NMR STUDY OF THE QUASI ONE DIMENSIONAL CONDUCTOR K₂Pt(CN)₄Br₀.₃ 3H₂O UNDER PRESSURE

W.H.-G. Müller,* H. Niedoba and D. Jérôme

Laboratoire de Physique des Solides,† Université Paris-Sud, 91 405 Orsay, France

(Received 4 December 1974 by P.G. de Gennes)

Investigation of spin lattice relaxation time $T_1$ and NMR line shift of Pt¹⁹⁵ in KCP under hydrostatic pressure are reported in the temperature range between 78 and 300 K up to 20 kbar. At temperature above 120 K, pressure decreases the relaxation rate appreciably.

THE MIXED valency platinum compound K₂Pt (CN)₄ Br₀.₃ · 3H₂O (KCP) is a well known example for a one dimensional quasimetallic system.¹ The physical properties have been extensively investigated for the last few years,² X-ray scattering,³ NMR⁴ and neutron diffraction⁵ studies have shown that KCP presents an insulating ground state provided by a three-dimensional (3-d) phase transformation occurring at low temperature.⁶

Due to the pronounced 1-d character the phase transition is smeared out over a wide temperature range in which fluctuations are expected.⁸ The fluctuations are probably responsible for the unusual NMR behaviour observed at temperatures higher than 100 K.⁹

There are several theoretical models for 1-d systems. The mean field theory¹⁰ provides a mean-field transition temperature $T_c^{mf}$ located well above the actual transition temperature. Lee et al.¹¹ have shown that the actual transition is given by a temperature $T_{3d}$ which is about a factor 4 smaller than $T_c^{mf}$.

The question of the quasi one-dimensionality of the system is very important because actual solids have no ideal 1-d character.

Hydrostatic pressure investigations give the possibility to vary the intra- as well as the inter-chains coupling.¹² Previous pressure investigations of KCP have exhibited an important pressure dependence of the conductivity at room temperature¹² and indicate a decrease of the “apparent gap” under pressure.¹³

The temperature dependence of the Knight shift and nuclear relaxation rate $T_1^{-1}$ have been measured at ambient pressure by Niédoba et al.⁹

In this communication we report the data of Knight-shift and $T_1$ at 0.2, 8 and 20 kbar.

Relaxation time measurements at 0.2 and 8 kbar were performed in a helium gas pressure equipment. For $T_1$ and Knight shift measurements at 20 kbar as well as Knight shift at 8 kbar a liquid pressure equipment with isopentane as pressure transmitting medium was used. The gas set-up provides a perfect hydrostatic pressure down to the freezing point of the fluid and a better NMR signal because of the larger sample volume in this case. However gas pressure NMR is limited to 10 kbar at low temperature for safety reasons. The 20 kbar experiment is performed with the help of a two-stage pressure equipment which will be published elsewhere.¹⁴

The samples orientated with their C-axis along the magnetic field have a volume of 200 and 80 mm³ for gas and liquid pressure equipments respectively.

<footnote>* Present address: Physikalisches Institut, Universität Karlsruhe, W. Germany.</footnote>

<footnote>† Laboratoire associé au CNRS.</footnote>
<figure><img src="image_1.png" /><figcaption>FIG. 1. Temperature dependence of the Pt$^{195}$ Knight shift at different pressures. The reference signal is $H_2PtCl_6$.</figcaption></figure>

The NMR signal is measured with a pulse and boxcar integrator technique.¹⁵ Above 100 K the signal to noise ratio is very poor, leading to considerable error bars. Especially, the exact location of the signal center, giving the Knight shift is difficult to determine. The measurements were performed at a frequency of 13 MHz with a field of about 14 kOe provided by a superconducting solenoid.

The temperature was measured with a copper-constantan thermocouple inside the pressure vessel. To be sure that the RF pulse does not heat up the sample, the temperature has been determined by comparative $T_1$ measurements of metallic platinum powder at several temperatures for the 8 kbar experiment. Within the error bars no difference between the vessel temperature and the sample temperature has been detected, indicating a good thermal contact provided by the pressure transmitting medium.

The results for the Knight shift are summarized on the Fig. 1. The ambient pressure values taken from Niedoba *et al*.⁹ indicate a continuous change of $K$ between 100 K and the room temperature where $K$ has not yet reached a constant value. Since $K$ is proportional to the density of states at Fermi level $n(E_F)$, a continuous change of $n(E_F)$ is observed in a very broad transition region. The broadness of the transition was attributed to the smearing out of the phase transition by strong fluctuations.⁹ Practically no difference can be observed between the ambient

<figure><img src="image_2.png" /><figcaption>FIG. 2. Rate of relaxation $T_1^{-1}$ for Pt$^{195}$ at several pressures.</figcaption></figure>

pressure and 8 kbar experiments within the error bars. However at 20 kbar the variation of $K$ with temperature seems to be steeper.

In Fig. 2 we have presented the $T_1^{-1}$ results. At low temperature, namely $T < 77$ K, the increase of $T_1^{-1}$ occuring under pressure is in agreement with the decrease of the low temperature band gap, for free carriers induced relaxation.¹⁶ The decrease of the relaxation rate observed at $T > 120$ K under pressure hints obviously a decrease of the mechanism responsible for the short $T_1$ at ambient pressure, Fig. 2.

Since the chains are already metallic at high temperature and ambient pressure we do not expect much change occuring under pressure through the increase of the intra-chain coupling. As far as the transverse direction is concerned, pressure is going to admix 3-d character into the 1-d character through the increase of the interchain coupling. The data of $T_1^{-1}$ show undoubtedly that pressure tends to diminish very strongly the enhancement effect observed previously.⁹ As seen from the temperature dependence of the Knight shift, the application of pressure reduces the width of the metal-insulator transition. This is another manifestation of the enhanced 3-d coupling occuring under pressure. We expect that at increasing pressure the phase transition would resemble more and more a traditionnal 3-d phase transition.

$T_1^{-1}$ can be evaluated with the knowledge of the frequency and wave vector dependent susceptibility.
Any decisive fit with the theory will await further
development of the influence of the 3-d coupling on
the susceptibility of quasi 1-d systems.<sup>17-20</sup>

*Acknowledgments* — We are very grateful to Dr. H. Launois for stimulating discussions. Discussions with Profs. J. Friedel, S. Barisic and M. Weger, are also gratefully acknowledged. We also thank G. Delplanque, G. Malfait and R. Delaplace for their technical assistance

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