# Structural and magnetic investigations of the Peierls transition of a-(Per2 m(mnt)2 with M = Fe and Co

V. Gama, R. Henriques, Manuel Almeida, Claude Bourbonnais, Jean Pouget, Denis Jerome, Pascale Auban-Senzier, B. Gotschy

► To cite this version:

V. Gama, R. Henriques, Manuel Almeida, Claude Bourbonnais, Jean Pouget, et al.. Structural and magnetic investigations of the Peierls transition of a-(Per2 m(mnt)2 with M = Fe and Co. Journal de Physique I, 1993. hal-04988885

HAL Id: hal-04988885

https://hal.science/hal-04988885v1

Submitted on 13 Mar 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Classification
*Physics Abstracts*
76.30P — 76.60E — 61.65

# Structural and magnetic investigations of the Peierls transition of α-(Per)₂M(mnt)₂ with M = Fe and Co

V. Gama (¹), R. T. Henriques (¹), M. Almeida (¹), C. Bourbonnais (²), J. P. Pouget (³), D. Jérôme (³), P. Auban-Senzier (³) and B. Gotschy (³, *)

(¹) Laboratorio Nacional de Engenharia e tecnologia Endustrial, Departamento de Quimica, P-2685 Sacavem, Portugal

(²) Département de physique, Université de Sherbrooke, Sherbrooke, Québec, J1K 2R1, and Laboratoire de Physique des Solides (CNRS URA 2), Université de Paris-Sud, Bâtiment 510, Orsay, F-91405, France

(³) Laboratoire de Physique des Solides (CNRS URA 2), Université de Paris-Sud, Bâtiment 510, Orsay, F-91405, France

*(Received 18 September 1992, revised 7 January 1993, accepted in final form 16 January 1993)*

**Abstract.** — In this work we present and discuss the temperature variation of X-ray, EPR and nuclear spin-lattice relaxation rate measurements for the two quasi-one-dimensional compounds α-(Per)₂Fe(mnt)₂ and α-(Per)₂Co(mnt)₂. From X-ray and EPR intensity data, a Peierls instability of the Perylene stacks is found to take place at 58 K and 75 K for the Fe and Co derivatives respectively. In contrast to other members of this series of two-chain compounds, the M(mnt)₂ stacks are found to play no role in the Peierls instability. As for diffusive excitations of localized spins in dimerized Fe(mnt)₂ stacks, their influence is shown to dominate the temperature variation of both the spin-lattice nuclear relaxation rate and the previously measured Faraday spin susceptibility.

## 1. Introduction.

Among the large variety of charge transfer organic conductors so far studied, a special place is taken by the (Per)ₙ M(mnt)₂ series, where Per is perylene, mnt = maleonitriledithiolate and M = Ni, Cu, Pd, Pt, Au, Fe and Co [1]. This is justified for materials of stoichiometry n = 2 by the presence in the α phase of segregated stacks of highly conducting Per chains and, for the elements M = Ni, Pd, Pt, of dithiolate magnetic chains. These materials are quasi one dimensional quasi-(1D) conductors which undergo whatever M a well defined low temperature

<footnote>(*) *Permanent address:* Physikalisches Institut der Universitaet Bayreuth, DW8580 Bayreuth, Germany.</footnote>
metal-insulator transition, suggesting the occurrence of a $2 k_F$ Peierls instability on the perylene stacks. In the case of Pt and Pd derivatives however, an earlier structural investigation has clearly shown the onset of spin-Peierls like fluctuations on the $M(mnt)_2$ stacks at twice the $2 k_F$ wave vector which drive a lattice dimerization at about the metal-insulator transition [2], raising several puzzling questions concerning the real role of the two kinds of stacks in the phase transition. A recent NMR study of the Pt derivative [3] confirmed the presence of a 1D spin-Peierls instability of the $Pt(mnt)_2$ stacks of localized spins, however the absence of quantum spin dynamics raised some basic questions about the microscopic driving force of the spin-Peierls instability. In the same study, an analysis of the EPR data confirmed, at least in a qualitative way, the existence of a Kondo-like interstack exchange coupling between localized spins and itinerant electrons, conjecturing that an RKKY induced interaction between localized spins could be involved in the dynamics of the transition [3].

In order to elucidate the respective role of the stacks in the phase transition, we investigate here two new compounds keeping the $\alpha$ phase structural type but where the organometallic counter ions have already undergone a chemically induced dimerization at room temperature [4]. In the case of the Co derivative, the $Co(mnt)_2$ stack is diamagnetic. In the case of the Fe derivative, the $Fe(mnt)_2$ stack is composed of antiferromagnetically coupled pairs of either $S = 3/2$ or $S = 1/2$ spins. According to transport measurements (resistivity and thermopower) [4], these compounds undergo a metal-insulator transition at 58 K and 73 K for the Fe and Co derivatives respectively, a temperature more than two times higher than those at which the others members of the series undergo a metal-insulator transition. The gap opens up for the Perylene stacks and it also affects the spin degrees of freedom as shown by the rapid drop in the magnetic (Faraday) susceptibility data of $Per_2Co(mnt)_2$ at the transition [4]. In this paper, we complete the study of the structural and magnetic properties of $Per_2Fe(mnt)_2$ and $Per_2Co(mnt)_2$ using X-ray diffuse scattering, RPE and NMR measurements. As far as the NMR data of $Per_2Fe(mnt)_2$ are concerned, we establish a connection between the temperature variation of the nuclear relaxation rate and Faraday susceptibility data of reference [4]. The Fe and Co derivatives used are of the same batches as those investigated in reference [4].

## 2. X-ray scattering.

The X-ray investigation of the structural instabilities, exhibited by $\alpha$-$Per_2M(mnt)_2$ with $M = Fe$ and $Co$, has been performed between 17 K and 295 K by the so called fixed-crystal fixed film method previously used to study the Pt, Pd and Au derivatives [2]. In order to avoid the fluorescence of the Fe and Co under the $CuK\alpha$ X-ray radiation, the $MoK\alpha$ (0.709 Å) wavelength was used, with however, a serious loss of sensitivity for the detection of weak diffuse scattering.

X-ray patterns obtained on the Fe and Co derivatives show two distinct features. First, room temperature data present clear evidence for the stacks of $Fe(mnt)_2$ and $Co(mnt)_2$ to be already dimerized with an in-chain periodicity of $b' \approx 2b \sim 8.2$ Å, in agreement with the structural finding of reference [4]. This dimerization, reminiscent of the one taking place in the insulating compound [(Et)₄N][Fe(mnt)₂] [5], is not driven by the coupling to collective dynamical effects in the spin or charge degrees of freedom as it could be, for example, for the spin-Peierls or the Peierls transition.

Secondly, at low temperatures, the presence of superlattice reflections with the $b'*/2$ reduced component in chain direction are detected in both derivatives, indicating that the lattice has undergone a phase transition. Figure 1 gives the temperature dependence of the superlattice intensity, obtained from a microdensitometer reading of the X-ray films. This intensity extrapolates to zero at about 58 K and 75 K for the Fe and Co derivative respectively. At this temperature ($T_p$), a metal-insulator transition is observed in transport measurements
<figure><img src="image_2.png" /><figcaption>Fig. 1. — Temperature dependence of a superlattice reflection of α-(Per)₂Co(mnt)₂ and α-(Per)₂Fe(mnt)₂.</figcaption></figure>

[4]. Below $T_p$, the periodicity along the stacks increases two-fold (2b') which would correspond to a tetramerisation (4b) of the dithiolate chains or simply to a Peierls instability driven by the conducting perylene chains at the $2 k_F (\approx b'/2 = b'/4)$ critical wave vector. As we will see in the next section, it is the latter possibility that allows a consistent interpretation of EPR measurements.

Only 3D pretransitional fluctuations could be detected five degrees above $T_p$ in both derivatives (Fig. 1). However, not too much attention has been paid to their study because of the use of the MoKα radiation.

### 3. Electronic paramagnetic resonance.

EPR-Data were obtained on a X-band Brucker ESP-300 spectrometer at 9.3 GHz. The g value was obtained by measuring simultaneously the static field with a Brucker NMR gaussmeter ER 035 M and the frequency with a Hewlett packard 5350 B microwave frequency counter. The integration from the EPR spectra were carried using a Brucker ESP-1600 computer data system. Measurements were performed from 4.2 to 300 K using a He flow Oxford Instruments EST-900 cryostat, the temperature was measured with a gold (0.07 %Fe)-chrome thermocouple with an accuracy of the order of ≈ 3 K and the stability about 0.2 K. The crystals were glued with Apiezon-N to a teflon support which was placed inside a quartz tube. The applied magnetic field $B_0$ was oriented perpendicular to the crystals b axis (stacking direction).

The thermal dependence of the g factor and of the linewidth of the ESR signal for the Fe and Co derivatives are shown in figures 2a and 2b respectively, and that of the integrated signal intensity are given in figures 3a and 3b respectively. EPR results of the Fe derivative have been briefly reported in reference [6].

The EPR measurements provide important pieces of information about the spin degrees of
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2. — Thermal dependence of the *g* factor and of the linewidth of the ESR signal of a) α-(Per)₂Fe(mnt)₂ and b) α-(Per)₂Co(mnt)₂.</figcaption></figure>
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. — Thermal dependence of the integrated intensity of the ESR signal of a) α-(Per)₂Fe(mnt)₂ and b) α-(Per)₂Co(mnt)₂.</figcaption></figure>
freedom that are involved in the lattice instability at $T_p$. In both materials, the EPR line is characterized by a nearly isotropic *g* factor value of 2.004, which is almost temperature independent. Such a value is typically found for conduction electrons which here would be those of the perylene stacks. In the case of the Fe derivative, the absence of a significant temperature dependence for *g* also indicates that the spins are not coupled to localized spins of the neighbouring Fe(mnt)₂ stacks. Such a situation contrasts with the one prevailing in (Per)₂Pt(mnt)₂ [3] and (Per)₂Pd(mnt)₂ [7], where both spin systems are strongly coupled by mixing the *g* factor of both spin species with temperature.

Nevertheless, the EPR linewidth is about 5 times greater in the Fe derivative than in the Co one. In the Fe derivative, it is comparable to that observed in the materials with M(mnt)₂ paramagnetic units, such as M = Pt [8] and Ni [6], although in our case *g* value measurements show that there is no sizeable exchange interaction between the spins belonging to stacks of different nature.

The EPR integrated intensity (Fig. 3) behaves similarity in the Fe and Co materials : it shows a monotonic decrease from room temperature down to $T_p$, where it suddently drops off to zero. The EPR and resistivity data directly show the opening of a gap in both spin and charge degrees of freedom on the perylene stacks. As this opening occurs at the same temperature the metal-insulator and the structural transitions occur, it is highly suggestive that the perylene chains of both derivatives undergo a Peierls instability.

It is also instructive to compare the EPR spin susceptibility to the Faraday susceptibility which probes the contribution of all the spins of both kinds of chains. In the case of the Co derivative, where the dimerized Co(mnt)₂ stacks are non-magnetic, the EPR and Faraday [4] susceptibilities behave similarly in temperature, as expected. This is not the case of the Fe derivative, where it is found that the dimerized Fe(mnt)₂ stacks of localized spins contribute dominantly to the amplitude and thermal dependence of the Faraday susceptibility [4] (Fig. 4).

<figure><img src="image_7.png" /><figcaption>Fig. 4. — Temperature dependence of the spin susceptibility obtained by the Faraday method for α-(Per)₂Fe(mnt)₂ and α-(Per)₂Co(mnt)₂. After reference [4].</figcaption></figure>
As it will be discussed in the next section, the latter is closely connected to the temperature variation of the nuclear relaxation rate.

**4. Nuclear magnetic resonance.**

The NMR experiments have been performed on the protons ¹H which pertain to the perylene molecule, with the experimental set up already used in reference [3], using a standard saturation recovery technique. We have used powdered samples of α-(Per)₂Fe(mnt)₂, with a typical weight of 50 mg. The irradiation frequency of 45 MHz corresponds to a static field of 10.58 kG. The experimental error on T₁ is about 10%.

The temperature dependence of the nuclear relaxation rate ($T_1^{-1}$) given in figure 5 for α-(Per)₂[Fe(mnt)₂] shows a monotonic decrease from the high temperature region down to T_p ≈ 70 K. Below this temperature, the recovery of the magnetization becomes strongly non-exponential. In figure 5, we separate the time constant which is obtained after a long delay from the time constant corresponding to the growth of magnetization just after saturation. In the large temperature region where the relaxation is characterized by a single exponential, the monotonic decrease of $T_1^{-1}$ is at first sight reminiscent of the one found for a quasi-1D metal. In this respect, the comparison with the existing $T_1^{-1}$ vs. T data of Per₂Au(mnt)₂ compound which has no localized spins is relevant here. In that case, the source of nuclear relaxation comes from itinerant electrons of Perylene chains giving a $T_1^{-1}$ temperature profile similar to the one of figure 5. The amplitude of $T_1^{-1}$ is found to be quite different however. Indeed, in Per₂Au(mnt)₂ at ambient temperature one has for example, $T_1^{-1}$ [300 K] ~ 5 sec⁻¹ whereas $T_1^{-1}$ [300 K] ~ 30 sec⁻¹ for α-(Per)₂[Fe(mnt)₂] which can be considered as a value too large to be ascribed to the conduction electrons of the Perylene stacks. Such a large $T_1^{-1}$ amplitude is much closer to $T_1^{-1}$ ~ 20 sec⁻¹ found in (Per)₂[Pt(mnt)₂] and for which it has been shown [3] that localized spins of the Pt stacks are responsible for the large amplitude of $T_1^{-1}$. Assuming here that localized spin excitations of the dimerized Fe(mnt)₂ chains which are thermally activated and diffusive below room temperature [4] (see also Fig. 4), are coupled to ¹H through a dipolar type of interaction, one can use the analysis of reference [3] for

<figure><img src="image_8.png" /><figcaption>Fig. 5. — Temperature dependence of the spin-lattice relaxation rate ($T_1^{-1}$) for α-(Per)₂Fe(mnt)₂. Below 70 K, the recovery of the magnetization becomes non-exponential. Data are separated into long (open triangles) and short (closed circles) time constants.</figcaption></figure>
$T_1^{-1}$ with the result

$$T_1^{-1} \approx \frac{6}{5\sqrt{2}} K_B T (g\mu_B)^{-2} \frac{\gamma_N^2 \gamma_e^2}{r_0^6} \chi_s(T) (D\omega_N)^{-1/2} g(\omega_N). \quad (1)$$

Here $r_0$ is the average distance (~10 Å) between ¹H and the center of Fe(mnt)₂ molecule, $\omega_N$ is the nuclear Larmor frequency, $g(\omega_N)$ is a field dependent constant while the exchange coupling $J$ between localized spins lead to a temperature independent diffusion constant $D \propto J$ for the excitations of antiferromagnetically coupled dimers of spins. Therefore the essential source of intrinsic temperature dependence for the enhancement of the quantity $(T_1 T)^{-1}$ should be then linear in the spin susceptibility $\chi_s(T)$ of the Fe(mnt)₂ stacks. According to the results of reference [4] (Fig. 4), we verify that the contribution to $\chi_s(T)$ coming from the excitations of spin dimers largely dominates compared to the one of conduction electrons of Perylene stacks. From the expression (1), this will then favor large $T_1^{-1}$ values in comparaison to those found in (Per)₂Au(mnt)₂.

Finally, we want to compare the temperature dependence of the quantity $(T_1 T)^{-1}$ with the susceptibility data in order to check the validity of (1). As shown in reference [4], the temperature dependence of the Faraday susceptibility of $\alpha$-(Per)₂Fe(mnt)₂ can be ascribed to dimers of spin $s$ localized on each Fe(mnt)₂ unit with an intradimer antiferromagnetic exchange $J$ and negligible interdimer coupling. The assumption $s = 3/2$ corresponding to a high spin configuration of Fe(mnt)₂ units provides the best fit for the susceptibility [4]. Figure 6 shows an attempt to explain the temperature dependence of the quantity $(T_1 T)^{-1}$ by two localized spin excitations, one coming from localized impurity spins giving rise to a Curie tail which is dominant at low temperature and the other which is attributed to the susceptibility

<figure><img src="image_9.png" /><figcaption>Fig. 6. — Decomposition of the relaxation $(T_1 T)^{-1}$ of $\alpha$-(Per)₂Fe(mnt)₂ into a Curie contribution and the spin excitations of antiferromagnetic dimers of spins $s = 3/2$ with $-2 J/kB = 450$ K. The insert shows the low temperature data where a clear Curie behaviour prevails.</figcaption></figure>
of *s* = 3/2 spins coupled in dimers [4]. We have used an exchange interaction – 2 *J/k*<sub>B</sub> = 450 K. The quality of the fit is rather good in the temperature domain 100-250 K (within the error bars of experimental points). Some improvement of the fit around 100 K can be gained if one considers the contribution to the nuclear relaxation from the conduction electrons on perylene stacks which amounts to about one tenth of the contribution from the localized spin excitations. Therefore, *T*₁ data are consistent with the existence of *s* = 3/2 spins on each Fe(mnt)₂⁻ unit. However, it is fair to mention that a comparison of the *T*₁ data of α-(Per)₂Fe(mnt)₂ with the singlet-triplet excitations of *s* = 1/2 dimers leads to nearly the same quality of fit. NMR alone cannot discriminate between *s* = 1/2 or *s* = 3/2 spins on the dithiolate stack.

**Concluding remarks.**

X-ray scattering and EPR data clearly show that the Per stacks of α-(Per)₂Fe(mnt)₂ and α-(Per)₂Co(mnt)₂ undergo a Peierls transition at 58 K and 73 K, as previously inferred from transport measurements [4]. This temperature is substantially higher than the temperature at which the other members of the α phase undergo a metal-insulating transition. By contrast with the Ni, Pd and Pt derivatives for which T_p = 25 K [10a], T_p = 28 K [2,10b], and T_p = 7 K [2,10b] respectively, and where there is sizeable inter-stacks spin-spin coupling, it is tempting to suggest that the absence of such a coupling could allow the developpment of a Peierls instability in the conducting stack at relatively high temperature. However, this interpretation ignores the experimental fact that other salts with non-magnetic dithiolate chains, such as the Cu and Au derivatives, undergo also a low temperature (33 K and 12 K respectively) [10] metal insulator transition. The high value of T_p in the Fe and Co derivative could be due to stronger inter-stack coupling (Coulombic) between the perylene charge density wave (CDW), caused by structural modification related to the chemical dimerization of the dithiolate stacks.

Even in the Fe and Co derivative, the Peierls transition temperature of α-Per₂M(mnt)₂ is much lower than the one at which the Perylene stacks undergo a peierls transition in other 2:1 salts. For example, the substituted Perylene salt (CPP)₂PF₆(CH₂Cl₂) undergo a Peierls metal-insulator transition at about 150 K [9]. Furthermore, its structural transition is announced by a sizeable regime of 2 *k*<sub>F</sub> fluctuations, detectable up to room temperature [9], as expected for a conventional Peierls instability. No such a regime of 2 *k*<sub>F</sub> structural fluctuations of the Per stack has been observed in any of the α-Per₂M(mnt)₂ materials studied. The absence of important pre-transitional fluctuations in the Fe and Co derivatives is confirmed by the spin susceptibility measurements which do not show, except in the near vicinity of T<sub>p</sub>, until 20 K above T<sub>p</sub> for the Co derivative from Faraday measurements [4], the growth of a pseudo-gap in the density of states. The inhibited nature of the 2 *k*<sub>F</sub> CDW instability of the Per stacks is probably a clue to understand the puzzling nature of the phase transitions shown by the α-Per₂M(mnt)₂ series of organic conductors.

**Acknowledgments.**

This work is partially supported by European Economic Community (EEC) under contract Esprit-Basic Research Action 3121. Several discussions with S. Ram.
References

[1] For a recent review see ALMEIDA M., GAMA V., HENRIQUES R. T. and ALCACER L., in « Inorganic and Organometallic polymers with special properties », R. Laine Ed. (1991).

[2] HENRIQUES R. T., ALCACER L., POUGET J. P. and JÉROME D., *J. Phys. C* **17** (1984) 5197.

[3] BOURBONNAIS C., HENRIQUES R. T., WZIETEK P., KONGETER D., VOIRON J. and JÉROME D., *Phys. Rev. B* **44** (1991) 641.

[4] GAMA V., HENRIQUES R. T., BONFAIT G., PEREIRA L., WAERENBORGH J. C., SANTOS I. C., DUARTE M. T., CABRAL J. M. P. and ALMEIDA M., *Inorg. Chem.* **31** (1992) 2598.

[5] WEIMER J. F., MELBY L. R. and BENSON R. E., *J. Am. Chem. Soc.* **86** (1964) 4329.

[6] GAMA V. P., HENRIQUES R. T., ALMEIDA M. and ALCACER L., *Synth. Met.* **42** (1991) 2553.

[7] ALCACER L. and MAKI A. M., *J. Phys. Chem.* **80** (1976) 1912.

[8] HENRIQUES R. T., ACACER L., ALMEIDA M. and TOMIC S., *Mol. Cryst. Liq. Cryst.* **120** (1985) 237.

[9] PENVEN P., Thesis, unpublished;
PENVEN P., JÉROME D., RAVY S., ALBOUY P. A. and BATAIL P., *Synth. met.* **27** (1988) B405.

[10] a) GAMA V., ALMEIDA M., HENRIQUES R. T., SANTOS I. C., DOMINGOS A., RAVY S. and POUGET J. P., *J. Phys. Chem.* **95** (1991) 4263;

b) BONFAIT G., LOPES E. P., MATOS M. J., HENRIQUES R. T. and ALMEIDA M., *Solid State Commun.* **80** (1991) 391.