SITE OCCUPANCY, ISOMER SHIFT AND NUCLEAR QUADRUPOLE INTERACTION OF 57Fe IN Fe2(1-y) Mg1+yTiyO4 WITH y ≥ 0.5

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SITE OCCUPANCY, ISOMER SHIFT AND

NUCLEAR QUADRUPOLE INTERACTION OF 57Fe

IN Fe2(1-y) Mg1+yTiyO4 WITH y

≥ 0.5

E. de Grave, C. Dauwe, J. de Sitter, A. Govaert

To cite this version:

E. de Grave, C. Dauwe, J. de Sitter, A. Govaert. SITE OCCUPANCY, ISOMER SHIFT AND

NUCLEAR QUADRUPOLE INTERACTION OF 57Fe IN Fe2(1-y) Mg1+yTiyO4 WITH y _{≥ 0.5.}

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Abstract. — The room temperature Mossbauer spectra of the spinel compounds Fe2<i-2,)Mgi+j/Ti2,04

with compositions y = 0.5, 0.6, 0.7, 0.8 and 0.9 must be described by a sum of two doublet spectra. The two subspectra are assigned to actahedral and tetrahedral ferric ions. The cation distribution calculated from the doublet intensities, is in close agreement with the results obtained from other techniques. The isomer shifts of both subspectra remain constant with y, whereas the quadrupole splittings increase with increasing y. Thermal vacancies, introduced by quenching the samples from 1 150 °C, have no influence on the isomer shift but make the quadrupole splitting to incrase. The contributions of monopoles and oxygen dipoles to the electric field gradient (EFG) at the B sites are estimated. From a comparison with the results obtained in the normal spinels ZnFe204 and CdFe204, it is concluded that the oxygen polarizability depends on the space occupied by the oxygen ion.

1. Introduction. — The system values for y(y > 0.5). More recently, X-ray diffraction IU TV» — (\ — \ \n F n a n (* t n e r m° gr a vim et r i c a l analyses, reported by one of

y Mg2HU4 - (1 - y) Mgr-e2U4 t h e p r e s e n t a u t h o r s ^ h a v e confirmed these results

has the cubic spinel configuration. The metallic cations and conclusions.

are distributed among the tetrahedral (A sites) and In this paper, the influence of the structural proper-octahedral (B sites) interstitial sites of the close-packed ties on the Mossbauer parameters of the spinels oxygen ions. The cation distribution may be represent- Fe2 ( 1 -y$A.gx +J,Ti.,,04 has been investigated for

compo-ed by (Fe?-xMg^+) t F e ? i2 y+xM g i +y-xT i y+] 04 in sitions y 5* 0.5. All compounds show a paramagnetic

which the cations enclosed by ( ) occupy the A sites doublet spectrum at room temperature. The spectra while those enclosed by [ ] occupy the B sites. The were recorded on a time-mode spectrometer using a preference of the T i4 +i o n s for the octahedral coordi- constant acceleration drive and a Co57/Pd source,

nation is due to the principle of maximal charge

neutra-lization [1 ]. All menbers of the solid-solution series show 2. Results and discussion. — The room temperature ferrimagnetic properties. The Neel temperature decrea- Mossbauer spectra of the above mentioned compounds ses with increasing y. There are no indications that the consist of a slightly asymmetric quadrupole doublet spin structure should be other than collinear. Therefore, with broadened, non-Lorentzian absorption lines. For the distribution parameter x may be determined from all recorded spectra, an acceptable fitting function the magnetic moment at 0 K. Such calculations were could be obtained for a superposition of two doublets performed by Blasse [2] and by Tellier et al. [3]. Both with Lorentzian lineshape. An example of such a authors also mentioned the influence of the heat fitting is represented in figure 1 and refers to the treatment of the samples on the cation distribution, composition y = 0.7. The two subspectra were This influence was observed to disappear at higher assigned to tetrahedral and octahedral ferric ions on the

32

SITE OCCUPANCY, ISOMER SHIFT AND NUCLEAR QUADRUPOLE

INTERACTION OF

57

Fe IN Fe

2(1

_

y)

Mg

1+y

Ti

y

0

4

WITH y > 0.5

E. DE GRAVE, C. DAUWE, J. DE SITTER and A. GOVAERT University of Ghent, Laboratory of Magnetism, Proeftuinstraat 86,

B 9000 Ghent, Belgium

JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 12, Tome 37, Décembre 1976, page C6-497

Résumé. — Les spectres Môssbauer de quelques composés de structure spinelle ayant la formule générale Fe2(i-J,)Mgi+î,Tii,04 avec y = 0,5, 0,6, 0,7, 0,8 et 0,9 doivent être décrits par une

super-position de deux doublets. Ces doublets peuvent être attribués à des ions ferriques situés à des sites octaédriques et tétraédriques. La distribution cationique, calculée des intensités des raies des deux doublets, correspond bien avec les résultats obtenus par d'autres techniques. Le déplace-ment isomérique des deux sous-spectres se montre indépendant de la valeur de y, tandis que la décomposition quadrupolaire croît quand le taux de substitution y croît. Des vacances thermiques introduites par une trempe depuis 1 150 °C, n'influencent pas le déplacement isomérique, mais font croître la décomposition quadrupolaire. Les contributions par les monopôles et les dipôles d'oxygène au gradient du champ électronique aux sites octaédriques sont estimées. D'une compa-raison avec les résultats obtenus aux spinelles normaux ZnFe204 et CdFezO^ nous avons conclu que la polarisabilité de l'oxygène dépend de l'espace occupé par cet ion.

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C6-498 E. DE GRAVE, C. DAUWE, J. DE SITTER AND A. GOVAERT

FIG. 1. -Room temperature Mossbauer spectrum of slowly cooled Fe0.6Mg1.7Ti0.704. The full lines represent the quadru- pole doublets of tetrahedral and octahedral ferric ions and their

sum, fitted to the experimental spectrum.

basis of the line intensities. The calculated line widths and isomer shifts for both subspectra are listed in Table I, together with some structural data obtained from X-ray diffraction analyses [4]. All data refer to slowly cooled samples. The absorption lines of both subspectra have a full width at half maximum (FWHM) of about twice the natural linewidth of 57Fe. The local variations in ionic distributions and lattice distortions have the effect of making the electric field gradient to vary within a large range of possible values, corresponding to the different cation arrangements, and the spectral lines will therefore be broadened out. The ionic distribution parameter X, calculated from the area

ratio of the two doublets, is in close agreement with previously reported data [2,3,4]. The isomer shift data indicate a greater 4s-electron density for the A site ferric

ions than for the B site ones which is consistent with the results obtained in most spine1 ferrites [5]. The quadru-

pole splitting AEQ for both octahedral and tetra- hedral Fe3+ ions is plotted against the composition parameter y in figure 2. As observed in many other

compounds of different structure [6, 7, 81, the quadru-

pole coupling constant is greatest for the tetrahedral ferric ions.

o B

-

s ~ t e (slowly cooled sample1 m A

-

slte (slowly cooled sample)

o B - slte [quenched sample) m A- site (quenched sample1

FIG. 2. - The variation with composition parameter y of the quadrupole splitting AEQ of octahedral and tetrahedral ferric ions in quenched and slowly cooled samples of

Fez(l-~)Mgl+~Ti,04.

A second series of Mossbauer measurements at room

temperature were performed on samples with the same compositions but which were quenched from 1 150 OC

Chemical composition and 5 7 ~ e hyperfine parameters for slowly cooled samples of Mg, +,Fe,(, -,,Ti,O, with y 2 0.5 at room temperature

Y U(=) X(">

r~

G

8~ 6,

X-ray M. E. (mm/s) (mmls) (mmls) (mmls) (0.02)

(0.001)

('1

(0.04) (0.02) (0.02) (0.02) (0.02)

U = oxygen parameter.

x = number of tetrahedral Fe3+ ions.

r

= Iinewidth (FWHM). 6 = isomer shift (versus Pd). (a) Determined in reference [4].

(3 Assuming f~ = f ~ .

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SITE OCCUPANCY, ISOMER SHIFT AND NUCLEAR QUADRUPOLE INTERACTION OF 57Fe C6-499

and post-annealed for 15 minutes at 150 OC in order to reduce the number of.quenched lattice defects. The spectra were analysed in the same way and the obtained parameters show the same behaviour compared to the slowly cooled samples. The variation of AEo(A) and AEQ(B) with composition y is represented too in figure 2. The larger quadrupole splittings with respect to the corresponding slowly cooled samples, is explain- ed by the presence of thermal vacancies. The line widths however, seem not to be influenced by the expected random distribution of these vacancies. The isomer shifts 6, and 6, remain, whithin the experimental error limits, constant at respectively 0.09 4 0.02 mm/s and 0.12

,

0.02 mm/s. The distribution parameters X, derived from the intensities of the

Mossbauer lines, are, for all compositions, significantly lower than the corresponding values for the slowly cooled samples whereas, according to the X-ray diffraction analyses, the same results are expected. It is not clear whether the thermal vacancies are responsible for this discrepancy.

The observed quadrupole splittings are caused by the non-vanishing electric field gradient at the 57Fe nucleus. In the case of a ferric-ion, the EFG arises solely from the charges on distant ions and may be calculated by lattice summations. In an axially symmetric site, the quadrupole splitting may then be written as

in which A is the nuclear quadrupole moment (0.20 barns) and y, the Sternheimer antishielding factor (- 9.14) ; eq is the principal component of the EFG tensor and consists of two main contributions, arising respectively from the monopoles and the oxygen dipoles. All other contributions are small and are usually neglected [9, 101, partly in order to simplify the calculations, partly because the agreement with the experimentally observed quadrupole coupling constants can always be made acceptable due t o the great uncertainty of the magnitude of the oxygen dipole polarizability.

In order to investigate quantitatively the variation

of the two main contributions to the EFG as a function of the composition parameters y, we have calculated the monopole contribution eqM for the B site ferric ions in

the Fe2(,

-,,Mg,

+,Ti,O, compounds. For this purpose, the following simplifications have been introduced. 1) The B sites have trigonal symmetry due to the oxygen parameter U ; the principal a,xis of the EFG

is therefore along a local

<

11 1

>

direction ;

2) the charges z, and z, of the A- and B site cations are equal to the average value of the actual charges ;

3) the contribution of the quadrupole moments may be neglected.

The dipolar contribution eqD may than be found

from the difference between the experimental value

eq,,, which is assumed to have a negative sign [10, 1 l],

and the calculated monopole contribution eqM. The

results are presented in table 11. The B site ions make a positive contribution to eqM. Increasing the average

charge will therefore have the effect of making the calculated field gradient more positive. The A site ions yield a negative contribution and decreasing their charge makes eqM more positive too. These twe contri-

butions will therefore make the EFG more positive for higher values of y. The oxygen ions make a negative contribution, the magnitude of which increases rapidly with increasing oxygen parameter, i. e. with increasing y. The overall effect is that the calculated EFG becomes more negative at higher values of y.

The magnitude of eqD shows the tendency to decrease

with increasing y. This result is consistent with the lattice sum calculations of Evans et al. [10], applied

to the case of CdFe204, from which one may conclude that eqD becomes more negative with decreasing oxygen

parameter. The values of eqD obtained for our samples

are only qualitatively true due to the above mentioned simplifications. However, they are significantly lower than those for CdFe20, (eqD =

-

1.07 X 1014 esu)

and ZnFe204 (eqD =

-

1.02 X 10i4 esu). A possible

explanation for this behaviour may be found from the work of Kirsch et al. 1121 in which it was suggested that

Measured and calculated electric jield gradients at the B sites in Fe,(

,-,,

Mg,+,Ti,O, with y

2

0.5

Y ZA

(*l

ZB

(*l

U eqM eqexp eqD

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C6-500 E. DE GRAVE, C. DAUWE, 5. DE SITTER AND A. GOVAERT

the oxygen dipole polarizability increases with increas- ZnFe,04 shows the same oxygen parameter as ing space occipiedby the oxygen ion. A measure for the

available space is given by the parameter ro2- defined by :

in which

<

v,

>

is the average Goldsmidt radius of the B site cations and

dB-,

is the distance between the oxygen and B ion lattice sites, given by the expression :

with a the lattice parameter and 6 the displacement of the oxygen ions : 6 = U

-

0.375. The roz- values for the investigated compounds are listed in table 11. The dipole contribution eqD to the EFG at the B sites

FIG. 3.

-

Dipole contribution eqD to the EFG at the B sites of

Fe~(1-~)Mg1+E;ri~04 plotted versus the parameter roz

-

describing the space available for the oxygen ions. The results of CdFezO4

and ZnFezO4 are included for comparison.

of the Fe2~1_y,Mg1+,Tiy04 compounds is plotted against ro2- m figure 3. The results for ZnFe204

and CdFe,04 are also included, the eqD values of which are taken from Ref. [10]. The normal spinel

Refe [l] VERWEY, E. J. W. and HEILMAN, E. L., J. Chem. Phys. 15

(1947) 174.

[2] BLASSE, G., P h i l h Res. Rep. 3 (1964) 91.

[3] TELLIER, J. and LENSEN, M. Bull. SOC. Chim. F. (1966) 2502.

[4] DE GRAVE, E., DE SITTER, J. and VANDENBERGHE, R., Appl. Phys. 7 (1975) 77.

[5] VAN LOW, J. J., Physica 32 (1966) 2102.

[6] M ~ z o c u c ~ r , T. and TANAKA, M., J. Phys. Soc. Japan 18 (1963) 1301.

[7] NICHOLSON, W. J . and BURNS, G., Phys. Rev. 133 (1964) A 1568.

Fe, .,Mgl.5Ti,.50, whereas CdFe,04 and

have the same oxygen parameter and about the same A

and B site charges. The

<

rB

>

values are however markedly different. These results suggest that the size of the cations may play an important r61e in determining the magnitude of the oxygen dipoles and that the assumption of a space-dependent oxygen polarizability may indeed be true. It should however be emphasized that what has been presented above must be regarded only as a first and quite rough approxima- tion, especially because of the introduction of average

A and B site ionic charges. Therefore, more Mossbauer measurements on other, but much simpler spinel compounds would be very helpful in checking the model further on.

The expansion of the oxygen tetrahedra, described by the oxygen parameter U, does not disturb the cubic

point symmetry of the A site cations and is therefore not responsible for the observed quadrupole splitting of the tetrahedral ferric ions. There are however several other factors which may give rise to an electric field gradient at the spinel A sites : the presence of three kinds of cations with different charges, the appearance of small distortions of the oxygen tetrahedra due to the differences in charge and size of the B site cations, covalency effects such as charge transfer and overlap distortions [13]. I t must be emphasized that these factors may also influence in some extent the B site electric field gradient. Whether or not the above mentioned properties may completely explain the observed quadrupole splittings of the A site ferric ions will have to await further experimental studies and numerical calculations.

Acknowledgements.

-

The authors wish to thank Prof. Dr. G. Robbrecht for his continuous interest in this work. They are much indebted to F. K. F. 0. and I. W. 0. N. L. for financial support.

[8] DE SITTER, J., DAUWE, C., DE GRAVE, E., GOVAERT, A. and ROBBRECHT, G., to be published in Physica B. [9] HUDSON, A. and WHITFIELD, H. J., Mol. Phys. 12 (1967)

165.

[l01 EVANS, B. J., HAFNER, S. S. and WEBER, H. P., J. Chem. Phys. 55 (1971) 5282.

[l11 YAGNIK, C. M. and MATHUR, H. B., Mol. Phys. 16 (1969) 625.

[l21 Krrtsc~, R., G~RARD, A. and WAUTELET, M., J. Phys. C :

Solid State Phys. 8 (1974) 3633.