Short Notes Kl3

phys. stat. sol. (b) - 105, K13 (1981) Subject classification: 5; 22.8.2 Department of Physics, University of Rajasthan, Jaipur Mossbauer Study of Jahn-Teller Distortion in Systems c_oxM_"2. g-xF-e(). 1-04 and ixM32 .g -x Fe - 0.1- 0 4

BY V.K. SINGH, R. CHANDFtA, and S. LOKANATHAN

1)

It has been shown that Fe5' introduced in small concentrations in distorted spinellic lattice behaves as a sensitive probe /I/. It can provide quantitative information about distortion of the crystal lattice from the observed quadrupole interaction. In spinel compounds cations qre located at 8 a o r tetrahedral si tes

and 16d o r octahedral s i tes of a face centred cubic lattice of oxygen ions. The crystalline field is cubic at tetrahedral sites while at octahedral sites it has trigonal symmetry owing to the cations belonging to the second coordination sphere /2/. The Mossbauer spectra of the systems CoxMn2. 9-xFe0. O4 and NixMn2. 9-xFe0. O4 exhibit two quadrupole doublets which have been attributed to two inequivalent positions of Fe3+ ions in octahedral si tes. The outer quadru- pole doublet with large quadrupole splitting is considered to be due to the Fe (I) site, the second coordination sphere of which contains Mn3+ cations only, while the inner quadrupole doublet is ascribed to Fe

second coordination sphere of which contains Mn cations can be Fe3+ in normal spinel, divalent Co2+ o r Ni2+ in partially in- verted spinels. Deviation from normal distribution has already been noticed /4/. The ligand field contribution to the quadrupole splitting decreases with de- creasing tetragonal distortion of octahedral si tes. The replacement of the Mn& (d4) Jahn-Teller ion by Co

thereby reducing the ligand field contribution to the quddrupole splitting. This results i n reduction of intensity of the outer quadrupole doublet showing that Fe3+ sites a r e converting into Fe3+(II) sites.

3+

3-t (11) sites, the 3+ and other cations/3/. These

2+ o r Ni2+ weakens the tetragonal distortion

In the present work a comparative study to observe the relative concentra- tion of cobalt and nickel at which the outer quadrupole doublet merges into the inner quadrupole doublet has been made and it has been found that the values of

1) Jaipur-4, India.

physica status solidi (b) 105 K14

-1.8 -1.2 -06 0 0.6 1.2 1.8 -IB -7.2 -06 0 0.6 1.2 18 Y Irnml.5) - v(mm/s) -

Fig. 1. Room temperature Mossbauer spectra of a), b), c ) NixMn2. 9-xFe0.

(a) x = 0.1, b) 0.3, c ) 0.5) and d), e), f ) CoxMn2. 9-xFe0.

f ) 1 . 5 )

(d) x = 0.1, e ) 1 . 3 ,

x (at which concentration the outer doublet merges into the inner one) for the

two systems (Co Mn2. 9-xFe0. 0 and NixMn2. 9-xFe0. 04) are quite different:

x = 1 . 5 for the cobalt system, x = 0 .5 for the nickel system. After this value

of x Mossbauer spectra for both the systems do not show any change.

X

The samples were prepared by the dry ceramic method /5/. AFt agrade CoCO , NiC03, MnO, and a - F e 2 0 3 were taken in appropriate proportion,

thoroughly mixed in distilled water, pelleted and then fired at 1200 OC for 6 h,

followed by quenching in order to preserve a single phase structure. Samples of the cobalt system were prepared for x = 0 .1 , 0 .5 , 1 . 0 , 1 . l , 1 . 2 , 1 . 3 , 1 . 4 ,

1 . 5 , 2 . 0 a n d t h a t o f t h e n i c k e l s y s t e m f o r x = 0 . 1 , 0 .2 , 0 .3 , 0 .4 , 0 .5 , 1 . 0 ,

1 . 5 , 2 . 0 .

3

The Mossbauer spectrometer used is described elsewhere /6/. Co5? in rho-

dium matrix was the source used, mounted on a drive operating i n the constant

acceleration mode and spectra were recorded on a ND 1100 multichannel ana-

lyzer. The recorded Mossbauer spectra (Fig. 1 ) of both systems were analyzed

with the help of an LBM-360 computer using a standard programme /?/. The

relevant Mossbauer parameters are listed i n Table 1 . In the cobalt system, two doublets were observed for x < 1 . 5 and only one for x > 1 . 5 while in the

nickel system two doublets for x < 0 . 5 and one for x > 0 . 5 .

Short Notes

T a b l e 1

Mossbauer Parameters of NixMn2. 9-xFe0. O4

X E(I)

0.1 0.36(1)

0.2 0.33(1)

0.3 0.37(3)

0.4 0.35(2)

0.5 - .1.0 - 1.5 - 2.0 -

B(n) @(I) &s(II) INT(II)

0.24(1) 1.25(1) 0.49(1) 0.42(1)

0.22(1) 1.20(1) 0.56(2) 0.43(2)

0.25(2) 0.92(3) 0.59(2) 0.63(2)

0.22(1) 0.74(2) 0.58(2) 0.76(1)

0.25(2) - 0.52(1) 1.00

0.23(1) - 0.54(1) 1.00

0.26(2) - 0.53(1) 1.00

0.25(1) - 0.58(2) 1.00

Fe 0 2.9-x 0.1 4 Mossbauer Parameters of CoxMn

FWHM(1)

0.430

0.45(2)

0.41(2)

0.32(2) - -

I - I-

FWHM(II)

0.43(1)

0.39(2)

0.41(1)

O.U(l) 0.48(1)

0.42(1)

0 .540)

0.75(3)

Is(m

0.24(1)

0.2 7(6)

0.24(2)

0.29(2)

0.3211)

0.29(2)

0.24(2)

0.30(1)

0.30(1)

&s(II)

0.53(2)

0.54(6)

0.62(2)

0.59(2)

0.60(1)

0.54(2).

0.55(2)

0 .460)

0.47(1)

@(I)

1.340

1.13(5)

0.77(2)

0.78(3)

0.78(3)

80.63(5)

INT(II) FWHM(1) FWHM(II)

0.32(1) 0.46(1) 0.44(2)

0.36(1) 0.55(5) 0.49(4)

0.70(3) 0.27(2) 0.50(2)

0.78(2) 0.34(3) 0.44(2)

0.86(1) 0.34(2) 0.44(1)

0.88(3) 0.35(4) 0.51(2)

0.99(1) 0.52(5) 0.45(2)

1.00 - 0.44(1)

1.00 - 0.42(1)

K l 5

x

0.1

0.5

1.0

1.1

1.2

1.3

1.4

1.5

2.0

WI)

0.36(1)

0.31(5)

0.38(2)

0.38(3)

0.39(3)

0.38(5)

0.35(3) - -

IS isomer shift in mm/s with respect to iron; QS quadruple splitting in mm/s; INT(II) intensity of doublet 11; FWHM full width at half maxi- mum. *) Data not reliable due to small intensity of doublet (I).

K16 physica status solidi (b) 105

The isomer shift and the range of quadruple splitting observed at the Fe(1) and Fe(II) si tes in both the systems a r e what one expects for Fek (high spin) in octahedral sites. In oxidic spinels, Mnk with outer electron configuration d , forms coordinate covalent square bonds by hybridizing empty dsp orbitals

/a/. Co2+ with outer electron configuration d7, forms hypoligated octahedral bonds hybridizing empty s p d and is a weak Jahn-Teller ion in octahedral en-

2+ . 8 2+ vironment. Ni excepts that it is not a Jahn-Teller ion in octahedral environment /9/.

The different values of x (at which the outer quadruple doublet merges into

the inner one) for the cobalt and nickel systems may be attributed to the cation distribution in octahedral and tetrahedral s i tes of the spinel compound and the behaviour of Co2+ cation as a weak Jahn-Teller ion in octahedral sites.

4 2

3 2

with outer electron configuration d behaves similarly to Co

The cation distribution in octahedral si tes of a spinel compound depends upon octahedral si te preference energy / lo, 11/. The large octahedral si te pre- ference energy of Ni2+ cation favours i ts occupation of octahedral si tes /12, 13/

whereas Co preference for octahedral and tetrahedral si tes /14/. Thus only half of it is used to influence the quadruple splitting of probe nuclei while almost all Ni2+

5 2 is used for this purpose. Co2+ with outer electron configuration (t ) (e ) is a weak Jahn-Teller ion in octahedral environment due to asymmetric charge distribution i n t ( e ) is a non-Jahn-Teller ion in octahedral environment /12/. Both of these factors may lead to slow change of lattice distortion in cobalt systems and a fast change of lattice distortion in nickel systems.

2+ , due to low octahedral s i te preference energy, has almost equal

2g g

2g 2g g 6 2 while Ni2+ with outer electron configuration (t

References /1/ M. BOFWAZ, G.FILOT1, A. GELBEEZ;, and M. ROGENBERG, J. Phys. C

/2/ E.W. GORTER, Philips Res. Rep. 9, 295 (1954). /3/ G. FILOTI, A. GELBERG, M. ROSENBERG, V. S P h U , a n d P. TELNIC,

/4/ F.C.M. DRIESSENS, Inorg. Chem. Acta 1, 193 (1967). /5/ G. FILOTI, A. GELBERG, V. GOMELA, and M. ROSENBERG, Internat.

/6/ R. CHANDRA and S. LOKANATHAN, phys. stat . sol. (b) 83, 273 (1977).

- 2, 1008 (1969).

-

phys. stat. sol. 14, K91 (1972). - -

J . Magnetism - 2, 65 (1972).

-

Short Notes Kl7

/7/ E. VON MEERWAL, Computer Phys. Commun. - 9, 11 7 (1975). /8/ J.B. GOODENOUGH, Phys. Rev. - 98, 391 (1955). /9/ M.D. STURGE, Solid State Phys. - 20, 94 (1967).

/10/D.S. MCCLURE, J. Phys. Chem. Solids - 3, 311 (1957). /11/ J.D. DUNITZ and L.E. ORGEL, J. Phys. Chem.Solids - 3, 20 (1957). /12/ R.G. BURNS, Mineralogical Application of Crystal Field Theory, Cam-

/13/ F.C.. ROMEIGN, Philips Res. Rep. - 8, 304 (1953). J.H. VAN SANTEN and VAN WIERINGEN, Trav. Chim. 7l 420 (1952).

/14/ M. P. GUPTA, A. P.B. SINHA, and S. K. DATE, J. Phys. Chem. Solids

bridge University Press , 1970 (p. 111).

1

- 39, 1321 (1978).

(Received March 11, 1981)

2 physica (b)