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CHEM 312: Lecture 15 Americium and Curium Chemistry Part 2

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1 CHEM 312: Lecture 15 Americium and Curium Chemistry Part 2
Readings: Am and Cm chemistry chapters Link on web page Combined due to similar chemical properties of elements Cover Am then Cm Nuclear properties Production of isotopes Separation and purification Metallic state Compounds Solution chemistry Coordination chemistry

2 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 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 Alpha phase up to 658 °C Beta phase from 793 °C to 1004 °C Gamma above 1050 °C Some debate in literature Evidence of dhcp to fcc at 771 °C Interests in metal properties due to 5f electron behavior Delocalization under pressure Different crystal structures Conversion of dhcp to fcc Discrepancies between different experiments and theory Alloys investigated with 23 different elements Phase diagrams available for Np, Pu, and U alloys

3 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 Å Difficulty in stabilizing divalent Am Am2O3 Prepared in H2 at 600 °C Oxidizes in air Phase transitions with temperature bcc to monoclinic between 460 °C and 650 °C 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 lattice 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 damage Complete degradation in 5 months for 241Am hydroxide Am(OH)3+3H+,Am3++3H2O logK=15.2 for crystalline Log K=17.0 for amorphous

4 Am organic compounds From precipitation (oxalates) or solution evaporation Includes non-aqueous chemistry AmI3 with K2C8H8 in THF Yields KAm(C8H8)2 Am halides with molten Be(C5H5) forms Am(C5H5)3 Purified by fractional sublimation Characterized by IR and absorption spectra

5 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 AmCl2(H2O)6+ Outer sphere Cl may be present

6 Am coordination chemistry
Oxides Isostructural with Pu oxides AmO may not be correct Am(V)=O bond distance of Å Am2O3 has distorted Oh symmetry with Am-O bond distances of Å, Å, and 1.984

7 Am coordination chemistry
Cyclopentadienyl (CP) ligands Am(C5H5)3 Isostructural with Pu(III) species Not pyrophoric 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 compounds From laser ablation mass spectra studies, examination of molecular products Differences observed when compared to Pu and Np compounds Am 5f electrons too inert to form sigma bonds with organic, do not participate

8 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 mg Target for production of transactinide elements

9 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 247Cm are 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

10 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 5 waters at 13 M Stronger complexation with NO3- 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

11 Cm atomic and spectroscopic data
Cm(III) absorbance Weak absorption in near-violet region Solution absorbance shifted 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 5f7 has enhanced stability Half filled orbital Large oxidation potential for IIIIV Cm(IV) is metastable

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

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

14 Cm separation and purification: Similar to Am
Solvent extraction Organic phosphates Function of ligand structure Mixed with 6 to 8 carbon chain better than TBP HDEHP From HNO3 and LiCl 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 Ion exchange Anion exchange with HCl, LiCl, and HNO3 Includes aqueous/alcohol mixtures Formation of CmCl4- at 14 M LiCl From fluorescence spectroscopy Precipitation Separation from higher valent Am 10 g/L solution in base Precipitation of K5AmO2(CO3)3 at 85 °C Precipitation of Cm with hydroxide, oxalate, or fluoride

15 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 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 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

16 Cm oxide compounds Cm2O3 Thermal decomposition of CmO2 at 600 °C and 10-4 torr 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 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

17 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 around 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 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

18 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

19 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?

20 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?

21 Questions Comment on blog Provide response to PDF Quiz 15

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