1. 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

2. 15-2 Production of Am isotopes Am produced in reactors from neutron irradiation of Pu § 239 Pu to 240 Pu to 241 Pu, then beta decay of 241 Pu 241,243 Am main isotopes of interest §Long half-lives §Produced in kilogram quantity §Chemical studies §Both isotopes produced in reactor 241 Am §source for low energy gamma and alpha àAlpha energy 5.44 MeV and 5.49 MeV §Smoke detectors §Neutron sources  ( ,n) on Be §Thickness gauging and density § 242 Cm production from thermal neutron capture 243 Am §Irradiation of 242 Pu, beta decay of 243 Pu Critical mass § 242 Am in solution à23 g at 5 g/L àRequires isotopic separation

3. 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) àFormed from pulse radiolysis *Absorbance at 313 nm *T 1/2 of oxidation state 5E-6 seconds Am(III) §Easy to prepare (metal dissolved in acid, AmO 2 dissolution) àPink in mineral acids, yellow in HClO 4 when Am is 0.1 M  7 F 0  5 L 6 at 503.2 nm (  =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 àBased on fluorescence spectroscopy *Lifetime related to coordination  n H2O =(x/  )-y Øx=2.56E-7 s, y=1.43 ØMeasurement of fluorescence lifetime in H 2 O and D 2 O Am(IV) §Requires complexation to stabilize àdissolving Am(OH) 4 in NH 4 F àPhosphoric or pyrophosphate (P 2 O 7 4- ) solution with anodic oxidation àAg 3 PO 4 and (NH 4 ) 4 S 2 O 8 àCarbonate solution with electrolytic oxidation

4. 15-4 Am solution chemistry Am(V) §Oxidation of Am(III) in near neutral solution àOzone, hypochlorate (ClO - ), peroxydisulfate àReduction of Am(VI) with bromide § 5 I 4  3 G 5 ; 513.7 nm; 45 L mol cm -1 § 5 I 4  3 I 7 ; 716.7 nm; 60 L mol cm -1 Am(VI) §Oxidation of Am(III) with S 2 O 8 2- or Ag 2+ 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 àSmaller absorbance at 666 nm Am(VII) §3-4 M NaOH, mM Am(VI) near 0 °C §Gamma irradiation 3 M NaOH with N 2 O or S 2 O 8 2- saturated solution Am(VII) §Broad absorbance at 740 nm

5. 15-5 Am solution chemistry Am(III) luminescence § 7 F 0  5 L 6 at 503 nm àThen conversion to other excited state §Emission to 7 F J § 5 D 1  7 F 1 at 685 nm § 5 D 1  7 F 2 at 836 nm §Lifetime for aquo ion is 20 ns à155 ns in D 2 O §Emission and lifetime changes with speciation àAm triscarbonate lifetime = 34.5 ns, emission at 693 nm Autoreduction Formation of H 2 O 2 and HO 2 radicals from radiation reduces Am to trivalent states §Difference between 241 Am and 243 Am 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 àAm(V) slower than Am(VI)

6. 15-6 Am solution chemistry Disproportionation §Am(IV) àIn nitric and perchloric acid àSecond order with Am(IV) *2 Am(IV)  Am(III) + Am(V) *Am(IV) + Am(V)  Am(III) + Am(VI) ØAm(VI) increases with sulfate §Am(V) à3-8 M HClO 4 and HCl *3 Am(V) + 4 H +  Am(III)+2Am(VI)+2 H 2 O àSolution can impact oxidation state stability Redox kinetics §Am(III) oxidation by peroxydisulfate àOxidation due to thermal decomposition products *SO 4.-, HS 2 O 8 - àOxidation to Am(VI) àAcid above 0.3 M limits oxidation *Decomposition of S 2 O 8 2- àInduction period followed by reduction àRates dependent upon temperature, [HNO 3 ], [S 2 O 8 2- ], and [Ag +2 ] àIn carbonate proceeds through Am(V) *Rate to Am(V) is proportional to oxidant *Am(V) to Am(VI) ØProportional to total Am and oxidant ØInversely proportional to K 2 CO 3

7. 15-7 Am solution chemistry: Redox kinetics Am(VI) reduction §H 2 O 2 in perchlorate is 1 st order for peroxide and Am à2 AmO 2 2+ +H 2 O 2  2 AmO 2 + + 2 H + + O 2 §NpO 2 + à1 st 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 solutions àSlow reduction with dithionite (Na 2 S 2 O 4 ), sulfite (SO 3 2- ), or thiourea dioxide ((NH 2 ) 2 CSO 2 ) § Np(IV) and Np(V) àIn both acidic and carbonate conditions *For Np(IV) reaction products either Np(V) or Np(VI) ØDepends upon initial relative concentration of Am and Np àU(IV) examined in carbonate

8. 15-8 Am solution chemistry Radiolysis §From alpha decay à1 mg 241 Am release 7E14 eV/s §Reduction of higher valent Am related to dose and electrolyte concentration §In nitric acid formation of HNO 2 §In perchlorate numerous species produced àCl 2, ClO 2, or Cl - Complexation chemistry §Primarily for Am(III) àF - >H 2 PO 4 - >SCN - >NO 3 - >Cl - >ClO 4 - §Hard acid reactions àElectrostatic interactions *Inner sphere and outer sphere ØOuter sphere for weaker ligands §Stabilities similar to trivalent lanthanides àSome enhanced stability due to participation of 5f electron in bonding

9. 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) àAm 3+ +H 2 O  AmOH 2+ +H + : log K =-6.402 àAm 3+ +2H 2 O  Am(OH) 2 + + 2H + : log K =-14.11 àAm 3+ +3H 2 O  Am(OH) 3 +3 H + : log K =-25.72 Carbonate §Evaluated by spectroscopy §Includes mixed species àAm hydroxide carbonate species àBased on solid phase analysis §Am(IV)  Pentacarbonate studied (log  =39.3) §Am(V) solubility examined 1mM Am 3+ ; 1 mM Am, 1 mM carbonate

10. 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 NaClO 4, complexation constant for Am 2 orders greater than Sm

11. 15-11 Am solvent extraction Tributylphosphate (TBP) §Am extracted from neutral or low acid solutions with high nitrate §Am(VI) àOxidation with (NH 4 ) 10 P 2 W 17 O 61 to stabilize Am(VI) à100 % TBP from 1 M HNO 3 *Separation factor 50 from Nd §Am separation from lanthanides à1 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 HNO 3 §30 % TRPO in kerosene àAm, Cm, tetravalent Np and Pu, hexavalent U extracted *Actinides stripped with 5.5 M HNO 3 (Am fraction) àTRPO with C 6 -C 8 alkyl group

12. 15-12 HDEHP Am solvent extraction Bis(2-ethylhexyl)phosphoric acid (HDEHP) §Has been used to Am separation §Part of TALSPEAK àExtracts lanthanides stronger that actinides àTALSPEAK components *Bis(2-ethyl-hexyl)phosphoric acid (HDEHP) *HNO 3 *DTPA *Lactic acid Carbamoylphosphine oxide (CMPO) §Synthesized by Horwitz àBased on DHDECMP extractions *Recognized functional group, simplified ligand synthesis *Purified by cation exchange §Part of TRUEX àTRUEX (fission products) *0.01 to 7 M HNO 3 *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

13. 15-13 Am solvent extraction Tertiary amine salt §Low acid, high nitrate or chloride solution à(R 3 NH) 2 Am(NO 3 ) 5 Quaternary ammonium salts (Aliquat 336) §Low acid, high salt solutions àExtraction sequence of Cm<Cf<Am<Es §Studies at ANL for process separation of Am Amide extractants §(R 1,R 2 )N-C(O)-CR 3 H-C(O)-N(R 1 R 2 ) àDiamide extractant àBasis of DIAMEX process §N,N’-dimethyl-N,N’-dibutyl-2-tetradecyl-malonamide (DMDBTDMA) àDIAMEX with ligand in dodecane with 3-4 M HNO 3 *Selective extraction over Nd

14. 15-14 Am/Ln solvent extraction Extraction reaction §Am 3+ +2(HA) 2  AmA 3 HA+3 H + àRelease of protons upon complexation requires pH adjustment to achieve extraction *Maintain pH greater than 3 Cyanex 301 stable in acid §HCl, H 2 SO 4, HNO 3 àBelow 2 M Irradiation produces acids and phosphorus compounds §Problematic extractions when dosed 10 4 to 10 5 gray New dithiophosphinic acid less sensitive to acid concentration §R 2 PSSH; R=C 6 H 5, ClC 6 H 4, FC 6 H 4, CH 3 C 6 H 4 àOnly synergistic extractions with, TBP, TOPO, or tributylphosphine oxide àAqueous phase 0.1-1 M HNO 3 àIncreased 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. 15-15 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 2 àSeparation of Cm from Am was performed with 0.01 % ethylenediamine- tetramethylphosphonic acid at pH 3.4 in 0.1 M NaNO 3 àseparation factor of 1.4 Separation of gram scale quantities of Am and Cm by cation and anion exchange  use of  -hydroxylisobutyrate or diethylenetriaminepentaacetic acid as an eluting agent or a variation of eluant composition by addition of methanol to nitric acid àbest separations were achieved under high pressure conditions *separation factors greater than 400 Distribution coefficients of actinides and lanthanides into Dowex 1 8 resin from 10 M LiCl

16. 15-16 Extraction chromatography Mobile liquid phase and stationary liquid phase §Apply results from solvent extraction àHDEHP, Aliquat 336, CMPO *Basis for Eichrom resins *Limited use for solutions with fluoride, oxalate, or phosphate àDIPEX resin (Eichrom) *Bis-2-ethylhexylmethanediphosphonic acid on inert support *Lipophilic molecule ØExtraction of 3+, 4+, and 6+ actinides *Strongly binds metal ions ØNeed to remove organics from support §Variation of support àSilica for covalent bonding àFunctional organics on coated ferromagnetic particles *Magnetic separation after sorption

17. 15-17 Am separation and purification Precipitation method §Formation of insoluble Am species àAmF 3, K 8 Am 2 (SO 4 ) 7, Am 2 (C 2 O 4 ) 3, K 3 AmO 2 (CO 3 ) 2 *Am(V) carbonate useful for separation from Cm *Am from lanthanides by oxalate precipitation ØSlow hydrolysis of dimethyloxalate ØOxalate precipitate enriched in Am Ø50 % lanthanide rejection, 4 % Am §Oxidation of Am(VI) by K 2 S 2 O 8 and precipitation of Cm(III) Pyrochemical process §Am from Pu àO 2 in molten salt, PuO 2 forms and precipitates àPartitioning of Am between liquid Bi or Al and molten salts *K d of 2 for Al system àSeparation of Am from PuF 4 in salt by addition of OF 2 *Formation of PuF 6, volatility separation

18. 15-18 Am metal and alloys Preparation of Am metal §Reduction of AmF 3 with Ba or Li §Reduction of AmO 2 with La §Bomb reduction of AmF 3 with Ca §Decomposition of Pt 5 Am à1550 °C at 10 -6 torr §La or Th reduction of AmO 2 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 àAlpha phase up to 658 °C àBeta phase from 793 °C to 1004 °C àGamma above 1050 °C §Some debate in literature àEvidence of dhcp to fcc at 771 °C §Interests in metal properties due to 5f electron behavior àDelocalization under pressure àDifferent crystal structures *Conversion of dhcp to fcc àDiscrepancies between different experiments and theory Alloys investigated with 23 different elements §Phase diagrams available for Np, Pu, and U alloys

19. 15-19 Am compounds: Oxides and Hydroxides AmO, Am 2 O 3, AmO 2 §Non-stoichiometric phases between Am 2 O 3 and AmO 2 AmO lattice parameters varied in experiments §4.95 Å and 5.045 Å §Difficulty in stabilizing divalent Am Am 2 O 3 §Prepared in H 2 at 600 °C §Oxidizes in air §Phase transitions with temperature àbcc to monoclinic between 460 °C and 650 °C àMonoclinic to hexagonal between 800 °C and 900 °C AmO 2 §Heating Am hydroxides, carbonates, oxalates, or nitrates in air or O 2 from 600 °C to 800 °C §fcc lattice àExpands due to radiation damage Higher oxidation states can be stabilized §Cs 2 AmO 4 and Ba 3 AmO 6 Am hydroxide §Isostructural with Nd hydroxides §Crystalline Am(OH) 3 can be formed, but becomes amorphous due to radiation damage àComplete degradation in 5 months for 241 Am hydroxide §Am(OH) 3 +3H +,  Am 3+ +3H 2 O àlogK=15.2 for crystalline àLog K=17.0 for amorphous

20. 15-20 Am organic compounds From precipitation (oxalates) or solution evaporation Includes non-aqueous chemistry §AmI 3 with K 2 C 8 H 8 in THF àYields KAm(C 8 H 8 ) 2 §Am halides with molten Be(C 5 H 5 ) forms Am(C 5 H 5 ) 3 àPurified by fractional sublimation àCharacterized by IR and absorption spectra

21. 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 àAmCl 2 (H 2 O) 6 + *Outer sphere Cl may be present

22. 15-22 Am coordination chemistry Oxides §Isostructural with Pu oxides §AmO may not be correct §Am(V)=O bond distance of 1.935 Å §Am 2 O 3 has distorted O h symmetry with Am-O bond distances of 2.774 Å, 2.678 Å, and 1.984

23. 15-23 Am coordination chemistry Cyclopentadienyl (CP) ligands §Am(C 5 H 5 ) 3 àIsostructural with Pu(III) species *Not pyrophoric àAbsorbance 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(C 8 H 8 ) 2 §In THF with 2 coordinating solvent ligands §Decomposes in water, burns in air §XRD shows compound to be isostructural with Pu and Np compounds àFrom laser ablation mass spectra studies, examination of molecular products àDifferences observed when compared to Pu and Np compounds àAm 5f electrons too inert to form sigma bonds with organic, do not participate

24. 15-24 Curium: Nuclear properties Isotopes from mass 237 to 251 242 Cm, t 1/2 =163 d §122 W/g §Grams of oxide glows §Low flux of 241 Am target decrease fission of 242 Am, increase yield of 242 Cm 244 Cm, t 1/2 =18.1 a §2.8 W/g 248 Cm, t 1/2 = 3.48E5 a §8.39% SF yield §Limits quantities to 10- 20 mg §Target for production of transactinide elements

25. 15-25 Cm Production From successive neutron capture of higher Pu isotopes  242 Pu+n  243 Pu (  -, 4.95 h)  243 Am+n  244 Am (  -, 10.1 h)  244 Cm §Favors production of 244,246,248 Cm àIsotopes above 244 Cm to 247 Cm are not isotopically pure àPure 248 Cm available from alpha decay of 252 Cf Large campaign to product Cm from kilos of Pu 244 Cm separation §Dissolve target in HNO 3 and remove Pu by solvent extraction §Am/Cm chlorides extracted with tertiary amines from 11 M LiCl in weak acid àBack 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

26. 15-26 Cm aqueous chemistry Trivalent Cm 242 Cm at 1g/L will boil 9 coordinating H 2 O from fluorescence §Decreases above 5 M HCl §7 waters at 11 M HCl §In HNO 3 steady decrease from 0 to 13 M à5 waters at 13 M àStronger complexation with NO 3 - Inorganic complexes similar to data for Am §Many constants determined by TRLFS Hydrolysis constants (Cm 3+ +H 2 O  CmOH 2+ +H + ) §K 11 =1.2E-6 §Evaluated under different ionic strength

27. 15-27 Cm atomic and spectroscopic data Cm(III) absorbance §Weak absorption in near-violet region §Solution absorbance shifted 20-30 Å compared to solid àReduction 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 CmF 4 àCmF 3 under strong fluorination conditions 5f 7 has enhanced stability §Half filled orbital àLarge oxidation potential for III  IV àCm(IV) is metastable

28. 15-28 Absorption and fluorescence process of Cm 3 + Optical Spectra HGFHGF 7/2A Z Fluorescence Process Excitation Emissionless Relaxation Fluorescence Emission

29. 15-29 Cm fluorescence Fluoresce from 595-613 nm §Attributed to 6 D 7/2  8 S 7/2 transition §Energy dependent upon coordination environment àSpeciation àHydration àcomplexation constants

30. 15-30 Cm separation and purification: Similar to Am Solvent extraction §Organic phosphates àFunction of ligand structure *Mixed with 6 to 8 carbon chain better than TBP §HDEHP àFrom HNO 3 and LiCl §CMPO àOxidation state based removal with different stripping agent §Extraction of Cm from carbonate and hydroxide solutions, need to keep metal ions in solution  Organics with quaternary ammonium bases, primary amines, alkylpyrocatechols,  -diketones, phenols Ion exchange §Anion exchange with HCl, LiCl, and HNO 3 àIncludes aqueous/alcohol mixtures àFormation of CmCl 4 - at 14 M LiCl *From fluorescence spectroscopy Precipitation §Separation from higher valent Am à10 g/L solution in base àPrecipitation of K 5 AmO 2 (CO 3 ) 3 at 85 °C àPrecipitation of Cm with hydroxide, oxalate, or fluoride

31. 15-31 Cm metallic state Preparation of Cm metal §CmF 3 reduction with Ba or Li àDry, O 2 free, and above 1600 K §Reduction of CmO 2 with Mg-Zn alloy in MgF 2 /MgCl 2 Melting point 1345 °C §Higher than lighter actinides Np-Am §Similar to Gd (1312 °C) Two states §Double hexagonal close-packed (dhcp) àNeutron diffraction down to 5 K àNo structure change §fcc at higher temperature XRD studies on 248 Cm Magnetic susceptibility studies §Antiferrimagnetic transition near 65 K à200 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 àCmO 2 and H 2 heated to 1500 K in Pt, Ir, or Rh *Pt 5 Cm, Pt 2 Cm, Ir 2 Cm, Pd 3 Cm, Rh 3 Cm

32. 15-32 Cm oxide compounds Cm 2 O 3 §Thermal decomposition of CmO 2 at 600 °C and 10 -4 torr §Mn 2 O 3 type cubic lattice àTransforms to hexagonal structure due to radiation damage àMonoclinic at 800 °C CmO 2 §Heating in air, thermal treatment of Cm loaded resin, heating Cm 2 O 3 at 600 °C under O 2, heating of Cm oxalate §Shown to form in O 2 as low as 400 °C àEvidence of CmO 1.95 at lower temperature §fcc structure §Magnetic data indicates paramagnetic moment attributed to Cm(III) àNeed to re-evaluate electronic ground state in oxides Oxides §Similar to oxides of Pu, Pr, and Tb àBasis of phase diagram §BaCmO 3 and Cm 2 CuO 4 àBased on high T superconductors àCm compounds do not conduct

33. 15-33 Cm compounds Cm(OH) 3 §From aqueous solution, crystallized by aging in water §Same structure as La(OH) 3 ; hexagonal Cm 2 (C 2 O 4 ) 3. 10H 2 O §From aqueous solution §Stepwise dehydration when heated under He àAnhydrous at 280 °C àConverts to carbonate above 360 °C *TGA analysis showed release of water (starting at 145 °C) àConverts to Cm 2 O 3 around 500 °C Cm(NO 3 ) 3 §Evaporation of Cm in nitric acid §From TGA, decomposition same under O 2 and He àDehydration up 180 °C, melting at 400 °C §Final product CmO 2 §Oxidation of Cm during decomposition Organometallics §Studies hampered by radiolytic properties of Cm §Some compounds similar to Am àCm(C 5 H 5 ) 3 form CmCl 3 and Be(C 5 H 5 ) 2 àWeak covalency of compound àStrong fluorescence

34. 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

35. 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?

36. 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?

37. 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