magnetic properties of some oxides with spinel structure bound... · magnetic properties of some...

.R 565 Philips Res. Repts, 20, 528-555, 1965

MAGNETIC PROPERTIESOF SOME OXIDES WITH SPINEL STRUCTURE

by G. BLASSE

AbstractCrystallographic and magnetic properties of a number of systems withspinel structure are reported. Some physical pecularities are the follow-ing: (a) The Curie constants in the system ZnMntFe2-t04 are found tobe too low compared with the spin-only values. This is ascribed to clusterformation. (b) At low temperatures ZnMn204 can be described as aone-dimensional antiferromagnet. (c) It seems probable that in thesystem ZnMn2-tGat04 clustering of MnH ions occurs at high valuesof t..(d) The critical amount of Mnê+ ions on B-sites necessary to givea macroscopie tetragonal distortion is very low in the systemZnMntFe2-t04 and very high in the systems Lio.sMntFe2.s-t04 andLio.sAI2.s-tMnt04. An explanation is proposed partly based on short-range order, partlyon oxygen-anion polarization. (e) In the systems.MnAltFe2-t04 and MnFe2-tRht04 deviations from the Néel model forsmall values of t could be explained with the aid of a model proposedrecently by Lotgering. (f) In the ternary system NiFe204-ZnFe204-NÎ2Ge04 the diamagnetic Zn2+ and Ge4+ ions, well known for theirtetrahedral-site preference, were found on both sites, depending on thecomposition. This demonstrates clearly that it is in general impossibleto deduce site preferences from the cation distribution of a smallnumber of compounds with spinel structure. The following systemshave been studied: Lio.sVtFe2.s-t04, Lio.s+o.StFe2.s-1.StMnt04;ZnMntFe2-t04; Lio.sMntFe2.s-t04; Lio.sAI2.s-tMnt04; CoMntFe2-t04;MnAltFe2-t04; MnFe2-tRht04; Lio.s+o.stFe2.s-1.StGet04 and theternary system NiFe204-ZnFe204-Ni2Ge04.

1. IntroduetionThe magnetic properties of mixed metal oxides with spinel structure have

been investigated very extensively (for a recent review, see ref. 1). Recently wehave reported a number of iron spinels containing V3+, Rh3+, AI3+, Ti4+ andSb5+ 1). This work will henceforth be denoted' as I. The present investigationis partly additional to, partly an extension of I.

In our study of V3+-substituted iron(III) spinels the replacement of Fe3+ inLio.5Fe2'504 by V3+ was not described. Recently an investigation ofthis systemhas been published by Lenglet and Lensenê). Their results are quite differentfrom what one would expect from our earlier results (I, pp. 43-60) and fromGarter's results on Cr3+-substituted Lio.5Fe2.504 3). This prompted us to re-investigate this system. The results fit very nicely with our expectations.. The replacement of Fe3+ in the practically normal spinel MnFe204 by dia-magnetic AP+ and Rh3+, which are expected to occupy the octahedral B-sites(see I, pp. 78-81 and 60-76) was carried out especially to investigate the toolow moment of MnFe204 (4·6 {LB instead of 5 (LB, see e.g. ref. 4).

MAGNETIC PROPERTIES OF SOME OXIDES WITH SPINEL STRUCTURE 529

The replacement of Fe3+in iron(III) spinels by Mn3+ has not been investigatedvery extensively. The present work takes care of a number of gaps in theliterature.The Ge4+-substituted iron(lII) spinels were prepared to study the tetrahedral-

site preference of the Ge4+ ion in the spinel structure.

2. Experimental

In the present paper the same experimental procedures were used as in I.The notation is also the same.

3. The system Lio,sVtFe2'S_t04

3.1. Experimental

Materials with the above general formula were prepared for several valuesof t (LiFe02, LiV02, V203, Fe203; 24 h. 950°C in vacuum). The compoundLiFe02 was prepared by heating LhC03 and Fe203 at 950°C in an atmosphereconsisting of an 80-~0mixture of O2and CO2; LiV02 was prepared .by heatingLi2C03 and NH4V03 at 750°C in H2. The preparation of V203 has beendescribed previously (I, p. 43).Chemical analysis ofthe total reducing power was carried out for a number of

samples. Results are given in table r. The X-ray-diffraction patterns show thatthe samples have spinel structure up to t = 2. Preparations for t > 2 were nolonger single spinel phase. Similar results have been reported by Gorter for thesystem Lio'5CrtFe2'5-t04 3) and by ourselves for the system Lio'5Fe'2'5-tRht04(I, p. 66). Th~ Li+vion distribution has been calculated from the intensities ofthe X-ray reflexions, assuming all V3+ions to be on B-sites and u = 3/8.

The saturation moments, Curie temperatures·, cell edges and amount of Li+on A-sites are presented in figs 1, 2 and 3. The a-T curves of the compositionst = 0'75,0'80 and 0·85 showed a compensation temperature (Néel's type N),those ofthe compositions t = 1·0and 1'25 showed a maximum (Néel's type P).

TABLE IChemical analysis of some compositions Lio'5VtFe2.5-t04

composition total reducing power (mgaeq/gramme)t found experimentally theoretical

0·5 4·7 4·891·0 9·9 9·891·5 14·6 15·02·0 19·8 20·3

530 G. BLASSE

o

I'-,'\._'~ J

,~"I I I I I Ol- I I

2

2

-2o

Fig. 1. Saturation moments ne for the system Lio.sYtFe2.s-,04o The dashed line is the onecalculated for all Li+ and V3+ ions on B-sites.

1 _t 2

Fig. 2. Curie temperatures and cell edges for the system Lio_sVtFe2-s-t04.

xt().4

0·2

00 1 -t 2

Fig. 3. Amount of Li+ ions on A-sites (x) for the system Lio.sVtFe2.s-t04.X from the value of the magnetic moment,<::) from the intensities of X-ray reflexions.

3.2. Discussion of the resultsThe results obtained for the system Lio·5VtFe2·5-t04are in good agreement

with what one should expect from earlier work on NiVtFe2-t041) andLio.5CrtFe2.5-t043).For t ~ 1 the cation distribution of the system can be described by the

formula


This follows from the small amount of Li+ ions on A-sites in this region (fig. 3).The agreement between the experimental values of the magnetic moments andthose calculated using the Néel coupling scheme 5) are good (fig. I). In thiscalculation we used the spin-only moment of the V3+ ion (2 (lB), as we didsuccessfully for the system NiVtFe2-t04, and all Li+ and V3+ ions were assumedto be on B-sites. Moreover, the variation of the cell edge with composition islinear and small, as in the system NiVtFe2-t04, which has a similar cationdistribution.

The magnetic moment ?hanges sign at a value of t of about 0·9. For valuesof t < 0·9 a-T curves with a compensation point were found; for values oft > 0·9 a-T curves with a maximum were found. This is the same behaviouras reported for the system NiVtFe2-t04 and must be ascribed to strong andnegative BB interactions of the type V3+-V3+ and/or Fe3+-V3+ (see I, pp. 51and 88).

In the region 1 < t < 2, and more specifically 1·3 < t < 1'7, the amountof Li+ ions on B-sites decreases drastically (fig. 3). Gorter reported the samephenomenon for the system Lio·5CrtFe2.5-t04. This author found kinks in theCurie-temperature vs composition curve and the cell-edge vs composition curve,which were ascribed to the change of the Lit-ion distribution. The same sortof curves have been observed for the system Lio.5VtFe2.5-t04. There is a de-viation from linearity in the Curie-temperature vs composition curve at t = 1·6and a rapid increase in the cell-edge vs composition curve between t = 1·2 andt = 1·7 (fig. 2). This rapid increase is explained by the fact that the spinel systembecomes normal, since a normal spinel has a larger cell edge than the analogousinverse one 6).

These results are now compared with those obtained by Lenglet and Lensenfor this system 2) .The latter authors prepared their samples in nitrogen.Chemical analysis was not mentioned. Their plots of nB vs t and Tc vs tagreewith ours, but all magnetic moments were plotted positive (B-sublattice-magnetization dominant). Their conclusions on the cation distribution (viz.V3+ ions partlyon A-sites) can therefore not be endorsed by us. If one assumesfor t > 1·5 all Li+ ions to be on B-sites, the magnetic moments calculatedaccording to the Néel-coupling scheme agree rather well with the experimentalvalues. However, this is in contradiction with the results of our X-ray measure-ments. More seriously is the discrepancy between the cell-edge vs compositioncurves of Lenglet and Lensen and this work (they report a maximum att = 0'75, which is not easy to explain). Finally we note that their samples werespinels up to t = 2'5, whereas we obtained spinels only up to t = 2.0 *).

*) We tried to prepare a spinel Lio.sV2.s04 by heating a mixture of Li2C03 and V203 in H2at various temperatures. All our attempts were unsuccessful.

532 G. BLASSE

4. The system Lio,s+o'StFe2'S-1'StMnt04

4.1. Experimental

Materials with the above general formula were prepared for several valuesof t (LiZC03, FeZ03, MnC03; 24 h. 750°C in Oz).

Chemical analysis of the oxidizing power was carried out for a number ofsamples (table II). X-ray-diffraction patterns showed the preparations to bepure spinels. The Li+don distribution was calculated from the intensities ofthe X-ray reflexions, assuming all Mn4+ ions to be on B-sites (see I, p. 136)

and u = 3/8.The saturation moments, cell edges and amount of Li+ ions on A-sites are

given in figs 4 and 5.

2""r-,

\\,,\

oo -tFig. 4. Saturation moments liB for the system LiO.5+0.5tFe2.5-1.StMnt04.

a(A)t

6--_I

"XI -

"~ I1 -

~ Î'-.n-

/ -,~In

V ........Ó 5

().s

o-t YJ

Fig. 5. Celledges and amount of'Li+ ions on A-sites (x) for the system Lio.5+0.5tFe2.5-1.5tMnt04(crosses from X-ray intensities; squares from magnetic moments).

Attempts to prepare the analogous system Nh+tFez-ztMnt04 in a similarway were not successful. The oxygen content of the materials obtained was far

too low.


TABLE IIChemical analysis of the system Lio'5+o.5tFe2.5-1.5tMnt04

% active oxygencompositiont .

found experimentally

1/32/314/35/3

2·35·27·812·215'7

theoretical

2·685·588·8112·616·2

4.2. Discussion

In I it was shown that the inverse spinel Fe [Lio.5Fe1-5]04 becomes"normal" on replacement of Fe3+ by tLi+ + ~Ti4+ (approximate distributionLit/2Fel-t/2 [Lio·5Fel-5-tTit] 04). Itwas also found that spinels containing Li+and Mn4+ ions tend to have a larger part of the Li+ ions on A-sites than theanalogous spinels with Ti4+ ions (compare e.g. Li [Coo'5Mnl'5] 04 andLio'5Coo'5 [LiO·5TÏ!.5]04).From these facts the system LiO'5+o.5tFe2.5-l'5tMnt04 is expected to have

a larger amount of Li+ ions on A-sites than the system Lio'5+o.5tFe2-5-l-5tTit04for the same values of t. This has been verified experimentally by us. Forexample, the amount of Li+ ions on A-sites for t = 1 in the Mn4+ system is0·6 and in the Ti4+ system 0·5 and for t = 1·4 it amounts to 1·0 and 0'7,respectively, Consequently, the ordered spinel phase (1 : 3 on B-sites) foundfor 0 < t < 0·4 and 1·2< t < 1·5in the Ti4+ system has not been encounter-ed in the Mn4+ system except for very low values of t. This absence of super-structure formation follows immediately from the cation distributions. Forexample, for t = 1·4 we detected 1 : 3 order in Lio'7Feo'3 [Lio.5Feo'lTÏ!'4]04;this is impossible for Li [Lio'2Feo'4Mnl-4]04.For t ~ 2/3 the amount of Li+ ions on A-sites was also calculated

from the value of the magnetic moment assuming the distributi.onLizFel-z [Lio'5+o'5t-zFel'5-lo5t+zMnt]04. The agreement between the valuesof x deduced in this way and those calculated from X-ray intensities is good(see fig. 5). For t = 1, however, a marked disagreement is found (X-ray:x = 0'6; n»: x = 0'3), so that somewhere between t = 2/3 and t -: 1 the.Néel-coupling scheme breaks down, probably due to the large amount ofLi+ ions on A-sites.For t > approx. 1·3 all A-site cations are diamagnetic (Li+) (the (220)

X-ray reflexion is absent). Nevertheless, ferrimagnetism has been found inthis region too. This ferrimagnetism differs from ordinary ferrimagnetism in

534 G. BLASSE

the spinel structure. In the latter magnetic coupling occurs between paramagneticions on A- and B-sites. Ferrimagnetism for t > 1·3 is due to antiferromagneticcoupling between Fe3+ and Mn4+ ions on octahedral sites. This phenomenonhas been and will be discussed in more detail elsewhere 7,S).

5. The system ZnMntFe2_t04

5.1.Experimental

Materials with the above general formula were prepared for several valuesof t (ZnO, MnC03, Fe203; 4 h. 1200 °C in air).Chemical analysis of the oxidizing power was carried out for all samples

(see table Ill). X-ray-diffraction patterns showed the preparations to be purespinels. For t ~ 0·50 these were cubic, for t;;;:: 0·75 tetragonally distortedwith cla > 1.

Magnetic susceptibilities X were measured against temperature T in theregion 80-1300 "K using the Faraday method. For ZnMn204 the measurementswere extended down to 4·2 "K, The x-1vs T curves were straight lines in alarge temperature region. The Curie constants and asymptotic Curie temper-atures are given in table Ill.In addition we prepared ZnMnO'5Ga1-504(ZnO, MnC03, Ga203; 4h.1150 °C

in air; cooled slowly and quenched from 1150 "C), ZnMnO'6Feo'6Gao'S04(ZnO,MnC03, Fe203, Ga203; 4 h. 1150 °C in air), ZnMgo'5Mno'5Feo'5Tio'504 (ZnO,MgC03, MnC03, Fe203, Ti02; 4 h. 1150°C in air) and LiFeo·5Mnl·504(LhC03, Fe203, MnC03; 24 h. 750°C in air). Chemical analysis and suscep-tibility measurements are presented in table Ill. X-ray analysis showed these

TABLE III

Chemical analysis and magnetic properties of some compositions ZnMntFe2-t04. and related materials

% Curie constantactive oxygen per mole spinel asymptotic region where

composition found theo- foundCurie Curie-Weiss

spin-only temperature law wasexper- retical exper- (Fe3+ + eK) found (OK)iment- iment-ally ally Mn3+)

ZnFel.sMno.s04 1.65 1-66 6'54 8·07 - 35 80- 800ZnFel.25Mno.7s04 2·47 2·49 6'39 7·72 -110 100- 800ZnFeMn04 3·32 3·33 6·54 7-38 -135 300- 900ZnFeo.sMnl.504 5'02 5·02 6·27 6·69 -285 100- 900ZnMn204 6·67 6·69 5·95 6·00 -420 900-1300ZnMnO.5Gal.504 1'54 1·53 1·50 1·50 - 95 80-1300Zn'Mno.s'Gar.eO« *) 1·54 1·53 1·48 1·50 - 60 80-1300ZnMno.oFeo.6Gao.S04 1·93 1'91 4·42 4·43 -110 300-1300ZnMgo.sMno.5Feo.s Tio.s04 1·80 1·81 3-41 3·69 -110 300-1300LiFeo.5Mnl.504 10·8s 11'02 4·55 5·56 -105 80- 800

-*) Quenched from 1150 °C into water.


a{..8.)9·2

8·8 s-i->a 7 3d2"c

~~: r~

1/~v.1, 1 2

8·4

8·0

MScIatt.fO

1.()s

MO-t

Fig. 6. Cell edges and cja ratio for the system ZnMntFe2-t04 (crosses from the present work,squares from ref. 9).

2

~v+--pI-

~.

V ~I-r+ ++

I »>p-I-

I I I I I I400 600 t200

oo 800 1(JJO-T(oK)

Fig. 7. Reciprocal susceptibility per gramme vs temperature for ZnMn204.

2

f-

V~

V~P

I- V~::::;::=-0.......-'

v--""-r fF'

t--.~ I I I I I200 400 600 1200oo

800 tOoo-T(W)

Fig. 8. Reciprocal susceptibility per gramme vs temperature for ZnMno.sFel.s04.

536 G.BLASSE

compositions to be pure spinels (all cubic). From the absence of the (220) and(422) reflexion in the X-ray diagram of LiFeo·5Mn1·504it is concluded thatthe distribution is Li [FeO.5Mn1.5]04.

Figure 6 presents the cell edges and cla ratio for the system ZnMntFe2-t04,fig. 7 the x-1 vs T curve for ZnMn204 and fig. 8 that for ZnFe1.5Mno·504.

The resistivity of all samples described amounts to 104-106 ncm.

5.2. Discussion

5.2.1. The compound ZnMnZ04. The magnetic susceptibility of ZnMn204 was reported for the first time byBangers 10).His measurements. were carried out in the region 80-850 "K. Inthe region below room temperature three measurements were performed only.Bçlow about 200 "K the susceptibility was temperature-independent andBongers assumed that below 200 "K the compound was antiferromagnetic.Because of this result it seemed worth while to study the susceptibility of thiscompound in a larger temperature region.

Figure 7 presents our results. The susceptibility is temperature-independentin the region 45-225 "K, but increases at lower and decreases at higher temper-atures. A Curie-Weiss law is followed at very high temperatures only (seetable Ill).

For an explanation of this behaviour it is necessary to start with a consider-ation of the structure: ZnMn204 is a tetragonally (cja> 1) distorted spine:with a = 8·09 À and c = 9·23 Á. In the spinel structure there are rows olcations in the [110], [110], [101], [101], [011] and [011] directions. In the uioand [110] directions (in the ab plane) the distance between these cations i:ta y2 = 2·86 A, in the other directions (in the ac and bc planes) the dlstanebetween the cations is t (a2 + C2)1/2 = 3·07 Á. In the ab plane the t2g orbital(da;y) are half-filled and the eg orbitals (da;2-y2) empty, so that, due to tlurelatively short distance, strong and antiferromagnetic interactions in the [110and [110] rows are anticipated (direct overlap of da;y orbitals, see e.g. ref. 11JIn the ac and be planes the t2g orbitals (da;z and dyz orbitals) are half-filled anithe eg orbitals partly half-filled (dz2) and partly empty (da;2-y2). It is not possiblto predict the sign of the interactions in these planes, but it seems quite surthat they will be considerably weaker than in the ab plane, because the knowstrong BB interactions in oxygen spinels are always due to direct t2g overla(see e.g. refs 1 and 11).At higher temperatures, however, ZnMnZ04 becomes cubic. According t

Rosenberg and co-workers the transition temperature is at about 1400 "K 12I Sinha and Sinha 13)have deduced that aspinel with paramagnetic ions on'on B-sites has a temperature-independent susceptibility below the Néel tempe


ature, if the interaction in the ab plane is stronger than in the ac and be planes(J12 and J13, respectively). The following formulae were found:

and

BA = 2 S (S + 1)z (jJ121 + 2Ihal)/3 k.

With the values found experimentally in this investigation, viz. BA = -420 OKand XM (T < TN) = 0.398.10-2, the following values of Jiz and ha are found:

J12/k = -42 "K and Jislk = -5 "K *).

The value of BA, however, is not very accurate, becausea Curie-Weiss behav-iour is only found at very high temperatures. From fig. 7 it is clear that ourvalue is only an upper limit, the real value probably being smaller. Unfortu-nately, susceptibilitymeasurements above 1300"Kare not easy to carry out. If asmaller value of BA is used in the Sinha-and-Sinha formulae, the ratio betweenj12 and h3 becomes still larger, e.g. for BA= -375 OK, Jsslk = -470 OKand JI3/k=0 OK. This corresponds to strong and negative interactions in therows in the ab plane and no interactioris between the rows, i.e. a one-dimensionalantiferromagnet. It seems therefore mo~eappropriate to use the model of a one-dimensional antiferromagnet than the Sinha-and-Smha model. We have used theIsing linear-chain model, although this is probably not correct..

Our experimental results (fig. 7) do not fit curves calculated from the Isinglinear-chain model as have been found e.g. for CuCl2 and CuBr2 15). It mustbe remembered, however, that ZnMn204 is not a completely normal spinel 12).If a small amount of Zn2+ ions are on B-sites the one-dimensional linear chainsare no longerinfinite. Moreover, the presence of some Mn3+ ions on A-sitesintroduces AB interactions. Indeed it was found that the behaviour of thesusceptibility below 45 OK was dependent on the cooling rate. For an annealedsample the susceptibility was smaller below 45 OK than for a sample cooledslowly. A correction for the non-infinity of the Mnê+ chains was appliedfollowing Haseda et al. 16), who corrected for the non-infinity of linear Cu2+chains in copper dihydroxyparaquinone by subtracting the low-temperatureeffectusing a Curie-Weiss law. Our experiinental results can then approximatelybe described by the expression for Ising's linear-chain model (fig. 9). In ourealculation we have used the simplified formulae of BarracIough and Ng 15),ziz,

X = (Nf.L2/kT) exp (2J/kT), if T ~ TN

') Rosenberg and Nicolae 14)have recently also studied ZnMn204 and reported J12lk =-15 oKand Jsslk = -24 "K, but presumably there is an error in their calculations.

538 G.BLASSE

Fig. 9. Molar susceptibility of ZnMn2û4.Full line (with crosses): experimental results.Dashed line: correction for non-infinity of linear chains.Dotted line: calculated for Ising's linear-chain model with Jlk = -165 oK.Circles represent experimental susceptibilities after correction.

and

• which are correct approximations for our system at not too lowtemperatures;J is the exchange constant in the chain. From :fig.9 it is seen that we have :fittedthe experimental data with TN = 330 "K (maximum in the curve) and, con-sequently, Jlk = -165 "K, because Jlk = -t TN, as can be found fromdifferentiation of the :first equation. The agreement between calculated andexperimental values is satisfactory, if one considers the approximations thathave been made. At higher temperatures a discrepancy occurs. This is nolunexpected because of the decreasing tetragonality of the compound at highertemperatures, which implies a breakdown of the linear-chain model.

The JIk value, -165 "K, should merely be considered as an order of magnitude for the exchange interaction. It is very large, but not unreasonable for ~strong and antiferromagnetic BB interaction. For MgV204 with 8A =-750 OK 1) Jik is about -100 OK (see also ref. 17). A more extended stud.on ZnMn204 (as normal as possible) seems necessary to confirm the model

5.2.2. The system ZnMntFe2-t04The spinels Zn [CoMn] 04 and Zn [NiMn] 04 were investigated by Bon

gers 10), who found that manganese is tetravalent and cobalt and nickedivalent in these compounds. The present study was undertaken to investigatthe charge distribution of ZIi [FeMn] 04.

From the resistivity ofthe samples (~ 104Qcm for 0 < t < 2, and 106 Qcrfor t = 2) and from the tetragonal distortion, which occurs already at t = 0·6~it is immediately clear that the combination Fe3+ and Mn3+ is stable in th:

MAGNETIC PROPERTIES OF SOME OXIDES WITH SPINEL STRUCTURE 539.

system with respect to the combination Fe2+ and Mn4+. This distortion isbrought about by the Jahn-Teller effect acting on the Mn3+ ions. The relativelylow value of t, for which a tetragonal phase has already been found, will bediscussed further in sec. 6.This system was investigated earlier by O'Keeffe 9). His lattice parameters

are given too in fig. 6 and it is seen that the agreement between the two setsof values is good. The results of our magnetic study differ considerably, how-ever, from those of O'Keeffe, which is probably due to the fact that he firedin oxygen, whereas we used air (O'Keeffe did not give chemical analyses).

The most remarkable result of the susceptibility measurements is that thevalue of the Curie constant is far too low compared with the spin-only valuefor Mn3+tFe3+2_t and that in some cases it is even equal to the Curie constantcalculated for Fe2+ and Mn4+.We assume that this fact is fortuitous, since theresistivity and crystallographic measurements clearly point to trivalent ions.

This result prompted us to investigate the Mn3+ ion and the combination Fe3+and Mn3+ in a diamagnetic dilution on B-sites. This was realized by studyingZnMnO.5Gal'504, ZnMnO'6Feo'6GaO'S04and ZnMgo'5Mno'5Feo'5Tio'504.In ZnMnO.5Gal·504 the Mn3+ ion has the correct moment (see table Ill).

The quenched sample has a considerably lower asymptotic Curie temperature.It does not seem very probable that this effect is caused by migration of Mn3+ ,ions to A-sites, since there is a large amount of Ga3+ ions, which have a muchstronger preference for A-sites (see also I). Nevertheless this possibility cannotbe excluded at the moment. Another explanation seems more reasonable, viz.clustering of Mn3+ ions on the octahedral sublattice. Goodenough andRogers 18) have proposed that Mn3+ ions on a lattice of non-Jahn- Teller ionsmay cluster, due to a dynamic or static Jahn-Teller effect. Quenched andannealed samples of ZnMnO'5Gal'504 were investigated to see whether it ispossible to find some influence of clustering on the susceptibility. It wasanticipated that in the quenched sample the Mn3+ and diamagnetic ions wouldbe distributed more statistically among the B-sites and that in the annealedsample some clustering would occur.In this connection the following simple calculation is illustrative. Each

B-site cation in the spinel structure has six nearest B-site neighbours, Let pdenote the probability for a nearest-neighbour ion of a fixed Mn3+ ion inZn [MnO·5Gal.5]04 to be Mn3+ too. We have considered the (partly hypo-thetical) cases:(a) p = 0 (complete ordering of Ga3+ and Mn3+ ions as in lithium-iron oxide,

Fe [LjO.5Fel.5]04;:b) p = t (statistical distribution of Gaê+ and Mn3+ ions am~)llg the B-

sublattice); .'c) p = t and i (representing a certain clustering);d) p = 1 (representing one large cluster in a diamagnetic host sublattice).

540 G. BLASSE

We assume that the susceptibility of an Mn3+ ion can be described by

Xn = 3·00/(T- 67 n),

where n is the number of neighbouring Mn3+ ions (and 6 - n the number ofGa3+ neighbours). This is not unreasonable in view of the results found forZnMn204 at high temperatures (C = 2·98 and (JA. ~ -420 OK). The total

susceptibility is then

n=6 6! .X = \' pn (1 - p)6-n Xn.i...J n! (6- n)!

n=O

A similar treatment has been used by Schinkel 30) in a study of manganese-containing borate glasses. Susceptibilities were calculated in the region500-1300 "K (i.e. the region in which the experiments, from which C and (JA

are determined, are carried out) and plotted as I/x vs T. From the straightline found in this way C and (JA were calculated. The results are

p C (JACK)

0 3·00 01 2·96 -11042 2·98 -21043 3·00 -31041 3·00 -400

This means that the asymptotic Curie temperature (measured in the region500-1300 OK) increases if a statistically distributed sample (p = 1) becomesmore or less clustered (p > 1), so that the experimental results can also beinterpreted as clustering of Mn3+ ions in the annealed sample. We shall return

to this point later.The deviation from the spin-only value 3·00 found for the probabilities

p = 1and ican be accounted for in the following way. For these values of fthe number of Mn3+ ions with no paramagnetic neighbour ions, is relativel;large. This means that at low temperatures the contribution to the totasusceptibility of the term with n = 0 (i.e. X= 3·00jT) becomes very largrcompared to the contributions with n =1= O. The X-I vs T curve deviates strongl:from a straight line at lower temperatures (convex curve towards T = 0 OK)This phenomenon is discussed in more detail by Havinga+") and Schinkel ê'')The effect is still present at higher temperatures for p = 1 and i and giverise to a too low value of the Curie constant, if this is deduced from experiment

at too low temperatures.Having found that the MnH ion in ZtiMnO.5Gal·504 has the correct momeni

we now turn to the combination Mn3+ and Fe3+ in a diluted sublattice. Frot


table III it is seen that for ZnMnO'6Feo'6GaO'S04the correct moment has beenfound and that the deviation in the case of ZnMgo'5Mno'5Feo'5Tio'504 is small.From a comparison of the asymptotic Curie temperature of these compounds(BA = -110 OK) and that of slowly cooled ZnMnO'5Gal'504 (BA = -95 OK)it follows that the Fe3+-Mn3+ BB interaction is very weak. The Fe3+-Fe3+BB interaction is also very weak as has been demonstrated on ZnFe204 20,21).

Below 300"K the X-I vs T curve of the two compounds curves towards theorigin (T = 0 OK). This must be due to the effect of "free" paramagnetic ionsas argued above. Because of the weak Fe3+-Fe3+ and Fe3+-Mn3+ BB inter-actions the Fe3+ ion in these compounds can be considered as free, so that theeffect is more pronounced than in ZnMnO.5Gal.504, where we did not observethe curvature at temperatures down to 80 "K,

With these results in mind we now return to the problem ofthe too low Curieconstants observed in the system ZnMntFe2-t04. A number of reasons will begiven why these moments may be too low. In view of the complexity of theproblem no further approach is made.

(a) If there is a small amount of Fe3+ ions on A-sites, magnetic clusters areformed due to strong Fe3+-Fe3+AB interactions. This results in a too lowmoment as has been shown by Lotgering for ZnFe204 20).Too low values ofthe Curie constant have also been found by GeIler et al. 22)for garnets withFe3+ on one sublattice. Here also there may be some Fe3+on the other sublattice,so that clusters result. Clusters are also formed by Fe3+A-Mn3+BAB inter-actions. Further there may be the influence of the temperature dependence ofthese strong magnetic interactions, by thermal expansion (discussed in sec. 5.2of I) as well as by the temperature dependence of the c[a ratio, either in theMn3+ clusters (for t < 0.65) or in the complete lattice (for t > 0.65) (comparethe discussion on ZnMn204).

(b) In a similar way there may be some Mn3+ ions on À-sites, so thatMn3+A-Fe3+Bclusters are possible. These give even more complications, be-cause the energy difference between the combinations Mn3+A-Mn3+BandMn2+A-Mn4+Bis very smaIl23,24).

(c) The cation distribution may change with temperature, as has been shownfor MgMn204 24).

(d) At higher temperatures the combination Fe2+ and Mn4+ may becomeoccupied. In view of Bongers' results on ZnCoMn04 and ZnNiMn04 it is clearthat the energy difference between Fe2+ and Mn4+ and between Fe3+and Mn3+cannot be very large. Above 800-900 "K the susceptibility of our samplesdecreases more rapidly than corresponds to the Curie-Weiss law found at

______ ~_I

542 G.BLASSE

lower temperatures (see fig. 8). This may be due to other valency states. Therelatively low values of the resistivities at room temperatures for 0 < t < 2(p Rj 104 Qcm) also points to the presence of some Fe2+ on B-sites. If onlytrivalent ions were present a higher value is expected for our stoichiometriesamples.

(e) At lower temperatures the susceptibility may behave anomalously due to"free" pa~amagnetic ions, as argued above.

These possibilities complicate the problem very much, so that no furtheranalysis was performed. Itis noteworthy, however, that for ZnMno '6Feo'6Gao·804the correct moment has been found. Here the probability of finding para-magnetic ions on A-sites is small because of the large tetrahedral-site preferenceof the Ga3+ ion. In the case of ZnMgo'5Mno'5Feo'5Tio'504 the diamagneticions Mg2+ and Ti4+ do not show a pronounced tetrahedral-site preference, sothat the deviating magnetic moment may be related to the larger probabilityof finding paramagnetic ions on A-sites.The present investigation demonstrates that the interpretation of Curie

constants of compounds with more than one kind of paramagnetic ions mustbe performed with care. This is illustrated again in the series of compoundsLi [Meo'5Mn1-5104 (Me = Fe, Co, Ni). In the case of Co and Ni there aredivalent and tetravalent ions 25). The Curie constant of LiFeo·5Mnl·504 is4,55, to be compared to the spin-only values 5·56 (LiFe3+o.5Mn3+o.5Mn4+04)and 4·30 (LiFe2+o.5Mn4+1.504).One is tempted to conclude that the lattercharge distribution is to be preferred. However, above 800 "K we observed thesame deviation from the Curie-Weiss law as found for samplesZnMntFe2-t04.It is not possible to account for this phenomenon using the distributionLiFe2+o'5Mn4+1.504, because then the susceptibility is expected to increasemore rapidly than corresponds to the Curie-Weisslaw.Atpresent the distribu-tion LiFe3+o'5Mn3+o.5Mn4+04is preferred, although the experimental Curieconstant is too low. From the very weak (220) reflexion, which is due toA-site ions only, it can be calculated that there is still a small amount (0-5%)of an ion that is considerably heavier than the Li+ ion on the A-sites (thecontribution of a deviation from cubic close-packing to the intensity was takeninto account by using u = 0,390, which value is probably even too large). Thissmall amount of paramagnetic ions on A-sites induces magnetic clusters, whichlower the Curie constant. .

6. The system Lio·sMntFe2,s-t04

6.1. Experimental - .Materials with the above general formula were prepared for several values

of t (Li2C03, MnC03, Fe203; 6 h. 1100 °C in air).

MAGNETIC PROPERTffiS OF SOME OXIDES WITH SPINEL STRUCTURE 543

Chemical analysis of the oxidizing power-was carried out on a number ofsamples. The results are given in table IV. X-ray-diffraction patterns show that

TABLE IV

Chemical analysis of the system Lio.5MntFe2.5-t04.I

composition % active oxygent found experimentally theoretical

0·50 1·90 1·930·75 2·84 2·911·00 3·75 3·881·25 4·80 4·851·50 5·79 5·851·75 6·79 6·842·00 7·55 7·81

the samples have spinel structure up to t = 1·75 and hausmanniet structure fort = 2. Preparations for t > 2 were no longer single-phased. The Li't-iondistribution has been calculated from the intensities of the X-ray reflexionsassuming u = 3/8.

The saturation moments, cell edges and Curie temperatures, and amount ofLi+ ions on A-sites are presented in figs 10, 11 and 12.

51

(b)/I

"I~

~ (a) x..../

~V ""1>--/

~ r-,

3

2

oo 0·5 1·0 1·5-t 2·0

Fig. 10. Saturation moments nn for the system Lio.sMntFe2.5-t04.(a) experimental (crosses); .(b) calculated from the cation distribution found from Xvray-reflexion intensities;(c) calculated for all Li+ and Mn3+ ions on B-sites. .

544 G.BLASSE

fJ.43

fJ

600 8·39

40

8'35

1·0 1·5 2·0_tFig. 11. Curie temperature (Tc) and cell edges (a) for the system Lio.sMntFe2.s-t04.

005

~//

i/I i!

Rh/j !!

1,

/ ~n /// /

~_"./Cr,V

J..-.............

x

i0·40'3

0·2

04

oo ().s 1-0 1·5-t

2·0

Fig. 12. Amount of Li+ ions on A-sites (x) for the system Lio.sMntFe2.s-t04. The resultsfor the analogous Cr, V and Rh systems are given too (after refs 1, 3 and sec. 3 of this work,respectively).

6.2. Discussion of the resultsI

As in the system LiO'5Fe2.5-tMet04 (Me = V, Cr, Rh) a sudden migrationof Li+ ions from B-sites to A-sites has been found (fig. 12). Itwas discussed insec. 3.2 that the kink in the Tc vs t curve and the sudden increase in the a vs tcurve (fig. 11) can be ascribêd to this migration of Li+ ions.It is noteworthy that in the case of MnH this migration occurs at lower

t values than in the case of V3+ and Cr3+. In I we developed a theory which. states that the distribution in these types of systems is more normal accordingto the substituting ion being more covalently bonded. The covalent characterofthe Mn3+-02-bond is expected to resemble that ofthe V3+-0Z- and Cr3+-02-


bonds and not that of the Rh3+-02- bond. Two explanations can be offered.(a) If the Mn3+ ions form clusters on octahedral sites (compare sec. 5.2.2), the1 : 3 order of Fe [LiO.5Fel·5]04, which is the main reason why Lio.5Fe2'504is inverse (compare I) is spoilt and the Li+ ions migrate to A-sites.(b) If at higher temperatures Fe2+ and Mn4+ ions are formed (compare sec.5.2.2), the system at the reaction temperature contains a large amount ofMn4+ ions, due to which fact the Li+ ions migrate to A-sites (compare I andsec. 4.2). At lower temperatures ionic migration is too slow to maintainequilibrium. At room temperature all Mn3+ and Fe3+ ions will be trivalent,the resistivity of all samples being lOQ06 Dcm.Due to the migration of Li+ ions to A-sites, the superstructure of

Fe [LiO.5Fel.5]04 is spoilt at low values of t: only at t = 0·25 we observedvery weak superstructure reflexions.

The values ofthe magnetic moments calculated according to the Néel model,assuming all Mn3+ ions to be on B-sites and the maximum amount of Li+ ionscompatible with the X-ray results to be on A-sites, are lowerthan the experi-mental values (fig. 10). This may be due to a certain amount of Mn3+ ions onA-sites. However, this amount must be relatively large, thedifference betweenthe Mn3+ and Fe3+ moment being small (e.g. for t = 0·5 it must be 0,25, i.e .

. 50 % of the Mn3+ ions present). Another possibility is that the Néel-couplingscheme is locally no longer valid if clustering of Mn3+ ions occurs. A distri-bution with Li+ ions on B-sites and Mn3+ ions on A-sites is discarded in viewof the X-ray results, although the variation of the magnetic moment with tcan be explained in this way.It is also striking that the system Lio·5MntFe2.5-t04 remains cubic up to

t = 1·75. It has been found (see e.g. ref. 11) that the critical amount of trivalent,manganese on B-sites necessary to give a macroscopie tetragonal distortion isabout t = 1'2; ZnMlltFe2-t04 with critical value t = 0.65 is a remarkable'exception to one side, Lio·5MntFe2.5-t04 to the other side. For t = 1·75nearly all Li+ ions are on A-sites. This means that the critical value of t is inbetween t = 1·75 (if there are only Li+ and Fe3+ ions on A-sites) and t = 1·25(if there are only Li+ and Mn3+ ions on A-sites). The latter limit seems ratherimprobable.

In our opinion the transition cubic -+ tetragonal is not only determinedby a certain critical amount of Jahn-Teller ions, but also by other effects, of .which two are mentioned here:Ca) If the possibility of short- or long-range order between different ions exists(i.e, if there are cations with different charge and/or ionic radius), the criticalamount of Jahn-Teller ions necessary to give rise to macroscopie deformationis larger than without the possibility of order, because order prevents contactbetween the Jahn-Teller ions or possible clustering of these ions. This isdemonstrated on the following examples (taken from ref. 11):

546 G.BLASSE

Zn [Cr3+Mn3+]04 cla = 1·04Mn2+ [Cr3+Mn3+]04 cja = 1·05Zn . [Ga3+Mn3+)04 cja = 1·05Zn [Fe3+Mn3+]04 c[a = 1·06

Li [Mn3+Mn4+]04Ga3+ [Ni2+Mn3+]04Zn2+ [Mgo.5Mn3+Tio'5]04

cla = 1·00cla = 1·00cja = 1·00

In the first column ions with equal charge and radius occupy the octahedralsites and 50% of MnH is already enough to bring about a considerabledistortion. The low critical amount of Mn3+ in the system ZnMntFe2-t04 (32%)may be due to this effect. On the other hand the presence of ions with differentcharges may induce lower-symmetrical fields at the, site of the Mn3+ ion.For Cu=a similar effect is observed: Ga [CuGa] 04 26)and Zn [CuTi] 04 27)

are cubic. For the system Ge [C02-tCUt] 04 a critical value t = 0·25 has beenreported 2B).Here there are only ions with the same charge and radius on theB-sublattice. The compound CuFe204 seems to be a special case (distortiondue to Jahn-Teller and spin-orbit effect, see ref. 11).(b) If the anions surrounding the central Jahn-Teller ion are completelydifferently polarized, the cubic symmetry is lost. If this symmetry loweringoccurs in arbitrary directions no cooperative Jahn-Teller distortion can beexpected. This is illustrated by LiO'5Mnl·75FeO·7504,which is still cubic. Theapproximate cation distribution is Lio·45Feo·55[Lio.05Mnl.75Feo.20]04 (inter-.change of Mn and Fe among A- and B-sites may occur). Nearly all B-ionsare trivalent, and approximately one half of the A-ions is monovalent and theother half trivalent. No superstructure reflexions were observed so that onA-sites short-range order probably occurs. This means that the oxygen ionsare partly surrounded by Li(A) + 3Me3+(B) and strongly polarized, andpartly by Me3+(A) + 3Me3+(B) and weakly polarized (cf. sec. 1.3.4 of I). Ifthe octahedron of ligands distorts with cla > 1, the dz2 orbital is half-filledand the dZ2_y2 ~rbital empty. However, if the ligands on the z-axis are stronglypolarized towards the central cation, the electron may still prefer the dZ2_y2

orbital and the central Jahn-Teller ion is "lost" for a cooperative distortion.This effect may explain the large critical value (t = 1·75)found for the systemLiO.5MntFe2.5:"t04. Because of this we also investigated the analogous alumi-nium system.

6.3. The system Lio·5Alz.5-tMnt04, Materials with the above general formula were prepared for several valuesof t (LizC03, Al(OH)3, MnC03; 6 h. 1150 °C in air) ..X-ray analysis shows

(a) samples with t ~ 0·25 are single-phased spinels;(b) samples with 0·25 < t < 0·50 contain two spinel phases;


(c) samples with 0·50 < t ~ 1·55are single-phased (cubic) spinels;(d) samples with t ~ 1·75 were no longer single-phase.

The miscibility gap found on the aluminium-rich side of this system isanalogous to that found for Lio·5A}z.5-tFet0429)and is probably due to thedifference between the ionic radii of A}3+and MnH (see J, p. 82).As in the system Lio·5MntFe2.5-t04 the critical amount of Mn3+ necessary

for macroscopie distortion is very large (t > 1·50),which may be ascribed tothe different polarization state of the anions.

7.1. Expertmental

Materials with the above general formula were prepared for several valuesof t (CoC03, MnC03, Fe203, 4 h. 1200 °C in air). -Chemical analysis of the active-oxygen content was performed for the

l composition CoMnFe04 (found experimentally 3·39%, theoretical 3·42%).X-ray analysis showed the samples to be pure spinels. The sample with

t = 1 is still cubic, those with t ~ 1·5 are tetragonally distorted (cia> 1). .Resistivities were measured in the region 0 < t < I' on sintered discs. The

results aret = 0·25t = 0·50t = 0·75t = 1·00

p = 107 ncm,p = 105 ncm,p = 104ncm,p = I04·ncm.

Saturation moments, cell edges and Curie temperatures are presented infigs 13 and 14.

2

I--' -,\\

, , , , ~2

Fig. 13. Saturation moments nu for the system CoMntFe2-t04:

_-

'548 G. BLASSE

1_tFig. 14. Curie temperatures (Tc) and cell edges (a) for the system CoMntFe2-tû4. For t ;;. 1'5the value of (a2c)113 has been plotted.

7.2. DiscussionAn interpretation of the experimental data is not very easy, because all

cations present can have different valencies (Mn : 2, 3 and 4; Fe : 2 and 3;Co : 2 and 3). From the relatively high resistivity in the iron-rich region of thesystem it is concluded that the samples do not contain ions of the same elementwith different charge (see also ref. 41). From the saturation moments it isconcluded that the substitution of Mn for Fe in CoFe204 does not take placeaccording to the distribution Fe [CoMntFel-t], because then the momentwould decrease with increasing t, whereas it tends to increase. In view ofprevious work on substituted CoFe204 (see I) it seems even probable that partof the C02+ ions migrate to A-sites on manganese substitution. At the presenttime it seems impossible to interpret the results of this complicated system anyfurther.

8. The system MnAltFe2_t04

8.1. ExperimentalSamples with the above general formula were prepared for several values

of t (MnC03, AI(OH)3, Fe20a; 4 h. 1300 °C in nitrogen, containing traces of

-tFig. IS. Saturation moments nn for the system MnAltFe2-t04. Straight line calculatedaccording to the Néel-coupling scheme assuming all AI3+ ions to be on Bvsites.

- .


oxygen). Chemical analysis showed all samples to contain a trace of activeoxygen (i.e. Mn3+), but never more than 0·05%. Xvray analysis showed allsamples to be pure spinels.Saturation moments, cell edges and Curie temperatures are presented in

figs 15 and 16.

-tFig. 16. Curie temperatures (Tc) and cell edges (a) for the system MnAltFe2-tû4.

'.------.------. a(Á)

1---*__ +--_--l8.s1

8.2. Discussion

The compounds MnAl204 and MnFe204 are both nearly normal spinels 31,32).From fig. 15 it is seen that in a first approximation the saturation momentsfound experimentally can be accounted for using the Néel-coupling scheme andthe cation distribution Mn2+1-aFe2+1l[Mn2+aFe3+2-Il_tAI3+t] 04. Trivalentmanganese and divalent iron are excluded in view of the work of Lotgering 33).

For 0 < t < 0·6 the moments found experimentally are lower; for0·6 < t < 1·2 they are higher than those calculated. The too low moment ofMnFe204 has been explained by Lotgering 34)by assuming that those octa-hedral Mn2+ ions, which are surrounded by tetrahedral Mn2+ ions only, areantiparallel to the octahedral Fe3+ ions, i.e. the Mn2+-Fe3+ BB interaction ismore strongly negative than the Mn2+-Mn2+ AB interaction. The deviationfound by us for lower t values can also be explained in this way. At highervalues of t another effect obscures this phenomenon, viz. the migration of part ofthe A13+ions to A-sites observed in all Me2+Ab04 spinels and Me2+AltFe2_t04systems (see e.g. refs 1 and 31). Migration of diamagnetic ions to A-sites resultsin a higher magnetic moment. It seemed worth while to study the systemMnFe2-tRht04, because Rh3+ is also diamagnetic and does not enter A-sites(see e.g. I). For this system we expect therefore only too low moments comparedto the calculated ones.

9. The system MnFe2_tRht04

9.1. Experimental

Samples with the above general formula were prepared for several values

550 G.BLASSE

of t (MnC03, Fe203, Rh203; 24 h. 1150 °C in air). Chemical analysis showedall samples to contain maximally 0·05 % active oxygen. X-ray analysis showedthem to be pure spinels.

Saturation moments, cell edges and Curie temperatures are presented infigs 17 and 18.

Fig. 17. Saturation moments ns for the system MnFe2-tRht04. Straight line calculatedaccording to the Néel-coupling scheme assuming all Rh3+ ions to be on Bvsites.

Tcr~~-----+----~~1800i---------1~

1 -t.Fig. 18. Curie temperatures (Tc )and cell edges (a) for the system MnFe2-t~ht04.

9.2. DiscussionFrom fig. 17 it is seen that the saturation moments found experimentally are

lower than those calculated according to the Néel-coupling scheme for thedistribution Mn2+1-sFe3+s [Mn2+s Fe3+2_8_tRh3+t] 04 in the region 0 < t << 0,5, equal to the calculated values in the region 0·5 < t < approx. 1·4 andhigher than the calculated values in the region t > 1·4. This agrees very welwith what one should expect. In the region 0 < t < 0·5 there is obviousljsome Mn2+ on B-sites and part of these will be antiparallel to the Bvsublatticraccording to the Lotgering model. For higher Rh3+ concentration, however


all Mn2+ are expected to be o,n A-sites, because inverse spinels always becomenormal on Rh3+ substitution (see I). Moreover, the BB Interactions becomeweaker due to diamagnetic dilution. Consequently, there is no reason why thereshould be a deviation from the calculations following the Néel-coupling scheme.The fact that this scheme is not obeyed at high values of t is not unexpectedbecause of the low concentration of paramagnetic ions on B-sites. The com-pounds MnAh04 and MnRh204 have both been reported to be antiferro-magnetic, due to AA interactions 31,35).

10. The system LiO'5+0'5tFe2'5_1'5tGet04

Materials with the above general formula were prepared for several valuesof t (Li2C03, Fe203, Ge02, 24 h. 1100 °C in 02). X-ray analysis showsthe samples to be single-phased spinels up to t = 0·25. Preparations fort > 0·25 were no longer single-phased. The non-existence of LiFeGe04 wasreported earlier i). The relative intensity of the (220) reflexion increases forincreasing values of t, so that the greater part of the Li+ ions must be onB-sites.

The saturation moments are given in fig. 19. From the good agreement

2

-"""-=---- ~,.-:::::'-.

<,~<,-.

04 0.2_tFig. 19. Saturation moments nu for the system Lio.5+0.5tFe2.5-1.5tOet04.The straight linesare calculated from the Néel-coupling scheme assuming that(a) all Li+ ions are on B-sites and all OeH ions on A-sites,(b) all Li+ and OeH ions are on B-sites.

between the experimental values and those calculated for the distribution

Fe1-tGet [Lio'5+o.5tFe1.5-0.5t]

it is concluded that all Ge4+ ions are on A-sites. Similar results have beenobtained for Ge4+-substituted NiFe204 and CoFe204, although a small partof the Ge4+ ions is on A-sites in these systems 1,36).

552 G.BLASSE

11.1. ExperimentalMaterials whose composition is within the triangle NiFe204-ZnFe204-

NhGe04 were prepared in the usual way (NiC03, ZnO, Fe203, Ge02; 24 h.11500C in 02). Samples were left to cool in the furnace. Compositions in-vestigated are indicated in fig. 20 by small circles. The binary systems NiFe204-ZnFe204 and NiFe204-Ni2Ge04 have been investigated earlier 36,37).X-ray analysis showed that a miscibility gap occurs in this spinel system as

indicated (approximately) in fig. 20).

Fig. 20. The ternary system NiFe204-ZnFe204-NÎ2Ge04. The characters A, B, C and Dareexplained in the text.

Saturation moments, cell edges and Curie temperatures for compositions orthe joins NiFe204-A and B-C (compare figure) are presented in figs 21,22 am23. For Nio.5Zno.75Fel.5Geo.2504(D in fig. 20) we found nB = 2·65 !JoB antTc = 450 oK.

11.2. Discussion of the resultsThe occurrence of a miscibility gap in the system ZnFe204-Ni2Ge04 wa

explained earlier in I.The Madelung constant (see fig. 1.4 of I) shows a markeminimum in this series and, moreover, anion polarization is favourable for thlimiting compositions of the system, but not for intermediate compositions.

The investigation of the system Zn [Fe2104-Ge [Ni2] 04 was originallset up in order to study the Ni2+-Fe3+BB interaction (see also ref. 42). Th


Fig. 21. Curie temperatures (Tc) and cell edges (a) for the systemNio.25+l.5tZnO.75-o.75tFe2-1.5tGeO.75t04(join BC in fig. 20).

4

:3

Idl)//!

!/(b)

I IoFig. 22. Saturation moments ne for the system NiZntFe2-2tGet04;(a) calculated for all Zn2+ and Ge4+ ions on A-sites,(b) calculated for equal amounts of diamagnetic ions on A-sites and B-sites (e.g. Zn2+ onA-sites and Ge4+ on B-sites).

600

~<,

_u,.<Tc""

I I0·2 04-t

Fig. 23. Curie temperatures (Tc) and cell edges (a) for the system NiZntFe2-2tGet04.

should be possible if the diamagnetic Zn2+ and Ge4+ ions remain on A-sites.The strongly magnetic behaviour of Nio·5ZnO.75Fel.sGeO'2S04(D in fig. 20)points to the contrary. On the other hand, the miscibility gap prevents an

554 G. BLASSE

investigation of the complete system. Therefore, it was decided to study thejoin BC of fig. 20 (Nio.25Zno.75Fe204-Nh.75Feo.50Geo.7504).It has been foundearlier (see e.g. refs 1, 36 and 37) that for the limiting compositions Band Cnearly all diamagnetic ions are on A-sites. The marked maximum of the Curietemperature along thejoin BC (fig. 21)points to a partial migration of Feê+ionsto A-sites and of diamagnetic ions to B-sites.

A similar conclusion can be drawn from the results for the join NiFe204-A(NiZntFe2-2tGet04) (figs 22 and 23). For low values of t all diamagnetic ionsoccupy A-sites, but for higher values of t part of the diamagnetic ions occupyB-sites too.

Unfortunately it is not possible to differentiate between Zn2+ and Ge4+ionsby X-ray analysis, but nevertheless the following description of the cationdistribution of the ternary system seems obvious:(a) Ni2+ is always on B-sites;(b) in the NiFe204-rich region Zn2+ and Ge4+ are on A-sites;(c) in the Znf'e-Oa-rich region Zn2+ is on A-sites and Ge4+ on B-sites;(d) in the NhGe04-rich region Ge4+ is on A-sites and Zn2+ on B-sites.To account for this distribution we use the conclusion of I: the cation

distribution is determined mainly by polarization and site preference. In theinverse NiFe204 polarization is not very important and the ions are distributedaccording to their site preference, i.e. Zn2+ and Ge4+ (both 3d10) on A-sites(see I). In the normal Zn [Fe2] 04 polarization is important and can bemaintained only if high-charged ions (like Ge4+) enter Bssites, whereas inGe [Nie] 04 polarization is important too, but can be maintained only if low-charged ions (like Zn2+) enter B-sites.Perhaps the most important conclusion ofthis section is that it is not possible

to determine site preferences from the cation distribution of spinels as has beendone before many times (see e.g. refs 38 and 42). This may be illustrated by thefact that to our knowledge only four cations have been found on one and thesame sublattice in a number of compounds with spinel structure, viz. Cr3+ 39),

Mn4+, Rh3+ and Sb5+(see I and this work). These ions occupy exclusively octa-hedral sites, in other oxides too. For the first three ions this will be due to theirvery large octahedral-site preference, for the latter one anion polarization willbe mainly responsible for this fact, at least in the spinel structure (compareref.40).

Acknowledgement

The author gratefully acknowledges the experimental assistance of MissG. E. C. M. van den Berg and Mr D. J. Schipper (preparation), Mr J. F. Fastand Miss A. Reitsma (measurements) and Mr J. Visser (analysis).

Eindhoven, July 1965


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magnetic properties of some oxides with spinel structure bound... · magnetic properties of some...

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