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15-1 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.

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Presentation on theme: "15-1 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."— Presentation transcript:


2 15-1 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

3 15-2 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 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

4 15-3 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 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

5 15-4 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

6 15-5 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 àAmCl 2 (H 2 O) 6 + *Outer sphere Cl may be present

7 15-6 Am coordination chemistry Oxides §Isostructural with Pu oxides §AmO may not be correct §Am(V)=O bond distance of Å §Am 2 O 3 has distorted O h symmetry with Am-O bond distances of Å, Å, and 1.984

8 15-7 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

9 15-8 Curium: Nuclear properties Isotopes from mass 237 to 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 mg §Target for production of transactinide elements

10 15-9 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

11 15-10 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

12 15-11 Cm atomic and spectroscopic data Cm(III) absorbance §Weak absorption in near-violet region §Solution absorbance shifted 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

13 15-12 Absorption and fluorescence process of Cm 3 + Optical Spectra HGFHGF 7/2A Z Fluorescence Process Excitation Emissionless Relaxation Fluorescence Emission

14 15-13 Cm fluorescence Fluoresce from nm §Attributed to 6 D 7/2  8 S 7/2 transition §Energy dependent upon coordination environment àSpeciation àHydration àcomplexation constants

15 15-14 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

16 15-15 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

17 15-16 Cm oxide compounds Cm 2 O 3 §Thermal decomposition of CmO 2 at 600 °C and 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

18 15-17 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

19 15-18 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

20 15-19 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?

21 15-20 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?

22 15-21 Questions Comment on blog Provide response to PDF Quiz 15

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