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N. EDELSTEIN, W. EASLEY, and R. MCLAUGHLIN. Lawrence Radiation Laboratory, University of California, Berkeley, Calif. Lanthanide/Actinide Chemistry...
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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

Literature Cited (1) Abraham, M., Judd, B. R., Wickman, H. H., Phys. Rev. 130, 611 (1963). (2) Cohen, D., J. Inorg. Nucl. Chem. 18, 211 (1961). (3) Edelstein, N., Easley, W., McLaughlin, R., J. Chem. Phys. 44, 3130 (1966). (4) Fong, F. K., J. Chem. Phys. 41, 2291 (1964). (5) Friedman, E., Low, W., J. Chem. Phys. 33, 1275 (1960). (6) Gruber, J. B., Cochran, W. R., Conway, J. G., Nicol, Α., J. Chem. Phys. 45, 1423 (1966). (7) Guggenheim, H., Kane, J. V., Appl. Phys. Letters 4, 172 (1964). (8) Keenan, T. K., J. Am. Chem. Soc. 83, 3719 (1961). (9) Kiss, Z. J., Staebler, D. L., Phys. Rev. Letters 14, 691 (1965). (10) Kiss, Z. J., Yocom, P. N., J. Chem. Phys. 41, 1511 (1964). (11) Lãmmermann, H., Conway, J. G., J. Chem. Phys. 38, 259 (1963). (12) Lea, K. R., Leask, M. T. M., Wolf, W. P., J. Phys. Chem. Solids 23, 1381 (1962). (13) Marrus, R., Nierenberg, W. Α., Winocur, J., Phys. Rev. 120, 1429 (1960). (14) McClure, D. S., Kiss, Z., J. Chem. Phys. 39, 3251 (1963). (15) Merz, J. L., Pershan, P. S., Bull. Am. Phys. Soc. 11, 364 (1966). (16) Weber, M. J., Bierig, R. W., Phys. Rev. 134, A1492 (1964). RECEIVED

October 14, 1966.

Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.