20 Stability of Tetravalent Actinides in Perovskites CLAYTON W. WILLIAMS, LESTER R. MORSS, and IN-KYU CHOI
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Argonne National Laboratory, Chemistry Division, Argonne, IL 60439
This paper reports the first determination of the enthalpy of formation of a complex actinide(IV) oxide: ∆H°f (BaU0 ,s, 298 K) = -1690 ± 10 kJ mol . The preparation and properties of this and other actinide (IV) complex oxides are described and are compared with other perovskites BaM0. The relative stabilities of tetravalent and hexavalent uranium in various environments are compared in terms of the oxidation-reduction behavior of uranium in geological nuclear waste storage media; in perovskite, uranium(IV) is very unstable in comparison with uranium(VI). 3
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I n s i m p l e o x i d e s , t h e a c t i n i d e s a r e m o s t s t a b l e i n t h e +4 o x i d a t i o n s t a t e ; t h e d i o x i d e s , A n 0 , a r e known f o r a l l e l e m e n t s thorium through c a l i f o r n i u m . Although the p r o p e r t i e s o f T h 0 , U 0 , and Pu0 are e s p e c i a l l y important i n nuclear technology, c o m p l e x a c t i n i d e o x i d e s ( o x i d e s w i t h o n e o r more m e t a l i o n s i n a d d i t i o n t o a n a c t i n i d e ) a r e a l s o i m p o r t a n t s i n c e t h e y may b e found as f i s s i o n p r o d u c t s i n n u c l e a r f u e l s and they a r e models f o r p o s s i b l e m a t r i c e s i n which n u c l e a r wastes w i l l be s t o r e d . I t i s s u r p r i s i n g t h a t s o much e f f o r t h a s b e e n d e v o t e d t o c o m p l e x a c t i n i d e o x i d e s t h a t c o n t a i n a c t i n i d e s i n t h e +6 r a t h e r t h a n t h e +4 o x i d a t i o n s t a t e ( 1 ) . Because t h e p r e p a r a t i v e c o n d i t i o n s o f complex a c t i n i d e oxides i n d i c a t e t h a t they favor higher a c t i n i d e o x i d a t i o n s t a t e s , we h a v e s e l e c t e d a m o d e l s y s t e m i n w h i c h b o t h +4 a n d +6 actinides are e a s i l y prepared i n s i m i l a r oxide coordination. I n t h e d i o x i d e s each a c t i n i d e i s surrounded b y 8 e q u i d i s t a n t oxygens a t t h e c o r n e r s o f a cube, b u t i n complex o x i d e s t h e a c t i n i d e i o n i s u s u a l l y surrounded b y 6 oxygens a t t h e a p i c e s 2
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0097-6156/84/0246-0323S06.00/0 © 1984 American Chemical Society
Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE
of a regular or distorted octahedron (2) . Our model system i s perovskite, the mineral CaTi03, i n which structure type many complex actinide (IV) , (V), (VI), and even (VII) oxides c r y s t a l l i z e (1,2) (Figure 1). We have previously determined the enthalpies of formation of several perovskite +4 and +6 oxides (3,4). Our objective i n this study was to determine the enthalpy of formation of BaU0 and to evaluate the r e l a t i v e s t a b i l i t y of uranium(IV) and (VI) i n comparable complex oxides, especially i n comparison with binary oxides, halides, and aqueous ions. From a p r a c t i c a l point of view, these compounds are models of c r y s t a l l i n e matrices for nuclear waste disposal. One such storage material i s SYNROC, a synthetic mineral whose major constituents are the complex oxides h o l l a n d i t e , z i r c o n o l i t e , and perovskite. We have chosen perovskite as a model structural family because of i t s e f f i c i e n t packing and i t s accommodation of a wide range of cations, both i n s i z e and oxidation state (5).
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Previous Work. The best-known set of actinide(IV) perovskites i s BaM0 (M=Th through Cf) (1,6). The corresponding lanthanide(IV) oxides BaCeU" , BaPr0 , and BaTb0 are also known and have been w e l l characterized (7). A few compounds SrM0 (M=Pa, U, Np, Pu) are also known T l ) . Recently. EuU0 and EuNp0 were prepared and characterized as ( E u ) ( U ) ( 0 2 - ) (8,9). By f a r the greatest body of l i t e r a t u r e exists for BaU0 (10,11) . Very thorough studies of the Ba0-U0 system revealed a pseuclô-cubic BaU0 phase with a = 4.387 Â (12), or a = 4.410 Â (13) . In general, i t was found that BaU0 could only be prepared by heating BaO with U0 at 1200-1900°C i n inert gas or hydrogen. P a r t i a l oxidation and loss of Ba have been noted, as has the fact that BaU0 can take up more than 1 mol of BaO i n s o l i d solution (10,13). While our work was i n progress, a new report on BaU0 +x appeared (14) . Hydrogen reduction of BaU0i+ was incomplete even at 1100°C, y i e l d i n g BaU0 . or BaU0 . , a = 4.40 ft. Reaction of Ba U0 with U0 i n hydrogen at 1150-1200°C yielded BaU0 . or Ba .98U0 , a = 4.39 Â. These pseudocubic x-ray l a t t i c e parameters were resolved by powder neutron d i f f r a c t i o n into the orthohonibic space group Pnma with refined vacancy structures of 3
3
3
3
3
3
2+
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i+
3
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U
B^O.9H 0.909°3
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Q
and
U
Bao.976 1.001°3-
The two other most important actinide(IV) complex oxides are BaTh0 and BaPu0 . The former has been prepared many times (15) ; however, there are disputes concerning i t s properties. Fava et a l . (16) prepared BaTh0 from stoichiometric amounts of BaO ancf Tïït) and observed an ideal perovskite, whereas Nakamura (17) found that an excess of BaO i s necessary and i d e n t i f i e d a distorted perovskite structure. I t i s also not obvious why BaO i s not taken up i n s o l i d solution with BaTh0 (15) ; BaU0 i s 3
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Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
Downloaded by IOWA STATE UNIV on March 2, 2017 | http://pubs.acs.org Publication Date: March 8, 1984 | doi: 10.1021/bk-1984-0246.ch020
20.
Stability
WILLIAMS ET A L .
of Tetravalent
Actinides
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Figure 1. Perovskite structures: (a) simple perovskite, shown as idealized cubic BaU0 . (b) ordered perovskite 3
Ba MgU0 2
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(BaMgo.5Uo.5O3).
Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
326
GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE
reported to form a s o l i d solution with up to 2 moles of BaO (10). Some doubt remains concerning the Ba/Th r a t i o i n these samples, since no analyses were reported and loss of Ba i s possible by v o l a t i l i z a t i o n during heating or during methanol extraction. BaPu0 was thoroughly studied by K e l l e r (18) who found i t necessary to use a BaO:Pu0 r a t i o of 3:1 to react a l l of the Pu0 ; excess BaO was then extracted with methanol. 3
2
2
Experimental We prepared BaU0 twice, each time beginning with highpurity BaO (ŒRAC, assay 99.3% by acidimétrie t i t r a t i o n , found free of Ba(0H) or BaC0 by x-ray powder diffraction) and U0 .oo (reduced from hi^h-purity U 0e i n H at 1000°C; composition v e r i f i e d gravimetrically by i g n i t i o n to U 0 at 800°C). The stoichiometric amount of each oxide was weighed i n a dry box (
