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Lecture 10: Curium Chemistry

Lecture 10: Curium Chemistry. From: Chemistry of actinides Nuclear properties Production of Cm isotopes Atomic data Cm separation and purification Metallic state Classes of compounds Solution chemistry Analytical Chemistry. Cm nuclear properties. Isotopes from mass 237 to 251

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Lecture 10: Curium Chemistry

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  1. Lecture 10: Curium Chemistry • From: Chemistry of actinides • Nuclear properties • Production of Cm isotopes • Atomic data • Cm separation and purification • Metallic state • Classes of compounds • Solution chemistry • Analytical Chemistry

  2. Cm nuclear properties • Isotopes from mass 237 to 251 • Three isotopes available in quantity for chemical studies • 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 242Cm • 244Cm, t1/2=18.1 a • 2.8 W/g • 248Cm, t1/2= 3.48E5 a • 8.39% SF yield • Limited quantities to 10-20 mg • Target for production of transactinide elements

  3. Cm Production • From successive neutron capture of higher Pu isotopes • 242Pu+n243Pu (b-, 4.95 h)243Am+n244Am (b-, 10.1 h)244Cm • Favors production of 244,246,248Cm • Isotopes above 244Cm to 247Cmare not isotopically pure • 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 acid • 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

  4. Atomic and spectroscopic data • Ground state electron configuration • [Rn]5f76d17s2, Term symbol: 9D2 • Ionization limit (48 560 cm-1) • Cm3+ [Rn]5f7, 8D7/2 • X-ray data • Electron binding energies • K=128.24 KeV, LI=24.52 KeV, LII=23.65 KeV, LIII=18.9 KeV

  5. Cm atomic and spectroscopic data • 5f7 has enhanced stability • Half filled orbital • Large oxidation potential for IIIIV • Cm(IV) is metastable • Cm(III) absorbance • Weak absorption in near-violet region • Solution absorbance shifted 20-30 Å compared to solid • 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 • CmF3 under strong fluorination conditions

  6. Cm fluorescence • Fluoresce from 595-613 nm • Attributed to 6D7/28S7/2 transition • Energy dependent upon coordination environment • Speciation • Hydration • complexationconstants

  7. Absorption and fluorescence process of Cm3+ Optical Spectra Fluorescence Process H G F Emissionless Relaxation A 7/2 Excitation Fluorescence Emission Z 7/2

  8. Cm separation and purification • Solvent extraction • Fundamentally the same as Am • Organic phosphates • Function of ligand structure • Mixed with 6 to 8 carbon chain better than TBP • HDEHP • From HNO3 and LiCl • Use of membrane can result in Am/Cm separation • CMPO • Oxidation 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, b-diketones, phenols

  9. Cm separations • Ion exchange (similar to Am conditions) • Anion exchange with HCl, LiCl, and HNO3 • Includes aqueous/alcohol mixtures • Formation of CmCl4- at 14 M LiCl • From fluorescence spectroscopy • TEVA resins • Same range of organic phases • Precipitation • Separation from higher valentAm (as discussed in Am chapter) • 10 g/L solution in base • Precipitation of K5AmO2(CO3)3 at 85 °C • Precipitation of Cm with hydroxide, oxalate, or fluoride

  10. 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) • Neutron diffraction down to 5 K • No structure change • fcc at higher temperature • XRD studies on 248Cm • Magnetic susceptibility studies • Antiferrimagnetic transition near 65 K • 200 K for fcc phase • Metal susceptible to corrosion due to self heating • Formation of oxide on surface

  11. Cm metallic state • Preparation of Cm metal • CmF3 reduction with Ba or Li • 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 • CmO2 and H2 heated to 1500 K in Pt, Ir, or Rh • Pt5Cm, Pt2Cm, Ir2Cm, Pd3Cm, Rh3Cm

  12. Cm compounds • Hydrides • Reaction of metal with H2 at 250 °C • fcc from XRD, CmH2+x • Dihydride also forms • Halides • Complete CmX3 and CmF4 • CmF3 precipitates with excess F- • Anhydrous forms when compound placed over P2O5 • CmCl3 from treating Cm oxides with anhydrous HCl between 400-600 °C • Hexagonal UCl3 type structure • 9 Cl- coordination tricappedtrigonal prism • CmBr3 from treating CmCl3 with NH4Br between 400-450 °C • Orthorhombic structure (PuBr3) • Coordinated by 8 Br- • CmI3 from CmBr3 with NH4I • Also from reactions with elements • CmF4 • Fluoride oxidation of CmF3 • Monoclinic ZrF4 structure • Antiprismatic 8-coordination • Some evidence of CmF6 and trivalent oxyfluorides

  13. Cm oxides • Cm2O3 • Thermal decomposition of CmO2 at 600 °C and 10-4torr • Mn2O3 type cubic lattice • Transforms to hexagonal structure due to radiation damage • 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 °C • Evidence of CmO1.95 at lower temperature • fcc structure • Magnetic data indicates paramagnetic moment attributed to Cm(III) • Need to re-evaluate electronic ground state in oxides

  14. Cm compounds • Oxides • Similar to oxides of Pu, Pr, and Tb • Basis of phase diagram • BaCmO3 and Cm2CuO4 • Based on high T superconductors • Cm compounds do not conduct • S, Se, Te compounds • CmS2 and CmSe2 from Cm hydride and elements heated under vacuum • Tetragonal structure • Thermal treatment of CmS2 yields Cm2S3 (bcc) • 1,1 species from heating elements 700-750 °C • bcc structure • CmTe3 from heating at 400 °C

  15. Cm compounds • N, P, As, and Sb • 1,1 species • Cm metal or hydride with elements • Sealed tubes from 350-900 °C • All have NaCl structure • CmN and CmAs are ferromagnetic • Lower effective magnetic moments than expected for 5f7 configuration • Strong spin-orbit coupling and crystal field effects • Formation of Pu,CmN species • Lattice similar to known species parameters

  16. 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 • Anhydrous at 280 °C • Converts to carbonate above 360 °C • TGA analysis showed release of water (starting at 145 °C) • Converts to Cm2O3 above 500 °C’ • Cm(NO3)3 • Evaporation of Cm in nitric acid • From TGA, decomposition same under O2 and He • Dehydration up 180 °C, melting at 400 °C • Final product CmO2 • Oxidation of Cm during decomposition

  17. Cm compounds • Phosphates • CmPO4.0.5 H2O from aqueous solutions with Na2HPO4 or (NH4)2HPO4 • Unknown structure • Dehydrates at 300 °C • Monazite structure • Cm[Fe(CN)6] forms solids (dark red) • K3[Fe(CN)6] with Cm in 0.2 M HNO3 • Eu, Ce, and Pr do not form solids under the same conditions • Hexafluoroacetylacetone (HFAA) • Cs ion complex forms with Cm • 1,1,4 species

  18. Cm compounds • Organometallics • Studies hampered by radiolytic properties of Cm • Some compounds similar to Am • Cm(C5H5)3 form CmCl3 and Be(C5H5)2 • Weak covalency of compound • Strong fluorescence

  19. Cm aqueous chemistry • Inorganic complexes similar to data for Am • Many constants determined by TRLFS • Hydrolysis constants (Cm3++H2OCmOH2++H+) • K11=1.2E-6 • Evaluated under different ionic strength • 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 • 5 waters at 13 M • Stronger complexation with NO3-

  20. Cm solution chemistry • Polytungstate shown to quench Cm fluorescence • Cm(IV) species exhibit chemiluminescence upon reduction • Stronger complexes with bidentate carboxylic acids • Some data trends may result from experimental measurement differences • Organic complexation with same ligands as Am • CMPO, HDEHP, 8-hydroxyquinoline

  21. Cm Analytical chemistry • Typical alpha spectroscopy • Odd A isotopes have lower energy • May require separation prior to alpha spectroscopy • Utilization of TEVA resins or anion exchange • Fission • Even isotopes • Requires pure isotopic sample • TRLFS • No chemical separation needed

  22. Review • Nuclear properties • Long lived isotopes, fissile, SF decay route • Production of Cm isotopes • Capture and separation method • Classes of compounds • Oxidation state of Cm in compounds • Solution chemistry • Spectroscopic methods for speciation • Formation of tetravalent state • Analytical Chemistry • Methods of Cm detection

  23. 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?

  24. Pop Quiz • Why does Cm have fewer accessible oxidation states than Am?

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