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(Reçu Ze 9 septembre 1985, accept,4 sous forme définitive Ze 18 novembre 1985). Résumé: On décrit les propriétés conductrices et magnétiques de trois ...

J.

FÉVRIER 1986,

Physique 47 (1986) 157-161

157

Classification Physics Abstracts 76.50 - 72.80L - 61.10

PHYSICAL PROPERTIES OF THE DMtTTF SALTS: A NEW SERIES PRESENTING ANTIFERROMAGNETIC GROUND STATES.

C.

a

Coulona,J. Amiella,

D.

Chasseaub* ,

E.

Manhalc

and J.M.

Fabrec

CRPP, Domaine Universitaire, 33405 Talence cedex, France.

bChemical

Crystallography Laboratory,

9 Parks

Road, Oxford

0X1

3PD

(U.K. )

cLaboratoire Montpellier,

(Reçu

Ze 9

de Chimie France.

Structurale

septembre 1985, accept,4

sous

Organique,

USTL,

forme définitive Ze

34060,

18 novembre 1985)

décrit les propriétés conductrices et de trois composés de la série des sels de DMtTTF. On observe pour chacun d’ eux un état basse Selon antiferromagnétique température. l’anion, l’organisation cristalline soit est, triclinique (similaire à celle des sels de TMTTF), soit monoclinique. Comme attendu, la position des axes est fonction de la symétrie magnétiques cristalline. Finallement, l’origine de la phase magnétique est discutée au moyen des descriptions théoriques déjà utilisées pour des composés voisins, les sels de TMTTF.

Abstract : The conducting and magnetic properties of three salts of the DMtTTF series are described. Low are temperature antiferromagnetic ground states found for the three compounds. Depending on the anions the crystal organisation is either triclinic for the TMTTF salts) or monoclinic. As (as expected, the position of the magnetic axis is correlated to the crystal symmetry. Finally the origin of the magnetic phase is discussed using the same theoretical framework as for the related TMTTF

INTRODUCTION

phase transition has been experimentally observed for many salts of the seriesjlo. The distinctive properties of the TMTCF salts were recently related to their peculiar molecular organisation which gives rise to and comparable positive negative contributions to the transverse overlap1.1-. To check the validity of this argument the synthesis of new series of compounds presenting closely related crystal structures is clearly useful. A preliminary study of a new series of radical cation the DMt’l’TP (dimetylsalts on based molecule was trimethylenetetrathiafulvalene) The elemental analysis recently publishedlz . revealed the usual 2:1 stoichiometry. The EPR results of the a occurrence low suggested temperature antiferromagnetic ground state for three of these samples, although the moderate quality of the crystals did not allow any more detailed study. In this letter we report the structural and physical properties of three DKTTTF salts of higher quality, two of them were prepared with tetrahedral anions (clo4 and ReO4),and the last one with an octahedral anion (AsF6) and clearly demonstrates the magnetic nature of the low temperature ground state. We finally discuss the correlation existing between the

Résumé: On magnétiques

Until recently the radical cation salts low temperature antiferromagnetic a presenting the ground state were essentially found among and TMTSF1 2 3 (tetramethyltetraselenafulvalene) TMTTF (tetramethyltetrathiafulvalene) series4 5 6 7 These compounds all present the same raw formula: where X is a (TMTCF)2X ( where C = S orandse and the same have anion) diamagnetic at room organisation crystallographic are described with the they temperature 8 9 . The main features of triclinic PI space group. their crystal structure is the occurrence of sheets of anions separating the organic stacks which are characterised by a "zig -zag" stacking of the TMTCP molecules. As a consequence diads of TMTCF are formed (i.e. two different interchain overlaps should exist ) although the TMTCF chains are found to be only weakly dimerised. Moreover, if non-centrosymmetrical, the anions should be statistically disordered to restore the center of symmetry of the structure. If they get ordered at low temperature the pristine unit cell cannot be preserved and the condensation of a superstructure is necessary. such

salts. a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01986004702015700

158

physical properties organisation of these compounds. observed

and

the

crystal

EXPERIMENTAL was synthesis of the DMTTTF molecule as already describedl2 . However the last step (i.e. the separation of DMtTTP from TMTPF and flash chromatography on HMTTF) was improved : silica gel with CS 2 as eluant was used. The

The realized

residual

amount

of

TMTTP

or

HMTTF

after

The results given in precisely determined. figure 2 include a dashed line which represents an extrapolation between the high and low T behaviour. Nevertheless the comparison with the corresponding TMTTF salts (as an example the conductivity of the AsP6 salt is also given in figure 2) leads to the

be

conclusion that the metallic behaviour of the DMtTTF salts is more pronounced : a higher value of a 300KI a lower estimation of Tmax are found. Moreover a lower value of the gap in the localised regime for the a x 200 - 300K (instead of 600 - 800K corresponding TMTTF salts’s ) is also measured. Note that, cracks, despites the occurrence of

this

purification was shown to be less than 0.1% using a capillar gas chromatrograph. The samples were then prepared by the standard electrochemical technique from 1,1,2-trichloroethane. X ray intensities were collected on an Enraf-Nonius CAD4 diffractometer with graphite monochromatized CU-lCa radiation. The conductivity measurements were made using a standard four probe technique with The AFMR results were platinum paint contacts. obtained with a standard EPR Varian spectrometer operating at 9.3 GHz (X band).

RESULTS

The crystal structure of (DMtTTP )2AsP 6 has been publishedL3 . A triclinic structure similar to that of the TMTCF salts was found. On the contrary we find ( DMtTTF )2ClO 4 monoclinic (space group C 2/c). The unit cell parameters are

already

a -

27.768

A

J9 - 110.8603BF

b -

7.159 1

V - 2608

c -

14.040 A

Å3

A relevant projection of the structure is given in A "zig -zag" figure la. stacking of the DMtTTF molecules is found along the b axis. Within the

stacks the molecules are related through a screw Since their average plane is perpendicular to axis. b, only one interplanar distance is found (i.e. there is only one intrachain overlap). Noticeable interstack interactions are suggested by the short interchain S-S distances. The main difference between the triclinic (AaF 6) and the monoclinic structures is illustrated in figure lb which shows that the relative position of the molecules is different in these two cases. Last but not least, it should be noted that the Cl atoms lay on a 2 fold axis in the monoclinic structure (see fig. la). Thus no orientational disorder of the can exist. The results concerning the salts are Re04 still preliminary. However, the structure of this compound is also monoclinic and similar to that of the The occurrence of sonoclinic monoclinic symmetries is an important asset obtained with this new series of materials. It is striking that a similar structure has been recently found14 for the selenium analog the More structural details on DMtTTF salts will be published elsewhere. The electrical conductivity results are given in figure 2. The room temperature value of r is about 100, 170 and 250 Q -1.an -1. for the AsF6. Re04 and salts respectively. When decreasing the temperature Then an a metallic behaviour is first observed. an revealed localisation is electronic by exponential increase of a. Due to the formation of "cracks" (usually observed in these series of the and compounds15 ) the temperature amplitude of the maximum of the conductivity cannot

FIGURE la :: Crystal structure of (DMtTTF)2ClO4I projection perpendicular to the stacking axis.

(Cl04)

ClO4

Clo4 salt.

(DMtTSF)2ReO4.

FIGURE 3.b Projection of the crystal structure of the DMtTTP salts along the direction C3-Cl3 (see fig. la) to show the difference the between triclinic an monoclinic organisations.

C104

(Tmax )

results are found at low temperature observed the intrinsic origin of The slight differences found between localisation. the three salts may be real although an extrinsic influence of the sample quality cannot be excluded. different A more systematic comparison of the members of the series is in progress.

reproducible proving the

159

for around the (DMtTTF)2AsF6 The rotation around the direction of stacking axis. in gmax (long axis of the molecule) is alsothegiven T - 0 The fit was obtained using insert. with n /y - 3.35 KGauss, theory Nagamiya n* /y - 5.18 KGauss. The hard axis is found at about 130 from the stacking axis.

FIGURE 3 :

AFMR rotation

rotating

FIGURE 21 Electrical conductivity of the DMtTTF salts. Note that the low temperature activation also given energy is smaller than for

for

comparison.

(TMTTP)zAsP6

pattern at T the crystal

- 2 , 5 IC

pattern at T the crystal

- 2.5 IC

EPR results amount of

concerning samples containing a residual TMTTF were already published12 As of expected the occurrence of a few percent impurities does not drastically change the magnetic

behaviour and similar results were obtained with the new batch. The EPR linewidth is first a decreasing function of T down to 20-30K and then increases quickly at lower temperature. At the same time the paramagnetic susceptibility remains high and weakly This behaviour suggests temperature dependent12. the occurrence of growing antiferromagnetic (AP) fluctuations as T is reduced below 20K. One way to prove the condensation of an AF ground state is to detect an antiferromagnetic resonance (AFMR). This

technique

was

already successfully applied

using

a

conventional X band spectrometer to the study of the magnetic ground state of several TMTTF salts 7. With this method, characteristic rotation patterns are obtained which give the resonance field (usually far from wo/y - 3300 Gauss) as a function of the

of When the temperature the rotation angle. is low compared to the transition experiment temperature (Tn) the fit of the results requires two parameters n. and n- which are the zero field low T In this uniform spin wave frequencies16. limit, n_/y (where y is electron gyromagnetic ratio) the Moreover is also the field. spin-flop comparison between the experimental data and the theory gives an estimate of the position of the magnetic axes. we found the AFMR for the three studied compounds. The Neel temperature Tn is about 9 K for the AsF6 and ClO4 salts. The long range magnetic order is only observed below T n = 3.5 K for the Re04 salt. Rotation patterns obtained at 2.5X for the AsF6 and rotation the C104 are given in figures 3 and 4. theThedirection axes are either the stacking axis or of the maximum of the g factor (long axis of the organic molecule). Typical fits obtained using the standard AFMR theory16 are also given.

FIGURE 41

APMR rotation

for around the The rotation around the direction of stacking gmax (long axis of the molecule) is also given in The fit was obtained using the T = 0 insert. with Nagamiya theory 0_ /y - 2.30 KGauss, n4 /y - 3.55 KGauss. The stacking axis is the hard magnetic axis.

(DMtTTF)2ClO4 axis.

rotating

160

AsP6 salt a "bubble" is found when rotating the crystal around a. This pattern characterises the situation where slightly above the applied The rotation around g microwave frequenCy7. indicates a misorientation of the magnetic axes relative to the crystallographic frame since the minimum of the resonance field is not found in the direction perpendicular to a. A similar result was already obtained for the TMTTF salts presenting an AF ground state7 and is the expected consequence of The misorientation also the triclinic symmetry. affects the shape of the bubble which becomes slightly unsymmetrical. Finally the fits give ti_ and n+ and an estimate of the position of the hard axes magnetic axes. The easy, and a, in are respectively close found7 for directions close to that The angle between the and hard axis and a is about 13°. projection of the axis in the (b/, plane is situated easy approximatively along the direction of 9int (short axis of the DMTTTF molecules). Moreover we find P_/y m 3.35 kGauss and o..../y :ac 5.2 kGauss. salt are The rotation patterns obtained for the For this characteristic of the case n_ n+. Wo the salt, no misorientation is observed since resonance field is extremum at 900 from the stacking axis b which is exactly the hard axis. This is in agreement with the monoclinic crystal symmetry which implies that P is a principal magnetic direction. The easy and intermediate axes are at about 10° from The and gmax respectively. the direction of fits are ound for n_/y a- 2.3 kGauss and best n..../y :ac 3.5 kGauss. For the

O_-is

intermediate and already

to b - axc c*

(TMTTF)2SCN

(TMTTf)2SbF6.

T’he

c*)

Clo,

gint

For the

Re04 salt we got quasi-sinusoidal rotation as when the patterns expected experimental The temperature (2.5 lc) is only slightly below Tn. position of the magnetic axes is close to that found for the salt and already ClO4 0-/y =

0 . 9

kGauss,

n+/y m

1.3 kGauss.

The

determination of the spin-flop field now requires the extrapolation at T = 0 of n_(T) which gives the rough estimate n_( 0) :ac 1.5 - 2. kGauss. CONCLUDING ARKS

The first noticeable result concerning the DMtTTF series is the conducting behaviour. Although the MftTTF molecule is non-symmetrical, high conductivity can be obtained (a is better for a given anion than for the corresponding TIf1’TP salt). A8 expected, -.- trical molecules are easily incorporated to realise a zig-sag stacking of the conducting chains. Contrary to the TIf1’TP series, both triclinic and monoclinic samples are found with the DMt’1’TP molecule. It is noteworthy that monoclinic symmetry is found with the tetrahedral anions. Realizing that these anions are disordered in the triclinic (T) phase and ordered in the monoclinic (K) structure, it may be argued that the latter crystal organisation the allows anion ordering without the lack of elastic energy required by the condensation of a superstructure. As far as the organic lattice is concerned, figure 2b shows that the M and T crystal structures present some similarities. In particular both structures the existence of negative and positive imply contributions to the transverse overlap as already found in the TMTCF series11 and it may be argued that this singular situation favors the AP ground state. An alternative explanation for the occurence of antiferromagnetism may be the weaker localisation found in the DMtTTP series (compared with that observed for the corresponding TMTTF salts). Such an explanation has already been used to discuss the

competition between electronic instabilities in TMTTP

the

series’7.

With the exception of the position of the magnetic axes which directly results from the crystal symmetry, the parameters of the AF ground state (T , n and n... ) are similar for the triclinic AsF6 and the monoclinic salts. The obtained values are close to that already found for the TMTTP saltg7. Smaller values of n:t are found for the Reo 4 but this is the consequence of a smaller and the ratio Tn n+ / 0_ remains almost the same for all the

Cl04

compounds. In summery the physical properties of the TMTTF and DMtTTF salts are closely related. With this new series, we have shown that the possibility of finding antiferromagnetism is not restricted to the TirlrCP series (C - S or Se) but can be extended to new compounds grown with unsymmetrical molecules. Moreover the triclinic structure of the ’1’MTCP salts is not a necessary condition to observe the AP state either. However, some characteristics of the local structural organisation of these compounds may be relevant to determine the topology of their low

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