inorganic ferromagnets

lOUr,OI O[

ELSEVIER

Journal of Alloysand Compounds 262-263 (1997) 4!~-427

Copper hydroxide based organic/inorganic ferromagnets V. L a g e t a, M. Drillon a, C. H o r n i c k a, P. R a b u a'*, F. R o m e r o b, P. T u r e k ~, R. Ziessel b alPCMS UMR46 (CNRS-ULP-ECPM), Groupe des Mat~riam: bmrganiques, 23 me du Loess. 67037 Strasbour~ France bLaboratoire de Chunie, d'Electronique et de Photonique Mol~culaire, ECPM. I me Blaise Pascal. 67008 Strasbourg. France Clnstitut Charles Sadron, Unila,rsit~ Louis Pasteur. 6 me Boussing,mlt, 67083 Strasbourg Cedex, France

Abstract

Layered metal(il)-based carboxylate compounds are shown to be suitable for the design and preparation of new hybrid materials. The structure-magnetic property relationship of these kind of copper derivatives is discus~d. Two series of copl]er(li) n-alkyi carboxylates are compared; one exhibits unprecedented ferromagnetic interactions for distances as far as 40 A between the metal layers. Their properties are linked to a structural transformation of the inorganic layers from Brucite-like to Cu(OH)~. type arrangement. © 1997 Elsevier Science S.A. h'eywords: Molecular magnetism; Two-dimensional systems; Organic/inorganic materials; 3d electrons; ~ electrons

I. Introduction

Basically, tile design of 3d architectures in molecular chemistry is difficult to achieve, so that hybrid organicdnorganic compounds may be very promising for stabilizing new molecular materials such as high 7[, ferromagnets. Among the layered structures, the layered transition metal hydroxide nitrates have been shown to be good examples of planar triangular magnetic systems, with basal spa:ing of approx. 6.9 A [I,2]. From a magnetic point of view, the cobatt(ll) compounds are good prototypes of ferromagnetic 2d systems, while the behavior for the copper(ll) derivatives is closely dependent on the bridging ligand; it is 2d antiferromagnetic for the nitrate and 2d ferromagnetic for the acetate. At low temperature, a metamagnetic behavior is observed, mainly driven by the inter-layer exchange interaction. On the other hand, the layered compounds, M2(OH)3X (with M = Co, Cu) in which the exchange-

* Corresponding author. 0!]25-8388/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved P l l S0925.8388(97)00347-2

able anion, X ~ NO; or OAc ~-, is coordinated to the divalent metal ion through an oxygen atom 0 X. are shown to be good candidates for substitution teat* lions [3]. The structure may be viewed as a 2d triangular network of divalent metal, made up of [M2(OH)~Ox] layers held together through hydrogen bonds and involving the exchangeable-anion X. The in-plane metal=metal distances are very close for the different compounds (~ 3.15 /~)whereas the basal spacing (c !~arameter) differs according to the size of X, i.e. 6.9 A (nitrate) and 9.3 ,~ (acetate). Thus, tile use of large organic anions (aikylsulfotes, alkylcarboxylates) for the substitution of X, enables us to tune the basal spacing from l0/~ to 40/~ and accordingly to control t ' - magnetic dimensionality [4,5]. It has been shown ,ecently that for large inter-plane spacing, long-range ferromagnetism is stabilized [0-8], with an unexpected hysteresis cycle, due to the leading effect of dipolar interactions [8,9]. Finally, an anionic exchange reaction has been carried out with organic radical anions, leading to the new hybrid ferromagnet Co2(OH)~(p-IMB).I.OH20 (p-IMB = imino-nitroxide substituted benzoate

4~

E Lager ¢t aL / J¢nmtM of Alloys aml Comlnmmls a , ~ ~ :t

anion), made of inorganic magnetic layers interleaved with magnetic organic species [9]. In this paper, we focus on the series of copper(ll) n-alkybcarboxylates. The occurrence of very longrange ferromagnetic interactions is discussed on the basis of structural findings deduced from powder X-ray diffraction and infrared spectroscopy.

45 40 1~phases (Ferro)

35 30

2. ~ s u ~

and dismssion

Two ~fies of layered copper(ll) compounds Cu :(OH)~(C, H ~~,~ ~COO).mH :O were prepared according to the procedures re~rted elsewhere [5-9], which consist of an exchange reaction from copper hydroxide acetate powder, using the appropriate ~dium ~lts in water, An hydrated ~ries (noted it) and an anhydrous series (noted 13) characterized by the ~me C u / O H / C , H :, +~COO stoichiometty have l~en i~lated, respectively, after I day and 3 days of reaction. In all cases, the Cu:(OHh(C.H:,. ~COO).mH:O compounds exhibit inten~ (001) X-ray diffraction lines, in agreement with the layered-type structure of the materials (Fig. 1). As previously indicated, the coppedll) hydroxide acetate exhibits monoclinic symmetry, while the lattice parameters for longer al~l carboxylate chains are refined from monoelinic o~ hexagonal unit celh, depending on the degree of ordering of the exchanged anions, As might be anticio p~ted, varying the ~dkyl chain length changes the in|erolayer spacing tc parameter), while the inophme par~me!ers remain quasioconstant, The two dift~rent sffuclul~al varieties, , and t], are observed for lhe

)

[

|;'i~. I, St t~|Uta| m ~ e t for the iayc~d c~ff'~gll) hydroxklc-n.al.

25 20 15

a rhas~ (Antiferro)

10

0

2

4

6

8

IO

12

n Fig. 2. Variation of the interlayer spacing as a function of n in Cu :(OH)de,, H :,,, tCOO).m it :O.

largest n values. For imth series, the inter-layer spacing varies linearly with the aikvl chain length (n), according to the relation d(A)~ dq~+ 2.54n.cos¢ proposed by Meyn et al. [41 for hydroxide double salts (Fig. 2). The temperature dependence of the ,~,T product is illustrated in Fig. 3 for u ~ 7, which is representative of all the compounds. Whatever the n value is, the ct compounds show a decrea~ of ~,T down to 2 K, denoting the antiferromagnetic character of the exchange couplings. In turn, the behavior of the series differs drastically. Upon cooling, a decrease of T is first ob~rved down to a minimum at 60 K then a very sharp increa~ at lower temperature, pointing t~,ards a fertimagnetic or canted spin-arrangement within the coppet(ll) layers. The very large value of ~T,,,, is the signature of a net magnetic moment in the ground-state. The occurrence of long-range ferromagnetic order is illustrated by a characteristic hysteresis I~op in the M(H) curve at T ~ 4,2 K (in~t of Fig, 3), Similar behavior is observed for the different /] compounds. even for very lar~ inter-layer spacing [e.g. up to 40 ,~ for n ~ 12), The ordering tem~ratures (TeL are found to lie between 20 K and 15 K for n ranging from 7 to 12, Structural investigation was completed with infrared spectro~opy measurements. The s~ctra obtained from ~mpies c~rresponding to three stages

V. Lager et al. / Journal of AIIoys and Compo.nds 262-263 ff997~ 423-427

100 ~ ~

~.4

~

425

. T=4.2K

o.2.

~"

t o.o ~

0

10

-

f o

•

E [=,

t

.o,

% ~

-IS

,B

"

"

.

,

-10

-S

.

.

.

.

0 S Field( kOe )

I0

IS ,

m it D

0.1 )

50

tO0

Temperature (K)

150

Fig. 3. Magnetic behavior of a (square) and/3 (circle) Cu2(OH)~(CTH,sCOO).mH20.

during the exchange reaction are plotted in Fig. 4 and compared to that of the starting material. For the hydroxide acetate and the other hydrated phases (~ series), the broad absorption band in the range 3600-3200 cm -* is the signature of lattice water. This band disappears with the dehyoration proce~,~ and a hydroxyl stretching mode [10] appears at 3570 cm °~. For the anhydrous products, the absorption band at 3200 cm -t may be attributed to quite strong hydrogen bonding as previously mentioned tbr co!~per hydroxide Cu(OH)~ [1 la, I Ib, IIc]. Furthermore normal modes tbr C~lol are seen in the range 3|100o,2800 cm ~'* for all the .°alkyl derivatives (n > 1), Finally, the most complex feature appears in

the range 1600-1400 cm-'. Let us first focus on the hydroxide acetate Cu2(OH)3(CH3COO).H20. The characteristic doublet due to the carboxylate group is measured at 1550cm -I (C-O)and 1410 cm -! (C-O). Surprisingly, the difference between these two bands (Av= 130 cm -' ) suggests a bridging carboxylate [10], each oxygen atom being linked to a different metal. In fact, such a situation could be ruled out here, since it necessitates a charge compensation and consequently a variation of the molecular weight which has never been obtained, it remains that the bated positions suggest substantial electron delocaliz~tion on tile twO C - O bonds. An explanation could be ~hat the two oxygen atoms of the carbo~late groups are eno

-

intermediate phase

l

Cu2{OH)$CnX~CO2'X2o (~)

4000

3000

2000

!000

0

Wave number (cm"1) Fig. 4. Infrared spectra of ~, intermediate and/~ phases (. ~ 12) compared to Cu ~(OH)~(CH~COO).H~O.

42fi

E La~wt et at / Jtmm.+:#1of A.lh~vs at;d t~'+n+;t~+mmL,~2t~ o-20.++i ~ 7) 423 ~427

~a~d, one with a coppedll) ion, the other in a very strong hydrogen ~ n d with neigh~uring hydro~q ion. Clearly, it b difficult to draw a conclusion, in the a ~ n c c of a c~.stal structure. It is well established, however, that in almost all copper hydroxy-~lts, the OH- ions are likely to be involved in strong hydrogen bond Ilia, lib, lie]. When examining the results obtained for the exchanged compounds, rids doublet is still present and slightly shifted at i535 cm -~ and 1 ~ cm ~t. Additional bands -,ccur, at I ~ 5 cm-~, a~igned t~, ~CH2 scissoring and riCH3 antisymmetric bbending modes [12], and at 1 ~ cm -~ in the case of the hydrated phases (a). The latter stretch vibration, ~ogether with the overlapping band at 1410 cm- ~, may be attributed to the presence of unidentate COO groulm. Thus it can be assumed that, in a first step (a), two carboxy!ates doublets are superimposed, one wRh a unidentt+t¢ character, the other wRh a bridging character. The former disappears completely at the final stage of the reaction (,8). Note that polymorphism is very common in coppcr(ll) ~lts [I la, I lb, l lc]. Moreover, while the starting material Cu:(OHh(CH++COO).H+O exhibits a quasi-Brucite an-angement of the laye~ [3,7,8] Ca(OH)++ s h ~ s a strong corrugation of the layers, mainly due to the Jahn++Teller effect [lla, l lb, l lc]. In addition to the Ig band at 3200 cm =~, another comparim~n wlth t o p e r hydroxide can be made from the variation of inter-planar di~'tanc¢ with n. The ~lop~s of the ~wo straight lines (Fig. 2) arc very clo~ while d,, value are found to be 7/~ and IZI ~ for the ~ and B compounds, rcspeclivcly. This implieg that (i)in both ~erics, the alkyl chains are stacked head=tootail in bilayers, with a tilt angle a 25++30++with r~spgct to th~ c axis, (it) the main differo en¢¢ between the two ~ries is the (inorganic) layer thiekne~, as indicated by the d, values~ The additive term do i~ludes two contributions: the distance from the center of the layer to the carboxylate car~n atom and the distance ~tween the methyl grouj~s belonging to adjacent chains, fhe value do +~7 A obtained for the a ~ries (n ~ 0, X +++HCOO ° ) is ve~ similar to that to und for Cu++(OH)++(NO~) (6.93 A), whose himclare em':~i~ of Brucitc-like layers separated by nitrate groups [13L The layer thickness h linked to the stocking parameter of cobalt(II) or nickel(ll) hyo droxides, i,e, around 4,05 ~. In coml~arison, the stack° ing " in CulOH): ~s 10.59 A, namely appros, ~,9 which shed light on the difference ~ . tw¢'en d+,%aiu¢+d¢duced for (+ and O ~ri¢s (S,I ~), r a+ also relevant in e+laining the ob~rved magnetic ~havior o:f the two series. I , a structural modification of the hydro~desheet can induce significant changes in the aM angle along the Cu-O-Cu bridges, and con~,quently in the exchange pathways, giving a

change of sign (ferromagnetic or antiferromagnetic) of the interaction [14]. Moreover, the existence of different coordination sites, as already mentioned for other copper(ll) hydroxy-salt, [13,15] may promote a canting of magnetic sublattices, likely to be responsible of the behavior for the /3 phases. At low temperature, the long-range order is explained by a ferromagnetic alignment of the in-plane net moments through small inter-laye~ iat¢ractions. In view of the very large inter-plane distances (up to 40 ,g,), this coupling is understood by using a model of rigid ferromagnetic layers coupled only by dipole interactions. In 2d ferromagnets, the spins align within correlation domains, whose size diverges in the vicinit), of To. Thus, it stabilizes giant magnetic moments g(S) and the dipole interactions are shown to become the leading interaction mechanism between adjacent layers, even at very large distances. 3. Conclusion

The magnetic study of this series of hybrid organic-inorganic layered compounds shows that spectacular ferromagnetic or antiferromagnetic behavior may be observed, depending on the structure of copper(ll) layers. The different structural and mag,~etic features have been related to a structural modification of the inorganic magnetic sheets, presumably due to a 3ahn++Teller effect. Further experiments with model compounds arc in progress in ruder to specify I11¢ nature of the carboxylate bond. Particular attention is being paid to the formation of a suitable product ior crygtal structure resolution. Finally, it appears that self-assembled stacks of inter~netrating sublattices may hvor very long.range magnetic correlations. Thus, long-range ferromagnetic order has ~ ¢ n ob~rved ~tween copper(ll) layers up to ~ A apart, illustrating the key role of the dipole interactions. In this res~ct, this kind of layered hybrid systems a p ~ a ~ very promising for the design of new molecular multilayer magnets. Pe~e~en~s [1]

P. Rahu, S. Angclov, P. ~golk M ~laich¢, M. Drillon.

[2]

S, Rouha. P, Rabu, ~, Re.roche, L,.P, I~,¢~naui|, M, Drllhm,

[3]

S, Yamanaka. T, Sako. K ~ki, M, tlam~ri, .%did Slate h m ~ ~27 (I',~2) 53-,,%,

[5]

M. ~ i I ~ , . C. i.lomi~:L V. l~tt~l. ¢1 al.. M~d. C~ysl. Liq. Cr~. 273 (1995) 12,~, V. ~+~t. S. Rouba. P. Rabu. C Horaick. M. Drilhm. J. M~gn. M ~ . M++lcr. 154 { 1 ~ ) L~.. W. Fujita. K. Ax,+.~ga.Ino¢g. Chore..~ (PF+{O i+~l~. P. R~b~. S. R~b~. V. La~el. C. Homick. M. Drillon. J. Chem. ~ . Chore. Coma. (|9~) 1|fiT.

IZi.~,

[~] [?] [8]

V. Lager et al. ~Journal of AIIoys and Compotmds 262-263 ff997) 423-427 [9]

V. Laget, P. Rabu, C. Hornick, F.M. Romero, R. Ziessei, i'. Turek, M. Drillon, Mol. Cryst. Liq. Cryst. (1997) in press. [10] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed., John Wiley and Sons, 1986. [lla] H.R. Oswald, A. Relier, H.W. Schmalle, E. Dubler, Acta Crystallogr. C46 (1990) 2279. [lib] A. Riou, Y. Cudennec, Y. Gerault, Mater. Res. Bull. (1990) 987.

427

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