Physica 108B (1981) 118 7-1188 SC 5 North.Holland Publishing Company

COMPARATIVE X RAY DIFFUSE SCATTERING STUDIES OF SOME SUPERCONDUCTING AND

NON SUPERCONDUCTING I : 2 SALTS (TMTSF)2 X AND (TMTTF)2 X ; WITH X = PF6, AsF6,CI04, NO 3

J.P. Pouget and R. Com~s Laboratoire de Physiques des Solides - Universit~ Paris Sud - 91405 Orsay,(France)

K. Bechgaard H.C. Oersted Institute, Universitetsparken 5, DK-2IOO - Copenhagen -(Denmark)

J.M. Fabre and L. Giral Laboratoire de Chimie Structurale Organique (U.S.T.L.) - 34060 Montpellier -(France)

X-ray diffuse scattering experiments performed on the highly conducting(TMTSF)2 X com- pounds (X=PF6, AsF6, CI04, NO^) reveal no charge density wave instabilities, whether they display superconducting ~roperties or not. Instead, another type of phase transi- tion, not observed for the superconducting members (PF6, AsF6, CIO~), takes place in the non-superconducting (TMTSF) 2 NO3,and involves an ordering of the counter ions, which doubles the lattice constant in chain direction. Similar experiments performed on the less conducting (TMTTF)~X compounds (X =CIO4,PF6) , which do not present a superconducting state, either reveal ~he ordering transltlon of the counter ion (CIO4) or the quasi I-D 2k F scattering characteristic of a charge density wave instability (PF6). Consequences on the dimensionality of the Fermi surface and on the superconducting instability are discussed.

The observation of Kohn anomalies and Peierls transitions coming from the instability of a ~D electron gas towards the formation of charge density waves (CDW), at 2k F (and 4kF) wave vec- tor(s)has almost become a routine result of dif- fuse X-ray scattering measurements on organic conductors (1). Figure ! illustrates the typi- cal aspect of such diffuse scatteringsfrom TMTSF-DMTCNQ (2).

Figure I. X-ray diffuse scattering from TMTSF-DMTCNQ at !25 K. The 2k F (white arrows) and 4k F (black arrows) scatteringsappear as diffuse lines at respectively O.25a* and O.5Oa* from the layers of Bragg reflections , of the main lattice,perpendicular to the stacking di- rection. The diffuse intensity comes mainly _) from the selenium atoms of the TMTSF molecu~s ~.

This particular example has been chosen here for several reasons : (i) most of I-D diffuse inten- sity in figure ! arises from the movements or displacements of the Se atoms from the TMTSF molecules,also present in the title compounds which reveal superconducting properties (3-5) ; (ii) the positions of the 2k F and 4k F scatte- rin~ from TMTSF-DMTCNQ give a 0.5 electron charge transfer which is similar to the charge

transfer from the title compounds,where it is imposed by the chemical formula;(iii) in spite of the high value reached by the conductivity of TMTSF-DMTCNQ at low temperature and under pres- sure it does not become superconducting. In sum- mary TMTSF-DMTCNQ is the organic conductor,with a typical 2k F Peierls transition at 42 K,which is closest to the title compounds.

Both the selenim and sulfur title compounds are isomorphous. Compared to the previous case of TMTSF-DMTCNQ a first difference is provided by the zig zag chains of the slightly dimerized TMTSF or TMTTF molecules (6) which is imposed by the counter ions. The resulting doubling of the lattice constant in chain direction coincides with the doubling which tends to be driven by the 4k F electronic instability. It is therefore understandable that none of title compounds re- veal the 4kF scattering (see for example the cases in figure 2 and 3).

Among the title compounds,the (TMTSF)2X and (TMTTF)2X families present respectively very different physical properties (8,]o).Theselenium compounds, whether superconducting or not~are highly conducting ~ From a structural point of view they have also in common that they do not present the development of ]-D 2k F scattering for decreasing temperatures which is characte- ristic of the Peierls instability. This is c~ar- ly shown by the comparison of the X-ray pattern from figure l, with the pattern from (TMTSF) 2 Cl~ shown in figure 2. This experimental aSsen- ceO~f ]-D diffuse scattering is all the more meaningful since the high scattering factor from selenium (compared to sulfur) usually gives rise to well contrasted diffuse lines as in figure |. All the selenium title compounds have a similar behaviour regarding the absence of I-D 2k F scat- tering . At most it has been possible in the

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case of AsF6,between I00 and 50 K (4),and in the case of CIO. between 200 and IOO K, to detect q very weak I-D 2k F scattering which is at the border-line to emerge from the diffuse thermal background,but which disapears towards lower temperatures.

further reduce the density of states at the Fermi level, which is unfavourable for the deve- lopment of a superconducting state (9).

Figure 2 : X-ray pattern from (TMTSF) 2 CIO& at 11K. No particular I-D diffuse scattering is observed on top of the continuous background due to thermal scattering (phonons).

These results clearly show the mutual exclusion of the instabilities leading to the superconduc- ting state and to the Peierls insulator.

Within the family of the selenium compounds,ano- ther structural behaviour allows to differen- ciate between those which display a superconduc- ting state ( X = PFa, AsF 6, CIO~) and (TMTSF)2NO 3 which does not (7,8~. In (TMTS~) 2 NO~, another type of phase transition takes place~round 4OK and involves an ordering of the counter ions. In spite of the fact that this phase transition doubles again the lattice constant in chain direc- tion,as a Peierls transition would do, such a Peierls transition can be ruled out from the ab- sence of I-D precursor scattering, from the very strong intensity of the superlattice reflections, and more generally from the observed transport properties (8).

Similar experiments performed on the less conduc- ting" (TMTTF)~X salts (X = CiOa and PF6) ' which do not reveal'superconducting ~roperties, either show the quasi I-D scattering characteristic of a 2k F charge density wave instability (case of PF 6) (2) or a metal insulator transition driven by the ordering of the counter ions (case of CI04). The X-ray pattern from (TMTTF) 9 CIO 4 shown in Figure 3,illustrates the typic~l features ari- sing from an ordering of the counter ions.

Besides the clear demonstration on several exam- ples of the mutual exclusion of the superconduc- ting and the Peierls instability which is the main conclusion of this stud~, other perhaps more subtle effects are revealed by this struc- tural investigation. The absence of a metal- insulator transition in (TMTSF)pNO3, in spite of the ordering of the counter i~ns, shows that the Fermi surface must be far from one-dimension- nal in this compound,in order to allow the ope- ning of only partial gaps. Such partial gaps

Figure 3 : X-ray pattern from (TMTTF) 9 CIO A at 20 K. In addition to the Bragg reflecTions-from the main lattice (compare with the corresponding reflections from the isomorphous (TMTSF) 2 CIO 4 from figure 2), very strong superstructure reflections (arrows) are clearly visible. These superstructure reflections correspond to a dou- bling of the unit cell in chain direction and are due to the ordering of the CiO4--counter ions, below 70 K.

This is a direct structural confirmation of the more isotropic character of the selenium salts already invoqued for the explanation of some physical properties. In contrast is the observation made with (TMTTF)p CIO 4. There, a similar counter ion ordering drives a metal in- sulator transition, in agreement with a more quasi one-dimensional Fermi surface (IO).

[I] See for example Pouget et al in "1980 An- nual Conf. Condensed Matter Division of the European Physical Society, Antwerpen 1980, Plenum, Press (in press).

[2] J.P. Pouget in "Int. Conf. on Low Dimensio- nal Synthetic Metals" Helsingor 1980, Chemi- ca Scripta (in press).

[3] D. Jerome et al., J. Phys. Lett., 4__[],(1980), L 95.

[4] M. Ribault et al., J.Phys. Lett., 4_L,(198~, L 605.

[5] K. Bechgaard et al., J. Am. Chem. Soc., (submitted).

[6] J.L. Galign~ et al., Acta Cryst. B35, 1129, (1979) ; Brunet al. (prepint).

[7] Results quoted by Parkin et al., J. Phys. C (submitted).

[8] K. Bechgaard et al., Sol. State Co~., 33 (1980), 1119.

[9] J. Friedel, J. Phys. Lett.,36, (1975) L 279.

[IO] P. Delhaes et al., Mol. Cryst. Liq. Cryst., 50, (1979), 43.