Full text of DOI:10.1557/mrs2001.179

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as energy sources for space exploration
(e.g., the Cassini mission to Saturn). At
one time, they had even been considered
as energy sources for heart pacemakers.
Today, the U.S. Department of Energy has
an active program to produce 238Pu for
such power generators, and this program
itself will demand knowledge of pluto-
nium chemistry and materials science.
If the existing fissile plutonium (mainly
the 239Pu isotope) is not used for nuclear-
reactor fuels, it must be maintained in
storage or disposed of in some safe fashion.
One approach is to incorporate the pluto-
nium in special glass or ceramic matrices
and bury it in designated sites. This dis-
position has been considered for dealing
with excess quantities of weapons-grade
(mainly 239Pu content) plutonium. Each
concept for addressing these issues has
proponents and objectors. Regardless of
the approach or approaches taken, some
form of storage and processing of pluto-
nium materials will be required, and these
different issues will drive the need for
comprehensive science and technology.
Certainly, plutonium oxides, oxide-related
materials, and the different products formed
during corrosion-type reactions involving
plutonium metal are of central interest in
this regard. This interest has led to new in-
vestigations, both fundamental and tech-
nological in nature, using state-of-the-art
scientific tools and approaches that have
generated new and unexpected findings
regarding the chemistry of plutonium.
The volume of early work on plutonium
and materials containing it, compared with
that of the last two decades, shows that
recent efforts have been more limited in
number. The quantity of high-quality infor-
mation about plutonium gathered in thepast
is impressive, given the hazardous nature of
this element, its complex redox behavior (at
least five oxidation states are known), and
its relatively recent discovery. Much of the
information known up to the mid-1980s
is covered in several reviews; one of the
Plutonium Oxide
Systems and
Related Corrosion
Products
R.G.Haire and J.M.Haschke
Agency,1 this inventory exceeds 160 metric
tons from civilian sources alone, in addi-
tion to military supplies. This inventory
consists essentially of the fissionable 239Pu
isotope. Although the development and
deployment of nuclear reactors is uncer-
tain at the present time, both pure pluto-
nium and uranium–plutonium mixtures
(e.g., as mixed oxides, the so-called MOX
fuels) can be considered as sources of
energy. One concept for utilizing excess
weapons-grade plutonium from stockpiles
is to generate and use MOX reactor fuels
for power generation. In this regard, ton
quantities of plutonium have been placed
in MOX-type fuels worldwide.
This article is a short qualitative summary
of a chapter in the forthcoming bookAdvances
in Plutonium Chemistry: 1967–2000, edited
by Darleane Hoffman for the Amarillo National
Research Center. Readers are referred to the
chapter for more technical details.
Introduction
Elemental plutonium is considered to be
one of the most complex elements in the
periodic table and, by some, one of the more
toxic, given its moderate radioactivity and
that it is a heavy metal. Essentially a manu-
factured element (only traces are found in
nature), it was first discovered and pro-
duced some six decades ago. Amazingly,
hundreds of tons of plutonium are pres-
ently in existence. Crystalline solids formed
by plutonium and nonmetallic elements
(e.g., hydrogen, oxygen, halides, etc.) were
some of the first compounds investigated,
and oxides and oxide-related materials
have always been important compounds.
The determination of the chemical and
physical properties of compounds and the
elemental state was essential in the quest
to prepare large quantities of the metal for
military applications. Subsequently, infor-
mation for plutonium oxides, carbides,
nitrides, and so on became important for
nuclear-reactor fuels. As a result of these
different applications, plutonium has been
studied extensively, despite the experi-
mental difficulties involved.
New scientific findings regarding pluto-
nium continue to surface, which is a sign
of the complexity of this element. Athor-
ough understanding of the properties and
corrosion reactions of plutonium and its
compounds continues to be essential today.
This is important given the storage and
process operations involving this element
that must be addressed and the potential
for future accidental release or dispersion
of these materials, as well as their limited
presence in the environment.
4
more comprehensive is recommended.
Also recommended is a collection of ex-
tended abstracts from a conference dealing
with recent plutonium investigations.5
There are other isotopes of plutonium,
which are used for different applications.
For example, 238Pu is shorter-lived than
239Pu (88 years versus 2.4 ϫ 104 years, re-
spectively) and has different nuclear prop-
erties. The 242Pu and 244Pu isotopes have
low specific activities and are best for
basic research studies. Amajor application
for 238Pu is in electric-power generators.
These generators make use of the heat from
radioactive decay to produce electricity
by means of thermopiles.2,3 Such devices
often employ 238Pu oxide in a special irid-
ium container; they have been very useful
The recent reduction in the number of
studies on plutonium results in part from
a perception that plutonium materials are
thoroughly understood and additional in-
vestigations have limited value. Several of
the latest advances in the basic science of
plutonium have come as spin-offs of more
applied investigations, where unexpected
behaviors of plutonium were encountered,
prompting basic investigations of these
behaviors. Such has been the case with
the extensive studies of plutonium-oxide
systems, where it appears that the previous
Exploring and understanding the chem-
istry, physics, and materials science of non-
metallic plutonium solids remain important
and relevant today, if for no other reason
than the need to deal with the large world-
wide inventory of this element and the
different materials that contain it. Accord-
ing to the International Atomic Energy
MRS BULLETIN/SEPTEMBER 2001
689

Plutonium Oxide Systems and Related Corrosion Products
knowledge was not adequate to deal with
and understand the complexities of these
systems.
Plutonium Oxide Systems
oxide (lanthanide oxide designation), as
well as by other terminology.13 The situa-
tion with the sesquioxides can be confusing,
as the PuO1.52 phase, a bcc cubic (Mn2O3-
type) oxide (a0 ෇ 1.098 nm), forms at an
O/Pu ratio slightly above the ideal stoi-
chiometry for Pu(III). It is designated as
the ␣-phase, or the C-type sesquioxide.
The phase behavior of plutonium oxides
is discussed in detail in a later section.
Plutonium is known to exhibit oxidation
states III through VII, and the III and IV
states are well established in binary ox-
ides.4,9,10 Accordingly, there are multiple
phases of the sesquioxide, the very stable
dioxide, and intermediate phases between
the sesquioxide and dioxide. Crystallo-
graphic data for these oxides are given in
Table I.
The electronic behavior of plutonium
metal, which makes it the most complex
metal in the periodic table, arises from a
large number of electronic configurations
of similar energy. The complexity is more
subdued in plutonium’s compounds, and
in many instances their behavior can be
linked to that observed in other 4f electron
(lanthanide) and 5f electron (actinide) ele-
ments. This is especially true for the tri-
valent and tetravalent oxidation states of
plutonium. There is a tendency toward
decreased stability for plutonium’s higher
oxidation states in solid compounds; in
general, it is more difficult to achieve oxi-
dation states above four than for neptunium
(plutonium’s near neighbor) or uranium.
Except for plutonium hexafluoride, higher
halides of plutonium are conspicuously
absent. There is also a reduced tendency
for plutonium to form nonstoichiometric
compounds. For many years, and after
many controversies, it was accepted that
the highest binary oxide of plutonium was
the dioxide. Very recent work6–8 challenges
this concept, and it now appears that small
amounts of binary oxides with a higher
oxygen stoichiometry may indeed exist.
Many Pu(III) compounds are quite
lanthanide-like in their behavior and
properties. In several instances, their
properties can be predicted on the basis
of an ionic radius correlation. Examples of
this are found in the phase behavior of
plutonium sesquioxides and the chem-
istry of plutonium hydrides. This correla-
tion can be carried over to the Pu(IV) state,
but the comparison with lanthanide ele-
ments is limited, as cerium is the only
lanthanide that readily exhibits this oxida-
tion state.
16
The very stable dioxide has always domi-
nated the oxide chemistry of plutonium.
There have been some claims for the exis-
tence of solid PuO, especially in the form
of films on metal surfaces, but it is unlikely
that this material exists as a pure compound.
There is a potential for the formation of solid
solutions of the type Pu(O,N,C), which
would have fcc-type structures and would
be similar structurally to a monoxide. The
monocarbide and mononitride are accepted
compounds. However, the monoxide has
been established as a high-temperature
gas-phase product. Recently, evidence for
binary materials having oxygen/plutonium
ratios above 2.0 (PuO2ϩx; PuO3 and PuO4)
have been reported (see later discussion).
Specialized publications11–14 and an exten-
sive array of literature augment these listed
reviews. Several of the publications also
cover special topics involving equilibrium,
structure, thermodynamics, and magnetic
properties.
Thermodynamic predictions and re-
sults of numerous studies have indicated
that plutonium dioxide is the highest
composition binary phase in the Pu-O sys-
tem.4,9,10 This fcc (CaF2-type) phase has a
lattice parameter of a0 ෇ 0.5397(1) nm at
25ЊC. The PuO2.00 composition actually has
a narrow substoichiometric (PuO2Ϫx) range
at room temperature, where a0 ෇ 0.5400nm
17
at the boundary of PuO1.98
.
Ordered re-
moval of oxygen atoms from a fourth of
the anion sites on a diagonal in the fcc
PuO2 structure yields the C-type structure
of PuO1.5. Its unit cell appears as a “double”
unit cell of the dioxide cell; a0 ෇ 2a0 of the
fcc dioxide.18,19
As outlined in prior reviews,4,9,10 PuO2 is
prepared by air oxidation of the metal or
by calcination of plutonium precipitates
(e.g., oxalate, peroxide, hydroxide, carbon-
ate, etc.) at temperatures between 250ЊC
and 1000ЊC. A significant variation in the
oxide’s particle size and surface area can
The dissolution of oxygen in the different be encountered when the material and/or
polymorphs of plutonium metal is small;
the solubility limit for the ␧ form at 580ЊC
is reported to represent an O/Pu ratio of
calcining temperature are changed. These
differences are essentially eliminated by
heating the products at 1000ЊC or higher
to form a high-fired oxide, which is diffi-
cult to dissolve in aqueous HNO3 and
other mineral acids.4 Reaction of plutonium
and oxygen in the stoichiometric ratio of
the sesquioxide yields a 3Ϻ1 molar mix-
ture of PuO2 and elemental Pu. Hexagonal
Pu2O3 is only obtained by reducing the
4 ϫ 10 .
Ϫ4 15 The oxygen content of molten
plutonium metal may be higher.
The lowest-established stoichiometry in
oxides is found in the hexagonal sesquiox-
ide (La2O3 structure type; a0 ෇ 0.3841 and
c0 ෇ 0.5958nm), a phase often identified in
the literature as the ␤-phase or the A-type
In this brief article, an effort will be
made to present some important facets of
plutonium-oxide chemistry. It is hoped that
from this overview, the reader will gain
new perspectives and interest into the fas-
cinating science of plutonium.
TableI:Crystallographic Data for Plutonium Oxides.a
Formula
X-ray
Density
(g/cc)
The focus of this article is on plutonium
oxides, hydroxide materials, and products
that accompany corrosion-type reactions
of metallic plutonium. Also presented are
some scientific findings regarding fun-
damental plutonium-oxide chemistry that
have been acquired recently using advanced
research tools. These include studies of
gas-phase clusters of plutonium oxide or
oxide–hydroxide, claims for the existence
Structure
Type
Space
Group
Unit Cell
(nm)
Units
Phase
Symmetry
(per cell)
PuO1.50
(A-type)
PuO1.51
(C-type)
PuO1.61
(C-type)
PuO2
La2O3
Mn2O3
Fe2O3
hexagonal
P3m1
a ෇ 0.3841(6)*
c ෇ 0.5958(5)
a ෇ 1.104(2)
1
16
...
11.47
10.2
...
bcc
bcc
Ia3
Ia3
a ෇ 1.095–1.101
CaF2
NaCl
fcc
fcc
Fm3m
Fm3m
a ෇ 0.53960(3)
a ෇ 0.496
4
4
11.46
13.9
of PuO and PuO4 (a new formal oxidation
3
[PuO]**
state, VIII, for plutonium), the formation
of PuO2ϩx materials, establishing catalytic
activity by Pu2O3, and the gas-phase oxi-
dation of Pu and PuO ions by oxygen.
a From Reference4.
*Uncertainties of last digits are in parentheses.
**Probably a Pu(O,C)-type material—given only for comparison purposes.
690
MRS BULLETIN/SEPTEMBER 2001

Plutonium Oxide Systems and Related Corrosion Products
dioxide with excess plutonium metal,
under flowing dry H2 or with excess C,
in compacts at temperatures between
1500–2000ЊC. The latter process often leaves
a carbon contamination in the product.
Confusion may develop when one ex-
amines the proposed preparation schemes
for the sesquioxide materials. The silver-
gray PuO1.52 (␣-Pu2O3) involves reduction
of PuO2 with carbon or dry hydrogen
(often in the presence of titanium turnings)
at 1520–1800ЊC.4,9,10 However, the prepara-
tive conditions are indistinguishable from
those for preparing ␤-Pu2O3. The reported
preparation of C-type PuO1.52 by heating
PuO2 in dynamic vacuum at 1650–1800ЊC
is questionable, as the O/Pu ratio of con-
gruently vaporizing PuO2Ϫx is 1.85–1.90 in
they consist of very small particles (e.g.,
1–1000 nm in diameter) that may display
special chemical/physical behavior. One
form of plutonium colloid has been inves-
tigated by studies29 using extended x-ray
absorption fine structure (EXAFS) and
XANES (x-ray absorption near edge struc-
ture), which confirmed that it consisted of
tiny forms, smaller than those frequently
reported by others. Studies of this “poly-
mer” material are complicated by the wide
variation of forms that may be encoun-
tered. One polymer form is crystalline,
simulates a PuO2 product with small crys-
tallites, and yields a fluorite-type diffrac-
tion pattern. Other colloids are amorphous
in nature.
and this possibility will be investigated in
future work.
In other gas-phase studies, using mass
high-temperature spectrometry, Ronchi
et al.7 studied vapor species over pluto-
nium dioxide and found evidence for
volatile PuO gas molecules, but only at a
level of 0.023ppm of the total plutonium
content. Thermochromatography studies6,34
indicate that both PuO3 and PuO4 exist as
trace volatile species in a gas stream. The
behavior of PuO4 was reported to be simi-
lar to that of OsO4 and RuO4. Both of these
oxide species are new—and surprising,
based on the previous understanding of
plutonium oxide chemistry. These species
are also unexpected for high-temperature
products, based on general entropy con-
APu(V) and Pu(VI) hydroxide material
has been reported1.0,30 Hydroxides of Pu(III),
20
19
this temperature region. As suggested
siderations, which should favor lower
previously,17 involvement of a Ta crucible,
which serves as a reductant, is necessary
to approach an oxide with an O/Pu ratio
of 1.5. Incomplete reduction can yield prod-
ucts with O/Pu ratios greater than 1.52,
and diphasic mixtures of PuO1.52and PuO1.98
can form upon cooling to room tempera-
ture. Indeed, the oxide system between
the sesquioxide and dioxide stoichiome-
tries can be complex.
which are very susceptible to oxidation to
oxidation states. The existence of these
products must be confirmed and addi-
tional properties investigated.
4,14
Pu(IV) by air,
and Pu(III)-oxide hy-
22,23,31
drides have also been reported.
In a recent work32 on the solubility of
Pu(IV)-hydroxide materials, where the
solid is identified as Pu(OH)4, a log of the
solubility product constant Ksp of Ϫ58.7
was given. This value generates a stan-
dard free energy of formation of Ϫ1446 kJ
molϪ1 for this compound. The difficulty in
such Ksp assignments is the correct deter-
mination of the plutonium concentration—
Fourier transform ion cyclotron mass
spectrometer studies have also been em-
ployed recently to investigate plutonium
in the gas phase. Among other reactions,
ϩ
the oxidation of [PuO] molecular ions by
oxygen was examined, which addressed
reaction rates and sequential chemical re-
actions of ligated ions.35
Although the preparation of pure␣-Pu2O3
by high temperatures alone is difficult, the
Another area of recent research has con-
cubic oxide forms as a micrometer-thick in this case, verifying that colloidal
cerned the existence and formation of plu-
8,36
surface layer when PuO2-coated metal is
heated at 150–200ЊC in vacuum.21 Gram
amounts of cubic PuO1.46Ϯ0.04 powder can
be obtained by reacting plutonium metal
with water in a near-neutral salt solution
at room temperature.22,23
Pu-containing species were not present.
A wealth of high-quality information
has been generated for the plutonium
oxygen system, and the validity of these
data and the understanding of pluto-
nium’s science have withstood the test of
time. However, advances in scientific tech-
niques and continued efforts have opened
new aspects of plutonium science. Exam-
ples of new pursuits are investigations that
employ EXAFS/XANES studies at synchro-
trons, neutron-diffraction studies, and ad-
vanced gas-phase chemistry that often is
employed in studies of transactinide work.
Some of the recent efforts involve mass
spectrometry, with and without the use of
trapped-ion techniques. These new find-
ings must now withstand the test of time
as well as confirmatory studies.
tonium oxides of the form PuO2ϩx,
where x ෇ 0.25. This newly recognized
material is described as a fluorite-related
oxide, in which some of the Pu oxidation
states are greater than IV and composi-
tions reach and exceed PuO2.25. Its behavior
resembles that observed in uranium oxides,
where oxides between UO2 and UO2.25
The formation and the stability of PuO2
have dominated plutonium oxide chem-
istry, and investigations have focused
largely on the dioxide properties and be-
havior. The refractory dioxide has been
identified as the terminal binary phase in
the Pu-O system, and as the stable binary
oxide in the environment.24,25 The situation
reflects an observation initially applied to
corrosion chemistry of plutonium,26 that
investigators are inclined to concentrate
their attention on plutonium dioxide and
overlook other potential compounds that
may contribute to the overall behavior of
plutonium.
are known. A material assigned to be
30,37
PuO3 и0.8H2O has also been reported
,
but the latter compound may be a per-
oxide containing Pu(IV). Solid peroxides
with a “high” oxygen content but con-
taining Pu(IV) can be obtained by precipi-
tation processes.4 The observation and
evidence for PuO2ϩx came out of studies
looking at the sorption of water on pluto-
nium oxide surfaces, where chemical in-
dications suggested that an oxide higher
than the dioxide may have formed. The
One recent investigation examined sev-
eral plutonium oxide and oxide–hydroxide formation of a metastable PuO(OH) sur-
2
Plutonium hydroxides or hydrated di-
oxides have been discussed over many
years. The most significant in the context
discussed here is the so-called Pu(IV)
polymer, which is best described as con-
sisting of very small particles with a
“PuO2 backbone.” These particles become
a dispersed colloid when the surface
atoms generate a charged particle that is
clusters that can form in the gas phase.33
These clusters and their chemical reactions
can contribute to the general understand-
ing of PuOx chemistry, and they represent
materials that bridge the gap between
gaseous materials containing one pluto-
nium atom and the condensed, solid
phases that have large numbers of pluto-
nium atoms. These plutonium clusters
have been found to consist of 6–18 pluto-
nium atoms. Other clusters with more
plutonium atoms could very well exist,
face by dissociative chemisorption of H2O
is believed to be the first step in a rela-
tively slow process that forms the PuO2ϩx
.
The reported formation of a higher oxide
by reaction between the dioxide and
water8,36–39 contradicts previous theoreti-
cal and experiment results. Previously, it
was accepted that the dioxide was the bi-
nary phase with the highest oxygen (O2Ϫ
stoichiometry.
)
27,28
counterbalanced by an anion layer.
When suspended, these colloids can ap-
pear as pseudo solutions, but in reality
Support for this PuO2ϩx material comes
from x-ray diffraction analysis of the oxide
MRS BULLETIN/SEPTEMBER 2001
691

Plutonium Oxide Systems and Related Corrosion Products
formed on plutonium metal during cor-
rosion by water vapor at 200–350ЊC. The
data show a new fluorite-related phase
(PuO2.27) in addition to PuO2 and␣-Pu2O3.38
X-ray photoelectron spectroscopy (XPS)
analyses of oxide found at the interface
show high-binding-energy peaks in the
plutonium’s 4f5/2 (441.9 eV) and 4f7/2
(429.0eV) electronic levels, which are con-
sistent with the presence of Pu(VI). The
appearance of an O 1s band at 530.1 eV in-
dicates the presence O2Ϫ. Additional evi-
dence for this PuO2 ϩ H2O reaction were
obtained by a microbalance technique and
by oxygen titration methods at constant
water pressure (30 mbar) and tempera-
tures of 25–350ЊC.36,39,40 Isothermal meas-
urements show that hydrogen is generated
at linear temperature-dependent rates, and
mass spectrometric analyses indicated that
H2 is the only gaseous product. Constant
oxidation rates are described by a single
ln(dH2/dt) versus 1/T relationship, where
reader is referred to a separate report (and
the references therein) in the article by
Muller and Weber in this issue. A discus-
sion of these different plutonium waste
materials is beyond the scope of this
overview.
Below 335ЊC, PuO1.52 and PuO2Ϫx coexist;
above that temperature, a fourth oxide,
PuO1.6Ϯ␦, forms over a broad composition
range. On the metal-rich side, this phase
coexists with PuO1.52 until it disproportion-
ates peritectically above 450ЊC into ␤-Pu2O3
and PuO1.6.
Thermodynamic Properties
On the oxygen-rich side, PuO1.6 coexists
with PuO2Ϫx below 630ЊC, and with metal-
rich PuO2ϩx at higher temperatures, until
it transforms congruently into PuO2Ϫx at
1180ЊC. The PuO1.6 phase is either fcc (a0 ෇
0.5549 Ϯ 0.02 nm) or bcc (a0 ෇ 1.098 Ϯ
0.03nm). Hexagonal Pu2O3Ϫ␦ coexists with
oxygen-saturated metal and exhibits an
increasing extent of nonstoichiometry until
the congruent melting point at 2080ЊC is
approached. A congruent melting point of
2425ЊC is recommended for PuO2.13
Thermodynamics data are well defined
for PuO2 and to a lesser extent for hexago-
nal Pu2O3. Such data are limited for cubic
PuO1.52 and PuO1.6 and are not available
for other known intermediate oxides. The
13
literature has been extensively reviewed.
Recommended thermodynamics values
for plutonium oxides at 289 K14 are given
in Table II. However, a comparison of en-
thalpies and free energies of formation for
hexagonal Pu2O3 and PuO1.52 with those of
an earlier assessment1 show significant
differences. The value of standard entropy,
The most recent diagram13 is given in
Figure 2 and differs from earlier propos-
als4 in two important ways. In the earlier
assessments, the PuO1.6 phase is stable to a
temperature in excess of 2350ЊC, instead of
1180ЊC, and the miscibility gap between
Pu(s,l) closes below the 2080ЊC melting
point of ␤-Pu2O3. In the more recent evalua-
tion, the Pu ϩ ␤-Pu2O3Ϫ␦ two-phase region
extends to temperatures in excess of 2500ЊC.
This interpretation is consistent with ex-
perimental data indicating that liquid plu-
tonium metal and molten plutonium oxide
coexist at temperatures above 3230ЊC, the
boiling point of the metal.43,44 However,
the time of coexistence (Ͻ0.5 s) may be
too short for equilibration/equilibrium to
have been reached.
SЊ, for PuO , derived from the most recent
2
Ϫ1
heat-capacity measurements,42 is 16 J K
Ϫ1
the activation energy is 39Ϯ3 kJ mol .
molϪ1 lower than reported by others.
Enthalpies of formation are also reported
The lattice parameters of the products
increased progressively with composition
(O/Pu) from 2.016 to 2.265, suggesting the
formation of a continuous oxide solid so-
lution. Structural and XPS results indicate
that this “higher oxide” of plutonium forms
by the accommodation of hexavalent plu-
tonium and interstitial oxygen in the
for gaseous oxides formed during vapor-
14
ization of the PuO2Ϫx
.
The enthalpies of
formation of PuO(g) and PuO2(g) are
listed as Ϫ92 and Ϫ422 kJ molϪ1 at 298 K,
respectively. Free energies of formation
for the hexagonal and cubic sesquioxides
should help resolve the questions of phase
relationship and their stability. However,
comparison of ⌬GfЊ (standard free energy)
fluorite PuO2 structure, as found for the
36,40
formation of UO2ϩx
.
Although this
Ϫ1
higher plutonium oxide is supported by
experimental results, and its existence can
account for unexplained experimental be-
havior, the findings are still controversial
among scientists and will be a subject of
future discussions and thermodynamic
considerations.
for hexagonal Pu2O3 (Ϫ1580 kJ mol )
with the composition-adjusted enthalpy
Ϫ1
for bcc Pu2O3 (Ϫ1574Ϯ 22 kJ mol ) shows
an uncertainty well in excess of the differ-
ence, emphasizing the need for signifi-
cantly better thermodynamics data.
The relationship between the cubic and
hexagonal forms of Pu2O3 is complex and
uncertain. The two oxides are described as
distinct and separate equilibrium phases
at low temperature, instead of dimorphic
forms of the sesquioxide. As outlined in the
most recent review,13 reasons for recom-
mending two phases are (1)the hexagonal
Pu2O3 to cubic Pu2O3 transformation has
not been observed; (2)the phases have been
observed to coexist; and (3)their composi-
tions do not overlap. With the lanthanide
oxide, Nd2O3, the transformation of the
hexagonal form to the cubic form is re-
There are also several mixed-oxide sys-
tems with plutonium, including MOX fuels
(uranium–plutonium dioxides); ternary ox-
ides of plutonium with transition elements,
alkali metals, and the alkaline earths; and
Phase Equilibria
The recently proposed plutonium–
oxygen equilibrium phase diagram shown
in part by Figure 1 is based on a reassess-
ment of previous high-temperature data.13
This region of the diagram is similar to that
4
oxychalcogenide-type materials. Oxides
4,11
and silicate materials are formed when
plutonium is placed in various glasses for
waste forms. There are also important and
complex ternary (or higher) oxide systems
generated with transition metals, which
have promise for ceramic waste forms.
One compound (Pu2Zr2O7), referred to as
being a plutonium pyrochlore oxide (py-
rochlore oxide refers to a crystal form of
the mineral pyrochlore). This plutonium
presented in prior reviews. The three
stable phases at room temperature are
hexagonal Pu2O3 (identified as ␤-Pu2O3),
PuO1.52 (identified as ␣-Pu2O3) and PuO2Ϫx
.
Table II:Thermodynamic Properties of Plutonium Oxides at 298 K.a
CpЊ
(J K؊1 mol؊1
117.0(5)*
...
SЊ
(J K؊1 mol؊1
163.0(6)
57.7
⌬HfЊ
⌬GfЊ
(kJ mol؊1
Ϫ1580
Ϫ795
Oxide Phase
Pu2O3 (hexagonal,␤)
PuO1.52 (bcc,␣)
PuO1.6 (cubic)
)
)
(kJ mol؊1
)
)
41
compound is considered as one of sev-
eral important waste forms. When pre-
pared with neutron “poisons” (e.g., Gd
and Hf, both of which absorb neutrons), a
material of composition (Pu, Gd)2Hf2O7 is
obtained, which reduces nuclear criticality
concerns regarding the plutonium. With
regard to these potential waste forms, the
Ϫ1656
Ϫ836(11)
Ϫ884(17)
...
...
...
PuO2 (fcc)
66.3(3)
66.1 (3)
Ϫ1056.2(7)
Ϫ998.0(7)
a From Reference14.
*Uncertainties of last digits are in parentheses.
692
MRS BULLETIN/SEPTEMBER 2001

Plutonium Oxide Systems and Related Corrosion Products
150–200ꢀC; (2) the coexistence of this
product with the metal;21 (3) the formation
of cubic PuO1.46ꢁ0.04 at 23ꢀC;23 and (4) the
formation of PuO1.52 during cooling of
17
structurally related PuO1.6 and PuO2ꢂx
.
Formation of the cubic sesquioxide seems
to be kinetically favored, if the Pu sub-
lattice of the parent phase has cubic (fcc)
symmetry.
A cubic plutonium sesquioxide is formed
at low temperatures by facile movement
of anions in the preexisting fcc metal lattice;
after formation, the phase cannot trans-
form into ꢃ-Pu2O3 unless the temperature
is above 450ꢀC. Although the recent phase
diagram for plutonium oxides may be
correct, it is inconsistent with known rela-
tionships of isomorphous lanthanide oxides
and with behavior indicating that cubic
Pu2O3 is either stable or metastable at low
temperatures. The cubic sesquioxide ap-
pears to catalyze reactions of plutonium
with hydrogen to form hydride and with
oxygen to form dioxide.8,45 Intermediate
oxides similar to those known for the
cerium-, praseodymium- and terbium-
oxygen systems18,19 are not observed, as
the formation of the dioxide is favored ki-
netically when the sesquioxide is exposed
to oxygen.
Figure 1. Phase diagram for the plutonium–oxygen system (based on the assessment
by Wriedt13).
Effects of kinetics factors in establishing
the equilibrium diagram of Pu-O are also
suggested by results of oxygen titration
experiments, where H2O was used for the
controlled oxidation of plutonium metal to
the dioxide at room temperature over a
one-year period.22,23 The quantity of hy-
drogen formed, and the time-dependence
of the H2/Pu ratio (which defines the O/Pu
ratio in the solid product) were used to
monitor the extent of reaction. The H2/Pu
ratio time data show a sequence of well-
defined linear segments resulting from
generation of H2 at progressively slower
constant rates, a behavior consistent with
sequential free-energy-controlled reactions
that must be completed before the next re-
action begins. The reactions correspond to
sequential formation of the PuOH and
Pu7O9H3 oxide-hydrides, Pu2O3, members
of the PunO2nꢂ2 homologous series (n ꢀ 7,
9, 10, and 12), PuO2, and the PuO2ꢄx
phases. Absence of O2 in the gaseous
product shows that the pressure rise is
not driven by the radiolysis of water. The
terminal product obtained, PuO2.265, had a
fluorite structure with a lattice parameter
a0 ꢀ 0.5404(1) nm; it decomposes upon
heating to 500ꢀC in vacuum to form PuO2
[a0 ꢀ 0.5395(1) nm].
Figure 2. Revised phase diagram for the plutonium–oxygen system (combined information
from high-temperature data and oxygen titration data between 25–350ꢀC).
Results of these PuO2 ꢄ H2O reactions22,23
suggest that the equilibrium phase dia-
gram is significantly more complex than
indicated by Figure 1. Proposed phase re-
lationships for O/Pu within the 1.5–2.0
versible near 600ꢀC.18,19 However, a similar
transformation process may be sluggish
at lower temperatures (e.g., the transition
of Pu2O3 phases), given the kinetic energy
needed to rearrange metal atoms. Alterna-
tive interpretations are also possible. These
include (1) the formation of a cubic ses-
quioxide during autoreduction of PuO2 at
MRS BULLETIN/SEPTEMBER 2001
693

Plutonium Oxide Systems and Related Corrosion Products
range and at temperatures below 700ЊC in
Figure 2 are derived by analogy to those
of the praseodymium–oxygen system, in
which the corresponding intermediate
oxides are well known.18,19 It should be
emphasized that formation of the different
phases in the plutonium–oxygen system
can be strongly dependent on the experi-
mental method used and the kinetic be-
havior of the materials involved.
tendency to ignite spontaneously if exposed
to air (see subsequent discussion).
not fully in accord with reports of rapid
corrosion in moist inert atmospheres. As-
sessment of the chemical and kinetics data
suggests that diffusion of reactive gases
through an oxide layer of constant thick-
ness is most likely responsible for the ob-
served linear corrosion rate.53
Atmospheric corrosion of plutonium
metal is dependent on the diffusion of
oxygen through an adherent dioxide layer
during reaction with dry air or oxygen, as
demonstrated by the parabolic depend-
ence of the corrosion rate on the reaction
time.12 The rate is found to be inversely
proportional to the thickness of the prod-
uct layer. Similarly, UO2ϩx (with x Ͻ 0.25)
Literature data for reactions of unalloyed
metal with oxygen and water vapor at
25–400ЊC show that moisture-enhancement
of the corrosion rate in air is confined to a
54
Plutonium Corrosion
is the only product obtained by the reactions rate-temperature envelope, as suggested
The corrosion chemistry and reactivity
of plutonium during storage is an impor-
tant aspect of plutonium technology with
relevance to the safe handling and storage
of plutonium metal and its alloys. It is also
important with regard to mechanisms for
the dispersal of plutonium into the envi-
ronment.26,44 Amassive form of oxide-free
solid metal is essentially nondispersible
and presents a lower risk than powdered
(fine) materials, which have a greater
probability of igniting and of becoming
airborne. Mechanical pressures generated
by the corrosion of plutonium metal are
known to have ruptured sealed storage
of uranium metal with dry air and flowing
water vapor, which prevents accumula-
tion of hydrogen near the metal’s surface.
Plutonium chemistry may be similar in
this regard. Data for kinetics studies of the
corrosion of plutonium by air and water
vapor show a paralinear time-dependence
and have been examined on the basis of
each allotropic form of the metal.11 The na-
ture of the linear stage of reaction is espe-
in Figure 3. The upper boundary of this
envelope is defined by the Arrhenius
equation for corrosion by the equilibrium
vapor pressure of water vapor (e.g., pres-
sure as a function of temperature) and at
0.2 bar pressure of O2 (e.g., air). The lower
boundary is fixed by theArrhenius equa-
tion for reaction in dry (Ͻ0.5 ppm H2O)
air. This envelope is closed by intersection
of these regions at Ϫ25ЊC and by a sharp
cially important, as only a thin (micrometers decrease in the Pu ϩ H2O rate between
thick) oxide layer forms during the para-
bolic stage, and most corrosion occurs
during a constant-rate process. The linear
behavior of the rate is explained by forma-
tion of an intact inner oxide layer and a
110ЊC and 200ЊC (attributed to a mecha-
nism change that eliminates enhancement
by moisture). Corrosion rates and activation
energies within the envelope are independ-
ent of O2 pressure, but vary systematically
with the square root of the vapor pressure
of water.
8
containers. The hazard posed by pluto-
46
nium oxides formed by corrosion depends
on several factors. These include the cor-
rosion rate that produces the oxide, the
size distribution of particles formed, and
the time interval for the corrosion (from
periods of an hour or less for accidents, and
up to decades for materials in storage).
The perplexing nature of plutonium cor-
rosion is indicated by a discussion of fac-
tors associated with changes in the metal’s
reactivity.26 Observations suggest that
moisture can be a more important factor
than temperature. Corrosion can be rapid
in moist helium or argon, and moisture
offers a greater potential for corrosion
than dry oxygen. Kinetics data for pure, un-
alloyed plutonium metal in the presence
of moisture show paralinear behavior,
where an initial parabolic stage (consistent
with diffusion control) of the reaction of
the product layer is followed by a slow,
constant-rate (linear) stage of reaction and
then by a more rapid linear stage.
porous outer oxide layer. Mechanisms
proposed for rate enhancement by water
include participation of a reactive PuH2
intermediate and the rapid diffusion of
The chemistry of moisture-enhanced
corrosion in air55 corresponds closely with
the formation of PuO2ϩx by the water-
Ϫ
OH ions through the oxide diffusion
barrier.
catalyzed PuO ϩ O2 reaction. Formation
2
The corrosion kinetics of plutonium metal
and its alloys has been reviewed previ-
ously,26 and an assessment of kinetics data
and proposed mechanisms has been pub-
of PuO2ϩx by that reaction at the gas–oxide
interface increases the oxygen gradient
and promotes diffusion of oxygen through
the oxide layer to the oxide–metal inter-
face. This interpretation is supported by
identification of PuO2ϩx, PuO2, and ␣-Pu2O3
in succession between the gas–oxide and
oxide–metal interfaces of a metal sample
reacted with water vapor at 250ЊC.38
45
lished. In addition, studies of oxide prop-
erties and the kinetics of their formation
have been described,11,12 and the ignition/
pyrophoric nature of plutonium metal has
been discussed.26,47
Substantial progress toward understand-
ing plutonium corrosion resulted from a
series of careful kinetics studies.47–52 Sev-
eral observations reported in earlier work
were addressed by measuring rates for
unalloyed plutonium in dry air, moist
O2-N2 mixtures, and moist N2 at 90ЊC.48
Tests were conducted with Pu samples
that were prepared by rolling and casting
techniques and that contained different
levels of impurities due to radioactive
decay. Results showed that PuO2 was the
only product; PuH was not detected. Ki-
netics data suggest2that the corrosion rate
is not appreciably altered by impurities,
radiation damage, or even differences in-
duced by casting or rolling, although micro-
cracks in the cast metal caused uneven
corrosion (pitting). The corrosion rate is
markedly enhanced by moisture, with oxi-
dation being slightly faster in moist O2-N2
(or air) mixtures than in moist N2 alone,
A mechanism for accelerated oxidation
by water vapor or moist air has been pro-
posed,46 but its validity remains uncertain.
Mechanisms that involve transport of hy-
drogen or hydroxide to the product–metal
interface and the return of hydrogen to the
53
gas–solid interface do not seem credible.
The hydride is stable, with an equilibrium
hydrogen pressure of only 70 nanobar at
300ЊC,54 and it must appear as the major
product, if these mechanisms are correct.
However, only small amounts of hydride
are found in the interface at 250ЊC.38
Pronounced decreases in the corrosion
rate of plutonium metal are found near
200ЊC (where the transition from the
␤-phase to the ␥-phase occurs) and also near
400ЊC. The corrosion rate is also reduced
by alloying and is markedly reduced for
the fcc,␦-phase gallium alloys (containing
a few percent of gallium) of plutonium
compared to the monoclinic, ␣ phase of
the pure metal. Corrosion can occur by dif-
ferent reactions during storage of bulk plu-
tonium in sealed containers; in such cases,
the resulting plutonium metal (fines) often
has a greater surface area and increased
A spallation-type process that main-
tains a constant thickness of an oxide layer
has also been proposed, and an empirical
relationship defining the thickness of
suchan adherent oxide infusion layer as a
function of reaction temperature has been
described.56 The rate-limiting phase is
probably a stoichiometric dioxide and not
a bcc Pu O3. Alikely enhancement mecha-
2
694
MRS BULLETIN/SEPTEMBER 2001

Plutonium Oxide Systems and Related Corrosion Products
netics data for 25–100ꢀC give an activation
ꢂ1
energy of 29 ꢁ 4 kJ mol for the metal-
plus-water reaction.
Concluding Comments
Understanding the chemistry and ac-
quisition of scientific data for plutonium
oxides and related products remains as
important today as it has been in past
decades. Although the focus and the spe-
cific knowledge sought regarding these
materials may have shifted since the dis-
covery of this element, new scientific data
and knowledge continue to be essential.
Large amounts of plutonium exist, and it
is critical that its existence be addressed.
Plutonium is a valuable material, certainly
in the sense of being a potential source of
energy, but even its disposition as a waste
will require careful consideration.
It is essential that the critical parameters
be well defined and understood for safely
handling, storing, disposing of, and using
this element. The migration behavior of
plutonium presently in the environment,
or plutonium that might be released acci-
dentally into the environment, are addi-
tional important issues that will require
study. The data from studies involving
plutonium are also important for develop-
ing rational models to consider various
potential scenarios that may be encoun-
tered with plutonium materials. Clearly,
additional studies are needed to be able to
understand and control the complex
chemistry of plutonium and to establish
fully its fundamental science and techno-
logical applications.
Figure 3. Arrhenius-type plot for the corrosion rate R of both pure and alloyed plutonium
metal in dry (ꢆ0.5 ppm water) and moist (0.2 bar water vapor) air atmospheres (from
Reference 45). Curve 1, metal in air with moisture at the equilibrium pressure fixed by
temperature; Curve 2, metal with moist air; Curve 3, metal at 110–200ꢀC with moist air;
Curve 4, metal ꢆ200ꢀC with dry air and ꢇ200ꢀC with either dry or moist air; Curve 5, Ga-Pu
alloy with moist air; Curve 6, Ga-Pu alloy with dry air; Curve 7, corrosion at 500ꢀC (independent
of moisture or alloying); Curve 8 is for burning bulk metal in static air (temperature-independent);
Curve 9 is for burning, free-falling droplets of metal in air; and Curve 10 represents the
temperature-independence of the rate in air when a hydride-catalyzed reaction is involved.
Acknowledgments
R.G. Haire’s research was sponsored in
part by the Division of Chemical Sciences,
Geosciences, and Biosciences, Office of Basic
Energy Sciences, U.S. Department of Energy,
under contract No. DE-ACO5-00OR22725
with Oak Ridge National Laboratory, man-
aged and operated by UT-Battelle, LLC.
nism suggested by the occurrence of the
water-catalyzed reaction is that water in-
creases the rate by increasing the oxygen
gradient across a constant thickness of
oxide by forming PuO2ꢄx at the gas–solid
interface.8,53
The increased chemical reactivity of metal
stored in sealed containers is consistent with
the emerging chemistry of plutonium
with oxygen and water.8,36,44,53 If moisture
and air are present, the pressure in the
container first decreases as oxygen reacts
via the water-catalyzed cycle, then in-
creases at the identical rate as hydrogen is
formed by the metal-plus-water reaction,
and then decreases as the hydrogen reacts
to form a hydride.57 An autoreduction
process transforms the dioxide layer on
metal to ꢅ-Pu2O3 during extended storage.
Upon exposure to air or oxygen at a pres-
sure of 1 bar, surface layers of the ses-
quioxide and PuH2ꢁx catalyze the reaction
of plutonium with nitrogen and oxygen
(ꢀair) at a rate of ꢀ6 cm/h; with oxygen,
oxidation is intense at ꢀ0.3 meters/h.45
These reactions may be sufficient to ignite
chips or fines of the plutonium metal,
which have very high surface areas.
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