Effect of Anion Disorder on the Peierls Transition in ... - ACS Publications

Hikari Nakayama, and Tokiko Uchida. Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278-8510, Japan. J. Phy...
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J. Phys. Chem. B 1999, 103, 7565-7572

7565

Effect of Anion Disorder on the Peierls Transition in the Commensurate One-Dimensional Conductors (TTF)(SCN)0.56(ClO4)0.01, (TTF)(NO3)0.59(Cl)0.28, and (TTF)(SCN)0.09(Br)0.59 Masagi Mizuno,* Kazumasa Honda, and Jyunji Akimoto National Institute of Materials and Chemical Research, Tsukuba Research Center, Tsukuba, Ibaraki 305-8565, Japan

Hikari Nakayama and Tokiko Uchida Faculty of Science and Technology, Science UniVersity of Tokyo, 2641 Yamazaki, Noda, Chiba 278-8510, Japan ReceiVed: February 16, 1999; In Final Form: June 29, 1999

Three new quasi-one-dimensional tetrathiafulvalenium (TTF) conductors, (TTF)(SCN)0.56(ClO4)0.01, (TTF)(NO3)0.59(Cl)0.28, and (TTF)(SCN)0.09(Br)0.59, were obtained by electrochemical crystallization. They are all nonstoichiometric compounds with commensurate anion ratios and behave as Mott’s 1D variable range hopping conductors because of their disordered anion structure. Evidenced by their unusual structure-conductivity characteristics, they are considered as in the intermediate regime, ranging from the highly conducting incommensurate compounds to the almost insulating commensurate salts with Peierls structure typified by (TTF)3(BF4)2. The Peierls instability is thus suppressed giving incomplete Peierls structures for these compounds despite their commensurate band filling ratio. This was most probably derived from the random potential exerted over TTF conducting chain by the disordered anion structure.

Introduction The nature of the electrical conduction and the compositionstructural relations of the low dimensional materials have been subjects of great interest in the past three decades. Among them, the quasi-one-dimensional (1D) conductors based on tetrathiafulvalene (TTF, C6H4S4) are thought to be the prototype of organic conductors having a simplified composition, highly symmetric crystal packing, and single conduction column with various kinds of instabilities.1-7 The effect of the positional disorder has been revealed upon the temperature dependence of dc conductivity along the TTF radical stacking when it obeys to Mott’s variable range hopping (VRH) model of electrical conduction as in amorphous semiconductors. Another effect is also anticipated in the condensation of the Peierls instability8,9 which is inherent to an equally spaced quasi-1D chain. Recently, it has been shown that TTF salts with mixed anions form a new series of 1D conductors which are inherently disordered in the anion lattice because of its anion mixing.10,11 When the 1D conducting chain is under the influence of random potentials, the electron-phonone coupling in the chain is smeared out, reducing to an incomplete structural change as was exemplified by the following new TTF conductors. Here, we report about the preparation, structure, and conduction properties of three mixed anion TTF conductors with commensurate stoichiometry:12 (TTF) (SCN)0.56(ClO4)0.01 (1), (TTF)(NO3)0.59(Cl)0.28 (2), and (TTF)(SCN)0.09(Br)0.59 (3).

dichloroethane solution of TTF and tetrabutylammonium nitrate was used for (2), and an acetonitrile solution of TTF and tetrabutylammonium salts of corresponding anions was used for (1) and (3). The details of the preparation were described in a previous report.10 (TTF) (SCN)0.56(ClO4)0.01 (1) crystallized in very fine, long needles and conglomerated into a bundle several mm long and 0.03 to 0.05 mm wide. (TTF)(NO3)0.59(Cl)0.28 (2) crystallized into black-brown needles and grew into lath-shaped bundles of crystals 1.6 to 2.5 mm long and 0.08 mm wide. (TTF)(SCN)0.09(Br)0.59 (3) crystallized into black-purple needles and also grew into lath-shaped bundles of crystals typically 3 mm long and 0.3 to 0.6 mm wide. Crystal 3 is slightly unstable in ambient atmosphere and its surface deteriorates after exposure to the air for several months at room temperature in much the same way as other bromines containing TTF conductors do. The anion stoichiometry was determined by chemical analysis and the results for the samples of each compound are given below. calc. obs.

The quasi-one-dimensional TTF conductors with mixed anions of commensurate stoichiometry were obtained by a conventional electrochemical crystallization technique. A 1,2* E-mail: [email protected]. fax: +81-298-54-4709.

(TTF)(SCN)0.56(ClO4)0.01 (1) H, 1.70 N, 3.30 S, 61.47 H, 1.85 N, 3.29 S, 61.53

ClO4, 0.42% ClO4, 0.40%

(TTF)(NO3)0.59(Cl)0.28 (2) calc. C, 28.66 H, 1.60 N, 3.34 S, 51.10 O, 10.03 Cl, 5.37% obs. C, 28.42 H, 1.74 N, 3.33 S, 51.25 O, 9.98 Cl, 5.29% calc. obs.

Experimental Section

C, 33.12 C, 32.93

C, 28.50 C, 28.41

(TTF)(SCN)0.09(Br)0.59 (3) H, 1.57 N, 0.49 S, 51.09 H, 1.68 N, 0.38 S, 51.05

Br, 18.36% Br, 18.48%

In the case of compounds (1) and (3) the contents of perchlorate and bromide, respectively, were determined by difference. Structural Analysis for (TTF)(SCN)0.56 (ClO4)0.01 (1). A black needle crystal of dimensions 1.48 × 0.47 × 0.25 mm3

10.1021/jp990558x CCC: $18.00 © 1999 American Chemical Society Published on Web 08/19/1999

7566 J. Phys. Chem. B, Vol. 103, No. 36, 1999 TABLE 1: Crystal Data (TTF)(SCN)0.56(ClO4)0.01

(TTF)(NO3)0.59(Cl)0.28

crystal system: tetragonal space group: P42/mnm (#136) a ) b ) 11.2095(7) Å

crystal system: orthorhombic space group: Pnnm (#58) a ) 11.864(4) Å b ) 10.991(5) Å c ) 3.593(2) Å V ) 468.5(4) Å3 Z)2 Dm ) 1.767(4) Dc ) 1.783(2) R ) 0.100 Rw ) 0.120

c ) 3.624(1) Å V ) 455.32(9) Å3 Z)2 Dm ) 1.73(5) Dc ) 1.735(1) R ) 0.035 Rw ) 0.054

was used for the structural analysis. The density of the crystals was determined by flotation in an aqueous solution of zinc bromide. The reflection data were collected on an Enraf-Nonius CAD4 diffractometer with graphite-monochromated Mo KR radiation using the 2θ-ω scan technique (3.0° e 2θ e 79.9°). Accurate unit cell parameters were obtained by the least-squares refinement based on the setting angle of 25 reflections. The empirical absorption correction based on psi(Ψ) scans was applied. A total of 1698 reflections were collected of which 421 with (|Fo| g 3σ|Fo|) were used for the refinement. The intensities of three representative reflections were measured after every 50 reflections. No decay correction was applied. The X-ray structure was identified by direct methods (SHELEXS 86)13 and refined using the standard full-matrix least-squares and the Fourier procedures. All of the hk0 reflection data were eliminated from the analysis of TTF lattice to avoid the overlapping reflections due to the nearly commensurate anion lattice. All non-hydrogen atoms were refined anisotropically to give R ) 0.034 and Rw ) 0.047 for 20 parameters. The Fourier calculations based on the atomic parameters of the TTF radical showed the anion was located at (0, 0.5, 0 ∼ 1.0) disordered along the c-axis. The crystal data are shown in Table 1 along with those of other salts. Structural Analysis for (TTF)(NO3)0.59 (Cl)0.28 (2). This was performed in a way quiate similar as that for compound (1) using 1250 reflections for the refinement. All of the hk0 reflection data were eliminated from the analysis of TTF lattice to avoid the overlapping intensity due to the nearly commensurate anion lattice. All non-hydrogen atoms were refined anisotropically to give R ) 0.10 and Rw ) 0.12 for 32 parameters. The crystal data are shown in Table 1. Structural Analysis for (TTF)(SCN)0.09 (Br)0.59 (3). An oscillation photograph of a brown block crystal was taken by a MAC MO3X with CuKR. It showed that the TTF lattice might have a 3-fold structure due to its nearly commensurate anion stoichiometry of 0.68. A Weissenberg photograph of the layer showed reflections accompanied by diffuse lines along the aand b-axes, indicating the existence of a severe structural instability within this plane. Another Weissenberg photograph of the second layer of the 3-fold structure showed prismatical splitting into four reflections instead of having one normal Bragg reflection spot. This indicates the development of an anti phase domain structure along the ab plane in the crystal. Conductivity Measurement. The conventional four-probe dc conductivity measurements at various temperatures down to 50 K were carried out on individual crystals. Details of the measurements were given in a previous report.10 Results and Discussion Preparation of Crystals of Mixed Anion TTF Conductors. There are two methods of electrochemical preparation of mixed

Mizuno et al. anion TTF conductors. One is to employ 1,2-dichloroethane as solvent together with an anion source such as tetrabutylammonium nitrate. This always brings in a TTF salt which contains chloride as a partner of mixed anions. It has been suggested that during the electrolysis, 1,2-dichloroethane is decomposed to produce free chloride in solution. The chloride is taken into crystal growing as mixed anion salt. Most of the chloride containing mixed anion compounds was prepared in this way. Another way is to use a nonchlorinated solvent such as acetonitrile, dissolving two kinds of supporting electrolytes as anion sources. Most of the TTF salts containing thiocyanate as mixed anions were prepared by the latter method. Crystal structure of (TTF)(SCN)0.56(ClO4)0.01 (1). Based on the systematic absence of 0kl with k + l ) 2n + 1, packing consideration, and molecular symmetry of the TTF radical, the space group was determined to be P42/mnm (#136). This is a very common way of packing among this series of TTF conductors. In most cases, the anion lattice is fully disordered along the TTF stack because of anion mixing.10,11 This compound also has a nearly commensurate anion stoichiometry of 0.57 ∼ 4/7 ) 0.571. This was reflected upon the oscillation photograph when progressions of very weak reflection spots appeared between the first Bragg layer lines at (C* (C* is the reciprocal lattice vector corresponding to the c-axis) as a result of average TTF lattice. The progression is listed in Table 2 where the presence of two kinds of series is apparent. The major series has a repeating unit of 1/7C*, and another additional progression is described by a repeating unit of 1/20C* or 1/40C*. The first progression coincides with the anion ratio and must have originated from the commensurate total anion stoichiometry of 4/7. The second one was likely derived from the minor anion perchlorate. When the period is 1/40C*, the content of perchlorate is calculated to be 0.025. This is very close to the observed value of 0.01 ( 0.02, and within the experimental error. The presence of these series of very weak reflection spots other than strong layer lines due to average TTF structure indicates the anion lattice is in the earliest stage of ordering. The effect of overlapping reflections between the TTF and anion lattices was small enough that the TTF lattice structure was analyzed separately to obtain a well converged average structure of R ) 0.034 and Rw ) 0.047. Although the band filling is 5/7, the degree of anion ordering was apparently insufficient to induce a Peierls-type structural change. This consideration is in accordance with the observation of high conductivity at room temperature and a relatively large dTTF value of 3.624(1) Å, despite the commensurate stoichiometry of the anions for this compound. Eliminating hk0 reflections from the observed intensity in a similar way to the previous analysis performed the structural analysis of the TTF lattice. For the anion lattices, structural analysis was not possible because only insufficient refection data were available. Figure 1 shows the c-axis projection of the unit cell, Figure 2 illustrates molecular geometry, and Table 3 contains the positional parameters for the TTF radicals in the cell. TTF radicals lay in the ab plane and stack equatorial along the c direction. The interplane distance of 3.624(1) Å was short enough to guarantee its high conductivity along this direction, although it is the largest dTTF value found so far for this series of conductors. This highly symmetric crystal packing is very often seen in this series of TTF conductors, and (TTF)(SCN)0.56(ClO4)0.01 (1) was found to be isostructural with (TTF)(Cl)0.42(ClO4)0.14,10 (TTF)(Cl)0.60(BF4)0.02, and (TTF)(Cl)0.66(SCN)0.09.11 Crystal Structure of (TTF)(Cl)0.28(NO3)0.59 (2). This com-

Effect of Anion Disorder on the Peierls Transition

J. Phys. Chem. B, Vol. 103, No. 36, 1999 7567

TABLE 2: Progression Analysis of the Additional Weak Reflections in the Oscillation Photographs of the Commensurate Compounds TFF(NO3)0.59(Cl)0.28

TFF(SCN)0.56(Cl4)0.01

TTF(SCN)0.09(Br)0.59

δa ) 0.87 = 7/8 ) 0.875

δ ) 0.57 = 4/7 ) 0.571

δ ) 0.68 = 2/3 ) 0.667

progression

n/8

progression

n/7

0.36C*b 0.63C* 0.90C*

3/8 ) 0.375 5/8 ) 0.625 7/8 ) 0.875

0.424C* 0.576C* 0.901C* 1.002C* 1.149C* 1.351C* 1.426C* 1.572C* 1.806C* 2.000C*

3/7 ) 0.429 4/7 ) 0.571 7/7 ) 1.000 8/7 ) 1.143 10/7 ) 1.429 11/7 ) 1.571 14/7 ) 2.000

m/20

progression 0.298C* 0.698C* 0.898C* 1.000C* 1.297C* 1.346C* 1.681C* 1.792C* 1.980C*

18/20 ) 0.900 27/20 ) 1.350 36/20 ) 1.800

n/3

m/10 3/10 ) 0.300 7/10 ) 0.700 9/10 ) 0.900

3/3 ) 1.000

13/10 ) 1.300

4/3 ) 1.333 5/3 ) 1.667

18/10 ) 1.800

6/3 ) 2.000

δ is the total anion stoichiometry and equal to the degree of partial oxidation of the TTF radical. b C* stands for the reciprocal lattice vector resulting from the average TTF-TTF distance in the stack. a

Figure 2. The molecular geometry of TTF radicals (Å and deg). Figure 1. The c-axis projection of the TTF radical arrangement in a unit cell. The central radical lies at a position c/2 above or below the surrounding radicals. The positions of the mixed anions disordered along the c-axis are denoted by circles.

pound has nearly commensurate anion stoichiometry of 0.87 ∼ 7/8 ) 0.875, the effect of which was reflected upon the oscillation photograph as the additional two layer lines with weak but clear reflection spots between the Bragg layer lines at (C* because of the average TTF structure. These sublayer lines are located at 0.36C* and 0.63C* corresponding to 3/8C* and 5/8C*, respectively, as is shown in Table 2. This indicates the development of partial ordering of an anion lattice with the repeating distance of 8 × of dTTF. No satellite reflection spots were observed and there is no appreciable structural modulation in the TTF stacks. Based on the systematic absence of 0kl with k + l ) 2n + 1 and h0l with h + l ) 2n + 1, packing consideration, and molecular symmetry of the TTF radical, the space group was determined to be Pnnm (#58). The X-ray oscillation photograph has revealed that the anion lattice is partially ordered as a result of the nearly commensurate stoichiometry of 0.87 which is close to 7/8 ) 0.875. It showed one progression of very weak reflection layers with a period of 8 × dTTF, a part of which overlaps the reflections due to the TTF lattice. The structural analysis of the TTF lattice has been performed separately from the anion lattice structure to avoid the effect of

TABLE 3: Fractional Atomic Coordinates and Thermal Parameters TTF(SCN)0.56(ClO4)0.01 Atom

X

Y

Z

Beq (Å2)a

S1 C1 C2 H(C2)

0.30591(5) 0.4565(2) 0.2601(2) 0.184(3)

0.49035(6) 0.4565b 0.3435(3) 0.336(3)

0.0000b 0.0000b 0.0000b 0.0000b

4.26(2) 3.30(3) 5.12(6) 5.3(6)

TTF(NO3)0.59(Cl)0.28 Atom

X

Cl(1) S(1) S(2) N(1) C(1) C(2) C(3)

0.0000b 0.1821(4) -0.0005(5) 0.0000b 0.039(1) 0.218(2) 0.129(2)

Z

Beq (Å2)a

0.5000b

0.5000b

0.0183(4) 0.1975(4) 0.5000b 0.046(1) 0.177(2) 0.256(2)

0.0000b 0.0000b 0.5000b 0.0000b 0.0000b 0.0000b

5.700b 5.9(1) 6.0(1) 5.700b 4.6(3) 7.7(6) 6.6(5)

Y

a Beq ) (8π2/3)(U (aa*)2 + U (bb*)2 + U (cc*)2 + 2U aa*bb*cosγ 11 22 33 12 + 2U23bb*cc*cosR + 2U13aa*cc*cosβ). b Fixed.

partial ordering of anion lattices by eliminating the observed hk0 reflection data from the calculation. It was not possible to analyze the anion structure because only insufficient reflection data were available. The c-axis projection of the unit cell is almost identical to that of (1) in Figure 1. Table 3 contains the positional parameters for the TTF radicals in the cell.

7568 J. Phys. Chem. B, Vol. 103, No. 36, 1999 TABLE 4: Degree of Partial Oxidation (DPO), Average TTF-TTF Distance dTTF, and Room Temperature Conductivity σ (300 K) TTF conductors (1) (TTF)(SCN) 0.56(ClO4)0.01 (2) (TTF)(NO3)0.59(Cl)0.28 (3) (TTF)(SCN)0.09(Br)0.59 (4) (TTF)3(BF4)2a

DPO dTTF (Å) 4/7 7/8 2/3 2/3

3.624 3.593 3.54 3.4537

σ(300 K) (S/cm) 2.9-53, 19 ( 14 2.3-5.9, 3.9 ( 1.3 0.7-1.6, 1.0 ( 0.4 8.2 × 10-5 - 1.5 × 10-4

a The conductivity was measured along the elongated crystal axis which was identified to be the reported TTF stacking axis with the lattice constant of 10.792(2) Å.

Crystal Structure of (TTF)(SCN)0.09(Br)0.59 (3). The cell dimensions measured from X-ray photographs were as follows: a ) b )11.10(2) Å, c ) 3.54(1) Å. Although the crystal structure has not been analyzed yet, the TTF radicals presumably stack along the c-axis as in all the other crystals of the series. If so, the obtained dTTF of 3.54(1) Å is the shortest distance found in this series of TTF conductors. Anion stoichiometry corresponding to one TTF radical is 0.68 which is close to 2/3 ) 0.67. It can be said that TTF and anion lattices are nearly commensurate to each other in a very simple fraction. It would be quite useful to compare the structural data of (TTF)(SCN)0.09(Br)0.59 (3) with those of (TTF)3(BF4)2 (4) where anion stoichiometry is exactly commensurate with TTF in a fraction 2/3 as reported earlier,14 (TTF)3(BF4)2 crystallizes in the triclinic system, space group P1, and the repeating unit in the TTF column consists of one neutral molecule and one dimer pair of the radical cations. They have two inter planar distances, 3.40 and 3.48 Å, with an average of 3.453 Å. This distance is the shortest ever found for TTF compounds. As far as the average dTTF distance is concerned, the (TTF)(SCN)0.09 (Br)0.59 (3) crystal is situated between the incommensurate TTF conductors with mixed anions and the commensurate (TTF)3(BF4)2 (4) crystal. Anti phase domain structure is rarely seen in organic crystals.15,16 The presence of strong streaks along the a- and b-axes reveals that structural instability is present in the TTF lattices along these directions. This is in accordance with the observation of anti phase domain structure developed within the ab plane. No satellite reflections have been observed in any mixed anion TTF conductors examined so far, showing that the TTF lattice is not periodically modulated. Molecular Geometry. The molecular geometry of the TTF radical in each salt is shown in Figure 2. Conduction Properties. The results of conductivity are summarized in Table 4 together with other related parameters. (TTF)(SCN)0.56(ClO4)0.01 (1). The room-temperature conductivity was measured on a bundle of very thin needles of average size 5.6 × 0.24 × 0.20 mm3 and it was found to be in the 3 to 53 S/cm range with an average value of 19 ( 14 S/cm. This value falls in the same range of conductivity as other compounds of this series. The value of dTTF in the stack is 3.624(1) Å, which is the longest distance found for the mixed anion TTF conductors. The temperature dependence is illustrated in Figure 3(a) where natural logarithmic conductivity and its derivatives are plotted against inverse temperatures from 50 to 300 K. Thermal hysteresis is noticeable from slightly below room temperature down to 230 K as is shown in Figure 3(a). This is similar to the other thiocyanate-containing TTF conductors. Because of the linear shape of the thiocyanate ion, it has more degrees of freedom on the lattice than other anions with spherical symmetry. Once randomized at higher temperatures, linear shaped anions take a long time to settle down in a stable conformation

Mizuno et al. in crystal lattice. This was expected from their long relaxation time of more than 1 h confirmed at a fixed temperature in the hysteresis region. The metal-insulator transition temperature which was estimated from the temperature at which the derivative value ∂(lnσ)/∂(1/T) has the maximum value and was estimated to be 142 ( 13 K. The temperature dependence of conductivity is well described by 1D Mott’s variable range hopping (VRH) equation for other mixed anion TTF conductors as shown in Figure 4(a). It is described as

σ(T) ) σ0 exp[-(T0/T)1/2] where σ0 and T0 are characteristic constants. VRH parameter T0 was estimated as (2.54 ( 0.40) × 104 K for this salt which is less than half of those of other TTF conductors as shown in Table 5. (TTF)(NO3)0.59(Cl)0.28 (2). The room-temperature conductivity of single crystals of (2) was in the 2.3 to 5.9 S/cm range with an average value of 3.9 ( 1.3 S/cm. This conductivity is 1 order of magnitude smaller than those of other compounds of this series while dTTF is 3.593(2) Å, one of the shortest distances found so far for the mixed anion TTF conductors. The temperature dependence of the conductivity is illustrated in Figure 3(b) where the natural logarithmic conductivity and its derivative are plotted against inverse temperature from 50 to 300 K. The activation energy is dependent upon temperature and is in accordance with Mott’s 1D VRH model as is seen from the plot against inverse square root temperature shown in Figure 4(b). This energy happened to be less dependent on temperature than that of other VRH conductors of this series. The effect of anion partial ordering is visible in its conduction behavior where activation energy is less dependent upon temperature than other 1D VRH conductors, as shown in Figure 4(b). It behaves as if the Arrhenius character were partly mixed into the VRH conduction. Most likely this mixed character of compound (1) originated from the partial ordering of the anion lattice induced by nearly commensurate anion stoichiometry 7/8. The low conductivity at room temperature also reveals the existence of modulation of the TTF lattice as a result of the partial anion ordering. Since the band filling of the TTF stack is calculated to be 9/16, the 8-fold modulation does not open a band gap at the Fermi surface, yet the conductivity drops by 1 order of magnitude. We do not suspect Peierls-type structural change but distortions of the TTF stacks might have been involved in this conductivity drop. This may be different from the case of metal chain compounds where the band filling is the same as the commensurate ratio (n - 1)/n and the condensation of Peierls instability takes place in a static form. The metal-insulator transition temperature was 271 ( 4 K, the highest value found for this series of compounds. The temperature dependence of conductivity is well described by 1D Mott’s VRH equation as in other mixed anion TTF conductors, and the VRH parameter T0 was estimated as (5.38 ( 0.69) × 104 K, similar to those of other TTF conductors as shown in Table 5. (TTF)(SCN)0.09 (Br)0.59 (3). The conductivity at room temperature is in the range of 0.7 to 1.6 S/cm with an average value of 1.0 ( 0.4 S/cm. This is another low value as was found for (TTF)(NO3)0.59(Cl)0.28 (1). Its temperature dependence is shown in Figure 3(c), which is well described by the 1D Mott’s VRH model shown in Figure 4(c). The T0 value was estimated as (2.06 ( 0.17) × 104 K shown in Table 5, and the metal insulator transition temperature was obtained as 260 ( 11 K.

Effect of Anion Disorder on the Peierls Transition

J. Phys. Chem. B, Vol. 103, No. 36, 1999 7569

Figure 3. Temperature dependence of conductivity and its derivative. (a) (TTF)(SCN)0.56(ClO4)0.01, (b) (TTF)(NO3)0.59(Cl)0.28, and (c) (TTF)(SCN)0.09(Br)0.59. The arrows indicate the direction of the thermal process. EA is defined as EA ) -kB [∂(lnσ(T))/∂(T-1)] where kB is Boltzmann’s constant.

A thermal hysteresis of conductivity is seen in Figure 3(c) from slightly below room temperature down to 90 K. The appearance of the hysteresis differs from crystal to crystal. Every thiocyanate-containing conductor examined so far has displayed thermal hysteresis in this region without exception.10,11 The anion stoichiometry is a nearly commensurate 0.68 ∼ 2/3 ) 0.67 and a band filling of 2/3. This is a very simple fraction so we can expect a Peierls-type structure and conduction property for this salt. In the oscillation photograph, along the crystal c-axis there appear two distinct layer lines between the Bragg layer lines as a result of the average TTF structure. One

is of weak and the other of moderate intensity, at the positions 1/3C* and 2/3C*, respectively. This tells us that 3-fold anion and/or TTF lattice ordering is developing according to the commensurate anion stoichiometry. Actually, we have observed unusually short dTTF as 3.54(1) Å and 1 order of magnitude lower conductivity at room temperature compared with other TTF conductors. However, this partial ordering is apparently not big enough to induce a complete Peierls-type change as described in the structural and conduction study of (TTF)3(BF4)2 crystal.14 It has a very short average dTTF of 3.453 Å and single crystal conductivity of 1.5

7570 J. Phys. Chem. B, Vol. 103, No. 36, 1999

Mizuno et al.

Figure 4. The VRH plotting of the conductivity of (a) (TTF)(SCN)0.56(ClO4)0.01, (b) (TTF)(NO3)0.59(Cl)0.28, and (c) (TTF)(SCN)0.09(Br)0.59. The straight line is a visual fit to the data. The arrows indicate the direction of the thermal process.

TABLE 5: Values of T0 Obtained from Regressing eq 1 onto the Conduction Data TTF conductors

T0 (K)

ref

TTF(ClO4)0.14(Cl)0.42 TTF(BF4)0.02(Cl)0.60 TTF(SCN)0.09(Cl)0.66 TTF(SCN)0.56(ClO4)0.01 TTF(NO3)0.59(Cl)0.28 TTF(SCN)0.09(Br)0.59 K2Pt(CN)4Br0.32.3 H2O

6.37 × 6.66 × 104 4.40 × 104 (v)a, 4.01 × 104 (V)a 2.54 × 104 5.38 × 104 2.06 × 104 5.79 × 104

10 11 11 this work this work this work 19

a

104

Arrows indicate the direction of the thermal process.

× 10-4 to 8.2 × 10-5 S/cm which is much lower than those of other incommensurate TTF conductors.

Commensurability and the Peierls Structure. The composition, average dTTF, and room-temperature conductivity for these compounds are summarized in Table 4 in order of the value of dTTF. The correlation between the conductivity and structure of TTF stacks is particularly interesting from the point of view of the instability of a linear conducting chain against a charge density wave (CDW). The effect of commensurate stoichiometry upon structure and conductivity was recognized in several 1D systems from bis(oxalato)platinate compounds.17 They have a simple fractional commensurate stoichiometry presented as (n - 1)/n where n is a small integer. They all show very low conductivity such as

Effect of Anion Disorder on the Peierls Transition 10-3 ∼ 10-5 S/cm despite the unusually short average Pt-Pt distance of the conducting chain. The detailed structural analysis of one of these compounds showed a strongly disproportionate platinum chain structure.18 This is referred to as a Peierls structure when Peierls instability has condensed into a static form instead of an equally spaced linear chain structure. The features of the Peierls structure found in bis(oxalato)platinates are summerized. (i) The Fermi wavevector kF ) (n - 1)π/n(dPt-Pt) where the Pt chain is n-fold; (ii) The roomtemperature conductivity σ is 3 to 5 orders of magnitudes less than that of the incommensurate salts of the same series; and (iii) The average Pt-Pt separation dPt-Pt is relatively short compared with those of the incommensurate salts of the same series. When the 1D chain has a simple fractional band filling such as 1/2, 2/3, etc, the structural change takes place easily as was noted by Peierls.8 In the TTF conductors, the effect of commensurate stoichiometry on the structure and properties is less apparent than in metal chain conductors, (TTF)3(BF4)2 (4) has a commensurate ratio of 2/3. Compared with (TTF)3(BF4)2 (4), the commensurate TTF conductors with mixed anions did not undergo any drastic change in conductivity and structure, but remained moderate. (TTF)(SCN)0.56(ClO4)0.01 (1) did not show any appreciable structural or conduction change despite its nearly commensurate anion ratio 4/7 and behaved similarly to other incommensurate conductors. (TTF)(NO3)0.59(Cl)0.28 (2) did show a slight change, lowering its conductivity by 1 order of magnitude and shortening dTTF distance to some extent. (TTF)(SCN)0.09(Br)0.59 (3) underwent somewhat more structural change because one of the simplest commensurate fractions, was 2/3. Yet this change was not big enough to induce a complete Peierls-type change as had happened in (TTF)3(BF4)2 (4). The degree of structural change due to the commensurate ratio of (3) can be assigned to the midpoint of the transition to the Peierls structure typified by (4). The TTF conductors with commensurate anion ratio have revealed themselves to have the common features of the Peierls structure summerized previously with Pt-chain compounds in this text except feature (i). All of the Peierls structures of 1D systems found so far, bis(oxalato)platinates metal chain compounds and TTF conductors, have revealed that the1D chains shrinks upon the Peierls transition which brings the shortening of the average repeating distance shorter than those in the corresponding compounds with equally spaced 1D chains. Yet, the conductivity at r.t. drops sharply by several orders of magnitude because of the disproportional structural change. However, the structural and conductivity changes expected from the commensurate TTF conductors with mixed anions are not enough as to reach to a complete Peierls structure. As the reason for the incomplete Peierls transition, increased crosssection for the conduction path compared with metal chain compounds could be ruled out because of the existence of the typical examples of the Peierls structure in (TTF)3(BF4)2. Another most plausible explanation would be addressed to the effect of disorder as is discussed below. Effect of Disorder Structure of Anions. The effects of disorder structure on the electronic states in the 1D conductors were studied both theoretically and experimentally in the seventies.19-22 These experimental situations have been reviewed with the conclusion that the disorder does not play a dominant role.23 We have pointed out in our previous report9 that the purification of the [NMP][TCNQ] did not eliminate the variable range hopping behavior from the observed conductivity data, which should be attributed to the intrinsic disorder structure in

J. Phys. Chem. B, Vol. 103, No. 36, 1999 7571

Figure 5. The average TTF-TTF distance dTTF and room-temperature conductivity σ (300 K) of the commensurate TTF conductors; (1) (TTF)(SCN)0.56(ClO4)0.01, (2) (TTF)(NO3)0.59(Cl)0.28, (3) (TTF)(SCN)0.09(Br)0.59, and (4) (TTF)3(BF4)2. The area “A” enclosed by broken lines represents the region where incommensurate TTF conductors with mixed anions are located.

the NMP column.21 Introduction of disorder for controlling the band filling of the conducting chain was examined in the molecular alloys of [NMP/Phen][TCNQ], reaching the conclusion that the central physics is determined by the Peierls interaction and not by disorder.24 However, because of the anion disorder structure in the mixed anion TTF conductors, the spatial coherence between the positions of the anion and TTF lattice is broken, the effect of commensurability of the compound becomes blurred even if it is of minor effect of the disorder. This violation of the positional periodicity of the anion would make it difficult for both lattices to undertake the cooperative segregation into a bigger and better defined repeating unit. Thus, the anion disorder could prevent the TTF lattice from having a distinct Peierls structure. In other words, the disorder causes the Peierls distortion to shift to an incommensurate periodicity.24 The structure and conduction properties of the mixed anion TTF conductors with commensurate stoichiometry presented here thus can span a material spectrum depending on the degree of the effect of the disorder recovered partially by the anion ordering in the lattice. The four-probe conductivity at 300 K vs dTTF distance of these conductors is summarized in Table 4 and illustrated in Figure 5 in the order of degree of the resultant structural change. Both (TTF)(SCN)0.09(Br)0.59 (3) and (TTF)3(BF4)2 (4) have the same commensurate ratio 2/3. The essential difference between the two compounds is that compound (3) has a disorder structure due to the mixed anion lattice while compound (4) has well-defined structure. The disorder structure of (3) was revealed by X-rays and evidenced by the VRH conduction behavior observed in the temperature dependence of conductivity as is shown in Figure 4(c). The VRH conduction induced by the anion disorder is discussed in some of our previous reports.10,11,28 Mobility Edge and Peierls Transition. The electronic structure of disordered materials was fully investigated by N. F. Mott,25 leading to the variable range hopping model for the conductivity at low-temperature region. The band structure of the disordered conductors was reported by Cohen et al.26,27 It was predicted that there are continuous bands extended states with tails of localized states in the band gap, named as mobility edges. The existence of the mobility edge means the narrowing of the band gap due to the disorder. As the randomness increases

7572 J. Phys. Chem. B, Vol. 103, No. 36, 1999 in a conductor, mobility edges move inward into the band making the gap narrower. This should be true also for a newly opened band gap at the Fermi surface when Peierls transition takes place in the commensurate 1D conductors. In this way the decreased band gap eventually reduces the electronic energy gained by the structural phase transition and suppresses the structural change as an incomplete one. This explanation could be evidenced by observing the incomplete Peierls structure of several commensurate TTF conductors with mixed anions as shown in this text. Partial Ordering of Mixed Anions. In most cases, the mixed anions are arranged in a disordered way in the lattice parallel to TTF stacks, inducing VRH conduction along the stacks. When TTF forms a mixed anion salt in a commensurate ratio, it often happens that anions are partially ordered. This partial ordering is evidenced by the appearance of additional layer lines or very weak spots between the Bragg layer lines due to the average TTF structure shown on an oscillation photograph taken along the stacks. Actually, these additional layer lines have the same period of progression as the commensurate ratio and have been attributed to the partial ordering of anions as exemplified by the new compounds presented here. No satellite reflection was observed, revealing that the TTF lattice is not modulated periodically. The intensity and number of these additional reflections reflect the degree of anion ordering. Such reflections observed so far have been very weak and few in number, suggesting the anion ordering is of a very limited degree. When the anions order positionally to some extent, then TTF conductivity restores an Arrhenius conduction from room temperature down to 50 K as was found in (TTF)(SCN)0.47(NO3)0.10.28 The structural analysis of partial ordering of anions could not be performed because of their insufficient intensity and number of reflections. When the degree of anion ordering increases while the disorder is still dominant, the crystal likely recovers its commensurability and TTF stacks eventually degenerate into a Peierls structure. The more commensurability increases, the more dTTF and conductivity decrease and the compound becomes insulating. Figure 5 illustrates the relationship between the average TTF-TTF distance dTTF and room-temperature conductivity of these commensurate compounds. The other 19 mixed anion TTF conductors with incommensurate stoichiometry are confined in a small area A enclosed by the broken lines. These three mixed anion TTF conductors presented here are considered to be the suitable examples to indicate the various degrees of incomplete structural change suppressed by the coexisting disorder structure. Conclusions The crystal structure and conduction properties of the quasione-dimensional tetrathiafulvalenium (TTF) conductors with mixed anions (TTF)(SCN)0.56(ClO4)0.01 (1), (TTF)(NO3)0.59(Cl)0.28 (2), and (TTF)(SCN)0.09(Br)0.59 (3) were reported. Their commensurate stoichiometry restores the partial ordering of an anion lattice, giving very weak reflections at the commensurate

Mizuno et al. positions in X-ray photographs. The structure and conductivity of these commensurate TTF conductors are in the intermediate regime between the equally spaced TTF stacks for the incommensurate salts and the Peierls structure with completely disproportionate TTF stacks for the commensurate ones. The random potential created by the disorder structure of the anion lattice suppresses the Peierls instability leading to the incomplete Peierls structures of these compounds. These results could be explained by using the concept of “mobility edge” presented by Cohen et al.26, 27 References and Notes (1) Wudl, F.; Smith, G. M.; Hufnagel, E. J. J. C. S. Chem. Commun. 1970, 1453. (2) Etemad, S.; Penny, T.; Engler, E. M.; Scott, B. A.; Seiden, P. E. Phys. ReV. Lett. 1975, 34, 741. (3) Wudl, F.; Schafer, D. E.; Walsh, W. M. Jr.; Rupp, L. W.; DiSalvo, F. J.; Waszezak, J. V.; Kaplan, M. L.; Thomas, G. A. J. Chem. Phys. 1977, 66, 377. (4) Bozio, R.; Pecile, C. J. Phys. C 1980, 13, 6205. (5) Shchegolev, I. F.; Yagubskii, E. B. Extended Linear Chain Compounds; Miller, J. S., Ed.; Plenum Press: New York, 1982; Vol 2; pp 385-434. (6) Shibaeva, R. P. Extended Linear Chain Compounds; Miller, J. S., Ed.; Plenum Press: New York, 1982; Vol 2; pp 435-467. (7) Miller, J. S.; Epstein, A. J. Angew. Chem., Int. Ed. Engl. 1987, 26, 287. (8) Peierls, R. E. Quantum Theory of Solids, Clarendon: Oxford, 1955; Chapter 5.3. (9) Come`s, R.; Lambert, M.; Launois, H.; Zeller, H. R. Phys. ReV. B 1973, 8, 571. Come`s, R.; Shirane, G. Highly Conducting One-Dimensional Solids; Devreese, J. T., Evrard, R. P., van Doren, V. E., Eds.; Plenum: New York and London, 1979; Chapter 2. (10) Mizuno, M.; Gotoh, M.; Honda, K.; Shibuya, I.; Aoki, K. Synth. Met. 1994, 63, 29. (11) Mizuno, M.; Gotoh, M.; Honda, K.; Shibuya, I.; Aoki, K. Phys. ReV. B 1995, 51, 6237. (12) Preliminary results are reported in Mizuno, M.; Honda, K.; Nakayama, H.; Kokubo, H.; Uchida, T. Synth. Met. 1999, 103, 2138. (13) Sheldrick, G. M. Crystallographic Computing 3; Sheldrick, G. M., Kruger, C., Goddard, R., Eds.; Oxford University Press: Oxford, 1985, pp 175-189. (14) Legros, J.-P.; Bousseau, M.; Valade, L.; Cassoux, P. Mol. Cryst. Liq. Cryst. 1983, 100, 181. (15) Kobayashi, H. Acta Crystallogr. B 1978, 34, 2818. (16) Bertinotti, A.; Luzet, D. Europhys. Lett. 1986, 1, 181. (17) Mizuno, M. Synth. Met. 1987, 19, 963. Lee, K. H.; Kim, J. H.; Mizuno, M. Bull. Korean Chem. Soc. 1987, 8, 137. (18) Kobayashi, A.; Sasaki, Y.; Kobayashi, H. Bull. Chem. Soc. Jpn. 1979, 52, 3682. (19) Bloch, A. N.; Weisman, R. B.; Varma, C. M. Phys. ReV. Lett. 1972, 28, 753. (20) Ehrenfreund, E.; Etemad, S.; Coleman, L. B.; Rybaczewski, E. F.; Garito, A. F.; Heeger A. J. Phys. ReV. Lett. 1972, 29, 269. (21) Coleman, L. B.; Cohen, A. F.; Heeger, A. J. Phys. ReV. B 1973, 7, 2122. (22) Etemad, S.; Penney, T.; Engler, E. M.; Scott, B. A.; Seiden, P. E. Phys. ReV. Lett. 1975, 34, 741. (23) Garito, A. F.; Heeger, A. J. Acc. Chem. Res. 1974, 7, 232. (24) Miller, J. S.; Epstein, A. J. Angew. Chem., Int. Ed. Engl. 1987, 26, 287. (25) Mott, N. F. AdV. Phys. 1967, 16, 49. (26) Cohen, M. H.; Fritzsche, H.; Ovshinsky, S. R. Phys. ReV. Lett. 1969, 22, 1065. (27) Economou, E. N.; Cohen, M. H. Phys. ReV. Lett. 1970, 25, 1445. (28) Mizuno, M.; Honda, K.; Gotoh, M.; Nakayama, H.; Uchida, T. Phys. ReV. B 1996, 54, 6023.