Sodium–sodium halide co-intercalated graphite: chemistry, structure and electrical transport

Journal of Physics and Chemistry of Solids 60 (1999) 475–482

Sodium–sodium halide co-intercalated graphite: chemistry, structure and electrical transport L. Edman a, A. He´rold b, P. Jacobsson a,1, M. Lelaurain b, E. McRae b,*, B. Sundqvist a b

a Department of Experimental Physics, Umea University, S-90187 Umea, Sweden Laboratoire de Chimie du Solide Mine´ral, UMR 7555, Universite´ Henri Poincare´ Nancy 1, B.P. 239, 54506 Vandoeuvre les Nancy Ce´dex, France

Received 3 August 1998; accepted 4 October 1998

Abstract This study deals with the second to fourth stage compounds resulting from the co-intercalation of sodium and sodium halides into graphite. The charge transfer was determined through chemical analyses and X-ray diffraction and the results are compatible with Raman spectroscopy data. We present detailed results on c axis conduction between 4.2 K and 295 K and for hydrostatic pressures as high as 1.6 GPa. Possible mechanisms explaining the c axis conduction are discussed. 䉷 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Inorganic compounds; C. High pressure; D. Electrical conductivity; D. Transport properties

1. Introduction The intercalation of sodium into graphite has a long history. While the reaction of graphite with lithium and the heavy alkali metals (M ˆ K, Rb, Cs) under appropriate conditions leads to first stage LiC6 and MC8 respectively, that with sodium invariably leads to a much poorer sixth stage compound [1]. More recent studies have shown that high pressure synthesis of sodium binary compounds is possible [2], leading to products as metal-rich as C2Na [3,4]. The synthesis of high sodium content materials under less extreme conditions necessitates a third element or molecule and over the past years, high sodium content compounds have indeed been found containing the hydride [5], the hydroxide [6] and the peroxide [7] or when the third species is a polar molecule such as NH3 [8], THF or DMSO [9,10]. A number of interesting reviews have treated some aspects of the chemistry, structure and physical properties of ternary GICs over the past years [11–14]. The recent cointercalation of sodium and its halides (NaBr, NaCl, NaI) * Corresponding author. 1 Present address:. Department of Physics, Chalmers University of Technology, S-41296 Go¨teborg, Sweden.

into graphite has given rise to a rich new family of donor GICs [15–17], and it is these compounds that will be examined here, developing in more detail some preliminary results [15,16,18]. As we shall see, while the intercalation of NaBr and NaCl leads to compounds with well defined inplane unit cells, that of NaI results in a range of chemical and structural possibilities, as wide as what has been observed in certain acceptor GICs, such as those containing SbCl5 [19]. After indicating the synthesis conditions and experimental techniques employed, we summarise details concerning the chemical composition, the structure and some results of Raman spectroscopy, from which we can extract charge transfer data. Results are then furnished on selected samples concerning the c axis resistivity as a function of temperature and hydrostatic pressure, r c(T,p) for 4.2 ⱕ T ⱕ 300 K and 1 bar ⱕ p ⱕ 1 GPa. The discussion will dwell on the many particularities of these new donor GICs.

2. Sample syntheses and experimental techniques The pyrographite or single crystal (Kish) based samples were synthesised in stainless steel reactors under argon atmosphere. The graphite host was placed in the reactor

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with the required solid halide and a large excess of liquid sodium, then heated at temperatures between 450⬚C and 850⬚C for durations from several hours to 10 days [15,17]. After terminating the synthesis, the holder was quenched in water; the excess sodium and halide was removed from the sample faces by washing in alcohol, and then water. It is important to note that the transfer of the halide to graphite requires the liquid sodium as an intermediary, and is thus a function of the solubility of the former in the latter, varying as the inverse of the boiling point. Thus, while NaI (Tb ˆ 661⬚C) can be intercalated at temperatures above 450⬚C, that of NaCl (Tb ˆ 801⬚C) can only be intercalated at temperatures above 550⬚C. However, the temperature affects not only the solubility of the halide in liquid sodium, but also the intercalation thermodynamics. Indeed, while NaI can be intercalated to result in second stage compounds, between 450⬚C to 850⬚C, halide-rich NaBr and NaCl GICs are obtained only above 800⬚C, suggesting that these are unstable, endothermic phases. Unfortunately, at such high temperatures, the graphite host is strongly attacked by the halide such that the synthesis of samples suitable for physical measurements is rendered more difficult. Chemical compositions were determined using chemical analysis and energy dispersive X-ray analysis to extract overall and local information, respectively. For the former, the sample to be analysed was weighed and then burned to eliminate the graphite matrix. After dissolving the ash in water, acidimetry allowed evaluating the content of sodium not bonded with the halogen, the latter then determined through the use of the initial weight uptake. For the X-ray studies, we used a combination of photographic and diffractometric techniques. Room temperature studies were carried out with a classical, vertical-axis counter diffractometer, a 1 kW molybdenum source, a quartz monochromator and using a long focal distance (510 mm) yielding an angular resolution of du FWHM ˆ 0.04⬚. The sample was placed at the centre of the goniometer, successively with the c axis perpendicular to the incident beam (u ˆ 0⬚, (0 0 l) reflections) then parallel (h k 0). The sample and counter angles could further be positioned to determine the intensities along a given hk reciprocal rod. As a complementary technique, h k 0 and h k 1 reflections could be recorded using an Inel CPS 120 curved detector placed in a horizontal then a vertical position [20] Study of the (0 0 l) reflections yields information on the stage purity of the compounds. Through the peak positions we can obtain: the c axis repeat distance Ic, separating 2 layers of the intercalate; the stage s, or number of graphene sheets between these intercalate layers; the interplanar distance di or sandwich thickness. If the unfilled galleries in the intercalation compound retain the 335 pm spacing of graphite, then Ic ˆ di ⫹ (s-1) × 335 pm. When the sample examined contains a mixture of phases, the reflections from each phase can be separated at large l provided that the correlation lengths along the c axis of the individual phases are sufficiently long ( ⬎ 10 nm). When this is

not the case, what is observed is interpreted in terms of interstratification. A small amount of dispersion in peak positions with respect to the theoretical positions (i.e., single phase) and different FWHM values are the signs of such an admixture of phases. The (h k 0) reflections furnish information on the planar organisation of the intercalate and also allow determining the C–C distance dCC within the graphene layers. Raman inelastic light scattering studies were carried out at room temperature using a Dilor spectrometer and a triple monochromator with a 1800 lines/mm holographic grating to resolve the backscattered spectrum. The slits were set to give a resolution of 3 cm ⫺1. Basal plane electrical resistivity was measured inductively at 35 kHz using a well established non-contact technique [21]. For the r c(T) measurements, the sample was placed in a holder comprising two coaxial platinum contacts exerting light spring pressure on the sample faces. The outer furnished the DC measuring current (1–5 mA), the inner serving as voltage probes. All these resistivity measurements were made through cooling of the sample from 295 K to 4.2 K over a 6 to 10 h period. The high pressure measurements were carried out in a piston cylinder device integrated with an external heater and a single stage Leybold–Heraeus cooling head type RGS 120 connected to a Leybold–Heraeus closed cycle helium refrigerator [22,23]. By adjusting the current through the heater with a PID-regulator it was possible to obtain isothermal conditions in the temperature interval 150– 300 K during pressure runs. As pressure containers, cylindrical Teflon娃 cells with an outer diameter of 45 mm were used. The cell was filled with a 50/50 mixture by volume of n- and iso-pentane, a pressure transmitting medium with good hydrostatic properties down to at least 150 K in the pressure region of interest (p ⬍ 1.0 GPa) [24]. In each cell one (or two) sample(s), a Chromel–Alumel type K thermocouple and a Zeranin娃 (pressure gauge were placed. The resistances of the sample and the pressure gauge were measured with a DC four-probe technique. The Zeranin pressure gauge has a maximum inaccuracy in pressure of 1% down to 160 K, while the calibrated thermocouple has a maximum inaccuracy of 0.1 K at room temperature and ⬍ 1 K at 80 K [23]. Probably because of the extremely high basal plane elastic modulus of graphite and the small thickness of the samples, standard sample holders proved to be unsuitable during pressure runs. To overcome this, a simple but functional set-up was developed in which two thin copper wires were attached with silver paint directly to each of the two flat surfaces of the sample. To further minimise the tension at the contact points between sample and wires during preparation and execution of experiments, all four wires were bundled together in a relief point. Nevertheless, as in our previous measurements on pure graphite [25], pressure changes with time had to be carried out very slowly to avoid contact movement: Dp/Dt ˆ 0.001 GPa/min.

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Table 1 Structural data on sodium halide GIC samples examined. The 2D unit cell refers to the intercalate layer. Sample: NaBr-3H NaCl-2H NaCl-3H NaCl-3K NaCl-4H NaI-180 NaI-28 NaI-52

Graphite HOPG HOPG HOPG Kish SC HOPG HOPG Kish SC HOPG

Stage 3 2 3 3 4 2 2 2 (3)

Ic (pm)

2D cell

1441 1091 1426 1426 1760 1109 1104 1130

√ √ ( 3 × 3)R30⬚ √ √ ( 3 × 3)R30⬚ √ √ ( 3 × 3)R30⬚ √ √ ( 3 × 3)R30⬚ √ √ ( 3 × 3)R30⬚ Orthorombic Hexagonal (3aG) Several large

3. Results and discussion While the 2D intercalate layer structure in the bromide and chloride GICs is well defined, such is not the case for the iodide compounds and we give here the results on three specific samples which represent only a very small percentage of all those examined. Indeed, a wide range of NaI GICs were synthesised on which chemical analyses and diffraction studies were performed, the conclusion being that such materials have a far more complex relationship between synthesis conditions and resulting chemical composition, structure, and properties than do those containing NaBr and NaCl. A detailed discussion on the crystallochemistry of the NaI GICs will be presented elsewhere [26]. Some structural details on the samples studied are given in Table 1. The feature common to all the three halide families is that the intercalate involves a double layer of sodium. The interplanar distances are 760 ^ 5 and 775 ^ 5 pm for the NaCl and NaBr GICs respectively. The uncertainty can be attributed to the fact that for s ⬎ 2, one cannot affirm perfect stage purity and some degree of interstratification is generally present. At room temperature, the NaCl and NaBr compounds both possess commensurate √ √ hexagonal ( 3 × 3)R30⬚ in-plane unit cells independent of stage for 2 ⱕ s ⱕ 4. Taking two Na and one halogen atom per six carbon atoms for the unit cell, leads to a ‘‘crystallographic formula’’ of C3sNaX0.5 where X ˆ Br or Cl. Chemical analyses give somewhat smaller Na/C and X/Na ratios. The sheets on either side of the intercalate layer are stacked in an A/A arrangement but the intercalate layers, while possessing strong 2D character, have no ordered sequences along the c direction. Structural data on the three NaI GICs studied here are also summarised in Table 1. Both XRD and electron microscopy show that the intercalate layer in sample NaI-28 comprises a hexagonal cell, larger than that in the bromide and chloride GICs. The stacking sequence is A/AB/BC/CA but with many stacking faults. Sample NaI-52 is multiphased with several large unit cells simultaneously present. Sample NaI180 is particularly interesting since the cell is incommensu√ rate (unlike the 3aG and 3aG cells, aG being the unit cell of the graphite host) and yet surprisingly this is the only sample

dC–C (pm) ^ 0.05 142.45 142.26

142.51 142.33 142.2

in which there is three dimensional correlation between intercalate layers. This is highly unusual for a second stage material. For a number of samples, we measured the change in inplane carbon–carbon distance, DdC–C (Table 1) from which it is possible to evaluate the charge transfer per carbon atom, fc (Table 2) using the Pietronero–Stra¨ssler relationships [27]: DdC–C …s ˆ 1†…pm† ˆ 15:1fc ⫹ 14:6兩fc 兩3=2 ⫹ 23:6fc2

…1†

DdC–C …s ⱖ 2†…pm† ˆ …2=s†DdC–C …s ˆ 1†; s ˆ stage

…2†

It should be noted that the precise values thus determined depend strongly on the exact value taken for dC–C in the host. Pietronero and Stra¨ssler themselves used 142.1 pm and the literature more often gives 142 pm. Even using the (3 0 0) reflection and a molybdenum X-ray source, distinguishing between these two values is close to our limits of resolution. In Table 2, we thus indicate an uncertainty of ^ 0.05 pm. The charge transfers determined through overall chemical analysis (fc,chem) for the bromide and chloride GICs (Table 2) are somewhat greater than those determined from Eqs 1 and 2. This is often found in GICs and attributed to the fact that all the charge transferred between graphite and the different elements of the intercalate layer is not necessarily delocalised within the graphene layers. The distance dC–C was also determined for the three NaI GICs (Table 1) and we observe that the sample with the orthorhombic cell has the highest value of fc, considerably greater than that for sample NaI-28 with the hexagonal cell and for NaI-52 containing several cells and an admixture of stage 3. As we will see later, these differences seem to play a fundamental role in determining the electronic properties. The close similarities between the third stage NaCl and NaBr GICs was further confirmed using room temperature Raman light scattering. A doublet was observed in both cases, with peaks at 1580.4 and 1606 cm ⫺1 for the chloride GIC and at 1579 and 1605.3 cm ⫺1 for the bromide GIC. The more intense upper and less intense lower components of the doublet are attributed to intercalate bounding and inner graphene layers respectively [28]. The upper frequencies in both cases are slightly lower than what is observed in

0.018

0.053 0.053

0.025 0.016 0.009

0.032

0.047

NaBr-3H NaCl-2H NaCl-3H NaCl-3K NaCl-4H NaI-180 NaI-28 NaI-52

fC (dC–C)

fC,chem

Sample:

r a (4.2 K) (m V cm) 5.5 ^ 0.5

4.5 ^ 0.5 2.4

r a(295 K)(m V cm)

17 ^ 2

18 ^ 2 23 ^ 2 17 ^ 2 9 1.4 1.4 1.2 5.0 1.1 0.7

0.9

r c (295 K) (v cm)

0.9 0.7 4.1

0.6

r c (4.2 K)(V cm)

A (4.2 K) 1.1 × 10 5

1.6 × 10 5 1.7 × 10 6

A (295 K) 5.2 × 10 4 7.8 × 10 4 6.1 × 10 4 7.1 × 10 4 6.2 × 10 5

⫺ 0.23 ⫺ 0.19

⫺ 0.20 ⫺ 0.12 ⫺ 0.37 ⫺ 0.37

d(lnR)/dp (GPa ⫺1)

Table 2 Charge transfer and electrical resistivity data. Second column gives charge transfer determined via chemical analysis assuming full ionisation of sodium and halogen. Third column based on relations 1 and 2, using dC–C ˆ 142.05 pm. A ˆ r c/r a.

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Fig. 1. (a) Basal plane and c axis resistivity versus temperature under ambient pressure for sample NaI-180; (b) Semilogarithmic plot of c axis resistivity of NaI-180 compared to the third stage NaBr-3H and fourth stage NaCl-4H.

the third stage alkali metal GICs MC36, in which full ionisation is generally assumed, leading to fc ˆ 0.028. The Raman frequencies are therefore compatible with the charge transfers determined from the DdC–C measurements. Electrical resistivity measurements were made on several samples as summarised in Table 2. As expected, the charge transfer strongly modifies the electronic transport properties. Pristine graphite is an anisotropic semi-metal with r a(295 K) in the range 40–50 m V cm. Because of the

small band overlap the carrier density changes with temperature, making r a a strongly non-linear function of T. Intercalation increases the number of carriers and significantly decreases r a, as shown in the Table 2 and illustrated in Fig. 1 by the data for sample NaI-180. The observed values of r a (295 K)/ r a (4.2 K) indicate a significant degree of residual scattering. It is possible that much of this arises from local compositional and structural disorder in the intercalate planes. However, the temperature dependent (phonon

Fig. 2. Rc(p)/Rc(p ˆ 1 bar) at 293 K for third stage samples (a) NaBr-3H, (b) NaCl-3H and (c) NaCl-3K.

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Fig. 3. Rc(p)/Rc(p ˆ 1 bar) at 293 K for 2 NaI GICs examined simultaneously within the same pressure cell.

scattering limited) resistivities observed are in the range 8– 25 m V cm at room temperature, comparable to that of many elemental metals. As in several other GIC cases, r a(T) can be well fit by a second order polynomial. The results for the c-axis resistivity are much more variable and difficult to analyse. Natural graphite is an anisotropic semi-metal with r c/r a (⬇100 and both the T and p dependence of r a and r c can be predicted in simple models [25]. For the GICs studied here the anisotropies are of the order of 10 5 (c.f. Table 2) and band theories are no longer valid. The T dependencies of r c for several samples are shown in Fig. 1, and further data are summarised in Table 2. All the chloride and bromide samples are ‘‘metallic’’ at all pressures in the sense that dr c/dT ⬎ 0, although the magnitudes of r c(295 K) are much higher than for typical metals. This feature is also common to high temperature superconductors [29] and has been discussed elsewhere [30]. As illustrated in Fig. 1, the behaviour of NaI-180 is

metallic, but possesses a peak around 250 K, similar to what is observed in a few other ternary donor compounds such as those containing Bi and an alkali metal [30,31]. A detailed explanation has never been put forward, although Sugihara has tentatively suggested that the mechanism responsible might be small polaron hopping across the intercalate layer [32]. The effects of pressure on the c axis resistance at 293 K of the bromide and chloride samples are illustrated in Fig. 2. For NaBr-3H, Rc decreases smoothly between 0.1 GPa and the maximum pressure of 1.6 GPa; the relative variation with pressure is somewhat greater for the two NaCl samples, but almost identical beyond 0.2 GPa for both kish and HOPG based hosts. Fig. 3 shows results for two NaI samples, examined simultaneously within the same pressure cell. The behaviour is very similar to that of Fig. 2. As illustrated in these figures, and indeed as observed for most known GICs, pressure causes the c axis resistance to

Fig. 4. Normalised c axis resistance, Rc(T,p)/Rc(293 K, 1 bar) for sample NaBr-3H. Pressures from top to bottom are 1 bar, 0.55 GPa and 1 GPa.

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Fig. 5. Logarithmic pressure coefficient versus T. Dashed lines give coefficients as calculated from isobaric temperature runs.

decrease. No sharp transitions are observed contrary to the case in some acceptor GICs we have studied in which the structural state of the intercalate is a function of the pressure [33] or in the GICs containing the alkali metals in which pressure application provokes a stage change above a few kilobars [34] For sample NaI-52 which included a very small amount of stage 3, maintaining a pressure of 1 GPa over a period of several days resulted in an Rc decrease of about 1% per day and 0 0 l X-ray analysis following the experiment showed a slightly greater third stage content. We show on Fig. 4 the normalised thermal variation of Rc at ambient pressure from 4.2 to 295 K, and at 0.55 and 1 GPa over a more limited temperature range for NaBr3H. For all the three values of pressure, dlnRc/dT evaluated at 293 K is 1.1 ( ^ 0.1) × 10 ⫺3 K ⫺1. Fig. 5 summarises the results of our studies by presenting the logarithmic pressure derivatives dln(Rc)/dp as a function of temperature for the different samples examined. All values are negative, as is the case for graphite itself, for which dln(Rc)/dp is about ⫺ 27%/GPa at ambient temperature [25] Only the third stage NaCl GIC has an absolute value slightly greater than this. These pressure coefficients are all very large considering that the c axis interplanar distance in graphite and typical GICs change by less than 3% up to 1 GPa: r c is thus extremely sensitive to the effect of pressure on the interplanar distance. Numerous authors have attempted over the past years to unravel the complexities involved in explaining c axis conduction in lamellar solids as a function of temperature, pressure and charge transfer between intercalate and host. While it initially seems fairly straightforward to explain a simple transfer of charge carriers between parallel layers, the large number of theories put forward to date concerning not only intercalated graphite, but also superlattices and high temperature super-

conductors attest to the underlying difficulties. The materials examined here add some new enigmas, notably as concerns the NaI sample with the incommensurate intercalate sublattice. The pressure sensitivity of r C suggests the plausibility of a tunnelling or hopping mechanism where conduction is mediated by the exponential tails of the overlap integrals. A tunnelling mechanism is also suggested by the very weak temperature dependenace for both r C and dr C/dT in these GICs. It should be noted that even larger pressure coefficients were recently found for the phonon-limited c-axis resistivity of single crystal graphite [25]. The reason is the extremely anharmonic lattice properties of graphite in the c direction, resulting in a Gru¨neisen parameter of the order of 10, similar to that of C60 [35]. In the present materials, a phonon origin is ruled out by the absence of a temperature dependence in dr C/dp.

4. Conclusions This work illustrates the complex nature of the interactions linking chemical composition and intercalate layer structure to the elastic and electronic properties of GICs. One of the most unexpected results is the inverse relationship between charge transfer and c axis conductivity in the NaI GICs. In all the sodium halide GICs, r C is remarkably more sensitive to hydrostatic pressure than what the compressibility would lead to expecting and Fig. 4 illustrates that unlike many ordinary metals, the ‘‘residual’’ resistivity is strongly modified by the applying pressure. While it is difficult to explain the microscopic origins of these enigmatic points, there is strong evidence that a tunnelling phenomenon is at least partially responsible.

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