On magnetic properties of some oxides with delafossite-type structure

Mat. Res. Bull., Vol. 21, p p . 745-752, 1986. Printed in the USA. 0025-5408/86 $3.00 + .00 Copyright (c) 1986 Pergamon Journals Ltd.

ON MAGNETIC PROPERTIES OF SOME OXIDES WITH DELAFOSSITE-TYPE STRUCTURE Jean-Pierre

351,

Doumerc, Aree Wichainchai, Abdelaziz M i c h e l P o u c h a r d a n d Paul H a g e n m u l l e r

Ammar,

Laboratoire de Chimie du Solide du CNRS Universit~ de Bordeaux I cours de la Liberation, 33405 TALENCE Cedex,

France

( R e c e i v e d March 27, 1986; Communicated b y P. Hagenmuller)

ABSTRACT

Magnetic properties of ABO 2 (A = Cu, Ag, Pd and B = Cr, Fe) d e l a f o s s i t e - t y p e compounds are investigated on the base of a 2D-Heisenberg model using the expansion series m e t h o d p r o p o s e d for reciprocal magnetic s u s c e p t i b i l i t y by Rushbrooke and Wood. Exchange l uti Cr~--Cr ~

integrals have been determined and c o r r e l a t e d to the variation of the distance as the A cation varies.

their

3D-long range ordering which takes place at low temperature has been tentatively determined from the thermal v a r i a t i o n of the susceptibility. The investigation+leads to the conclusion that interlayer B- -Binteractions which occurs through (CuO2)3groups are relatively strong in spite of the important structural anisotropy. Magnetic behavior of CuAIO~,z CuCOO^z also been d e t e r m i n e d and dlscussed. MATERIALS

INDEX

: magnetic

and AgCoO 2 have

properties, delafossite 2D-magnetic interactions

structure,

Although the structure of the mineral delafossite CuFeO^ was established many years ago after several controversial studies as reported in ref. i, only a few researchers have investigated properties of synthetic materials exhibiting the so-called delafossite-type structure. To our knowledge only one series of important review papers published by a Dupont de Nemours group is now available (1-3); they concern mainly synthesis, cristallographic and electrical properties. Other articles, dealing mainly with crystal chemistry of those materials, were p u b l i s h e d during the last ten years, but they are outside of the scope of the present paper mainly devoted to m a g n e t i c properties. 745

746

J . - P . DOUMERC, et al.

Vol. 21, No. 6

MSssbauer resonance has been used in i n v e s t i g a t i n g CuFeO~ by Muir et al. (4, 5). If one excepts the latter references and some other papers (5-9) only little attention has been paid to magnetic properties of ABO 2 d e l a f o s s i t e - t y p e compounds especially concerning dependence of the magnetic behavior and anisotropy of the structure. In addition some d i s c r e p a n c i e s appear between the magnetic data r e p o r t e d by various authors (Table i). Magnetic

TABLE I data of some d e l a f o s s i t e - t y p e

~eff ( ~B ) CuCrO 2

PdCrO 2 AgCrO 2

CuFeO 2

3.77 3.70 3.88 3.83

ep(K)

TN(K)

-176 -146 -199

32 27

(7) This work

108

This work

34 22 30

(6) (8) (9) This work

References

(6)

-500

~ 4.1

compounds

-129 3.79 3.96 -211 3.818(EPR) I -165 3.94 -168 3.83(EPR) 5.58(EPR)

-50

25 14 12.5

5.72 5.68

-90

13

Experimental

(7) (5) cited in (5) as private comm. This work This work (Rushbrooke and Wood fitting)

section

CuAIO2, CuCrO 2 and CuFeO~z have been p r e p a r e d reaction according to the reactlon schemes : 2 CuO + B203 or

: Cu20

~

2 CuBO 2 + 1/2 02

+ B203

÷

2 CuBO 2

by solid

state

(i) (2)

S t o i c h i o m e t r i c mixtures of CuO (Merck, 99%) or Cu?O(Merck, 99,9%)and the c o r r e s p o n d i n g B^O~ o x i d e s ( C r ? O ~ : M e r c k , 9 9 , 9 % ; F e g C ~ : • z JMC) were homogenelzed, pressed and t h e n - h ~ a t e d at 1 0 5 0 ° C - f o r about 48 hours. The p r o c e d u r e was repeated until complete reaction i.e. a single phase was detected by the X-ray d i f f r a c t i o n patterns. For reaction (i) heating was p e r f o r m e d in air in an alumina boat and for reaction (2) in an e v a c u a t e d and sealed silica tube. In the case of CuFeO2, although X-ray diffraction pattern showed a single phase, traces of a ferro- or ferrimagnetic impu-

DELAFOS SITE STRUCTURES

Vol. 21, No. 6

747

r i t y was g e n e r a l l y d e t e c t e d . It c o u l d be e l i m i n a t e d in r e a c t i o n ( 2 ) u s i n g an e x c e s s of C u 2 0 ( e . g . C u p O : 3 . 5 4 4 g and F e 9 0 ~ : i . 3 1 8 g). P u r e C u F e O 2 was r e c o v e r e d by l e a c h i n g w i t h d i l u t ~ ~ i t r i c a c i d (IN). C u C o O ^ a n d P d C r O _ w e r e p r e p a r e d u s i n g an e x c h a n g e r e a c t i o n z . z in c o p p e r or p a l l a d l u m m o l t e n c h l o r i d e s r e s p e c t i v e l y , as d e s c r i b e d p r e v i o u s l y by S h a n n o n et al. (2) : CuCl

+ LiCoO 2

÷

Pd + P d C l 2 + 2 L i C r O 2

CuCoO 2 + LiCl + 2 PdCrO 2 + 2 LiCl

S t a r t i n g m i x t u r e s w e r e p r e s s e d a n d h e a t e d in an e v a c u a t e d s e a l e d s i l i c a tube for 4 days at 4 9 0 ° C in the c a s e of C u C o O . a n d 7 9 0 ° C in the c a s e of P d C r O _ . R e a c t i o n p r o d u c t s w e r e r e c o v e r e d as a b l a c k p o w d e r by w a s h l n g in w a t e r . W h e n t r a c e s of Pd m e t a l r e m a i n e d l e a c h i n g in a q u a r e g i a was a l s o u t i l i z e d . AgCoOp and AgCrOp were prepared d e s c r i b e d Dy S h a n n o n e~ al. (2) u s i n g ver n i t r a t e :

a c c o r d i n g to the m e t h o d an o x i d i z i n g f l u x of sil-

AgNO 3 + LiCoO 2

÷

AgCoO 2 + LiNO 3

AgNO 3 + LiCrO 2

÷

AgCrO 2 + LiNO 3

A f t e r h e a t i n g in an e v a c u a t e d and s e a l e d s i l i c a t u b e at 3 5 0 ° C for 4 d a y s the p r o d u c t was r e c o v e r e d by l e a c h i n g w i t h water a n d s u b s e q u e n t l y w i t h an a m m o n i a s o l u t i o n to r e m o v e t r a c e s of s i l v e r c h r o m a t e w h i c h are o f t e n p r e s e n t . Results

and discussion

CuAIO 2 C u A I O ^ w a s e x p e c t e d to be d i a m a g n e t i c or s l i g h t l y p a r a m a • z g n e t l c . The t h e r m a l v a r i a t i o n of the m a g n e t i c s u s c e p t i b i l i t y of our s a m p l e s s h o w s t h a t t h e y c o n t a i n some p a r a m a g n e t i c i m p u r i t i e s leading3~o a C u r i e Iaw. EPR m e a s u r e m e n t s have shown that they w e r e Fe ions w h i c h w e r e c o n t a i n e d in the s t a r t i n g _0~ oxide. ~z J . F r o m the C u r i e c o n s t a n t (C ~ 0.01) the a m o u n t of Fe d o p l n g in our C u A I O 2 s a m p l e can be e s t i m a t e d as ~ 0.25%. CuCoO 2 and AgCoO 2 Both CuCoO~ and AgCoO. exhibit a weak paramagnetism varying • . z z • slightly wlth ~mperature. T h e r e f o r e the e l e c t r o n l c c o n f i g u r a t i o n of the Co ~ ions is c l e a r l y low s p ~ (LS). The v a ~ g e of magnetic susceptibility ( × ~ 0 0 K = 5 3 0 x i 0 v u e m CGS m o l e ~ for i n s t a n c e for C u C o O p ) is h o w e ~ e r m u c h h i g h e r t h a n e x p e c t e d for an o x i d e c o n t a i n i n g j~st c o p p e r (I) a n d c o b a l t (III) (LS) c a t i o n s . The o r i g i n of t h i s b e h a v i o r has not yet b e e n e x p l a i n e d but it co~d r e s u l t f r o m s m a l l a m o u n t s of m a g n e t i c i m p u r i t i e s s u c h as Co ions which could be due to slight departure from stoichiometric composition. they

Heating both compounds b e g i n to d e c o m p o s e did

up to 950 K, t e m p e r a t u r e at w h i c h not a l l o w us to d e t e c t any t r a n s i -

748

tion

J.-P.

towards

another

DOUMERC, et 8/.

spin c o n f i g u r a t i o n

Vol.

21, No.

of Co 3+"

Two c o m p e t i n g effects can be c o n s i d e r e d i n I ~ d e r plain the s t a b i l i t y of the LS c o n f i g u r a t i o n of Co

to

ex-

(i) the trigonal s y m m e t r ~ of the ~ c t a h e d r o n d i s t o r t i o n split the t~ o r b i t a l s into two e-- and a a ~ o r b i t a l s and t h e r e f o r e tends t6 g d e c r e a s e the e n e r g y gap b e t w e e n the e and the less stable o c c u p i e d 3d orbitals g (ii) e~ch oxygen atom is c o o r d i n a t e d to three Co III atoms and one Cu . The value of the C o - O - C o angle, equal to 96 ° , should lead to an i m p o r t a n t c o n t r i b u t i o n of the oxygen 2p orbitals to the Co-O bonding. As 2p o r b i t a l s are less stable than 2s o r b i t a l s and then c l o s e r to c a t i o n i c o r b i t a l s one expects in the p r e s e n t case a Co-O b o n d i n g more c o v a l e n t than e.g in the cas~ of a regular octahedra with an angle of 109 ° and a sp h y b r i d i z a t i o n or in the case of p e r o v s k i t e s t r u c t u r e with a s p hybridization. This effect should lead to a r e l a t i v e l y high value for i0 Dq. It can finally be c o n c l u d e d that the s t a b i l i t y of the LS c o n f i g u r a t i o n results from the r e l a t i v e l y strong covalency of the Co-O bonding (ii) which overcomes the c o n t r i b u t i o n of o c t a h e d r o n distortion. CuCrO2,

P d C r O 2, A g C r O 2, CuFeO2

a) R e s u l t s The thermal v a r i a t i o n of the r e c i p r o c a l m a g n e t i c s u s c e p t i b i l i t y is given in Fig. 1 for C u C r O . , P d C r O ^ , A g C r O ^ and CuFeO^ z ~ . . " As the t e m p e r a t u r e d e c r e a s e s they all u n d e r g o a ~ r a n s l t l o n ~rom a p a r a m a g n e t i c to an a n t i f e r r o m a g n e t i c phase as a l r e a d y r e p o r t e d p r e v i o u s l y for CuerO? (7), AgerO~ (8, 9) and C u F ~ O ~ (5, 6, 7). ~-- v s . T curves e x h i D i t a n e a r l y ~ l ~ n e a r part ~ ~,±~, t e m p e r a t u re. As T d e c r e a s e s the slope 8× - /a T d e c r e a s e s p r o g r e s s i v e l y and then u n d e r g o e s a sharp d i s c o n t i n u o u s change from a r o u g h l y zero value to a n e g a t i v e one at a t e m p e r a t u r e w h i c h for a 2D-type m a t e r i a l s is u s u a l l y c o n s i d e r e d as e x c e e d i n g slightly a long range o r d e r i n g t e m p e r a t u r e . We a c t u a l l y i d e n t i f y both here as for CuFeO~ this t e m p e r a t u r e (13 K) c o r r e s p o n d s e x a c t l y to a N~el point ( ~ = 14 K) as shown by the M 6 s s b a u e r r e s o n a n c e study of Muir et a~. (5). It is indeed w o r t h w h i l e to notice that in spite of the highly a n i s o t r o p i c 2 D - c h a r a c t e r of the d e l a f o s s i t e structure (see below) T~ seems to be relatively easy to d e t e [ ~ i n e simply on h a n d o~ s u s c e p t i b i l i t y m e a s u r e m e n t s . B e l o w TN × increases and tends to a finite value as T b e c o m e s zero. In the p a r a m a g n e t i c range data can be fitted using r i e - W e i s s law whose p a r a m e t e r s are given in Table I. b) M a g n e t i c

susceptibility

of the c h r o m i u m o x i d e s

For the Cr 3+ ions in o c t a h e d r a l s u s c e p t i b i l i t y is g i v e n by (I0) : ×

= × S O ( l - 8 k 2 ~0/10Dq) g = 2(1-4

a Cu-

sites

+ 8k2~

k 2 10/10Dq)

(4A2g 2 ~B/10Dq

ground

state)

6

Vo]. 21, No.

6

DELAFOSSITE STRUCTURES

749

SO

where × is the spin only c o n t r i b u t i o n , 10 the spin orbit coupling, 10Dq the c r y s t a l field energy, k the d e l o c a l i z a t i o n factor. Using the values of the g factor obtained from EPR measurements (~ = 1.978 for both C u C ~ ) and AgCrO?) and values of ~= 92 cm-- and 10Dq 17000 cm-- ~ a k e n from Tef. i0 it is p o s s l ~ l e to e s t i m a t e a p p r o x i m a t e l y k = 0.7 for CuCrO 2 and A g C r O 2 .The same value will be a s s u m e d for PdCrO 2 w h i c h could not be s t u d i e d by EPR due to its m e t a l l i c type c o n d u c t i v i t y . A similar v a l u e of k = 0.7 was o b t a i n e d by Delmas et al. (ii) for the alkali c h r o m i u m oxides LiCrO2, NaCrO 2 and KCrO 2. 1

|

'

r

l

|

l

|

|

|

|

~

400x-CuemCC m° ) ...,./

300 200

CrO2J//

/J

S

0 0

AgCr02 200

400 FIG.

'

'

T'(K)

1

M a g n e t i c s u s c e p t i b i l i t y of d e l a f o s s i t e - t y p e compounds. Solid lines q~sult from a R u s h b r o o ~ a n d W o o d series e x p a n s i o n of x . Values of J/k are given in Table II. c) E v a l u a t i o n

of e x c h a n g e

integral

J

The s t r u c t u r e of the ABO? d e l a f o s s i t e phases is h i g h l y anisotropic. It c o n s i s t s of 5exagonal comp~t oxygen double layers h a v i n g o c t a h e d r a l sites ~ c c u p i e d by B-- ions and w h i c h are linked to each other ~. A ions linearly b o n d e d to two o x y g e n atoms giving (O-Cu-O) - entities p a r a l l e l to the z-axis. Such a structure suggests a large difference between the m a g n i t u d e s of m a g n e t i c i n t e r a c t i o n s w h i c h are e x p e c t e d to be stronger in a layer than p e r p e n d i c u l a r l y , since the c ~ r r e s p o n ding Cr-Cr d i s t a n c e s are about 2.9-3 A and 5.7-5.8 A respectively.

750

J . - P . DOUMERC, et al.

Vol. 21, No. 6

In order to estimate the intralayer exchange integral J i.e. within the ( B O . ) n - l a y e r s we have used an e x p a n s i o n of the z n . reciprocal susceptlblllty ~n ascending powers of reciprocal temperature. The c o e f f i c i e n t s have been d e t e r m i n e d for a Heisenberg model by R u s h b r o o k e and Wood (12) up to the 6th power for various lattices including the triangular one of interest in our case. Two remarks can be made when using such a procedure : (i) Down to which t e m p e r a t u r e does the series expansion used remain a good a p p r o x i m a t i o n to the suscep36 tibility ? Fig. 2 gives the variation of a reduced suscept i b i l t y ~ ( d e f i n e d by × _ = 26 z z rea IJI×/~g ~ ~) as a functlon of kT/IJI f o ~ a triangular arrangement and a spin S = 3/2. The curves correspond to series expansions limited to the ist, 5th and 6 4' kuj 6th power respectively. It appears FIG. 2 clearly from Fig. 2 that the R e d u c e d reciprocal suscepv a l i d i t y of the a p p r o x i m a t i o n is t i b i l i t y c a l c u l a t e d using limited to temperatures larger the model of R u s h b r o o k e than about 8 times IJl/k. and W o o d (12) limiting the (ii) Fig. 2 shows that the expanexpansion to the ist, 5th sion p r o c e d u r e when limited to the and 6th p o w e r , r e s p e c t i v e l y . Ist term (Curie-Weiss law) is a poor approximation, since the second term has still a strong influence even at r e l a t i v e l y high temperatures. Furthermore it is obvious als_~ that fitting the high temperature nearly linear part of the × curve with a Curie-Weiss law should lead to an o v e r - e s t i m a t i o n of ~eff and lepl.

48 ~ ' " ' / -

16 i6

61

Finally using the values ~ g and k o b t a i n e d from EPR and the e x p a n s i o n coefficients of X~ c a l c u l a t e d from ref. 12 (numeric values can be found in ref. Ii) we have d e t e r m i n e d the exchange integrals of the c h r o m i u m delafossite compounds by usual least square r e f i n e m e n t methods. They are given in Table II. Table

II

Exchange integrals of some d e l a f o s s i t e - t y p e oxides.

i i f

J/k(K)

a(A)

I AgCrO 2

-9.0

2.984

I CuCrO 2

-11.4

2.975

I PdCrO 2

-23.0

2.923

P f

I l

For CuFeO~ both effective moment and J/k R a v e been adjusted during the fitting process giving 5.68 ~B and 2.2 K respectively. Discussion The crystal structure suggested a 2D-character for the magnetic properties of d e l a f o s s i t e - t y ~ compounds. The shape of the × vs.T curves confirms this trend.

Vol. 21, No. 6

DELAFOSSITE STRUCTURES

The e v i d e n c e is p a r t i c u l a r l y for w h i c h the d e p a r t u r e from a b e g i n s by r o o m temperature.

strong linear

751

in the case of ~ d C r O ^ v a r i a t i o n of ×vs.~

Various types of m a g n e t i c i n t e r a c t i o n s o c c u r i n g in triangular lattice a r r a n g e m e n t s with edge sharing CrO. o c t a h e d r a have been d i s c u s s e d by Delmas et al. (ii). They p o i n t e d out that the s u p e r e x c h a n g e i n t e r a c t i o n s are f e r r o m a g n e t i c (except the e -s-e c o r r e l a t i o n s w h i c h however are weak) and finally c o n c l u d e ~ tha~ direct t 2 -t 2 antiferromagnetic i n t e r a c t i o n s overcome superexchange i~te~ctions. Table II shows that the J/k values o b t a i n e d using a Heisenberg model and R u s h b r o o k e and W o o d c a l c u l a t i o n s vary strongly from AgCrO~ to PdCrO~. This v a r i a t i o n is c o m p a r e d to that of the a - l a t t l c e c o n s t a n t w ~ i c h is equal to the Cr-Cr distance within nthe (BO~)_ layers. As e x p e c t e d J/k decreases as a Increases due o b v i o u s l y n to a more r a p l d decrease of the c o n t r l.b u t l o n of the direct interactions. R o s e n b e r g et al. have even p o i n t e d out that the Cr-Cr i n t e r a c t i o n s in c h a l c o g e n i d e s become f e r r o m a g n e t i c for dcr_C r > 3.6 A (8). We have c o n s i d e r e d i n t e r a c t i o n s as u s u a l l y for d 3 and d 5 (HS) c o n f i g u r a t i o n s to fit w i t h a H e i s e n b e r g model. For such a 2 D - H e i s e n b e r g a n t i f e r r o m a g n e t no internal field is t h e o r e t i c a l l y e x p e c t e d for T > 0 (13). However in the case of CuFeO^ the M ~ s s b a u e r i n v e s t i g a t i o n by Muir et al. (5) has shown that an internal field is o b s e r v e d at low t e m p e r a t u r e and t h e r e f o r e that 3D long range o r d e r i n g occurs for T < T N. Muir et al. have p r o p o s e d .for CuFeO^ a m a g n e t i c s t r u c t u r e J+ . z . w h i c h consists of layers of Fe lons f e r r o m a g n e t l c a l l y c o u p l e d with alternating antiferromagnetic coupling between parallel sheets. However as far as we c o n s i d e r the i n t e r a c t i o n s w i t h i n the layers to be stronger than those b e t w e e n them, the p r e s e n t i n v e s t i g a t i o n leads to a d i f f e r e n t c o n c l u s i o n since it supports that m a g n e t i c i n t e r a c t i o n s w i t h i n the layers have an a n t i f e r r o m a g n e t i c character. In fact m a g n e t i c i n t e r a c t i o n s in t r i a n g u l a r lattices lead generally to relatively complex helimagnetic structures (14, 15) r e s u l t i n g from a t o p o l o g i c a l frustration. T h e r e f o r e it is d i f f i c u l t to account in a simple manner for possible magnetic arrangements at low t e m p e r a t u r e (T < TN). D e t e r m i n a t i o n of the m a g n e t i c s t r u c t u r e s by neutron d i f f r a c t l o n is in progress. References 1 - C.T. Prewitt, 719 (1971).

R.D.

2 - R.D. Shannon, D.B. i0, 713 (1971).

Shannon Rogers

and D.B. and

C.T.

Rogers,

Prewitt,

3 - D.B. Rogers, R.D. Shannon, C.T. Inorg. Chem. i0, 723 (1971).

Prewitt

4 - A.H. Muir Jr. 65 (1967).

J. Phys.

and

H.

Wiedersich,

Inorg.

and

Chem.

Inorg. J.L.

Chem.

i0,

Chem.,

Gillson,

Solids,

28,

752

J.-P.

5 - A.H. Muir Jr., R.W. the Conference on (Tihany, 1969). 6 - R. K o h l m u l l e r (1968). 7 - A. Apostolov,

and C.R.

DOUMERC, et al.

Vol. 21, No. 6

Grant and H. Wiedersich, P r o c e e d i n g s of the Application of M6ssbauer Effect J.

Omaly,

Acad.

Sc.

Bull.

Soc.

Bulgarie,

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22,

8 - M. R o s e n b e r g and A. Kndlle, H. S a b r o w s k y Phys. Chem. Solids, 43, 87 (1982).

ii,

1968,

4383

1969.

and Chr.

Platte, •

J. .

V

9 - M. K r y g o w s k a - D o n i e c , J. Kwiatkowska, M . S t o j i 6 A c t a P h y s i c a P o l o n i c a A64, 143 (1983).

and V.Splrlc,

10 - B.N. Figgis, I n t r o d u c t i o n N e w - Y o r k (1961).

Interscience,

ii

-

to

Ligand

C. Delmas, G. Le Flem, C. F o u a s s i e r Phys. Chem. Solids, 39, 55 (1978).

12 - G.S.

Rushbrooke

13 - L.J. de Jongh (1974).

et P.J.

and A.R.

Wood,

Molec.

Miedema,

Fields.

and P. Hagenmuller, Phys.,

Advances

in Physics,

14 - J.L. Soubeyroux, D. Fruchart, J . C . M a r m e g g i , C. Delmas and G. Le Flem, phys. stat. sol., 15 - F.M.R. Engelsman, G.A. Wiegers, Laar, J. Sol. State Chem., 6, 574

6, 409

J.

(1963). 23,

1

W.J.Fitzgerald, 67, 633 (1981).

F. J e l l i n c k (1973).

and

B.

Van