Effect of mechanical pressure on the adsorption-induced electrical conductivity of ferrocene

Abstract

The u s e o f mechanical pressure to modulate electrical transport properties has proven to be one of the most fruitful techniques available. During the past three decades three reports have been published about the effect of the mechanical pressure on the electrical properties of ferrocene (bis(cyclopentadienyl) iron; (CsH5)gFe) the firstsynthesized and perhaps the best-known metalloorganic sandwich compound. Okamoto et al. [1] reported a slight increase of the electrical conductivity of ferrocene with increasing pressure up to 400 MPa; above which the conductivity decreased and continued to do so at higher pressures up to 3.3 GPa. In contrast, Maksimychev et al. [2] found a monotononically decreasing conductivity in the entire pressure region of 100 MPa to 3.6 GPa. Both research teams reported good reproducibility. To justify the dissimilar conductivity-pressure variation reported in [1, 2], Karvally et al. [3] investigated again the pressure dependence of the d.c. conductivity of ferrocene at room temperature up to 670 MPa. It has been reported [3] that a slow continuous phase transition may occur in ferrocene under pressure at room temperature. The adsorption of minute amounts of moisture on ferrocene may cause additional modifying effects on the pressure dependence of the conductivity. It appears that systematic pressure-dependent conductivity studies can provide new insights into both the bulk and the surface properties. We were interested in investigating the pressure-dependent conductivity variation with temperature of ferrocene under carefully controlled conditions. Some interesting findings were encountered, which are briefly discussed below. High-purity microcrystalline ferrocene was obtained from Aldrich Chemical Co. Inc., Milwaukee, Wisconsin, USA, and was used after further purification by repeated crystallization. Reagent chemicals used were of spectrograde quality. The current-temperature characteristics were measured using a cylindrical pressure cell placed inside a suitably designed conductivity chamber made of brass and fashioned with Teflon. The pressure cell used was almost as designed by Karvally et al. [3], with some modifications for temperature control. 30 mg samples were loaded into the stainless steel pressure cell lined with an appropriate Teflon tubing and placed inside the chamber. The sample was evenly distributed over the base plate. Teflon insulators were used at all critical places where leakage could have interfered with the conductivity measurements. To allow various vapours inside the chamber, dry nitrogen gas was passed at a constant rate through a bubbler which contained the adsorbate chemical kept at the desired temperature to maintain a specific vapour pressure. Repeated heating and cooling of the sample in dry nitrogen atmosphere ensured desorption of any preadsorbed gas or vapour. After an exposure of 30 min of the sample cell at 299 K to a vapour of 5.33 kPa vapour pressure inside the chamber, adsorption was ended. The pressure-transducing piston was then introduced into the cylindrical cavity and the pressure was applied. The pressure cell was then cooled to 288 K. After 1 h the cell was heated slowly (heating rate 0.01 Ks -1) and the current and temperature noted. The baseplate and the piston introduced into the pressure cell served as electrodes. Current readings were taken at a steady 40 V d.c. bias. We applied pressures up to 5 MPa. Details of the experimental arrangement and procedure will be given elsewhere. With the application of mechanical pressure o n the ferrocene cell adsorbed with ethanol at 5.33 kPa, the conductivity (proportional to the current) increased very slowly as shown in Fig. lc. Curve a of Fig. i shows the variation of the conductivity of pure ferrocene with pressure within the applied pressure range. Adsorption of methanol vapour showed a very small change in conductivity (Fig. 1, curve b). The pressure had very little effect on the conductivity of pure ferrocene in this pressure range, and for ethanol and methanol adsorption the conductivity was found to increase linearly, although slowly, with the applied mechanical pressure. When the ferrocene cell adsorbed with ethanol at 5.33 kPa was heated without any mechanical pressure applied, the conductivity varied with the cell temperature as shown in Fig. 2, curve b. Here a broadband peak has