Lecture 8: Plutonium Chemistry
• From: Chemistry of actinides
 Nuclear properties and isotope production
 Pu in nature
 Separation and Purification
 Atomic properties
 Metallic state
 Compounds
 Solution chemistry
8-1
Pu nuclear properties
• Isotopes from 228≤A≤247
• Important isotopes
238Pu

 237Np(n,g)238Np
* 238Pu from beta decay of 238Np
* Separated from unreacted Np by ion exchange
 Decay of 242Cm
 0.57 W/g
 Power source for space exploration
* 83.5 % 238Pu, chemical form as dioxide
* Enriched 16O to limit neutron emission
 6000 n s-1g-1
 0.418 W/g PuO2
 150 g PuO2 in Ir-0.3 % W container
8-2
Pu nuclear properties
•
239Pu
 2.2E-3 W/g
 Basis of formation of higher Pu isotopes
 244-246Pu first from nuclear test
• Higher isotopes available
 Longer half lives suitable for experiments
8-3
8-4
Pu in nature
• Most Pu due to anthropogenic sources
• 239,244Pu can be found in nature
 239Pu from nuclear processes occurring in U ore
 n,g reaction
* Neutrons from
 SF of U
 neutron multiplication in 235U
 a,n on light elements
* 24.2 fission/g U/hr, need to include neutrons from
235U
• 244Pu
 Based on Xe isotopic ratios
 SF of 244Pu
 1E-18 g 244Pu/g bastnasite mineral
8-5
8-6
Pu separations
• 1855 MT Pu produced

Current rate of 70-75 MT/years

225 MT for fuel cycle

260 MT for weapons
• Large scale separations based on manipulation of Pu oxidation
state

Aqueous (PUREX)

Non-aqueous (Pyroprocessing)
• Precipitation methods

Basis of bismuth phosphate separation
 Precipitation of BiPO4 in acid carries tri- and tetravalent
actinides
* Bismuth nitrate and phosphoric acid
* Separation of solid, then oxidation to Pu(VI)
 Sulfuric acid forms solution U sulfate, preventing
precipitation

Used after initial purification methods

LaF3 for precipitation of trivalent and tetravalent actinides
8-7
8-8
Pu separations
• Solvent extraction
 Some novel chemistry with third phase formation
http://www.nap.edu/books/0309052262/html/41.html
8-9
Pu separations
•
•
•
Extraction chromatography

Extractant on solid support
Ion-exchange

Both cation and anion exchange
 Anion exchange based on formation of appropriate species in acidic
solution
 Change of solution impact sorption to column
Pu separation

Sorb Pu(IV,VI) in 6 M acid, reduce to Pu(III)
8-10
Pu anion exchange
8-11
8-12
8-13
Pu cation exchange
• General cation exchange trends for Pu

HN03, H2S04, and HC104 show stronger influence than HC1

Strong increase in distribution coefficient in HClO4 at high
acidities exhibited for Pu(III) and Pu(VI)
8-14
8-15
Pu separations
• Alkaline solutions
 Need strong ligands that can compete with hydroxide
to form different species
 F-, CO32-, H2O2
* High solubility, based on oxidation state
* Stabilize Pu(VII)
• Room temperature ionic liquids
 Quaternary ammonium with anions
 AlCl4-, PF6O
O
N
 Liquid-liquid extraction
S
S
CF
F C
O
O
 Electrochemical disposition
3
3
O
NTf2
N
N
N
NTf2
N
NTf2
N R
8-16
NTf2
Pu separations
• Halide volatility (PuF6, PuCl6)
 PuO2 in fluidized bed reactor with fluorine at 400°
C
 Can substitute NH4HF2 for some fluorination
 Also use of O2F2
 PuF6 decomposes to PuF4 and F2 in a thermal
decomposition column
• Supercritical fluid extraction
 Most research with CO2
 Use complexants dissolved in SCF
 TBP.HNO3, TTA for extraction from soil
 Change of pressure to achieve separations
8-17
Pu atomic properties
• Ground state configuration [Rn]5f67s2
• Term symbol 7F0
• Optical emission Pu I spectra

Within 3 eV (24000 cm-1)
 5f56d7s2, 5f66d7s, 5f56d27s, 5f67s7p, 5f57s27p, and 5f56d7s7p
* Large number of electronic states and thousands of
spectral lines

Isotopic influence on spectra
8-18
Pu atomic properties
• Moessbauer spectroscopy
 238,239,240Pu
238Np beta decay, 44 keV photon
239Np beta decay, 57.3 keV photon
Alpha decay of 244Cm, 42.9 keV photon
8-19
Metallic Pu
• Interests in processing-structure-properties
relationship
• Reactions with water and oxygen
• Impact of self-irradiation
−3
Density
Liquid density at m.p.
Melting point
19.816 g·cm
−3
16.63 g·cm
912.5 K
Boiling point
3505 K
Heat of fusion
Heat of vaporization
Heat capacity
2.82 kJ·mol
−1
333.5 kJ·mol
−1
−1
(25 °C) 35.5 J·mol ·K
−1
8-20
Preparation of Pu metal
• Ca reduction
• Pyroprocessing

PuF4 and Ca metal
 Conversion of oxide to fluoride
 Start at 600 ºC goes to 2000 ºC
 Pu solidifies at bottom of crucible

Direct oxide reduction
 Direct reduction of oxide with Ca metal
 PuO2, Ca, and CaCl2

Molten salt extraction
 Separation of Pu from Am and lanthanides
 Oxidize Am to Am3+, remains in salt phase
 MgCl2 as oxidizing agent
* Oxidation of Pu and Am, formation of Mg
* Reduction of Pu by oxidation of Am metal
8-21
Pu metal
• Electrorefining
 Liquid Pu oxidizes from anode ingot into
 molten salt electrode
 740 ºC in NaCl/KCl with MgCl2 as oxidizing
agent
Oxidation to Pu(III)
Addition of current causes reduction of
Pu(III) at cathode
Pu drips off cathode
8-22
Pu metal
• Zone refining (700-1000 ºC)

Purification from trace impurities
 Fe, U, Mg, Ca, Ni, Al, K, Si, oxides and hydrides

Melt zone passes through Pu metal at a slow rate
 Impurities travel in same or opposite direction of melt
direction

Vacuum distillation removes Am

Application of magnetic field levitates Pu
8-23
http://arq.lanl.gov/source/orgs/nmt/nmtdo/AQarchive/98fall/magnetic_levitation.html
Pu phase stability
• 6 different Pu solid phases

7th phase at elevated pressure

fcc phase least dense
• Energy levels of allotropic phases are very close to each other

Pu extremely sensitive to changes in temperature, pressure,
or chemistry
• Densities of the allotropes vary significantly

dramatic volume changes with phase transitions
• Crystal structure of the allotropes closest to room temperature are
of low symmetry

more typical of minerals than metals.
• Pu expands when it solidifies from a melt
• Low melting point
• Liquid Pu has very large surface tension with highest viscosity
known near the melting point.
• Pu lattice is very soft vibrationally and very nonlinear
8-24
8-25
8-26
8-27
8-28
8-29
Pu metal phases
• Low symmetry ground state for a phase due to 5f
bonding
 Higher symmetry found in transition metals
• f orbitals have odd symmetry
 Basis for low symmetry (same as p orbitals Sn, In,
Sb, Te)
 odd-symmetry p orbitals produce directional
covalent-like bonds and low-symmetry noncubic
structures
• Recent local density approximation (LDA) electronicstructure calculations show narrow width of f bands
leads to low-symmetry ground states of the actinides
 Bandwidths are a function of volume.
 narrower for large volumes
8-30
Pu metal phase
•
•
•
•
ground-state as a function
of bandwidth for Nb and U
and

bct (body-centered
tetragonal) and ort
(orthorhombic), bcc
(body-centered
cubic)
When the f band in
uranium is forced to be
broader than 7 eV, the
high-symmetry bcc
structure is stable
Demonstrates narrow
bands favor lowersymmetry structures for U,
not that niobium
true equilibrium
bandwidths (Weq) are
narrow (larger volumes)
for the light actinides.
8-31
Pu metal phase
• atomic-sphere approximation
calculations for contributions to
orbitals

d fcc phase
• If Pu had only an f band contribution
equilibrium lattice constant smaller
than measured
• Contribution from s-p band stabilizes
larger volume
• f band is narrow at larger volume (low
symmetry)
• strong competition between the
repulsive s-p band contribution and the
attractive f band term induces
instability near the ground state
• density-of-states functions for different
low-symmetry crystal structures
• are very similar

total energies for different lowsymmetry crystal structures are
very close to each other
8-32
Pu metal phase
• For actinides f electron bonding increases up to Pu

Pu has the highest phase instability
• At Am the f electrons localize completely and become nonbonding

At Am coulomb forces pull f electrons inside the valence shell

leaving 2 or 3 in the s-p and d bands
• f-f interaction varies dramatically with very small changes in interatomic
distances

lattice vibrations or heating
• f-f and f-spd interactions with temperature may results in localization as
Pu transforms from the α- to the δ-phase
• Low Pu melting temperature due to f-f interaction and phase instability

Small temperature changes induce large electronic changes

small temperature changes produce relatively large changes in free
energy
• Kinetics important in phase transitions
8-33
8-34
Pu metallic radii
based on 12
coordinate and
extrapolated to
room
temperatures
8-35
Metallic Pu
• Pu liquid is denser that 3
highest temperature
solid phases
 Liquid density at
16.65 g/mL
 Pu contracts 2.5 %
upon melting
• Pu alloys and the d
phase
 Ga stabilizes phase
 Complicated phase
diagram
8-36
8-37
Phase
never
observed,
slow
kinetics
8-38
8-39
8-40
Metallic Pu
•
•
•
•
•
Other elements that stabilize d phase

Al, Ga, Ce, Am, Sc, In, and Tl stabilize
phase at room temperature

Si, Zn, Zr, and Hf retain phase under
rapid cooling
Microstructure of d phase due to Ga
diffusion in cooling
Np expands the a and b phase region

b phase stabilized at room
temperature with Hf, Ti, and Zr
Pu eutectics

Pu melting point dramatically reduced
by Mn Fe, Co, or Ni
 With Fe, mp=410 °C, 10 % Fe
 Used in metallic fuel

Limit Pu usage (melting through
cladding
Interstitial compounds

Large difference in ionic radii (59 %)

O, C, N, and H form interstitial
compounds
8-41
Metallic Pu
•
•
Electronic structure shows
competition between itinerant and
localized behavior

Boundary between magnetic
and superconductivity

5f electrons 2 to 4 eV bands,
strong mixing
 Polymorphism
 Solid state instability
 Catalytic activity
Isolated Pu 7s25f6, metallic Pu
7s26d15f5

Lighter than Pu, addition f
electron goes into conducting
band

Starting at Am f electrons
become localized
 Increase in atomic
volume
8-42
Metallic Pu
•
•
Modeling to determine electronic structure and bonding properties

Density functional theory
 Describes an interacting system of fermions via its density not via
the many-body wave function
 3 variables (x,y,z) rather than 3 for each electron
* For actinides need to incorporate
 Low symmetry structures
 Relativistic effects
 Electron-electron correlations

local-density approximation (LDA)
 Include external potential and Coulomb interactions
 approximation based upon exact exchange energy for uniform
electron gas and from fits to the correlation energy for a uniform
electron gas

Generalized gradient approximation (GGA)
 Localized electron density and density gradient
Total energy calculations at ground state
8-43
8-44
Modeling Pu metal electronic configuration
• Pu metal configuration 7s26d15f5
 From calculations, all eight valence electrons are
in the conduction band,
 5f electrons in α-plutonium behave like the 5d
electrons of the transition metals than the 4f of the
lanthanides
• Bonding and antibonding orbitals from sum and
differences of overlapping wavefunctions
 Complicated for actinides
 Small energy difference between orbital can
overlap in solids
 Accounts for different configurations
8-45
•
•
•
bandwidth narrows with increasing orbital angular momentum

Larger bands increase probability of electrons moving
 d and f electrons interact more with core electrons
Narrowing reflects

decreasing radial extent of orbitals with higher angular momentum, or
equivalently

decrease in overlap between neighboring atoms
8-46
Enough f electrons in Pu to be significant

Relativistic effects are important
Transition at Pu
8-47
• For Pu,
degree of f
electron
localization
varies with
phase
8-48
•
•
•
•
5f electrons extend
relatively far from the
nucleus compared to
the 4f electrons

5f electrons
participate in
chemical
bonding
much-greater radial
extent of the
probability densities
for the 7s and 7p
valence states
compared with 5f
valence states
5f and 6d radial
distributions extend
farther than shown by
nonrelativistic
calculations
7s and 7p
distributions are
pulled closer to the
ionic cores in
relativistic
calculations
8-49
Pu metal physical and thermodynamic
properties
• Already reviewed density and thermal expansion
• Heat capacity
 Difficulties in measurement due to self-heating and
damage
 239Pu 2.2 mW/g
 Low temperature measurements do not permit
annealing
 Use of 242Pu helps overcome decay related issues
8-50
The Specific Heat of Plutonium
and Other Metals
•
The low-temperature specific
heat of a metal is the sum of
a lattice term an electronic
term
•
In this figure, the line for
copper represents the
behavior of most metals
whereas the lines for α- and
δ-plutonium have the highest
values of γ (intercept values)
of any pure element

Indicating that
conduction electrons
have an enhanced
effective mass
•
The compound UBe13 has an
extremely high electronic
specific heat, which
continues to increase until it
is cut off by the compound’s
transition to
superconductivity just below
1K

The
superconductivity of
UBe13 proves that
its large heat
capacity must be
associated with the
conduction electrons
8-51
Pu metal magnetic behavior
• Magnetic susceptibility (c)
 Internal response of material to applied
magnetic field
 M = χB,
M is magnetization of the material
B is the magnetic field intensity
• Large values for Pu and alloys
8-52
• Susceptibilities of Pu
higher than most
metals,
• lower than materials
with local moment
• Variation in
susceptibility as
plutonium changes
phase
• increase slightly as
temperature
decreases
8-53
8-54
Pu metal mechanical properties
• Related to crystal structure and melting point
 Pu has a range of structures with different
melting points
Results in a variety of mechanical
properties
* Stress, oxidation, corrosion,
pyrophoricity, self-irradiation
• Sensitive to chemistry (alloying) and processing
(microstructure)
8-55
Pu metal mechanical properties
• Stress/strain properties
 High strength properties bend or deform rather
than break
 Beyond a limit material abruptly breaks
* Fails to absorb more energy
8-56
Pu metal mechanical properties
• α-plutonium is strong and brittle, similar to cast iron
 elastic response with very little plastic flow
 Stresses increase to point of fracture
 strength of the unalloyed α-phase decreases
dramatically with increasing temperature
 Similar to bcc and hcp metals.
• Pu-Ga δ-phase alloys show limited elastic response
followed by extensive plastic deformation
 low yield strength
 ductile fracture
8-57
• For α-Pu elastic limit is basically fracture
strength
• The Pu-Ga alloy behaves more like Al
 Fails by ductile fracture after elongation 8-58
• Tensile-test results for
unalloyed Pu

Related to temperature
and resulting change in
phases
• Strengths of α- and β-phase
are very sensitive to
temperature

Less pronounced for γphase and δ-phase
• data represent work of several
investigators

different purity
materials, and different
testing rates
 Accounts for
variations in values,
especially for the αPu phase
8-59
Pu metal mechanical properties
• Metal elastic response due to electronic structure and resulting
cohesive forces

Metallic bonding tends to result in high cohesive forces and
high elastic constants
 Metallic bonding is not very directional since valence
electrons are shared throughout the crystal lattice
 Results in metal atoms surrounding themselves with as
many neighbors as possible
* close-packed, relatively simple crystal structures
• The Pu 5f electrons have narrow conduction bands and high
density-of-states

energetically favorable for ground-state crystal structure to
distort to low-symmetry structures at room temperature

Pu has typical metal properties at elevated temperatures or
in alloys
8-60
Pu metal corrosion and oxidation
• Formation of oxide layer

Can include oxides other than dioxide

Slow oxidation in dry air
 Greatly enhanced oxidation rate in presence of water or
hydrogen
• Metal has pyrophoric properties
• Corrosion depends on chemical condition of Pu surface

Pu2O3 surface layer forms in absence or low amounts of O2
 Promotes corrosion by hydrogen
• Pu hydride (PuHx, where 1.9 < x < 3) increases oxidation rate in O2
by 1013
• PuO2+x surface layer forms on PuO2 in the presences of water

enhances bulk corrosion of Pu metal in moist air
8-61
• O2 sorbs on Pu surface to
form oxide layer
• Oxidation continues but O2
must diffuse through oxide
layer

Oxidation occurs at
oxide/metal interface
• Oxide layer thickness initially
increases with time based on
diffusion limitation
• At oxide thickness around 4–5
μm in room temperature
surface stresses cause oxide
particles to spall

oxide layer reaches a
steady-state thickness
 further oxidation
and layer removal
by spallation
• Eventually thickness of oxide
layer remains constant
8-62
Steady state in dry air at room temperature
• steady-state layer of Pu2O3 at oxide-metal interface
 Pu2O3 thickness is small compared with the oxide
thickness at steady state
 Autoreduction of dioxide by the metal at the oxide
metal interface produces Pu2O3
 Pu2O3 reacts with the diffusing O2 to form
dioxide
8-63
Arrhenius Curves for Oxidation of Unalloyed and Alloyed Plutonium in
Dry Air and Water Vapor
•
•
•
•
•
•
•
•
ln of the reaction rate R versus 1/T

slope of each curve is proportional to
the activation energy for the
corrosion reaction
Curve 1 oxidation rate of unalloyed
plutonium in dry air or dry O2 at a pressure
of 0.21 bar.
Curve 2a increase in the oxidation rate when
unalloyed metal is exposed to water vapor up
to 0.21 bar, equal to the partial pressure of
oxygen in air
Curves 2b and 2c show the moistureenhanced oxidation rate at water vapor
pressure of 0.21 bar in temperature ranges of
61°C–110°C and 110°C–200°C, respectively
Curves 1’ and 2’ oxidation rates for the δphase gallium-stabilized alloy in dry air and
moist air (water vapor pressure ≤ 0.21 bar),
respectively
Curve 3 transition region between the
convergence of rates at 400°C and the onset
of the autothermic reaction at 500°C
Curve 4 temperature-independent reaction
rate of ignited metal or alloy under static
conditions

rate is fixed by diffusion through an
O2-depleted boundary layer of N2 at
the gas-solid interface
Curve 5 temperature-dependent oxidation
rate of ignited droplets of metal8-64
or alloy
during free fall in air
Oxide Layer on Plutonium Metal under Varying Conditions
•
•
•
•
•
•
corrosion rate is strongly dependent on the metal
temperature

varies significantly with the isotopic
composition,quantity, geometry, and
storage configuration
steady-state oxide layer on plutonium in dry air at
room temperature (25°C) is shown at the top

(a) Over time, isolating PuO2-coated
metal from oxygen in a vacuum or an
inert environment turns the surface oxide
into Pu2O3 by the autoreduction reaction

At 25°C, the transformation is slow

time required for complete reduction of
PuO2 depends on the initial thickness of
PuO2 layer
 highly uncertain because reaction
kinetics are not quantified
above 150°C, rapid autoreduction transforms a
several micrometer-thick PuO2 layer to Pu2O3
within minutes

(b) Exposure of the steady-state oxide
layer to air results in continued oxidation
of the metal
Kinetic data indicate that a one-year exposure to
dry air at room temperature increases the oxide
thickness by about 0.1 μm
At a metal temperature of 50°C in moist air (50%
relative humidity), the corrosion rate increases by a
factor of approximately 104

corrosion front advances into unalloyed
metal at a rate of 2 mm per year
150°C–200°C in dry air, the rate of the
autoreduction reaction increases relative to that of
the oxidation reaction
8-65

steady-state condition in the oxide shifts
toward Pu2O3,
PuO2+x Study
• Examined Pu oxides by two methods
 X-ray diffraction (XRD)
Gives information about structure
* Lattice parameters
 X-ray photoelectron spectroscopy (XPS)
Used to evaluate binding of oxygen
• Examined reaction of PuO2 with H2O from 25 °C
to 350 °C
8-66
Results
• Mass spectrometric analysis shows production
of H2(g)
 PuO2(s)+xH2O(abs) <--> PuO2+x(s)+ xH2(g)
350°C
H2 pressure
formation
300°C
250°C
200°C
Time (hr)
8-67
Results
• Lattice parameter change
 Attributed to increase in Pu:O ratio
Cubic lattice parameter
variation
ao=5.3643+0.01746 (O:Pu)
Relative insensitivity
attributed to formation of
Pu(VI)
Extra O forms plutonyl
O/Pu ratio
8-68
Results
• X-ray photoelectric spectroscopy data
 High binding energies for the oxide
442 eV, 429 eV
* 4f5/2, 4f7/2
 Pu(VI) or Pu(VII)
 No Pu(V)
 O 1s spectrum consistent with oxide
Absence of OH- attributed to continued
reaction of water
8-69
Results
• PuO2+x formed via catalytic cycle
Driven by H2O sorbed
to surface
If O2 present, H
reforms water
Formation of water
drives catalytic cycle
8-70
Pu oxide coating reaction with H2
•
•
•
•
•
•
•
•
•
•
Plutonium hydride (PuHx)

fcc phase

forms a continuous solid solution for 1.9 < x < 3.0
 Pu(s) + (x/2)H2(g) → PuHx(s)
observed value of x depends on hydrogen pressure and temperature
hydride is readily oxidized by air
decomposes back to its component
elements when heated in continuously pumped vacuum
Hydriding occurs only after the ubiquitous dioxide layer on the metal is penetrated
Unlike oxidation the reaction of hydrogen initiates at a limited number of
nucleation sites
a single nucleation site typically appears only after a lengthy, but unpredictable,
induction period
Once formed sites are the most reactive areas of the surface

Hydriding rate is proportional to the active area covered by the hydride

Increases exponentially over time to a maximum value as sites grow and
ultimately cover the surface

. At that point, the rate
Temperatures between –55°C and 350°C and a H2 pressure of 1 bar

reaction at a fully active surface consumes plutonium at a constant rate of
6–7 g/cm2 min
8-71

Advances into metal or alloy at about 20 cm/h
Rates for Catalyzed Reactions of Pu with H2, O2, and Air
• Diffusion-limited oxidation
data shown in gray
compared to data for the
rates of reactions catalyzed
by surface compounds
• oxidation rates of PuHxcoated metal or alloy in air
• the hydriding rates of
PuHx- or Pu2O3-coated
metal or alloy at 1 bar of
pressure,
• oxidation rates of PuHxcoated metal or alloy in O2
• rates are extremely rapid,
• values are constant

indicate the surface
compounds act as
catalysts
8-72
Hydride-Catalyzed Oxidation of Pu
•
•
•
•
•
•
After the hydride-coated metal or alloy is exposed to
O2, oxidation of the pyrophoric PuHx forms a surface
layer of oxide and heat
H2 formed by the reaction moves into and through the
hydride layer to reform PuHx at the hydride-metal
interface
sequential processes in reaction

oxygen adsorbs at the gas-solid interface as
O2

O2 dissociates and enters the oxide lattice as
an anionic species

thin steady-state layer of PuO2 may exist at
the surface

oxide ions are transported across the oxide
layer to the oxide-hydride interface
 oxide may be Pu2O3 or PuO2–x (0< x
<0.5

Oxygen reacts with PuHx to form heat (~160
kcal/mol of Pu) and H2
H2 produced at the oxide-hydride interface moves
through the PuHx layer to the hydride-metal interface
reaction of hydrogen with Pu produces PuH2 and heat
8-73
rupture in inner container for Pu metal
8-74
• increase in the
inner
vessel’s
diameter near
the ruptured
end
shows the
extent of
hydridecatalyzed
corrosion
during a 3hour period
8-75
Radiation damage
• Decay rate for 239Pu is sufficient to produce radiation
damage
 Buildup of He and radiation damage within the
metal
• radiation damage is caused mainly by the uranium
nuclei
 recoil energy from the decay to knock plutonium
atoms from their sites in the crystal lattice of the
metal
 Vacancies are produced
• Effect can produce void swelling
• On the microscopic level, the vacancies tend to diffuse
through the metal and cluster to form voids
• Macroscopically, the net effect the metal swells
8-76
Pu Decay and the Generation of Defects
• α particle has a range of about 10 μm through the Pu
• uranium nucleus range is only about 12 nm
• Both particles produce displacement damage

Frenkel pairs
 namely vacancies and interstitial atoms

Occurs predominantly at the end of their ranges
• Most of the damage results from the uranium nucleus and is
confined to the collision cascade region
8-77
Stages for Radiation-Induced Void Swelling
8-78
Predictions for Radiation-Induced Damage in Pu
• predicted contributions to
volume distortion in
stabilized plutonium at
70°C
• Distortions due to void
swelling are likely to be
much larger than those due
to helium-bubble formation
• large uncertainty in the
transient period prevents
estimating when the void
swelling should begin its
linear growth rate
• figure shows several
possible swelling curves
8-79
Pu Compounds
• Original difficulties in producing compounds

Amount of Pu

Purity
• Aided by advances in microsynthesis and increase in amount of
available starting material
• Much early effort in characterization by XRD
Pu Hydrides
• PuHx

x varies from 1.9< x <3.0

Pu + x/2 H2PuHx
 H2 partial pressure used to control exact stoichiometry
 Variations and difficulties rooted in desorption of H2
• Pu hydride crystallizes in a fluorite structure
8-80
Pu hydride
•
•
Pu hydride oxidation state

PuH2 implies divalent Pu, an unstable oxidation state Pu(II)

measurements show Pu as trivalent and PuH2 is metallic
 Pu(III), 2 H- and 1e Electron in conduction band

Consistent with electrical conductivity measurements showing PuHx progressively
changes from a metallic to semiconductor with increasing x

Electrons removed from conduction band and bound as H– on octahedral sites as the
hydride increases
Phase relationships

Two differing phase diagrams
 Temperature, pressure, reaction rates differ
 Dependence on the formation of saturated Pu hydride
* Hydrogen saturated Pu (PuHs) forms and co-exists with Pu hydride
* 5f electrons localized in Pu(III) compounds

High pressure hydride synthesis results in more complex diagram
 Coexistence of cubic (PuH2.77) and hexagonal PuH2.88 (Region III and IV).
 Orthorhombic PuH2.95 (Region V)
 Region VI in high pressure hydride synthesis phase diagram is unknown
8-81
8-82
8-83
Pu hydride
• Solid state structure
 Similar to lanthanide trifluoride system
 Cubic from PuHx 1.9 to 2.7
 Decrease in lattice parameter with increasing x
* 5.36 to 5.34 Å
 Hexagonal beyond x=2.9
 Hydrogen mobility in structure
 H found at octahedral and tetrahedral sites
 Replacement with deuterium
 Neutron scattering to correlate H location with
stoichiometry
 Structure becomes complicated at x near 3
8-84
8-85
8-86
Aries process
• Hydride used to prepare
metal
 Formation of
hydride from metal
 Heated to 400 °C
under vacuum to
release hydrogen
 Can convert to oxide
(with O2) or nitride
(N2) gas addition
during heating
8-87
Pu borides
•
•
•
•
•
Range of compounds

PuBx x= 2, 4, 6, 12, 66

Potential storage or waste form for Pu

High melting points
 Little work performed on compounds
Prepared from heating elements

Under vacuum between 900 °C and 1200 °C

Arc melting under Ar
Pu hydride can also act as starting material
Structure

Dominated by B-B bonding
 Similar to most metal borides
 Pu occupies vacant sites
* Power XRD, no single crystal data
Little data on properties

Some data on magnetic properties
 Suggest tetravalent Pu for diborides
* Based on comparison to Np complexes
8-88
8-89
8-90
8-91
Pu carbides
•
•
Four known compounds

Pu3C2, PuC1-x, Pu2C3, and PuC2

PuC exists only as substoichiometric compound
 PuC0.6 to PuC0.92

Compound considered candidate for fuels
Synthesis

At high temperatures elemental C with:
 Pu metal
 Pu hydrides
 Pu oxides
* Oxygen impurities present with oxide starting material
* High Pu carbides can be used to produce other carbides
 PuC1-x from PuH2 and Pu2C3 at 700 °C

Final product composition dependent upon synthesis temperature,
atmosphere (vacuum or Ar) and time
8-92
Pu carbides
• Structure
 Lattice constant depends upon composition
 As C content increases, C are replaced by C2 units
 Pu2C3 from Pu4(C2)3
* Cubic structure with 12 C2 units in cell
* Studied by XRD and neutron scattering
 XRD not accurate for C
 Neutron scattering shows C-C of 1.295
Å in Pu2C3
 C2 bond in acetylene is 1.20 Å
 PuC2
 Variation of XRD data with temperature
* High temperature (1710 °C) cubic
* Room temperature tetragonal unit cell
8-93
Pu carbides
• Chemical properties

PuC1-x oxidizes in air starting at 200 °C

Slower reaction with N2
 Formation of PuN at 1400 °C

Pu2C3 has reactions similar to PuC1-x

All Pu carbides dissolve in HNO3-HF mixtures
 Liberation of CO2 with oxidizing acids
 With lower carbides formation of other organics
* Mellitic and oxalic acids
• Thermodynamic properties

PuC1-x evaporates upon heating
• Ternary phases prepared

Pu-U-C
 M3C2, MC1-x, M2C3, and MC2 observed

Pu-Th-C

Mixed carbide-nitrides, carbide-oxides, and carbide hydrides
have been prepared
8-94
8-95
Pu-silicon system
•
•
•
Five known Pu-Si compounds

5:3, 3:2, 1:1, 3:5, and 1:2 (Pu:Si)

Highest melting point for 3:5 at 1646 °C
Synthesis

Reaction with PuF3 at 1200 °C under vacuum
 4 PuF3 + (3+4x)Si4 PuSix + 3 SiF4
* SiF4 is volatile and removed
 Arc melting of Si and Pu or PuHx under Ar
 PuO2 with Si or SiC at 1400 °C under vacuum
Structures

Commonalities with borides

Production of isolated Si2 units or structures (chains, layers, networks) with
increased Si content
12.0
Properties
11.5

Metallic appearance
11.0

Pyrophoric

Oxidize in air to form PuO2
10.5

Reacts with water

High melting point and high densities 10.0
 8.96 g/cm3 (Pu3Si5)
9.5
3
 10.151 g/cm (PuSi)
9.0
 11.98 g/cm3 (Pu5Si3)
Density
•
8.5
0.4
8-96
0.6
0.8
1
1.2
Pu/Si
1.4
1.6
1.8
8-97
8-98
Pu pnictides
• Basic compounds

Highest order PuX2

Prepared by reaction of Pu metal or hydride in sealed quartz
tube at 400-750 °C
• Pu-nitrogen system

Only PuN known with certainty
 Narrow composition range
 Liquid Pu forms at 1500 °C
* PuN melting point not observed

Preparation
 Pu hydride with N2 between 500 °C and 1000 °C
 Can react metal, but conversion not complete
 Formation in liquid ammonia
* PuI3 + NH3 +3 M+ PuN + 3 MI+ 1.5 H2
 Intermediate metal amide MNH2 formation
8-99
 Pu precipitates
8-100
Pu nitride
•
•
Structure

fcc cubic NaCl structure

Lattice 4.905 Å
 Data variation due to impurities, self-irradiation

Pu-N 2.45 Å

Pu-Pu 3.47 Å
Properties

High melting point (estimated at 2830 °C)

Compatible with steel (up to 600 °C) and Na (890 °C, boiling point)

Reacts with O2 at 200 °C

Reaction rates increase with H2O vapor

Dissolves in mineral acids
 Rapidly with HNO3

Moderately delocalized 5f electrons
 Increases with atomic number of ligand
 Behavior consistant with f5 (Pu3+)
 Supported by correlated spin density calculations
8-101
Pu-P system
• Formed from
 Pu hydride with PH3 at elevated
temperature
 Pu hydride with excess red phosphorus in
pressure vessel at 600-800 °C under Ar
Excess P removed by distillation at 300
°C
• Melts with decomposition at 2600 °C
• Pu As and Pu Sb compounds also form
 Pu4Sb3 formed in addition to mono species
8-102
Pu oxide
• Pu storage, fuel, and power generators
• Important species

Corrosion

Environmental behavior
• Different Pu oxide solid phases

PuO

Pu2O3
 Composition at 60 % O
 Different forms at PuOx
* x=1.52, bcc
* x=1.61, bcc

PuO2
 fcc, wide composition range (1.6 <x<2)
8-103
8-104
Pu oxide preparation
• PuO
 Existence of phase uncertain
 Definitely identified in gas phase
* IR spectrum
 Not indicated in phase diagram
 Surface film on Pu metal
 Molten Pu metal with stoichiometric Ag2O
 Reduction of PuO2 with C at 1500-1800 °C
 Reduction of PuOCl or PuO2 with Ba vapor
• PuO reacts violently with O2
 Some discussion on PuO actually PuOC
 Oxycarbide does not react violently with O2
8-105
Pu oxide preparation
•
Pu2O3

Hexagonal (A-Pu2O3) and cubic (C-Pu2O3)
 Distinct phases that can co-exist
 No observed phase transformation
* Kinetic behavior may influence phase formation of cubic
phase
 C-Pu2O3 forms on PuO2 of d-stabilied metal when
heated to 150-200 °C under vacuum
 Metal and dioxide fcc, favors formation of fcc Pu2O3
 Requires heating to 450 °C to produce hexagonal
form
 Not the same transition temperature for reverse
reaction
 Indication of kinetic effect

Formed by reaction of PuO2 with Pu metal, dry H2, or C
 A-Pu2O3 formed
 PuO2+Pu2Pu2O3 at 1500 °C in Ta crucible
* Excess Pu metal removed by sublimation
 2PuO2+CPu2O3 + CO
8-106
Pu oxide preparation
• Hyperstoichiometric sesquioxide (PuO1.6+x)
 Requires fast quenching to produce of PuO2 in
melt
 Slow cooling resulting in C-Pu2O3 and PuO2-x
 x at 0.02 and 0.03
• Substoichiometric PuO2-x
 From PuO1.61 to PuO1.98
 Exact composition depends upon O2 partial
pressure
 Single phase materials
 Lattice expands with decreasing O
8-107
8-108
Pu oxide preparation
• PuO2

Pu metal ingited in air

Calcination of a number of Pu compounds
 No phosphates
 Pu crystalline PuO2 formed by heating Pu(III) or Pu(IV)
oxalate to 1000 °C in air
* Oxalates of Pu(III) forms a powder, Pu(IV) is tacky
solid
 Rate of heating can effect composition due to
decomposition and gas evolution

PuO2 is olive green
 Can vary due to particle size, impurities

Pressed and sintered for heat sources or fuel

Sol-gel method
 Nitrate in acid injected into dehydrating organic (2ethylcyclohexanol)
 Formation of microspheres
8-109
* Sphere size effects color
Pu oxide preparation
• PuO2+x, PuO3, PuO4
 Tetravalent Pu oxides are favored
 Unable to oxidize PuO2
* High pressure O2 at 400 °C
* Ozone
 PuO2+x reported in solid phase
 Related to water reaction
* PuO2+xH2OPuO2+x + xH2
* Final product PuO2.3, fcc
 PuO3 and PuO4 reported in gas phase
 From surface reaction with O2
* PuO4 yield decreases with decreasing O2
partial pressure
8-110
8-111
8-112
Pu oxide structures
• Lattice changes with O/Pu ratio
 fcc commonality with PuO2
 Related to fluorite structure
8-113
8-114
8-115
8-116
Pu oxide
•
•
•
Oxygen at 1.84 Å

Similar to Pu=O in Pu(V) complexes
 1.85 Å

Interpreted as mixture of Pu(IV) and Pu(V)
 Oxidation by electron transfer to O
 f4 to f3 transition
Properties

Interstitial excess O and O vacancies are mobile
 Interstitials are more mobile

Similar to O behavior in U
 Studied by gas phase isotope exchange
Vaporization complicated

Some composition change upon heating
 C-Pu2O3 decomposes to A-Pu2O3 and PuO1.6+x
 Data on vaporization conflicting
* Dependent upon technique, interaction with matrix

PuO2 goes to PuO1.831
 Gas phase PuO2+ and PuO+
8-117
8-118
Pu oxide
• Chemical properties

Thermodynamic parameter available for Pu oxides

Dissolution
 High fired PuO2 difficult to dissolve
 Rate of dissolution dependent upon temperature and
sample history
* Irradiated PuO2 has higher dissolution rate with
higher burnup
 Dissolution often performed in 16 M HNO3 and 1 M HF
* Can use H2SiF6 or Na2SiF6
 KrF2 and O2F2 also examined
 Electrochemical oxidation
* HNO3 and Ag(II)
 Ce(IV) oxidative dissolution
8-119
Pu S, Se, and Te systems
• Forms PuX, Pu2X3, PuX2-x
 PuTe3
• Prepared from stoichiometric reaction of PuHx and
elements in sealed quartz
 1 week at 350-750 °C
 PuX2-x
 Decomposition to form other ratios
• Direct from elements
• Structures
 All PuX are fcc
 All Pu2X3 are bcc or orthorhombic
 PuX2-x are tetragonal with a PuS2 monoclinic
 PuTe3 pseudo-tetragonal
8-120
8-121
Pu S, Se, Te
• Properties
 Metallic luster
PuS: Au color
PuSe: Cu color
PuTe: Black
 Nonmagnetic
Semiconductors
 f-d hybridization
8-122
Alkali metal oxoplutonates
• Formed from PuO2 and alkali metal oxides, hydroxides, peroxides, or
carbonates

Variation in atmosphere
 O2, inert gas, vacuum
• Pu(IV)

Li8PuO6
• Pu(V)

Li7PuO6, Li3PuO4, Na3PuO4
 Oxidizing atmosphere
• Pu(VI)

M6PuO6, M4PuO5 (Li, Na)

M2PuO4 (K, Rb, Cs)
 From oxides in oxygen atmosphere
• Pu(VII)

M5PuO4 (Li, Na)

M3PuO5 (Rb, Cs)
8-123
Group II Pu oxides
• Pu(III)

BaPu2O4
 BaO, Pu, and PuO2 in H2
* Formation of PuHx from Pu metal
* Atmosphere switched to inert
• Pu(IV)

Sr and Ba compounds only

MPuO3
• Pu(V)

Ba3PuO5.5
 From Ba3PuO6, PuO2, and BaO
 Oxidation state of compound is uncertain
• Pu(VI)

MPuO4, M3PuO6 (Ca, Sr, and Ba)

Ba2MPuO6 (Sr, Mn, Pb, Mg, Ca)
8-124
Structures
• Perovskites
 CaTiO3 structure (ABO3)
 Pu(IV, VI, or VII) in octahedral PuO6n Cubic lattice
 BO6 octahedra with A cations at center unit cell
• Double perovskites
 (Ba,Sr)3PuO6 and Ba(Mg,Ca,Sr,Mn,Zn)PuO6
 M and Pu(VI) occupy alternating octahedral sites
in cubic unit cell
8-125
8-126
8-127
8-128
• Pu-Ln oxides
 PuO2 mixed with LnO1.5
 Form solid solutions
Oxidation of Pu at higher levels of Ln
oxides to compensate for anion defects
 Solid solutions with CeO2 over entire range
8-129
Ternary oxides of Pu and actinides
• Prepared with Th, Pa, U, and Cm
• ThO2
 Solid solutions over entire range
 Follows Vegard’s Law
 At 1000 °C
 Melting points constant up to 25 % wt ThO2,
increase linearly with increasing ThO2
 At 1650 °C under Ar partial phase separtion
 C-Pu2O3
8-130
U-Pu-Oxides
• MOX fuel
 2-30 % PuO2
• Lattice follows Vegard’s law
• Different regions
 Orthorhombic U3O8 phase
 Flourite dioxide
Deviations from Vegard’s law may be
observed from O loss from PuO2 at
higher temperature
8-131
8-132
• Prepared by precipitation process or co-milling
• Properties examined
 O potential
 Thermal
8-133
Plutonium halides
• General formula PuX3
 PuF4 stable solid, PuCl4 can be found in gas phase
 PuF6 gas phase
 f2 electron configuration
• Trivalent oxyhalides
 PuOX
 Some different oxyfluorides can be formed
 PuOF3, PuOF4, PuO2F2
 A range of fluoride salts with monovalent cation
 General formula MxPuF4+x
* x = 1, 2, 3, 4
* M7Pu6F31 and MPuF6
8-134
Pu fluoride preparation
• Used in the preparation of Pu metal
• 2PuO2 + H2 +6 HF 2 PuF3 + 4 H2O at 600 °C
• Pu2(C2O4)3 + 6 HF2 PuF3 + 3 CO + 3 CO2 + 3 H2O at
600 °C
 At lower temperature (RT to 150 °C) Pu(OH)2F2
or Pu(OH)F3 forms
 PuF3 from HF and H2
 PuF4 from HF and O2
 Other compounds can replace oxalates (nitrates,
peroxides)
• Stronger oxidizing conditions can generate PuF6
 PuO2 + 3 F2 PuF6 + O2 at 300 °C
 PuF4 + F2  PuF6 at 300 °C
8-135
Pu fluoride preparation
• PuF3
 Insoluble in water
 Prepared from addition of HF to Pu(III) solution
 Reduce Pu(IV) with hydroxylamine (NH2OH) or
SO2
 Purple crystals
 PuF3.0.40H2O
* Anhydrous PuF3 formed by heating in HF gas
at 200-300 °C
* heating starting material in H2 from 150 to
600 °C, then HF at 200-300 °C
* Heating PuF4 in H2 from 600 °C
8-136
Pu fluoride preparation
• PuF4
 In soluble in H2O
 From the addition of HF to Pu(IV) solution
* Pale pink PuF4.2.5H2O
* Soluble in nitric acid solutions that form fluoride
species
 Zr, Fe, Al, BO33 Heating under vacuum yields trifluoride
 Formation of PuO2 from reaction with water
* PuF4+2H2OPuO2+4HF
 Reaction of oxide with fluoride
* 3PuF4+2PuO24PuF3+O2
 Net: 4PuF4+2H2O4PuF3+4HF+O2
* High vacuum and temperature favors PuF3
formation
8-137
 Anhydrous forms in stream of HF gas
Pu fluoride preparation
• PuF6

Formation from reaction
of F2 and PuF4

Fast rate of formation
above 300 °C
 Reaction rate
* Log(rate/mg PuF4
cm-2hr-1=5.9172719/T)
 Faster reaction at 0.8
F2 partial pressure

Condensation of product
near formation
 Liquid nitrogen in
copper condenser
near PuF4

Can be handled in glass
8-138
Pu fluoride structures
• PuF3
 Each Pu surrounded
by 9 F
8-139
Pu fluoride structures
• PuF4
 Isostructural with An
and Ln tetraflourides
 Pu surrounded by 8 F
Distorted square
antiprism
• PuF6
 Gas phase Oh
symmetry
8-140
Pu fluoride properties
• PuF3
 Melting point: 1425 °C
 Boiling point: decomposes at 2000 °C
• PuF4
 Melting point: 1037°C
• PuF6
 Melting point: 52°C
 Boiling point: 62°C
 ΔsublH°=48.65 kJ/mol, ΔfH°=-1861.35 kJ/mol
 IR active in gas phase, bending and stretching
modes
 Isotopic shifts reported for 239 and 242
8-141
8-142
• PuF6
• Equilibrium
constant measured
for PuF6PuF4+F2
 ΔG=2.55E4+5.2
7T
 At 275 °C,
ΔG=28.36
kJ/mol
 ΔS=-5.44 J/K
mol
 ΔH=25.48
kJ/mol
8-143
Pu halides
• PuF6 decomposition

Alpha decay and temperature
 Exact mechanism unknown

Stored in gas under reduced pressure
• Higher halide preparation

PuCl3 from hydrochlorination
 Pu2(C2O4)3.10H2O+6HCl2PuCl3+3CO2+3CO+13H2O
 Reaction of oxide with phosgene (COCl2) at 500 °C
 Evaporation of Pu(III) in HCl solution

PuCl4
 PuCl3+0.5Cl2PuCl4
* Gas phase
* Identified by peaks in gas phase IR
8-144
Pu halides
• PuBr3
 Combination of elements
 HBr with PuHx
 Pu(III) oxalate with HBr, 400-600 °C
• PuI3
 Pu metal with HI at 400 °C
 2Pu+3HgI2 2PuI3+3Hg
• Structures (PuCl3)
 9 Cl for each Pu
 Tricapped trigonal prism
8-145
Pu halides
• PuBr3 and PuI3 structure
 Isostructural
 6 Pu-Br of 3.08 Å
 2 Pu-Br caps of 3.06 Å
8 coordinate Pu
Br and I larger than Cl
* Enhanced electron repulsion
• Properties
 PuCl3 free energy of formation determined
ΔG=-924.7+0.22292T
8-146
 PuOX from reaction with H2O
Pu oxyhalides
• Only prepared with Pu(III) and
Pu(VI)
 PuOX for all halides from
water reaction with PuX3
 Pu(VI) species
 Excess PuF6 with water
and HF
* PuO2F2
 PuO2Cl2 from vacuum
evaporation of PuCl6 at
room temperature
8-147
Ternary halogenoplutonates
• Pu(III-VI) halides with ammonia, group 1, group 2,
and some transition metals
• Preparation
 Metal halides and Pu halide dried in solution
 Metal halides and PuF4 or dioxide heat 300-600 °C
in HF stream
 PuF4 or dioxide with NH4F heated in closed vessel
at 70-100 °C with repeated treatment
 PuF6 or PuF4 with group 1 or 2 fluorides
8-148
8-149
8-150
8-151
Pu solution chemistry
• Originally driven by the need to separate and purify Pu
• Species data in thermodynamic database
• Complicated solution chemistry
 Five oxidation states (III to VII)
 Small energy separations between oxidation states
 All states can be prepared
* Pu(III) and (IV) more stable in acidic solutions
* Pu(V) in near neutral solutions
 Dilute Pu solutions favored
* Pu(VI) and (VII) favored in basic solutions
 Pu(VII) stable only in highly basic
solutions and strong oxidizing conditions
 Some evidence of Pu(VIII)
8-152
Pu solution chemistry
• Pu3+ and Pu4+ simple hydrates free species
• Plutonyl oxo species for Pu(V) and Pu(VI)
 Pu(V) effective charge 2.2
 Pu(VI) effective charge 3.2
• PuO4• Redox chemistry instrumental in identifying species
8-153
Pu solution chemistry
• Coordination number varies

Large values, 8 to 10 for water coordination
• Spectroscopic properties

A few sharp bands
 5f-5f transitions
* More intense than 4f of lanthanides
* Relativistic effects accentuate spin-orbit coupling
* Transitions observed spectroscopically
 Forbidden transitions
 Sharp but not very intense
• Pu absorption bands in visible and near IR region

Characteristic for each oxidation state
8-154
8-155
8-156
8-157
8-158
8-159
8-160
8-161
8-162
Pu solution chemistry
• Other spectroscopic methods employed in Pu
analysis
 Photoacoustic spectroscopy
 Thermal lensing
• Vibrational spectroscopy
 Oxo species
Asymmetric stretch 930-970 cm-1
* 962 cm-1 in perchloric acid
Linear arrangement of oxygen
 Raman shifts observed
Sensitive to complexation
* Changes by 40 cm-1
8-163
Pu solution chemistry
•
•
•
Redox chemistry

Potentials close to 1 V for 4 common states

Kinetics permit coexistance of oxidation states
 Pu(IV) and Pu(V) tend toward disproportionation
* 3Pu4++2H2O2Pu3++PuO22++4H+
 K=0.0089 at 1.0 M I
* 3PuO2++4H+Pu3++2PuO22++2H2O
 Pu concentration
 Ionic strength
 pH

Kinetics for disproportionation based on time and Pu concentration
 Moles seconds (M s)
Some redox couples are quasi- or irreversible

Breaking or forming oxo bonds
 i.e., Pu(V)/Pu(III), Pu(VI)/Pu(III)
Equilibrium between redox states

K=Pu(III)Pu(VI)/Pu(IV)Pu(V)
 K=13.1, corrected for hydrolysis
8-164
8-165
8-166
8-167
Pu solution chemistry
•
Preparation of pure oxidation states

Pu(III)
 Generally below pH 4
 Dissolve a-Pu metal in 6 M HCl
 Reduction of higher oxidation state with Hg or Pt cathode
* 0.75 V vs NHE
 Hydroxylamine or hydrazine as reductant

Pu(IV)
 Electrochemical oxidation of Pu(III) at 1.2 V
* Thermodynamically favors Pu(VI), but slow kinetics due to oxo
formation

Pu(V)
 Electrochemical reduction of Pu(VI) at pH 3 at 0.54 V (vs SCE)
* Near neutral in 1 micromole/L Pu(V)

Pu(VI)
 Treatment of lower oxidation states with hot HClO4
 Ozone treatment

Pu(VII)
 Oxidation in alkaline solutions
8-168
* Hexavalent Pu with ozone, anodic oxidation
Pu solution chemistry
•
•
Pu(VI) oxo oxygen exchange with water
18O enriched water exchange

 need to maintain hexavalent oxidation state
* Exchange rate increases with lower oxidation state

Exchange half life = 4.55E4 hr at 23 °C
 Two reaction paths
* Reaction of water with Pu(VI)
* Breaking of P=O bonds by alpha decay
 Faster exchange rate measured with 238Pu
Pu redox by actinides

Similar to disproprotionation

Rates can be assessed against redox potentials
 Pu4+ reduction by different actinides shows different rates
* Accompanied by oxidation of An4+ with yl bond formation

Reduction of Pu(VI) by tetravalent actinides proceeds over
pentavalent state

Reactions show hydrogen ion dependency
8-169
8-170
Pu solution chemistry
•
Pu reduction by other metal ions and ligands

Rates are generally dependent upon proton and ligand concentration
 Humic acid, oxalic acid, ascorbic acid

Poor inorganic complexants can oxidize Pu
 Bromate, iodate, dichromate

Reactions with single electron reductants tend to be rapid
 Reduction by Fe2+

Complexation with ligands in solution impacts redox
 Different rates in carbonate media compared to perchlorate
 Mono or dinitrate formation can effect redox
* Pu(IV) formation or reaction with pentavalent metal ions proceeds
faster in nitrate than perchlorate
* Oxidation of Pu(IV) by Ce(IV) or Np(VI) slower in nitrate

Pu(VI) reduction can be complicated by disproportionation

Hydroxylamine (NH2OH), nitrous acid, and hydrazine (N2H4)
 Used in PUREX for Pu redox control
 Pu(III) oxidized
* 2Pu3++3H++NO3-2Pu4++HNO2+H2O
* Re-oxidation adds nitrous acid to the system which can initiate an
autocatalytic reaction
8-171
Pu 2 phase redox system
transfers
Pu(IV)
HNO3
Fe(II)
Fe(III)
N2H4
HAN
NO2-
Fe(II)
HNO2
HN3
HNO2
H 2O
H+/NO3-
Fe(III)
PuO2+
PuO22+
N2
N2O4
NO2
Azides
Na
Ag
Pu
N2/NH4+
Pu(III)
Inter-Phase Layer
8-172
Pu aqueous chemistry
• Reduction of Pu(IV) to Pu(III) by HAN is fast
• Two Reactions are possible:
2NH3OH   4Pu4  4Pu3  N2O  H2O  6H 
2NH3OH   2Pu4  2Pu3  N2  2H2O  4H 
• Preferred Reaction depends on the ratio R
3
3
4

3
PuNO  Pu  NO
Kd
Kh
Pu4  H 2O 
PuOH3  H 
[ Pu( IV )]0
R
[ NH 3OH  ]0
Ka
NH 3OH  
 NH 2OH  H 
3 k3 / k3
PuOH3  NH 2OH K
 Pu3  NH 2O  H 2O
k4
2NH 2O 
N 2  2H 2 O
8-173
Pu aqueous chemistry
• kinetic of the reaction derived using the steady state approximation
applied to NH2O
[ Pu( IV )]2 [ NH 3OH  ]2
d[ Pu( IV )]

k
dt
[ Pu( III)]2 [ H  ]4 ( K d  [ NO3 ])2
• Reoxidation of Pu
3
k  k4 K32 Kh2 Kd2 Ka2
4

2
Pu  N 2O4  Pu  NO2  NO rate control
3

4
Pu  H  HNO3  Pu  NO  HNO2
• With
H   HNO2  NO3 
 N 2 O4  H 2 O
8-174
Pu aqueous chemistry
• In addition to the scavenging of nitrous acid,
hydrazine also may reduce Pu(IV) to Pu(III)
 Excess of Pu
4Pu
4
3
 N 2 H 4  4Pu  N 2  4H

 Excess of hydrazine
4
3

2Pu  2N2 H4  2Pu  2NH4  N2
 Net
1

4
3
N 2 H 5  Pu  Pu  N 2  NH 4  H 
2
• Autocatalytic reaction may result from Pu redox
cycling in HAN/N2H4 system
8-175
Pu aqueous chemistry
• Autoradiolysis
 Formation of radicals and redox agents
 Low reaction if concentrations below 1 M
 With nitrate can form other reactive species
(HNO2)
 Formation of Pu(IV).H2O2
 Rate proportional to Pu concentration and dose
rate
 Pu(VI) reduction proceeds over Pu(V)
 Formation of HNO2 and disproportionation
8-176
Pu hydrolysis
• Size and charge
 Smaller ions of same charge higher hydrolysis
 For tetravalents
* Pu>Np>U>Pa>Th
8-177
Pu hydrolysis 10 mM
8-178
Pu(III) 10 mM
8-179
Pu(IV) 10 mmol/L
8-180
Pu(V) 10 mmol/L
8-181
Pu(VI) 10 mmol/L
8-182
Pu aqueous chemistry
•
Hydrolysis/colloid formation

In many systems
solubility derived
Pu(IV) concentrations
vary due to colloid
formation

Colloids are 1- to
1000-nm size particles
that remain suspended
in solution

x-ray diffraction
patterns show Pu(IV)
colloids are similar to
the fcc structure of
PuO2
 Basis for theory
that colloids are
tiny crystallites
PuO2,
* May include
some water
saturated of
hydrated
surface

Prepared by addition
of base or water to
acidic solutions
8-183
Pu colloid model
8-184
Pu aqueous chemistry: colloids
•
•
Characterization

SANS
 Long, thin rods 4.7 nm x 190 nm

Light scattering
 Spherical particles
 1 nm to 370 nm

Laser induced breakdown
 12 nm to 25 nm
XAFS studies of Pu(IV) colloids

demonstrated that average fcc structure is overly simplistic

additional chemical forms are present that affect solubility

Variations in measured Pu(IV) concentrations may be related to the local structure

colloids displays many discrete Pu–O distances
 2.25 Å Pu-OH to 3.5 Å

amplitude of Pu–Pu is reduced, decrease in number of nearest neighbors
 four H atoms incorporated into the Pu(IV) colloid structure could result in one
Pu vacany.

EXAFS reveals that many atoms in the colloid structure are distributed in a nonGaussian way when
 several different oxygen containing groups are present
* O2–,, OH-, and OH2
8-185
8-186
Pu aqueous chemistry
• Complexing ions

General oxidation state trends for complexation constants
 Pu(IV)>Pu(VI)≈Pu(III)>Pu(V)
• Oxoanions

Pu complexes based on charge and basicity of ligand
 ClO4-<IO3-<NO3-<SO42-<<CO32-<PO43* 7 to 12 ligands (higher value for Pu(IV)
• Carbonate

Inner and outer sphere complexation with water
 Outer interaction form chains and layer structures

Bidentate with small bite angle

Pu(III) carbonate
 Oxidize rapidly to tetravalent state
 Complexation values consistent with Am(III)

Pu(IV) carbonate
 Pu(CO3)n4-2n, n from 1 to 5
* n increases with pH and carbonate concentration 8-187
8-188
Pu aqueous chemistry
• Pu(V) carbonates
 Carbonates to Pu(V) solution
 Reduction of Pu(VI) carbonates
Mono and triscarbonato species
• Pu(VI) extension of U(VI) chemistry
8-189
8-190
8-191
8-192
Pu solution chemistry
• Pu nitrates

First Pu complexes and important species in reprocessing and
separations

Bidentate and planar geometry
 Similar to carbonates but much weaker ligand

1 or more nitrates in inner sphere

Pu(III) species have been prepared but are unstable

Pu(IV) species
 Pu(NO3)n4-n, n=1-6
* Tris and pentanitrato complexes not as prevalent
 Removal of water from coordination sphere with nitrate
complexation
* Pu-O; 2.49 Å for Nitrate, 2.38 Å for H2O
 Spectrophotometric determination of complexation
constants with nitrate and perchlorate

Pu(NO3)66- complexes with anion exchange resin

For Pu(IV) unclear if penta- or hexanitrato species
 Evidence suggests hexanitrato species in the presence of
8-193
resins
Pu solution chemistry
• Pu nitrates
 Nitrate solids from precipitation from nitric acid
solutions
 Orthorhombic Pu(NO3)4..5H2O
 M2Pu(NO3)6.2H2O; M=Rb, Cs, NH4+,
pyridinium in 8 to 14 M HNO3
* Pu-O 2.487 Å
 Mixed species
 TBP complexes, amide nitrates
 No inner sphere Pu(V) nitrate complexes found
 Only Pu(VI) mononitrate in solution
 Solid phase PuO2(NO3)2.xH2O; x=3,6
characterized
8-194
Pu solution chemistry
• Sulfate

Pu(III)
 Mono and disulfate complexes
 Solid K5Pu(SO4)4.8H2O
* Indicates Pu(SO4)45- in solution
* Likely Pu(SO4)n3-2n in solution

Pu(IV)
 High affinity for sulfate complexes
 Mono and bisulfate solution species
 Solid K4Pu(SO4)4.2H2O
 hydrated Pu(SO4)2 n=4, 6, 8, 9
 Mixed Pu2(OH)2(SO4)3(H2O)4
* Should be in basic solution with high sulfate

Pu(V) species not well characterized

Pu(VI) forms mono- and bisulfate from acidic solutions
 Examined by optical and IR spectroscopy
 Solids of M2PuO2(SO4)2
8-195
Pu solution chemistry
•
Phosphate complexes

Low solubility
 Range of solid species, difficult characterization
* Range of protonated phosphates
* P2O74-, (PO3)nn* Ternary complexes
 Halides, organics, uranium

Pu(III)
 Not characterized but proposed
 Pu(H2PO4)n3-n n=1-4

Pu(IV)
 Wide range of complexes
 Only Pu(HPO4)2.xH2O examined in solution phase

Pu(V)
 Ammonium monohydratephosphate Pu(V) tetrahydrate species
 Evidence of PuO2HPO4
Pu(VI)
 MPuO2PO4.yH2O
* Solution complexes from Pu(VI) hydroxide and H3PO4
8-196
Pu solution chemistry
•
•
•
Iodate

Pu(IO3)4 precipitate
 Not well characterized
 Prepared by hydrothermal methods
* Preparation of Pu(VI) diiodate species

Mixed Pu(VI) trishydroxide species
 From Pu(IV) and H5IO6 in hydrothermal reaction, forms (PuO2)2(IO3)(mOH)3

Pu(V) forms Pu(IV/VI) species
Perchlorate

No pure solution or solid phases characterized

Most likely does not form inner sphere complexes in aqueous solution
Oxalates

Previously discussed, forms microcrystals

Mono and bidentate forms

Pu(III) form trivalent oxalates with 10 and 6 hydrates

Pu(IV) forms with 2, 4, and 5 oxalates with n waters (n=0,1,2,or 6)
 Tetra and hexa monovalent M salts
 Mono hydroxide mixed solid species formed

Pu(V) disproportionates

Pu(VI)O2 oxalates
8-197
Pu solution chemistry
• Peroxide

Used to form Pu(IV) from higher oxidation states
 Further reduction of Pu(IV), mixed oxidation states

Pu(IV) peroxide species determined spectroscopically
 Two different absorbances with spectral change in
increasing peroxide

No confirmed structure
 Pu2(m-O2)2(CO3)68- contains doubly bridged Pu-O core

Formation of peroxide precipitate that incorporates
surrounding anions
 High acidity and ionic strength
 In alkaline media, Pu(VI) reduced to Pu(V) with
formation of 1:1 complex
8-198
Pu solution chemistry
•
Carboxylate complexes

Single or multiple carboxylate ligands for strong complexes with Pu
with typical oxidation state stability trend

Tend to stabilize Pu(IV)

Pu(III)
 Oxidation to Pu(IV) at pH > 5
 Range of mixed species
* Degree of protonation (HxEDTA)
* Mixed hydroxide species

Pu(IV)
 Stabilized by complexation
 Solution phase at relatively high pH
 1:1 Pu to ligand observed (Pu:EDTA, Pu:DTPA)
* Range of mixed species can be formed
 EDTA used in the dissolution of Pu(IV) oxide or hydroxide
solids

Pu(V) complexes to be unstable
 Oxidation or reduction solution dependent
8-199

Pu(VI) species observed
Pu solution chemistry
• Halides

Studies related to Pu separation and metal formation

Solid phase double salts discussed
• Cation-cation complexes

Bridging over yl oxygen form plutonyl species

Primarily examined for Neptunyl species

Observed for UO22+ and PuO2+
 6 M perchlorate solution

Formation of CrOPuO4+ cation from oxidation of Pu(IV)
with Cr(VI) in dilute HClO4
8-200
Pu non-aqueous chemistry
• Very little Pu non-aqueous and organometallic chemistry

Limited resources
• Halides useful starting material

Pu halides insoluble in polar organic solvents

Formation of solvated complexes
 PuI3(THF)x from Pu metal with 1,2-diiodoethane in THF
* Tetrahydrofuran
 Also forms with pyridine, dimethylsulfoxide

Also from the reaction of Pu and I2

Solvent molecules displaced to form anhydrous compounds

Single THF NMR environment at room temperature
 Two structures observed at -90 °C
8-201
Pu non-aqueous chemistry
• Pu oxidation with Tl or Ag
hexafluorophosphate in acetonitrile
(CH3CN)
 Pu(CH3CN)9(PF6)3.CH3CN
Pu trivalent cation complex
surrounded by PF6- anion
9 coordinate tricapped trigonal
prism
• PuCl4 can be stabilized
 PuCl4L2or3 from Cs2PuCl6 with amide
(RCONR’2) or phosphine oxide (R3PO)
Oh based symmetry for L2
8-202
Pu non-aqueous chemistry
• Amides

Uses PuI3(THF)4 starting material
 3 NaN(SiMe3)2 yields Pu complex with 3
NaI
* IR shows asymmetric PuNSi2 stretch at
986 cm-1
 Structure based on U and La
complexes
• Alkoxides

Reaction of Pu(N(SiMe3)2)3 with 3 2,6Bu2C6H3OH yields Pu(O-2,6-Bu2C6H3)3
 Will coordinate Lewis base (donate
electron pair)
8-203
Pu non-aqueous chemistry
• Borohydrides
 PuF4 + 2Al(BH4)3Pu(BH4)4+ 2Al(BH4)F2
 Separate by condensation of Pu complex in dry ice
 IR spectroscopy gives pseudo Td
 12 coordinate structure
• Cyclooctatraene (C8H8) complexes
 [NEt4]2PuCl6 + 2K2C8H8 Pu(C8H8)2+4KCl +
2[NEt4]Cl in THF
 Slightly soluble in aromatic and chlorinated
hydrocarbons
 D8h symmetry
 5f-5f and 5f-6d mixing
* Covalent bonding, molar absorptivity
8-204
approaching 1000 L mol-1cm-1
Pu non-aqueous chemistry
• Cyclopentadienyl (C5H5), Cp
 PuCl3 with molten (C5H5)2Be
trisCp Pu
* Reactions also possible with Na, Mg,
and Li Cp
 Cs2PuCl6+ 3Tl(C5H5) in acetonitrile
 Formation of Lewis base species
CpPuCl3L2
* From PuCl4L2 complex
 Characterized by IR and Vis spectroscopy
8-205
Pu electronic structure
•
•
Ionic and covalent bonding models

Ionic non-directional electrostatic bonds
 Weak and labile in solution
* Core 5f

Covalent bonds are stronger and exhibit stereochemical orientation

All electron orbitals need to be considered
 Evidence of a range of orbital mixing
PuF6

Expect ionic bonding
 Modeling shows this to be inadequate

Oh symmetry

Sigma and pi bonds
 t2g interacts with 6d
 t2u interacts with 5f or 6p and 7p for sigma bonding
 t1g non-bonding

Range of mixing found
 3t1u 71% Pu f, 3% Pu p, 26% F p characteristics

Spin-orbital coupling splits 5f state
 Necessary to understand full MO, simple electron filling does not
describe orbital
* 2 electrons in 5f orbital
 Different arrangements, 7 f states
8-206
8-207
Pu electronic structure
• PuO2n+

Linear dioxo
 Pu oxygen covalency
 Linear regardless of number of valence 5f electrons
 D∞h, no

Pu oxygen sigma and pi bonds
 Sigma from 6pz2 and hybrid 5fz3 with 6pz
 Pi 6d and 5f pi orbitals

Valence electrons include non-bonding orbital

d and f higher than pi and sigma in energetics
 5f add to non bonding orbitals

Weak ionic bonds in equatorial plane

Spin-orbital calculations shown to lower bond energy
8-208
8-209
8-210
Pu electronic structure
• Plutonocene

8 C 2 pi orbitals form pi bonds
 Combine with 6d Pu atomic
orbitals
 5f form s, p, d, and f
orbitals
* 5fd directed toward C8
rings
 Bond 49 % f
character
 Demonstrates
covalency
 Other interactions
weaker
 Stronger d interactions
* Orbital 11% d character
8-211
8-212
Pu electronic structure
• Plutonocene
 Experimental
evidence of
splitting
 Need to consider
spin-orbital
coupling
 Different
occupancy of
orbitals
8-213