15-1

RFSS: Lecture 15 Americium and Curium Chemistry

• Readings: Am and Cm chemistry chapters§ Nuclear properties§ Production of isotopes§ Separation and

purification § Metallic state§ Compounds § Solution chemistry§ Coordination chemistry

15-2

Production of Am isotopes• Am produced in reactors from neutron irradiation of Pu

§ 239Pu to 240Pu to 241Pu, then beta decay of 241Pu• 241,243Am main isotopes of interest

§ Long half-lives§ Produced in kilogram quantity§ Chemical studies§ Both isotopes produced in reactor

• 241Am § source for low energy gamma and alpha

à Alpha energy 5.44 MeV and 5.49 MeV§ Smoke detectors § Neutron sources

à (a,n) on Be§ Thickness gauging and density§ 242Cm production from thermal neutron capture

• 243Am§ Irradiation of 242Pu, beta decay of 243Pu

• Critical mass§ 242Am in solution

à 23 g at 5 g/Là Requires isotopic separation

15-3

Am solution chemistry• Oxidation states III-VI in solution

§ Am(III,V) stable in dilute acid§ Am(V, VI) form dioxo cations

• Am(II)§ Unstable, unlike some lanthanides (Yb, Eu, Sm)

à Formed from pulse radiolysis* Absorbance at 313 nm* T1/2 of oxidation state 5E-6 seconds

• Am(III)§ Easy to prepare (metal dissolved in acid, AmO2

dissolution)à Pink in mineral acids, yellow in HClO4 when Am is

0.1 M§ 7F05L6 at 503.2 nm (e=410 L mol cm-1)§ Shifts in band position and molar absorbance indicates

changes in water or ligand coordination§ 9 to 11 inner sphere waters

à Based on fluorescence spectroscopy* Lifetime related to coordination

Ø nH2O=(x/t)-yØ x=2.56E-7 s, y=1.43Ø Measurement of fluorescence lifetime

in H2O and D2O• Am(IV)

§ Requires complexation to stabilizeà dissolving Am(OH)4 in NH4Fà Phosphoric or pyrophosphate (P2O7

4-) solution with anodic oxidation

à Ag3PO4 and (NH4)4S2O8

à Carbonate solution with electrolytic oxidation

15-4

Am solution chemistry• Am(V)

§ Oxidation of Am(III) in near neutral solutionà Ozone, hypochlorate (ClO-),

peroxydisulfateà Reduction of Am(VI) with bromide

§ 5I43G5; 513.7 nm; 45 L mol cm-1

§ 5I43I7; 716.7 nm; 60 L mol cm-1

• Am(VI)§ Oxidation of Am(III) with S2O8

2- or Ag2+ in dilute non-reducing acid (i.e., sulfuric)

§ Ce(IV) oxidizes IV to VI, but not III to VI completely

§ 2 M carbonate and ozone or oxidation at 1.3 V§ 996 nm; 100 L mol cm-1

à Smaller absorbance at 666 nm• Am(VII)

§ 3-4 M NaOH, mM Am(VI) near 0 °C§ Gamma irradiation 3 M NaOH with N2O or

S2O82- saturated solution

• Am(VII)§ Broad absorbance at 740 nm

15-5

Am solution chemistry• Am(III) luminescence

§ 7F05L6 at 503 nmà Then conversion to

other excited state§ Emission to 7FJ

§ 5D17F1 at 685 nm§ 5D17F2 at 836 nm§ Lifetime for aquo ion is 20

nsà 155 ns in D2O

§ Emission and lifetime changes with speciationà Am triscarbonate

lifetime = 34.5 ns, emission at 693 nm

• Autoreduction • Formation of H2O2 and HO2 radicals from

radiation reduces Am to trivalent states§ Difference between 241Am and 243Am

• Rate decreases with increase acid for perchloric and sulfuric

• Some disagreement role of Am concentration§ Concentration of Am total or

oxidation state• Rates of reduction dependent upon

§ Acid, acid concentration, § mechanism

à Am(VI) to Am(III) can go stepwise

§ starting ion à Am(V) slower than Am(VI)

15-6

Am solution chemistry• Disproportionation

§ Am(IV) à In nitric and perchloric acidà Second order with Am(IV)

* 2 Am(IV)Am(III) + Am(V)* Am(IV) + Am(V)Am(III) + Am(VI)

Ø Am(VI) increases with sulfate§ Am(V)

à 3-8 M HClO4 and HCl* 3 Am(V) + 4 H+Am(III)+2Am(VI)+2

H2Oà Solution can impact oxidation state stability

• Redox kinetics§ Am(III) oxidation by peroxydisulfate

à Oxidation due to thermal decomposition products

* SO4.-, HS2O8

-

à Oxidation to Am(VI)à Acid above 0.3 M limits oxidation

* Decomposition of S2O82-

à Induction period followed by reductionà Rates dependent upon temperature,

[HNO3], [S2O82-], and [Ag+2]

à In carbonate proceeds through Am(V)* Rate to Am(V) is proportional to

oxidant* Am(V) to Am(VI)

Ø Proportional to total Am and oxidant

Ø Inversely proportional to K2CO3

15-7

Am solution chemistry: Redox kinetics• Am(VI) reduction

§ H2O2 in perchlorate is 1st order for peroxide and Amà 2 AmO2

2++H2O22 AmO2+ + 2 H++ O2

§ NpO2+

à 1st order with Am(VI) and Np(V)* k=2.45E4 L / mol s

§ Oxalic acid reduces to equal molar Am(III) and Am(V)• Am(V) reduction

§ Reduced to Am(III) in NaOH solutionsà Slow reduction with dithionite (Na2S2O4), sulfite (SO3

2-), or thiourea dioxide ((NH2)2CSO2)

§ Np(IV) and Np(V)à In both acidic and carbonate conditions

* For Np(IV) reaction products either Np(V) or Np(VI)Ø Depends upon initial relative concentration of

Am and Npà U(IV) examined in carbonate

15-8

Am solution chemistry• Radiolysis

§ From alpha decayà 1 mg 241Am release 7E14 eV/s

§ Reduction of higher valent Am related to dose and electrolyte concentration

§ In nitric acid formation of HNO2

§ In perchlorate numerous species produced

à Cl2, ClO2, or Cl-

• Complexation chemistry§ Primarily for Am(III)

à F->H2PO4->SCN->NO3

->Cl->ClO4-

§ Hard acid reactionsà Electrostatic interactions

* Inner sphere and outer sphereØ Outer sphere for weaker

ligands§ Stabilities similar to trivalent

lanthanidesà Some enhanced stability due to

participation of 5f electron in bonding

15-9

Am solution chemistry

• Hydrolysis§ Mono-, di-, and trihydroxide species§ Am(V) appears to have 2 species,

mono- and dihydroxide§ Am hydrolysis (from CHESS

database)à Am3++H2OAmOH2++H+: log K

=-6.402à Am3++2H2OAm(OH)2

++2H+: log K =-14.11

à Am3++3H2OAm(OH)3+3H+: log K =-25.72

• Carbonate§ Evaluated by spectroscopy§ Includes mixed species

à Am hydroxide carbonate speciesà Based on solid phase analysis

§ Am(IV)à Pentacarbonate studied (log

b=39.3)§ Am(V) solubility examined

1mM Am3+;

1 mM Am, 1 mM carbonate

15-10

Am solution chemistry: Organics• Number of complexes examined

§ Mainly for Am(III)• Generally stability of complex

increases with coordination sites• With aminopolycarboxylic acids,

complexation constant increases with ligand coordination

• Natural organic acid§ Number of measurements

conducted§ Measured by spectroscopy

and ion exchange• TPEN (N,N,N’,N’-tetrakis(2-

pyridylmethyl)ethyleneamine)§ 0.1 M NaClO4, complexation

constant for Am 2 orders greater than Sm

15-11

Am solvent extraction

• Tributylphosphate (TBP)§ Am extracted from neutral or low acid solutions with high nitrate § Am(VI)

à Oxidation with (NH4)10P2W17O61 to stabilize Am(VI)à 100 % TBP from 1 M HNO3

* Separation factor 50 from Nd§ Am separation from lanthanides

à 1 M ammonium thiocyanate aqueous phase• Dibutyl butylphosphonate (DBBP)

§ Phosphonate functional group§ Similar to TBP, stronger extractant of Am

• Trialkylphophine oxide (TRPO)§ Increase in basicity of P=O functional group from TBP to DPPB to TRPO§ Am and Cm extraction from 1-2 M HNO3

§ 30 % TRPO in kerosene à Am, Cm, tetravalent Np and Pu, hexavalent U extracted

* Actinides stripped with 5.5 M HNO3 (Am fraction)à TRPO with C6-C8 alkyl group

15-12

HDEHP

Am solvent extraction• Bis(2-ethylhexyl)phosphoric acid (HDEHP)

§ Has been used to Am separation§ Part of TALSPEAK

à Extracts lanthanides stronger that actinidesà TALSPEAK components

* Bis(2-ethyl-hexyl)phosphoric acid (HDEHP)* HNO3

* DTPA* Lactic acid

• Carbamoylphosphine oxide (CMPO)§ Synthesized by Horwitz

à Based on DHDECMP extractions* Recognized functional group, simplified ligand synthesis* Purified by cation exchange

§ Part of TRUEXà TRUEX (fission products)

* 0.01 to 7 M HNO3

* 1.4 M TBP * 0.2 M Diphenyl-N,N-dibutylcarbamoyl phosphine oxide (CMPO)* 0.5 M Oxalic acid* 1.5 M Lactic acid * 0.05 M DTPA CMPO

15-13

Am solvent extraction

• Tertiary amine salt§ Low acid, high nitrate or chloride solution

à (R3NH)2Am(NO3)5

• Quaternary ammonium salts (Aliquat 336)§ Low acid, high salt solutions

à Extraction sequence of Cm<Cf<Am<Es§ Studies at ANL for process separation of Am

• Amide extractants§ (R1,R2)N-C(O)-CR3H-C(O)-N(R1R2)

à Diamide extractantà Basis of DIAMEX process

§ N,N’-dimethyl-N,N’-dibutyl-2-tetradecyl-malonamide (DMDBTDMA)à DIAMEX with ligand in dodecane with 3-4 M HNO3

* Selective extraction over Nd

15-14

Am/Ln solvent extraction• Extraction reaction

§ Am3++2(HA)2AmA3HA+3 H+

à Release of protons upon complexation requires pH adjustment to achieve extraction

* Maintain pH greater than 3

• Cyanex 301 stable in acid§ HCl, H2SO4, HNO3

à Below 2 M• Irradiation produces acids and

phosphorus compounds§ Problematic extractions when

dosed 104 to 105 gray• New dithiophosphinic acid less sensitive

to acid concentration§ R2PSSH; R=C6H5, ClC6H4,

FC6H4, CH3C6H4 à Only synergistic extractions

with, TBP, TOPO, or tributylphosphine oxide

à Aqueous phase 0.1-1 M HNO3

à Increased radiation resistance

Distribution ratios of Am(III ) and Ln(III ) in 1.0 M Cyanex 301‐heptane (16 mol% of Cyanex 301 neutralized before extraction contacts)

15-15

Ion exchange separation Am from Cm

• LiCl with ion exchange achieves separation from lanthanide

• Separation of tracer level Am and Cm has been performed with displacement complexing chromatography § DTPA and nitrilotriacetic acid in presence of

Cd and Zn as competing cations§ displacement complexing chromatography

method is not suitable for large scale• Ion exchange has been used to separate trace levels of

Cm from Am § Am, Cm, and lanthanides sorbed to a cation

exchange resin at pH 2à Separation of Cm from Am was

performed with 0.01 % ethylenediamine-tetramethylphosphonic acid at pH 3.4 in 0.1 M NaNO3

à separation factor of 1.4• Separation of gram scale quantities of Am and Cm by

cation and anion exchange § use of a-hydroxylisobutyrate or

diethylenetriaminepentaacetic acid as an eluting agent or a variation of eluant composition by addition of methanol to nitric acidà best separations were achieved under

high pressure conditions* separation factors greater than 400

Distributioncoefficients of actinides and lanthanides into Dowex 1 8 resin from 10 M LiCl

15-16

Extraction chromatography• Mobile liquid phase and stationary liquid phase

§ Apply results from solvent extractionà HDEHP, Aliquat 336, CMPO

* Basis for Eichrom resins* Limited use for solutions with fluoride, oxalate, or

phosphateà DIPEX resin (Eichrom)

* Bis-2-ethylhexylmethanediphosphonic acid on inert support* Lipophilic molecule

Ø Extraction of 3+, 4+, and 6+ actinides* Strongly binds metal ions

Ø Need to remove organics from support§ Variation of support

à Silica for covalent bondingà Functional organics on coated ferromagnetic particles

* Magnetic separation after sorption

15-17

Am separation and purification• Precipitation method

§ Formation of insoluble Am speciesà AmF3, K8Am2(SO4)7 , Am2(C2O4)3, K3AmO2(CO3)2

* Am(V) carbonate useful for separation from Cm* Am from lanthanides by oxalate precipitation

Ø Slow hydrolysis of dimethyloxalateØ Oxalate precipitate enriched in AmØ 50 % lanthanide rejection, 4 % Am

§ Oxidation of Am(VI) by K2S2O8 and precipitation of Cm(III)• Pyrochemical process

§ Am from Puà O2 in molten salt, PuO2 forms and precipitatesà Partitioning of Am between liquid Bi or Al and molten

salts* Kd of 2 for Al system

à Separation of Am from PuF4 in salt by addition of OF2

* Formation of PuF6, volatility separation

15-18

Am metal and alloys• Preparation of Am metal

§ Reduction of AmF3 with Ba or Li§ Reduction of AmO2 with La§ Bomb reduction of AmF3 with Ca§ Decomposition of Pt5Am

à 1550 °C at 10-6 torr§ La or Th reduction of AmO2 with distillation

of Am• Metal properties

§ Ductile, non-magnetic§ Double hexagonal closed packed (dhcp) and

fcc§ Evidence of three phase between room

temperature and melting point at 1170 °C à Alpha phase up to 658 °C à Beta phase from 793 °C to 1004 °Cà Gamma above 1050 °C

§ Some debate in literatureà Evidence of dhcp to fcc at 771 °C

§ Interests in metal properties due to 5f electron behaviorà Delocalization under pressureà Different crystal structures

* Conversion of dhcp to fccà Discrepancies between different

experiments and theory• Alloys investigated with 23 different elements

§ Phase diagrams available for Np, Pu, and U alloys

15-19

Am compounds: Oxides and Hydroxides• AmO, Am2O3, AmO2

§ Non-stoichiometric phases between Am2O3 and AmO2

• AmO lattice parameters varied in experiments§ 4.95 Å and 5.045 ŧ Difficulty in stabilizing divalent Am

• Am2O3

§ Prepared in H2 at 600 °C§ Oxidizes in air§ Phase transitions with temperature

à bcc to monoclinic between 460 °C and 650 °C

à Monoclinic to hexagonal between 800 °C and 900 °C

• AmO2

§ Heating Am hydroxides, carbonates, oxalates, or nitrates in air or O2 from 600 °C to 800 °C

§ fcc latticeà Expands due to radiation damage

• Higher oxidation states can be stabilized§ Cs2AmO4 and Ba3AmO6

• Am hydroxide§ Isostructural with Nd hydroxides§ Crystalline Am(OH)3 can be formed, but

becomes amorphous due to radiation damageà Complete degradation in 5 months for

241Am hydroxide§ Am(OH)3+3H+

,Am3++3H2Oà logK=15.2 for crystallineà Log K=17.0 for amorphous

15-20

Am organic compounds• From precipitation (oxalates) or solution evaporation• Includes non-aqueous chemistry

§ AmI3 with K2C8H8 in THF

à Yields KAm(C8H8)2

§ Am halides with molten Be(C5H5) forms Am(C5H5)3

à Purified by fractional sublimationà Characterized by IR and absorption spectra

15-21

Am coordination chemistry• Little known about Am coordination chemistry

§ 46 compounds examined§ XRD and compared to isostructural lanthanide compounds§ Structural differences due to presence of oxo groups in oxidized Am

• Halides§ Coordination numbers 7-9, 11§ Coordination include water

à AmCl2(H2O)6+

* Outer sphere Cl may be present

15-22

Am coordination chemistry

• Oxides§ Isostructural with Pu oxides§ AmO may not be correct§ Am(V)=O bond distance of 1.935 Å § Am2O3 has distorted Oh symmetry with Am-O

bond distances of 2.774 Å, 2.678 Å, and 1.984

15-23

Am coordination chemistry• Cyclopentadienyl (CP) ligands

§ Am(C5H5)3

à Isostructural with Pu(III) species* Not pyrophoric

à Absorbance on films examined* Evaluated 2.8 % relative bond covalency * Indicates highly ionic bonding for species* Data used for calculations and discussion of 5f and 6d

orbitals in interactions• Bis-cyclooctatetraenyl Am(III) KAm(C8H8)2

§ In THF with 2 coordinating solvent ligands§ Decomposes in water, burns in air§ XRD shows compound to be isostructural with Pu and Np

compoundsà From laser ablation mass spectra studies, examination of

molecular productsà Differences observed when compared to Pu and Np

compoundsà Am 5f electrons too inert to form sigma bonds with

organic, do not participate

15-24

Curium: Nuclear properties• Isotopes from mass 237 to

251• 242Cm, t1/2=163 d

§ 122 W/g§ Grams of oxide glows§ Low flux of 241Am

target decrease fission of 242Am, increase yield of 242Cm

• 244Cm, t1/2=18.1 a§ 2.8 W/g

• 248Cm, t1/2= 3.48E5 a§ 8.39% SF yield § Limits quantities to

10-20 mg§ Target for production

of transactinide elements

15-25

Cm Production• From successive neutron capture of higher Pu isotopes

§ 242Pu+n243Pu (b-, 4.95 h)243Am+n244Am (b-, 10.1 h)244Cm§ Favors production of 244,246,248Cm

à Isotopes above 244Cm to 247Cm are not isotopically pureà Pure 248Cm available from alpha decay of 252Cf

• Large campaign to product Cm from kilos of Pu• 244Cm separation

§ Dissolve target in HNO3 and remove Pu by solvent extraction§ Am/Cm chlorides extracted with tertiary amines from 11 M LiCl

in weak acidà Back extracted into 7 M HCl

§ Am oxidation and precipitation of Am(V) carbonate• Other methods for Cm purification included NaOH, HDEHP, and

EDTA§ Discussed for Am

15-26

Cm aqueous chemistry• Trivalent Cm• 242Cm at 1g/L will boil• 9 coordinating H2O from fluorescence

§ Decreases above 5 M HCl§ 7 waters at 11 M HCl§ In HNO3 steady decrease from 0 to 13 M

à 5 waters at 13 Mà Stronger complexation with NO3

-

• Inorganic complexes similar to data for Am§ Many constants determined by TRLFS

• Hydrolysis constants (Cm3++H2OCmOH2++H+)§ K11=1.2E-6§ Evaluated under different ionic strength

15-27

Cm atomic and spectroscopic data• Cm(III) absorbance

§ Weak absorption in near-violet region

§ Solution absorbance shifted 20-30 Å compared to solidà Reduction of intensity in solid

due to high symmetry* f-f transitions are symmetry

forbidden§ Spin-orbit coupling acts to reduce

transition energies when compared to lanthanides

• Cm(IV) absorbance§ Prepared from dissolution of CmF4

à CmF3 under strong fluorination conditions

• 5f7 has enhanced stability§ Half filled orbital

à Large oxidation potential for IIIIV

à Cm(IV) is metastable

15-28

0

10

20

30

Wa

ve

nu

mb

er

(10

3 c

m-1

)Absorption and fluorescence process of Cm3+

Optical Spectra

HGF

7/2A

Z 7/2

Fluorescence Process

Excitation

EmissionlessRelaxation

FluorescenceEmission

15-29

Cm fluorescence

• Fluoresce from 595-613 nm§ Attributed to

6D7/28S7/2 transition

§ Energy dependent upon coordination environmentà Speciationà Hydrationà complexation

constants

15-30

Cm separation and purification: Similar to Am

• Solvent extraction§ Organic phosphates

à Function of ligand structure* Mixed with 6 to 8 carbon chain better than TBP

§ HDEHPà From HNO3 and LiCl

§ CMPOà Oxidation state based removal with different stripping agent

§ Extraction of Cm from carbonate and hydroxide solutions, need to keep metal ions in solutionà Organics with quaternary ammonium bases, primary amines, alkylpyrocatechols,

b-diketones, phenols• Ion exchange

§ Anion exchange with HCl, LiCl, and HNO3

à Includes aqueous/alcohol mixturesà Formation of CmCl4

- at 14 M LiCl* From fluorescence spectroscopy

• Precipitation§ Separation from higher valent Am

à 10 g/L solution in baseà Precipitation of K5AmO2(CO3)3 at 85 °Cà Precipitation of Cm with hydroxide, oxalate, or fluoride

15-31

Cm metallic state• Preparation of Cm metal

§ CmF3 reduction with Ba or Lià Dry, O2 free, and above 1600 K

§ Reduction of CmO2 with Mg-Zn alloy in MgF2/MgCl2

• Melting point 1345 °C§ Higher than lighter actinides Np-Am§ Similar to Gd (1312 °C)

• Two states§ Double hexagonal close-packed (dhcp)

à Neutron diffraction down to 5 Kà No structure change

§ fcc at higher temperature

• XRD studies on 248Cm• Magnetic susceptibility studies

§ Antiferrimagnetic transition near 65 Kà 200 K for fcc phase

• Metal susceptible to corrosion due to self heating§ Formation of oxide on surface

• Alloys§ Cm-Pu phase diagram studied§ Noble metal compounds

à CmO2 and H2 heated to 1500 K in Pt, Ir, or Rh

* Pt5Cm, Pt2Cm, Ir2Cm, Pd3Cm, Rh3Cm

15-32

Cm oxide compounds• Cm2O3

§ Thermal decomposition of CmO2 at 600 °C and 10-4 torr§ Mn2O3 type cubic lattice

à Transforms to hexagonal structure due to radiation damageà Monoclinic at 800 °C

• CmO2

§ Heating in air, thermal treatment of Cm loaded resin, heating Cm2O3 at 600 °C under O2, heating of Cm oxalate

§ Shown to form in O2 as low as 400 °Cà Evidence of CmO1.95 at lower temperature

§ fcc structure§ Magnetic data indicates paramagnetic moment attributed to

Cm(III)à Need to re-evaluate electronic ground state in oxides

• Oxides§ Similar to oxides of Pu, Pr, and Tb

à Basis of phase diagram§ BaCmO3 and Cm2CuO4

à Based on high T superconductorsà Cm compounds do not conduct

15-33

Cm compounds• Cm(OH)3

§ From aqueous solution, crystallized by aging in water§ Same structure as La(OH)3; hexagonal

• Cm2(C2O4)3.10H2O

§ From aqueous solution§ Stepwise dehydration when heated under He

à Anhydrous at 280 °Cà Converts to carbonate above 360 °C

* TGA analysis showed release of water (starting at 145 °C)à Converts to Cm2O3 around 500 °C

• Cm(NO3)3

§ Evaporation of Cm in nitric acid§ From TGA, decomposition same under O2 and He

à Dehydration up 180 °C, melting at 400 °C§ Final product CmO2

§ Oxidation of Cm during decomposition• Organometallics

§ Studies hampered by radiolytic properties of Cm§ Some compounds similar to Am

à Cm(C5H5)3 form CmCl3 and Be(C5H5)2

à Weak covalency of compoundà Strong fluorescence

15-34

Review

• Production and purification of Am and Cm isotopes§ Suitable reactions§ Basis of separations from other actinides

• Formation of Am and Cm metallic state and properties§ Number of phases, melting points

• Compounds § Range of compounds, limitations on data

• Solution chemistry§ Oxidation states

• Coordination chemistry§ Organic chemistry reactions

15-35

Questions

• What is the longest lived isotope of Am?• Which Am isotope has the highest neutron induced fission

cross section?• What are 3 ligands used in the separation of Am?

§ What are the solution conditions?• What column methods are useful for separating Am from

the lanthanides?• Which compounds can be made by elemental reactions

with Am?• What Am coordination compounds have been produced?• What is the absorbance spectra of Am for the different

oxidation states?• How can Am be detected?

15-36

Questions

• Which Cm isotopes are available for chemical studies?

• Describe the fluorescence process for Cm§ What is a good excitation wavelength?

• What methods can be use to separate Cm from Am?

• How many states does Cm metal have? What is its melting point?

• What are the binary oxides of Cm? Which will form upon heating in normal atmosphere?

15-37

Pop Quiz

• How can high valent oxidation states of Am be made?

• Why does Cm have fewer accessible oxidation states than Am?

• Respond to lecture in blog• Provide pop quiz answers