Hyperfine Interactions 28 (1986) 627-630 627

MOSSBAUER EFFECT STUDY OF Li~+Me4+Fe~+O~-SPINELS H. SALEH, M. ELNIMR, N. NADA and M. FAYEK M6ssbauer Laboratory, Qatar University, Qatar

Mossbauer effect spectra are obtained for the cubic

ferrites Li 2 Me Fe 6 012 (where Me 4+ is one of the

tetravalent elements Ge, Si and Ti) at 78K up to their

magnetic transition temperatures. In a rather large

temperature range they clearly are a broadened six

lines pattern. Each spectrum is successfully analysed

to two sets of magnetic hyperfine splitting Zeeman

patterns with isomer shifts corresponding to ferric ions

in both tetrahedral and octahedral sites. The tendency

of the present non-magnetic elements to occupy the

octahedral site led to an antiparallel Neel spin structure.

Debye and Neel temperatures of these substituted lithium

ferrites are determined and their hyperfine parameters

are discussed.

] . INTRODUCTION: The solid solutions of Li2MeFe6012 (Me 4+

Ge, Si or Ti) with the spinel structure have been the subject of

recent investigation~l~. Watanabe et.al./i/, reported for the first

time the crystal chemistry of these compounds. It has shown that

magnetization values of such compounds are consistent with a Neel

collinear ferrimagnet. Detailed X-ray diffraction patterns were

suffering from unknown diffraction peaks not related to its spinel

structure. They also carried out Mossbauer effect measurements on

these materials at room temperature where six broad absorption ~aks

were observed, and not subjected to data analysis techniques.

Further, as far as we know the mentioned solid solutions have never

been subjected to an extensive Mossbauer effect study. In this

paper the Mossbauer results from liquid nitrogen up to the Neel

transition temperatures for each composition are described and ten-

tatively explained.

2. EXPERIMENTAL: Syntheses of powder polycrystalline ferrites

with the composition Li2MeFe6012 where Me 4+ one of the elements

(Ge, Si and Ti) were done using the usual ceramic technique. The

constituent reagents of Li203, O~ Fe203 and Me 4+ 03 - were mixed and

fired in air at 1200~ for 48 hours, then pelleted, sintered at

1350~ for 3 days and cooled slowly. Chemical analysis confirmed the

non-vaporization of lithium ions on firing samples at high tempera-

tures. X-ray diffraction diagrams for all prepared materials using

Co K~ radiation showed single phase patterns and have the spinel

type structure of the desired compositions with no extra lines. No

difficulties arising from unreaoted starting materials are manifest

in either X-ray or Mossbauer results. The lattice constants were

found to be (8.3451, 8.3482 and 8.3501) • 0.0002~ for Ge, Si and Ti

lithium substituted ferrites respectively, in agreement with the

values reported previously in /i/. Mossbauer effect (ME) measurements

were carried out on these samples in the temperature range between

78K and the magnetic transition temperature for every compound,

using a conventional spectrometer operated in a constant acceleration

�9 J.C. Baltzer A.G., Scientific Publishing Company

628 H. Saleh, et al., M.S. of Lil2+Me**F~+O~l]spinels

mode with a laser calibration. The source was 57Co in palladium

matrix at 25~

3. RESULTS AND DISCUSSION: The measured ME spectra for the

studied compounds at room temperature showed that the average full width at half maximum for the Mossbauer lines are large compared

with those of ~ Fe203 suggesting the overlapping sextets. In fact the analysis of the spectra lines shows a good fit to two six lines Zeeman patterns with different populations and confirms the occu- pation of two different crystallographic sites by ferric ions, one due to Fe 3+ ions at the [B] site and the other due to Fe 3+ ions at

the (A) site. Each Zeeman component is approximated by a symmetric

spectrum with six Lorentzian distribution peaks. The spectra of the

compositions Li2GeFe6012 at various temperatures are shown in Fig.1.

- W - I - ~ - & - ] Q 2 & i I '~ - t 0 4 - ~ - 4 - ~ 2 & $ I I0 VELOCI] Y I ~ l / i l

Fig.l Mossbauer spectra at

various temperatures for

Li2GeFe6012

k, , o c t

_'.\ oT,~

�9 �9 0 3 - o ~ ~

U : ) O ]

' I I I I I I ~ , 0 2 0 0 4 0 0 6 0 0 8 0 0

T(K)

Fig.2 Temperature depen-

dence of isomer shift

values ($) in Li2TiFe6012

In the figure the black dots represent the experimental points and

the continuous lines, the least squares fit of the experimental

spectrum for each site. The vertical lines above each spectrum

indicate the position of the individual lines. The dashed lines represent the best fit to a six line spectrum for each site. The

determined isomer shift (~S) and internal magnetic field (Hell) values were used to identify the six lines correspondin~ to the

octahedral Fe 3+ site and the remaining six lines to the tetrahedral Fe3+ site. The obtained magnetic spectra at 78~ for all samples

showed a splitting of iron on (A) sites smaller than for [B] sites and did not change dramatically compared with that at room tempera-

ture and confirmed the stability of the magnetic structure of such

compounds at low temperatures. Through systematic analysis of the

ME spectra of the studied samples at different temperatures, the following are the specific results:

i, The line widths of the A- site pattern are quite narrow, in-

dicating the presence of virtually no disorder. On the other hand

there is a broadening of the lines of the B-site pattern due to

those fractions of octahedral cations which are surrounded by the

different numbers of nearest neighbour tetrahedral cations because

of the random distribution of the different ions among the tetra-

hedral sites. Empirically it is known that most of the Li and Ti

cations prefer to lie on the octahedral sites while Si and Ge ions tend to occupy the tetrahedral site

H. Saleh, et al., M.S. of Li~+Me'+F~+O~l;spinels 629

tend to occupy the tetrahedral site. This was confirmed in many spin~ ferrites using neutron diffraction techniques~2~. The present ME

patterns at 78~ show that the ratio of the integrated intensities of ferric ions on (A) and [B] subspectra is in good agreement with those expected for cation distribution. Debye temperature was deter-

mined for the studied samples by measuring the temperature depen- dence of the absorption area intensity of the ME spectra. The ob- tained values were 495, 529 and 442K for the substituted Li ferrites Ge, Si and Ti respectively.

ii. The magnitude of the apparent quadrupole splitting (QS) is equal to zero within an experimental error for all the spectra below the Neel temperature. Above the Neel temperature QS is also found to be absent and the spectra have been fitted with two singlets corres-

ponding to Fe 3+ ions in tetrahedral and octahedral sites.

iii. The isomer shift (IS) values obtained from the ME spectra of

the three compositions taken at different temperatures relative to iron metal for the ferric ions at (A) and [B] sites (ie. between

0.1-0.5 mm/sec). These values are very similar to those reported

for LiFes08 /3/. Fig. 2 illustrates how IS of the ferric ions (~) on both crystallographic sites vary with temperature. It turns out that the ME hyperfine spectra pertaining to iron nuclei at the two

lattice sites are shifted with respect to each other. In fact the

temperature dependence of ~ for all specimens were linear within

experimental errors. The increase in the values of IS with decrease in temperature for a given site is due to the second order Doppler

effect. Although the difference between IS for Fe 3+ at A and B

sites at a fixed temperature is small, the B site values are never- theless more positive than A site values. This difference can be

attributed to the slight sp 3 covalency of the tetrahedral ions. The

observed difference is further in good agreement with similar resu~s

of other spinel ferrites /4/.

iv. As in other ferrites, the present analysis showed that the sex-

tet with the higher hyperfine magnetic field is due to the octahedral site iron ions and the pattern with the smaller field arises from

iron ions at tetrahedral site. The smaller A site field is primarily

due to a largercovalency and therefore due to a greater degree of spin delocalisation at A-sites. The difference between the hyperfine

field values of both sites is in agreement with the results in classical spinels /5/. High temperature ME measurements up to Neel

points were obtained. (See Fig.l for Li2GeFe6012) (to restrict the

figure to a reasonable length only eight sample spectra are shown).

For all compounds as the temperature approaches 800K it becomes ve~ difficult to distinguish between the two sites. Above that tempera-

ture a strong paramagnetic component exists in the spectra

simultaneously with the magnetic component. The spectra of lithium

germanium ferrite recorded at 803~ shows two small absorption dips

between which a sharper absorption line. This indicates that at that temperature some magnetic ordering is still observable, whereas

a great deal of this material already behaves paramagnetically. At

the temperatures 809, 873 and 751K for the Ge, Si and Ti lithium

ferrites, the magnetic transition is completed in every compound and

the magnetic six lines spectrum has completely disappeared. These

are thus taken as Neel temperatures of these compounds, the values of which for Ge and Si are in good agreement with the results

obtained from magnetization measurements /i/. For all samples, the

values obtained for both sites are plotted against temperature.

630 H. Saleh, et al., M.S. o f L/2+Me*+F~+O~t;spinels

bOO'

500

~400 0

.-r

2oo

IOC

�9 $i %

I I I l t I I I ~ / ~ iO0 2.o0 ~:)0 4O0 500 600 700 BOO 900

T(K)

Fig.3 Temperature dependence

of the hyperfine magnetic fields

I 0.6 ~,

.~ 0,4 \

o.~

i i I I 1 O.Z O~l 0.6 n B LO

( T / T N )

Fig.4 Thermal variation

of the hyperfine field

f o r L i 2 G e F e 6 0 1 2

T h e h y p e r f i n e f i e l d H ( 0 ) i s e s t i m a t e d b y e x t r a p o l a t i o n s o f t h e s e H ( T ) v e r s u s T c u r v e s t o a b s o l u t e z e r o . T h e i n t e r s e c t i o n o f s u c h c u r v e s o n t h e t e m p e r a t u r e a x i s i n d i c a t e s t h e N e e l p o i n t s f o r t h e c o m p o u n d s s t u d i e d . T h e f a c t t h a t ~Bf ( 0 ) e x c e e d s H~f ( 0 ) i s e a s i l y e x p l a i n e d b y t h e g r e a t e r c o v a l e n t c h a r a c t e r o f t h e F e ~ + - 0 2 - b o n d . T h e r a t e o f d e c r e a s e o f t h e m a g n e t i c h y p e r f i n e f i e l d w i t h t e m p e r a - t u r e f o r b o t h s i t e s ( F i g . 3 ) s h o w s t h a t t h e A - s i t e i o n s a r e s u b j e c t t o t h e s t r o n g e s t m o l e c u l a r f i e l d a n d t h e r e f o r e w i t h i n c r e a s i n g t e m p e r a t u r e i t w i l l b e h a r d e r t o d i s r u p t t h e m a g n e t i c a l i g n m e n t i n t h e A - s u b l a t t i c e t h a n t h e B - s u b l a t t i c e . G o o d a g r e e m e n t w a s o b t a i n e d f o r t h e r e d u c e d h y p e r f i n e f i e l d v e r s u s T / T N p l o t f o r t h e d i f f e r e n t s a m p l e s w i t h B r i l l o u i n f u n c t i o n w i t h S = 5 / 2 . An e x a m p l e i s s h o w n i n F i g . 4 f o r t h e f e r r i t e L i 2 G e F e 1 6 0 1 2 . T h i s r e f l e c t i n g t h e s a m e spin state for the iron situated in the two sites. Even in the high

temperature range, it may be concluded that the electronic relaxation- ~ime in the ~• electronic crystal field levels of 6S5/257Fe3+

ions is large enough compared to the nuclear Larmor precession time. The analysis of ME data showed that the one third power law/4/ H =

D(l-T/TN)~Which describes the sublattice magnetism is valid for the studied compositions having measured hyperfine fields with suffici~t

accuracy. The log-log plot of Heff against T/T N for both sites in

every compound was straight line and the exponent ~ on the average is quite close to 1/3 .

REFERENCES

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/ 3 / J . L . D o r m a n n , R e v u e P h y s . A p p . 15 ( 1 9 8 0 ) 1 1 1 3

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