195Pt—A Survey of Mossbauer Spectroscopy - Advances in Chemistry

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195

Pt—A

Survey of Mossbauer Spectroscopy

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N. BENCZER-KOLLER Department of Physics, Rutgers University, New Brunswick, N. J. 08903

The recoilless resonance absorption and scattering of the 99 keV transition in Pthas been studied to determine the lifetime [τ = (1.7 ± 0.2) χ 10 sec], conversion coeffi­ cient (α= 7.0 ± 0.3), magnetic moment (μ = —0.615 ± 0.041 nm), and radius relative to the ground state radiu ( δ R / R

r

ι

Ta=Ts=20°K ta=0.0063 in.

1.00 J99 .98

\\ J f

J97 36 .95 .94

€= «052*.002 -5

-4

-3

-2

- 1 0 1 2

3

4

5

RELATIVE VELOCITY CM/SEC)

Physical Review

Figure 2. Typical Mossbauer absorption spec­ trum obtained with a Au in a Pt matrix source and a 0.0063-inch thick Pt absorber; both source and absorber were at 20°Κ (9) 195

In Platinum Group Metals and Compounds; Rao, U. V.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

10.

BENCZER-KOLLER:

Survey of Mossbauer

137

Spectroscopy

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ture a n d vast a v a i l a b l e d a t a o n heat capacity, lattice constants, a n d elastic constants. I n a b a s i c M o s s b a u e r e x p e r i m e n t , the r e d u c t i o n i n t r a n s m i s s i o n ( 9 ) ( F i g u r e 2 ) or the increase i n scattered i n t e n s i t y of r a d i a t i o n ( 2 ) ( F i g u r e 3 ) is o b s e r v e d as a f u n c t i o n of the r e l a t i v e v e l o c i t y b e t w e e n a source a n d an absorber. T h e f u l l w i d t h at h a l f m a x i m u m of the resonance c u r v e Γ is r e l a t e d to the m e a n l i f e of the r a d i a t i n g state b y the u n c e r t a i n t y r e l a ­ tion Γ 2fr/r. T h e d e p t h of the c u r v e , C., is r e l a t e d to / , t h e m a g n i t u d e of the recoilless f r a c t i o n of g a m m a rays e m i t t e d , a n d h e n c e to the c r y s ­ t a l l i n e properties of the s o l i d . F i n a l l y , the d i s p l a c e m e n t of the c u r v e f r o m zero r e l a t i v e v e l o c i t y indicates the e n e r g y difference b e t w e e n e m i t t e d a n d a b s o r b e d r a d i a t i o n a n d is p r o p o r t i o n a l to the s-electron

τ

I

I

I

I

Γ

Physics Letters

Figure 3. Typical Mossbauer scattering spectrum obtained with 32 mg/cm Pt scatterer at 29°Κ and 560 mg/cm Pt scatterer at 92°Κ (2)

2

2

d e n s i t y difference at the nucleus i n the source a n d absorber, a n d to the difference i n the m e a n square r a d i u s of the nucleus i n its e x c i t e d state and its g r o u n d state.

Resonance Effect and Line Width of the 99 and 130 keV Transitions T h e 99 k e V S t a t e . T y p i c a l l y , a source of 1-3 m C i of p l a t e d o n a p l a t i n u m f o i l m a y be used.

1 9 5

A u electro­

T h e resonance effect for the

In Platinum Group Metals and Compounds; Rao, U. V.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

138

PLATINUM

GROUP

METALS

AND

COMPOUNDS

99 k e V t r a n s i t i o n f o r s u c h a source a n d a 0.006-inch n a t u r a l p l a t i n u m f o i l absorber, b o t h at 20 °K, is c Ξ== 5% a n d the l i n e w i d t h Γ = 2.1 c m / s e c . T h e w i d t h of the resonant l i n e depends o n the absorber thickness, t . T h e a

e x t r a p o l a t e d l i n e w i d t h at t = 0, Γ = a

h a l f l i f e of t h e state, τ

1 / 2

=

(1.7 ±

(1.75 ± 0.18) c m / s e c , y i e l d s t h e

0.2) Χ 10"

10

sec, i f one assumes n o

b r o a d e n i n g of t h e l i n e o w i n g to hyperfine interactions. T h i s v a l u e agrees w i t h the electronically measured value r

1/2

«

1.4 Χ 10"

10

sec (4). A s

p l a t i n u m m e t a l is a c u b i c c r y s t a l , n o l i n e b r o a d e n i n g interactions are

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e x p e c t e d (9). T h e 130 k e V S t a t e . T h e d e c a y of the 130 k e V state has b e e n s t u d i e d extensively, a n d several inconsistencies a r e b e i n g r e s o l v e d .

T h e results

of different measurements of t h e m e a n l i f e a n d d e c a y m o d e of t h e 130 k e V state a r e discussed b y F i n k a n d B e n c z e r - K o l l e r (8).

T h e half-life

of t h e state has b e e n m e a s u r e d e l e c t r o n i c a l l y , a n d t h e t r a n s i t i o n m a t r i x e l e m e n t for excitation has b e e n d e r i v e d f r o m C o u l o m b e x c i t a t i o n d a t a T h e c o m b i n a t i o n of t h e C o u l o m b e x c i t a t i o n y i e l d , t h e i n t e r n a l

(12).

c o n v e r s i o n coefficient (8) a =

1.76 ± 0.19, a n d t h e b r a n c h i n g r a t i o

(8)

P o = 0.060 ± 0.008 f o r t h e crossover d e c a y to g r o u n d , y i e l d s a h a l f - l i f e C

τι/2 =

0.014) ns i n excellent agreement w i t h a recent

(0.414 ±

M o s s b a u e r d e t e r m i n a t i o n of t h e l i n e w i d t h , Γ = e q u i v a l e n t to π /

2

=

(4.4 ±

(0.49 ± 0.05) ns. W i l e n z i c k et al. (15)

(15)

mm/sec,

0.4)

do not i n ­

d i c a t e t h e thickness of t h e P t absorber u s e d . T h e resonance l i n e f r o m t h e 130 k e V state is a factor of 5 n a r r o w e r t h a n that f r o m t h e 99 k e V t r a n s i t i o n a n d therefore m o r e a m e n a b l e to i s o m e r shift studies. recoilfree f r a c t i o n , / =

H o w e v e r , e v e n at 20°K, t h e 130 k e V t r a n s i t i o n 2.8%, is m u c h s m a l l e r t h a n that of t h e 99 k e V

t r a n s i t i o n , a n d t h e effect o b s e r v e d 0.26%

w i t h a 0.011-inch absorber is o n l y

(10).

Lattice Dynamics T w o of t h e m o r e d i r e c t t e c h n i q u e s u s e d i n t h e s t u d y o f l a t t i c e d y ­ n a m i c s of crystals h a v e b e e n t h e s c a t t e r i n g of neutrons a n d of x-rays f r o m crystals. I n a d d i t i o n , t h e p h o n o n v i b r a t i o n a l s p e c t r u m c a n b e i n ­ f e r r e d f r o m c a r e f u l analysis of measurements of specific heat a n d elastic constants.

I n studies of B r a g g reflection of x-rays ( w h i c h i n v o l v e s n o

loss of energy t o t h e l a t t i c e ) , i t w a s f o u n d that t e m p e r a t u r e has a strong influence o n t h e i n t e n s i t y of t h e reflected lines.

T h e i n t e n s i t y of the

scattered x-rays as a f u n c t i o n of t e m p e r a t u r e c a n b e expressed b y =

I e~ 0

2wm

w h e r e 2W(T)

is c a l l e d t h e D e b y e - W a l l e r factor.

I(T)

Similarly;

i n t h e M o s s b a u e r effect, g a m m a rays are e m i t t e d or a b s o r b e d

without

loss o f energy a n d w i t h o u t change i n t h e q u a n t u m state of t h e lattice b y

In Platinum Group Metals and Compounds; Rao, U. V.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

10.

BENCZER-KOLLER

Survey

of Mossbauer

139

Spectroscopy

a n u c l e u s b o u n d i n a c r y s t a l . T h e f r a c t i o n / of recoilless g a m m a rays e m i t t e d or a b s o r b e d b y a n u c l e u s i n a c r y s t a l l a t t i c e is e~

2W

w h e r e R = r e c o i l e n e r g y of n u c l e u s a n d G (ω) =

where

frequency distribution,

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n o r m a l i z e d so that

J

G(G>)

άω =

1

ο If t h e lattice is a s s u m e d to h a v e a D e b y e f r e q u e n c y s p e c t r u m w i t h c o = k6 /h, then DW

m

where 6

DW

is t h e effective D e b y e t e m p e r a t u r e .

s p e c t r u m , G (ω),

Since t h e r e a l f r e q u e n c y

m a y b e c o n s i d e r a b l y different f r o m a h a r m o n i c

Debye

s p e c t r u m , t h e effective D e b y e t e m p e r a t u r e a p p r o p r i a t e to a n y other q u a n ­ t i t y i n v o l v i n g a different average over t h e s p e c t r u m m a y b e a p p r e c i a b l y different f r o m 6 DW

T h u s , for example, the D e b y e temperature 0 perti­ C

nent to the heat c a p a c i t y w i l l v a r y m u c h m o r e r a p i d l y w i t h t e m p e r a t u r e than 0 . DW

I n p r i n c i p l e , h o w e v e r , f r o m k n o w l e d g e of t h e heat c a p a c i t y ,

lattice constants, a n d elastic constants as a f u n c t i o n of t e m p e r a t u r e , one c a n l e a r n e n o u g h a b o u t the f r e q u e n c y s p e c t r u m to p r e d i c t t h e D e b y e W a l l e r factor.

F e l d m a n a n d H o r t o n ( 7 ) have m a d e s u c h a c a l c u l a t i o n

in the quasiharmonic approximation i n w h i c h the lattice potential i n P t m e t a l is a s s u m e d to b e h a r m o n i c , a n d of a l l a n h a r m o n i c effects, o n l y the

_ 240

^23I°K

229 K>^ e

Ξ 230 ο ®

225 K^^ e

220 210 0

100

50

150

200

250

300 Physical Review

Figure 4. Flot of ^ ( T ) vs. T . The solid line gives the value of ^ D W ( T ) referred to the 0°K volume. The broken line shows the effect of thermal expansion ( 7 ) . D

W

In Platinum Group Metals and Compounds; Rao, U. V.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

140

PLATINUM

t h e r m a l expansion is t a k e n i n t o account. D e b y e temperature should b e 0

DW

=

GROUP

METALS

A N D COMPOUNDS

T h e i r conclusions are that the

(231 ± 3 ) ° K at 0 ° K a n d s h o u l d

have a very slight temperature dependence ( F i g u r e 4 ) . Experimentally, f r o m the analysis of the t e m p e r a t u r e d e p e n d e n c e of the a b s o r p t i o n s p e c t r a o b t a i n e d w i t h a source o f

1 9 5

A u embedded in a natural platinum matrix

a n d a n a t u r a l p l a t i n u m absorber, H a r r i s , R o t h b e r g , a n d B e n c z e r - K o l l e r (JO) obtained an extrapolated 0

DW

=

(234 ± 6 ) ° K at 0 ° K i n excellent

a g r e e m e n t w i t h t h e o r e t i c a l p r e d i c t i o n s f o r the p l a t i n u m c u b i c l a t t i c e .

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A c t u a l l y , the q u a n t i t y t h a t is m e a s u r e d is f/(l

+ a), n a m e l y a c o m b i n a ­

t i o n o f the recoilless f r a c t i o n / a n d o f the c o n v e r s i o n coefficient « . T h e determination of 6

DW

is t h e n v e r y sensitive to the c h o i c e o f a ( F i g u r e 5 ) .

If the e x p e r i m e n t a l 2 W is fitted to the t h e o r e t i c a l p r e d i c t i o n s o f F e l d m a n a n d H o r t o n , t h e n a v a l u e for the c o n v e r s i o n coefficient a c a n b e o b t a i n e d , a = 7.0 ± 0.3. T h i s v a l u e o f the i n t e r n a l c o n v e r s i o n coefficient is m o r e

β no

K)

2030405060708090100 TfK) Physical Review

Figure 5. Experimental values ( 9 ) of 0 for various values of the internal conversion coefficient vs. temperature. The solid line was obtained from the analysis of specific heat measurements and other thermody­ namic data ( 7 ) . D W

In Platinum Group Metals and Compounds; Rao, U. V.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

B E N C Z E R - K O L L E R

Survey of Mossbauer

141

Spectroscopy

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10.

Physics Letters

Figure 6.

Mossbauer spectra with Pt in ferromagnetic

alloys

In Platinum Group Metals and Compounds; Rao, U. V.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

(2)

142

GROUP

METALS

AND

COMPOUNDS

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PLATINUM

Physical Review

Figure

7. Mossbauer spectra for Pt-Fe absorbers in different field configurations (1)

In Platinum Group Metals and Compounds; Rao, U. V.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

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10.

BENCZER-KOLLER

Survey of Mossbauer

143

Spectroscopy

Physical Review

Figure 8. Mossbauer spectra for absorbers of Pt-Co, Pt-Ni, and Pt (1) precise t h a n t h a t w h i c h c a n b e o b t a i n e d b y a n y of the n u c l e a r spectroscopy m e t h o d s a v a i l a b l e to date.

Hyperfine Interaction T h e M o s s b a u e r a b s o r p t i o n of scattering experiments w e r e c a r r i e d out b y B e n c z e r - K o l l e r , H a r r i s , a n d R o t h b e r g ( 3 ) , A t a c , D e b r u n n e r , a n d F r a u e n f e l d e r (2),

Agresti, Kankeleit, a n d Persson ( J ) , a n d B u y r n , G r o d -

zins, B l u m , a n d W u l f f (6) Pt-Fe

w i t h a single l i n e source a n d absorbers

alloys r a n g i n g i n c o m p o s i t i o n

from

P t - C o , a n d P t - N i alloys as w e l l as P t F e . 3

Pto.03Feo.97 to

of

Pt .5oFe .5o, 0

0

A t y p i c a l s p e c t r u m is s h o w n

i n F i g u r e 6. T h e e x p e c t e d 6-line p a t t e r n c o r r e s p o n d i n g to a 3 / 2 - »

1/2

m a g n e t i c d i p o l e t r a n s i t i o n is not r e s o l v e d , a n d o n l y t w o peaks c a n b e o b served. F r o m a 6-line fit to the o b s e r v e d p a t t e r n , H a r r i s et al. ( 3 )

In Platinum Group Metals and Compounds; Rao, U. V.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

were

144

PLATINUM

Table I.

GROUP

AND

COMPOUNDS

Magnetic Moment of the 99 k e V Excited State in T.,

Absorber

°K

T.,

°K

Pto.iFeo.9 Pt Fe

229 418

20 20

20 20

Pto.sFeo.7 Pto.07Coo.93 Pto.07Nio.93

166 71.5 140

29 29 29

29 29 29

Pto.03Feo.97 Pto.03Coo.97 Pto.03Nio.97

30-60 50 50

4.2 4.2 4.2

4.2 4.2 4.2

Pto.03Feo.97 •

52

4.2

4.2

3

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METALS

• 521

Pto.50Feo.50

Calorimetric determination P o l a r i z e d n e u t r o n - s p i n resonance

Pto.03Feo.97 Pto.iFeo.9 Average

Table II.

W i d t h , Isomer Shift, Magnitude of the A u in Various Matrices and 1 9 5

(Mv\

t Source Matrix

\Cm2J

Pt

Cu Be Ir Pt 195mp

t

p

m

t

T.,

°K

Foil

326 326 610 104

4.2 4.2 4.2 20.0

4.2 4.2 4.2 20.0

326

4.2

4.2

o n l y a b l e to o b t a i n a r e l a t i o n s h i p b e t w e e n H n u c l e u s , a n d the r a t i o μ*/μ

ΰ

i n t

2.04 ± 0.10 ~2.5 3.05 ± 0.15 2.1 =fc 0.2

, t h e effective field at the

of m a g n e t i c m o m e n t s

of the e x c i t e d

and

g r o u n d states. T h e three o t h e r groups a l t e r e d t h e a b s o r p t i o n or s c a t t e r i n g patterns b y p o l a r i z i n g t h e absorbers w i t h a n e x t e r n a l field a p p l i e d e i t h e r p a r a l l e l or p e r p e n d i c u l a r to the g a m m a r a y e m i s s i o n ( F i g u r e s 7 a n d 8 ) . T h e y thus w e r e able to o b t a i n u n i q u e v a l u e s for the s i g n a n d m a g n i t u d e of μβ a n d H

i n t

.

T h e s e results, as w e l l as i n t e r n a l fields o b t a i n e d f r o m a

c a l o r i m e t r i c d e t e r m i n a t i o n (11)

a n d a p o l a r i z e d n e u t r o n s p i n resonance

In Platinum Group Metals and Compounds; Rao, U. V.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

10. 1 9 5

Spectroscopy

145

P t and Internal Magnetic Field for Various P t - F e Alloys

μ

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Survey of Mossbauer

B E N C Z E R - K O L L E R

Hint MG

β

Ref.

-0.9