10-1

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

10-2

Cm nuclear properties• Isotopes from mass 237 to

251• Three isotopes available in

quantity for chemical studies§ 242Cm, t1/2=163 d

à 122 W/gà Grams of the oxide

glowsà Low flux of 241Am

target decrease fission of 242Am, increase yield of 242Cm

§ 244Cm, t1/2=18.1 aà 2.8 W/g

§ 248Cm, t1/2= 3.48E5 aà 8.39% SF yield à Limited quantities

to 10-20 mgà Target for

production of transactinide elements

10-3

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

10-4

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

à K=128.24 KeV, LI=24.52 KeV, LII=23.65 KeV, LIII=18.9 KeV

10-5

Cm atomic and spectroscopic data

• 5f7 has enhanced stability§ Half filled orbital

à 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

10-6

10-7

Cm fluorescence

• Fluoresce from 595-613 nm§ Attributed to

6D7/28S7/2 transition

§ Energy dependent upon coordination environmentà Speciationà Hydrationà complexation

constants

10-8

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

10-9

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

10-10

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 (as discussed in Am

chapter)à 10 g/L solution in baseà Precipitation of K5AmO2(CO3)3 at 85 °Cà Precipitation of Cm with hydroxide, oxalate, or fluoride

10-11

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

10-12

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

10-13

Cm compounds

• Hydrides§ Reaction of metal with H2 at 250 °C

à fcc from XRD, CmH2+x

à Dihydride also forms• Halides

§ Complete CmX3 and CmF4

§ CmF3 precipitates with excess F-

à Anhydrous forms when compound placed over P2O5

§ CmCl3 from treating Cm oxides with anhydrous HCl between 400-600 °Cà Hexagonal UCl3 type structureà 9 Cl- coordination tricapped trigonal prism

§ CmBr3 from treating CmCl3 with NH4Br between 400-450 °Cà Orthorhombic structure (PuBr3)à Coordinated by 8 Br-

§ CmI3 from CmBr3 with NH4Ià Also from reactions with elements

§ CmF4

à Fluoride oxidation of CmF3

* Monoclinic ZrF4 structureØ Antiprismatic 8-coordination

à Some evidence of CmF6 and trivalent oxyfluorides

10-14

Cm oxides

• Cm2O3

§ Thermal decomposition of CmO2 at 600 °C and 10-4 torr§ Mn2O3 type cubic lattice

à Transforms to hexagonal structure due to radiation damage

à 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

10-15

Cm compounds• Oxides

§ Similar to oxides of Pu, Pr, and Tbà Basis of phase diagram

§ BaCmO3 and Cm2CuO4

à Based on high T superconductorsà Cm compounds do not conduct

• S, Se, Te compounds§ CmS2 and CmSe2 from Cm hydride and elements

heated under vacuumà Tetragonal structureà Thermal treatment of CmS2 yields Cm2S3 (bcc)

§ 1,1 species from heating elements 700-750 °Cà bcc structureà CmTe3 from heating at 400 °C

10-16

Cm compounds

• N, P, As, and Sb§ 1,1 species

à Cm metal or hydride with elements* Sealed tubes from 350-900 °C

à All have NaCl structure§ CmN and CmAs are ferromagnetic

à Lower effective magnetic moments than expected for 5f7 configuration* Strong spin-orbit coupling and crystal field

effects§ Formation of Pu,CmN species

à Lattice similar to known species parameters

10-17

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

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

10-18

Cm compounds

• Phosphates§ CmPO4

.0.5 H2O from aqueous solutions with Na2HPO4 or (NH4)2HPO4

§ Unknown structure§ Dehydrates at 300 °C

à 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

à 1,1,4 species

10-19

Cm compounds

• Organometallics § Studies hampered by radiolytic properties

of Cm§ Some compounds similar to Am

à Cm(C5H5)3 form CmCl3 and Be(C5H5)2

à Weak covalency of compoundà Strong fluorescence

10-20

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

10-21

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

10-22

Cm Analytical chemistry

• Typical alpha spectroscopy§ Odd A isotopes have lower energy

à May require separation prior to alpha spectroscopy* Utilization of TEVA resins or anion

exchange• Fission

§ Even isotopesà Requires pure isotopic sample

• TRLFS§ No chemical separation needed

10-23

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

10-24

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?

10-25

Pop Quiz

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