Platinum Group Metals and Compounds

10

195

Pt—A

Survey of Mossbauer Spectroscopy

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 23, 2016 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0098.ch010

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


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


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


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


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,


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


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 .


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


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

Survey of Mossbauer

141

Spectroscopy


10.

Physics Letters

Figure 6.

Mossbauer spectra with Pt in ferromagnetic

alloys


(2)

142

GROUP

METALS

AND

COMPOUNDS


PLATINUM

Physical Review

Figure

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



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 )


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


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


10. 1 9 5

Spectroscopy

145

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

μ


Survey of Mossbauer

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

Hint MG

β

Ref.

-0.9

Platinum Group Metals and Compounds

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