Solid State Communications, Vol. 82, No. 9, pp. 701-703, 1992. Printed in Great Britain.

0038-1098/92 $5.00 + .00 Pergamon Press Ltd

X-RAY DIFFRACTION STUDY OF THE CATION DISTRIBUTION IN THE Mn-Zn-FERRITES

Tahir Abbas, Y. Khan*, Mushtaq Ahmad and Shahid Anwar

Department of Physics, Bahauddin Zakariya University, Multan-60800, Pakistan

(Received 10 February 1992 by M. Cardona)

Lattice parameter, oxygen positional parameter and cation distribution in spinel structural ferrites MnxZnl_xFe204 (x = 0.00, 0.25 and 0.50) were determined for the best fitting of the curve between In (I~b~hkt/ I~l kt) and (sin 02/22). There is a continuous increase in the lattice par- ameter a with the compositional parameter x, in these mixed spinels. It has been observed that Zn 2+ ions prefer the tetrahedral sites in the spinel structure.

1. INTRODUCTION

FERRITES are widely used magnetic materials due to their high electrical resistivity, low eddy currents and dielectric loss. These materials are extensivley used in microwave devices, computer memory chips, magnetic recording media etc. Technically, fine ferrites have been of interest due to their applications for prep- aration of high density ferrites as suspension materials in ferrofluids at low temperature. High frequency applications of these materials find use in the fabri- cation of radiofrequency coils, transformer cores, and rod antennas. Ferrites should also be mechanically strong to resist damage during machining and assembly of parts. Manganese-zinc ferrites belong to the group of soft ferrite materials characterized by high magnetic permeabilities and low losses [1] and have numerous electronic applications. The Mn-Zn ferrites used in magnetic suspension should have minimum spatial and time-dependent variations in permeability [2]. Control of the electrical conductivity of Mn-Zn ferrites is important in high frequency applications because of eddy current damping losses [3]. The investigation of cation distribution at octahedral and tetrahedral pos- itions and the oxygen positional parameter u, which quantifies the degree of distribution of the anion sublattice, are very important in order to control the domain of applications of the Mn-Zn ferrites. The control over cation distribution and the oxygen pos- itional parameter provides a means to develop the desired physical properties which helps in their proper use in the industry. The general formula for spinel is AB204 where A is a divalent and B a trivalent metal ion. There are eight formula units per unit cell, i.e.

* Present address: Institut fur Werkstoffe der Elek- trotechnik, Ruhr University, Bochum, F.R.G.

AsB16032. The spinel structure can be described in terms of a nearly cubic close-packed arrangement of anions with one-half of the octahedral positions (B-sites) and one-eighth of the tetrahedral positions (A-sites) occupied by the cations. The cation arrange- ment can vary between two extremes. One is the normal spinel in which all divalent cations occupy A-sites, and all trivalent cations are on B-sites. The value of the oxygen positional parameter u is 0.375 for this case. The other limiting case is the inverse spinel, in which the divalent cations occupy B-sites and the trivalent cations are equally divided among A-sites and B-sites. Spinels with cation distribution intermediate between normal and inverse are known as mixed spinels. This structure is common for these materials. In the non- ideal spinel structure the oxygen ions are displaced from their ideal positions along [1 1 1] direction away from the tetrahedral position. This deviation is quan- tified by the oxygen positional parameter u which is usually greater than 0.375 for this case. A study of the structure of a spinel should involve the determination of the lattice parameter, the oxygen positional par- ameter, and the cation distribution among the tetra- hedral and octahedral sites of the anion close-packed lattice. Inthis paper we have studied the oxygen pos- itional parameter u and the cation distribution in the MnxZnt_xFe204 for x = 0.00, 0.25 and 0.50 and the results are discussed.

2. EXPERIMENTAL DETAILS

MnxZnl_xFe204 (i.e. x = 0.0, 0.25 and 0.5) spinel ferrites were prepared in polycrystalline form by high temperature solid-state reaction method. The parent oxides (i.e. MnO 99.9%, ZnO 99.9%, Fe203 99.9%) were highly pure, purchased from Merck. Mixture(s) of the parent materials were heated for prolonged periods at 1000°C and, finally, 2h at

701

702

I x -O.5

X-RAY DIFFRACTION STUDY

113 !

022 ~ 333 4 4 0

. _ L 3 L _ 7 .. . . . . 7 _ L _ Z _ X 10.25 113

022/L.__ ~ :533440

x 'O 113

15 70 28 (*) - - - - - -

Vol. 82, No.,9

Table 1. Miller Indices h k 1 and interplaner spacing for Mnx Znl _ x Fe2 O4 ferrites

Sr. No. x = 0.0 x = 0.25 x = 0.50

h k l d(A) h k l d(A) h k l d(A)

1 022 2.9785 022 2.9883 022 3.0079 2 113 2.5495 113 2.5495 113 2.5636 3 004 2.1081 004 2.1175 004 2.1128 4 333 1.6261 333 1.6314 224 1.7277 5 440 1.4946 440 1.4990 333 1.6274 6 . . . . 440 1.4990

Fig. 1. Diffraction patterns of the ferrites (i.e. MnxZn~ _xFe204).

1300°C for complete conversion. All the heat treat- ments were done in air and alumina crucibles were used throughout the experiment. The ferrite samples were removed from the furnace, quenched in air, re- ground, and X-ray diffraction patterns were taken at room temperature. When no traces of the parent ox- ides were found in the diffraction patterns, it was assumed that the reaction has been completed. The X-ray diffraction of the Mn-Zn ferrites (i.e. MnxZnj_~Fe204) was taken with a Shimadzu X-ray diffractometer XD-5A which is equipped with a re- search goniometer VG-108R. A copper X-ray source with a nickel filter was used and the diffractograms were taken at room temperature.

3. RESULTS AND DISCUSSION

The X-ray diffraction patterns of the Mn-Zn ferrites are shown in Fig. 1. We observe from the X-ray diffraction patterns that the intensity of the diffraction lines increases with increasing manganese content. When increasing the manganese contents in Mn-Zn ferrites, the interplaner spacings are also increased monotonically. The structure of the Mn-Zn ferrites is face centred cubic (fc c); its parameters are listed in Table 1. The relationship between lattice parameter a and compositional parameter x is shown in Fig. 2. The oxygen positional parameter u and the cation distribution at tetrahedral and octahedral pos- itions were determined on the basis of the equation [4]:

In (Io~'/I~ l) = In (K) - 2Belt (sin 02/22), (1)

where Io*~ t is the observed intensity of the h k I line, I ~ i the calculated intensity of the h h l line, K the scale factor, Bar the effective Debye parameter or the effec- tive temperature factor, 0hkt the Bragg angle, 2 the wavelength of the incident X-rays.

Using the experimental values of a and oxygen positional parameter u of each Mn-Zn ferrite in the

equations [5]:

dAx = ax/3(u -- 1/4) tet bond, (2)

dBx = a(3u 2 - l l/4u + 43/64) I/2 oct bond, (3)

d~,, = ax/~(2u - 1/2) tet edge, (4)

d~ = a~/2(l - 2u) shared oct edge, (5)

d~ = a(4u 2 -- 3u + 1i/16) 1/2 unshared oct edge,

(6) we calculate the selected interionic distances and the radius of the tetrahedral and octahedral intersites of MnxZnl_~Fe204 from dA~ and dB~, taking 1.38A as the radius of the divalent oxygen ion. The lattice parameters a, oxygen positional parameters u, octa edge, radius of the tetrahedral and octahedral inter- sites in Mn~Znl_xFe~O4 are shown in Table 2. The cation distribution of the spinel ferrites Mn~Zn~_x- FezO4 is shown in Table 3. We see from the Table 2 that there is a continuous variation of the lattice par- ameter a with the compositional parameter x in Mn~Zn,_xFe204. The lattice parameter a increases with the manganese content of the material which may be a direct consequence of the larger ionic radius of

85

T o

84 0 0.25 05

x ----,4[,-

Fig. 2. Lattice parameter, a as a function of com- position, x in MnxZnl_xFe204.

Vol. 82, No. 9

Table 2. Oxygen positional parameter u, Tet edge, Octa edge, radius of tetrahedral and octahedral intersites

X U Tet edge Octa edge (A)

Shared Unshared (A) (A)

X-RAY DIFFRACTION STUDY 703

0.375 [6]. From the above observations we conclude that the MnxZnl _~Fe204 is a mixed ferrite and coor- dination of the Fe 3+ ion is very little affected by

rte t r~ changes in compositional parameter x. Among the (A) (A) divalent ions, Zn 2+ shows stronger tetrahedral pos-

itional preference than Mn 2+ because Zn 2÷ is non magnetic ion. Mn 2+ and Fe 3÷ are magnetic ions;

0.4481 0.7309 they prefer the oetahedral position than tetrahedral 0.4532 0.7367 position. 0.4553 0.7392

Acknowledgements - This work was completed from the grant provided by the National Scientific Research and Development Board (NSRDB) of the Government of Pakistan. One of the authors (Y.K.) acknowledges the grant provided by UNDP for a visit to B.Z. University, Multan, Pakistan.

0.0 0.375 2.8519 0.25 0.375 2.9864 0.50 0.375 2.9970

2.8519 2.9852 2.9864 2.9864 2.9970 2.9970

the Mn 2+ (i.e. 0.80A) as compared to Zn 2+ (i.e. 0.74 A). For the oxygen positional parameter u, an approximately constant value 0.375 was found for all the samples. In [5], the value of u for CoFexGa~_xO4 spinel ferrites was found to be 0.382 for all the con- centration of Ga and the value of u for GeCo204 is

Table 3. Cation distribution in Mn~Zn~_~Fe204 f errites

x Cation distribution

Tetrahedral sites Octahedral sites

Zn 2+ Mn 2+ g e 3+ Z n 2+ Mn 2+ Fe 3+

0.0 0.716 - 0.284 0.284 - 1.716 0.25 0.916 0.0427 0.0413 0.044 0.0413 1.9147 0.50 0.9120 0.0453 0.0427 0.048 0.0427 1.9093

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