Solid State Communications, Vo1.48,No.2, pp.99-103, 1983. Printed in Great Britain.

0038-1098/83 $3.00 + .OO Pergamon Press Ltd.

HYDROGEN BOND FORMATION AND ANION ORDERING IN SUPERCONDUCTING (TMTSF),C~~, AND (TMTsF),AsF,.

M. A. Beno, 6. S. Blackman, P. C. W. Leung and Jack M. Williams*

Chemistry Division, Argonne National Laboratory Argonne, Illinois 60439

(Received 24.May 1983 by R. H. Silsbee)

An examination of the low temperature (125K) X-ray diffraction deter- mined structure of (TMTSF),ClO,, the only ambient pressure organic superconductor known, reveals a previously unseen nearest-neighbor hydrogen bonding environment surrounding the anion resulting in short oxyqen atom to methyl-hydroqen atom interactions that are likely respon- sibie for the order;ng of the ClO,- anion associated with a transition observed at 24K.' Suoerconductivitv is observed in (TMTSF),CIOs only when the anions adopt'an ordered (relaxed') configuration..-In contrast to the previously reported single short Se l **O distance, the O***H-C contacts are both short and numerous. The observed librational motion of the perchlorate anion provides further support for the importance of the O***H interactions in the pinning of the anion. Additional dis- cussion regarding the influence of F .**H hydrogen bonds on the AsF6- anion disorder in (TMTSF)2A~F,, a superconductor under applied pressure only, is also presented.

1. INTRODUCTION

The recent reports of a sluggish anion ordering transition at 24K (etiology unknown) as a prerequisite to the onset of superconductivity (Tc = l.lK) in the quasi-one-dimensional conduc- tor (TMTSF)iClOt,, the only known ambient pres- sure organic superconductor derived from tetra- methyltetraselenafulvalene,(TMTSF), raises the question of the forces driving this ordering phenomenon.1 It appears that in quickly cooled samples, which lead to the "Q = quenched state", neither anion ordering nor superconductivity are observed even at the lowest attainable tempera- tures. However, in slowly cooled samples pos- sessing the "R = relaxed state", anion ordering is accompanied b transformati0n.l $

a (a, 21, c) crystallographic AlT reporTed (TMTSF)$, X =

monovalent anion, salts are triclinic (Pl). Diffraction data (298K, 125K)2ss of (TMTSF)$l04 have shown that in the anion disordered struc- ture the Cl atom is located at the i site with 8-half oxygen atom positions observed for the tetrahedral C101,- anion as required by an inver- sion center. From X-ray data it appears that at 24K the centers of symmetry disappear resulting in a single ordered C101,- configuration in the unit cell (see Fig. l)." To date the cause of this transformation, first noted in electrical resistivity studies,4 has been elusive, but the formation of one short Se=**0 interaction (bond distance 3.3911)A at I25K, versus a van der Waals radius sum of 3.4A) has been suggested as a possible driving force for the pinning of the anions.5 An understand- ing of the origin of the anion ordering phe-

*Author to whom correspondence is to be addressed.

99

nomena could provide the insight needed for the rational synthesis of new organic superconduc- tors based on TMTSF if the anion ordering is predictable and can be controlled.6

2. RESULTS AND DISCUSSION

From a detailed examination of low tempera- ture X-ray diffraction datasg6 of (TMTSF)2C104, in which all methyl group H-atoms were located and their positional parameters refined using full matrix least-squares procedures (Biso fixed), and taking special note of the immediate surroundings of the Clot,- ion in its cavity between TMTSF molecules in the crystamice, we have discovered a very asymmetric distribu- tion of oxygen atom to methyl-group hydrogen- atom [HzC-H***O-ClOs-1 bonding interactions (see Fig. 2 and Table 1). These appear to be of seminal importance in pinning the anion in an ordered configuration which may be associated with the phase transition and subsequent dis- appearance of the center of symmetry that occurs at 24K. In fact, it is possible that the Cl atom of the anion never resides at the i site at any temperature, i.e., the anion lnay always be statically ordered (in a single unit cell), and slightly displaced about-center ofsyrmnetry, but the "average" crystal structure derived from diffraction data requires a disordered lattice due to the two (crystallographically imm anion configurations. For comparison, it is known that in (TMTSF)zReOb the phase transition at 180K is accompanied by the formation of an observable superlattice and involves the order- ins and displacement of the anion as well as a

"2kF" distortion of the TMTSF stack.7 However, using existing diffraction data316 for (TMTSF)z- C104, in which there is no indication of a

100 (TMTSF)2C104 AND (TI'fTSF)2AsF6 Vol. 48, No. 2

Fig. 1. Stereoview of the postulated ordered structure of (TMTSF),ClO,

at 24K.l' The center of symmetry at the Cl atom of the tetrahedral

anion has been removed resulting in a (a, b, 5) crystallographic

transformation. Ordered ClO,- ions, and two types of TMTSF stacks

with (faint lines inside unit cell boundary), and without short

Se. e.0 interactions are formed.

superstructure at 125K, it is not possible to resolve any slight displacement of the anion from the center of symmetry because of the large oxygen atom thermal motion at low temperature. This results in inexact Cl-0 distances (viz., to O(l), 1.519(7M; to O(2), 1.453(7)A; to O(3), 1.392(9)A; and to O(4), 1.368(9lA, respectively, versus an accepted Cl-0 bond distance of 1.46Aa). The relative thermal inactivity of O(1) and O(2), compared to O(3) and O(4), un- doubtedly results from the greater involvement of O(1) and O(2) in H-bond formation (see - Table 2). A heat capacity study and residual WttrOPY

determination might shed considerable light on the degree of lattice order at very low tempera- tures.g

When considering the methyl group H-atom to oerchlorate ion oxvqen atom (HPC-H***O-C~O~-) &tact distances hid geometry-given in Table 1 and Figure 2, it is clear that the oxygen atoms of the anion reside in very dissimilar environ- ments in terms of H*.*O distances.10 This is the case when using either the hydrogen atom oositions determined from the diffraction data or the "idealized" (C-H = l.O8A, angles =109.5') positions (see Table 1). The observed C-H***0 distances are well within the range for weak hydrogen bonds," with "strength", as determined by the length and number of short O*=*H interaction distances in the order O(2)- >0(1)>0(4)=0(3). We suggest that while these H..*O hydrogen bonding forces may be weak when compared to room temperature thermal excita- tions, as the temperature is reduced and as the lattice shrinks, these interactions become

increasingly significant. Therefore, the pro- nounced asymmetry of the H-bonding environment, and reduced atom thermal motion at low tempera- ture, definitely promote pinning of the C104' anion. This hypothesis is further supported by the observed oxygen atom thermal motion. The size and orientation of the oxygen atom thermal ellipsoids clearly indicates that two of the oxygen atoms [O(l) and O(2)] exhibit much small- er and more isotropic thermal motion than the remaining two [O(3) and O(4)]. This is indica- tive of anion pinning at O(1) and O(2) suggest- ing that the anion is rocking or pivoting about an imaginary vector connecting O(1) and O(2). If the predominant interaction behind the anion ordering process involved a short Se**=0 inter- action,-then O(4) would be expected to have the smallest thermal vibration ellipsoid. However, the reverse is observed, further suggesting that the Senteraction is not likely the primary driving force for anion ordering at any tempera- ture unless the hydrogen bonding interactions are reduced, and the Se***0 interaction increas- ed, by asymmetric shifts of the TMTSF molecules at temperatures G25K.

The effect of H-bonding on the probable orientations of the (octahedral) AsFs- anion within the methyl-group cavity in (TMTSFlsAsFs is much different from that for the (tetrahe- dral) C101+- anion (vide supral because in the former case the anion F***H hydrogen bonding environment is, by contrast, very symmetric (see Fig. 3 and Table 31. We have very recently demonstrated the existence of AsFs- anion disorderI in (TMTSFlzAsF6.

Vol. 48, No. 2 ANION ORDERING IN SUPERCONDUCTING (TMTSF)2C104 AND (TMTSF)2AsF6

w - Fig. 2. The ordered Cl0 anion environment in the low temperature (125K) '3 6

X-ray determined ' crystal structure of (TMTSF),ClO,. Short

H,C-H . * .o-c10,- hydrogen bonding interactions (drawn as faint

lines for 0s.. Hc3.OA) exist for O(1) and O(2) which limit the

thermal motion of these atoms and may be responsible for "pinning"

the Cl0 L1- anion in the lattice.

Table 1. Interatomic distances involving hydro- gen atoms in (TMTSF)~CIOL, at 125K.

Distance(A) Symmetry Oper. I II

U(l) H(5A) O(l)=H(4C)

2 55 2:66

2 49 2:71

l-x,-y,-2

O(l)--H(9B) 2.70 2.53 O(l)--H(9C)* 2.75 2.60 O(l)--H(lOB) 2 .a7 2.69 O(Z)--H(4C) 2.36 2.40 O(Z)--H(5A) 2.44 2.39 O(Z)--H(9A) 2.50 2.35 O(Z)--H(lOC) 2.77 2.57 O(Z)--H(9B) 2.90 2.68 O(3)--H(lOB) 2.73 2.60 O(3)--H(5A) 2.75 2.69 O(3)--H(4A) 2.76 2.68 O(3)--(9B) 2.85 2.56 O(3)--Se(3) 3.60 O(4)--H(5A) 2.62 2.56 O(4)--H(lOA) 2.76 2.76 O(4)--H(9B) 2.77 2.51 O(4)--H(lOC) 2.80 2.64 O(4)--Se(3) 3.39 C(4)--H(4A) 0.99 1.08 C(4)--H(4B) 0.92 1.08 C(4)--H(4C) 1.16 1.08 C(5)--H(5A) 1.03 1.08 C(5)--H(5B) 0.94 1.08 C(5)--H(5C) 1.18 1.08 C(9)--H(9A) 0.92 1.08 C(9)--H(9B) 0.84 1.08 C(9)--H(9C) 0.89 1.08 C(lO)--H(lOA) 1.06 1.08 C(lO)--H(lOB) 1.04 1.08 C(lO)--H(lOC) 0.93 1.08

l-x,-y,-2 l-x,-y,l-2 x,y-1 ,z-1 x,y-l,z-1

x-l,Y,Z x-1 ,Y ,z 1-x,1-y,l-z 1-x,1-y,l-z x-l,y,z-1 -x,1-y,l-z x-l,Y,Z x-l ,y-1 ,z 1-x,-y,l-z l-x,-y,l-z l-x,-y,-z x-l ,y-1 ,z-1 x-l ,y,z-1 x-l ,y-1 ,z-1 x-l ,y,z-1

All hydrogen atoms within a sphere of 4A in radius around the Cl atom are included in this table. [*H(SC) is outside the sphere]. This atom is included in the table because of the short O--H distance. (I) The hydrogen positions are determined by least-squares refinement. (II) Idealized hydrogen positions fixed at 1.08A from their respective carbon atoms.

101

Table 2. Thermal components along principal axes (AZ) for the C10~,- anion from the low temperature (125K) X-ray crystal structure of (TMTsF)~c~~~.

?iTOM RMSDl* RHSDZ RMSD3

oc11, 0.1596 0.1196 0.2146 0.1842 0.2545 0.2039

:I:; 0.1338 0.1533 0.2216 0.1965 0.4326 0.2623

O(4) 0.1605 0.2292 0.5014

*Root-mean-square amplitudes.

Fig. 3. The H-bonding environment about the AsF6- anion in the low temp-

erature (125K) X-ray crystal structure of (TMTSF)2AsF, is far

more symmetrical than that of the ClO,- anion as depicted in

Fig. 2. For clarity the AsFG- anion has been drawn in an ordered

configuration with six F-atom positions. A disordered model with

twelve partially occupied fluorine positions

results I2 (see Table 3).

Table 3. Interatomic distances involving hydrogen atoms in

gives similar

(TMTSFlzAsFc at 125K.

Distance IA) Ordered AsF6- Disordered AsFs-

I II* I IB II* IIB* Sym .

;(1&4~~ (1) (5 1 2:80 2 61 2 2:80 18 2:70 2 60 2 3:04 65 2 2:70 77 2 3:04 82 x-l,Y,Z x-l ,y-1 ,z

F(l)--H(5C) 2.97 2.97 3.01 2.88 3.02 2.89 x-l,Y,Z F(2)--H(5A) 2.58 2.65 2.49 2.81 2.56 2.88 x-I,Y,Z F(Z)--H(4C) 2.63 2.53 2.54 2.83 2.45 2.74 x-I,Y,Z F(2)--H(9C) 2.69 2.74 2.73 2.61 2.79 2.66 1-x,1-y,l-z F(2)--H(lOC) 2.78 2.74 2.86 2.59 2.82 2.55 1-x,1-y,l-z F(2)--H(9C) 3.00 2.99 2.99 3.03 2.98 3.03 x-l,y,z-1 F(3)--H(9B) 2.66 2.64 2.65 2.72 2.63 2.71 1-x,-y,l-z F(3)--H(9A) 2.77 2.67 2.78 2.79 2.68 2.69 x,y-l,z-1 F(3)--H(lOC) 2.81 2.80 2.82 2.81 2.81 2.81 x,y-l,z-1 F(3)--H(lOB) 2.90 2.96 2.95 2.78 3.01 2.83 x,y-l,z-1 F(3)--H(lOC) 2.99 2.97 2.89 3.30 2.86 3.27 1-x,1-y,l-z

*Idealized hydrogen positions C*..H dist. = 1.08A. All distances less than 3.OA are included in the table. For the disordered model, six independent F-atom positions were refined, F(l), F(lB), F(2), F(2B), F3, and F(3B). The distances to these fluorine atom positions are given under I and IB for the experimental hydrogen positions and II and IIB for the idealized positions.

Vol. 48, No. 2 ANION ORDERING IN SUPERCONDUCTING

Using the same arguments presented earlier in this paper regarding the effects of the hydrogen bonding environment on the anion orientation, we find that for the AsFs- anion (see Table 31, none of the F**=H separations are as short as those for O***H in the C101,- case. One expects F-*-H H-bonding distances to be 2.58 or less in the AsFs- case.11 Therefore, if the slight asymmetry in the hydrogen bonding environment noted for the AsFs- case (Table 3) is sufficient to pin the anion in two (disordered) config- urations, then it is not at all surprising that the C104- anion is more radically affected by its H-bonding environment.13

In surrnnary it should be noted that while we have pointed out the importance of O***H and F*=*H interactions in pinning the anions in (TMTSF~~X, X = C101+- and AsF6-, respectively, it is extremely difficult to predict the occurrence and quantitative effects of this phenomena from one anionic derivative to another. We do note, however, that within the class of oxygen con- taining tetrahedral anions (ClO4-, BrO4-, Re04-, 104-l the metal-insulator transltion tempera- tures, generally associated with anion ordering, increase with anion size which is to be expected if the strength of the H-bonding interactions

(TMTSFj2C104 AND (TMTSF)2AsF6 103

between the oxygen atoms and the methyl group hydrogen atoms also increase at the same time. Anion ordering schemes different from that observed for the Cl0 -

4 ion, as is the case for

(TMTSF12Re04, may or ginate from different methyl group orientations at low temperature. Finally, the lack (or weakness) of these H-bonding interactions may have a profound effect on the low temperature properties of (TMTSFlsX materials. For examole. in iTMTSFj;BFb, which contains the smallest tetrahedral ion TMTSF derivative known, and which has not yet been shown to be a super- conductor, the anion may never completely order at even the lowest temperatures because it may be too small for its methyl group cavity and does not form H-bonds of sufficient strength to cause ordering.

Acknowledgments - Work at Argonne National Laboratory is supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences, under Contract W-31-109-Eng-38. Support was provided for G. S. Blackman as a Student Research Participant under the auspices of the Division of Educational Programs, Argonne National Laboratory.

REFERENCES

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(a) Mu-Yong Choi, P. M. Chaikin, P. Haen, and R. L. Greene, Solid State Commun. 41, 225 (19821; (b) P.Garoche,.Drusetti, and K. Bechgaard, Phys. Rev. Lett. 49, 1346 (19821; (cl J. P. Pouget,G.Shirane, K. Bechgaard, and J. M. Fabre, Phys._Rev. B, 5203 (19831. K. Bechgaard, K. Carneiro, F. B. Rasmussen, M. Olsen. G. Rindorf. C. S. Jacobsen. H. J. Pedersen, and J. C. Scott, J. Am. Chem. Sot. 103. 2440 (1981).

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%?. Beno, J. M. Williams, M. M. Lee, and 0. 0 . Cowan, Solid State Commun. 44, 1195 (1982). - --- 0. U. Gubser, W. W. Fuller, T. 0. Poehler, J. Stokes, D. 0. Cowan, M. M. Lee, and A. N. Bloch, Mol. Cryst. Liq. Cryst. 2, 225 (19821; S. Tomi~.~e~~ and K. Bechgaard, J. Phys. (Paris) Lett. 43, L839 (19821. ----- N. ThoruD and G. Rindorf, private commun- ication.'

-

J. M. Williams, M. A. Beno, J. C. Sullivan, L. M. Banovetz. J. M. Braam. G. S. Blackman, C. D. Carlson,-D. L. Greer,-and D. M. Loesing, J. Am. Chem. Sot. 105, 643 (19831.

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R Mo t n.-i%uget, R. Comes, and K. Bechg&i Phys. Rev. Lett. 49 1008 (19821. E. Maveri:k,GGrossemer,'and K. N. Trueblood, Acta Cryst. 835, 2233 (19791.

9. For a discussion of anion order/disorder and heat capacity/residual entropy determinations in hydrogen bonded solids see: J. M. Williams, J. Chem. Ed. 52, 210 (1975).

10 Using datafrom Ref. we have drawn (Fig. 21 the tetrahedral ClOk- anion and its immediate environment. Anisotropic thermal ellipsoids depict only four of the total of eight one- half oxygen atom positions used in the refinement. Attempts to resolve possible disorder associated with atoms O(3) and O(4) always converged to only 8 disordered one-half occupancy oxygen atom positions.

11 W. C. Hamilton and J. A. Ibers, "Hydrogen Bonding in Solids", W. A. Benjamin, New York, p. 16, (19681. Calculated van der Waals radius sums for O***H and F***H are 2.6A and 2.5A, respectively.

12. J. M. Williams, M. A. Beno, J. C. Sullivan, L. M. Banovetz, J. M. Braam, G. S. Blackman, C. D. Carlson, D. L. Greer, D. M. Loesing and K. Carneiro, J. de Phys. (in press).

13. The difference in H-bonding interactions formed by the AsFs- and Cl01+- ions, respec- tively, is that in the case of AsFs- the average negative charge is somewhat lower and more isotropic being spread over six F-atoms while in the C101,- anion the charae is dis- tributed over four atoms at the corners of a tetrahedron.