# Pressure Induced Magnetic State in (TMTTF)2PF6

F Creuzet, Denis Jerome, A Moradpour

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F Creuzet, Denis Jerome, A Moradpour. Pressure Induced Magnetic State in (TMTTF)2PF6. Molecular Crystals and Liquid Crystals, 2014, 119, pp.297 - 302. 10.1080/00268948508075174. hal-05010422

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# Pressure Induced Magnetic State in (TMTTF)₂PF₆

F. Creuzet<sup>a</sup>, D. Jerome<sup>a</sup> & A. Moradpour<sup>a</sup>

<sup>a</sup> Laboratoire de Physique des Solides (associé au CNRS), Bat. 510, Université Paris-Sud, 91405, Orsay Cédex, France

Published online: 17 Oct 2011.

**To cite this article:** F. Creuzet, D. Jerome & A. Moradpour (1985) Pressure Induced Magnetic State in (TMTTF)₂PF₆, Molecular Crystals and Liquid Crystals, 119:1, 297-302

**To link to this article:** http://dx.doi.org/10.1080/00268948508075174

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PRESSURE INDUCED MAGNETIC STATE IN $(TMTTF)_2PF_6$

F. Creuzet, D. Jérôme, A. Moradpour

Laboratoire de Physique des Solides (associé au CNRS)

Bât. 510, Université Paris-Sud, 91405 Orsay Cédex (France)

**Abstract** - We present $^1$H NMR relaxation data on $(TMTTF)_2PF_6$. The low temperature insulating state is shown to be non magnetic at ambient pressure and antiferromagnetic (SDW) at 13kbar.

INTRODUCTION

$(TMTTF)_2X$ organic conductors have been of great interest since the discovery of superconductivity in $(TMTTF)_2PF_6$ under pressure.$^1$ In absence of any anion ordering, the metal insulator transition observed at low temperature in the $(TMTTF)_2X$ series has been characterized as the condensation of a spin-density-wave (SDW) phase $^2$. The sulphur analog family $(TMTTF)_2X$ has been interpreted as more one-dimensional at ambient pressure $^3$ and a study of the temperature-pressure phase diagram $^4$ has clearly shown the similarities between both series, the critical pressure for the suppression of insulating phase being shifted to higher value for the sulphured compounds. Experimental investigations of the microscopic nature of the insulating state have already extended the comparison, showing for example the magnetic character of the low-temperature semiconducting state in $(TMTTF)_2Br$. $^{5-6}$ For $(TMTTF)_2PF_6$, the results at ambient pressure were less convincing and the nature of the insulating state was not clear so far.$^5$ In the present work, $^1$H NMR relaxation rates measurements show unambiguously that the low-temperature state is non-magnetic at ambient pressure. Consequently, the results support the existence of a spin-Peierls (SP) state. We also show that a pressure of 13 kbar is sufficient to induce a spin density wave (SDW) state.

EXPERIMENTAL

We have performed $^1$H NMR measurement on single crystals of $(TMTTF)_2PF_6$ with magnetic field perpendicular to the conductivity axis at 1 bar and 13 kbar. It is very important to note that the
previous results at ambient pressure reported in ref. 5 were
collected using powdered samples prepared by metathesis (anion
exchange with (TMTTF)₂BF₄) and these probably contain magnetic
impurities. As the electronic properties of the Bechgaard salts
are very sensitive to impurities ⁷, a new batch of samples prepa-
red by usual electrochemical method ⁸ has been used here.

A home-made pulsed NMR spectrometer was operated at a
frequency of 45 MHz (resonance field ~ 10.6 kOe). The free induc-
tion and spin-echo signals were digitized and analysed by a
microcomputer. The spin-lattice relaxation time T₁ was measured
by observing, on more than one decade, the time recovery of the
free induction after the saturation comb pulse. The spin-spin
relaxation time T₂ has been determined by the echo signal method
whereas, assuming an exponential shape, T₂'' is given by the width
of the echo.

RESULTS

For the ¹H spin-lattice relaxation measurements in these materials we know that an important contribution is due to the quantum tunnelling of methy-groups ⁵⁻⁹. At high enough temperature, this contribution is dominant and can be extrapolated with the assump- tion of an activated behaviour. So, we have plotted on the figu- res the only point for which this contribution is negligible.

At ambient pressure, no anomaly is detected. The linewidth is
constant between 2 and 50 K (T<sub>2</sub>'' ≈ 22 µs). At low temperature, the
relaxation rate is very small and a clear departure is observed
at ~ 14 K (Fig. 1). This temperature corresponds with the drop in
static susceptibility ¹⁰⁻¹¹. These new NMR data show unambiguously
that the low temperature phase (below 15 K) is non-magnetic in
character and its nature will be discussed later. Note that the
high-temperature data (T > 25 K) are not far from those of ref. 5
and we can interprete the previous large excess of relaxation
rate at low temperature as due to magnetic impurities.

A pressure of 13 kbar on (TMTTF)₂PF₆ induces important
<figure><img src="image_3.png" /><figcaption>FIGURE 1</figcaption></figure>

¹H spin-lattice relaxation at 1 bar.

<figure><img src="image_4.png" /><figcaption>FIGURE 2</figcaption></figure>

¹H spin-lattice relaxation at 13 kbar.

changes. From resistivity measurements ⁴, a nearly metallic phase is stabilized down to ~19 K, temperature at which a metal-insulator transition occurs. NMR data show a clear peak of the
spin lattice relaxation rate around T ~ 17 K (Fig. 2). In a narrow
temperature region (± 5 K), the relaxation was found non exponen-
tial, but we can define the initial (0) and the final (+) slopes
of the decay. We note the apparent constant value of T₁⁻¹ above the
transition (25-35 K). The anomalies of T₂ and T₂⁽⁺⁾ (Fig. 3) are

<figure><img src="image_5.png" /><figcaption>FIGURE 3</figcaption></figure>

evidences for the onset of an inhomogeneous local field. T₂
increases by about 30 % while T₂⁽⁺⁾ decreases by a factor 3. With
T₂⁽⁺⁻¹⁾ values, we can estimate the increase of the inhomogeneous
local field at ¹H sites as about 40 Oe. The increase of T₂ below
17 K is the consequence of nuclear spin decoupling due to the
inhomogeneous local fields. All these results allow us to asso-
ciate the metal-insulator transition with the condensation of a
magnetic (SDW)phase. We want to point out that, by many features,
(TMTTF)₂PF₆ at 13 kbar behaves like (TMTTF)₂Br at 1 bar. Especial-
ly the amplitude of the magnetization and the constant value of
T₁⁻¹ above the transition are the same. This is in agreement with
the picture already mentionned ⁴ that the T-P phase diagrams
of (TMTSF)₂X and (TMTTF)₂X series are identical (without anion
effect) but shifted in pressure.

DISCUSSION

At ambient pressure, the resistivity measurements clearly show
the existence of a well defined Hubbard correlation gap at high
temperature ($T \lesssim 200$ K). This has been interpreted by a weak localization of the electrons on the chain, which is consistent with the prediction of a $4 k_F$ charge density wave in a g-dogy model, when the Umklapp scattering processes (related to the dimerization of the organic molecules along the stack) are relevant. At this point, the observation of weak diffuse sheat at $2 k_F$ by X-ray measurements below $60 \text{ K}^{13}$ and the absence of any magnetic effect by NMR strongly suggest that the low temperature state is of Spin-Peierls type in agreement with susceptibility measurements$^{14}$.

At $13 \text{ kbar}$, the present work shows that the low-$T$ state is characterized as a SDW state. This possibility of a pressure induced magnetic state strongly support the interpretation of the T-P phase diagrams$^{4-5}$. In absence of anion ordering effects, the phase diagrams and the nature of the low-$T$ phases are identical for both $(TMTSF)_2X$ and $(TMTTF)_2X$ families but are shifted on the pressure axis. Apparently, the critical pressure depends on the magnitude of the resistivity correlation gap or the relevance of Umklapp processes. From a theoretical point of view$^{14}$, the constant value of $T_1^{-1}$ above the transition may be attributed to the existence of a small correlation gap, which can be inferred from a flat variation of the resistivity between 50 and 19 K at this pressure$^4$. In this sense, the depression of the relaxation rate observed in $(TMTTF)_2\text{Br}$ between 1 bar and 13 kbar above the transition, may be attributed to a gradual reduction of the correlation gap. This should be correlated with the reduction of the Umklapp scattering under pressure$^{12-15}$.

## CONCLUSION

The present results confirm that the properties of the electron gas in $(TMTTF)_2X$ and $(TMTSF)_2X$ are quite similar with an appropriate change of the pressure scale. Furthermore, the $(TMTTF)_2\text{PF}_6$ compound seems to be a nice candidate to check the predictions of theories, as it undergoes by a continuous change of the lattice parameters, non magnetic, magnetic and possibly superconducting
low temperature states. We believe that the most relevant effect of pressure is the reduction of the Umklapp scattering¹² (and consequently the Hubbard gap) while SDW phase appears and finally superconductivity takes place.

REFERENCES

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