39 Polymolybdates as Plutonium (IV) Hosts R. A. PENNEMAN Los Alamos Scientific Laboratory, University of California, P. O. Box 1663, Los Alamos, N M 87545 R. G. HAIRE and M. H . L L O Y D Oak Ridge National Laboratory, P. O. Box X, Oak Ridge, T N 37830
Dissolution of Zr-clad plutonium-bearing fuel can result in a residue which contains Pu, Zr and fission-product mo lybdenum. In addition, solutions of these elements can yield in soluble products on standing, or with further treatment such as extended heating. Initial data suggested that molybdates or poly meric molybdic acids were responsible for formation of the solids. In the case of polymeric molybdic acids, interstices in the matrix of MoO octahedra could accept foreign ions, such as Pu(IV) and Zr(IV). It should be noted here that the ion size disparity be tween Zr(IV) and Pu(IV) will generally require that Zr and Pu oc cupy different coordination sites, with Pu(IV) demanding larger coordination. Recently, the existence of Zr molybdates, Pu molyb dates and Zr-Pu molybdate mixtures (containing up to several per cent Pu) have been identified; it was found that the individual Zr and Pu molybdate products were not isostructral. Detailed studies of the formations of these materials from aqueous media are still in progress (M. H. Lloyd and R. L. Fellows, results to be pub lished). A major concern is the a s s o c i a t i o n o f Pu w i t h the Zr-molybdenum p r e c i p i t a t e s , which r e s u l t s in r e l a t i v e l y high losses o f Pu. In an e f f o r t t o determine the mechanism f o r the Pu a s s o c i a t i o n , several p o s s i b i l i t i e s were e x p l o r e d . Presented here are chemical/ s t r u c t u r a l c o n s i d e r a t i o n s o f the molybdenum systems, the e x p e r i mental evidence c o l l e c t e d t o date on these m a t e r i a l s and the i n i t i a l conclusions t h a t have been reached about them. 6
Chemical/Structural Considerations o f the System Jiirgensen and Penneman (1) discussed the behavior o f the transiawrenciurn elements Ζ = 104, 105, e t c . and c a l l e d a t t e n t i o n to the s t r i k i n g c o n t r a s t in p o l y m e r i z a t i o n behavior, and charac t e r i s t i c aquo species o f the hexavalent d - and f - b l o c k elements. For simple aquo i o n s , it turns out, e m p i r i c a l l y , t h a t the t o t a l
0-8412-0527-2/80/47-117-571$05.00/0 © 1980 American Chemical Society
572
ACTINIDE SEPARATIONS
h y d r a t i o n energy of a given M is - ζ κ , where κ is a c h a r a c t e r i s t i c constant f o r each t r a n s i t i o n group £2). However, oxo com plexes of the (V) and (VI) s t a t e s are more s t a b l e than accounted f o r by such a simple theory. I t is f r e q u e n t l y argued t h a t a s p e c i f i c property of 5f gçoup M(VI) $nd M(V) is the formation of l i n e a r dioxo complexes, M 0 and M0 . T h e i r most s t r i k i n g property is the coexistence of oxo and aquo l i g a n d s , which is exceedingly rare in the r e s t of the elements. There are c h a r a c t e r i s t i c d i f f e r e n c e s found f r e q u e n t l y between the oxo ions of the V- and V I - v a l e n t a c t i n i d e s , uranium through americium, and those of the d elements in the same valence states. A. A c t i n y l V ' s and V I s g i v e monomeric oxo c a t i o n s in a c i d , in c o n t r a s t to the d element p r o c l i v i t y towards p o l y m e r i z a t i o n (isopoly acid formation). P r o t a c t i n i u m is s i m i l a r t o d e l e ments in t h i s regard. B. The a c t i n y l ( V ) and (VI) oxo ions d i s p l a y t r a n s o r i e n t a t i o n of the " y l " oxygens, rare in d-element compounds. Trans oxygen b i n d i n g may r e q u i r e enhanced s p a r t i c i p a t i o n , a r e l a t i v i s t i c e f f e c t found in the c a l c u l a t i o n a l e f f o r t s on superheavy e l e ments. C. The " y l " oxygens are u s u a l l y of higher bond o r d e r , w i t h much s h o r t e r metal-oxygen bond lengths than the e q u a t o r i a l oxygen c o o r d i n a t i o n (1.75 Â vs 2.45 Â ) . D. A c t i n y l ions r e q u i r e higher oxygen c o o r d i n a t i o n (7-8) w h i l e d-block elements in the same valence s t a t e o f t e n have (4 or 6) oxygen c o o r d i n a t i o n . The s t r u c t u r e of a t y p i c a l oxygen c o o r d i n a t i o n sphere around uranyl has s i x oxygens in the e q u a t o r i a l plane and two short oxygen bonds p e r p e n d i c u l a r to t h a t plane. Bidentate oxygen-donors ( w i t h 2 oxygens attached to same element n i t r a t e , carbonate) have a s h o r t e r b i t e than i n d i v i d u a l oxygens, and can be accommodated without puckering the e q u a t o r i a l oxygen r i n g ; two n i t r a t e s and two water molecules are common ( 2 ) . Hydroxyl groups r e p l a c e water as the pH is increased. A s t r u c t u r e c o n t a i n i n g d i m e r i c u n i t s is known, w i t h r e t e n t i o n of the uranyl oxygens ( 4 ) . In c o n t r a s t , hexavalent t r a n s i t i o n metals, such as molybdenum and tungsten, polymerize in a c i d by aggregation to s p e c i f i c a l l y favored geometries c o n t a i n i n g oxide octahedra of Mo0 and W0 (the i s o p o l y a c i d s ) (5a). These oxide s t r u c t u r e s have c a v i t i e s which accept f o r e i g n i o n s , to form heteropoly ions (5b). _The c e n t r a l c a v i t y is a t e t r a h e d r a l s i t e , o f t e n occupied by P0| , SiO| , e t c , a f a c t u t i l i z e d in phosphate p r e c i p i t a t i o n : 2
2
2
2
1
6
6
( N H ) M o 0 2 4 - 4 H 0 + phosphate -> ( N H ^ a P M o ^ C W i e H g O Φ 4
6
7
2
The i s o l a t e d Mo0 octahedron is known only in condensed u n i t s , where the Mo0 polyhedra share c o r n e r s , edges and faces in combi6
6
39.
PENNEMAN
Poly molybdates as Pu(IV) Hosts
ET AL.
573
n a t i o n s , p r o v i d i n g s i t e s f o r hetero atoms w i t h c o o r d i n a t i o n num bers: 4, 6, 8, and 12. Although condensed u n i t s c o n t a i n i n g 6, 12, and 18 Mo or W atoms are common, others ( c o n t a i n i n g 5, 9, 10, 11, or 17 M atoms) are formed by removal of one u n i t and a l s o by dime r i z i n g , eg., ( Ρ Μ ο 0 ) , ( Χ Μ ^ ) , ( X M ^ O ^ ) , ( X M 0 ) . Some known s t r u c t u r e s w i t h guest metal ions in 4, 8 and 12 c o o r d i n a t i o n are l i s t e d in the f o l l o w i n g t a b l e . 2
5
2 3
2
1 7
6 1
Known S t r u c t u r e s X-coordination X X
n +
Mo
W 0
4
l o
0 o ~ ~ ( 8
1 2
n )
4
3 6
H
6 2
"
or
Réf.
X
4
(5)
Si,P,As,Zr,Ti
8
(6,7,8)
Ce,Th,U,Np,Pu
12
(9,10,11)
X Wio0 6 ~ 4+
3
Χ
+ η
Μο
1 2
8
0
( 1 2 4 2
"
η )
"
Ce,Th,U,Np
Since t e t r a h e d r a l c o o r d i n a t i o n is not a p p r o p r i a t e f o r p l u t o nium c o o r d i n a t i o n by oxygen, it w i l l not be discussed h e r e I f one removes from an octahedron the c e n t e r metal M and an a p i c a l oxygen, four p l a n a r oxygens remain a v a i l a b l e f o r c o o r d i nation. I f t h i s is done from 2 W u n i t s , there are 8 oxygens and a c a t i o n vacancy t h a t can coordinate a l a r g e c a t i o n . The first example of such a s t r u c t u r e contained C e ( I V ) , and l a t e r U(IV) (6-7). In these s t r u c t u r e s the 8-0 s approximate an a n t i p r i s m f o r C ë U V ) which is d i s t o r t e d in the U(IV) case. The M o u n i t provides a 12-coordinated s i t e , an icosahedron of oxygens which can accommodate l a r g e c a t i o n s ; such a s t r u c t u r e c o n t a i n i n g Ce(IV) was determined by Dexter and S i l v e r t o n (9). The s i z e s of the c o o r d i n a t i o n c a v i t i e s are i l l u s t r a t e d by the f o l l o w i n g data: +
6
6
1
12
(Experimental) M Ce
M-0 d i s t a n c e s I V
Ce
I V
Ce
I V
U
I V
2.38
- 2.40 A
Coord. No.
Material
Structure 6-
8
CeW 0 H l o
3 6
0-antiprism
2
8-
2.51
12
CeMo 0
2.50
12
(NH ) Ce(N0 )
2.29
1 2
4 2
0-icosahedron - 2.32
4
2
3
6
8-
8
UW 0 1 0
3 6
0-antiprism
574
ACTINIDE SEPARATIONS
Since there are no known s t r u c t u r e s of such compounds w i t h plutonium, it is useful to u t i l i z e Z a c h a r i a s e n ' s r u l e s (123 f o r e s t i m a t i n g a c t i n i d e - o x y g e n bond lengths which would be r e q u i r e d in a p a r t i c u l a r c o o r d i n a t i o n geometry. Z a c h a r i a s e n ' s Bond Length - Bond Strength R e l a t i o n s (12). 1. A bond s t r e n g t h s . . = s., is assigned to a bond between the
j
j*
i t h and j t h atoms of a s t r u c t u r e so t h a t 1
1
) *u
=
*i
·
]
-H
S
=
*j
where v. and y . are the valences of the two atoms. 2.
The length of a bond D
AB
between two atoms of species A and Β
is a f u n c t i o n only of the strength of the bond. A u n i v e r s a l f u n c t i o n Dflg(s), v a l i d f o r all s t r u c t u r e s c o n t a i n i n g A - Β bonds, is p o s t u l a t e d : D = D - Bins where D is normalized to u n i t bond strength ( s = l ) . Zachariasen has t a b u l a t e d D and Β values f o r d and f block oxides a n d h a l i d e s (12). His bond length formulas f o r Ce-0 d i s tances in A are: C e ( I I I ) , D = 2.18 - O.338 Ins; and Ce(IV), D = 2.117 - O.326 Ins. We estimate the f o l l o w i n g values: x
x
x
0
$
Cerium-Oxygen Distances, Â Coord. D Ce(III)-0
s*
s*
2.51
3/8
8
1/2
2.65
1/4
12
1/3
D Ce(IV)-0
2.34 2.48
s*: the bond strengths are d i v i d e d e q u a l l y among the Ce-0 bonds in a given c o o r d i n a t i o n , where a c t u a l bond distances are unknown. Note t h a t r e d u c t i o n of Ce(IV) to C e ( I I I ) in the 8-coordinated case would cause an opening o f t h e c a v i t y by i n c r e a s i n g the Ce-0 bond length from 2.34 to 2.51 A and t h a t the estimated Ce(IV)-0 d i s tance agrees w e l l w i t h the experimental measurements given e a r lier. For plutonium, data from s i n g l e c r y s t a l determinations are l a c k i n g . The f o l l o w i n g values are estimated as in the cerium case above using Z a c h a r i a s e n s formulas f o r plutonium: P u ( I I I ) - 0 , D = 2.142 - O.35 Ins; and Pu(IV)-0, D = 2.094 - O.35 Ins. 0
1
s
g
39.
PENNEMAN ET
Polymolybdates as Pu(IV) Hosts
AL.
575
Plutonium-Oxygen D i s t a n c e s , A Pu(III)-0
Pu(IV)-0
D, c a l c d
2.49
2.34
8-coord.
D, c a l c d
2.63
2.48
12-coord.
Here a g a i n , the expansion on r e d u c t i o n of Pu(IV) to P u ( I I I ) would loosen the b i n d i n g and open the oxygen c a v i t y . Since base depolymerizes the polymolybdates, a b a s i c reducing agent should break the plutonium-bearing framework. However, a good reducing agent would be r e q u i r e d as the Pu(IV) is c o n s i d e r a b l y s t a b i l i z e d (by O.9V in P W 0 e f ) ( 8 ) . We w i l l have occasion to r e f e r to such estimated Pu(IV)-0 d i s t a n c e s in the f o l l o w i n g s e c t i o n , in which the more extreme case of s u b s t i t u t i n g Pu(IV) f o r Z r ( I V ) is discussed. 2
1 7
The S t r u c t u r e s of Some Simple Molybdates The " s i m p l e molybdate s a l t s have been the s u b j e c t of study by several authors (13-24). There is s t i l l confusion as to t h e i r degree of common s t r u c t u r a l f e a t u r e s ; f o r example we question the unusual c o o r d i n a t i o n polyhedra and the metal-oxygen d i s t a n c e s in the s t r u c t u r e s of Th(Mo0 ) and H f ( M o 0 ) as deduced by Thoret (13). L i k e l y the oxygen p o s i t i o n s are in e r r o r s i n c e the m e t a l oxygen bond strengths from Zachariasen*s formula give unreasonable valence sums f o r the coordinated metals. In c o n t r a s t , we f i n d the s t r u c t u r e of the hydroxy z i r c o n i u m polymolybdate hydrate (14) q u i t e s a t i s f a c t o r y when analyzed using Z a c h a r i a s e n ' s formulas. For atom numbering in the f o l l o w i n g a n a l y s i s see F i g . 1, which was taken from reference 14. Using Z a c h a r i a s e n s values f o r molybde num-oxygen bonds, and f o r zirconium-oxygen bonds, we f i n d f o r Zr(Mo 0 (0H) (H 0) : 11
4
2
4
2
1
2
7
2
2
2
Summation of bond strengths around Mo and Zr. Neighbor Mo-0 0 0 0 0 0
3 4 5 6 7 8
Distance 1.722 2.310 1.797 1.755 2.113 2.034
hi 1.706 O.2627 1.343 1.538 O.4918 O.6319
ΣΜο = 5.97
Neighbor Zr-0 0 0 0 0 0 0
5 5 6 6 7 7 8
Distance 2.088 2.088 2.173 2.173 2.175 2.175 2.141
O.679 O.679 O.523 O.523 O.520 O.520 O.578 ZZr = 4.02
ACTINIDE SEPARATIONS
Figure L
Structure of
ZrMo 0 (OH) (H 0) 2
7
2
2
2
39.
Polymolybdates as Pu(IV) Hosts
PENNEMAN ET AL.
577
Note t h a t the seven Z r - 0 bond strengths are f a i r l y uniform but t h a t there is q u i t e a range in Mo-0 d i s t a n c e s , w i t h a r e s u l t a n t range in t h e i r bond s t r e n g t h s ; in s p i t e of t h i s , they sum q u i t e w e l l f o r hexavalent molybdenum. One of the u s e f u l extensions o f Z a c h a r i a s e n s bond s t r e n g t h a n a l y s i s is t h a t by summing the oxygen bond s t r e n g t h s , as w e l l as those o f the metals, one can o f t e n t e l l which oxygens in the s t r u c t u r e a r i s e from coordinated water or hydroxyl. The 0-H 0 1
bond strength is d i v i d e d as f o l l o w s :
83
0
17 H —*—O.
Summation of bond strengths around the oxygens: ο
Bond D i s t a n c e , Α
Σ , Metal-Oxygen only
1.722
0 -Mo
2.310
O4-M0
1.797, 1.755, 2.113, 2.034,
1.71
3
2.088 2.173 2.175 2.141
0 0 0 0
5 6 7 8
0
O.26
-Mo,Zr -Mo,Zr -Mo,Zr -Mo,Zr
0
2.02 2.06 1.01 1.84
0
3 4
7
Σ w i t h H-Bonding
= 1.71 + .17 + .17 = 2.05 = O.26 + .83 + .83 = 1.92
= 1.01 + .83 + .17 = 2.01
Thus, 0 c l e a r l y o r i g i n a t e s from a water molecule, and is c o o r d i nated s o l e l y t o Mo; 0 is a hydroxyl oxygen which bridges Z r and Mo. The summation of oxygen bond strengths makes the assignments unequivocal. Consequences o f S u b s t i t u t i n g Pu(IV) f o r Z r in Z r M o 0 ( 0 H ) - ( H Q ) * There is a d i r e c t l i n k provided by 0 between Z r in one one chain and a Mo in the adjacent chain and the Z r - 0 bond v e c t o r is essen t i a l l y a l i g n e d along the t e t r a g o n a l a a x i s . S u b s t i t u t i o n o f Pu(IV) f o r Z r ( I V ) a t the same c o o r d i n a t i o n number and bond strengths would i n v o l v e an increase in the P u ( I V ) - 0 bond d i s tance o f O.14 A over the Z r ( I V ) - 0 d i s t a n c e . This would c l e a r l y cause expansion between chains. S i m i l a r l y , chain lengthening would r e s u l t . Since Pu(IV) g e n e r a l l y r e q u i r e s more than an oxygen c o o r d i n a t i o n o f seven, it would not seem l i k e l y t h a t a s o l i d s o l u t i o n w i l l r e s u l t over any extended range, nor t h a t the then un known Pu(IV) molybdate species would be i s o s t r u c t u r a l w i t h t h i s Zr compound. This p r e d i c t i o n has been borne out by the X-ray work which has shown t h a t two d i f f e r e n t s t r u c t u r e s are i n v o l v e d . 4
7
2
7
2
2
2
5
5
5
Experimental Data on Zirconium Molybdate In n i t r i c a c i d s o l u t i o n c o n t a i n i n g uranium, plutonium, z i r conium, molybdenum and other f i s s i o n p r o d u c t s , p r e c i p i t a t i o n of z i r c o n i u m molybdate occurs p r e f e r e n t i a l l y . S o l u t i o n s c o n t a i n i n g only z i r c o n i u m and molybdenum y i e l d the "same" p r e c i p i t a t e d mater-
578
ACTINIDE SEPARATIONS
i a l as s o l u t i o n s c o n t a i n i n g a d d i t i o n a l l y uranium, plutonium, f i s s i o n products, e t c . The d i f f e r e n c e in t h i s l a t t e r case is t h a t the p r e c i p i t a t e contains about 2 wt.% Pu or g r e a t e r . The p r e c i p i t a t i o n of the zirconium-molybdenum m a t e r i a l is a f u n c t i o n of a c i d s t r e n g t h , temperature and time. The r a t e of p r e c i p i t a t i o n is lower w i t h lower temperatures, low z i r c o n i u m concent r a t i o n s and higher a c i d strengths. There a l s o appears t o be an i n d u c t i o n p e r i o d before the onset of p r e c i p i t a t i o n . Discussed here is the c h a r a c t e r i z a t i o n of the zirconium molybdate s o l i d s , as obtained s e p a r a t e l y from n i t r i c a c i d s o l u t i o n s ; chemical analyses, thermogravimetry and X-ray powder d i f f r a c t i o n were used t o charact e r i z e these s o l i d s . Chemical analyses of the Zr-Mo p r e c i p i t a t e s y i e l d a Mo/Zr mole r a t i o o f two. Thermogravimetry on c a r e f u l l y a i r d r i e d p r e c i p i t a t e s a l s o provide a Mo/Zr mole r a t i o of two, based on the weight l o s s being Mo0 and the f i n a l product being Z r 0 . The data i n d i c a t e 2.5 H 0/Zr in the o r i g i n a l m a t e r i a l . X-ray powder d i f f r a c t i o n p a t t e r n s of the i n i t i a l p r e c i p i t a t e show the m a t e r i a l is i d e n t i c a l t o the compound reported by C l e a r f i e l d and B l e s s i n g (14) t o be Z r M o 0 ( 0 H ) ( H 0 ) . (tetragonal, a = 11.45(1)A and c = 12.49(1)A). Heating t h i s m a t e r i a l leads t o the formation of annydrous Z r ( M o 0 ) , which on f u r t h e r heating decomposes t o give monoclinic Z r 0 . The anhydrous Z r ( M o 0 ) m a t e r i a l was indexed as having hexagonal symmetry, w i t h a = 10.0 Â and c = 11.6 Â, in agreement w i t h l a t t i c e parameters f o r Z r ( M o 0 ) r e ported by Trunov and Kovba (15a) and by F r e u n d l i c h and Thoret (15b). 3
2
2
2
B
2
4
2
2
2
2
4
2
4
2
Experimental Data on Plutonium Molybdate In the absence of z i r c o n i u m , a plutonium-molybdenum compound can be p r e c i p i t a t e d from n i t r i c a c i d s o l u t i o n s . The presence of zirconium in the same s o l u t i o n is d e t r i m e n t a l t o formation of t h i s m a t e r i a l , as zirconium molybdate is formed p r e f e r e n t i a l l y . However, the amount o f Pu molybdate s o l i d s t h a t form is a f u n c t i o n of hydrogen i o n c o n c e n t r a t i o n s ; a t 1M HN0 or l e s s , s o l i d s form but a t higher a c i d c o n c e n t r a t i o n s the q u a n t i t y of p r e c i p i t a t e decreases. At 3M HN0 s o l i d s are j u s t b a r e l y d e t e c t a b l e . By chemical analyses, the plutonium molybdate p r e c i p i t a t e s contains 2 moles of molybdenum per mole of plutonium(IV). Based on thermogravimetry, the plutonium molybdate g r a d u a l l y loses ~2 moles of water t o form an anhydrous Pu(Mo0 ) . Continued heating of t h i s compound r e s u l t s in a l o s s of Mo0 (>750°C) t o y i e l d fee Pu0 . X-ray powder d i f f r a c t i o n of the c a r e f u l l y a i r d r i e d p l u t o nium molybdate p r e c i p i t a t e provided data t h a t were indexed as having orthorhombic symmetry, w i t h a = 3.34, b = 10.97 and c = 6.32 Â. The m a t e r i a l appears t o Be i s o s t r u c t u r a l w i t h orthorhomb i c U(Mo0 ) ( a = 3 . 3 6 , b = 11.08, c = 6.42 Â) reported by 3
3
4
3
2
4
2
Q
Q
Q
2
39.
Polymolybdates as Pu(IV) Hosts
PENNEMAN ET AL.
579
C. Skvortsova and Sidorenko (16). The X-ray p a t t e r n s obtained bef o r e and a f t e r the l o s s of water from the plutonium-molybdenum compound were i d e n t i c a l , suggesting the waters were not important to the s t r u c t u r e . A d d i t i o n a l heating of the plutonium compound produced a new phase, a l s o orthorhombic and i d e n t i c a l t o Pu(Mo0 ) reported by Tabuteau ( 1 7 ) , by Prokoshin ( 1 8 ) , and by Ustinov (19) ( a = 9.42, b = 10.057~c = 13.98 À ) . TiïTs m a t e r i a l is i s o s t r u c t u r a l w i t h the compounds 8-NpMo 0 and crThMo 0 ( 2 0 ) . For p r i o r work, having more a p p l i c a t i o n t o process c o n d i t i o n s , see r e f . 25 and 26. 4
2
Q
2
P r e c i p i t a t e s from HN0
3
8
2
8
s o l u t i o n s c o n t a i n i n g Mo/Zr/Pu
The f o l l o w i n g t a b l e l i s t s some a n a l y t i c a l r e s u l t s on p r e c i p i t a t e s c o n t a i n i n g Mo, Z r and Pu. X-ray r e s u l t s are not a v a i l a b l e on these m a t e r i a l s a t present. In all cases, the amount of z i r c o nium in the i n i t i a l s o l u t i o n was in excess of the amount necessary to form Z r M o 0 ( 0 H ) ( H 0 ) 2
7
2
2
2
I n i t i a l Concn. g/1 Mo Zr Pu
HN0 ,M
mg Pu * mg (Zr+Mo)
3
1.5
5.0
1.47
1
O.086
1.5
1.0
1.47
1
O.122
1
1.5
1.5
2
O.023
1
1.5
1.5
3
O.013
1
1.5
1.5
4
O.009
*Each value average of 5 experiments. There is evidence t h a t the Pu a s s o c i a t e d w i t h the Zr-molybdate p r e c i p i t a t e s is not sorbed on the s o l i d ' s surface but is an i n t e g r a l p a r t o f the m a t e r i a l . This is borne out by the f a c t t h a t the Pu cannot be leached out o f the s o l i d phase without d e s t r o y i n g ( d i s s o l v i n g it) it. A l s o , when the f r e s h l y prepared p r e c i p i t a t e is added t o Pu(IV) s o l u t i o n s , Pu is not c a r r i e d down (sorbed) by the Zr s o l i d phase. Conclusions 1) Plutonium molybdate p r e c i p i t a t e d in the absence o f Z r is a d i f f e r e n t s t r u c t u r e ( i s o s t r u c t u r a l w i t h U ( M o 0 ) (16) than the 4
2
580
ACTINIDE SEPARATIONS
Z r M o 0 ( 0 H ) ( H 0 x whose s t r u c t u r e was determined by C l e a r f i e l d and B l e s s i n g ( 1 4 j . 2) Because of the d i f f e r e n c e between Pu-0 and Z r - 0 bond lengths (Δ= O.14A) a t the same bond s t r e n g t h , it is expected t h a t the Z r M o 0 ( 0 H ) ( H 0 ) l a t t i c e w i l l not accomodate extensive sub s t i t u t i o n of the l a r g e r Pu(IV) f o r Z r ( I V ) . 3) The plutonium-bearing p r e c i p i t a t e obtained from n i t r i c a c i d s o l u t i o n s ( c o n t a i n i n g molybdenum, zirconium and plutonium) gives an X-ray powder d i f f r a c t i o n p a t t e r n not d i s t i n g u i s h a b l e from t h a t of Z r M o 0 ( 0 H ) ( H 0 ) p r e c i p i t a t e d without plutonium. How ever, since the Pu content is low, the Pu could be present e i t h e r in the Pu molybdate s t r u c t u r e or r e p l a c i n g Zr in the Zr molybdate s t r u c t u r e and not be detected in the X-ray p a t t e r n s . 4) Present data on the Zr and Pu molybdates have not p r o vided evidence f o r the e x i s t e n c e of heteropoly molybdate s t r u c tures f o r those p r e c i p i t a t e s obtained from 1-5M HN0 . 5) Z a c h a r i a s e n ' s e m p i r i c a l r u l e s r e l a t i n g bond strengths and bond lengths in 4 f and 5f oxides is demonstrated t o be useful when a p p l i e d to the simple and complex molybdate s t r u c t u r e s (12). 2
7
2
2
2
Q
2
7
2
2
7
2
2
2
2
2
3
Acknowledgments We g r a t e f u l l y acknowledge the h e l p f u l comments of R. L. F e l l o w s , Chemical Technology D i v i s i o n , Oak Ridge National Laboratory. References 1. 2. 3.
4. 5.
6. 7. 8. 9.
Jørgensen, C. K.; Penneman, R.A., "Heavy Element Properties"; North-Holland Publishing Co., Amsterdam, 1976, p.117. Jørgensen, C. Κ., Chimia (Switz.), 1969, 23, 292. a. Taylor, J. C.; Mueller, Μ. Η., Acta Cryst., 1965, 19, 536. b. Dalley, Ν. K.; Mueller, M. H.; Simonsen, S. Η., Inorg. Chem., 1971, 10, 323. c. Eller, P. G.; Penneman, R. A. Inorg. Chem., 1976, 15, 2439. Åberg, M.; Acta Chem. Scand. 1969, 23, 719-810. a. Cotton & Wilkinson, Adv. Inorg. Chem., Interscience Publishers,Ed., 1966. b. Weakley, T. J. R., Struct. Bonding, 1974, 18, 131-176. Iball, J.; Low, J. N.; Weakley, T. J. R., J. Chem. Soc., Dalton Trans, 1974, 2021. Golubev, A. M.; Kazanskii, L. P., Dokl. Akad Nauk SSSR, 1975, 221, 351, 826, Zh. Neorg. Khim, 1975, 20, 867. Saprykin, Dokl. Acad. Nauk 1976, 228, 649; Radiokhim, 1976, 18, 101. Dexter, D. D.; Silverton, J. V., J. Am. Chem. Soc., 1968, 90, 3599.
39.
PENNEMAN
ET
AL.
Polymolybdates as Pu(IV) Hosts
581
10. Kazanskii, L. P.; Torchenkova, Ε. Α.; Spitsyn, V. I., Dokl. Akad. Nauk SSSR, 1973, 201, 141. 11. Torchenkova, Ε. Α.; Golubev, A. M., Dokl. Akad. Nauk SSSR, 1974, 216, 1073. 12. Zachariasen, W. H. "Bond Lengths in Oxygen and Halogen Com pounds of d and f Elements." Journal of the Less-Common Metals, 1978, 62, 1-7. 13. Thoret, J., Rev. Chem. Min., 1974, 11, 237. 14. Clearfield, Α.; Blessing, R. H. J. Inorg. Nucl. Chem., 1972, 34, 2643. 15. a. Trunov, V. K.; Kovba, L. M. Russian J. Inorg. Chem., 1967, 12, 1703. b. Freundlich, W.; Thoret, J. C.R. Acad. Sc. Paris, C, 1967, 265, 96. 16. Skvortsova, Κ. B.; Sidorenko, T. A. Zapiski, Vses. Mineralog. Obshch. 1965, 94, 548. (JCPDS 18-1425). 17. Tabuteau, Α.; Pages, M.; Freundlich, W. Mater. Res. Bull., 1972, 7, No. 7, 691. 18. Prokoshin, A. D.; Ustinov, O. Α.; Dogaev, Yu. D.Zavod. Lab. 1973, 3, 305. 19. Ustinov, O. A.;Novoselov, G. P.; Chebotarev, N. T.; Prokoshin, A. D.; Andrianov, Μ. Α.; Matyushin, E. A. Radiokhimiya, 1976, 18, (No. 1), 115. 20. Freundlich, W.; Pages, M. C.R. Acad. Sc. Paris, C, 1969, 269, 392. 21. Golub, A. M.; Maksin, V.I.; Perepelitsa, A. P. Zhurnal Neorganicheskoi Khimii, 1975, 20, 867-870. 22. Brixner, L. H. J. Solid State Chem. 1973, 6, 550. 23. Srivastava, J. Radioanal. Chem., 1977, 40, 7. 24. Termes, S. C.; Pope, M. T. Transition Met.Chem. 1978, 3, 103. 25. Lloyd, M. H. "Solution Instabilities and Solids Formation in LWR Reprocessing Solutions." Trans. Am. Nuclear Soc. 1967, Vol. 24. 26. Lloyd, M. H. "Chemical Behavior of Plutonium in LWR Fuel Reprocessing Solutions." Conference: Plutonium Fuel Cycle Process, ANS National Topical Melting, Miami, Florida, May 1977. RECEIVED
July 24, 1979.
