965

INFLUENCE OF Fe 2+ CONCENTRATION ON THE MOSSBAUER SPECTRA OF I R O N - C O B A L T AND I R O N - N I C K E L FERRITES

H. F R A N K E and M. R O S E N B E R G Ruhr-Universitiit Bochurn, NB 03/34, Postfach IO 21 48, 4630 Bochum I, BRD

The investigation of Fe3 ,Me,O4 (M = Co or Ni) with M6ssbauer spectroscopy gives evidence for substitutional disorder in these compounds and for Fe"*-FC* electronic relaxation with frequencies higher as 10MHz at room temperature.

As reported by us in a previous paper [1] the M6ssbauer spectra of cobalt-iron ferrites F e s _ x C o x O 4 with 0.1 ~< x ~<0.7 could be splitted in an A-site Fe3+-sextet and starting with x = 0.1 in an increasing number of B-site Fe3+-sextets, thus allowing us to ascribe up to 5 different hyperfine fields at the B-site Fe-nuclei for x = 0.5 in contrast to only two sextets mentioned very recently in a paper by Murray and Linnett [2].

The experimentally observed M6ssbauer B- spectra have to be related to the occurrence of five types of surroundings of the B-site Fe-ions with probabilities which obviously depend on the Co2+-concentration. We have tried to inter- pret the appearance of the five different sur- roundings of the B-sites as a result of a random (statistical) distribution of the Co2+-ions over the B-sublattice, i.e. assuming that the Fe3_xCoxO4 system at room temperature in the composition range 0 ~< x ~< 0.7 behaves as a substitutional al- loy with compositional disorder. Both the ions C o 2÷ and Fe z÷ have about the same radius (0.74 and 0.76.A, respectively) a statistical distribu- tion of the Co 2+ on the B-sites is very probable.

For each composition a Co 2* distribution is established and the Fe ions have to adapt their valencies to this charge distribution as to fulfil the condition of electrical neutrality (Co 2* can- not change its valency by electron transfer!). Thus the Co 2+ statistical distribution gives rise to a distribution of the Fe z* ions with, on average, more Fe 2* ions in the Co2+-poor regions and less Fe 2÷ ions in the Co2+-rich regions.

Assuming a fast electron transfer with the neighrouring Fe 3* o r F e z+ ions the valency state of Fe-ion at a given B-site will fluctuate in time between the +3 and +2 states in order to preserve locally the average charge and spin (nd) necessary to fulfil the electrical neutrality con-

dition, i.e. a higher (nd) in the Co-rich regions and a lower (no) in the Co-poor regions. The same mechanism holds for other substitutional systems as for instance Fe3_xNixO4.

Assuming further that the hyperfine field at a given 57Fe-nucleus will depend on the number of B-nearest neighbours with which the electronic exchange may take place and that up to x = 0.7 the hopping occurs so fast that only single valued average hyperfine fields can be observed with the M6ssbauer technique for every type of surrounding of the B-ions, it was possible to assign four of the B-Iorentzian fitting sextets to four different cationic configurations in groups of

(1) 3 Fe 3÷ + 3 Co 2÷ (Bs-sextet), (2) 3 Fe 3÷ + 2 Co 2+ + 1 Fe z÷ (B4-sextet), (3) 3 Fe 3÷ + i Co 2÷ + 2 Fe z÷ (B3-sextet), (4) 3 Fe 3÷ + 3 Fe 2* (B2-sextet).

The fifth Bt-sextet had a stronger Fe z+ character as the other ones.

Under the same experimental conditions as reported in [i] we have prepared and in- vestigated the M6ssbauer spectra of six com- positions in the system Fes_xNixO4 with x = 0.1; 0.3; 0.5; 0.7; 0.8; 0.9; 1.0 and nine compositions in the system Fes_xCoxO4 with x = 0.5; 0.6; 0.7; 0.75; 0.8; 0.85; 0.9; 0.95; 1.0.

The B-part of the M6ssbauer spectra of the nickel-iron ferrites could be fitted over the whole compositional range by up to five Iorent- zian-sextets as in the case of iron-cobalt ferrites as shown in fig. 1 where the intensities of the fitting spectra for four of them (B2-B5) are plotted as function of x.

Assuming as in the case of cobalt- iron ferrites that in Fe3_xNixO4 the Ni2*-ions are statistically distributed over the B-sites and that the ob-

Physica 86-88B (1977) 965-967 © North-Holland

966

(4 1,4 i

\

A - - B 5 o - - B4 + B3 o - - - B2

c

-~ 0.6 <

0.2 !~

0,2 OZ. 0.6 0,8 ~ 1,0

Fig. I. Measured ( B 2 - O , B3-+, B4-O, B5-A) and calculated (B2 - - - , B3 + + +, B4 - . - . - , B5 ) intensities of the B2-B5 sextets in the Ni, Fe:. ~O~ M6ssbauer room tem- perature spectra.

520

kGAUS~

500

&80

L60

44C

} +

iT

T

I T I i

0.5

k

i

9 9

I <:5,

r

i

i

0,7

+}++++I

B2

/ t r

0.9 ,,0 x +

Fig. 2. B1-B5 hyperfine fields versus Co concentrat ion in the Co, Fe, xO, sys tem and corresponding linewidth of the M6ssbauer fitting spectra (bars).

served values of the B2-B4 hyperfine fields originates in the above-mentioned distributions (1)-(4) with Ni 2+ instead of Co 2+ we obtain by using the binomial distribution the curves plot- ted in fig. 1 which are in qualitative agreement with the experimental data. At x = 0.8 it is pos- sible to separate three of the fitting sextets B2, B 4 and B~ but only two (B 4 and Us) for x = 0.9 in agreement with the calculated probabilities given by the binomial distribution. As shown in fig. 2 the results for Fe3_xCoxO4 with x/>0.7 does not agree any more with the assumption of the statistical disorder of Co 2+ over B-sites only. In the concentration range 0 .7<x<~ 1.0 four different values of the B-Fe 5+ hyperfine field could be found, lying between about 440 and 520 kOe. In the case of pure Fe2CoO4 the ob- served splitting was first reported by Sawatzky et al. [4] and attributed to the A-site Co 2÷ which occurs because, as compared to Fe2NiO4, cobalt ferrite is not a totally inverted spinel.

Unfortunately, there are no available data about the Co 2+ ionic distribution on A and B sites for x < 1.0, and about the influence of x on the reduction of the B-site -SVFe-hyperfine field owing to the presence of Co 2+ ions on A-sites. However, we tried to apply the binomial dis- tribution to both the six B-site and A-site

9 0,5

i \ + \ / " O . .

" ~ ' . + . / B3 ,-

0.3 / &

So,1 \ ,\

, , ~ B1 • 0.'5 0,6 017 0,8 0,'9 1,0 x

Fig. 3. Measured (Bi -O, B 2 - O , B3-+, B/I-O, B5-A) and calculated (Bi - . - . - . - , B2 - - - , B3 + + + , B4 , B~ - - ) intensities of the B i - B 5 sextets in the Co~Fe, xO4 M6ssbauer room tempera ture spectra.

nearest-neighbour cation surroundings of a given B-site 57Fe nucleus under the simplifying assumptions that 24% of the Co2+-ions are on A-sites in Fe2CoO4 and 0.24 x in Fe3_xCoxO4. The calculated curves are plotted against x in fig. 3 and compared with the measured inten- sities of the fitting Iorentzian-sextets. Only a qualitative agreement could be reached, as ex- pected, in view of the crude approximations used in our computation model. In conclusion our investigation of Fe-Co-ferri tes and Fe-Ni-

f e r r i t e s wi th the M 6 s s b a u e r s p e c t r o s c o p y at r o o m t e m p e r a t u r e has g iven s t rong e v i d e n c e for subs t i tu t iona l d i s o r d e r in these c o m p o u n d s owing to the s ta t i s t i ca l d i s t r i bu t ion of Ni 2÷ ions on the B-s i tes and of the Co 2~ ions on bo th the A- and B-s i tes .

The d e p e n d e n c e of the hype r f ine fields on the s ta t i s t ica l conf igura t ions of the nea r e s t neigh- b o u r s ca t ions in the c o n c e n t r a t i o n range x > 0.6 where , a c c o r d i n g to e lec t r i ca l p rope r t i e s , e lec- t ron ic hopp ing occu r s , m e a n s that the hopp ing f r e q u e n c y is h igher as the nuc l ea r L a r m o r f r e q u e n c y at r o o m t e m p e r a t u r e in a g r e e m e n t with the t r a n s p o r t p r o p e r t i e s in the N i - F e - f e r -

967

r i te s y s t e m [5]. An inves t iga t ion of the low- t e m p e r a t u r e M 6 s s b a u e r s p e c t r a of b o t h C o - F e

and N i - F e - f e r r i t e s is now in p rogres s .

References

[1] H. Franke and M. Rosenberg, Verh. der D P G 4 (1976) 303, to be published in J. Magn. Magn. Mater.

[2] P. J. Murray and J. W. Linnett, J. Phys. Chem. Solids 37 (1976) 619.

[3] J.W. Linnett and M.M. Rahman, J. Phys. Chem. Solids 33 (1972) 1465.

[4] G.A. Sawatzky, F. Van Der Woude and A.H. Morrish, Phys. Rev. 187 (1969) 747.

[5] P. Nicolau, I. Burget, M. Rosenberg and I. Belciu, IBM J. Res. Develop. 14 (1970) 249.