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15 Optical and Electron Paramagnetic Resonance Spectroscopy of Actinide Ions in Single Crystals N. EDELSTEIN, W. EASLEY, and R. McLAUGHLIN Lawrence Radiation Laboratory, University of California, Berkeley, Calif.
The formation and stabilization of various oxidation states of actinide positive ions in CaF crystals are described. Para magnetic resonance and optical spectra are reported for divalent Am and trivalent Cm in these crystals. Tetravalent Cm and Pu, formed as a consequence of the intense alpha radiation, are identified by their optical spectra. 2
" Ο are e a r t h ions are s t a b i l i z e d i n the d i v a l e n t state i n crystals of a l k a l i n e ·"·
e a r t h h a l i d e s (14).
T h i s o x i d a t i o n state is u s u a l l y f o r m e d b y r e d u c
t i o n of the t r i v a l e n t rare e a r t h i o n to the d i v a l e n t f o r m b y one of three m e t h o d s — g a m m a i r r a d i a t i o n of the crystals (14), (4, 7 ) , or a l k a l i n e e a r t h m e t a l r e d u c t i o n (10).
s o l i d state electrolysis T h e last t w o t e c h n i q u e s
are m o r e efficient since u n d e r some c o n d i t i o n s a l l of the t r i v a l e n t r a r e e a r t h ions c a n b e r e d u c e d .
R e c e n t l y w e r e p o r t e d the s t a b i l i z a t i o n of
d i v a l e n t A m i n C a F , the first w e l l c h a r a c t e r i z e d d i v a l e n t a c t i n i d e 2
(3).
I n this p a p e r w e w i l l briefly r e v i e w the A m w o r k a n d s u m m a r i z e o u r f u r t h e r attempts to find other d i v a l e n t a c t i n i d e s . W e w i l l also r e p o r t o n the p a r a m a g n e t i c resonance ( P M R ) spectra of C m
3 +
in CaF . 2
Experimental The
actinide-doped
single crystals of
Bridgman-Stockbarger technique. n i d e i n 10-50λ of d i l u t e H N 0 CaF
2
containing 2 w t . %
CaF
2
were
grown by
the
A c o n c e n t r a t e d s o l u t i o n of the a c t i
s o l u t i o n w a s p i p e t e d onto a p o w d e r of
3
P b F , w h i c h had been placed i n a 2
carbon
crucible. T h e crucible a n d sample were then placed i n a furnace, melted u n d e r v a c u u m , a n d t h e n the c r u c i b l e w a s l o w e r e d s l o w l y t h r o u g h the hot z o n e of the f u r n a c e .
P M R measurements w e r e t a k e n at 4.2°K. a n d 203
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
204
L A N T H A N I D E / A C T I N I D E
4000
4000
5000
5000 Wavelength
6000
7000
6000
7000
C H E M I S T R Y
(A)
Figure 1. (a) Optical spectrum of a radiation reduced Am -CaF crystal, (b) Optical spectrum of Am-CaF crystal after annealing, (c) Optical spectrum of an electrolytically reduced Am -CaF crystal 2+
2
2
2+
2
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
15.
EDELSTEiN E T A L .
Single
205
Crystals
1°K. at a f r e q u e n c y o f a p p r o x i m a t e l y 9.0 G c / s e c . w i t h a s u p e r h e t e r o d y n e spectrometer.
O p t i c a l measurements w e r e m a d e at r o o m t e m p e r a t u r e
a n d 77 °K. w i t h a C a r y M o d e l N o . 14 spectrometer a n d a J a r r e l l - A s h F - 6 spectrometer u s i n g p h o t o g r a p h i c plates. Results
and
Discussion
A m in CaF . 2
Crystals g r o w n w i t h A m are initially light pink. O n
s t a n d i n g f o r w e e k s t o m o n t h s t h e y d a r k e n t o a b r o w n color. T h e i n i t i a l a b s o r p t i o n s p e c t r u m shows lines c h a r a c t e r i s t i c of t r i v a l e n t A m ( F i g u r e lb).
A s t h e c r y s t a l d a r k e n s because of r a d i a t i o n d a m a g e , n e w b r o a d
b a n d s g r o w i n as s h o w n i n F i g u r e l a . F i g u r e l c shows t h e s p e c t r u m o b t a i n e d f r o m a n e l e c t r o l y t i c a l l y r e d u c e d c r y s t a l . T h e o r i g i n of these n e w b r o a d b a n d s is p r o b a b l y a t t r i b u t e d t o f t o d transitions o f d i v a l e n t A m . T h e crystals w h i c h h a v e d a r k e n e d s h o w at 4 ° a n d 1°K. a six-line i s o t o p i c PMR 8
SÎ/2
s p e c t r u m w h i c h is assigned to t h e Γ
6
crystal
field
state o f t h e
electronic c o n f i g u r a t i o n o f d i v a l e n t A m i n c u b i c s y m m e t r y .
(f)
Since both
2 4 1
A m and
2 4 3
A m h a v e n u c l e a r spins of 1=5/2, t h e l i n e i s split
i n t o six h y p e r f i n e c o m p o n e n t s .
T h e m e a s u r e d parameters of t h e s p i n
Hamiltonian 'β = for A m
2 +
in C a F
2
gβïί'~^+M'S
r
are g i v e n i n T a b l e I . T h e g v a l u e c a l c u l a t e d f o r t h e Γ
c r y s t a l field state of A m
2 +
6
using wavefunctions given b y L e a , Leask, and
W o l f ( 1 2 ) a n d the L a n d e g value taken from atomic b e a m data o n atomic Am
(13) is 4.517.
T h e agreement b e t w e e n e x p e r i m e n t a n d t h e o r y is
satisfactory. Table I. A m
2 +
in CaF
(S' =
2
1/2, ί = 5 / 2 )
g = 4.490 ± 0.002 A Χ 1 0 (cm." ) 2
2 4 1
Am
1.837 ± 0.002
2 4 3
Am
1.821 ± 0.002
A i ! ! ^ ) A ( ^Am) 4
=
1
1.009 ± 0.001
24
Spin Hamiltonian parameters of A m - C a F . 2 +
2
A f t e r the crystals h a v e a g e d f o r several w e e k s o r longer, o n h e a t i n g to a b o u t 5 0 0 ° C . t h e y e m i t a n intense green t h e r m o l u m i n e s c e n c e , c h a r a c teristic o f t r i v a l e n t A m i n n o n c u b i c sites. F i g u r e 2 shows t h e e m i s s i o n
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
206
LANTHANIDE /ACTINIDE CHEMISTRY
s p e c t r u m of this t h e r m o l u m i n e s c e n c e
photographed
at 500 ° C . a n d the
a b s o r p t i o n s p e c t r u m of t r i v a l e n t A m t a k e n at 77 °K.
T h e r e is a shift i n
the centers of the lines c a u s e d b y different c r y s t a l fields at t h e
two
temperatures, b u t the e m i s s i o n c l e a r l y arises f r o m ions i n sites t h a t are the same as those w h i c h cause the a b s o r p t i o n s p e c t r u m .
T h i s t y p e of
t h e r m o l u m i n e s c e n c e has b e e n o b s e r v e d i n rare e a r t h - d o p e d C a F , a n d 2
a r e a c t i o n m e c h a n i s m has b e e n g i v e n (9, 1 5 ) .
Figure 2. (Top) Emission spectrum of a radiation reduced Am -CaF crystal at ^500°C. (Bottom) Absorption spectrum of Am -CaF at 77°K. 2+
2
3+
Cm
8 +
in CaF . 2
2
C r y s t a l s g r o w n w i t h C m are i n i t i a l l y p a l e y e l l o w or
almost colorless after a n n e a l i n g . B e c a u s e of the d a m a g e f r o m the h i g h r a d i a t i o n l e v e l t h e y are rose c o l o r e d after one h o u r . A f t e r 3 - 4 h o u r s the color has c h a n g e d to b u r g u n d y , a n d i n a b o u t 15 hours the crystals are black.
A t a l l temperatures the c h a r a c t e r i s t i c orange
g l o w of C m
present w h i c h is a t t r i b u t e d to e m i s s i o n f r o m c r y s t a l field levels of first e x c i t e d electronic state d o w n to the g r o u n d electronic state.
3 +
is the
The
c h a n g e i n c o l o r of t h e c r y s t a l is c a u s e d b y the g r o w t h of a b r o a d a b s o r p t i o n b a n d c e n t e r e d at 5000A. w i t h a b o u t 2000A. h a l f - w i d t h . Besides this b r o a d a b s o r p t i o n b a n d w h i c h grows i n w i t h t i m e , there are a n u m b e r of r e l a t i v e l y sharp lines w h i c h start to a p p e a r after a n n e a l i n g . lines h a v e b e e n assigned to C m
4 +
the e n e r g y l e v e l d i a g r a m of C m
3 +
i n the C a F and C m
2
i n the C a F
4 +
2
crystal.
c o m p a r i s o n w e s h o w the d a t a of G r u b e r a n d C o n w a y o n C m ( 6 ) , a n d the d a t a o b t a i n e d b y K e e n a n o n C m F
4
These
c r y s t a l . F i g u r e 3 shows 3 +
For
in L a C l
3
(8).
T r i v a l e n t rare e a r t h or a c t i n i d e ions c a n b e i n c o r p o r a t e d i n the a l k a l i n e e a r t h h a l i d e s i n sites of v a r i o u s symmetries.
S i n c e the c r y s t a l as a
whole must be electrically neutral, charge-compensating
ions m u s t also
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
15.
EDELSTEiN E T A L .
Single
Cm LaCU
Crystals
Cm CaF
3 +
3 +
C1T1F4 ?
207
Cm CaF
4 +
?
282624 225
xlO c m
~'
x 10
-3
181614 12
9 10
10
I I I 2
8-J
Figure
3. Energy level diagram of and Cm in various matrices
Cm
3+
4+
b e present.
T h e a r r a n g e m e n t of these c h a r g e - c o m p e n s a t i n g
ions
about
the rare e a r t h ( o r a c t i n i d e ) i o n determines the s y m m e t r y site of i m p u r i t y i o n a n d its c r y s t a l field splittings. T h e most c o m m o n
the
symmetry
sites present i n these types of crystals are c u b i c , t e t r a g o n a l , a n d t r i g o n a l T h e sites present d e p e n d o n the w a y the crystals are g r o w n a n d
(16).
annealed (5).
I n our C m - C a F
2
crystals w e find t r i v a l e n t C m i n the c u b i c
site a n d i n t w o different t r i g o n a l sites. P r e l i m i n a r y values of the g tensor i n the t w o t r i g o n a l sites are g i v e n i n T a b l e I I . A l s o i n c l u d e d is the v a l u e for the c u b i c site. I n a l l three sites the c r y s t a l field s p l i t t i n g is large, a n d at 4 ° K . a n d 1°K. w e see o n l y resonance lines f r o m the g r o u n d c r y s t a l field state. T h e g v a l u e for t h e c u b i c site is, w i t h i n e x p e r i m e n t a l error, t h e same as d i v a l e n t A m i n C a F , 2
a n d therefore the Γ c r y s t a l field state is the l o w e s t for this i o n also. T h i s 6
g v a l u e also agrees w i t h the w o r k of A b r a h a m , J u d d , a n d W i c k m a n o n Cm
3 +
in L a C l
3
(I).
T h e n u m b e r of a b s o r p t i o n lines w e o b t a i n f r o m the C m o n the t e m p e r a t u r e at w h i c h r a d i a t i o n d a m a g e
4 +
takes p l a c e .
depends If,
after
a n n e a l i n g , the c r y s t a l is k e p t at r o o m t e m p e r a t u r e , m o r e a b s o r p t i o n lines are f o u n d t h a n w h e n the c r y s t a l is p l a c e d at 77 °K.
M o r e d i f f u s i o n of
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
208
LANTHANIDE/ACTINIDE CHEMISTRY
charge-compensating
ions takes p l a c e i n the c r y s t a l at t h e h i g h e r t e m -
p e r a t u r e w i t h the c o n s e q u e n c e that m o r e s y m m e t r y sites appear. Table II.
Cm
3 +
in CaF
2
(S' =
1/2)
C u b i c Site g = 4.492 ±
.002
Trigonal Site I g
= 3.41 ± .02
n
g_ = 6.88 ± .02
Trigonal Site II g
= 2.69 ± .02
M
g ^ = 5 . 9 1 ± .02
Spin Hamiltonian parameters of C m - C a F . 3 +
P u i n C a F . Crystals grown w i t h
2 3 0
2
2
P u are l i g h t b l u e i n a p p e a r a n c e
a n d g r a d u a l l y c h a n g e to a d e e p e r b l u e i n p e r i o d s of m o n t h s . with
2 3 8
Crystals
P u c h a n g e d to a d e e p , d a r k b l u e i n a f e w days. O p t i c a l spectra
of the t r i v a l e n t
Pu-CaF
2 3 9
2
crystals shows three groups of sharp lines a n d
a n u m b e r of groups of diffuse lines. Lammermann and Conway ( I I )
T h i s result is s i m i l a r to that of
w h o f o u n d i n the spectra of t r i v a l e n t
P u i n L a e t h y l sulfate o n l y three groups of sharp lines. T h e center of the three groups of sharp lines of P u i n C a F
2
agree w i t h the centers of
the three s h a r p l i n e groups i n the e t h y l sulfate c r y s t a l w i t h i n 300 c m . " . 1
I n the
2 3 8
Pu-CaF
2
c r y s t a l , t w o types of a b s o r p t i o n lines a p p e a r e d
with
t i m e after a n n e a l i n g the c r y s t a l ; b r o a d b a n d s of — 1 0 0 A . h a l f - w i d t h a n d s h a r p lines of >—LA. h a l f w i d t h . W e
have considered
the sharp l i n e
s p e c t r u m separately f r o m the other s t r u c t u r e a n d assign it to P u
4 +
i n the
crystal. F i g u r e 4 shows a n energy l e v e l d i a g r a m of P u T h e first c o l u m n is the d a t a of C o h e n ( 2 ) o n P u w e felt the agreement w i t h o u r d a t a o n the P u 3 i n F i g u r e 3) 10 w t . % P u
4 +
i n various diluents.
in 1M HC10 . 4
in C a F
2
w a s not c o n c l u s i v e , w e c o - p r e c i p i t a t e d
with C a F
4 +
4 +
4 +
2
Because
crystal ( column approximately
a n d took the o p t i c a l s p e c t r u m of the p r e c i p i t a t e
i n a m i n e r a l o i l m u l l . T h e s e d a t a are s h o w n i n c o l u m n 2. T h e agreement of the m u l l d a t a w i t h the c r y s t a l d a t a is q u i t e satisfactory.
The broad
b a n d s are l i k e l y c a u s e d b y c o l o r centers f o r m e d i n the c r y s t a l or associated w i t h Y
3 +
impurities. T h e broad bands formed i n the
system s h o w no correspondence Cm-CaF
2
If the
2 3 8
Pu-CaF
2
w i t h the b r o a d b a n d f o r m e d i n the
system. 2 3 8
Pu-CaF
2
c r y s t a l after a n n e a l i n g is k e p t at 77 °K., n o sharp
lines a p p e a r , i n d i c a t i n g that no P u
4 +
is f o r m e d at this t e m p e r a t u r e .
The
b r o a d b a n d s d o a p p e a r w h i c h g i v e the c r y s t a l a different shade of b l u e
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
15.
EDELSTEiN E T A L .
Single
Crystals
209
t h a n that w h i c h appears at r o o m t e m p e r a t u r e . A g a i n this result m u s t b e a t t r i b u t e d to the d i f f u s i o n of v a r i o u s species w h i c h c a n or cannot t a k e p l a c e i n the c r y s t a l , d e p e n d i n g o n the t e m p e r a t u r e . Pu
Pu
4 +
in IM HC10
4 +
in CaF mull
2
4
Pu
4
in CaF
+
2
crystal
2826-
4
-
5
-
6
-
7
—
-
8
-
9
—
-
10
-
1 1 12
24-
— —
2220-
=
—
1816141210-
=
8-
Figure
4.
—
L
Energy level diagram in various matrices
of
Pu*
+
Conclusion O u r results s h o w that as expected A m is the a c t i n i d e element w h i c h forms t h e d i v a l e n t o x i d a t i o n state most easily. I n o u r attempts to f o r m d i v a l e n t P u a n d C m w e f o u n d i n s t e a d sharp l i n e spectra c a u s e d b y the t e t r a v a l e n t state.
O u r experiments d o not e x c l u d e the p o s s i b i l i t y t h a t
d i v a l e n t ions of these elements are f o r m e d because w e d o not h a v e a n u n a m b i g u o u s m e t h o d of detection. I n o u r attempts to m a k e d i v a l e n t ions w e h a v e f o u n d i n t e r e s t i n g solid-state c h e m i c a l effects a t t r i b u t e d to the h i g h l e v e l of r a d i a t i o n i n t h e crystals. Acknowledgments W e w i s h to t h a n k Β. B . C u n n i n g h a m , B . R . J u d d , J . G . C o n w a y for m a n y v a l u a b l e c o m m e n t s , a n d R . W h i t e for c o l l a b o r a t i o n w i t h t h e P u experiments.
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
210
LANTHANIDE /ACTINIDE CHEMISTRY
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October 14, 1966.
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
