15-1

Americium and Curium Chemistry• From: Chemistry of actinides

§ Nuclear properties§ Production of Am isotopes§ Am separation and purification § Metallic state§ Compounds § Solution chemistry§ Coordination chemistry§ Analytical Chemistry

15-2

Production of Am isotopes• Am first produced 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 solutionà 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• 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 solution

à Ozone, hypochlorate (ClO-), peroxydisulfateà Reduction of Am(VI) with bromide

• 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

• Am(VII)§ 3-4 M NaOH, mM Am(VI) near 0 °C§ Gamma irradiation 3 M NaOH with N2O or S2O8

2- saturated solution

15-5

Am solution chemistry• Am(III) has 9 inner sphere waters

§ Others have calculated 11 and 10 (XAFS)§ Based on fluorescence spectroscopy

à Lifetime related to coordination* nH2O=(x/t)-y

Ø x=2.56E-7 s, y=1.43Ø Measurement of fluorescence lifetime in H2O and

D2O

15-6

Am solution chemistry• Thermodynamic data available (NEA data)

§ Systematic differences at Amà Thermodynamic changes with atomic numberà Deviation at Am due to positive entropy of vaporization

15-7

Am solution chemistry• Autoreduction

§ Formation of H2O2 and HO2 radicals from radiation reduces Am to trivalent statesà Difference between 241Am and 243Am

§ Rate decreases with increase acid for perchloric and sulfuric

§ Some disagreement role of Am concentrationà Concentration of Am total or oxidation state

§ Rates of reduction dependent uponà Acid, acid concentration, à mechanism

* Am(VI) to Am(III) can go stepwise à starting ion

* Am(V) slower than Am(VI)

15-8

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 H2O

à Solution can impact oxidation state stability

15-9

Am solution chemistry: Redox Kinetics

• Am(III) oxidation by peroxydisulfate§ Oxidation due to thermal

decomposition productsà SO4

.-, HS2O8-

§ Oxidation to Am(VI)à 0.1 M to 10 nM Am(III)

§ Acid above 0.3 M limits oxidationà Decomposition of S2O8

2-

§ Induction period followed by reduction

§ Rates dependent upon temperature, [HNO3], [S2O8

2-], and [Ag+2]§ 3/2 S2O8

2- + Am3++2 H2O3 SO42-

+AmO22++4H+

à Evaluation of rate constants can yield 4 due to peroxydisulfate decomposition

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

oxidantà Am(V) to Am(VI)

* Proportional to total Am and oxidant

* Inversely proportional to K2CO3

15-10

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

15-11

Am solution chemistry

• Radiolysis§ From alpha decay

à 1 mg 241Am release 7E14 eV/s§ Reduction of higher valent Am related to

dose and electrolyte concentration

§ In nitric acid need to include role of HNO2

§ In perchlorate numerous species produced

à Cl2, ClO2, or Cl-

15-12

Am solution chemistry

• 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

15-13

Am solution chemistry: Organics• Number of complexes examined

§ Mainly for Am(III)• Stability of complex decreases with

increasing number of carbon atoms• 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

15-14

Am(IV) solution chemistry

• Am(IV) can be stabilized by heteropolyanions§ P2W17O61

anion; formation of 1,1 and 1,2 complexà Examined by absorbance at 789 nm and 560 nmà Autoradiolytic reduction

* Independent of complex formationà Displacement by addition of Th(IV)

* Disproportionation of Am(IV) to Am(III) and Am(VI)

§ EXAFS used with AmP5W30O11012-

• Cation-cation interaction§ Am(V)-U(VI) interaction in perchlorate

à Am(V) spectroscopic shift from 716-733 nm to 765 nm

15-15

Am separation and purification• 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• 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)

15-16

Am solvent extraction• Am from lanthanides

§ HDEHP extract lanthanides better than actinidesà Harder metal-ligand interactionà Basis of TALSPEAKà Preferential removal of actinides by contact with DTPA

solution* Reverse-TALSPEAK* Also useful with DIDPA

à Selective actinide extraction with DTPA and 0.4 M NaNO3

* Ce/Am Df of 72

§ Recent efforts based on soft donor moleculesà Sulfur and nitrogen containing ligandsà Tripyridyltriazene (TPTZ) (C5H4N: pyridyl, (R-N:, azene)

and dinonylnapthalene sulfonic acid (HDNNS) in CCl4 and dilute nitric acid* Preferential extraction of Am from trivalent

lanthanides

15-17

Am solvent extraction• Am from lanthanides

§ Initial work effected direction of further researchà Focus on nitrogen and sulfur containing

ligands* Thione (Phosphine SO), pyridenes,

thiophosphonic acid§ Research does not follow CHON principles§ Efforts with Cyanex 301 achieved

lanthanide/actinide separation in pH 3 solutionà Bis (2,4,4-trimethylpentyl)dithiophosphinic

acid

15-18

Am solvent extraction• Lanthanide/actinide separation

§ Extraction reactionà Am3++2(HA)2AmA3HA+3 H+

* Release of protons upon complexation requires pH adjustment to achieve extractionØ Maintain pH greater than 3

§ Cyanex 301 stable in acidà HCl, H2SO4, HNO3

* Below 2 M§ Irradiation produces acids and phosphorus compounds

à Problematic extractions when dosed 104 to 105 gray§ New dithiophosphinic acid less sensitive to acid concentration

à R2PSSH; R=C6H5, ClC6H4, FC6H4, CH3C6H4 * Only synergistic extractions with, TBP, TOPO, or

tributylphosphine oxide* Aqueous phase 0.1-1 M HNO3

* Increased radiation resistance

15-19

15-20

Ion exchange• Cation exchange

§ Am3+ sorbs to cation exchange resin in dilute acidà Elution with a-hydroxylisobutyrate and

aminopolycarboxylic acids• Anion exchange

§ Sorption to resin from thiocyanate, chloride, and to a limited degree nitrate solutions

• Inorganic exchangers§ Zirconium phosphate

à Trivalents sorb* Oxidation of Am to AmO2

+ achieves separation§ TiSb (titanium antimonate)

à Am3+ sorption in HNO3

à Adjustment of aqueous phase to achieve separation

15-21

Ion exchange separation Am from Cm• Separation of tracer level Am and Cm has been performed with displacement

complexing chromatography § separations were examined with DTPA and nitrilotriacetic acid in the

presence of Cd and Zn as competing cations§ use of Cd and nitrilotriacetic acid separated trace levels of Am from Cm§ displacement complexing chromatography method is too cumbersome to use

on a large scale• Ion exchange has been used to separate trace levels of Cm from Am

§ Am, Cm, and lanthanides were sorbed to a cation exchange resin at pH 2à separation was achieved by adjusting pH and organic complexantà Separation of Cm from Am was performed with 0.01 %

ethylenediamine-tetramethylphosphonic acid at pH 3.4 in 0.1 M NaNO3 with a separation factor of 1.4

• Separation of gram scale quantities of Am and Cm has been achieved by cation and anion exchange § methods rely upon use of a-hydroxylisobutyrate or

diethylenetriaminepentaacetic acid as an eluting agent or a variation of the eluant composition by the addition of methanol to nitric acidà best separations were achieved under high pressure conditionsà repeating the procedure separation factors greater than 400 were

obtained

15-22

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

* 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

15-23

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

15-24

Am metal, alloys, and compounds• Alloys investigated with 23 different

elements• Phase diagrams available for Np, Pu, and

U alloys• Am compounds

§ Oxides and hydroxidesà AmO, Am2O3, AmO2

* Non-stoichiometric phases between Am2O3 and AmO2

à AmO lattice parameters varied in experiments

* 4.95 Å and 5.045 Å* Difficulty in stabilizing

divalent Amà Am2O3

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

temperatureØ bcc to monoclinic

between 460 °C and 650 °C

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

15-25

Am compounds• Am oxides and hydroxides

§ AmO2

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

à fcc lattice* Expands due to radiation damage

§ Higher oxidation states can be stabilizedà Cs2AmO4 and Ba3AmO6

§ Am hydroxideà Isostructural with Nd hydroxidesà Cystalline Am(OH)3 can be formed, but becomes amorphous due

to radiation damage* Complete degradation in 5 months for 241Am hydroxide

à Am(OH)3+3H+,Am3++3H2O

* logK=15.2 for crystalline* Log K=17.0 for amorphous

§ Am hydrolysis (from CHESS database)à Am3++H2OAmOH2++H+: log K =-6.402à Am3++2H2OAm(OH)2

++2H+: log K =-14.11à Am3++3H2OAm(OH)3+3H+: log K =-25.72

15-26

Solution absorption spectroscopy• Am(III)

§ 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§ Solution spectroscopy compared to Am doped in crystals§ Absorbance measured in acids and carbonate

15-27

Solution absorption spectroscopy • Am(IV)

§ In acidic media, broad absorption bandsà 13 M HF, 12 M KF, 12 M H3PO4

§ Resembles solid AmF4 spectrum

15-28

Solution absorption spectroscopy• Am(V)

§ 5I43G5; 513.7 nm; 45 L mol cm-1

§ 5I43I7; 716.7 nm; 60 L mol cm-1

à Collected in acid, NaCl, and carbonate

15-29

Solution absorption spectroscopy• Am(VI)

§ 996 nm; 100 L mol cm-1

§ Smaller absorbance at 666 nmà Comparable to position in Am(V) à Based on comparison with uranyl, permits analysis based on uranyl core with addition of electrons

15-30

Solution absorption spectroscopy

• Am(VII)§ Broad absorbance at 740 nm

• Am(III) luminescence§ 7F05L6 at 503 nm

à Then conversion to other excited state§ Emission to 7FJ

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

à 155 ns in D2O§ Emission and lifetime changes with speciation

à Am triscarbonate lifetime = 34.5 ns, emission at 693 nm

15-31

15-32

Am spectroscopy

• Vibrational

§ AmO2+

à Antisymmetric vibration in solids at 802 cm-1

§ Raman of Am(III) phosphate

à Symmetric stretch of PO43- at 973 cm-1

à PO3- groups at 1195 cm-1

• X-ray absorption§ Absorption edge at 18504 eV

à 4 eV difference between Am(IV) and Am(III)

15-33

Cm nuclear properties• Isotopes from mass 237 to 251

§ Three isotopes available in quantity for chemical studiesà 242Cm, t1/2=163 d

* 122 W/g* Grams of the oxide glows* Low flux of 241Am target decrease fission of

242Am, increase yield of 242Cmà 244Cm, t1/2=18.1 a

* 2.8 W/gà 248Cm, t1/2= 3.48E5 a

* 8.39% SF yield * Limits quantities to 10-20 mg* Target for production of transactinide

elements

15-34

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

15-35

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

15-36

15-37

Cm atomic and spectroscopic data• 5f7 has enhanced stability

§ Half filled orbitalà Large oxidation potential for

IIIIVà Cm(IV) is metastable

• 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

15-38

Atomic and spectroscopic data

• Cm fluorescence§ Fluoresce from 595-613 nm

à Attributed to 6D7/28S7/2 transition

à Energy dependent upon coordination environment* Speciation* Hydration* complexation constants

15-39

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

15-40

15-41

15-42

Cm separation and purification

• Solvent extraction§ Fundamentally the same as Am§ Organic phosphates

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

§ HDEHPà From HNO3 and LiCl

* Use of membrane can result in Am/Cm separation§ CMPO

à 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

15-43

Cm separations

• Ion exchange (similar to Am conditions)§ Anion exchange with HCl, LiCl, and HNO3

à Includes aqueous/alcohol mixturesà Formation of CmCl4

- at 14 M LiCl* From fluorescence spectroscopy

§ TEVA resinsà Same range of organic phases

• 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

15-44

Cm metallic state

• 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

15-45

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

• 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

15-46

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

15-47

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 above 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

à Oxidation of Cm during decomposition

15-48

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

15-49

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?

15-50

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?

15-51

Pop Quiz

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

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