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
10-1
Cm nuclear properties
• Isotopes from mass 237 to
251
• Three isotopes available in
quantity for chemical
studies
242Cm, t =163 d

1/2
 122 W/g
 Grams of the
oxide glows
 Low
flux of
241Am target
decrease
fission
242
of Am,
increase
yield of
242Cm
244Cm, t =18.1 a

1/2
 2.8 W/g
248

Cm, t1/2= 3.48E5 a
 8.39% SF yield
 Limited
quantities to 1020 mg
 Target for
production of
transactinide
elements
10-2
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
10-3
Atomic and
spectroscopic data
• Ground state electron
configuration
 [Rn]5f76d17s2, Term symbol:
9D
2
 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
10-4
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
10-5
10-6
Cm fluorescence
• Fluoresce from 595-613
nm
 Attributed to
6D 8S
7/2
7/2
transition
 Energy dependent
upon coordination
environment
Speciation
Hydration
complexation
constants
10-7
Absorption and fluorescence process of Cm3+
Optical Spectra
Fluorescence Process
30
Wavenumber (10
3
-1
cm )
H
G
F
Emissionless
Relaxation
20
A
7/2
Excitation
10
Fluorescence
Emission
10-8
0
Z
7/2
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
10-9
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 valent Am (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
10-10
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
10-11
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
10-12
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 tricapped trigonal 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
10-13
 Some evidence of CmF6 and trivalent oxyfluorides
Cm oxides
• 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
10-14
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
10-15
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
10-16
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
10-17
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
10-18
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
10-19
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 2+ +
3+
(Cm +H2OCmOH +H )

K11=1.2E-6

Evaluated under
different ionic strength
10-20
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
10-21
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
10-22
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
10-23
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?
10-24
Pop Quiz
• Why does Cm have fewer accessible oxidation
states than Am?
10-25