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The plutonium-oxygen system
JOURNAL OF NUCLEAR MATERIALS 12, 2 (1964) 131-141, © NORTH-HOLLAND PUBLISHING CO., AMSTERDAM
THE PLUTONIUM-OXYGEN
SYSTEM t
T. D. CHIKALLA, C. E. McNEILLY and R. E. SKAVDAHL
General Electric Company, Han]ord Atomic Products Operation, Han]ord Laboratories, Richland, Washinflton Received 30 November 1962 and in revised form 18 September 1963
The crystallographic, thermodynamic, and physical properties of plutoninm-oxygen compounds with varying O/Pu ratios are presented and discussed relative to the construction of a phase equilibria diagram for this system. Property determinations included melting and dissociation temperatures, thermal expansion, lattice constants, and electrical resistivity. The most unusual feature of the diagram is a two-phase solid immiscibility region at low temperatures for O/Pu ratios between 1.70 and 1.98. Certain portions of the disgram are still uncertain because of the experimental difficulties associated with the ease with which PuO2.00 loses oxygen at high temperature and with which suboxides gain oxygen.
Les propri@t@s cristallographiques, thermodynamiques et physiques des composds Pu-O avec des rapports variables O-Pu sent pr@sentdes et discut@es coneernant la construction d'un diagramme d'dquilibre de phase dans ce syst@me. Les ddterminations des propri@t@s comportaient les temperatures de fusion et de dissociation, la dilatation thermique, les constantes r@ticulaires et la r@sistivitd @lectrique. La caractdristique la plus anormale du diagramme
1.
Introduction
P l u t o n i u m ceramics are becoming increasingly i m p o r t a n t in nuclear fuel cycles. P l u t o n i u m dioxide mixed with u r a n i u m dioxide has a l r e a d y seen service in t h e r m a l reactgrs and the mixtures have p e r f o r m e d well. Because of the interest in fast reactors a n d space systems, fuels containing higher concentrations of p l u t o n i u m oxides will u n d o u b t e d l y be utilized. This requires a knowledge of the phase equilibria, first in the
est une r@gion d'immiscibilit@ k deux phases dans l'@tat solide observ6e aux basses temp@ratures dans le domaine de rapport O-Pu, situ@ entre 1,70 et 1,98. Certaines parties du diagramme sent encore incertaines en raison des difficult@s exp@rimentales associ4es avec la facilitd avec laquelle PuO~.00 perd de l'oxyg@ne aux temp@ratures @lev~ies et les sous-oxydes gagnent de l'oxyg~ne. Die kristallographisehen, thermodynamischen und physikalischen Eigenschaften von Plutonium-Sauerstoff-Verbindungen mit verschiedenen O/Pu-Verh~ltnissen werden angegeben und im Zusammenhang mit der Aufstellung des Zustanddiagramms fiir das System diskutiert. Die Eigensehaftsbestimmungen umfassen die Schmelz- und Dissoziationstemperaturen, die thermische Ausdehnung, die Gitterkonstanten und den elektrischen Widerstand. Die am meisten hervortretende Charakteristik des Diagramms ist ein Bereich bei goringen Temperaturen for O/Pu-Verh~ltnisse zwischen 1,70 und 1,98, in dem Unvermisehbarkeit zweier fester Phasen vorliegt. Gewisse Teile des Diagramms sind noch ungewiss. Dies hat seinen Grund in experimentellen Schwierigkeiten, welche mit dem leichten Sauerstoffabgang von PuO2 bei hohen Temperaturen lind der Sauerstoff-Aufnahme durch Suboxide zusammenh~ngt.
p l u t o n i u m - o x y g e n system, and secondly in the b i n a r y s y s t e m in which P u 0 2 is incorporated. This r e p o r t summarizes observations on t h e phase relations in the P u - O s y s t e m which have been o b t a i n e d during the course of e x p e r i m e n t s involving various p r o p e r t y d e t e r m i n a t i o n s such as melting point, t h e r m a l expansion, electrical resistivity, a n d t h e r m a l stability. A correlation of these various observations has led to the c o n s t r u c t i o n of a probable b i n a r y equilibrium
Work performed under Contract No. AT (54-1)-1350 for the U.S. Atomic Energy Commission. 131
132
T. D. CHIKALLA, C. E. MCNEILLY AND R. E. SKAVDAHL
diagram. I t should be emphasized t h a t oxygen pressure has a large effect on composition, particularly above O / P u = 1.5 and is thus an extremely important variable. Consequently, to represent the true equilibrium in this system, one should construct a pressure-temperaturecomposition space diagram. In view of the very low oxygen pressures and high temperatures involved, a three dimensional diagram would be a formidable task. A phase diagram for the Pu-O system was first reported by Holley et al 1) (fig. 1). Beta Pu2Os was found to coexist with plutonium metal at low oxygen concentrations. Beta Pu20a is also shown as a compound existing at the composition suggested by the formula, and was found to melt peritectically at 2240 ° C. At high temperatures there may be slight deviations from stoichiometry toward an oxygen excess. The composition of alpha Pu~O3 was not determined in this work but the compound was thought to exist slightly above the O/Pu = 1.50 suggested by the formula. The diagram shows this compound to decompose peritectoidally to beta Pu203 and PuO2-z. Between about PuO1.58 and PuO2.00 a two-phase region consisting of ~-Pu~0s and Pu02 is shown. Pu02.00 became oxygen deficient at high temperatures and a melting point of 2240°C was observed for Pu02-~.
4oo1 F 2000~
1
r
~
2.
Crystallography of the Plutonium Oxides
2.1.
PLUTONIUM DIOXIDE
The structure of plutonium dioxide was determined by Zaehariasen 2) in 1943 using a sample which weighed only 10 #g. The structure is fluorite, isomorphous with U02, Th02 and CeO2, and contains four plutonium and eight oxygen atoms per unit cell. The oxygen atoms are in simple cubic packing within the unit cell which is bounded by plutonium atoms at the cell corners and face-centered positions. The space group is Fm3m and the room temperature lattice constant 3) of 5.3960 =k 0.0003 A gives a calculated density of 11.46 g/cm 3. The interatomic distances based on this lattice constant are : PuOO-Pu -
The oxygen ion spacing gives an ionic radius for O = o f 1.35 A which is close to the 1.401 reported by Pauling 4). The radius of Pu +4 is then calculated as 0.93 A. PuO2 loses oxygen readily at elevated temperatures in reducing atmosphere. The oxygen deficiency results in formation of the larger Pu +3 ion which causes a unit cell expansion. 1
!
~
I ~"
Liquid
80 =2.336 4Pu = 2.336 A 60 =2.698 12Pu = 3.815/~
P.O2/
[~ ,
1 1 I/
1600-
/
I~*~"
hex-Pu203\
[ l[f ......
/
1
h ex - Pu203 I
Liquid + hex*Pu203
~ 1200
~
hex-Pu203 + _ cubic Pu203
cubic-Pu203 +PuO 2
800 - -
5~ 6
400 --
1
Pu + hex-Pu203
7
0.2
0.4
0.6 0.8 1.0 1.2 1.4 1.6 Solid composition oxygen-to-plutonium ratio
1.8
2.0
Fig. 1. Plutonium-oxygen system phase relationships as determined by Holley et al 1).
THE P L U T O N I U M - O X Y G E N
No systematic variation of unit cell dimension as a function of composition has been reported. Unlike its uranium analogue, plutonium dioxide does not readily incorporate excess oxygen in interstitial holes and PuO2+x compositions are rarely prepared. Also no real evidence has been found here or presented in the literature to indicate the prsence of any compounds with O/Pu ratio greater than 2. However, Drummond and Welch 5) and Roberts et al 6) have prepared compositions with ratios as high as 2.09, due perhaps to excess oxygen dissolved in the lattice.
2.2.
ALPHA PLUTONIUM SESQUIOXIDE
The structure of ~-Pu~Oa was indexed by Zachariasen in 1944 7). Alpha Pu203 is bodycentered cubic with 16 formula units per unit cell. I t is a defect structure similar to the C modification of the rare earth sesquioxides (and also Mn208 and TleO3) and can contain more than the 48 oxygen atoms indicated by the composition formula. For this reason the compound is frequently referred to as Pu2OaPu407 or cubic Pu203. The lattice parameter has been found to extend from 10.98 ± 0.02 to 11.06 ± 0.02 A for oxygen rich and oxygen deficient structures respectively. The space group is Ia3 and the atomic positions and position parameters have been solved for the isostructural compound (Fe, Mn)2Oa s). The structure of a-Pu20z is based on a pseudo face-centered lattice with one-half the cell length of the true unit cell. This may be viewed as a clustering of eight PuO2 fluorite cubes with removal of 25 per cent of the oxygen atoms from the simple cubic arrangement in the stacked fluorite structure. There is also a slight shift of the remaining plutonium atoms. This gives an atomic distribution very similar to the fluorite structure and a cell side which is slightly greater than twice t h a t noted for oxygendeficient PuO2, e.g, 11.00 A. This similarity also introduces some degree of arbitrariness into indexing the diffraction patterns of various compositions around the O/Pu ratio of 1.7 onto
133
SYSTEM
one structure or the other. This large open cell can easily accommodate excess oxygen as was originally predicted by Zachariasen. 2.3.
ALPHA-PRIME PLUTONIUM SESQUIOXIDE
This compound is a high temperature form of ~-Pu~Oa. As quenched from the melt, this phase is cubic with a lattice constant of 5.409 ± 0.001 A. The structure is apparently of the fluorite type with oxygen vacancies. This phase is very unstable with respect to oxidation and if one cools slowly from the melt, the products are ~-PueO8 and PuO1.98. This is the result of oxidation during cooling which results in even scavenged inert atmospheres. The ~'-Pu203 phase can only be retained by using very rapid quenching rates, estimated to be on the order of 20 000 ° C/minute. A melting point of 2360 ± 20°C has been found for ~'-Pu208 and X-ray diffraction on samples quenched from about 2000 ° C has shown the compound to be congruently melting. 2.4.
BETA PLUTONIUM SESQUIOXIDE
This is not an allotrope of ~-Pu20a but rather defines a phase of the Pu2Oa composition discovered after ~-Pu~Oa. The structure is hexagonal, similar to the A modification of the rare earth sesquioxides such as LaeOa, and was indexed independently by Ellinger 9) and by Templeton and Dauben 10). There is one formula unit per unit cell which gives a calculated density of 11.47 g/cm a based on a0=3.841 ± 0.006 A and c0=5.958 ± 0.005 • (Ellinger). The space group is P 3 m l and the interatomie distances are : Pu - 3Pu
3.57/~
Pu - 6Pu
3.84/k
P u - 3Pu
3.86 X
Pu
2.36/k
-
4Oii
P u - 3Oi
2.62 A
Oi
2.62 A 2.36/k
O ~ -
-
6Pu 4Pu
The Pu-O distance gives an ionic radius of 0.96/k for pu+a which deviates from Zacha-
134
T.D.
CHIKALLA,
C.
E.
MeNEILLY
riasen's radius of 1.01 A 11). This indicates the presence of Pu +4 and points toward a composition containing an excess of oxygen over the composition Pu203. 2.5.
PLUTONIUM MONOXIDE
PuO was first found by Zachariasen 12) as an impurity phase. To date few pure samples of this material have been formed and it is generally seen as a film on plutonium metal. The structure is in space group Fm3m and is face-centered cubic of the NaC1 type with four formula units per cell. Lattice parameters of 4.96 =i= 0.01 A 9), 4.960 ± 0.003/~27), and 4.948 ± 0.002 A 12), have been reported, and give a calculated density of 13.9 g/cm 3. Each plutonium atom is bonded to six oxygen atoms at a distance of 2.48/~. 3.
Thermodynamics
Compared to the uranium oxides, very few tiiermodynamic data are available in the plutonium-oxygen system. The bulk of the work has been performed on Pu02. Estim~tes have been made for other oxides of plutonium. 3.1.
PLUTONIUM DIOXIDE
In an early paper, Brewer et al 13) estimated the free energy functions for several plutonium compounds based largely on reported heat data in the corresponding uranium systems. On the basis of the heat of formation, AH29s=-251 kcal/mole, the following free energy functions were calculated for Pu02: TABLE 1 F r e e e n e r g y f u n c t i o n s c a l c u l a t e d for PuO2 Temperature (°K)
(A F - - zIH298)/T
298 500 1000 1500
43 42 41 41
(e.u.)
Brewer 14) found the original estimate of AH2o8 to be inconsistent with the heats of formation of ThO2 and UO2. Based on these analogies, a revised estiinate of Pu02 of AH298 = - 246 5- 5
AND
R.
E.
SKAVDAHL
kcal/mole was made. Holley et al 1) determined the heat of combustion of plutonium and calculated for Pu02, dH298=-252.87 5- 0.38 kcal/mole which agrees well with the AH29s= -252.4 5- 1.1 kcal/mole found by Russian workers in their heat of combustion experiments is). Osborne and Westrum 16) estimated values for the absolute entropy of actinide dioxides based on their valuv of S°29s for ThO2 calculated from heat capacity measurements. The S°29s thus obtained for Pu02 was 19.7 cal/deg/mole which is well above the value of S°29s= 16.3 cal/deg/mole obtained by Sandenaw 17). Taking S°29s= 12.30 cal/deg/mole for plutonium metal and S°29s=49.01 cal/dcg/mole for 02(g), the entropy of formation AS°298 for PuO2 is found to be - 4 1 . 6 and - 4 5 . 0 cal/dcg/mole using the data of Osborne and Westrum and Sandenaw, respectively. The former value appears to be more consistent with entropy data on other actinide dioxides and also with Coughlin's 18) estimate of AS°29s=--41.9 cal/deg/mole. Using Coughlin's AS°29s and ACp of 1.5 cal/deg, Holley et al. 1) calculated the heat and free TABLE 2 H e a t a n d free e n e r g y o f f o r m a t i o n o f PuO2 Temperature (°K) 298.16 383 (a)
383 (fl) 4O0 5O0 503 593 6OO 7OO 800 9O0 913 913 1000 ll00 1200 1300 1400 1500
(~) (6)
(e) (liq)
--AH (eal/mole) 252 252 253 253 253 253 254 254 253 253 253 253 254 254 253 253 253 253 253
900 ( ± 5 0 0 ) 800 700 700 500 400 100 100 900 800 600 600 100 000 800 700 500 400 200
_AF ° (cal/mole) 240 236 236 236 231 228 228 227 223 219 215 214 214 211 207 203 199 195 191
400 800 800 100 900 000 000 700 600 500 400 900 900 400 300 300 200 200 2 0
( ± 800)
THE P L U T O N I U M - O X Y G E N SYSTEM e n e r g y of f o r m a t i o n of PuOs u p to 1500 ° K , b a s e d on their AH2gs of - 2 5 2 . 9 kcal/mole. These d a t a are t a b u l a t e d in t a b l e 2 a n d included the h e a t s of t r a n s i t i o n a n d fusion for t h e allotropic t r a n s f o r m a t i o n shown. T h e free e n e r g y can be r e p r e s e n t e d b y the following e q u a t i o n o v e r the i n t e r v a l 298-1500 ° K : A F ° = - 253480 - 3.45 T log T + 52.48 T.
(1)
S a n d e n a w aT) m e a s u r e d the h e a t c a p a c i t y of P u 0 2 at low t e m p e r a t u r e s . T h e h e a t c a p a c i t y b e t w e e n 11 a n d 100 ° K was f o u n d to be a n a m a l ous a n d irreproducible, changing slightly a f t e r e v e r y one of seven liquid helium coolings. A f t e r seven coolings, the h e a t c a p a c i t y versus t e m p e r a t u r e curve was quite u n i f o r m o v e r the r a n g e 4 - 300 ° K . A least squares fit gives the following e q u a t i o n for the low t e m p e r a t u r e h e a t c a p a c i t y per g r a m f o r m u l a weight (gfw):
C r = A T + B T a (11.9 ° K to 28.8 ° K )
(2)
where
3.2.
ALPHA P L U T O N I U M
A F~, = - 414 + 0.066T keal/mole.
S a n d e n a w iv) calculated values of e n t h a l p y (H) a n d e n t r o p y (S) for the u n i f o r m c u r v e of h e a t c a p a c i t y versus t e m p e r a t u r e , a p p l y i n g t h e T h i r d Law. These calculated values, along w i t h values of h e a t c a p a c i t y r e a d f r o m a s m o o t h e d curve, are s u m m a r i z e d for a few selected t e m p e r a t u r e s in t a b l e 3.
25 50 75 100 150 200 250 298 300 320
0.80 2.62 4.56 6.38 9.28 11.9 14.3 16.4 16.4 17.2
(3)
240
220
200
TABLE 3 Values of C~, S, and AH at different temperatures ST (eal/gfw. °K)
SESQUIOXIDE
To date, there h a v e been no t h e r m o d y n a m i c d a t a published in the open literature. As a result of the s t u d y of e x p e r i m e n t a l d a t a o b t a i n e d in this l a b o r a t o r y a n d of the p r e v i o u s l y m e n tioned results a n d e s t i m a t e s for the t h e r m o d y n a m i c functions of o t h e r p l u t o n i u m - o x y g e n c o m p o u n d s , a r o u g h e s t i m a t e of the AH298 a n d ASegs for a - P u 2 O s has been obtained. I f the values of AHsgs a n d AS29s per g r a m f o r m u l a weight of c o n t a i n e d p l u t o n i u m as r e p o r t e d above, are p l o t t e d as a f u n c t i o n of the O / P u ratio, t w o s t r a i g h t lines result (see fig. 2). The value of AH29s for P u O e m p l o y e d was the a r i t h m e t i c m e a n of - 1 3 0 a n d - 1 4 0 , or - 1 3 5 kcal/mole. F r o m these t w o lines, values of AH298 a n d AS298 for an O / P u r a t i o of 1.62 are seen to equal - 2 0 7 k c a l / m o l e a n d - 3 3 cal/°K, respectively. A first a p p r o x i m a t i o n of the free e n e r g y of f o r m a t i o n for ~ - P u 2 0 s would t h e n be :
A = 1.350 × 10 -z B = 2.877 × 10 -5 ( s t a n d a r d deviation) = 3.0 × 10 -2.
TemperVia ature (eal/gfw. °K) (°K)
135
ST--So (cal/gfw)
180
50
160
40
140
30
120
2o
i
0.434-0.03 1.504-0.04 2.944-0.05 4.504-0.05 7.644-0.07 10.684-0.10 13.614-0.13 16.334-0.16 16.434-0.16 17.524-0.20
7.0 48 139 276 668 1200 1858 2600 2632 2969
4- 0.5 4- 1 4- 2 4- 3 4-7 4- 10 4- 13 4- 26 4-26 4-30
1.0
1.2
1.4
1.6
Oxygen-to-plutonium,
Fig. 2.
1.8
I0 2.0
atomic ratio
Variation of ASsgs and ASsgs with oxygen-to-plutonium ratio.
~'
i
136
T.D.
CHIKALLA,
C.
E.
MCNEILL¥
AND
R.
E.
SKAVDAHL
As a second approximation, the hydrogen reduction of Pu02 is considered. We have observed th at reduction of Pu02 below about PuOLgs takes place initially at a temperature of l l 0 0 to 1200°C (approximately 1400°K) when using continuously flowing hydrogen with approximately 100 ppm water vapor. Assuming the reduction reaction to be
Since the above results are based on comparison with the A modification of the rare earth s:,squioxides, we have taken them to be representative of/~-Pu203, rather than ~-Pu203. In addition, the us~ of the above values of AH2gs and ASegs to calculate the free energy of Pu203 by the relation
2PuO2 + 0.76He --> Pu20~.2a + 0.76H20
and the use of the free energies as calculated by cq. (6) in predicting reduction of PuOe by various materials, yields large, positive values of the LIF of reaction for reactions which are known to occur experimentally and which yield ~-Pu20a as the reduced product.
(4)
it is possible to calculate the value of the LJF of formation of .~-Pu20a (Pu203.24) at 1400 ° K required to obtain a partial pressure o~" H20 equivalent to 100 ppm in hydrogen gas at one atmosphere. (The significance of Pu203.2a2 PuOL62=a-Pu20a will be discussed in more detail later in the report). The value calculated was about - 3 4 0 kcal/mole for ~-Pu20a (at 1400 ° K). In order to obtain an estimate of the free energy of formation of a-Pu203 as a function of temperature, the value o f / I F = - 340 kcal/ mole at 1400 ° K was assumed to be the " t r u e " value. Then, since the line representing the values of ASegs as a function of O/Pu atomic ratio in fig. 2 has the less steep slope of the two lines plotted, the value of LJS29s-- --33 c.u. per gram formula weight of plutonium at O / P u = 1.62 was also assumed to be a " t r u e " value. Subsequent use of eq. (6) yields the value of A H 2 ~ s = - 4 3 2 kcal/mole for ~-PueOa and therefore a rough estimate of the free energy of formation of ~-Pu20a as a function of temperature (°Kelvin) would be zJ F T ° ~
3.3.
--
432 + 0.066 T kcal/m~.)le.
(5)
BETA PLUTONIUM SESQUIOXIDE
Brewer la) has estimated d H 2 9 s 387 kcal/ mole for Pu20a based on heat data for PuC13 and the analogous sesquioxide and chloride in the lanthanum system. A constant entropy of formation of - 6 3 e.u. was assumed in accord with the entropies of formation of other oxides. Roberts 19) has estimated that/1H29g= -393 ± l0 kcal/mole for Pu2Os, which is in good agreement with Brewer. This estimate was based on the heats of formation of Pu +3 and pr+3 and Pr203.
LJ F T
3.4.
- - LJH298 - TzJS298
(6)
P L U T O N I U M MONOXIDE
Based on early data on phase equilibria in the P u - O system, Brewer 13) has estimated that the heat of formation of PuO lies between - 1 3 0 and 140 kcal/mole. He also estimated A S 2 9 s to be - 2 1 cal/°K. Qualitative confirmation of the above values of zJH29s and AS29s has been obtained here at Hanford 2o) by the reaction of Pu02 and carbon to form PuO at 1800° C in helium and which is predicted, by applying the values of A H 2 9 s and AS29s in eq. (6), to have an equilibrium partial pressure of CO at 1800°C of about 250 mm Hg.
4. 4.1.
The Phase Diagram O/Pu
2 . 0 0 - 1.62
The proposed phase diagram is shown in figs. 3a and b. Pu02 loses a slight amount of oxygen after heating to only 1100 1200° C in inert or reducing atmospheres. Oxidation gives a slight weight gain which corresponds to a composition of PuO1.gs. X-ray diffraction gives lattice constants of about 5.398 to 5.400 A, and thermal emf measurements on PUO1.99 give indications of n-type conductivity. Both of these measurements are indicative of an oxygen deficient Pu02 lattice. The boundary of this defect phase has been placed at PuO1.gs. Heating of PuO2 to higher temperatures in vacuum, inert or reducing atmosphere results
THE
PLUTONIUM-OXYGEN
in further oxygen deficiency, and upon cooling to room temperature, a two-phase product is formed. The coexisting phases are ~-Pu2Os and PuO1.gs. The O/Pu atomic ratio then is dependent upon the relative proportions of the two phases present. Heating such a mixture in
2400
'
I
I
'
I
'
air to temperatures near 800°C oxidizes the material to Pu02.00 and therefore produces a weight increase which m ay then be used to calculate the O/Pu ratio of the original sample. The relative intensities of the X-ray diffraction spcctra may also be used to obtain an estimate
I
I
'
137
SYSTEM
I
'
I
2~9 2085 /
2000
--1900
Liq +
~
PuO2_x_
80
L~n
/
1600
f
/
/
(9
L + PuO Pu203
/
/ 1200 - -800
/
/
+
/
Pu02 -x
C
Liq
~ fi ,
PuO +
Pu 0
__/ •
+
~+~
P u + PuO
--
/
~/p~l02_x,
/ f
8 l
i~
I
I
i
l
0.4
i
0.6
I 0.8
1.0
1.2
a - Pu203 + , W2-~,
1.6
1.4
2(_~ \- -
~\~' ~O2.x. !
g 0.2
PuO
a - Pu203
Pu203
400
Pu02 - x ,
B-Pu2oa
i/
_
1.8
2.0
Oxygen-to-Plutonium
Fig.
3a. Proposed phase equilibria diagram for the system plutonium-oxygen. 800
I
a+~ - --/Pu203 \
I
a - PU203
600
I \\
(9
/
/
i 400
\
\!
\/ .\,
o
o
.
0
•
O
200
0 Thermal Expansion (Hanford) - Pu203 a - Pu203
1.6
o Electrical Resistivity (Hanford) • Thermal Expansion (Harwell)
+ PuO2-x
,
I
,
1.7
1.8
1.9
2.0
Oxygen-to-Plutonium
Fig.
3b.
Low temperature-high oxygen content portion of diagram showing experimental points.
138
T. D. CHIKALLA, C. E. MCNEILLY AND R. E. SKAVDAHL
measurements on PuO2.00 show good continuity as a function of temperature indicating no allotropes for this compound. Reduced oxides, however, show discontinuities at various temperatures on both heating and cooling 21-23). X-ray diffraction on these types of specimen show ~-Pu203 and PuO1.9s to be present both before and after the experiment. Specimens of composition PUO1.72, PuO1.Te, PuOl.s2, PuOl.s4, PuO1.90, PuO1.92, Pu01.96, and PUO1.97 all gave a discontinuity between 280-340 ° C, in both dialatometric and resistivity experiments. An additional higher temperature discontinuity was noted, the exact temperature of which was composition dependent. Samples between PuOl.s and Pu01.9 showed this discontinuity at 650 ° C while at PuO1.72 the temperature had dropped
1.8---
1.6--
1.4-O 1.2
1.0--
eq
0.8--
0.6--
0.4--
0.2
\ 0.0
1.5
[
I
1.6 1.7 1.8 Oxygen-to-plutonium
[
1.9
\~
2.0
Fig. 4. Dependence of X-ray diffraction intensity ratio of Pu203(222)-to-PuO2(lll) lines on oxygen-toplutonium ratio.
of the O / P u ratio after an i n t e n s i t y / c o m p o s i t i o n calibration has been made. Such a calibration is shown in fig. 4 where the ratio of the line intensities of the ~ - P u 2 0 3 (222) a n d P u 0 2 (111) planes is p l o t t e d as a function of a t o m i c ratio. Compositions of Pu01.9: a n d below h a v e shown p - t y p e c o n d u c t i v i t y , a p p a r e n t l y due to ~ - P u 2 0 3 which contains excess o x y g e n a n d which t h u s is e x p e c t e d to b e h a v e as a p - t y p e semi-conductor. T h e r m a l e x p a n s i o n a n d electrical r e s i s t i v i t y TABLE
to 490°C (table 4). H i g h t e m p e r a t u r e X - r a y diffraction on s a m ples b e t w e e n PuO1.7 a n d PuO1.9 showed a twop h a s e region j u s t a b o v e 300°C consisting of two different cubic phases, b o t h of PuO2-x composition, a l t h o u g h one of these could be considered as a Pu203+z composition. Only the low angle s p e c t r a were recorded a n d on the basis of previous s t a t e m e n t s it is a s s u m e d t h a t a - P u 2 0 3 containing excess o x y g e n coexists with PuO1.gs below the t w o - p h a s e loop whose t e m p e r a t u r e boundaries are defined b y the o b s e r v e d discontinuities. Compositions below PuO1.79 are e x t r e m e l y unstable a n d could not be p r e p a r e d b y the n o r m a l h y d r o g e n reduction techniques. Therefore, the lower limit of the d i a l a t o m e t r i c a n d resistivity analyses was a b o u t Pu01.7. At r o o m t e m p e r a t u r e , c¢-Pu203 has shown lattice 4
Summary of electrical resistivity measurements of plutonium oxides O/Pu ratio
I
I
1.72 1.84 1.92 1.96 2.00 t
Extrapolated.
~298
[
~1250
(ohm-era)
]
(ohm-cm)
/~E( ,~ TT) (eV)
5 × 10-2 4 × 10-1 9 × l0 -1 4.5 8 × 102
0.95 1.11 1.24 1.28 1.8
6 × 104 1.0 × 105 8 × 104 t 3 × 105 t 4 × 101° t
TT
490 650 630 575
AE(~TT) (eV) 0.54 0.49 0.52 0.51 1.8
THE
PLUTONIUM-OXYGEN
constants from 10.98 to as high as 11.06 A. This points toward a varying oxygen solubility in the structure and is not surprising in view of the defect nature of the cell. Thus a ~-Pu~O3+z solid solution region is shown. This compound is apparently very unstable with respect to oxidation, and we have not been able to produce it free from other phases at room temperature. A lamellar, pearlitic type mierostructure has been observed in reduced oxides with the quantity of the lamellae increasing as compositions approach PuOLT0. Fig. 5 is a photomicrograph of a two-phase structure of composition PuOl.ss. This type of structure indicates the possibility of a eutectoid reaction in the system. Due to the refractory nature of plutonium oxides, especially those with higher O/Pu ratios, phase identification via normal metallographic techniques is, at best, difficult. In addition, the suboxides are quite susceptible to
Fig. 5. P h o t o m i c r o g r a p h of a n e t c h e d PuOl.s5 s a m p l e s h o w i n g t h e p r e s e n c e of t w o p h a s e s a n d a e u t e c t o i d like s t r u c t u r e . 300 x
SYSTEM
139
oxidation during grinding and polishing so that without great precautions, one may readily change the composition of a sample surface. For these reasons metallography was not used to a great extent in phase identification. A eutectoid reaction based on metallographic evidence and extrapolation of the discontinuities as shown in fig. 3b is given at 300°C and PuO1.70. At the invariant point, the reaction is of the form: PuO2-x ~ Pu01.98 ÷ ~-Pu203+y
(7)
where x is about 0.3 and y is somewhat greater than 0.24. As mentioned previously, high temperature diffraction showed two cubic phases coexisting above 300 ° C for O/Pu ratios greater than that of the eutectoid. The fact that p-type semiconductivity is observed in the two phase loop, indicates that one of these may have a Pu2Os+z composition. This oxygen excess causes a partial oxidation to Pu +4, i.e., an eight fold coordination of plutonium with respect to oxygen. Because of the close similarity between the C type rare earth and fluorite structures, this phase may represent a metastable transition between the ~-Pu203 and the PuO2 lattices. Thus the transformation corresponds more to an atomic rearrangement or position transfer than to the customary nucleation and growth type reaction. Adopting this hypothesis, the transition phase is labeled PuO2-x,. Along the two phase loop boundary it transforms along with PuO~-x to the defect fluorite phase PuO2-x. This solid immiscibility gap is similar to that occuring in the metallic system A1-Zn. Above 300°C the PuO2-x, is also shown coexisting with alpha Pu203+z for compositions below PuOi.70. This has not been verified experimentally due to difficulty in obtaining samples at these low O/Pu ratios, and is included to complete the phase diagram. The observation by high temperature diffraction of a single suboxide phase at 1000 ° C has been reported 23). In that report, however, it was not possible to determine whether the structure type was fluorite or that of the C type
140
T.D.
CHIKALLA,
C.
E.
MCNEILLY
rare earth sesquioxides. Quench tests were employed to clarify this point. Suboxide samples were quenched from 900 ° C into liquid nitrogen and from 1200 ° C by shutting off the power in a low heat capacity furnace. Both experiments frequently resulted in some quantity of ~-Pu2Oa indicating a quite rapid transformation from the sing!e-phase region. When the single-phase was retained by quenching, a very clear powder pattern with a good high angle doublet resolution was obtained. A composition of PuO1.77 when quenched from 900 ° C, gave a single fluorite type phase with a0=5.404 ± 0.001 A. This is in excellent agreement with the lattice constant predicted on the basis of a linear lattice spacing composition plot between PuOe (a0 = 5.396/k) and ec'-Pu208 (a0=5.409 A). High temperature thermal emf measurements have shown that suboxide compositions change from p- to n-type conductivity above about 650 ° C. This points toward a metal excess conductor and indicates that the high temperature single-phase is of the oxygen deficient fluorite type. The melting point of PuOe.00 has been reported 24) as 2400°C and the equilibrium oxygen pressure over PuO~.00 is between 0.1 and 1,0 atmospheres. In some 40 determinations in helium we have noted an apparent melting point of 2280 ± 30 ° C for Pu02. Accompanying melting is an evolution of oxygen since the melted composition consistently analyzes Pu01.62. This is thought to be the composition of the ~-Pu20a phase which Zachariasen predicted would exist with an oxygen excess and for which Holley et al 1) estimated PuOLas. This composition corresponds more closely to a formula of Pus08 than Pu2Oa. To be consistent with previous terminology this phase has been labelled a'-Pu2Oa. Since PuOz dissociates to this composition, and since this compound has a slightly but noticeably, higher melting point than the apparent Pu02 melting point, we have shown the 2280°C dissociation temperature as a melting minimum at a composition very close
A:ND R .
E.
SKAVDAHL
to PuOL62. A liquidus and solidus is shown joining the minimum and Pu02.00. a'-Pu20a undergoes an allotropic transformation to the C type rare earth (body centered cubic) a-Pu2Oa. It is difficult to determine the exact temperature primarily because compositions as low as PuOL62 oxidize so readily; however, it has been shown that it is below 900 ° C. 4.2. O / P u = 1 . 6 2 - 1.50 Beta Pu203 has been formed by heating Pu02 and carbon in the appropriate proportions at 1800° C in helium. The product was singlephase as determined by X-ray diffraction with lattice constants of a0=3.835 ± 0.004 -~, c0=5.944 ± 0.005 A. Heating of Pu02 with 20 per cent excess Pull3 in vacuum at 1600 ° C did not produce single-phase fl-Pu203 presumably due to vaporization of the plutonium metal after decomposition of the hydride. This type of reaction resulted in a mixture of e¢- and fl-Pu203. Melting of PuO~ in a hydrogen atmosphere has resulted in an unidentified product 25) and the reported interplaner spacings can be indexed very well on single-phase fl-Pu203. Workers at Los Alamos Scientific Laboratory have apparently been able to prepare fl-Pu~Oa by hydrogen reduction of PuO2 at 1900°C utilizing a small amount of high surface area powder 26). Six determinations gave a melting point of 2085 ± 25°C for fl-Puz08 in helium. This material as quenched from the melt and as slowly cooled through the solidification point gave only a single phase on X-ray powder patterns with a0=3.845 ± 0.006 A and Co= 5.948 ± 0.004/k. The melting of fl-Pu~Oa was therefore taken to be congruent. This is in contrast to the 2240°C melting point and incongruent melting reported previously 1). A eutectic has been drawn midway between fl-Pu20a and ~'-Pu20a at an arbitrarily selected temperature of 2030 ° C. Not enough samples of fl-Pu203 were run to determine the extent of high temperature oxygen solubility in this compound. Holley et al. 1) have reported some solubility of oxygen in fl-PuzO3 above 2000 ° C.
THE P L U T O N I U M - O X Y G E N SYSTEM
This is shown in the proposed diagram as a solubility loop near the eutectic temperature. When a pressed mixture of a 1 : 1 molar ratio of plutonium powder and fl-Pu203 were heated to 600 ° C, only plutonium metal and fi-Pue03 were observed in the sample.
a)
4.3.
9)
O / P u ( : 1.50
PuO has been prepared by reacting PuO2 and carbon in stoichiometric proportions at 1800°C in helium and at 1550° C in vacuum. The monoxide prepared in this manner m a y actually be a carbon-stabilized form or an oxycarbide. Non-carbon-bearing PuO was formed by Akimoto 27) employing the reaction of molten Pu metal and oxygen at 800 ° C. PuO is believed to melt congruently at about 1900 ° C. The region between O/Pu = 1.00 and 1.50 has been shown as a two-phase field of PuO ~nd fi-Pu203. An eutectic has been placed arbitrarily at O/Pu = 1.25 ~nd 1700 ° C. All samples of O/Pu ratio between 1.00 and 1.50, prepared by carbon reduction of PuO2, have shown the two-phase mixture by X- r ay powder pat t er n ~nalysis. The region below O / P u = 1.00 is shown as a two-phase mixture of PuO and Pu metal, although there have been no reliable samples to date to confirm this.
Acknowledgements The authors are grateful for the assistance of L. D. Bruggeman, R. L. Ring, C. T. Groswith, and J. P. Keiser in m any phases of the experimental work. Special thanks are due Dr. D. R. de Hulas, Dr. I. D. Thomas, and O. J. Wick for the benefit of m any helpful discussions.
References
~) 7) s)
10)
11) 12) 13)
14) la) 26) 17) 18) 19)
2o) 21) e~) 23)
24)
2)
C. E. Holley et al, Thermodynamics and phase relationships for plutonium oxides, Proceedings of the Second Geneva Conference (1958) 2) W . H . Zachariasen, Metallurgical Project Report No. CK 1096 (Technical Information Division USAEC 1943) ~) ~V. H. Zachariasen, Acta Cryst. 2 (1949) 388 4) L. Pauling, The Nature of the Chemical Bond, Third edition (Cornell University Press, Ithaca, N.Y., 1960)
25)
26) 27)
141
J. L. D r u m m o n d and G. A. Welch, J. Chem. Soc. (1957) 4781 L. E. J. Roberts et al, The Actinide Oxides, A/Conf. 15/8/26 Vol. 28 (1958) W . H . Zachariasen, AECD-1787, undated (Technical Information Division USAEC) R. W. G. Wyckoff, Crystal Structures, Vol. 2 (Interscience Publishers, New York 1948) A. S. Coffinberry and F. I-I. Ellinger, The intermetallic compounds of plutonium, Proceedings of the International Conference on the Peaceful Uses of Atomic Energy 9 (1956) 138 D . H . Templeton and C. H. Dauben, UCRL-1886 (Office of Technical Services, Washington 25, D.C., 1952) W . H . Zachariasen, MDDC-67, undated (National Nuclear Energy Series, McGraw-Hill Publ.) W . H . Zachariasen, CK-1367 (Office of Technical Services, Washington 25, D.C., 1944) L. Brewer et al, The thermodynamic properties and equilibria at high temperatures of the compounds of plutonium; The T r a n s u r a n i u m Elements, NNES, IV-14B (1949) L. Brewer, Chem. Rev. 52 (1953) 1 M.M. Popov and I. M. Ivanov, Soviet J. Atomic Energy (English translation) 2 (1957) 439 D. Osborne and E. F. Westram, J. Chem. Phys. 21 (1953) 1884 T. A. Sandenaw, J. Nucl. Mat. 10 (1963) 165 J. P. Coughlin, Contributions to Data on Theoretical Metallurgy, USBM Bulletin 542 (1954) L. E. J. Roberts, Plutonium Dioxide in Fuel Elements, A E R E - C - M - 3 2 5 (Office of Technical Services, Washington 25, D.C., 1957) R. E. Skavdahl, The Reactions Between PuO2 and Carbon, HW-SA-3117 C. E. McNeilly, J. Nucl. Mat. 11 (1964) 53 T. D. Chikalla and J. M. Taylor, Thermal Expansion of Plutonium Oxides, HW-74788 (1962) N. H. Brett and L. E. Russell, The thermal expansion of Pu02 and some other actinide oxides between room temperature and 10000 C, International Conference on Plutonium Metallurgy, Grenoble, France, 1960 (Cleaver-Hume Press, London 1961) p. 397, 426 L . E . Russell, Discussion of the Grenoble Papers, ibid, p. 492 S. ~V. Pijanowski and L. S. DeLuca, Melting Points in the System PuO~--UO2, KAPL-1957 (Office of Technical Services, Washington 25, D. C., 1960) Vf. M. Olson, Los Alamos Scientific Laboratories, Private communication (1963) Y. Akimoto, Preparation of AmO and PuO, Chemistry Division Semiannual Report November 1959, Report No. UCRL-9093, p. 74 (Office of Technical Services, Washington 25, D.C.)
THE PLUTONIUM-OXYGEN
SYSTEM t
T. D. CHIKALLA, C. E. McNEILLY and R. E. SKAVDAHL
General Electric Company, Han]ord Atomic Products Operation, Han]ord Laboratories, Richland, Washinflton Received 30 November 1962 and in revised form 18 September 1963
The crystallographic, thermodynamic, and physical properties of plutoninm-oxygen compounds with varying O/Pu ratios are presented and discussed relative to the construction of a phase equilibria diagram for this system. Property determinations included melting and dissociation temperatures, thermal expansion, lattice constants, and electrical resistivity. The most unusual feature of the diagram is a two-phase solid immiscibility region at low temperatures for O/Pu ratios between 1.70 and 1.98. Certain portions of the disgram are still uncertain because of the experimental difficulties associated with the ease with which PuO2.00 loses oxygen at high temperature and with which suboxides gain oxygen.
Les propri@t@s cristallographiques, thermodynamiques et physiques des composds Pu-O avec des rapports variables O-Pu sent pr@sentdes et discut@es coneernant la construction d'un diagramme d'dquilibre de phase dans ce syst@me. Les ddterminations des propri@t@s comportaient les temperatures de fusion et de dissociation, la dilatation thermique, les constantes r@ticulaires et la r@sistivitd @lectrique. La caractdristique la plus anormale du diagramme
1.
Introduction
P l u t o n i u m ceramics are becoming increasingly i m p o r t a n t in nuclear fuel cycles. P l u t o n i u m dioxide mixed with u r a n i u m dioxide has a l r e a d y seen service in t h e r m a l reactgrs and the mixtures have p e r f o r m e d well. Because of the interest in fast reactors a n d space systems, fuels containing higher concentrations of p l u t o n i u m oxides will u n d o u b t e d l y be utilized. This requires a knowledge of the phase equilibria, first in the
est une r@gion d'immiscibilit@ k deux phases dans l'@tat solide observ6e aux basses temp@ratures dans le domaine de rapport O-Pu, situ@ entre 1,70 et 1,98. Certaines parties du diagramme sent encore incertaines en raison des difficult@s exp@rimentales associ4es avec la facilitd avec laquelle PuO~.00 perd de l'oxyg@ne aux temp@ratures @lev~ies et les sous-oxydes gagnent de l'oxyg~ne. Die kristallographisehen, thermodynamischen und physikalischen Eigenschaften von Plutonium-Sauerstoff-Verbindungen mit verschiedenen O/Pu-Verh~ltnissen werden angegeben und im Zusammenhang mit der Aufstellung des Zustanddiagramms fiir das System diskutiert. Die Eigensehaftsbestimmungen umfassen die Schmelz- und Dissoziationstemperaturen, die thermische Ausdehnung, die Gitterkonstanten und den elektrischen Widerstand. Die am meisten hervortretende Charakteristik des Diagramms ist ein Bereich bei goringen Temperaturen for O/Pu-Verh~ltnisse zwischen 1,70 und 1,98, in dem Unvermisehbarkeit zweier fester Phasen vorliegt. Gewisse Teile des Diagramms sind noch ungewiss. Dies hat seinen Grund in experimentellen Schwierigkeiten, welche mit dem leichten Sauerstoffabgang von PuO2 bei hohen Temperaturen lind der Sauerstoff-Aufnahme durch Suboxide zusammenh~ngt.
p l u t o n i u m - o x y g e n system, and secondly in the b i n a r y s y s t e m in which P u 0 2 is incorporated. This r e p o r t summarizes observations on t h e phase relations in the P u - O s y s t e m which have been o b t a i n e d during the course of e x p e r i m e n t s involving various p r o p e r t y d e t e r m i n a t i o n s such as melting point, t h e r m a l expansion, electrical resistivity, a n d t h e r m a l stability. A correlation of these various observations has led to the c o n s t r u c t i o n of a probable b i n a r y equilibrium
Work performed under Contract No. AT (54-1)-1350 for the U.S. Atomic Energy Commission. 131
132
T. D. CHIKALLA, C. E. MCNEILLY AND R. E. SKAVDAHL
diagram. I t should be emphasized t h a t oxygen pressure has a large effect on composition, particularly above O / P u = 1.5 and is thus an extremely important variable. Consequently, to represent the true equilibrium in this system, one should construct a pressure-temperaturecomposition space diagram. In view of the very low oxygen pressures and high temperatures involved, a three dimensional diagram would be a formidable task. A phase diagram for the Pu-O system was first reported by Holley et al 1) (fig. 1). Beta Pu2Os was found to coexist with plutonium metal at low oxygen concentrations. Beta Pu20a is also shown as a compound existing at the composition suggested by the formula, and was found to melt peritectically at 2240 ° C. At high temperatures there may be slight deviations from stoichiometry toward an oxygen excess. The composition of alpha Pu~O3 was not determined in this work but the compound was thought to exist slightly above the O/Pu = 1.50 suggested by the formula. The diagram shows this compound to decompose peritectoidally to beta Pu203 and PuO2-z. Between about PuO1.58 and PuO2.00 a two-phase region consisting of ~-Pu~0s and Pu02 is shown. Pu02.00 became oxygen deficient at high temperatures and a melting point of 2240°C was observed for Pu02-~.
4oo1 F 2000~
1
r
~
2.
Crystallography of the Plutonium Oxides
2.1.
PLUTONIUM DIOXIDE
The structure of plutonium dioxide was determined by Zaehariasen 2) in 1943 using a sample which weighed only 10 #g. The structure is fluorite, isomorphous with U02, Th02 and CeO2, and contains four plutonium and eight oxygen atoms per unit cell. The oxygen atoms are in simple cubic packing within the unit cell which is bounded by plutonium atoms at the cell corners and face-centered positions. The space group is Fm3m and the room temperature lattice constant 3) of 5.3960 =k 0.0003 A gives a calculated density of 11.46 g/cm 3. The interatomic distances based on this lattice constant are : PuOO-Pu -
The oxygen ion spacing gives an ionic radius for O = o f 1.35 A which is close to the 1.401 reported by Pauling 4). The radius of Pu +4 is then calculated as 0.93 A. PuO2 loses oxygen readily at elevated temperatures in reducing atmosphere. The oxygen deficiency results in formation of the larger Pu +3 ion which causes a unit cell expansion. 1
!
~
I ~"
Liquid
80 =2.336 4Pu = 2.336 A 60 =2.698 12Pu = 3.815/~
P.O2/
[~ ,
1 1 I/
1600-
/
I~*~"
hex-Pu203\
[ l[f ......
/
1
h ex - Pu203 I
Liquid + hex*Pu203
~ 1200
~
hex-Pu203 + _ cubic Pu203
cubic-Pu203 +PuO 2
800 - -
5~ 6
400 --
1
Pu + hex-Pu203
7
0.2
0.4
0.6 0.8 1.0 1.2 1.4 1.6 Solid composition oxygen-to-plutonium ratio
1.8
2.0
Fig. 1. Plutonium-oxygen system phase relationships as determined by Holley et al 1).
THE P L U T O N I U M - O X Y G E N
No systematic variation of unit cell dimension as a function of composition has been reported. Unlike its uranium analogue, plutonium dioxide does not readily incorporate excess oxygen in interstitial holes and PuO2+x compositions are rarely prepared. Also no real evidence has been found here or presented in the literature to indicate the prsence of any compounds with O/Pu ratio greater than 2. However, Drummond and Welch 5) and Roberts et al 6) have prepared compositions with ratios as high as 2.09, due perhaps to excess oxygen dissolved in the lattice.
2.2.
ALPHA PLUTONIUM SESQUIOXIDE
The structure of ~-Pu~Oa was indexed by Zachariasen in 1944 7). Alpha Pu203 is bodycentered cubic with 16 formula units per unit cell. I t is a defect structure similar to the C modification of the rare earth sesquioxides (and also Mn208 and TleO3) and can contain more than the 48 oxygen atoms indicated by the composition formula. For this reason the compound is frequently referred to as Pu2OaPu407 or cubic Pu203. The lattice parameter has been found to extend from 10.98 ± 0.02 to 11.06 ± 0.02 A for oxygen rich and oxygen deficient structures respectively. The space group is Ia3 and the atomic positions and position parameters have been solved for the isostructural compound (Fe, Mn)2Oa s). The structure of a-Pu20z is based on a pseudo face-centered lattice with one-half the cell length of the true unit cell. This may be viewed as a clustering of eight PuO2 fluorite cubes with removal of 25 per cent of the oxygen atoms from the simple cubic arrangement in the stacked fluorite structure. There is also a slight shift of the remaining plutonium atoms. This gives an atomic distribution very similar to the fluorite structure and a cell side which is slightly greater than twice t h a t noted for oxygendeficient PuO2, e.g, 11.00 A. This similarity also introduces some degree of arbitrariness into indexing the diffraction patterns of various compositions around the O/Pu ratio of 1.7 onto
133
SYSTEM
one structure or the other. This large open cell can easily accommodate excess oxygen as was originally predicted by Zachariasen. 2.3.
ALPHA-PRIME PLUTONIUM SESQUIOXIDE
This compound is a high temperature form of ~-Pu~Oa. As quenched from the melt, this phase is cubic with a lattice constant of 5.409 ± 0.001 A. The structure is apparently of the fluorite type with oxygen vacancies. This phase is very unstable with respect to oxidation and if one cools slowly from the melt, the products are ~-PueO8 and PuO1.98. This is the result of oxidation during cooling which results in even scavenged inert atmospheres. The ~'-Pu203 phase can only be retained by using very rapid quenching rates, estimated to be on the order of 20 000 ° C/minute. A melting point of 2360 ± 20°C has been found for ~'-Pu208 and X-ray diffraction on samples quenched from about 2000 ° C has shown the compound to be congruently melting. 2.4.
BETA PLUTONIUM SESQUIOXIDE
This is not an allotrope of ~-Pu20a but rather defines a phase of the Pu2Oa composition discovered after ~-Pu~Oa. The structure is hexagonal, similar to the A modification of the rare earth sesquioxides such as LaeOa, and was indexed independently by Ellinger 9) and by Templeton and Dauben 10). There is one formula unit per unit cell which gives a calculated density of 11.47 g/cm a based on a0=3.841 ± 0.006 A and c0=5.958 ± 0.005 • (Ellinger). The space group is P 3 m l and the interatomie distances are : Pu - 3Pu
3.57/~
Pu - 6Pu
3.84/k
P u - 3Pu
3.86 X
Pu
2.36/k
-
4Oii
P u - 3Oi
2.62 A
Oi
2.62 A 2.36/k
O ~ -
-
6Pu 4Pu
The Pu-O distance gives an ionic radius of 0.96/k for pu+a which deviates from Zacha-
134
T.D.
CHIKALLA,
C.
E.
MeNEILLY
riasen's radius of 1.01 A 11). This indicates the presence of Pu +4 and points toward a composition containing an excess of oxygen over the composition Pu203. 2.5.
PLUTONIUM MONOXIDE
PuO was first found by Zachariasen 12) as an impurity phase. To date few pure samples of this material have been formed and it is generally seen as a film on plutonium metal. The structure is in space group Fm3m and is face-centered cubic of the NaC1 type with four formula units per cell. Lattice parameters of 4.96 =i= 0.01 A 9), 4.960 ± 0.003/~27), and 4.948 ± 0.002 A 12), have been reported, and give a calculated density of 13.9 g/cm 3. Each plutonium atom is bonded to six oxygen atoms at a distance of 2.48/~. 3.
Thermodynamics
Compared to the uranium oxides, very few tiiermodynamic data are available in the plutonium-oxygen system. The bulk of the work has been performed on Pu02. Estim~tes have been made for other oxides of plutonium. 3.1.
PLUTONIUM DIOXIDE
In an early paper, Brewer et al 13) estimated the free energy functions for several plutonium compounds based largely on reported heat data in the corresponding uranium systems. On the basis of the heat of formation, AH29s=-251 kcal/mole, the following free energy functions were calculated for Pu02: TABLE 1 F r e e e n e r g y f u n c t i o n s c a l c u l a t e d for PuO2 Temperature (°K)
(A F - - zIH298)/T
298 500 1000 1500
43 42 41 41
(e.u.)
Brewer 14) found the original estimate of AH2o8 to be inconsistent with the heats of formation of ThO2 and UO2. Based on these analogies, a revised estiinate of Pu02 of AH298 = - 246 5- 5
AND
R.
E.
SKAVDAHL
kcal/mole was made. Holley et al 1) determined the heat of combustion of plutonium and calculated for Pu02, dH298=-252.87 5- 0.38 kcal/mole which agrees well with the AH29s= -252.4 5- 1.1 kcal/mole found by Russian workers in their heat of combustion experiments is). Osborne and Westrum 16) estimated values for the absolute entropy of actinide dioxides based on their valuv of S°29s for ThO2 calculated from heat capacity measurements. The S°29s thus obtained for Pu02 was 19.7 cal/deg/mole which is well above the value of S°29s= 16.3 cal/deg/mole obtained by Sandenaw 17). Taking S°29s= 12.30 cal/deg/mole for plutonium metal and S°29s=49.01 cal/dcg/mole for 02(g), the entropy of formation AS°298 for PuO2 is found to be - 4 1 . 6 and - 4 5 . 0 cal/dcg/mole using the data of Osborne and Westrum and Sandenaw, respectively. The former value appears to be more consistent with entropy data on other actinide dioxides and also with Coughlin's 18) estimate of AS°29s=--41.9 cal/deg/mole. Using Coughlin's AS°29s and ACp of 1.5 cal/deg, Holley et al. 1) calculated the heat and free TABLE 2 H e a t a n d free e n e r g y o f f o r m a t i o n o f PuO2 Temperature (°K) 298.16 383 (a)
383 (fl) 4O0 5O0 503 593 6OO 7OO 800 9O0 913 913 1000 ll00 1200 1300 1400 1500
(~) (6)
(e) (liq)
--AH (eal/mole) 252 252 253 253 253 253 254 254 253 253 253 253 254 254 253 253 253 253 253
900 ( ± 5 0 0 ) 800 700 700 500 400 100 100 900 800 600 600 100 000 800 700 500 400 200
_AF ° (cal/mole) 240 236 236 236 231 228 228 227 223 219 215 214 214 211 207 203 199 195 191
400 800 800 100 900 000 000 700 600 500 400 900 900 400 300 300 200 200 2 0
( ± 800)
THE P L U T O N I U M - O X Y G E N SYSTEM e n e r g y of f o r m a t i o n of PuOs u p to 1500 ° K , b a s e d on their AH2gs of - 2 5 2 . 9 kcal/mole. These d a t a are t a b u l a t e d in t a b l e 2 a n d included the h e a t s of t r a n s i t i o n a n d fusion for t h e allotropic t r a n s f o r m a t i o n shown. T h e free e n e r g y can be r e p r e s e n t e d b y the following e q u a t i o n o v e r the i n t e r v a l 298-1500 ° K : A F ° = - 253480 - 3.45 T log T + 52.48 T.
(1)
S a n d e n a w aT) m e a s u r e d the h e a t c a p a c i t y of P u 0 2 at low t e m p e r a t u r e s . T h e h e a t c a p a c i t y b e t w e e n 11 a n d 100 ° K was f o u n d to be a n a m a l ous a n d irreproducible, changing slightly a f t e r e v e r y one of seven liquid helium coolings. A f t e r seven coolings, the h e a t c a p a c i t y versus t e m p e r a t u r e curve was quite u n i f o r m o v e r the r a n g e 4 - 300 ° K . A least squares fit gives the following e q u a t i o n for the low t e m p e r a t u r e h e a t c a p a c i t y per g r a m f o r m u l a weight (gfw):
C r = A T + B T a (11.9 ° K to 28.8 ° K )
(2)
where
3.2.
ALPHA P L U T O N I U M
A F~, = - 414 + 0.066T keal/mole.
S a n d e n a w iv) calculated values of e n t h a l p y (H) a n d e n t r o p y (S) for the u n i f o r m c u r v e of h e a t c a p a c i t y versus t e m p e r a t u r e , a p p l y i n g t h e T h i r d Law. These calculated values, along w i t h values of h e a t c a p a c i t y r e a d f r o m a s m o o t h e d curve, are s u m m a r i z e d for a few selected t e m p e r a t u r e s in t a b l e 3.
25 50 75 100 150 200 250 298 300 320
0.80 2.62 4.56 6.38 9.28 11.9 14.3 16.4 16.4 17.2
(3)
240
220
200
TABLE 3 Values of C~, S, and AH at different temperatures ST (eal/gfw. °K)
SESQUIOXIDE
To date, there h a v e been no t h e r m o d y n a m i c d a t a published in the open literature. As a result of the s t u d y of e x p e r i m e n t a l d a t a o b t a i n e d in this l a b o r a t o r y a n d of the p r e v i o u s l y m e n tioned results a n d e s t i m a t e s for the t h e r m o d y n a m i c functions of o t h e r p l u t o n i u m - o x y g e n c o m p o u n d s , a r o u g h e s t i m a t e of the AH298 a n d ASegs for a - P u 2 O s has been obtained. I f the values of AHsgs a n d AS29s per g r a m f o r m u l a weight of c o n t a i n e d p l u t o n i u m as r e p o r t e d above, are p l o t t e d as a f u n c t i o n of the O / P u ratio, t w o s t r a i g h t lines result (see fig. 2). The value of AH29s for P u O e m p l o y e d was the a r i t h m e t i c m e a n of - 1 3 0 a n d - 1 4 0 , or - 1 3 5 kcal/mole. F r o m these t w o lines, values of AH298 a n d AS298 for an O / P u r a t i o of 1.62 are seen to equal - 2 0 7 k c a l / m o l e a n d - 3 3 cal/°K, respectively. A first a p p r o x i m a t i o n of the free e n e r g y of f o r m a t i o n for ~ - P u 2 0 s would t h e n be :
A = 1.350 × 10 -z B = 2.877 × 10 -5 ( s t a n d a r d deviation) = 3.0 × 10 -2.
TemperVia ature (eal/gfw. °K) (°K)
135
ST--So (cal/gfw)
180
50
160
40
140
30
120
2o
i
0.434-0.03 1.504-0.04 2.944-0.05 4.504-0.05 7.644-0.07 10.684-0.10 13.614-0.13 16.334-0.16 16.434-0.16 17.524-0.20
7.0 48 139 276 668 1200 1858 2600 2632 2969
4- 0.5 4- 1 4- 2 4- 3 4-7 4- 10 4- 13 4- 26 4-26 4-30
1.0
1.2
1.4
1.6
Oxygen-to-plutonium,
Fig. 2.
1.8
I0 2.0
atomic ratio
Variation of ASsgs and ASsgs with oxygen-to-plutonium ratio.
~'
i
136
T.D.
CHIKALLA,
C.
E.
MCNEILL¥
AND
R.
E.
SKAVDAHL
As a second approximation, the hydrogen reduction of Pu02 is considered. We have observed th at reduction of Pu02 below about PuOLgs takes place initially at a temperature of l l 0 0 to 1200°C (approximately 1400°K) when using continuously flowing hydrogen with approximately 100 ppm water vapor. Assuming the reduction reaction to be
Since the above results are based on comparison with the A modification of the rare earth s:,squioxides, we have taken them to be representative of/~-Pu203, rather than ~-Pu203. In addition, the us~ of the above values of AH2gs and ASegs to calculate the free energy of Pu203 by the relation
2PuO2 + 0.76He --> Pu20~.2a + 0.76H20
and the use of the free energies as calculated by cq. (6) in predicting reduction of PuOe by various materials, yields large, positive values of the LIF of reaction for reactions which are known to occur experimentally and which yield ~-Pu20a as the reduced product.
(4)
it is possible to calculate the value of the LJF of formation of .~-Pu20a (Pu203.24) at 1400 ° K required to obtain a partial pressure o~" H20 equivalent to 100 ppm in hydrogen gas at one atmosphere. (The significance of Pu203.2a2 PuOL62=a-Pu20a will be discussed in more detail later in the report). The value calculated was about - 3 4 0 kcal/mole for ~-Pu20a (at 1400 ° K). In order to obtain an estimate of the free energy of formation of a-Pu203 as a function of temperature, the value o f / I F = - 340 kcal/ mole at 1400 ° K was assumed to be the " t r u e " value. Then, since the line representing the values of ASegs as a function of O/Pu atomic ratio in fig. 2 has the less steep slope of the two lines plotted, the value of LJS29s-- --33 c.u. per gram formula weight of plutonium at O / P u = 1.62 was also assumed to be a " t r u e " value. Subsequent use of eq. (6) yields the value of A H 2 ~ s = - 4 3 2 kcal/mole for ~-PueOa and therefore a rough estimate of the free energy of formation of ~-Pu20a as a function of temperature (°Kelvin) would be zJ F T ° ~
3.3.
--
432 + 0.066 T kcal/m~.)le.
(5)
BETA PLUTONIUM SESQUIOXIDE
Brewer la) has estimated d H 2 9 s 387 kcal/ mole for Pu20a based on heat data for PuC13 and the analogous sesquioxide and chloride in the lanthanum system. A constant entropy of formation of - 6 3 e.u. was assumed in accord with the entropies of formation of other oxides. Roberts 19) has estimated that/1H29g= -393 ± l0 kcal/mole for Pu2Os, which is in good agreement with Brewer. This estimate was based on the heats of formation of Pu +3 and pr+3 and Pr203.
LJ F T
3.4.
- - LJH298 - TzJS298
(6)
P L U T O N I U M MONOXIDE
Based on early data on phase equilibria in the P u - O system, Brewer 13) has estimated that the heat of formation of PuO lies between - 1 3 0 and 140 kcal/mole. He also estimated A S 2 9 s to be - 2 1 cal/°K. Qualitative confirmation of the above values of zJH29s and AS29s has been obtained here at Hanford 2o) by the reaction of Pu02 and carbon to form PuO at 1800° C in helium and which is predicted, by applying the values of A H 2 9 s and AS29s in eq. (6), to have an equilibrium partial pressure of CO at 1800°C of about 250 mm Hg.
4. 4.1.
The Phase Diagram O/Pu
2 . 0 0 - 1.62
The proposed phase diagram is shown in figs. 3a and b. Pu02 loses a slight amount of oxygen after heating to only 1100 1200° C in inert or reducing atmospheres. Oxidation gives a slight weight gain which corresponds to a composition of PuO1.gs. X-ray diffraction gives lattice constants of about 5.398 to 5.400 A, and thermal emf measurements on PUO1.99 give indications of n-type conductivity. Both of these measurements are indicative of an oxygen deficient Pu02 lattice. The boundary of this defect phase has been placed at PuO1.gs. Heating of PuO2 to higher temperatures in vacuum, inert or reducing atmosphere results
THE
PLUTONIUM-OXYGEN
in further oxygen deficiency, and upon cooling to room temperature, a two-phase product is formed. The coexisting phases are ~-Pu2Os and PuO1.gs. The O/Pu atomic ratio then is dependent upon the relative proportions of the two phases present. Heating such a mixture in
2400
'
I
I
'
I
'
air to temperatures near 800°C oxidizes the material to Pu02.00 and therefore produces a weight increase which m ay then be used to calculate the O/Pu ratio of the original sample. The relative intensities of the X-ray diffraction spcctra may also be used to obtain an estimate
I
I
'
137
SYSTEM
I
'
I
2~9 2085 /
2000
--1900
Liq +
~
PuO2_x_
80
L~n
/
1600
f
/
/
(9
L + PuO Pu203
/
/ 1200 - -800
/
/
+
/
Pu02 -x
C
Liq
~ fi ,
PuO +
Pu 0
__/ •
+
~+~
P u + PuO
--
/
~/p~l02_x,
/ f
8 l
i~
I
I
i
l
0.4
i
0.6
I 0.8
1.0
1.2
a - Pu203 + , W2-~,
1.6
1.4
2(_~ \- -
~\~' ~O2.x. !
g 0.2
PuO
a - Pu203
Pu203
400
Pu02 - x ,
B-Pu2oa
i/
_
1.8
2.0
Oxygen-to-Plutonium
Fig.
3a. Proposed phase equilibria diagram for the system plutonium-oxygen. 800
I
a+~ - --/Pu203 \
I
a - PU203
600
I \\
(9
/
/
i 400
\
\!
\/ .\,
o
o
.
0
•
O
200
0 Thermal Expansion (Hanford) - Pu203 a - Pu203
1.6
o Electrical Resistivity (Hanford) • Thermal Expansion (Harwell)
+ PuO2-x
,
I
,
1.7
1.8
1.9
2.0
Oxygen-to-Plutonium
Fig.
3b.
Low temperature-high oxygen content portion of diagram showing experimental points.
138
T. D. CHIKALLA, C. E. MCNEILLY AND R. E. SKAVDAHL
measurements on PuO2.00 show good continuity as a function of temperature indicating no allotropes for this compound. Reduced oxides, however, show discontinuities at various temperatures on both heating and cooling 21-23). X-ray diffraction on these types of specimen show ~-Pu203 and PuO1.9s to be present both before and after the experiment. Specimens of composition PUO1.72, PuO1.Te, PuOl.s2, PuOl.s4, PuO1.90, PuO1.92, Pu01.96, and PUO1.97 all gave a discontinuity between 280-340 ° C, in both dialatometric and resistivity experiments. An additional higher temperature discontinuity was noted, the exact temperature of which was composition dependent. Samples between PuOl.s and Pu01.9 showed this discontinuity at 650 ° C while at PuO1.72 the temperature had dropped
1.8---
1.6--
1.4-O 1.2
1.0--
eq
0.8--
0.6--
0.4--
0.2
\ 0.0
1.5
[
I
1.6 1.7 1.8 Oxygen-to-plutonium
[
1.9
\~
2.0
Fig. 4. Dependence of X-ray diffraction intensity ratio of Pu203(222)-to-PuO2(lll) lines on oxygen-toplutonium ratio.
of the O / P u ratio after an i n t e n s i t y / c o m p o s i t i o n calibration has been made. Such a calibration is shown in fig. 4 where the ratio of the line intensities of the ~ - P u 2 0 3 (222) a n d P u 0 2 (111) planes is p l o t t e d as a function of a t o m i c ratio. Compositions of Pu01.9: a n d below h a v e shown p - t y p e c o n d u c t i v i t y , a p p a r e n t l y due to ~ - P u 2 0 3 which contains excess o x y g e n a n d which t h u s is e x p e c t e d to b e h a v e as a p - t y p e semi-conductor. T h e r m a l e x p a n s i o n a n d electrical r e s i s t i v i t y TABLE
to 490°C (table 4). H i g h t e m p e r a t u r e X - r a y diffraction on s a m ples b e t w e e n PuO1.7 a n d PuO1.9 showed a twop h a s e region j u s t a b o v e 300°C consisting of two different cubic phases, b o t h of PuO2-x composition, a l t h o u g h one of these could be considered as a Pu203+z composition. Only the low angle s p e c t r a were recorded a n d on the basis of previous s t a t e m e n t s it is a s s u m e d t h a t a - P u 2 0 3 containing excess o x y g e n coexists with PuO1.gs below the t w o - p h a s e loop whose t e m p e r a t u r e boundaries are defined b y the o b s e r v e d discontinuities. Compositions below PuO1.79 are e x t r e m e l y unstable a n d could not be p r e p a r e d b y the n o r m a l h y d r o g e n reduction techniques. Therefore, the lower limit of the d i a l a t o m e t r i c a n d resistivity analyses was a b o u t Pu01.7. At r o o m t e m p e r a t u r e , c¢-Pu203 has shown lattice 4
Summary of electrical resistivity measurements of plutonium oxides O/Pu ratio
I
I
1.72 1.84 1.92 1.96 2.00 t
Extrapolated.
~298
[
~1250
(ohm-era)
]
(ohm-cm)
/~E( ,~ TT) (eV)
5 × 10-2 4 × 10-1 9 × l0 -1 4.5 8 × 102
0.95 1.11 1.24 1.28 1.8
6 × 104 1.0 × 105 8 × 104 t 3 × 105 t 4 × 101° t
TT
490 650 630 575
AE(~TT) (eV) 0.54 0.49 0.52 0.51 1.8
THE
PLUTONIUM-OXYGEN
constants from 10.98 to as high as 11.06 A. This points toward a varying oxygen solubility in the structure and is not surprising in view of the defect nature of the cell. Thus a ~-Pu~O3+z solid solution region is shown. This compound is apparently very unstable with respect to oxidation, and we have not been able to produce it free from other phases at room temperature. A lamellar, pearlitic type mierostructure has been observed in reduced oxides with the quantity of the lamellae increasing as compositions approach PuOLT0. Fig. 5 is a photomicrograph of a two-phase structure of composition PuOl.ss. This type of structure indicates the possibility of a eutectoid reaction in the system. Due to the refractory nature of plutonium oxides, especially those with higher O/Pu ratios, phase identification via normal metallographic techniques is, at best, difficult. In addition, the suboxides are quite susceptible to
Fig. 5. P h o t o m i c r o g r a p h of a n e t c h e d PuOl.s5 s a m p l e s h o w i n g t h e p r e s e n c e of t w o p h a s e s a n d a e u t e c t o i d like s t r u c t u r e . 300 x
SYSTEM
139
oxidation during grinding and polishing so that without great precautions, one may readily change the composition of a sample surface. For these reasons metallography was not used to a great extent in phase identification. A eutectoid reaction based on metallographic evidence and extrapolation of the discontinuities as shown in fig. 3b is given at 300°C and PuO1.70. At the invariant point, the reaction is of the form: PuO2-x ~ Pu01.98 ÷ ~-Pu203+y
(7)
where x is about 0.3 and y is somewhat greater than 0.24. As mentioned previously, high temperature diffraction showed two cubic phases coexisting above 300 ° C for O/Pu ratios greater than that of the eutectoid. The fact that p-type semiconductivity is observed in the two phase loop, indicates that one of these may have a Pu2Os+z composition. This oxygen excess causes a partial oxidation to Pu +4, i.e., an eight fold coordination of plutonium with respect to oxygen. Because of the close similarity between the C type rare earth and fluorite structures, this phase may represent a metastable transition between the ~-Pu203 and the PuO2 lattices. Thus the transformation corresponds more to an atomic rearrangement or position transfer than to the customary nucleation and growth type reaction. Adopting this hypothesis, the transition phase is labeled PuO2-x,. Along the two phase loop boundary it transforms along with PuO~-x to the defect fluorite phase PuO2-x. This solid immiscibility gap is similar to that occuring in the metallic system A1-Zn. Above 300°C the PuO2-x, is also shown coexisting with alpha Pu203+z for compositions below PuOi.70. This has not been verified experimentally due to difficulty in obtaining samples at these low O/Pu ratios, and is included to complete the phase diagram. The observation by high temperature diffraction of a single suboxide phase at 1000 ° C has been reported 23). In that report, however, it was not possible to determine whether the structure type was fluorite or that of the C type
140
T.D.
CHIKALLA,
C.
E.
MCNEILLY
rare earth sesquioxides. Quench tests were employed to clarify this point. Suboxide samples were quenched from 900 ° C into liquid nitrogen and from 1200 ° C by shutting off the power in a low heat capacity furnace. Both experiments frequently resulted in some quantity of ~-Pu2Oa indicating a quite rapid transformation from the sing!e-phase region. When the single-phase was retained by quenching, a very clear powder pattern with a good high angle doublet resolution was obtained. A composition of PuO1.77 when quenched from 900 ° C, gave a single fluorite type phase with a0=5.404 ± 0.001 A. This is in excellent agreement with the lattice constant predicted on the basis of a linear lattice spacing composition plot between PuOe (a0 = 5.396/k) and ec'-Pu208 (a0=5.409 A). High temperature thermal emf measurements have shown that suboxide compositions change from p- to n-type conductivity above about 650 ° C. This points toward a metal excess conductor and indicates that the high temperature single-phase is of the oxygen deficient fluorite type. The melting point of PuOe.00 has been reported 24) as 2400°C and the equilibrium oxygen pressure over PuO~.00 is between 0.1 and 1,0 atmospheres. In some 40 determinations in helium we have noted an apparent melting point of 2280 ± 30 ° C for Pu02. Accompanying melting is an evolution of oxygen since the melted composition consistently analyzes Pu01.62. This is thought to be the composition of the ~-Pu20a phase which Zachariasen predicted would exist with an oxygen excess and for which Holley et al 1) estimated PuOLas. This composition corresponds more closely to a formula of Pus08 than Pu2Oa. To be consistent with previous terminology this phase has been labelled a'-Pu2Oa. Since PuOz dissociates to this composition, and since this compound has a slightly but noticeably, higher melting point than the apparent Pu02 melting point, we have shown the 2280°C dissociation temperature as a melting minimum at a composition very close
A:ND R .
E.
SKAVDAHL
to PuOL62. A liquidus and solidus is shown joining the minimum and Pu02.00. a'-Pu20a undergoes an allotropic transformation to the C type rare earth (body centered cubic) a-Pu2Oa. It is difficult to determine the exact temperature primarily because compositions as low as PuOL62 oxidize so readily; however, it has been shown that it is below 900 ° C. 4.2. O / P u = 1 . 6 2 - 1.50 Beta Pu203 has been formed by heating Pu02 and carbon in the appropriate proportions at 1800° C in helium. The product was singlephase as determined by X-ray diffraction with lattice constants of a0=3.835 ± 0.004 -~, c0=5.944 ± 0.005 A. Heating of Pu02 with 20 per cent excess Pull3 in vacuum at 1600 ° C did not produce single-phase fl-Pu203 presumably due to vaporization of the plutonium metal after decomposition of the hydride. This type of reaction resulted in a mixture of e¢- and fl-Pu203. Melting of PuO~ in a hydrogen atmosphere has resulted in an unidentified product 25) and the reported interplaner spacings can be indexed very well on single-phase fl-Pu203. Workers at Los Alamos Scientific Laboratory have apparently been able to prepare fl-Pu~Oa by hydrogen reduction of PuO2 at 1900°C utilizing a small amount of high surface area powder 26). Six determinations gave a melting point of 2085 ± 25°C for fl-Puz08 in helium. This material as quenched from the melt and as slowly cooled through the solidification point gave only a single phase on X-ray powder patterns with a0=3.845 ± 0.006 A and Co= 5.948 ± 0.004/k. The melting of fl-Pu~Oa was therefore taken to be congruent. This is in contrast to the 2240°C melting point and incongruent melting reported previously 1). A eutectic has been drawn midway between fl-Pu20a and ~'-Pu20a at an arbitrarily selected temperature of 2030 ° C. Not enough samples of fl-Pu203 were run to determine the extent of high temperature oxygen solubility in this compound. Holley et al. 1) have reported some solubility of oxygen in fl-PuzO3 above 2000 ° C.
THE P L U T O N I U M - O X Y G E N SYSTEM
This is shown in the proposed diagram as a solubility loop near the eutectic temperature. When a pressed mixture of a 1 : 1 molar ratio of plutonium powder and fl-Pu203 were heated to 600 ° C, only plutonium metal and fi-Pue03 were observed in the sample.
a)
4.3.
9)
O / P u ( : 1.50
PuO has been prepared by reacting PuO2 and carbon in stoichiometric proportions at 1800°C in helium and at 1550° C in vacuum. The monoxide prepared in this manner m a y actually be a carbon-stabilized form or an oxycarbide. Non-carbon-bearing PuO was formed by Akimoto 27) employing the reaction of molten Pu metal and oxygen at 800 ° C. PuO is believed to melt congruently at about 1900 ° C. The region between O/Pu = 1.00 and 1.50 has been shown as a two-phase field of PuO ~nd fi-Pu203. An eutectic has been placed arbitrarily at O/Pu = 1.25 ~nd 1700 ° C. All samples of O/Pu ratio between 1.00 and 1.50, prepared by carbon reduction of PuO2, have shown the two-phase mixture by X- r ay powder pat t er n ~nalysis. The region below O / P u = 1.00 is shown as a two-phase mixture of PuO and Pu metal, although there have been no reliable samples to date to confirm this.
Acknowledgements The authors are grateful for the assistance of L. D. Bruggeman, R. L. Ring, C. T. Groswith, and J. P. Keiser in m any phases of the experimental work. Special thanks are due Dr. D. R. de Hulas, Dr. I. D. Thomas, and O. J. Wick for the benefit of m any helpful discussions.
References
~) 7) s)
10)
11) 12) 13)
14) la) 26) 17) 18) 19)
2o) 21) e~) 23)
24)
2)
C. E. Holley et al, Thermodynamics and phase relationships for plutonium oxides, Proceedings of the Second Geneva Conference (1958) 2) W . H . Zachariasen, Metallurgical Project Report No. CK 1096 (Technical Information Division USAEC 1943) ~) ~V. H. Zachariasen, Acta Cryst. 2 (1949) 388 4) L. Pauling, The Nature of the Chemical Bond, Third edition (Cornell University Press, Ithaca, N.Y., 1960)
25)
26) 27)
141
J. L. D r u m m o n d and G. A. Welch, J. Chem. Soc. (1957) 4781 L. E. J. Roberts et al, The Actinide Oxides, A/Conf. 15/8/26 Vol. 28 (1958) W . H . Zachariasen, AECD-1787, undated (Technical Information Division USAEC) R. W. G. Wyckoff, Crystal Structures, Vol. 2 (Interscience Publishers, New York 1948) A. S. Coffinberry and F. I-I. Ellinger, The intermetallic compounds of plutonium, Proceedings of the International Conference on the Peaceful Uses of Atomic Energy 9 (1956) 138 D . H . Templeton and C. H. Dauben, UCRL-1886 (Office of Technical Services, Washington 25, D.C., 1952) W . H . Zachariasen, MDDC-67, undated (National Nuclear Energy Series, McGraw-Hill Publ.) W . H . Zachariasen, CK-1367 (Office of Technical Services, Washington 25, D.C., 1944) L. Brewer et al, The thermodynamic properties and equilibria at high temperatures of the compounds of plutonium; The T r a n s u r a n i u m Elements, NNES, IV-14B (1949) L. Brewer, Chem. Rev. 52 (1953) 1 M.M. Popov and I. M. Ivanov, Soviet J. Atomic Energy (English translation) 2 (1957) 439 D. Osborne and E. F. Westram, J. Chem. Phys. 21 (1953) 1884 T. A. Sandenaw, J. Nucl. Mat. 10 (1963) 165 J. P. Coughlin, Contributions to Data on Theoretical Metallurgy, USBM Bulletin 542 (1954) L. E. J. Roberts, Plutonium Dioxide in Fuel Elements, A E R E - C - M - 3 2 5 (Office of Technical Services, Washington 25, D.C., 1957) R. E. Skavdahl, The Reactions Between PuO2 and Carbon, HW-SA-3117 C. E. McNeilly, J. Nucl. Mat. 11 (1964) 53 T. D. Chikalla and J. M. Taylor, Thermal Expansion of Plutonium Oxides, HW-74788 (1962) N. H. Brett and L. E. Russell, The thermal expansion of Pu02 and some other actinide oxides between room temperature and 10000 C, International Conference on Plutonium Metallurgy, Grenoble, France, 1960 (Cleaver-Hume Press, London 1961) p. 397, 426 L . E . Russell, Discussion of the Grenoble Papers, ibid, p. 492 S. ~V. Pijanowski and L. S. DeLuca, Melting Points in the System PuO~--UO2, KAPL-1957 (Office of Technical Services, Washington 25, D. C., 1960) Vf. M. Olson, Los Alamos Scientific Laboratories, Private communication (1963) Y. Akimoto, Preparation of AmO and PuO, Chemistry Division Semiannual Report November 1959, Report No. UCRL-9093, p. 74 (Office of Technical Services, Washington 25, D.C.)