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A study of Co3−xNixO4 (O≤x≤1) system

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Proc.1Jldian Acad. Sci., Vol. 87 A (Chern,Sei.),No.4,April1978, pp. 1lS-120,©printedin India.

A study of C03-xNix04 (0 «»

~

1) system

N K APPANDAIRAJAN and J GOPALAKRISHNAN*

Department of Chemistry, Indian Institute of Technology, Madras 600 036

-Present address: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012

MSreceived4December1977

Abstract. A series of solid solutions of compositionCo3 _xNixO. has been studied.

ForO<x<H,the members crystallize in the cubic spinel structure. Measurements of de electrical conductivity and Seebeck coefficient show that there is a transition from semiconducting state to semi-metallic state around x=0·5-0·6. All compositions withx>0'5are semimetallic and ferrimagnetic, The results have been rationalised on the basis of the cation distribution (CoH)r [Ni~1ICo~~x]00.for the solid solutions which permits formation of o·(e,) band through strong covalent interaction between octahedral-site low spin CoIllfNilll and oxide ion in the spinel structure.

Keywords. Oxide spinels; C03 _xN i,,0 . system; structure and electrical properties.

1. Introduction

Recently many cobaItites AC020 .. (A = Zn, Ni, Fe, Mn and Cu) have been investigated (Miyatani et of 1966; Holgerson and Karlson 1929; Kawano and Achieva 1966; Boucher et of 1970; Shimada et al 1975). Of these NiCo,O.

is an interesting case showing ferrrimagnetism (p.B = 1,25). To account for the magnetic behaviour, Blasse (1963)suggested that the valence and site distribution of cations in this compound is (Co2+)r [NilltColltlo 0. (where the roman superscripts indicate low spin oxidation state and the arabic superscripts high spin state) while Knop et of (1968) suggested a distribution (Coa+)r [Nill+COIIIJo 0. that accounts equally well for the observed ferrimagnetic moment. The results of neutron diffrac- tion study are not inconsistent with either model. A survey of the literature reveals that no systematic study of electrical transport properties of cobaltite-spinel oxides seem to have been made excepting for reports that Coa04is a p-type semiconductor (Rao and Subbarao 1974). It was thought that a study of the electrical behaviour of NiC0204and C03_xNixO.(O~x::;::;l) system would shed light on the oxidation state and cation-distribution in this system. The present paper reports the results of prepa- ration and studies of crystallographic and electrical behaviour of solid solutions of the system Coa_xNixO.(O~x~l).

2. Experimental

AnalaR grade hydrated cobalt and nickel nitrates were made use ofinthe preparation of Cos_xNixO. solid solutions. Compositions corresponding to different values of 115

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116 NK AppandairajanMUiJGopalakrishnan

x were made by mixing requisite volumes of aqueous solutions (I M) of the nitrates, evaporating them to almost dryness on a water bath and decomposing the dried mass in air at higher temperatures. Co3

o

Cwas obtained by firing at 600°C whereas for members containing nickel, a lower temperature(300-350°C)was employed. Deter- mination of the oxidising power and hence the average oxidation state of the transition metal ion in the samples were carried out by oxidimetric analysis. About 100 mg of the sample is treated with excess Fe2+ solution in 4N H2SOc' After dissolution of the sample, the excess Fez+is determined by titration with standard K2CrZ07• From the volume of Fez+consumed, the oxidising power and the average oxidation state of the transition metal ions are calculated. The results are given in table 1.

The analytical results (table l) indicate that in the C03 -JCNi,,04 solid solutions for

O~x~l,two of the three transition metal ions exist in the trivalent state as in COaOc' The samples were further characterised by x-ray powder diffraction usingCoKa radiation. Electrical conductivity and Seebeck coefficient measurements on sintered cylindrical pellets were made by the methods described elsewhere (Palanisamy 1974).

3. Results and discussioD

X-ray diffraction data of the Coa_JCNi,,04solid solutions (table 1) clearly indicate that monophasic spinel type products are formed up to x=1·0. For values ofx~1·I, apart from the diffraction lines characteristic of the spinel structure, additional lines,

Table. 1. Crystallographic and analytical data or Coa_xNixO, samples.

x

0'00 0'10 0'25 0'40 0'50 0-60 0'75 0'90 1'00

8'080 8'083 8'087 8'093 8'099 8'106 8'114 8'116 8'119

Average oxidation state orT.M. ion

2-66 2'70 2'69 2'68 2'67 2'70 2-66 2-68 2'70

8·'22

-e.

8.QQa

o

8 ·074L - - - J L . - - - - I ' - - - ' - - - ' - - : - - '

o 02 0·4 06 08 1·0

"

. . . 1. Variationorlattice parameter in Co3 _xNixO,syatem,

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A study of Coa_xNi,p,(0 ~x ~1)system 117 mostly of NiO, appear in the x-ray pattern, indicating that monophasic spinel solid solutions can be obtained for the system Cos_xNixO, onlyin the composition range

o

~x ~1·0. The variation of unit cell parameters refined by Nelson-Riely plots, as a function of compositionx, is given in figureI. The unit cell parameters of CosO, (8'08A) and NiCo1lOi8'119A) obtained in this work compare well with those reported in the literature (Knop et al 1968). The variation of unit cell parameters across the series Cos_xNixO", is almost linear following Vegard's law in the region O~x~1

(figureI). CosO,. a normal spinel has the cation distribution (C02+), [COIIICOIIIJo0,.

Assuming that the substitution of nickel takes place in the octahedral site as low spin NilIl, one can write the cation-distribution for NiCOlO, as (C02+), [NiIIlColII] 0, as suggested by Blasse (1963). The fact that NiCol\O, crystallizes in a slightly larger unit cell than C0304is understandable in terms of the larger ionic radius of low spin NiIIl(0'56A) as compared to low spin COlli (0-525 A) in the octahedral oxygen co- ordination. The general cation distribution for the solid solution series may then be written as (COI+), [Ni~1 CO~xlo 0,. The alternative distribution suggested by Knopet al(1968), (C03+), [Nill+Collllo0, is not consistent with the electrical proper- ties as seen below.

3.1. Electrical behaviour of COs_xNi,P. system

The variations of room temperature electrical resistivity and Seebeck coefficient with composition x in Coa_xNixO, are given in figures 2 and 3. Co:P, is a p-type semiconductor (P""'" 10' ohms em) with a large positive Seebeck coefficient(+ 600 poVfK). Substitution of nickel up to x --0·5 brings about a drastic decrease of both the resistivity and Seebeck coefficient. In the composition range O·S<x< 1,0, there is no significant variation, the values remaining almost constant around P""'"10 ohm em and a""'"30 poVrK. These values of resistivity were obtained by two-probe measurements. For compositions with

x>

0·5 showing low resistivity,

as 80

Q" x

"'

8'2·!l >~eo

oJ 0

1 !l 40

4.:1

r---,

FIpreZ. Room temperature resistivity in Coa-"Ni"O, system.

100 , . , . . . - - - ,

Ftpre3. Variation of Seebeck coeftlcient at room temperature.

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118 N K Appandairajan andJGopalakrishnan

measurements by four-probe technique have been carried out. The results show that for these samples the resistivity is much lower (...,0·2 ohm em) and its variation with temperature from 30- 100°0 is almost constant indicating semi-metallic nature.

For samples withx ~0,5, the variation of electrical resistivity with temperature in the range 30-400°C is shown in figure 4. Activation energiesfor conduction obtained from these plots of logpvs1

IT

are given in figure 5. The data show that the activa- tion energy steadily decreases with x and disappears almost completely between x=0'5and 0·6.

The electrical transport behaviour of Cos_xNixO.. is very similar to that of LaCo1_x NixOs (Raoet 011975; Gopalakrishnanet a11976)wherein a semiconductor-to-metal transition is observed. Recently a large number of such extrinsic semiconductor systems showing a transition to metallic or semimetallic-state has been discussed (Mott 1972; Rao et a11975; Rao and Om Prakash 1977). The transition to semi- metallic nature in Coa_xNixO, system for x>0'5 is understandable in terms of the cation distribution suggested by Blasse (1963) for NiC020..; (Coz+MNiIlICOIlI]oO...

The solid solutions would then have the distribution (Co2+MNi~I1Co1~,.Jo04' Goodenough (1965, 1969) has discussed the electrical transport properties of spinels in terms of qualitative one-electron energy band schemes. It has been suggested that the electrical behaviour in oxide spinels result mainly from the octahedral B-site cations and interactions thereof. In the spinel structure the B-site octahedra share their edges so that both directB-Binteraction as well as 900B-0 -B interaction are important in deciding the electronic behaviour. For CoaO.., Goodenough (1965) has suggested that a u*(e,) band is formed by strong covalentB-O-Btype interaction between the octahedral site low-spin CollI ions to account for its anti-ferromagnetic

T. OK

800 600 500 400 300

4

Q,.

'" 3 o -J

,,=0·'

x e025

,,=0·4 _~....o-<>-<.M'x •0 5

FIgure4. Blectrical resistivity in Col_xNixO, system.

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A study of Coa_xNi,PiO ~x ~1)system

04

r---'---,

0·3

~• 0·2

<7

0·1

x

Figure 5. Activation energy for resistivity in COa_xNixO, system.

119

nature (TN= 400K ). However COS04 is only a semiconductor because the 0'* (eg)

band is empty (ColII has the electronic conguration t26ge~). In Co3_xNix04 system, if the nickel ions substitute for ColII in the octahedral site as low spin NilII (t~ge~)

then it would result in placing charge carriers in the O'*(eg) band. However, for small values ofx,and at low temperatures (below about 4500 K )the samples of Co3_xNix04 system show still only semiconducting behaviour, perhaps because the charge carriers are ' Anderson-localised' (Mott 1972), since the donor centres (NilII) would be randomly distributed. Plots of In p vs T-l/4 for samples with x<O·5 show approximately linear behaviour belowo -450°K.

Qualitative magnetic measurements have shown that the samples become ferri- magnetic for values x>O·5. NiCo204has already been reported to be ferrimagnetic with magnetization 1·25fl-B. Further work on the magnetic behaviour of this system is in progress.

Acknowledgements

The authors thank Professor M V C Sastri for his keen interest and encourgement throughout this work. Special thanks of the authors are due to Professor C N R Rao, for helpful suggestions and comments on this work. Acknowledgement is made to the Department of Science and Technology, New Delhi for the financial support of this work.

References

BlasseG 1963Philips Res. Rep. 18 383

Boucher B, BulI R,diDelIa R and Perrin M 1970J.Phys. 31 113

GoodenoughJB1965ColIoques Internationaux du CNRS No. IS7, Proprietes Thermadynamiques Physiques et structurales des Derives, Semi-mUa//{ques,Orsa)' (paris: CNRSed1967) p. 263

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120 N K Appandairajan andJGopalakrislman

GoodenoughJB1969J.Phys. Chem. Solids 30 261

Gopalakrishnan J, Colsmann G and Reuter B1976Z. anorg.allg. Chem.424 155 Holgerson S and Karlson A1929Z.anorg,ollg.Chem,183 384

Kawano Sand Achieva N 1976Mat. Res. Bull.11911

Knop 0, Reid K Io,Sutarno and Nakagawa Y1968Can.J. Chem.463463 Miyatani K, Kohn K, Kamimura H and Iida S1966J.Phys. Soc.Jpn.21464 Mott N F 1972 Adv:Phys. 21 785

PalanisamyT1974Solid State aspects0/some ternary oxides ofvanadiumPh. D. thesis Indian Institute of Technology, Madras.

Rao C N R, Bhide V G and Mott N F1975Phil. Mag.321277 Rao C N R and Om Parkash1977Phil. Mag. 351111

Rao C N R, Om Prakash and Ganguly P 1975J.SolidSytate Chem,15 186

Rao C N Rand Subbarao G V1974Transition metal oxides(Washington: NBS US Dept. of Com- merce).

Shimada M, Kanamaru F and Kolzumi M 1975Mat. Res. Bull. 10733

References

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