Proc. Indian Acad, Sci., Vol. 87A (Chemical Sciences-S), No. 10, October 1978, pp. 331-335,© printed in India.

Electrical and magnetic properties of the system LaNit- xFex0 3

OM PARKASHAdvanced Centre for Materials Science, Indian Institute of Technology,Kanpur 208 016

MS~received 7 July 1978

Abstract. Electrical and magnetic properties of the system LaNi1_xFexOs (0<xsO'3)have been investigated. The system shows metallic behaviour similar to LaNiOsupto x=O'l while the sample with x=0'3 shows semiconducting behaviour. Oxygenpartial pressure dependence of electrical resistivity is noticed in the compositionLaNio.9Feo.10s·

Keywords. Spin state equilibrium; perovskite oxides; electrical resistivity; Seebeckcoefficient; lanthanum nickelate, lanthanum ferrite.

1. Introduction

Transition metal oxide perovskites of the type ABOa where A is a rare earth ion and Bis a transition metal ion have attracted considerable attention in recent years becauseof their interesting electrical and magnetic properties (Goodenough 1963, 1966, 1967,1972, 1974; Frederikse 1970; Rao and Subbarao 1970; Rao 1974; Subbarao et al1971) as well as potential applications as components of solid electrolyte fuel cells(Tedmon et al 1969), as cathode materials in alkaline solution zinc air batteries(Meadowcraft 1970; Kudo et a11975) (used in urban transport) and as active cata­lysts for the oxidation of carbon monoxide in auto-exhaust gases (Libby 1971; Voor­hoeve et aI1972; Om Parkash et a11974) and so on. Electronic and magnetic pro­perties of these materials are largely determined by the spin state S, of the transitionmetal ion and this aspect has been discussed in detail in the literature (Goodenough1967; 1974; Rao and Subbarao 1970, Rao 1974). Accordingly, LaNiOa with lowspin Nis+ in S=·! state is Pauli-paramagnetic metal due to the presence of itinerant­d-electrons while LaFeOa with high-spin Fes+ in the S = 5/2 state is an anti-ferro­magnetic insulator due to presence of localized d-electrons. LaCoOs with Co'"in the low (S=O) or the high (S=2) spin states exhibits localized or itinerant pro­perties of d-electrons depending on the conditions. Recently, we reported results ofour studies (Rao et al 1975) on LaNi1_xCoxOa and LaC01_xFexOs over a widerange of compositions and these studies clearly showed the interplay of localizedversus itinerant behaviour of d-electrons in these systems. Thus, LaNi1_xCoxOswith x~0'5 shows itinerant electron behaviour while compositions with x> 0·5show behaviour of localized electrons. We considered it interesting to investigatethe electronic and magnetic properties of the system LaNi1_xFexOa wherein a high­spin ion (Fe'") is incorporated into metallic LaNiOa containing low-spin Nis+.

331

332 Om Parkash

Our interest in the LaNil_xFex03 system was further strengthened by the possi­bility of using these materials as interconnection materials in fuel cells. Interconnec­tion materials must satisfy the following criteria: (a) they must be stable both in airand fuel atmospheres, (b) they should be good electronic conductors, (c) they shouldnot react with other components of the battery, (d) they should not contain pores.Kleinschmager and Reich (1972) have reported that the electrical resistivity of thesystem, LaOl.sl Ni0lo5 1C00l.33 (= 10 91) varies only marginally with oxygen partialpressure. This composition of KIeinschmager aud Reich (1972) roughly correspondsto LaNio.9 COO•l 03' It was, therefore, considered worthwhile to actually preparethe solid solution in the related system LaNil_xFexOa and investigate its electronicproperties.

2. Experimental

The members of the system LaNil_xFex03 were prepared using lanthanum oxalate,nickel oxalate and ferric oxide (all of better than 99·99 %purity) as starting materials.Stoichiometric quantities of these materials were mixed and the mixtures decomposedby heating slowly in air up to 975 K. The resulting mixtures were dissolved in aqua­regia and the solutions evaporated to dryness. The solid residue was dissolved inwater and this solution was dripped very slowly into a large excess of concentratedsodium carbonate solution under constant stirring. The precipitate was filtered andwashed with water a number of times. The precipitate was dried and heated slowlyin air at 1075 K. The resulting material was digested with hot water a number oftimes to remove Na- ions. It was then dried, pelletized and fired at 1175 K in anoxygen atmosphere for 24-48 hr. Compound formation was confirmed by recordingx-ray diffraction patterns using a GE XRD-5 diffractometer.

TGA was recorded using MOM derivatograph at a heating rate of lOo/min. Resis­tivity measurements were made on thick sintered pellets employing the four-probetechnique. Seebeck coefficients were measured relative to platinum using a set-upfabricated locally. Magnetic susceptibility measurements were carried out employinga Gouy balance.

3. Results and discussion

All the samples studied were found to have rhombohedral structure. The lattice para­meters of the system LaNil_xFexOa are listed in table 1. We see that aR and aR are

Table 1. Lattice parameters of LaNi1_xFexO.

LaNh_xFexO. QR,AO OR°X

0 5'41 60'690'01 5043 60-620'05 5044 60-550'10 NO 60'730'30 5048 60-94

Electrical and magnetic properties of the system LaNi1_xFe:Pa 333

nearly constant. This is probably because Nis+ and Fes+ have nearly equal ionicradii.

Plots of the logarithm of electrical resistivity (log p) against the reciprocal of abso­lute temperature for the LaNi1_xFexOs (0'0<x~0'3) system are shown in figure 1.The results clearly show that incorporation of the high-spin Fes+ ions into LaNiOahas more drastic effects than the incorporation of Cos+ ions into LaNiOa•LaNi1_xFexOa remains metallic only up to x ~ 0·1 while LaNi1_xCoxOa exhibitsmetallic behaviour up to x=0'5 (Rao et at 1975). When x=0'3, LaNi1_xFexOabecomes semiconducting although the temperature dependence of resistivity is notvery significant. The resistivity behaviour of this sample (x =0,3) is interesting, theresistivity is essentially temperature independent at low temperatures and decreasesonly slightly at higher temperatures.

Seebeck coefficients of LaNi1_xFexOa are plotted against temperature in figure 2.Consistent with the resistivity data, the material behaves like the parent LaNiOswhen x<0'3 over the entire temperature range of 400-1000 K (withn-typebehaviour).However, the composition with x=0'3 shows a distinct increase in Seebeckcoefficientwith increase in temperature at higher temperatures. Increase of a with T in this

1

E<.>

E;c0 '2Cl..CJI0-'

3

x =0·30

'2310001 T (K-1)

Figure 1. Plot of logarithm of electrical resistivity versus reciprocal of absolutetemperature for LaNi1_xFexOa.

o0...~ -20>~

o -40

~=0.30~c~o 0·10

00 0.01

400 600 800 1000 1200T(K)

Figure 2. Variation of Seebeck coefficient with temperature in LaNi1_xFexOa.

334 Om Parkash

sample is suggestive of an electron hopping mechanism operating at higher tempera­ture in this sample.

Magnetic susceptibility data LaNi1_xFex03 have been analysed in terms of plotXT versus temperature as shown in figure 3. Such X T-T plots are known to reflectvariation of spin state population in such a system (Rao et aI1975). When x<0·3,the plots are linear with positive slopes just as in the case of LaNi1_xCox03 (x <0,5).LaNi1_xFex03 with x=0'3, shows an increasing trend of xT with increase in tempe­rature (with a slope similar to samples with x<O·3), but xT seems to approcah a

40

• x=0·30

'2' 30Kt-*20

10

0

Figure 3. Variation of xTwith temperature in LaNi1_xFexOa.

jl/l

0.4

O~--""""'=-------------------

'"E0·2

573 773T(K)

Figure 4. TGA curve of LaNio.9FeO'lOa.

973 1173

E() -E 2-s:~ - •o •Q...

g'3-...J

I I

• Nitrogen (pure)

o Oxygen (pure)

I I I

1·61·0 1'2 1'4

1000/T (K-1)

Figure 5. Plot of logarithm of electrical resistivity against reciprocal of absolutetemperature for LaNio.9Feo'lOa.

Electrical and magnetic properties of the system LaNi1_xFexOs 335

constant value at high temperatures, a behaviour typical of simple paramagneticmaterials. Samples of LaNi1_xFexOa (x>0'3) showed field-dependent magneticsusceptibility similar to LaFeOa and we, therefore, did not pursue studies on thesesamples.

LaNiOa loses oxygen in well defined equilibrium stages (Gai and Rao 1975; Oba­yashi and Kudo 1975). At 1390 K LaNiOa decomposes to La2Ni04 and NiO;slightly below this temperature (1210 K), LaNiOs seems to undergo a rhombohedral­cubic transition. Consistent with the oxygen loss, the electrical resistivity of LaNiOavaries with oxygen partial pressure (Obayashi and Kudo 1975). The TGA curve ofthe sample LaNio.gFeo.lOs shown in figure 4 indicates that with the incorporationof iron in LaNiOa, oxygen loss (with the increase in temperature) is reduced consider­ably. We have measured the electrical resistivity of LaNio.gFeo.lOs in the range of600-1000 K in atmospheres of pure nitrogen (Po. < 10-6 atm.) and pure oxygen. Theresults are shown in figure 5. The results indicate that there is some oxygen partialpressure dependence of resistivity in this system. Therefore, LaNio.gFeo.lOS may notbe a good candidate for interconnection materials in fuel cells.

Acknowledgement

The author is extremely grateful to Professor C N R Rao for suggesting the problemand guidance during this work.

References

Frederikse H P R 1970 IBM J. Res. Develop. 14 295Gai P and Rao C N R 1975 Z. Naturforsch AJO 1092Goodenough J B 1963 Magnetism and the chemical bond, (New York: Interscience)Goodenough J B 1966J. Appl. Phys. 37 1415Goodenough J B 1967 Czech. J. Phys. B17 304Goodenough J B 1972 Prog. Solid State Chem. 5 145Goodenough J B 1974 Solid State Chemistry ed C N R Rao (New York: Marcel Dekker)Kleinschmager H and Reich A 1972 Z. Naturforsch A27 363Kudo T, Obayashi Hand Gejo T 1975J. Electrochem. Soc. 122159Libby W F 1971 Science 171 199Meadowcraft D B 1970 Nature 224 847Obayashi H and Kudo T 1975 Jpn. J. Appl. Phys. 14 330Om Parkash, Ganguly P, Rama Rao G, Rao C N R, Rajoria D Sand Bihde V G 1974 Mat. Res.

Bull.91173Rao C N R and Subbarao G V 1970 Phys. Status Solidi Al 597Rao eN R 1974 J. Indian Chem. Soc 51 979 and references thereinRao eN R, Om Parkash and Ganguly P 1975J. Solid State Chem. 15 186Subbarao G V, Wanklyn B M and Rao C N R 1971J. Phys. Chem. Solids 32345Tedmon Jr. C S, Spacil H Sand Mitoff S P 1969J. Electrochem. Soc. 116 1170Voorhoeve R J H, Remeika J P, Freeland P L and Matthias B T 1972 Science 177 353