# Coexistence of metallic character and charge density wave on Ni(dmit)2 stacks in TTF[Ni(dmit)2]2

A. Vainrub, Denis Jerome, M.-F. Bruniquel, P. Cassoux

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A. Vainrub, Denis Jerome, M.-F. Bruniquel, P. Cassoux. Coexistence of metallic character and charge density wave on Ni(dmit)2 stacks in TTF[Ni(dmit)2]2. EPL - Europhysics Letters, 1990. hal-05000153

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*Europhys. Lett.*, **12** (3), pp. 267-272 (1990)

# Coexistence of Metallic Character and Charge Density Wave on Ni(dmit)₂ Stacks in TTF[Ni(dmit)₂]₂.

A. VAINRUB(*)<sup>(§)</sup>, D. JÉROME(*), M.-F. BRUNIQUEL(**) and P. CASSOUX(**)

(*) Laboratoire de Physique des Solides, Université de Paris-Sud
91405 Orsay, France

(**) Laboratoire de Chemie de Coordination du CNRS, Université P. Sabatier
31077 Toulouse, France

(received 25 January 1990; accepted 27 March 1990)

PACS. 74.70K – Organic superconductors.

PACS. 71.45L – Charge-density-wave systems.

PACS. 76.60 – Nuclear magnetic resonance and relaxation.

**Abstract.** – We report ¹³C NMR spectra and spin-lattice relaxation measurements performed on the ¹³C enriched Ni(dmit)₂ molecules in the two-chain organic conductor TTF[Ni(dmit)₂]₂. The average Knight shift value for all Ni(dmit)₂ carbon positions amounts to about 100 p.p.m. between 300 and 100 K and decreases down to about 50 p.p.m. at 10 K. The temperature dependence of the spin-lattice relaxation rate is faster than the linear one. Our data clearly show that the CDW observed previously by structural investigations involves Ni(dmit)₂ stacks. However, at variance with other organic conductors, the onset of the CDW state in the present material does not induce any metal-insulator transition. Different possible shapes of Fermi surfaces are discussed.

*Introduction.* – High-conducting donor-acceptor compounds of TTF (tetrathiofuvalene) and transition metal (Ni or Pd) complex of «dmit» (dmit = 1,3-dithia-2-thione-4,5-dithiolato) exhibit interesting phase transitions and superconducting properties [1-4]. Among them, TTF[Ni(dmit)₂]₂ has metallike conductivity at ambient pressure [1] and is superconducting below 1.6 K under 7 kbar [2]. On the other hand, an X-ray study has shown the occurrence of one-dimensional (1D) structural fluctuations at wave vector 0.4*b* (TTF and Ni(dmit)₂ stacks are along *b*-axes) which become substantially three-dimensionally (3D) ordered below 40 K [4]. This superlattice has been attributed to the well-known charge density wave instability provoked by electron-phonon interaction in quasi-1D conductors.

Although a 3D network of S···S short contacts exists in the material, the electronic structure calculations [5] show 1D type of TTF[Ni(dmit)₂]₂ conduction band in contrast to the conductors based on sulphur containing BEDT-TTF molecule. More recent theoretical studies [6] have supported one-dimensionality and have shown in addition that because of

<footnote>(§) Permanent address: Institute of Chemical Physics and Biophysics, Estonian Academy of Sciences, 200001 Tallinn, Estonia, USSR.</footnote>
the small energy difference of 0.4 eV between the lowest unoccupied (LU) and highest occupied (HO) molecular orbitals (MO) of the Ni(dmit)₂ molecule the latter is also involved in the conduction band formation. According to these calculations, multiconduction band is thus formed consisting of one almost degenerate band of TTF HOMOs and two groups (each containing four branches) of LUMOs and HOMOs of Ni(dmit)₂.

Usually CDW induces a Peierls metal-insulator transition in an organic low-dimensional conductor and superconductivity is consequently suppressed. However, both instabilities occur in TTF[Ni(dmit)₂]₂. To gain insight into this problem separate investigations of Ni(dmit)₂ and TTF subsystems are needed. Local ¹H NMR studies of TTF stacks have shown their metallic character down to 1 K [7]. The superconducting instability is believed to be determined by the Ni(dmit)₂ subsystem because of the observation of the superconductivity in (CH₃)₄N[Ni(dmit)₂]₂, where the closed-shell (CH₃)₄N⁺ cations do not contribute to the conductivity [8]. Which stacks—Ni(dmit)₂, TTF or both—are involved in the CDW instability is still unknown. To study locally the Ni(dmit)₂ subsystem we have performed ¹³C NMR measurements of TTF[Ni(dmit)₂]₂ enriched by ¹³C in the dmit species. We have clearly observed the consequence of the CDW development on the Ni(dmit)₂ stacks towards low temperature although no insulating behaviour can be detected down to 10 K.

*Experimental section.* – Partially ¹³C-enriched TTF[Ni(dmit)₂]₂ was obtained as microcrystalline powder following a procedure derived from those described in [1], which involves five steps: i) preparation of the dmit²⁻ ligand through reduction of CS₂ by Na and stabilization of this ligand as the (n-Bu₄N)₂[Zn(dmit)₂] complex; ii) preparation of the stable intermediate dmit(COPh)₂; iii) regeneration of the dmit²⁻ ligand from this intermediate compound and precipitation of the (n-Bu₄N)₂[Ni(dmit)₂] complex; iv) iodine oxidation of this complex for obtaining the (n-Bu₄N)[Ni(dmit)₂] complex; v) slow interdiffusion of acetonitrile solutions of (n-Bu₄N)₂[Ni(dmit)₂] and (TTF)₃(BF₄)₂. Starting with ¹³CS₂ (Cambridge Isotope Laboratory), only the carbon atoms of the Ni(dmit)₂ moieties in TTF[Ni(dmit)₂]₂ are ¹³C-enriched. Given the cost of ¹³CS₂, a large and critical down-scaling (1:100) of the amounts of reagents used and of the size of glassware, compared to those of the typical standard procedure, was necessary. Typical optimized yields for steps i)-iv) are around 75%; the yield of step v) could not be made higher than 30%. Starting from a mixture of 2g ¹³CS₂ and 1.73 g CS₂, we were able to isolate 50 mg of partially ¹³C-enriched TTF[Ni(dmit)₂]₂.

37.175 MHz ¹³C NMR measurements were performed using a high-power pulse BRUKER MSL spectrometer with a superconducting magnet in the (10÷295) K temperature range. Field stability of 2 p.p.m. during some hours was achieved by stabilization of the magnet current. To exclude slow drift errors we have inserted inside the sample a small capillary tubing filled with KBr and have used the 37.03 MHz ⁷⁹Br NMR line as the internal reference. KBr is convenient because of the proximity with the ¹³C Larmor frequency and the fast quadrupolar relaxation at low temperatures [9]. To express our data in the usual ¹³C TMS scale we have accurately measured the ⁷⁹Br shift of KBr with respect to this reference. The spin-lattice relaxation time was measured by the inversion recovery method.

*Results.* – Figure 1 shows some examples of ¹³C NMR spectra. The asymmetric lineshape similar to the powder pattern for the case of axial symmetry of the magnetic shift tensor [10] is observed at room temperature. The resonance becomes symmetric and shifts slightly to high field as the temperature decreases. The spin-lattice relaxation rate is inhomogeneous throughout the line at room temperature as shown in fig. 1. The high-field part relaxes 2.4 times faster than the down-field one. The temperature dependence of the relaxation rates in different regions of the line (fig. 2) shows that the relaxation becomes homogeneous below ca. 80 K. The insert of fig. 2 gives an illustrative example of the nuclear magnetization
<figure><img src="image_2.png" /><figcaption>Fig. 1. – $^{13}$C NMR spectra of $^{13}$C-enriched TTF[Ni(dmit-$^{13}$C)$_2$]$_2$ at different temperatures: a) 10 K, b) 50 K, c) 110 K, d) 295 K and e) spin-lattice relaxation rate for d) spectrum.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 3.</figcaption></figure>

Fig. 2. – Temperature dependence of $^{13}$C spin-relaxation rate of TTF[Ni(dmit-$^{13}$C)$_2$]$_2$ for slow (x) and fast (○) relaxing parts of the line. Insert: inversion-recovery of nuclear magnetization; exponential fit is shown by the solid line.

Fig. 3. – Temperature dependence of the average over all Ni(dmit)$_2$ carbon positions $^{13}$C isotropic Knight shift of TTF[Ni(dmit-$^{13}$C)$_2$]$_2$. Insert: formula of Ni(dmit)$_2$.
inversion-recovery. An exponential recovery was observed for all parts of the NMR line and at all the temperatures.

*Discussion.* – All Ni(dmit)$_2$ molecules are crystallographically equivalent in TTF[Ni(dmit)$_2$]$_2$ but the six carbon positions in a given Ni(dmit)$_2$ moiety (see insert of fig. 3 for structural formula) are nonequivalent [1]. So, the lineshape in fig. 1 is the sum of six powder patterns. Each pattern is determined by the local magnetic shift tensor plus the dipolar broadening. The strongest dipolar interaction is the one between two nearest carbons separated by $r = 1.41$ Å and equal to $(3\gamma^2\hbar/2r^3) = 4.04$ kHz or about 100 p.p.m. So the linewidth in fig. 1 of about 400 p.p.m. is determined mainly by the difference of isotropic shifts and/or by an anisotropy of the shifts of the individual lines rather than by dipolar broadening.

In the case of anisotropic Knight shift tensor [K] the relaxation rate depends on the single-crystal orientation. When the Ox main axis of the [K] tensor is along the magnetic field, the shift is proportional to the longitudinal component $K_{xx}$, whereas the relaxation rate is determined by the transverse components of the tensor, $1/T_1 \propto (K_{yy}^2 + K_{zz}^2)$. So, the slowest relaxation rate corresponds to the orientation with the largest Knight shift. This fact qualitatively explains the inhomogeneous relaxation shown by curve e) in fig. 1, namely a decrease of the relaxation rate vs. the shift. A quantitative comparison with the theory is not possible in our case because of the composite character of the spectrum formed by six overlapping lines.

Although it is impossible to deconvolute the line shape into six lines, useful information can be obtained by the analysis of the moments of such composite spectrum as suggested in [11]. For individual carbon position which we denote by the index n the orientational dependence of NMR frequency is given by

$$ \omega_n = \omega_0[1 - (\delta_n^{xx} \sin^2 \varphi + \delta_n^{yy} \cos^2 \varphi) \sin^2 \varphi - \delta_n^{zz} \cos^2 \varphi], \quad (1) $$

where $\omega_0$ is the Larmor frequency, $\delta_n^{ii}$ ($i = x, y$ or $z$) are the main values of the magnetic-shift tensor [$\delta_n$]. $x, y$ and $z$ are the principal axes of [$\delta_n$] and $\varphi, \vartheta$ are the polar angles of the external magnetic field in these coordinates. The first moment is calculated by the simple integration

$$ M_1 = (1/4\pi) \int_0^{2\pi} d\varphi \int_0^{2\pi} d\vartheta \sin\varphi \omega_n(\varphi, \vartheta) = \omega_0(1 - \delta_n), \quad (2) $$

where $\delta_n = (1/3) \text{Tr}[\delta_n]$ is the isotropic shift. The dipolar broadening is symmetrical and obviously does not change the first moment. We sum the expression (2) over all the lines and obtain for the composite line moment $M_1 = \omega_0(1 - \bar{\delta})$, where $\bar{\delta} = \sum (\delta_n/6)$ ($n = 1, 2, \dots, 6$) is the average isotropic shift.

In a metal the total NMR shift is $\delta = \delta^{cs} + K$, where $\delta^{cs}$ is the chemical shift and K is the Knight shift. The chemical shifts of Ni(dmit)$_2$ being unknown we have used the values $\delta_1 = 125.5$ p.p.m. for the inner and $\delta_2 = 210.5$ p.p.m. for the outer carbons of the similar 4,5-ethylene-1,3-dithia-2-thion ($C_5S_5H_4$) molecule measured by a magic angle spinning experiment [12]. We believe that shifts change only slightly for Ni(dmit)$_2$ since the largest deviation of the shifts from their average for CS$_2$-group carbon of $C_5S_5H_4$, TTF and BEDT-TTF is 8 p.p.m. [12]. So, we assume $\delta^{cs} = (4\delta_1 + 2\delta_2)/6 = 154$ p.p.m. to be an average $^{13}$C chemical shift of Ni(dmit)$_2$.

Figure 3 shows the temperature dependence of the $^{13}$C average Knight shift of Ni(dmit)$_2$ moieties determined by calculation of the first moments of the lines in fig. 2. The
paramagnetic Knight shift is constant, $K \approx 100$ p.p.m. between room temperature and 100 K and drops to ca. 50 p.p.m. at 10 K. In spite of some error connected with the moments determination and our choice of chemical shift the values obtained in fig. 3 are large enough to conclude for the existence of a Knight-shift in the entire temperature domain including the low-temperature part. These results provide the evidence for the metallic character of the $Ni(dmit)_2$ subsystem. On the other hand, since the Knight shift decreases in the same temperature region where the CDW becomes 3D ordered [4], $Ni(dmit)_2$ stacks should be involved in the CDW formation.

The relaxation data (fig. 2) support the metallic character of the $Ni(dmit)_2$ stacks. Even for the slow-relaxing part of the spectrum, the relaxation rate is 7 times at 295 K and 20 times at 10 K faster than the Korringa relation $K^2 T_1 T = (\hbar/4\pi k)(\gamma_e/\gamma_n)^2$ predicts. Such a fast relaxation can be induced only by a hyperfine interaction with conduction electrons and gives evidence for a metallic state. It should be noted that because of the overlapping lines our data provide only some average description of the relaxation of all six nonequivalent carbons. The case is too complicated for a detailed analysis. Therefore, we do not discuss the observed relaxation enhancement and the faster than the linear temperature dependence of the relaxation rate although both phenomena are known to exist for organic metals [13].

Coexistence of a metallic state with a CDW distortion is unusual for organic conductors. It shows that even 3D ordered CDW does not open the gap throughout the entire Fermi surface and instead decreases the density of states at Fermi level as shown by the temperature dependence of the Knight shift. A multisheet Fermi surface for the $Ni(dmit)_2$ subsystem has been predicted by band calculations [6]. Therefore, CDW could open a gap solely on the sheets connected by a Fermi wave vector equal to the CDW wave vector, namely $0.4b^*$ without affecting the other sheet of the Fermi surface. Another possibility is that the planar sheet of the quasi-1D Fermi surface is sufficiently distorted by 2D or 3D couplings to prevent perfect nesting. In such a case the gap is opened only on some parts of a single sheet. At present, we feel unable to decide between the two possibilities. To solve this problem, high-resolution NMR experiments are in progress, and will be reported in a forthcoming publication.

***

The authors would like to thank E. CANADELL and S. RAVY for helpful discussions. We also thank P. WZIETEK for useful NMR software. The stay of AV at Orsay has been supported by the European contract ESPRIT 3121. The work by MFB and PC was sponsored by the PIRMAT-CNRS under contract «ATP Superconducteurs 89» No. 89N83/0231.

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