The Mössbauer study of polynuclear trivalent iron complexes

Volume 24A, number 2

THE

PHYSICS LETTERS

MOSSBAUER

16 January 1967

STUDY OF POLYNUCLEAR IRON COMPLEXES *

TRIVALENT

V . G . B H I D E ~, S . K . D A T E $, G . K , SHENOY and P . H . U M A D I K A R

Department of Physics, Institute of Science, Bombay 1, India Received 23 December 1966

M~ssbauer studies of some polynuelear trivalent iron complexes have been made. The spectra show spin relaxation effects which can be clearly seen at 78°K.

It is w e l l - k n o w n that h . f . s t r u c t u r e can be obs e r v e d in the M ~ s s b a u e r s p e c t r u m of p a r a m a g ne t i c s a l t s if e l e c t r o n i c ( s p i n - s p i n and s p i n - l a t t i c e ) r e l a x a t i o n t i m e s a r e l a r g e c o m p a r e d to L a r m o r p r e c e s s i o n t i m e . C o n s e q u e n tl y , m o s t of the o b s e r v a t i o n s of h . f . s p e c t r a in p a r a m a g n e t i c s have been made on f e r r i c ions by e i t h e r taking low c o n c e n t r a t i o n of t h e s e p a r a m a g n e t i c ions in a d i a m a g n e t i c l a t t i c e [1] o r by taking such s a l t s in -&

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Citrate

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Fig. 1. M~ssbauer spectra of ferric citrate (60 mg/cm 2) matched against 57Co:Cu standard source. * Work done under the auspices of the National Bureau of Standards, Washington, D.C. Now at National Physical Laboratory, New Delhi.

Table 1 The M~ssbauer parameters for various complexes are given. The isomer shift is given w . r . t . 310 stainless steel. Complex

T(°K)

Isomer Quadrupole Total shift splitting line width (mm/sec) (mm/sec) (mm/sec)

1. F e r r i c citrate

29, 200

0.52=e0,04 0.52±0.04 0.57±0,04 0.54±0.04

1.25±0.05 1.30±0.05

2. F e r r i c benzoate

298 200

0.50±0,04 0.45±0.05 0.58:~0.04 0.45±0.05

0.90±0.05 0.95:L0.05

3. F e r r i c malate

298 200

0.58~-0.04 0,60±0.05 1.25±0.05 0.64±0.04 0,65:~0.05 1.30±0.05

which F e 3 + - F e 3+ d i s t a n c e is so l a r g e that the spin r e l a x a t i o n t i m e s a r e long enough [2, 3]. We r e p o r t h e r e the r e l a x a t i o n ef f ect s in p o l y n u c l e a r f e r r i c c o m p l e x e s , such as f e r r i c c i t r a t e , f e r r i c b e n z o a t e and f e r r i c m a l a t e . The s u s c e p t i b i l i t y of t h ese and s i m i l a r c o m p l e x e s has been explained by taking exchange i n t e r a c t i o n s between two of the t h r e e f e r r i c ion p a i r s as equal and d i f f e r e n t f r o m the t h i r d [4-6]. The M 6 s s b a u e r p a r a m e t e r s of t h ese c o m p l e x e s used in conjunction with 57Co:Cu s o u r c e at 298°K and 200°K a r e given in table 1 and few typical s p e c t r a a r e p l o t t ed in the f i g u r e s . The s p e c t r a at r o o m t e m p e r a t u r e a r e n o n - L o r e n t z i a n s and have a width 1.25 ± 0.05 m m / s e c which is about f i v e t i m e s that f o r a s t a n d a r d a b s o r b e r . Both the i s o m e r shift and q u a d r u pole i n t e r a c t i o n a r e c h a r a c t e r i s t i c of h i g h - s p i n f e r r i c s t a t e . The d i f f e r e n c e in exchange i n t e r a c tion b e t w e e n f e r r i c ion p a i r s is not e x p e c t e d to influence any of t h e s e p a r a m e t e r s , but if the c h e m i c a l s u r r o u n d i n g s of these f e r r i c ions a r e diff e r e n t , then t h e r e will be a s p r e a d in both q u a d r u pole splitting and i s o m e r shift, which may cause a s y m m e t r y and p e r h a p s b r o a d e n i n g of the l i n e s . 91

Volume 24A, number 2

-10

-8

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PHYSICS LETTERS

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VELOCITY

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(mm/sec)

Fig. 2. M6ssbauer spectra of 57Co:Cu against (a) ferric citrate, (b) ferric benzoate, (c) ferric malate, at 78°K. A l t e r n a t i v e l y , n o n - L o r e n t z i a n b r o a d s p e c t r a may as well be due to r e l a x a t i o n e f f e c t s , as in t h e s e c o m p l e x e s the d i p o l a r and exchange i n t e r a c t i o n s a r e v e r y weak. Indeed, t h e r e is an indication of the wings in the s p e c t r a e v e n at r o o m t e m p e r a t u r e . F u r t h e r , the width of the l in e s is s e e n to i n c r e a s e on l o w e r i n g the t e m p e r a t u r e . The c r y s t a l l i n e f i e l d s p l i t s 6S~ s t a t e of Fe 3+ into t h r e e K r a m e r s ' doublet I ± ~ , t±~> and 1±½) s t a t e s . E a c h of t h e s e p r o d u c e s an e f f e c t i v e m a g netic f i el d if e l e c t r o n i c r e l a x a t i o n t i m e s a r e long enough, as c o m p a r e d to L a r m o r p r e c e s s i o n t i m e .

92

16 January 196'

J u s t i f i a b l y , n e g l e c t i n g the off diagonal t e r m s f o r I ± §> and I + ~> s t a t e s , one should e x p e c t in the hi r e l a x a t i o n t i m e l i m i t two s e t s of s i x - f i n g e r p a t t e r n with fields n e a r l y 550 kOe and 330 kOe r e s p e c t i v e l y . As the off diagonal t e r m s f o r 1±½) st at e a r e s i g n i f i c a n t , one e x p e c t s e l e v e n lines [7]. The indication of the wings on both s i d e s of the b r o a d c e n t r a l l i n es at r o o m t e m p e r a t u r e s u g g e s t s that r e l e x a t i o n t i m e f o r s o m e s t a t e p r e s u mably, I±~> st at e is sufficiently long so that h . f f i e l d does not t i m e a v e r a g e to z e r o . The c e n t r a l l i n es b e c o m e slightly b r o a d e r at 200°K and at 78°K, not only the c e n t r a l lines have b r o a d e n e d s t i l l f u r t h e r (F = 1.50± 0.05 m m / s e c ) but the h . f . e f f e c t s can be c l e a r l y s e e n (fig. 2a and 2b). H o w e v e r , in the c a s e of f e r r i c m a l a t e (fig. 2c), the wings a r e j u s t developing at 78°K. Indeed, in all t h e s e t h r e e c a s e s the s p e c t r a show few m o r e lin, in addition to those e x p e c t e d and m a r k e d by the a r r o w s . This may be due to the fact that h . f . f i e l d at two of the t h r e e f e r r i c ions has one value w h e r e a s the f i el d at the t h i r d is d i f f e r e n t . It i s, h o w e v e r , not p o s s i b l e f r o m the s p e c t r a to decide this point unambiguously. It may be m e n t i o n e d that c e n t r a l l i n es have not only b r o a d e n e d at 78 °] but have d e c r e a s e d in intensity. This s e e m s to indicate the e m e r g e n c e of h . f . f i el d a s s o c i a t e d with I± ½) s t a t e . Taking the line p o s i t i o n s market in fig. 2b, the h . f . f i e l d f o r I±~> s t a t e is about 500 kOe. D e t a i l i n v e s t i g a t i o n s using e . p . r , and m a g n e t i c s u s c e p t i b i l i t y m e a s u r e m e n t s a r e in hand.

References 1. G.K. Wertheim and J . P . R e m e i k a , Phys. Letters 10 (1964} 14. 2. J.W.G. Wignall, J. Chem. Phys. 44 (1966) 2462. 3. L. Campbell and S. De Benedetti, Phys. Letters 20 (1966} 120. 4. L.A.Welo, Phil.Mag. 6 (1928)481. 5. KoKambe, J. Phys.Soc. Japan5 (1950} 48; J. Yvon, J. Hoyowitz and A.Abragam, Rev. Mod. Phy 25 (1953) 165. 6. J . F . Duncan, C.R. Kanekar and F. K. Mok, to be published. 7. C.E. Johnson, T . E . Cranshaw and M.S.Ridout, Pro~ Intern. Conf. on Magnetis, (1964} p. 549.