~ Solid State Communications, Vol.51,No.12, pp.931-934, 1984 0038-1098/84 $3.00 + .00 Printed in Great Britain. Pergamon Press Ltd.

ANION-DONOR COUPLING IN (TMTSF)2X SALTS: SYMMETRY CONSIDERATIONS

Thomas J. Kistenmacher

Milton S. Eisenhower Research Center, Applied Physics Laboratory, The Johns Hopkins Univers i ty , Laurel , Maryland 20707, USA

(Received 25 April 1984; in revised form 02 July 1984 by O. M. Rowell)

A study of the geometry of the structural cavity in which the anion resides in the novel (TMTSF)2X salts has revealed s stronger than previ- ously realized symmetry basis for the rationalization of the contrasting crystal physics displayed by salts with anions of grossly different molecular symmetries. Moreover, the asymmetry of the anion-cavity interactions in salts with tetrahedral anions leads to the identifica- tion of a principal structural driving force for anion ordering. Final- ly, additional symmetry arguments suggest that salts with dipolar anions with C2v molecular symmetry are candidates for a low-temperature ferro- electric ground state.

It has now become quite clear I-S that trends in the structural and physical properties for the series of salts (TMTSF)2X, where TMTSF is the electron donor tetramethyltetraselena- fulvalene (Figure I) and X is any one of a variety of complex inorganic anions, are most readily manifested and interpreted when salts with anions of grossly different symmetries are treated as members of independent subsets. Perhaps the most striking difference in physical property between these subsets concerns the temperature at which a metal-lnsulator transi- tion sets in. Salts with octahedral AX 6- anions exhibit nearly identical transition temperatures [TM_ I = 12K], while the range of transition temperatures is at least 140K [TM_ I = 40K for (TMTSF)2BF 4 and TM_ I = 180K for (TMTSF)2ReO 4] for salts with tetrahedral AX 4- anions. 4 Moreover, the absence of any conceivable correlation of TM- I with anion size for salts with AX 6- anions is in sharp contrast with a general increase in TM- I with increasing

CA SE SE ("

C5 C SE SE 15

TMTSF

Fig. I. Molecular structure of the electron donor tetramethyltetraselenafulvalene, TMTSF. Methyl groups strongly involved in the definition of the anion cavity have been highlighted.

anion size (convenlently~udged by a van der Waals-like estimate, RlVaW , for the anion radi- us) for salts with AXe- anions. 1

The antecedents of the differing physical behavior shown by subsets of the (TMTSF)2X salts are likely structural in nature. Firstly, from a molecular structure perspective, an idealized AX 6- anion exhibits O h symmetry and is intrinsi- cally centrosymmetrtc; in contrast, an idealized AX 4- anion displays T d symmetry and is intrinsi- cally non-centrosymmetrtc. Secondly, the local environment about the cavity in which the anion is positioned is at least centrosymmetrlc, with the average structural motif characterized by the trlclinlc space group PI. Thus, at a very fundamental level, there is a symmetry match between a centrosymmetrlc AX 6- anion and its immediate surroundings, while there is a symme- try mismatch for a non-centrosymmetrlc AX 4- anion (an immediate consequence of which is a positional disorder of the terminal atoms of the anion in (TMTSF)2AX 4 salts).

In this report, the incipient symmetry properties of the structural cavity in these salts and the relationship between cavity sym- metry and anion symmetry are explored in de- tail. From thls endeavor, several important realizations have evolved: (I) there is a much stronger than previously appreciated symmetry basis for the differentiation between salts with octahedral AX 6- anions and those with tetra- hedral AX 4- anions; (2) a principal structural component of the driving force for anion order- ing in salts with AX 4- anions is identified; and, (3) the role that cavity symmetry plays in inducing anomalous behavior for salts with anions possessing permanent dipole moments (e.g., FSO 3- and PO2F2-) is recognized.

Two important features of the anion cavity in these salts were first recognized by Williams and coworkers: $'6 (a) the immediate surface of the anion cavity is largely made up from the terminal methyl groups of the TMTSF donor (Fig- ure I); and, (b) while the cavity geometry itself is largely Invariant across the series of

931

932 ANION-DONOR COUPLING IN (TMTSF)2X SALTS

salts, anions of different symmetries adopt different orientations within the cavity. More recently, 2 it was noted that the same six methyl groups [C(4), C(14), and C(15) and their inverslon-related symmetry metes C(4'), C(14'), and C(15')] showed the shortest distances to the center of mass of the anion, regardless of the symmetry of the anion. It is further exposed here (Table I) that the angular disposition of the methyl group interaction vectors [C(4)... C(4'), C(14)...C(14'), and C(15)...C(15')] are also largely insensitive to anion type, with direction cosines (relative to the orthogonal axes ~, h' and ~* ) and intervector angles vary- ing only slightly on proceeding, for example, from (TMTSF)2AsF67 to (TMTSF)2CI04.8

It is also readily apparent from Table 1 that the C(4)...C(4') and C(14)...C(14') inter- action vectors are near to being normal to the donor stacking axis ~ and essentially orthogonal to each other. In contrast, the C(15)... C(15') vector lles at a rather acute angle (36-38 ° ) to ~. Thus, it is clear that there are distinc- tively recognizable, and pervasive, regularities in the geometrical properties of the anion cavity in addition to its local inversion sym- metry. As noted earlier, 2 the overall geometry of the structural cavity in these salts is near that of a skewed, trigonal antiprism.

Most interesting is the match-up between geometrical features of the anion cavity and symmetry axes for various types of anions. In Figure 2, the AsF 6- anion is oriented in its cavity based on published atomic coordinates. 7 It is immediately obvious that each of the three methyl group interaction vectors lles qualita- tively parallel to one of the three-fold sym- metry elements of the AsF 6- octahedron. Quanti- tatively, the angular deviation between the interaction vector and the normal to the appro- priate triangular face of the AsF 6- octahedron

is only 3 ° for C(4)...C(4'), 6 ° for C(14)... C(14'), and II ° for C(15)...C(15') - a trend which inversely parallels the strength of the methyl group interaction as Judged by its con- tact distance (Figure 2). On the whole, then,

C14 9

Vol. 51, No. 12

3

I L "oqs I Ii 4 . 2 Q 0 I I

6 c~2

c;

C4 C14 C15

Fig. 2.

(TMTSF)2AsF 6

Top: Disposition of the six closest methyl groups about the AX 6- anion in the structure of (TMTSF)2AsF 6 (T = 295K); Bottom: Projection views down the C(4)...C(4'), C(14)...C(14'), and C(15)...C(15') interaction vectors.

the cavity in (TMTSF)2AsF 6 seems remarkably well-suited to accom~modate an anion of O h sym- metry, a fact which probably accounts for the absence of significant positional disorder 7 for the AsF 6- anion.

Table I. Crystallographic direction cosines and Intervector angles for cavity defining methyl group interactions

Direction cosines Intervector angles (angle, deg.) (deg.)

a b' c*

a) (TMTSF)2AsF 6

C(4)...C(4') 0.0442 -0.3046 -0.9514 (87.5) (107.7) (162.1)

C(14)...C(14') 0.2266 -0.9253 0.3041 (76.9) (157.7) (72.3)

C(15)...C(15') 0.7853 0.1277 0.6058 (38.2) (82.7) (52.7)

b) (TMTSF)2CI04

C(4)...C(4') 0.0560 -0.2676 -0.9619 (86.8) (105.5) (164.1)

C(14)...C(14') 0.1778 -0.9491 0.2600 (79.8) (161.6) (74.9)

C(15)...C(15') 0.8079 0.0942 0.5817 (36.1) (84.6) (54.4)

(4,4')/(14,14') 89.9

(4,4')/(15,15') 125.5

(14,14')/(15,15') 75.9

(4,4')/(14,14') 89.2

(4,4')I(15,15') 122.6

(14,14')I(15,15') 78.1

Vol. 5], No. 12

Recalling that cavity symmetry is nearly invariant to anion type (Table I), it is likely that a tetrahedral anion would find the symmetry potentlal of the anion cavity somewhat less agreeable. This suggestion is borne out in Figure 3, where views of the anion-methyl group interactions for (TMTSF)2CI048 are presented. From Figure 3, it is particularly clear that the interaction vectors C(4)...C(4') and C(14)... C(14') lle nearly parallel (the angular devia- tion in each case is ca. 7 ° ) to a two-fold symmetry axis of the perchlorate anion, irre- spective of anion orientation. (Equivalently, these interaction vectors closely follow four- fold symmetry axes of the "cube" defined by the eight half-welghted, oxygen atom positions.) The near orthogonality of C(4)...C(4') and C(14)...C(14'), Table I, ideally matches the interaxial angle for the two-fold axes of the tetrahedra (or the four-fold axes of the "cube"). The strikin G feature of Figure 3 is the absence of near collnearlty of the C(15)... C(15') vector and any of the symmetry axes of the individual perchlorate anion tetrahedra or their composite "cube."

Two aspects of this non-allgnment of the C(15)...C(15') interaction vector with any of the symmetry axes of the T d anion seem particu- larly important. Firstly, the C(4)...C(4') and C(14)...C(14') vectors are incapable of inducing anion ordering, as they do not differentiate between the two oppositely oriented tetra-

C14 9

2 c15b.. 4"

....... -O

"6C265 3 15

i 4.2?3 I

6 c~.

A~ION-DONOR COUPLING IN (TMTSF)2X SALTS 933

hedra. Thus, while the contact distances for these two interaction vectors substantially decrease (from 4.20 and 4.21A to 4.13 and 4.14A, respectively) on reduction in temperature from 295K to 120K in (TMTSF)2Re04, 8 it is not expect- ed that they are critical factors in the order- ing of the perrhennate anion at 18OK. ~ However, the C(15)...C(15') Interaction vector shows an even larger thermal contraction (0.20A) on going from 295 to 120K (4.38 to 4.18A), 8 and since this interaction vector can potentially discrim- inate between the two orientations of the ReO 4- tetrahedra, it is likely a principal structural component leading to anion order- ing. It is also believed that this methyl group...anion interaction is largely repulsive in nature, and anion ordering follows from a minimization of unfavorable interaction terms. Such a notion is at variance with a previous suggestion of methyl group...anlon "hydrogen bonding" as a leading term in the anion ordering transitlon. 5 Moreover, the rapid decrease in the C(15)...C(15') contact distance with temper- ature would seem to provide a structural basis for the strong correlation I between anion size and metal-insulator transition temperature for (TMTSF)2AX 4 salts.

Secondly, it is interesting to briefly consider salts where the four-coordinate, nomi- nally tetrahedral anion possesses a permanent dipole moment, ~ • Two salts, (TMTSF)2FS039 and (TMTSF)2PO2F~, I0 have been well document- ed. For an AX2Y 2- anion [e.g., PO2F 2- (symmetry group C2v) , Figure 4], ~ is constrained by symmetry to lle along it~ unique molecular two- fold axis. If there is any tendency for ~ to - - o parallel the symmetry direction defined by C(4)...C(4') or C(14)...C(14'), then it seems possible that a cooperative, ferroelectrlc ground state could become competitive with the superconducting ground state at low tempera- ture. Such a proposal would be an unusual II

C4 realization of Matthlas' propositlon 12 (based on dlmensionality arguments) that a ferroelectric ground state ought to win out over a supercon- ducting ground state in low-dlmenslonal inor- ganic and organic solids and could offer an explanation of the absence of superconductivity in (TMTSF)2PO2F2, I0 even under relatively high external pressure (ca. 15 kbar). In contrast, for an AX3Y- or AXY 3- anion [e.g., FSO 3- (sym-

+ C 4 C14

TWo

Td C2v C3v

Fig. 4. Symmetry-oriented views of the perchlo- rate (Cl04-, Td), difluorophosphate (PO2F2- , C2v), and fluorosulphate (FS03- , C3v) anions. For the latter two anions, the symmetry-restricted orientation of the permanent dipole moment is indicated by ~o"

Fig. 3.

(TMTSF)2CIO 4

Top: Disposition of the six closest methyl groups about the AX4- anion In the structure of (TMTSF)2CI04 (T = 295K); Bottom: Projection views down the C(4)...C(4'), C(14)...C(14'), and C(15)...C(15') interaction vectors.

934 ANION-DONOR COUPLING

merry group C3v) , Figure 4], £ is constrained by symmetry to lie along its unique three-fold molecular axis. For this case, it seems unlike- ly that ~ will parallel any symmetry axis of the structural cavity (vide supra), making a ferroelectric ground state much less likely - (TMTSF)2FSO 3 becomes superconducting at ~2K under an applied pressure of ca. 7 Kbar! 9

In summary, consideration of the invarlant symmetry aspects of the structural cavity in which the anions reside in (TMTSF)2X salts appears to provide a strong structural basis for the empirical observation I that independent correlations in structural and physical proper- ties with anion size are demanded for salts with octahedral and tetrahedral anions. Moreover, a principal structural component leading to an anion-ordering transition in (TMTSF)2AX 4 salts has been identified, and it is further suggested

IN (TMTSF)2X SALTS Vol. 5], No. I~

that this structural component plays a major r o l e i n r a t i o n a l i z i n g t he e m p i r i c a l o b s e r v a t l o n I of a linear relationship between metal-lnsulator transition temperature and anion size for this subset of (TMTSF)2X salts. Finally, salts with four-coordinate anions possessing a permanent molecular dipole moment are identified as possi- ble candidates for a competitive ferroelectric ground state at low temperature. Particularly susceptible are salts with anions displaying C2v symmetry, such as PO2F2-.

Suppor t of t h i s r e s e a r c h by t he N a t i o n a l Sc i ence Founda t ion (unde r Gran t No. DMR-8307693) and the U. S. Naval Sea Systems Command (under C o n t r a c t No. N00024-83-C-5301) i s g r a t e f u l l y acknowledged. The a u t h o r i s a l s o i n d e b t e d to Drs . F. J . Adr ian and W. A. Bryden f o r s e v e r a l stimulating discussions.

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