Preparation and magnetic properties of some new layer rocksalt-and mixed delafossite-type oxides

Mat. R e s . B u l l . , Vol. 28, p p . 159-165, 1993. P r i n t e d in t h e USA. 0025-5408/93 $6.00 + .00 C o p y r i g h t (c) 1993 P e r g a m o n P r e s s L t d .

PREPARATION AND MAGNETIC PROPERTIES OF SOME NEW LAYER ROCKSALTAND MIXED DELAFOSSITE-TYPE OXIDES Y.J. Shin, J.P. Doumerc, M. Pouchard and P. Hagenmuller Laboratoire de Chimie du Solide du CNRS, Universit~ de Bordeaux I, 351, cours de la Liberation, 33405 Talence Cedex, France

( R e c e i v e d O c t o b e r 14, 1992; C o m m u n i c a t e d b y P. H a g e n m u l l e r ) ABSTRACT Three new mixed delafossite-type oxides of formula Ag2M'M"O 4 (M'M" = NiTi, CoTi, CoSn) have been obtained by exchange reaction using three new compounds of ~-NaFeO 2 structure and of Na2M'M"O 4 formula as precursors. They have all been characterized by X-ray diffraction and magnetic measurements. MATERIALS

INDEX:

delafossite-type oxides, nickel, rocksalt, cobalt, tin, silver

i.

titanium,

layered

Introduction

When A is monovalent copper, silver, palladium or platinum AMO 2 ternary oxides crystallize with the delafossite structure [i]. In dela~ossitemonovalent cations are coordinated to two oxygen atoms forming linear AO~- group~parallel to the hexagonal c-axis. Depending on the close packing modeof theAO~- groups delafossite-type compounds can exhibit either a 3R or a 2H polytype (Fig. 1) [4]. The M-cations are situated at the center of trigonally distorted octahedra which form MO 2 layers by edge-sharing. The size of the trivalent M element can vary from that of A1 ~+ to that of La 3+ and t h e o c t a h e d r a l s i t e c a n a l s o be occupied by a pair of M '2+ and M ''4+ cations [I-3]. Apart from the hydrothermal method, delafossite-type silver compounds are generally prepared by a cation exchange method in an oxidizing flux [i]. Consequently the elaboration of delafossite-type Ag-compounds depends on the existence of corresponding alkali AMO 2 phases appropriate for the exchange reaction. This approach has led us to prepare some new phases such as Na2CoTiO 4, Na2CoSnO 4 and Na2NiTiO 4. Using them as precursors in the cation exchange reactions we have obtained three new compounds: Ag2NiTi04, Ag2CoTi04 and Ag2CoSnO 4 .

2.

Experimental

conditions

The alklai compounds, Na2CoTiO4, Na2CoSn04, and Na2NiTiO 4 were prepared by direct solid-state reactions of stoichiometric mixtures of Na2CO 3, M'O (M' = 159

160

Y.J.

SHIN et al.

UWnP-~.,.R~

,

~lllWm---,.~I ~

Vol. 28, No. 2

,Im~'Y~.,lll

~I~;~

~I~J~-'~ ~I~ ~ ' ~ ~ I ~ 3 ~

~l~:~|]i

~ I ~

~ I ~

' l ~

' I ~

3R

~l~If

(a)

2H • A3+ 0

© (c)

(b) FIG. 1.

+

.

3R-delafossite type structure (a). Circles represent the AE ions. Projection on the (110) plane for 3R (b) and 2H (c) polytypes.

Co, Ni), and M"O 2 (M"= Ti,Sn) following the chemical relation:

Na2CO3 + M'O + M"O 2

>

Na2M'M"O 4 + CO 2

Pelletized mixtures were put into alumina boats and heated at ca. 1150 K in an argon atmosphere for 5 days. The reaction products were rapidly introduced into an argon-filled glove box because of their sensitivity to air moisture. Cation exchange reactions were carried out in oxidazing flux of AgNO 3 and KN03, following the reaction: 2 AENO 3 + Na2M'M"O 4

KNO 3 >

AE2M'M"O 4 + 2NaNO 3

An excess of about 50~ of AENO 3 was used to promote the reaction.

The starting

mixtures were put into pyrex tubes sealed under low pressure (ca. I0 Pa), and they were heated in vertical furnaces at 570 K for 5 days. Reaction products were recovered by leaching out the remaining nitrates in water. Powder X-ray diffractograms were obtained with a Philips 1050 diffractometer using a copper anticathode. Lattice constants were determined by a least square method from the d-values corrected using silicon as an internal standard (a= 5.4305 A at 300 K).

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DELAFOSSITES

161

Magnetic susceptibility data were collected between Manics DSM8 susceptometer. EPR measurements were carried 200 tt spectrometer. 3. Results and discussion

4 and 300 K using a out on a Brucker ER

3.1. X-ray diffraction The X-ray diffractograms show that the Na-compounds crystallize with the ~-NaFeO 2 type and 3R-delafossite

and Ag-compounds type structures,

respectively. We do not observe any extra-peak that could have resulted from a possible ordering of the M' and M" ions. Lattice constants are given in Table I. TABLE I Lattice Constants of some Mixed ~-NaFeO 2 type and Delafossite Samples

(a)A large

a(A)

c(A)

c/a

Na2NiTiO 4

3.008

16.09

5.35

Na2CoTiO 4

3.025

16.11

5.33

Na2CoSnO 4

3.171

16.43

5.18

Ag2NiTiO 4

3.013

18.64

6.19

Ag2CoTiO 4

3.035

18.62

6.14

Ag2CoSnO~ a)

3.16

18.7

5.9

XRD-line broadening

type Oxides.

limits the accuracy for Ag2CoSnO 4.

2+ 2÷ The a - p a r a m e t e r i s l a r g e r f o r t h e Co compounds t h a n f o r t h e Ni - o n e s , w h i c h c a n b ~ ÷ e x p l a i n e d m a i n l y by an i o n i c g ~ d i u ~ ± l a r g e r f o r Co z÷ ( 0 . 7 4 5 A) t h a n f o r Ni ( 0 . 6 9 0 A) [ 5 ] . However t h e N i ~ T / T i ~T p h a s e ^ e x h i b i t s a s l i g h t l y l a r g e r v a l u e o f t h e c / a r a t i o t h a n t h e c o r r e s p o n d i n g CoZ÷/Ti 4÷ o n e . S u c h a result can be a t t r i b u t e d to a stronger c o v a l e n c y o f t h e N i - 0 bond w h i c h , a c c o r d i n g to a p r e v i o u s work, d e c r e a s e s the t r i g o n a l distortion o f t h e MO6 octahedra [ 2 ] . & more c o v a l e n t Ni-O bond c a n he p r e d i c t e d f r o m an z+ negativity of Ni (1.96) higher than that of Co z+ (1.88) [6]. The sharpness

electro-

of the XRD lines of Ag_CoTiO 4 and Ag2NiTiO 4 could reflect

the good ordering between Na + and [M'2÷/Ti ~÷] ions in the precursors Na2CoTiO 4 and Na2NiTi04, noticeable

as a small Li/Ni

line-broadening

exchange

in LiNiO 2 has been shown to bring a

in the AgNiO 2 delafossite

obtained

by an exchange

reaction similar to that used in the present investigation [7]. Conversely broad lines of AE2CoSnO 4 could result from some cationic disorder in corresponding

Na2CoSnO 4 phase

(Fig. 2).

the the

162

Y.J.

SHIN e t al.

Vol. 28, No. 2

(b)

__k.

i

_I[ i

i

I

!

60

,50

40

30

I

20 ~

(a) I

29 10

FIG. 2.

X-ray diffractograms at room temperature of Ag2CoTiO 4

(a)

and of Ag2OoSnO 4 (b).

3.2. MaKnetic properties The reciprocal molar magnetic susceptibilities corrected for the ion diamagnetism [8] of Ag2NiTiO 4, Ag2CoTiO 4 and Ag2CoSnO 4 are plotted in Fig. 3 as a function of the absolute temperature between 4 and 300 K. The experimental points have been fitted using the usual Curie-Weiss law and including a temperature

independent

paramagnetic

Ag2CoSnO 4 C M is temperature high temperature.

dependent

term

(TIP):

XM = CM/(T-8 p)

and it has been determined

+ TIP.

For

at low and

Results are reported in Table II.

Ag2NiTiO 4. In an octahdral site, Ni 2+ ion exhibits a 3A term (t6e2; S= I) in its ground state, and the corresponding effective magnetic moment and TIP are given by: SO (l-4k2A /AE) and TIP= 8Nk2 ;/AE,9 ~eff = ~eff o so is the spin-only value of the effective magnetic moment, k the where ~eff delocalization factor, AE the crystal field parameter and A the spin-orbit 0

coupling constant for a free ion (A = -315 cm -I) [9]. The lack of experimental 0

data for AE prevents us frog+estimating accuratelYcm_~i~ If we use the value determined for the JNi(H20)6 ~ complex (AE= 8.900 [9], we obtain: ~eff= 2.828 + 0.4k 2 and TIP= 240k 2 • Comparing with the experimental data given in Table II leads to k= 0.75. Such

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163

DELAFOSSITES

X-1 CemuCGS-1MoI)' A kgzNiTiO4

/

200 _<~kg2CoTiO4

. /

-

o kg2CoSn04 0 ~ .

o o°

cmo

.

100

0 0

,

,

Ioo

200

T (K)

FIG. 3.

Thermal variation of the reciprocal molar magnetic susceptibility of Ag2NiTi04(~), Ag2CoTi04(~), and of Ag2CoSn04(o). a value is consistent with a strong covalent contribution in the Ni-O bond as already mentioned above. TABLE II Experimental Magnetic Parameters of some Ag-Delafossite Compounds

(a) CM (emuKMol -I )

Samples

Op TIP (K) (10-6emuMol -I)

3.06

2.83

-3

2.20

4.21

3.87

25

2.47 1.57

4.45 3.55

3.87 3.87

-13.5

1.17

Ag2C°SnO4 (a)

HT LT

obtained s.o

(b)

~eff (BM)

Ag2NiTiO 4 Ag2CoTiO 4

s.o

~eff (BM)

from Curie

130

constant

(b) geff = g

Ag2CoTiO 4 and Ag2CoSnO 4. The Curie constant values rep~rted~ ~n Table II show that Co 2+ ion exhibits a high spin configuration ~T (t~e-; S= 3/2) in both Ag2CoTi04 and Ag2CoSnO 4 compounds. On the other hand, the different signs of 8p suggest that the resultant of the magnetic interaction changes from ferromagnetic (J>0) for Ag2CoTiO 4 to antiferromagnetic (J
164

Y.J.

SHIN e t a l .

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follows. The direct interactlon between Co 2+ ions (involving t orbitals) is ferromagnetic [I0]. It competes with possible superexchange interactions whlch can be either ferromagnetic (such as the eg-~p-¢p,-eg correlation) or antiferromagnetlc

(such as the eg-¢p-~p,-eg delocalization

and

the t2g-P-eg

correlation). As already noticed the ferromagnetic interactions are predominant in Ag2CoTiO 4. However as the Co-Co distance--which is equal to the a lattice constant--increases we may assume that, as usual, the direct (J>O) coupling decreases more rapidly than the superexchange and that the latter may become prevailing as the Co-Co distance becomes large enough. The antiferromagnetic interaction found for Ag2CoSnO 4 shows that such a situation is actually reached and that among the superexchange interactions the antiferromagnetic ones overcome the ferromagnetic ones'. In Ag2CoSnO 4 ~eff increases gradually from 3.55 ~B to 4.45 ~B as the temperature increases up to 300 K. This result suggests a strong spin-orbit coupling as expected for a T ground term configuration. For Ag2CoTi04, such an evolution of ~eff is not clearly observed probably because of the magnetic 1 interactions . Nevertheless, the large value of ~eff at 300 K suggests that spin-orbit

coupling

is also

large.

Actually Ag2CoTiO 4 does not

exhibit

any

detectable EPR signal at room temperature. This ma~+be attributed to a short relaxation time as expected for high spin Coions [9,11]. A quite asymmetric signal at 9 K with two g components (ell= 1.47, g±= 5.05) (Fig. 4) accounts for a strong contribution of the angular momentum and a large axial (trlgonal) distortion.

I 1

I 2

I 3

~

I 5

H(kG)

FIG. 4. EPR spectrum of Ag2CoTiO 4 at 9K.

1

As T decreases

ferromagnetic

interactions

should

lift ~M over

the expected

paramagnetic value. However, Fig. 2 shows a tendency to the onset of an antiferromagnetic ordering which could be ascribed to a weak antiferromagnetic interaction between the CoTiO 4 planes.

Vol. 28, No. 2

DELAFOSSITES

4.

165

Conclusions

Three novel delafossite-type oxides of formula Ag2M'M"04 (M'M"=NiTi, CoTi, CoSn) have been obtained by exchange reaction using three new compounds of aNaFeCo 2 structure and of Na2M'M"O 4 formula as precursors. The present study confirms that in delafossite-type phases the trivalent element M can be replaced by an (M'2++M ''4+) couple, provided that the difference of ionic radii is not too large. The evolution of their structural and magnetic properties can be correlated to that of the covalency of the M'-O and M"-O bonds. It would be relevant in a further step to determine the electrical behavior of the materials in order to have a better understanding of the physical behavior of this type of substituted materials.

AcknowledHement One of the authors (Y.J.S.) is grateful for the financial Sunkyong Magnetics, Ltd. (Seoul, ROK).

support of the

References R.D. Shannon, D.B. Rogers and C.T. Prewitt, Inorg. Chem., 10, 713 (1971) and references therein. 2. J.P. Doumerc, A. Ammar, A. Wichainchai, M. Pouchard and P. Hagenmuller, J. Phys. Chem. Solids, 48, 37 (1987). 3 K. Hayashi and M. Kato, Nihon Kagaku Kaishi, 1975, 2241 (1975). 4. W. Von Stahlin and R.H. Oswald, Z. anorg, allg. Chem., 373, 69 (1970). 5. R.D. Shannon, Acta Crystallogr. A, 32, 751 (1976). 6. A.L. Allred, J. Inorg. Nucl. Chem., 17, 215 (1961). 7. Y.J. Shin, J.P. Doumerc, P. Dordor, C. Delmas, M. Pouchard and P. Hagenmuller, J. Solid State Chem., submitted. 8. Landolt-B~rnstein, Vol. II, Springer-Verlag, Berlin, Heidelberg, New-York (1966). 9. B.N. Figgis, Introduction to Ligand Fields, Interscience Publishers, New-York, London, Sydney (1966). 10. J.B. Goodenough, Magnetism and the Chemical Bond, Interscience Publishers, New-York, London, Sydney. (1966); Progr. Solid State Chem., 5, 145 (1971). Ii. R.L. Carlin, Magnetochemistry, Springer-Verlag, Berlin, Heidelberg, New-York, Tokyo (1986). i.