Electrical conductivity and x-ray diffuse scattering
study of the family of organic conductors
(perylene)₂M(mnt)₂, (M = Pt, Pd, Au)

R T Henriques†§, L Alcácer‡, J P Pouget† and D Jérome‡

† Laboratoire de Physique des Solides, Université de Paris Sud, Bâtiment 510, 91405 Orsay, France

‡ Laboratorio Nacional de Engenharia e Tecnologia Industrial. P2685 Sacavem, Portugal

Received 20 March 1984

**Abstract.** We report resistivity measurements along the high-conductivity axis of Per<sub>2</sub>Pt(mnt)<sub>2</sub>, Per<sub>2</sub>Pd(mnt)<sub>2</sub> and Per<sub>2</sub>Au(mnt)<sub>2</sub>. At low temperatures a clear metal-insulator phase transition is observed at 6.5 K and 28 K, respectively for the first two salts, while it appears very gradually for the Au salt. The x-ray study shows above these critical temperatures one-dimensional structural fluctuations for Per<sub>2</sub>Pt(mnt)<sub>2</sub> and Per<sub>2</sub>Pd(mnt)<sub>2</sub>, which are attributed to the M(mnt)<sub>2</sub> stacks, and no such scattering for Per<sub>2</sub>Au(mnt)<sub>2</sub> down to 12 K. The diffuse scattering 'condenses' in superstructure reflections below 28 K in Per<sub>2</sub>Pd(mnt)<sub>2</sub>, which thus might undergo a spin-Peierls phase transition. The nature of these various phase transitions is discussed.

**1. Introduction**

In the search of highly conductive charge-transfer compounds of one-dimensional (1D) character, perylene was one of the first donors used. Perylene bromine (Akamatu et al 1954, 1956) and iodine (Labes et al 1960, Kommandeur and Hall 1961, Uchida and Akamatu 1961) exhibit high but not metallic-like conductivities. Kao et al (1979) synthetised Per<sub>4</sub>I<sub>2</sub> which undergoes a metal-insulator transition above 200 K. A few years ago, some of us (Alcácer et al 1980) discussed the synthesis, crystal structure, electrical conductivity and magnetic susceptibility temperature dependence of Per<sub>2</sub>Pt(mnt)<sub>2</sub> (mnt = 1,2-dicyanoethylene-1,2-dithiol), a perylene charge-transfer salt with conductivity of metallic type down to low temperatures. Other members of the family Per<sub>2</sub>M(mnt)<sub>2</sub> with M = Cu, Ni, Pd are known (Alcácer and Maki 1974, 1976), the Cu and Ni salts being semiconductors. We report here some experimental results on single crystals of Pt, Pd and Au compounds, with evidence showing that metallic conduction is due to the perylene donor stacks and that electrons are localised on the dithiolates, which thus can be considered as counter ions.

Other perylene-based compounds have been synthetised (Keller et al 1980) within
the basic idea that conductivity is mainly due to the partially oxidised donor stacks as
revealed by the substitution of the usual planar π acceptors like TCNQ and derivatives

<footnote>§ Permanent address: LNETI, Instituto de Energia, P2685 Sacavem, Portugal.</footnote>
by highly symmetric inorganic counter-ions, with such fruitful results as the stabilisation of a superconducting state first observed under pressure in (TMTSF)₂PF₆ (Jérome *et al* 1980) but also at ambient pressure in (TMTSF)₂ClO₄ (Bechgaard *et al* 1981). Although this may be the main trend in the synthesis of new organic conductors, quasi-one-dimensional systems exhibit a wide variety of interesting features. Thus it is worth studying other systems which are good or even bad conductors. Some systems, with bis-ethylene dithiolates as counter-ions, are insulators but undergo spin-Peierls transitions (Interrante *et al* 1979, Bray *et al* 1983).

Underhill and Ahmad (1981) succeeded in their attempts to obtain conduction through the anion Pt(mnt)₂⁻, but single crystals of Li₀.₇₅Pt(mnt)₂ have a metal-insulator transition around 215 K.

Per₂M(mnt)₂, with M = Pt, Pd, Au, is an isostructural series, crystallising in the monoclinic system (space group P2₁/c). For Per₂Pt(mnt)₂ a detailed structure was obtained (Alcácer *et al* 1980). Lattice parameters are *a* = 16.612 Å, *b* = 4.194 Å, *c* = 30.211 Å, β = 118.70°. Perylene and Pt(mnt)₂ form segregated regular stacks in a skew arrangement, along the *b* axis. The perylene molecules are separated by 3.32 Å and Pt(mnt)₂ molecules by 3.65 Å, with a distance of 4.19 Å between consecutive Pt atoms in the chain direction. In the *ac* plane the packing of stacks is very close to a hexagonal array, with six perylene stacks surrounding a dithiolate stack. In this respect the situation is different from the Bechgaard series ((TMTSF)₂X salts) where there is only one organic stack per unit cell.

## 2. Experimental procedure and results

The crystals used in this study were grown by the methods previously described: (a) on the anode of an electrochemical cell with a dichloromethane solution of perylene and of the tetrabutylammonium salt of M(mnt)₂⁻, for the Au and Pt compounds, using a constant current in the range 1–10 µA (Alcácer *et al* 1980); (b) by oxidation of perylene with iodine in the presence of TBA Pd(mnt)₂ in a dichloromethane medium, with a low cooling rate (Alcácer and Maki 1974).

### 2.1. Electrical resistivity

Resistivity measurements were made using a low-frequency (70 Hz) AC lock-in detection technique with a measuring current of 10 µA. Four electrical contacts were made using gold paint (Demetron M 8001) on previously evaporated gold contacts. Room-temperature conductivities are typically 500–600 (Ωcm)⁻¹ in Per₂Pt(mnt)₂ and Per₂Au(mnt)₂ and 250–300 (Ωcm)⁻¹ for the Pd analogue. Sample sizes vary from 6 × 0.08 × 0.03 mm³, with the voltage contacts 1.5–2 mm apart for the bigger crystals of the Pt compound, to 2 × 0.01 × 0.01 mm³ for the Pd one with no more than 1 mm for the voltage contact distance.

In figure 1 the resistivity versus temperature curves are plotted. The previously observed transition in Per₂Pt(mnt)₂ is confirmed to be at 6.5 ± 0.5 K by the maximum slope of log ρ = f(1/T). In this temperature range the resistivity increases by five orders of magnitude, becoming almost constant or even slightly decreasing below 2.5 K down to 1.3 K. The observed maximum conductivity in this compound is about 3 × 10³ (Ωcm)⁻¹ around 20 K.

The resistivity of the second member of the family Per₂Pd(mnt)₂ was only measured
<figure><img src="image_1.png" /><figcaption>**Figure 1.** Electrical resistivity versus temperature for Per<sub>2</sub>Pt(mnt)<sub>2</sub> (circles) Per<sub>2</sub>Au(mnt)<sub>2</sub> (crosses) and Per<sub>2</sub>Pd(mnt)<sub>2</sub> (squares).</figcaption></figure>

down to 4.2 K. The maximum of slope of log ρ = f(1/T) gives a metal-semiconductor phase transition at T<sub>c</sub> = 28 ± 1 K. However the possibility of another transition around 11 K cannot be excluded, as suggested by the results presented in figure 1, but the severe experimental conditions prevent a definitive conclusion. In the insulating state the resistivity increases by more than four orders of magnitude: ρ<sub>(4.2K)</sub>/ρ<sub>min</sub> ~ 2 × 10<sup>4</sup>.

The temperature dependence of the resistivity of Per<sub>2</sub>Au(mnt)<sub>2</sub> is somewhat differ-
ent. The log ρ versus T curve shows a broad minimum around 55 K and below this
temperature the resistivity increases slowly. When fitted to an activated law ρ =
ρ<sub>0</sub> exp(Δ/k<sub>B</sub>T), a value of Δ = 1.2 meV is derived for the range 15–9 K. The maximum
slope of log ρ = f(1/T) is between 9 and 6 K, with an estimate of Δ = 1.9 meV. However
from the data shown in figure 1, a critical temperature of transition to an insulating state
cannot be precisely determined.

The anisotropy of the conductivity was measured at 300 K, only for Per<sub>2</sub>Pt(mnt)<sub>2</sub>, using the method described by Montgomery (1971). The ratio σ<sub>b</sub>/σ<sub>a</sub> = 10<sup>3</sup> is similar to that obtained for Per<sub>4</sub>I<sub>2</sub> by Kao *et al* (1979).

## 2.2. X-ray diffuse scattering

As in previous investigations of structural instabilities of 1D conductors, measurements
were performed with the fixed-film, fixed-crystal method, using monochromatised Cu
K<sub>a</sub> (1.54 Å) x-ray radiation. A cooling system (Displex DE 202, Air Products and
Chemicals) was used to obtain patterns in the range 10–300 K. X-ray patterns were read
using a Joyce Loeble microdensitometer. Several samples (two of Per<sub>2</sub>Pt(mnt)<sub>2</sub>, two of
Per<sub>2</sub>Pd(mnt)<sub>2</sub> and three of Per<sub>2</sub>Au(mnt)<sub>2</sub>), coming from different batches, were studied,
with the same result for each species.

Structural fluctuations observed in the metallic state of Per<sub>2</sub>Pd(mnt)<sub>2</sub> are best illus-
trated by the x-ray pattern shown in figure 2, taken at 40 K. It exhibits along *b*\* and
<figure><img src="image_2.png" /><figcaption>**Figure 2.** X-ray pattern of $Per_2Pd(mnt)_2$ showing at 40 K the $0.5b^*$ diffuse scattering (white arrows). On this pattern the chain axis *b* is horizontal.</figcaption></figure>

midway between layers of main Bragg reflections, i.e. at the reduced wavevector $q_b = \frac{1}{2}b^*$, intense and nearly continuous diffuse lines, which come from the intersection of the Ewald sphere with diffuse sheets perpendicular to the highly conducting *b* direction. Such diffuse sheets are currently observed in 1D conductors like TTF-TCNQ (see for example Khanna *et al* 1977) where they reveal the presence of one-dimensional structural

<figure><img src="image_3.png" /><figcaption>**Figure 3.** Microdensitometer readings along *b*\* of the $2.5b^*$ diffuse line of $Per_2Pd(mnt)_2$ for various temperatures.</figcaption></figure>
fluctuations. The temperature dependence of this diffuse scattering is shown by the microdensitometer readings along b* of one of these lines (figure 3). This figure shows that a broad scattering appears around 100 K. Below this temperature it sharpens in width and increases in intensity. More quantitatively the critical diffuse scattering measured at a reduced wavevector q, I(q), can be related (if kT > ω_c, where ω_c is a characteristic frequency related to the dynamics of the structural phase transition) to the static susceptibility associated to the order parameter, χ(q), by the relationship:

$$I(q) \approx kT\chi(q).$$

Taking around the critical wavevector $q_c$, a lorentzian q dependence of the susceptibility, one gets:

$$I(q) \approx kT \frac{\chi(q_c)}{1 + \xi_c^2 (q - q_c)^2},$$

where $\xi_c$ is the (in-chain) correlation length of structural fluctuations.

<figure><img src="image_4.png" /><figcaption>**Figure 4.** (a) Temperature dependence of the peak intensity I, and the peak intensity corrected by the temperature I/T, for the 0.5b* of $Per_2Pd(mnt)_2$. (b) Temperature dependence of the half width at half maximum (HWHM) along b* of the 0.5b* scattering of $Per_2Pd(mnt)_2$. (In this study we use the convention b·b* = 2π.) On the figure the critical temperature $T_c$ = 28 K, at which there is an anomaly in electrical conductivity, is shown.</figcaption></figure>

The intensity of the diffuse scattering at the critical wavevector $I(q_c) \equiv I$, corrected by the thermal population factor T, I/T, is thus a measure of χ(q_c). The temperature dependence of these quantities I and I/T is shown in the upper part of figure 4. One sees clearly the divergence of χ(q_c) when T approaches $T_c$ = 28 K. The half width at half maximum (HWHM) of I(q) gives the inverse correlation length $\xi_c^{-1}$. (Practically, one obtains this quantity after correction of the data by the experimental resolution. Taking here for simplicity a lorentzian resolution, $\xi_c^{-1}$ is given by the difference between the experimental HWHM and the resolution.) The lower part of figure 4 gives the temperature dependence of the HWHM of the profile of the diffuse sheets along b*. One sees clearly
that this quantity reaches the experimental resolution at $T_c = 28$ K (i.e. $\xi_{\parallel} \to \infty$ at this temperature). Above $T_c$ one gets from figure 4: $\xi_{\parallel} \sim 3b$ at 100 K and $\xi_{\parallel} \approx 37b \pm 10b$ at 32 K.

A few degrees above $T_c$ (up to about 35 K) one sees a broad modulation of the diffuse lines, showing that lateral correlation between 1D fluctuations takes place. At 28 K the short-range order has 'condensed' into well defined satellite reflections. This temperature is equal to that at which an anomaly is observed in electrical conductivity (figure 1). The structural transition coincides with the metal-insulator phase transition of $Per_2Pd(mnt)_2$.

Unfortunately, because of the poor quality of the crystals used it has not been possible to obtain the $q_a$ and $q_c$ components of the satellite reflections. In addition, with the present experimental set-up, we have not been able to cool below 12 K in order to check if the second anomaly shown by the resistivity, below this temperature (see figure 1), corresponds really to an additional structural modification.

<figure><img src="image_5.png" /><figcaption>**Figure 5.** X-ray pattern of $Per_2Pt(mnt)_2$ showing at 10 K the broad $0.5b^*$ diffuse scattering (white arrows) and the sharp $0b^*$ diffuse scattering (black arrow). On this pattern, the chain axis *b* is horizontal. This pattern is free from $\lambda/2$ contamination.</figcaption></figure>

Similar one-dimensional structural fluctuations are observed in the metallic state of $Per_2Pt(mnt)_2$, under the form of diffuse lines at the same reduced wavevector $q_b = \frac{1}{2}b^*$, as shown at 10 K in figure 5. These lines appear below 25 K, at a much lower temperature than in $Per_2Pd(mnt)_2$. However in the two compounds this temperature corresponds roughly to $4T_c$, where $T_c$ is the temperature at which electrical measurements show a metal-insulator phase transition. Unfortunately the critical temperature 6.5 K of $Per_2Pt(mnt)_2$ is too low to be reached with our experimental set-up. Thus it has not been possible to check whether the electrical transition of this salt is accompanied by a structural distortion. The microdensitometer readings of figure 6 show that the intensity of the $q_b = \frac{1}{2}b^*$ diffuse lines increases and its width sharpens slightly when the temperature decreases. This indicates the development of a structural instability as $T_c$ is approached from above. However the measurement of the HWHM of the profile, shown in figure 6, leads in fact to very short in-chain correlation lengths for structural fluctuations, namely $\xi_{\parallel} \sim 5.5$ Å (1.5*b*) at 20 K, $\xi_{\parallel} \sim 9$ Å (2*b*) at 15 K and $\xi_{\parallel} \sim 13$ Å (3*b*) at 10 K.
<figure><img src="image_6.png" /><figcaption>**Figure 6.** Microdensitometer readings along *b*\* of the 1.5*b*\* diffuse line of Per<sub>2</sub>Pt(mnt)<sub>2</sub> for various temperatures.</figcaption></figure>

Even scaled with the corresponding $T_c$, the HWHM of the $q_b = 0.5b^*$ diffuse line of Per<sub>2</sub>Pt(mnt)<sub>2</sub> has a much weaker temperature dependence than the HWHM of the corresponding lines of Per<sub>2</sub>Pd(mnt)<sub>2</sub> (figure 4). Thus there is no clear evidence that the intensity and the width of the $q_b = 0.5b^*$ diffuse lines of Per<sub>2</sub>Pt(mnt)<sub>2</sub>, diverge and reaches the experimental resolution at $T_c = 6.5$ K, respectively. The possibility that they correspond to structural fluctuations, not stabilised by a structural distortion below the 6.5 K phase transition, must also be considered.

Another interesting feature shown by Per<sub>2</sub>Pt(mnt)<sub>2</sub> (figure 5) and not by Per<sub>2</sub>Pd(mnt)<sub>2</sub> (figure 2), is the presence of sharp diffuse lines passing through the main Bragg reflections. They belong to diffuse sheets with the reduced wavevector $q_b = 0b^*$. Their width at 10 K gives an in-chain correlation length of $\xi_{\parallel} \sim 60$ Å (14*b*) larger than that of the $q_b = 0.5b^*$ scattering. They cannot be clearly observed above 150 K. Their physical origin (stacking faults or 2*b* structural fluctuations) is not at present understood. They might be also involved in the 6.5 K phase transition. Their study also needs further work at temperatures lower than 10 K.

Finally, one of the most unexpected results of this investigation is that we have failed to detect any diffuse lines in Per<sub>2</sub>Au(mnt)<sub>2</sub> down to 12 K. In this respect, this last salt differs markedly from Per<sub>2</sub>Pt(mnt)<sub>2</sub> and Per<sub>2</sub>Pd(mnt)<sub>2</sub> which show 1D structural fluctuations below ~25 K and ~100 K respectively.

### 3. Discussion

In these three members of the family Per<sub>2</sub>M(mnt)<sub>2</sub>, one may distinguish the Pd and Pt isoelectronic species from the Au one. The former two salts show a well defined metal-insulator phase transition from the electrical measurements accompanied by
one-dimensional structural precursor effects above $T_c$. The Au salt exhibits a gradual metal-insulator phase transition as the temperature decreases but no structural fluctuations. In $Per_2Pd(mnt)_2$ the structural transition coincides with the electrical transition, and is indicated above 28 K by the divergence of 1D structural fluctuations. In this respect it can be viewed as a Peierls or a spin-Peierls transition, depending on the fact that the lattice is linearly coupled to a charge-density-wave (CDW) or a spin-density-wave (SDW) instability respectively. In $Per_2Pt(mnt)_2$ the divergence of structural fluctuations at 6.5 K is not clearly established, and magnetic measurements (see below) performed below 6.5 K suggest a different ground state.

In this section we shall discuss first the value of the charge transfer and try to ascribe the structural fluctuation to the different stacks. Afterwards we shall discuss the electronic properties of these stacks, and then come back to the nature of the phase transitions.

### 3.1. Charge transfer and assignment of the diffuse scattering

The absence of diffuse sheets in $Per_2Au(mnt)_2$ suggests that the 1D structural fluctuations occur on the dithiolate stacks in the Pt and Pd salts. The strong intensity of the $q_b = \frac{1}{2}b^*$ diffuse sheets and, on a given sheet, its weak dependence with the *h* and *l* reciprocal indices, strongly support an x-ray scattering dominated by the Pt and Pd atoms of the dithiolate stacks.

Instabilities of 1D electronic gases may occur either at the $2k_F$ or the $4k_F$ wavevectors, where $k_F$, the Fermi wavevector of the non-interacting electron gas, is related to the charge per site, $\rho$, by the formula $\rho = 4k_F/b^*$. By linear coupling to the phonons, these (CDW or SDW) instabilities may induce structural instabilities at the same wavevector. First in case of $q_b = 2k_F$ assignment we may derive a charge transfer of one electron from the perylene bands to each dithiolate radical anion. Secondly, in the case of $q_b = 4k_F$ assignment, the charge transfer becomes half an electron per radical anion. The first situation, corresponding to the formula $Per_2^+M(mnt)_2^-$ with one electron per dithiolate entity, leads to a spin $S = \frac{1}{2}$ for each $Pt(mnt)_2$ and $Pd(mnt)_2$ anion and a closed-shell ion, with spin pairing, for the $Au(mnt)_2$ anion. This charge transfer is in agreement with the magnetic properties of $Per_2M(mnt)_2$ showing a Curie-Weiss law for the Pt (Alcácer et al 1980) and Pd (Alcácer and Maki 1976) species and no such behaviour for the Au species (Flandrois 1980) it is in agreement with the value of the spin $S = \frac{1}{2}$ per chemical formula deduced from the Curie constant of $Per_2Pt(mnt)_2$ (Alcácer et al 1979). It also explains simply why with a closed-shell anion (no available electrons) no diffuse scattering arises from the $Au(mnt)_2$ entity.

As the unit cell contains twice as many perylene stacks as dithiolate stacks, a charge transfer of one electron per dithiolate molecule leaves $\frac{1}{2}$ hole per perylene molecule. On the perylene stacks $2k_F$ fluctuations would occur at the wavevector $\frac{1}{2}b^*$ and $4k_F$ fluctuations would occur at $\frac{1}{2}b^*$. However we have not found any clear evidence of a 1D scattering at $\frac{1}{2}b^*$ in the three compounds nor even at $\frac{1}{2}b^*$ in $Per_2Au(mnt)_2$. But we cannot rule out the possibility of a $4k_F$ (perylene) scattering superimposed on the $2k_F$ (dithiolate) scattering in $Per_2Pt(mnt)_2$ and $Per_2Pd(mnt)_2$.

Finally it is interesting to note that the number of electrons obtained per acceptor molecule is identical to that found in TTF metal bisdithiolene (MBDT) complexes presenting at low temperature antiferromagnetic ($M = Pt$) or spin-Peierls ($M = Au, Cu$) phase transitions, the second kind of transition being due to pairing of uncompensated spins of the $TTF^+$ entities (Bray et al 1983).
## 3.2. Electronic properties in the metallic state

Transport measurements show that the three salts have, in their metallic phase, a similar value of the electrical conductivity in the chain direction and the same value of the thermopower at room temperature: $S_{RT} = 28 \mu V K^{-1}$ (Henriques *et al* 1984). This means that the dominant conduction process is by holes on the perylene stacks.

The temperature dependence of the thermopower of $Per_2Pt(mnt)_2$ has been measured (Henriques *et al* 1984). Taking the value of the slope of the $S(T)$ plot, for a ¾-filled band, within a tight-binding approximation, neglecting the energy-dependent scattering (Kwak *et al* 1976), the contribution of the $Pt(mnt)_2$ stack, and using the expression:

$$S = \frac{\pi^2 k_B T}{6|e|t} \frac{\cos \frac{1}{2} \pi \rho}{(1 - \cos^2 \frac{1}{2} \pi \rho)}$$

a longitudinal bandwidth $W = 4t = 0.74$ eV is derived for the perylene stack. Using the value of the anisotropy measured at room temperature in $Per_2Pt(mnt)_2$, $\sigma_b/\sigma_a \approx 10^3$, it is possible to obtain the band anisotropy from the relationship $\sigma_b/\sigma_\perp \approx (d_\parallel/d_\perp)^2(t/t_\perp)^2$, assuming a diffusive transverse conductivity (Soda *et al* 1977). Taking the ratio of conductivity quoted above, the intrastack distance $d_\parallel \approx 4.2$ Å and an average lateral distance $d_\perp \approx 10$ Å between perylene molecules, one gets a band anisotropy $t_\parallel/t_\perp \sim 75$. This is indicative of strong one-dimensionality of the electron energy dispersion.

Contrary to their transport properties, the three salts show quite different magnetic properties. The magnetic susceptibility of $Per_2Pt(mnt)_2$ and $Per_2Pd(mnt)_2$ increases strongly at low temperature (Alcácer and Maki 1976, Alcácer *et al* 1979, 1980). In these two salts the paramagnetic susceptibility can be explained in terms of two contributions: a small contribution, almost temperature-independent, and a larger contribution, with temperature dependence provided by the perylene and dithiolate stacks, respectively. In the case of $Per_2Pt(mnt)_2$, where magnetic measurements have been performed down to 4 K (Alcácer *et al* 1980), the latter contribution follows a Curie-Weiss law above its maximum, at about 15 K, with a spin $S = \frac{1}{2}$ per $Pt(mnt)_2$ molecule, in agreement with the charge-transfer discussed in § 3.1. In the case of $Per_2Au(mnt)_2$ the magnetic susceptibility is weak ($\chi_M \sim 10^{-4}$) and decreases slightly for decreasing temperatures (Flandrois 1980). This is in agreement with spin pairing on the $Au(mnt)_2$ ions, which provides only a diamagnetic contribution to the total susceptibility, similarly to what occurs on $(n-C_4H_9)_4NAu(mnt)_2$ (Davison *et al* 1963).

It is interesting to remark that the values of the electrical conductivity and thermoelectric power at room temperature of $Per_2M(mnt)_2$ are intermediate between that of the one-chain compounds ($TMTTF)_2X$ and ($TMTSF)_2X$ (see for example Jérôme and Schulz 1982). In all these compounds the charge transfer is the same, leaving $\frac{1}{2}$ hole per perylene, $TMTTF$ or $TMTSF$ molecule. The marked difference is due to the magnetic properties of the counter-ions: uncompensated localised spin for the dithiolate molecule ($M = Pt, Pd$) and spin pairing for the closed-shell anions $X$. Thus the coexistence in the same crystal structure of metallic chains and paramagnetic stacks makes $Per_2M(mnt)_2$ unique compounds in the field of 1D conductors.

## 3.3. Nature of the low-temperature phase transitions

Let us first discuss the $Per_2Au(mnt)_2$ salt, which has a closed-shell dithiolate anion. The temperature dependence of its electrical conductivity resembles that of the solid solution
[(TMTSF)$_{1-x}$(TMTTF)$_x$]$_2$ClO$_4$ (Coulon *et al* 1982), where, after an increase below room temperature, the electrical conductivity saturates around 50–100 K for $x \sim 0.05$, decreasing at lower temperature. This behaviour can be due to a gradual electron localisation with decreasing temperatures. It does not correspond to a true phase transition. With the Au salt, it is thus conceivable that a similar effect occurs at low temperature on the perylene stacks. Figure 1 shows that the decrease of the electrical conductivity of $Per_2Au(mnt)_2$ is indeed very gradual below 55 K.

Unlike the Au salt, there is a more pronounced increase of resistivity at low temperature in the salts where the dithiolate stacks are magnetic. The maximum of slope of $log \rho = f(1/T)$ occurs at 6.5 K in $Per_2Pt(mnt)_2$ and at 28 K in $Per_2Pd(mnt)_2$. We shall see below that these maxima indicate true metal-insulator phase transitions, which appear to be driven by instabilities occurring on the dithiolate stacks. In the case of $Per_2Pd(mnt)_2$ the signature of the phase transitions is the growth of satellite reflections below 28 K, while in the case of $Per_2Pt(mnt)_2$ preliminary data show a drastic change of the ESR spectrum at 6.5 K.

In the case of $Per_2Pd(mnt)_2$, the 28 K phase transition has the structural features of a common Peierls transition, as found in TTF-TCNQ (Khanna *et al* 1977). However the observation, above the phase transition, of $2k_F$ one-dimensional structural fluctuations cannot give the nature of the electronic instability. In the case of a Peierls transition the structural instability is coupled via the electron-phonon coupling to $2k_F$ CDW instability while, in the case of a spin-Peierls transition, it is coupled via the spin-phonon coupling to $2k_F$ SDW instability. These two kinds of instabilities can occur equally in a 1D electron gas. However, within the frame of the Hubbard model of a half-filled band, for $U$ increasing with respect to the bandwidth, the charge degree of freedom becomes more and more frozen and the $2k_F$ instability becomes more and more associated with the spin degree of freedom. Thus the CDW Peierls transition evolves into a SDW spin-Peierls transition (Bray *et al* 1983, Caron and Bourbonnais 1984). In the present case, the $2k_F$ fluctuations occur on the Pd(mnt)$_2$ stacks, where the electrons appear to be relatively well localised from the magnetic measurements. Thus the phase transition might be a spin-Peierls transition and the 1D structural fluctuations observed below 100 K might reflect the presence of 1D spin fluctuations. If this behaviour is confirmed by magnetic measurements performed at low temperature, $Per_2Pd(mnt)_2$ would be the first compound with a spin-Peierls transition showing 1D structural fluctuations as precursors of the phase transition. In the two other families of compounds showing a spin-Peierls transition, TTF Cu BDT (Moncton *et al* 1977) and MEM(TCNQ)$_2$ (Van Bodegom *et al* 1981), the structural fluctuations are 3D above the phase transition, and they do not have the anisotropy of the expected 1D magnetic fluctuations!

In the above section we have discussed the origin of the structural instability occurring on the dithiolate stack, but we have not considered the perylene metallic stack which undergoes the 28 K metal-insulator phase transition. With ½ hole per perylene molecule the stabilisation of a $2k_F = 0.5b^*$ periodicity on the dithiolate stacks is viewed on the perylene stacks as a $4k_F$ wavevector. Thus the potential created by the distortion of the dithiolate stacks cannot open an electrical gap at $\pm k_F$ on the perylene stacks. Another possibility might be that on these stacks, as for $Per_2Au(mnt)_2$, the electron gas becomes relatively well localised at low temperatures and that the instability of the dithiolate stacks induces a localisation of one electron to every two perylene molecules or between two perylene molecules with the concomitant establishment of a $4k_F$ distortion. This implies that a large-$U$ limit should be taken for the interpretation of the metal-insulator transition on the perylene stack. A similar mechanism of electronic localisation on the
TMTTF or TMTSF organic stacks, driven by the counter-anion lattice X, has been considered in the $(TMTSF)_2X$ and $(TMTSF)_2X$ salts (Emery *et al* 1982).

Preliminary ESR measurements performed in $Per_2Pt(mnt)_2$ show close to $T_c = 6.5$ K a vanishing of the signal intensity and a rapid broadening of the linewidth. They recall those obtained at low temperature in $(TMTSF)_2PF_6$ (Pedersen *et al* 1980), where it is now well established that an antiferromagnetic phase transition takes place below 19–12 K, depending on the samples. In this last compound the $2k_F$ SDW instability occurs on the metallic TMTSF stack and drives the metal–insulator phase transition on the same stack. In $Per_2Pt(mnt)_2$ electrons are localised on the dithiolate stack and if further studies confirm that an antiferromagnetic order develops below 6.5 K one has to consider a mechanism similar to that proposed for $Per_2Pd(mnt)_2$ to drive a metal–insulator phase transition on the perylene stacks.

1D structural fluctuations are also observed at the wavevector $q_b = 0.5b^*$ on the $Pt(mnt)_2$ stack where the electrons are well localised. As for $Per_2Pd(mnt)_2$, these structural fluctuations may be related to 1D SDW fluctuations, and may lead to a spin-Peierls instability. However unlike that of $Per_2Pd(mnt)_2$, they do not appear to diverge when the temperature decreases. Even if $Per_2Pt(mnt)_2$ undergoes an antiferromagnetic phase transition at low temperature, this compound must be situated at the borderline between the antiferromagnetic and spin-Peierls instabilities. Competition between these two instabilities have been considered recently by various theories (Kondo 1984, Inagaki and Fukuyama 1984).

## 4. Conclusion

The preliminary x-ray and transport studies of $Per_2M(mnt)_2$ compounds (with M = Au, Pd, Pt) have shown that the physical properties of these salts cannot be interpreted in a conventional picture of a 1D electron band ignoring interactions between electrons. Thus the electrons appear to be localised on the dithiolate stacks, while they are itinerant on the perylene stacks. It is suggested that the 6.5 K and 28 K phase transition of the Pt and Pd salts, respectively, are driven by $2k_F$ SDW instabilities of the acceptor stacks, leading probably to an antiferromagnetic and to a spin-Peierls ground state, respectively, below $T_c$. This probably induces a $4k_F$ localisation of electrons on the perylene stacks, leading to a metal–insulator phase transition.

It has been found that in one-chain compounds like $(TMTSF)_2X$ or $(TMTTF)_2X$, the ordering of extra degrees of freedom, related to the counter-anion lattice (in that case the orientation of non-centrosymmetric anions X) may have an influence on the electronic properties of the organic stacks (Pouget *et al* 1981). In the present work, we have shown that somewhat similar effects may occur with the spin degree of freedom of the dithiolate stacks. However a full understanding of the coupling between the organic lattice and these extra degrees of freedom requires further study of this interesting family of salts.

## Acknowledgments

We thank H Novais for his help in sample preparation, S Tomic for collaborating in the preliminary EPR experiments reported here and A Andrieux for skilful technical assistance. We had useful discussions with M Almeida, S Barisic, K Bechgaard, C Bourbon-
nais, F Creuzet, T Takahashi and C Weyl. RTH thanks the French Government for a
grant supporting his stay at Orsay. The work was partially supported by the JNICT
contract no 413.82.47.

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