form Ln 6 UO 12

6-35

• • •

Solid solutions with UO

2 Solid solutions formed with group 2 elements, lanthanides, actinides, and some transition elements (Mn, Zr, Nb, Cd)



Distribution of metals on UO 2 fluorite-type 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 M y U 1-y O 2+x



x is positive and negative

6-36

•

Solid solutions with UO

2 Lattice parameter change in



Changes nearly linearly with increase in y and x



M y U 1-y O 2+x



Evaluate by change of lattice parameter with change in y

*

δa/δy



a is lattice parameter

  

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

6-37

Solid solutions of UO

2

•

Tetravalent M y U 1-y O 2+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 O 2

*

1500 ºC partial pressure At 0.2 atm., y=0.383 at 700 ºC to y=0.068 at

6-38

Solid solutions of UO

2

• •

Tri and tetravalent M y U 1-y O 2+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)O 2 and (Ce,U)O 2-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 U 3 O 8



Some systematics in trends

*

For Nd, when y is between 0.3 and 0.5, x = 0.5-y

6-39

6-40

6-41

Solid solution UO

2

•

Oxygen potential



Zr solid solution



Lower than the UO

*

x=0.05, y=0.3



2+x system -270 kJ/mol for solid solution

  

-210 kJ/mol for UO 2+x Th solid solution



Increase in

D

G with increasing y



Compared to UO 2 Ce solid solution difference is small at y less than 0.1

 

Wide changes over y range due to different oxidation states Shape of the curve is similar to Pu system, but values differ

*

Higher

D

G for CeO 2-x compared to PuO 2-x

6-42

• •

Solid solution UO

2 Trivalent



Oxygen potential increases with increasing x



Inflection point



For lanthanides La has highest G due to larger ionic radius Divalent



Higher oxygen potential than trivalent system



Configuration change



Formation of pentavalent U



At low O of Mg 2 partial pressures cannot dissolve high levels

6-43

• • • • • •

Borides, carbides, silicides

UB 2 , UB 4 , UB 12 are known compounds Prepared by mixing elements at high temperature Other reactions



UCl 4 +2MgB 2



UB 4 2MgCl 2 UB and UB 4 Inert species + form in gas phase



Potential waste forms



UB 12 more inert Large amount of ternary systems



U 5 Mo 10 B 24 , UNi 4 B



Sheets with 6 and 8 member rings

A view down the c‐axis of the structure of UB 4 6-44

• • • • •

Uranium carbides

Three known phases



UC, UC 2 , and U 2 C 3 UC and UC higher temperature

 

2 are completely miscible at At lower temperatures limited Synthesized by mixture of elements at high temperature U 2 C 3 prepared by heating UC and UC 2 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



UC



2 Ternary carbides



Melting elements in carbon crucible

*

U 2 Al 3 C 4 reacts slowly in air With N 2 at 1100 °C to form UN

6-45

• • •

Uranium-silicon

Compounds



U 3 Si, U 3 3 Si 2 , USi, U 3 Si 5 , USi 1.88

, Complicated phase diagram



Number of low temperature points Forms ternary compounds with Al



U(Al, Si) 3

    

with Al Cu, Nb, and Ru ternary phases U 2 Nb URu 2 Si 2 3 Si 4 ferromagnetic Heavy fermion material

*

metallic materials mass enhancement



antiferromagnetic interaction between conduction moments (d- or f electron)

6-46

• •

N, P, As, Sb, and Bi uranium

Monopnictides



UN, UP, UAs



Cubic NaCl structure U-nitrides



UN, U 2 N 3 , UN 2



UN prepared by uranium metal with nitriding agents



N 2 , NH 3



Thermal decomposition of higher nitrides

*

Higher nitride unstable with respect to UN

 

Mixture of higher nitrides with uranium metal

*

Treat surface with HNO



3 and washed with organics Remove traces of oxides and carbides UN easily oxidized by air, unstable in water

6-47

6-48

6-49

P, As, Sb, Bi-uranium

• •

UX, U 3 X 4 , and UX 2



X=P, As, Sb, Bi



UX is cubic except

b

-UBi



U 3 X 4



UX 2 Preparation is body centered cubic is tetragonal

 

Synthesis from the elements in an autoclave



2U + P 4



2UP 2 Uranium hydride with phosphine or arsine



UH 3 +PH 3



UP+3H 2

6-50

• •

S, Se, Te-uranium

Uranium-sulfur



US, US 2 , U 2 S 3 , U 3 S 5



Preparation

*

Heating U metal or UH 2 3

* * *

Heating elements in Decomposition of higher under vacuum UCl 4 with Li 2 X



U 3



S 5 U mixed U valence 3+ and U 4+ Se and Te prepared as the sulfur complexes



UTe



2 U contains Te-Te valence states 3+ and Te 1-,2-

6-51

6-52

Uranium halides

• • •

Thoroughly studied uranium compound



Isotope separations



Molten salt systems and reactors



Preparation of uranium metal Tetravalent and hexavalent oxidation state compounds Covalent halide compounds have 5f electron interaction



Ionic property highest with higher U oxidation state and more electronegative halides



Exception UF 3 move covalent than UCl 3

6-53

Trivalent uranium halides

• • • • • •

Sensitive to oxidation Stability decreases with increasing atomic number of halide Hydroscopic Stable in deoxygenated solvents



Soluble in polar solvents Range of colors Synthesis



Oxygen free

 

Temperature 600 ºC Ta or Mo tubes to avoid reaction with Si

6-54

• •

Trivalent uranium halides

Electronic properties



5f 3

 

4 I 9/2 ground state configuration Crystal field analysis of low temperature compounds



Large range of compounds evaluated for free ion and crystal field parameters Absorption spectra for U 3+



Strong f-d bands

 *

Laporte rule halides examined Mixing of electrons from different quantum levels



First f-d transition at 23000 cm

* *

-1 for CsUCl 4 .

3H 2 O 5f 3



5f 2 6d 1 Shifted toward IR region for NH 4 UCl 4 .

4H 2 O by 5000 cm -1



27000 cm -1 =370 nm, 15000 cm -1 =666.7 nm

*

For substitution of U 3+ substitution with halides



Increase in covalence properties related to red shift in f d band

6-55

Trivalent uranium halides

• • • • •

Preparation of UF 3



Reduction of UF



With Al, place in graphite crucible and heat to 900 ºC 4 by Al metal



With UN or U 2 N 3 at 900 ºC Stable in air at room temperature Insoluble in water, dissolved in nitric-boric acid Structure is capped trigonal prism Hydrate species also forms, but oxidizes in air



U 3+ in 1 M HCl and precipitation with NH 4 F

6-56

• • • • • •

Trivalent uranium halides

UCl 3

 

Reaction of gaseous HCl with UH 3 at 350 ºC Reduction of UCl 4 with Zn or Al at 400 ºC

 

Thermal vacuum decomposition of NH 4 UCl 4 Disproportionates to U and UCl 4 at 837 ºC Olive green powder or dark-red crystals Soluble in polar organic solvents Easily oxidized Hexagonal symmetry Forms hexa- and heptahydrate



Water in inner coordination sphere



Heptahydrate built from separate [U and Cl -



ions 2 Cl 2 (H Uraniums connected over bridging Cl 2 O) 14 ] 4+



units A number of hydrated complexes prepared



MUCl 4

*

From U 3+ in 11 M HCl with MCl

*

Tri- and tetrahydrates show 5f 21500 cm -1 and 16000 cm -1 3



5f 2 6d 1

*

at Red shift indicates covalent character of water interaction



Bond lengths based on inner sphere complexes

6-57

• •

Trivalent uranium halides

UCl 3



with neutral ligands Ammonia adducts, UCl 3 .

7NH 3



From UCl 3 heated in ammonia

 

UBr 3

    

UCl 3 (THF) x Wide range of crown ether complexes



Prepared from ligand and UCl 4



Intense f-d transitions in visible

*

IR needed to identify



species



ligand coordination oxidized in air Prepared by reaction of UH Reduction of UBr 4 UBr 3 vessel 3 with by Zn at 600 ºC reacts with quartz at prepare in sealed Ta or Mo Hydroscopic and oxidizes more readily than UCl 3 Isostructural with UCl 3 of UBr 3 with oxygen free water



M 2



UBr 5 and M 3 UBr 6 Melting points are high and increase with M mass

6-58

Trivalent uranium halides

• • • • • •

UI 3

   

Prepared from I 2 UI 4 with Zn on U metal at 525 ºC Vacuum decomposition of UI 4 UH 3 with methyl iodide Hydroscopic and attacks glass Dissolves in aqueous solution, methanol, ethanol, acetic acid



Forms unstable U 3+ 5f 3



5f 2 6d 1 at 13400 cm -1



Shift from 23000 cm -1 for UF 3 Synthesis of neutral donor complexes with solvent, U metal and I 2 at 0 ºC Mixed oxide species prepared



UOX (X=Cl, Br, I)



Heating stoichiometric mixtures of UO 2 X 2 , UO 2 , and U or UX 4 , U 3 O 8 and U at 700 ºC for 24 hours

6-59

• • • • • • • •

Tetravalent uranium halides

UF



4 stable upon exposure to air Lattice energy responsible for enhanced stability over other tetravalent halides All expect UF 4



U 4+ soluble in polar solvents can be stabilized in solution Different structures for solids



UF 4 : square antiprism

 

UCl 4 : dodecahedron UBr 4 : pentagonal bipyramid Ground State electronic configuration 5f 2 ( 3 H 4 )



Compounds have 5f 2



5f 2 transitions



f-d transitions begin 40000 -50000 cm



Higher energies than U 3+ -1 (UV-region) Absorption data collected at low temperature for transition assignment Evidence of 5f Over 60 5f 2



1 5f 7p 2 1 for Cs 2 UBr 6 transitions identified



U 4+



doped in BaY 2 Cl 7 Absorption, excitation, luminescence spectra U 4+



Crystal field strength for U 4+ dominated by symmetry of central ion rather than ligand

*

Lower symmetry results in lower crystal field has strong anti-stokes emission

6-60

• •

Tetravalent uranium halides

Complexes with inversion symmetry (UCl 6 2 ) used to determine electronic transitions

 

Low temperature Evaluation of side bands Low temperature UF 4 absorbance identified 91 f



f transitions

6-61

Tetravalent uranium halides

• • • • •

UF



4 exploited in nuclear fuel production Conversion to UF 6



Based on chemical stability and insolubility in solution Formed by a number of reactions



Uranium oxides with HF (UO 2 , U 3 O 8 )

  

U 3 O 8 + 8 HF



2UO 2 F 2 + UF 4 + 4 H 2 O if no H 2 UO 3 with ammonia-hydrogen fluoride mixtures

*

UO 2 and heating with same compounds Can also be prepared by the reduction of UF 6 in system Dissolves in the presence of reagents that can form fluoride complexes



Fe 3+ , Al 3+ , boric acid Fitting of UF 4 spectra resulted in assignment of 69 crystal field levels Hydrates formed from aqueous fluoride solution



nH 2 O (0.5



n=2.5 most stable



Water completely removed at 550 ºC

6-62

• • •

Tetravalent uranium halides

Complex uranium fluorides



Metal fluoride uranium fused salts



Fuels and reactors

 

LiF-BeF MgUF 6 2 -UF 4 and NaF-BeF 2 -UF 4 and CaUF 6 metal production for uranium Produced in a number of reactions



Solid state reaction between metal fluorides in inert atmosphere

 

U oxides with metal fluorides or carbonates in HF or HF-O Reduction of UF 6 fluorides 2 with metal



Controlled decomposition of higher fluoro complexes



(NH 4 ) 4 UF 8 Structures of compounds known



UF 6 2 : octahedral

 

UF 7 3 : pentagonal bipyramid UF 8 4 : bicapped triangular prism



Some complexes differ

*

Chains tricapped trigonal prisms for

b

-K 2 UF 2

6-63

Tetravalent uranium halides

•

Uranium oxide- and nitride fluorides



Melting UO

2

(or other oxides) and UF



Mono- and dihydrate precipitates

4

 

Mixed oxidation states of U found

*

5+ and 6+

*

4+ and 5+ UN

1.33

and UF

4



Compounds between UNF and UN

0.9

F

1.2

)

6-64

• • • • •

Tetravalent uranium halides

Uranium tetrachloride



Starting material for a range of uranium compounds



Ease of preparation



Solubility in polar organic solvents

 

Synthesis



Chlorination of UO 2

 

Need reactive form of UO 2 Converts to U 3 O 8 in air at 600 ºC Isostructural with other actinide tetrachlorides



Tetragonal symmetry Range of complex chlorides



M 2 UCl 6 and MUCl 5

 

Monovalents include NR 4 , PR 3 H compounds Can be prepared from fused salts of UCl 4 with metal chlorides Chlorine atoms can be replaced



UCl 4 in non-aqueous media with decomposition reaction Species are paramagnetic



Temperature dependent up to 350 ºC Oxychloride species



From UO 2 in excess UCl 4 followed by sublimation



Dissolves in water and aqueous nitric acid



Isostructural with Th, Pa, and Np oxychloride

6-65

Tetravalent Uranium halides

• •

Uranium tetrabromide



Prepared from:



Oxides with bromine

   

Oxides or UOBr 3 UO 2 with CBr 4 and sulfur bromine mixture insoluble in non-polar organic solvents Soluble in polar solvents



HBr evolved in ethanol, methanol, phenol, acetic acid, or moist air

  

Absorption bands 5f 2



5f 1 6d 1 Charge transfer at 30165 cm -1 at 41400-32160 cm Forms compounds with numerous ligands -1

 

Pentagonal bipyramid around U M 2 UBr 6

 *

with group 1 elements Can coordinate with organic cations Soluble in water, aqueous HBr, polar non-aqueous solvents

 

fcc crystals O

*

h from solution spectroscopy 5f 2



5f 1 6d 1 27400 to 39000 cm -1

*

Vibronic side bands

*

Hydrogen bonding can distort O h Oxybromides similar to oxychlorides to permit f



f

6-66

Tetravalent uranium halides

• • • •

UI 4

  

Prepared by direct combination of the elements at 500 ºC Used in preparation of UI 3 M 2 UI 6



from components in anhydrous methyl cyanide Hydroscopic compounds



Used to obtained spectroscopic terms for electronic transitions UOI 2 from heating U 3 O 8 , U, and I 2 UNI from UI 4 with ammonia Mixed halides

 

Range of compounds sealed at 450 ºC Higher fluoride species are more stable

 

UClF 3 >UCl 2 F 2 Mixed Cl-Br and Cl-I, Br-I

6-67

• • • • • •

Pentavalent uranium halides

Strong tendency to hydrolyze and disproportionate to tetra- and hexavalent species Preparation



UO 3 with thionyl chloride under reflux Decomposes in CCl 4 , CH 2 Cl 2 Varied coordination geometry



Octahedral (

a

-UF 5 )

 

Pentagonal bipyramid (

b

-UF 5 ) Edge-sharing octahedral (U 2 Cl 10 ) 5f 1 UF 5 electronic configuration: 4 F 5/2 ground state



Two phases, alpha over 150 ºC

 

Oxidation of UF



4 or reduction of UF 6 Oxidation with HF, noble gas fluorides



Reduction with HN 3 , SOCl 2 Water causes disproportionation

  

2UF 5 +3H 2 O



UF 4 +UO 2 F 2 +4HF Reduced to UF 4 by H 2 or Ni Stable in 50 % HF solution

6-68

• • • •

Pentavalent uranium halides

Structure

 a

-UF 5 chains of UF 6 octahedral bridged by trans-fluorides Complex compound preparation



Alkali halides in inert atmosphere at 300 ºC



Ammonia reaction

 

Metal halides reaction in HF Bonds covalent Oxide fluorides



UF 4 in intermittent O 2 creates U 2 OF 8



flow at 850 ºC Complex compounds also form UCl 5

    

Unstable through thermal decomposition Prepared by oxide treatment with CCl 4 at 80-250 ºC and UCl 5 catalyst or UO 3 with SiCl 4

a

-Cl 5 (monoclinic)from recrystallization from CCl 4

b

-Cl 5 (triclinic) by recrystallization of UCl 6 in CCl 4 or CH 2 Cl 2 Absorbance spectra same for both phases



Similar to UCl 6 -

6-69

Pentavalent uranium halides

• • • • •

Complex compounds



Range of compounds with ligands containing N, P, As, S, Se, and Te donor



Variety of MUCl



6 Group 1 and organic cations oxide species and complex



UOCl 3 from MoCl 5 at 200 ºC

 

UCl 4 and UO 2 Cl 2 at 370ºC UO 2 Cl 2 at 200 ºC



with WCl 5 , ReCl 5 Dissolves in anhydrous ethanol Pentabromide



Bromination of metal or UBr 4



Intermediate uranium halides



UF 4 with UF 6 UF 5



UOBr 3



from UO 3 fluctuates between C with CBr 4 4v and D 3h at 55 ºC UO 2 Br can also be prepared from thermal decomposition of UO 2 Br 2 Participation of 5f orbitals in bonding



5f, 6p, and 6d



Low population of 7s and 7p

6-70

• • • • • • • • •

Hexavalent uranium halides

Stability decreases with increasing halide mass No simple bromine or iodine forms React with water to form uranyl halides



Uranyl forms weak halides except with fluoride Soluble in polar organic solvents Generally yellow compounds



UF 6 colorless, UCl 6 green 5f 0 : 1 S 0 ground state Spectra of UO 2 2+



has vibrational fine structure Coupling with O=U=O stretching modes UF 6



has similar spectroscopic properties Superimposed on charge transfer bands centered near 26670 cm -1 and 38460 cm -1



Coupling resulting fine structure based on transitions t 1u (

s

+

p

) to empty 5f orbitals Compounds show weak, temperature dependent paramagentism

6-71

•

Hexavalent uranium halides UF

6



Readily volatile uranium compound



Isotope enrichment

6-72

• • • • • • • • • •

Orthorhombic colorless crystals Sublime at 56.5 ºC Liquid and gas O h symmetry Temperature independent paramagnetism Reactive and moisture sensitive Oxidizing agent



1 st nUF 6 +M



nUF 5 +MF n



bond dissociation at 134 kJ/mol Similar to F 2 (153.2 kJ/mol) Formation of M from UF 6



x UF (6+x) x=1,2 and MF Based on UF 6 electron Reduction from a number of reagents or alpha decay Some eutectic phase with BrF 2 , BrF 3 , BrF 5

UF

6

6-73

• •

UF

6

species

Tend to decompose to UF 6 when heated Oxide species



In liquid HF



3UF 6 +SiO 2



3UOF 4 +SiF 4

 

3UF 6 +B 2 O 3



3UOF 4 +2BF 3 Orange solid, non-volatile, decomposes at 200-250 ºC

 

UOF UO



2 F 4 2 at 250 ºC in vacuum decomposes to UF 6 and UO 2 F 2 also formed from UO 3 in gaseous HF at 300 ºC UO 2 F 2 ethanol yellow compound, slightly soluble in H 2 O, methanol and



Hydrated species from recrystallization in water

6-74

Hexavalent uranium halides

• • • • •

UCl 6

     

From thermal decomposition of UCl 5 Moisture sensitive Melts at 177 °C at 120-150 °C in vacuo Reacts with water to form uranyl Hexagonal symmetry Charge transfer bands around 21000 cm -1 UO 2 Cl 2

 

From the oxidation of UCl 4 Insoluble in non-polar solvents



A large number of different oxychloride compounds produced Oxybromide compounds

  

From the reaction of O UO 2 Br 2 2 with UBr 4 loses Br even at room temperature Hydrates and hydroxide species form Iodine compounds



Extremely unstable UO 2 I 2 reported



Number of moieties with organic Mixed halogen species



M 2 UO 2 Cl 2 Br 2



X 3 I (X=Cl or Br)

6-75

• •

Chemical bonding

Tri- and tetravalent U mainly related to organometallic compounds



Cp 3 UCO and Cp 3 UCO +



Cp=cyclopentadiene

*

5f CO

p

backbonding



Metal electrons to

p

of ligands

 *

Decreases upon oxidation to U(IV) Nitrogen containing ligand (terpyridyl)shows greater backbonding than Ce(III) Uranyl(V) and (VI) compounds



yl ions in aqueous systems unique for actinides



VO 2 + , MoO 2 2+ , WO 2 2+

*

Oxygen atoms are cis to maximize (p

p

)



M(d

p

)



Linear MO 2 2+

*

known for compounds of Tc, Re, Ru, Os Aquo structures unknown

 

Short U=O bond distance of 1.75 Å for hexavalent, longer for pentavalent



Smaller effective charge on pentavalent U Multiple bond characteristics, 1

s

and 2 with

p

characteristics

6-76

• •

Uranyl chemical bonding

Bonding molecular orbitals

 s

g 2

s

u 2

p

g 4

p

u 4



5f

 d

Order of HOMO is unclear

* p

g <

p

u <

s

g <<

s

u proposed



Gap for

s

based on 6p orbitals interactions and 5f

f

LUMO



Bonding orbitals O 2p characteristics

 

Non bonding, antibonding 5f and 6d Isoelectronic with UN 2 Pentavalent has electron in non-bonding orbital

6-77

6-78

• • • •

Uranyl chemical bonding

Linear yl oxygens from 5f characteristic



6d promotes cis geometry yl oxygens force formal charge on U below 6



Net charge 2.43 for UO



2 (H 2 O) 5 2+ Net negative 0.43 on oxygens , 3.2 for fluoride systems



Lewis bases

* * * *

Can vary with ligand in equatorial plane Responsible for cation-cation interaction O=U=O- - -M Pentavalent U yl oxygens more basic Small changes in U=O bond distance with variation in equatoral ligand Small changes in IR and Raman frequencies



Lower frequency for pentavalent U



Weaker bond

6-79

• • • • •

As all complexes, characterization based on coordination geometry, coordination number and bond distances Relate solid state to solution structure Large number of hexavalent uranium compounds from aqueous solutions O=U=O axis inert



Coordination around equatorial plane



4 to 6 coordinating ligands



Labile in solution Uranyl(VI) compounds



Common coordination geometry pentagonal bipyramid



Other coordination geometries



Distorted O h



Distorted pentagonal bipyramid

 

Hexagonal bipyramid

*

MUO 2 (NO 3 ) 3 , K 4 UO 2 (CO 3 ) 3 Square bipyrimid

*

Can occur in complexes with strong steric interference

Structure and coordination chemistry

6-80

U(VI) structure and coordination

• • •

UO 2 CO 3(s)



3 oxygens for each uranium



Will not be composed of a discrete complex



Oxygens shared by U forming layered structure Six coordination also forms with correct ligands Peroxide complexation in both solid and solution phase



Some self-assembling nano-clusters with peroxide

6-81

6-82

• •

U(III) structure and coordination

Expected to be similar to other trivalent actinides



U(III) does not form stable compounds



Actinides tend to form most stable complexes than lanthanides



No large differences in bond distances or coordination geometries



Any differences based on variation in ionic radius, larger for actinides U(III) complexes have high coordination numbers



8 or 9



Distorted trigonal prism



No structural determination of simple inorganic ligands in solution

6-83

• •

U(IV) and (V) structure and coordination

U(IV)



Normal and basic salts with inorganic ligands



Basic salts due to hydrolysis or oxide

 

Large ionic radius and 8 to 10 coordination



Similar to Ce(IV) Carbonates form trigonal bipyramid U(V)



Few examples of structures

  

Hexagonal bipyramid for triscarbonate



Similar to U(VI) species Labile ligands in equatorial plane Weaker complexes compared to U(VI)

6-84

Uranium organic ligands

• • • •

Same trends as observed with inorganic ligands Organic ligands have geometric constraints Structural information obtained from different methods



EXAFS

 

NMR Quantum calculations Coordination may be through limited functional groups



Carboxyl acids



Chelation

6-85

• •

Uranium solution chemistry

Uranyl(VI) most stable in solution



Uranyl(V) and U(IV) can also be in solution



U(V) prone to disproportionation



Stability based on pH and ligands



Redox rate is limited by change in species



Making or breaking yl oxygens

*

UO 2 2+ +4H + +2e -



U 4+ +2H 2 O yl oxygens have slow exchange



Half life 5E4 hr in 1 M HClO 4



Rate of exchange catalyzed by UV light

6-86

•

Uranium solution chemistry

Trivalent uranium

 

Dissolution of UCl 3 in water Reduction of U(IV) or (VI) at Hg cathode



Evaluated by color change

*

U(III) is green

 

Very few studies of U(III) in solution No structural information



Comparisons with trivalent actinides and lanthanides

6-87

• •

Uranium solution chemistry

Tetravalent uranium



Forms in very strong acid



Requires >0.5 M acid to prevent hydrolysis



Electrolysis of U(VI) solutions

*

Complexation can drive oxidation



Coordination studied by XAFS



Coordination number 9±1

*

Not well defined



U-O distance 2.42 Å



O exchange examined by NMR Pentavalent uranium



Extremely narrow range of existence

   

Prepared by reduction of UO 2 2+ in water with Zn or H 2 or dissolution of UCl 5 UV-irradiation of 0.5 M 2-propanol-0.2 M LiClO 4 with U(VI) between pH 1.7 and 2.7



U(V) is not stable but slowly oxidizes under suitable conditions No experimental information on structure Quantum mechanical predictions

6-88

• •

Hexavalent uranium solution chemistry

Large number of compounds prepared

 

Crystallization Hydrothermal Structure examined by XAFS

6-89

• • • • •

Aqueous solution complexes

Strong Lewis acid Hard electron acceptor



F >>Cl >Br -



I -



Same trend for O and N group



based on electrostatic force as dominant factor Hydrolysis behavior



U(IV)>U(VI)>>>U(III)>U(V) Uranium coordination with ligand can change protonation behavior



HOCH



2 COO pKa=17, 3.6 upon complexation of UO Inductive effect 2

*

Electron redistribution of coordinated ligand

 *

Exploited in synthetic chemistry U(III) and U(V)



No data in solution Base information on lanthanide or pentavalent actinides

6-90

• •

Uranium hydrolysis

Determination of constants from spectroscopic and titration



Determine if polymeric species form



U(OH) 4

 

Polynuclear species present expect at lowest concentration structure May form hydrated species no evidence of anionic species formation



i.e., U(OH) 5 (H 2 O) n-1 -



U

*

4 (OH) 16 6 coordination

6-91

Nanomole/L UO

2 2+

Micromole/L UO 2 2+ Millimole/L UO 2 2+ pH 6 U(VI) variation 6-92

• • •

Inorganic complexes

Strong fluoride complexes with U(IV) and U(VI) Oxygen ligand complexes increase with charge and base of the ligand



i.e., carbonate, phosphate, nitrate



Complexes with strong bases HSiO 4 3 and SiO 4 4 difficult to OH Complex structure from central U and ligand geometry



XAFS and neutron data

6-93

Uranium solution chemistry

• • •

Organic ligands and functional groups



Carboxylic acids



Additional amino or hydroxyl group Aliphatic nitrogen donors are strong bases



Competition with proton prevents coordination with U below pH 6 Ternary uranium complexes

 

Addition of OH to complex



U x L y (OH) z Evaluate based on L and OH steric constraints



complexation with U and Most ternary complexes contain OH and F -

6-94

• • • • •

Ligand substitution reactions

Most data with U focuses on rate of reaction



Mechanism of reaction are speculative



Describes molecular details of a reaction Data available



Non-aqueous solvents



Redox



Multidentate ligands Enthalpy and entropy terms evaluated Methods



Stop-flow



NMR



Protons, 13 C, 17 O, 19 F

 *

i.e., water change followed by 17 O Water reactions



Fast outer sphere going to rate determining inner sphere (k 2 )

 

Overall rate can determined from k 2



K obs and equilibrium constant Associative, Dissociative, Interchange Water exchange smaller with complexes



UO 2 (oxalate)F(H 2 O) 2 -

*

2E3 s -1 compared to 1.3E6 s -1

6-95

• •

Experimental ΔH=26 kJ mol -1 Calculated



74 (D), 19 (A), 21 (I)



Base on similarity between experimental and calculated

6-96

6-97

6-98

•

Ligand substitution reactions

NMR data for coordination



3 different fluoride ligands

6-99

• •

Uranium chemistry in solution

U isotopic exchange



Exchange between oxidation states and phases



Isotopic purity for a given



species Separation and evaluation

*

Counting or mass spectroscopy U fluorescence



Excitation of uranyl



Different spectra and lifetime

 

Quantum yield impacted by solution chemistry



Quenching from heavy ions in solution



electron transfer Excited U state used in chemical

 

reactions No consensus on primary de excitation mechanism I/I o =

t/t

o



o is state without ligand, I is is lifetime



g and

s

u to empty f orbital

6-100

Organometallic and biochemistry

• • •

Uranocene Biochemistry

 

RNA and DNA interactions over phosphates



Photochemical oxidation polysaccharides over deprotonated OH Analytical chemistry



Separation and preconcentration



Titration

  

Electrochemical methods Nuclear techniques Spectrometric

 

Atomic absorption, AES, XRF Indicator dye

 

Fluorescence Mass spectrometry

6-101

Review

• • • • • • • • •

Understand trends in Uranium nuclear properties Range of techniques and methods for U purification Understand the atomic properties of uranium Techniques used in the preparation of uranium metallic state



Properties and phases of uranium metal Trends and commonalties in the synthesis of uranium compounds Uranium compounds of importance to the nuclear fuel cycle Structure and coordination chemistry of uranium compounds

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Roles of the electronic structure and oxidation state Solution chemistry

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Trends with oxidation state Methods for the concentration analysis of uranium

6-102

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Questions

What are the natural isotopes of uranium What are some methods for the purification of uranium ore How can one prepare the different phases of U metal Provide 5 reactions that use U metal as a starting reagent Describe the synthesis and properties of the uranium halides How is the O to U ratio for uranium oxides determined What are the trends in U solution chemistry What atomic orbitals form the molecular orbitals for UO 2 2+

6-103

Pop Quiz

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What low valent uranium compounds can be synthesized? Provide an example for the trivalent and tetravalent oxidation state. Describe some studies that can utilize these compounds.

6-104