Synthesis, Structure and Physical Properties of Arsenic-Heavy Alkali-Metal Alloys Based

Synthesis, structure and physical properties of arsenic-heavy alkali-metal alloys based graphite intercalation compounds.

(1) Jamal Assouika,* (2) Claire Héroldb and (3) Philippe Lagrangeb

aLaboratory of Applied Chemistry (LCA), Faculty of Science and Techniques, Sidi Mohamed Ben Abdellah University, B.P. 2202, Route d’Imouzzer, Fez 30050 Morocco.

*Corresponding author. E-mail address: . Fax 212 5 35 60 82 14

bLaboratoire de Chimie du Solide Minéral (UMR CNRS UHP 7555). Université Henri Poincaré Nancy I. B.P. 239- 54506 Vandoeuvre-lès-Nancy cedex - France.

1. Introduction

Graphite consists of hexagonal carbon planes stacked along c-axis by weak Van der waals forces. Therefore, many substances, what is called intercalates, can enter into the gallery of the graphite to form graphite intercalation compounds (GICs).

This work is part of the framework of ternary donor-graphite intercalation compounds (GICs). It is about reaction between liquid mixture composed of a heavy-alkali metal alloyed with a second weakly electronegative element (Hg, Tl, Bi, Sb, S, As …) and a bulk HOPG graphite sample. (High Oriented Pyrolytic Graphite). Some of these compounds include potassium-mercury-graphite [1,2,3,4], potassium-thallium-graphite [5,6,7,8], alkali metal-bismuth-graphite [9,10,11], cesium-antimony-graphite [12,13,17], and alkali metal-arsenic-graphite previously published [14,15,16,17,18]. These compounds are, essentially characterized by the highest value of their inter-planar distance which is the distance between two adjacent graphitic planes into sample. It can reach up to 1200 pm in the case of K-Tl GICs family. Their and common chemical formula can be written as KMxC4s (0,5  x  1,5 ; s = stage and M = Hg, Tl, Bi, Sn, As…). We have called this new compounds family resulting from the action of M-As (M = K, Rb, Cs) mixture alloys on graphite the arsenio-graphitides of heavy alkali-metal along with mrecuro-, thallo-, bismutho- or antimono-graphitides.This donor-graphite intercalation compounds family also including alkalimetal-hydrides intercalated graphite [19,20] alkali-metal sulfides intercalated graphite [21,22] and a new recently discovered ternary GICs resulting from the reaction with graphite of Li alloyed with alkaline-earth or with rare earth elements [23,24,25] or simply from action of heavy alkali-metal-gold alloys on graphite samples [26,27]. The main feature of these ternary GICs is their high content of metallic alloy located between graphitic gaps. This one is usually constituted of metallic sheets which are disposed in the form of successive layers perpendicularly to graphite c-axis. For two-dimensional structure, they present various structural networks going from those which are perfectly commensurate with graphitic unit cell,such as in the case of KHgC4 and KHgC8 compounds, and those with a very complex unit cells encountered with thallium [28,29], bismuth [31,32,33] and antimony -heavy alkali metal systems [30, 31].

In this paper, will be described the experimental method used to synthesis these ternary compounds when amixture of a heavy alkali-metal alloyed with arsenic element both of high purity is made under vacuum next to a platelet of HOPG graphite and heated to molten temperature.

In order to characterize the synthesized compounds, a more detailed investigation of intercalated alloy structure between graphitic layers into various isolated phases has been systematically carried out. This atomic arrangement investigation was performed out as well along the graphite c-axis than as along the in-plane graphitic planes. This task was achievedthinks to a set of appropriate modern analytical instruments.

In the last paragraph of this paper, will be described the physical behavior of these ternary phases from measurements of electrical resistivity, dynamical magnetic susceptibility and Ramanspectroscopy carried out on samples purely isolated in laboratory. These measurements have been performed in our laboratory and in association with exterior laboratories. Note that similar previous works carried out on homologous donor species-graphite systems about this reported that the electrical resistivity carried out on these ternary compounds showed a marked in-plane metallic behavior and a marked electrical anisotropy [34,35] and some ones of them become superconducting at low temperature..

2. Experiment

Compounds and phases are synthesized by mixing, into a glove box under a purely argon atmosphere, of an amount of M-As (M = K, Rb, Cs) alloy whose chemical composition is well defined and which was, in turn, previously prepared under vacuum, in contact with a platelets of HOPG sample. The reaction mixture is then heated in sealed glass tube to the corresponding alloy melting temperature. After a duration time of reaction which is often selected, for a given binary-alloy system, according to the atomic ratioof reactant alloy and its reaction temperature. After the reaction has ended, the sample which becomes immersed into an excess the liquid alloy is cooled and then taken out of the oven. The intercalation reaction is clearly expressed by a spectacular change of the sample graphite color. The sample is then recovered after being cleaned by removing of the excessive reagent alloy from the sample surfaces. Once recovered, the sample is introduced into a Lindemancapillary tube under argon atmosphere and characterized by using X-ray diffractometerapparatus by placing suitably the sample according to the molybdenum monochromatic X-ray beamsuch a manner to select only the (00l) Bragg reflexions family. A overview of the recorded X-ray pattern allows to determine the nature of the phase(s) present in the sample, their repeat distance (distance between two successive intercalated layers) and stage number (defined as the number of carbon layers comprising between two successive intercalated sheets) of the intercalation compound. C-axissample dilatation is also measured during its recuperation from the glass tube into the glove box.Stage value, sample dilatation and repeat distance of a GIC are linked by the following equation:

Where: e = sample thickness, Ic = repeat distance, e/e= relative c-axis dilatation of sample.

A schematic representation of a graphite intercalation compound (GIC) is illustrated in figure 1 below.

Figure .1

In table .1 are listed the overall of the arsenio-graphitides phases observed and isolated during this work in the case of the three heavy alkali metal systems andare given the corresponding operating conditions allowing to their preparation. Chemical formula which was provided by the elemental chemical analysisis placed into the last column of table. Figures 2,.3 and .4 representthe As-K binary phase’s diagram, X-ray 00l reflexion patterns of purely isolated phases and the corresponding EDS spectra provided by Scanning Electron microscopy analysis respectively.

Table1- Ternary phases of arsenic - heavy alkali metal - graphite intercalation compounds.

Fjgure .2

Figure .3

Figure .4

3. Intercalation process.

Analysis of overall results obtained throughout this work, some general aspects relating to reaction mechanisms during intercalation must be pointed out:

The intercalation reaction takes place very quickly and the resulting reaction rate is as so high as the alkali-metal is less electronegative (KRbCs).

When the reactant alloy is very rich in alkali-metal, despite the high-temperature reaction, the intercalation mechanism leading to the formation of the richest ternary phases (stage 1) is going through the formation of binary graphite-alkali-metal phases followed by intercalation of arsenic element. On the other hand, when the reactant alloy is very rich in arsenic, the rate formation of ternary phases becomes so fast that it is not possible to observe the first moments of intercalation process. Therefore, one can say nothing about the corresponding reaction mechanisms. In all cases, chemical processes taking place during intercalation show a series of steps which set up through chemical and structural transformations that affect intercalated metallic alloy.

However, and as it was developed by P. Lagrange in a previous work [36], there is an alkali metal concentration limit, below which the alloy, very poor in alkali metal, behaves with graphite like the pure second element (naturally unable to intercalate itself into graphite) and there is no reaction at all. Beyond this critical concentration, the alloy reacts systematically with graphite leading to a ternary compound.

On figure 5 below, we compare these limited regions between arsenic and itshomologous elements on periodic table antimony and bismuth when they are, every single one of them, associated with one of the three heavy alkali metals (K, Rb or Cs). For a given second element (As, Sb or Bi), we notice that the more electropositive the alkali metal is, the more shifted to pure second element the range is. And for a given alkali metal, the more electronegative the second element is the more shifted to pure alkali metal the range is.Note antimony (Sb) does not react with graphite when it is alloyed with Rb or cesium alkali metals. (See figure 5).

In fact, the more or less electronegative nature of the second element controls the nature of the heavy alkali metal which can be associated leading to the formation of a ternary compound. This behavior seems due to reasons of electronic equilibrium inside the ternary compound.

Figure 5

It’s also worth notingthat a more or less important de-intercalation phenomenon marks systematically the end of reaction, in particular when the reaction time being morelonger. Nevertheless, independently of heavy-alkali metal alloying with arsenic (K, Rb or Cs), an exception to this phenomenon was observed when the alloy composition rate is ranging around 37%.As.at corresponding to a K-As eutectic point. This is undoubtedly due to the low melting temperature value in this case and consequently the resulting temperature reaction. So, it would be preferable to synthesize samples corresponding to high inter-planar distance (thick intercalate sheet) and low stage (1, 2) in this chemical alloying range. Retrieved samples after reaction are often in good condition with less damage and presenting a remarkable metallic sheen.

4. Chemical stability.

To assess the chemical stability of the graphite-alkali-metal arsenic ternary compounds, they have been subjected to air or liquid water and we have followed their chemical evolution state over time by controllingperiodically the evolution of their 00l reflexions pattern by using X-ray diffraction measurements.

The first and second stage phases having high inter-planar distance (2-Cs-As, 2-Rb-As and 2-K-As phases) are generally very unstable when they are exposed to air or immersed in water. Indeed, they undergo a progressive degradation starting by loss of their color followed by sample surfaces oxidation operation which is therefore spreading progressively inside of the sample. After a few days exposure in air, the sample suffers an important exfoliation. The second stage, in contrary, like 2nd stage Rb -As and Cs-As β-typephases and 2α-type of K-As phase, are more much stable in air and water since they remainunalteredafter several weeks even several months when they are exposed to these two natural oxidizing agents. Figure .6 illustrates the remarkable chemical stability of these second stage arsenio-graphitides of heavy alkali metals.

Figure .6

5. Structural analysis.

Arsenio-graphitides of heavy alkali-metals were structurally studied to identify the structural arrangement of the intercalate alloy between graphitic layers. Note that the structure graphite remains unchanged during intercalation process. Alkali and arsenic atoms intercalate into pristine graphite in such a way that a novel equilibrium of the system is established. Various techniques were used to conduct this study. It has two parts: one describing the analytical method for determining the nature of intercalatesstacking along c-axis over the different phases and in the second part we’ll describe the in-plane structure of intercalated phases and the appropriate technical means used for this task.

5.1. Along c-axis.

In order to determine the structural arrangement of intercalated atomic alloyingof these phases along c-axis direction, a virtual modeling based ona proposed atomic stacking sequence of graphitic plans and intercalated metallic sheets throughout crystallographic c-axis. This first model is computing owing to a software program available in Laboratory. In first step, we calculatethe theoretical structure factors F00lcal associated to the proposed structural modeland afterwards is calculated the corresponding electron density profile by applying the Fourier transform function to calculated structure factors. This calculated density profile is, in turn, compared to the experimental one which is deduced from the Fourier transform function of the observed structure factors F00lobs deduced from the experimental intensities according to the equation (2) below. For this, some fundamental calculations are developed below.

The structure factor is expressed as:

.cos(2l (1)

Where fr is the diffusion coefficient of r atom located at zr.

The 00l reflexions intensityis calculated according to the formula:

= M.K..(2)

WhereF00l is the structure factor of the 00l reflexion, M is 00l reflexion multiplicity, K is athermal agitating coefficient and A is the absorption coefficient supposed constant, Lp is the Lorentz-polarization factor.

The calculated electronic density profile of the proposed model along c-axis is calculated from the Fourier transform functionof the calculated structure factors F00l. Its expression is given by the equation:

(z) = F000 + 2.

Where F000designates the structure factor value corresponding to the non-diffracted X-ray beam (Mo = 0), Ic the repeat distance of the phase (distance between two successive intercalatedgraphitic intervals) and l the Miller index of 00l reflection. The comparison between experimental and calculated electronic density profiles allows to judging the validity of the proposed model. This agreement can be assessed by means of a residual factor defined as following:

R =

WhereF00lexpand F00lecalrepresent experimental and calculated structure factors respectively.

The difference between the proposed model and the real structure of intercalated sheets can be reduced by adjusting suitably the coordinates and chemical composition of each one of the stacking layers along c-axis. Generally, the agreement between calculated and experimental data is considered so satisfied when the residual factor R becomes less than 15 %. Thus, a satisfactory c-axis sequenced layers model involves minimizing of the residual factor value. By this way, the compound’s chemical formula deduced from proposed modeling could be compared to that provided by elemental chemical analysis.

A various c-axis atomic arrangement modes have been observed over the different arsenio-graphitides phases examined. However, it should be pointed out that, in general, the three-layered sheet model represents frequently, with a good approximation, the way in which are stacked the intercalated metallic sheets through c-axis. This is particularly the case of -type rubidium and cesium compounds as well as it is the case of the -type potassium phase. Further, a slight modification of this tri-layer standard structure allowsthe setting upof too more complex intercalated sheets arrangements. This is the case of  and -type potassium-arsenic phases in which two alkali-arsenic mixed layers replace the simply alkali layers in contact to carbon layers and surrounding the core arsenic layer in this different architecture. One phase was found to be penta-layered; it is the -type cesium compound of stages 1 and 2. This structure correlates well with its high inter-planar distance along c-axis which is reaching a value of 1120 pm.

Figures going from 7 to 10 and tables from 2 to 5 represent the electron density profiles andcompared observed and calculated 00l intensities respectively.

Figure .7

Table 2 – Calculated and observed 00l reflexions intensities of -As-K-G of stage 1. (Trilayer model. Residual factor R = 3,21 %)

Figure .8

Table 3 – Calculated and observed 00l reflexions intensities of -RbAs0,6C4 of stage 1 (Trilayer model, residual factor R = 3.8 %).

Figure .9

Table 4 – Calculated and observed 00l reflexions intensities of -CsAsC8 of stage 1.

(Pentalayer model).

Figure .10

Table 5 – Calculated and observed 00l reflexions intensities of -CsAsC8 of stage 2.

(Pentalayer model).

5.2. In-plane structure.

After describing the c-axis atomic stacking of metallic intercalated sheets in the most isolated phases, we attempted to determining how are, in 2D-structure, organized theseintercalated metallic sheets located between graphitic layers into these phases. For this, many technical tools have been employed for this purpose: (-2)-MoKα1X-Ray diffractometer, Monochromatic Laue assembly using a synchrotron-X-Ray source based in L.U.R.E. (Laboratoire de l’utilisation du rayonnement électromagnétique of Orsay-Paris), Precession camera equipped with a MoKa1 X-Ray source from the Laboratory of Mineralogy of Nancy I University and electronic diffraction Microscopy TEM (LCSM-Nancy I).

The combination of this range of equipmentsallowed us to perform a detailed study of the in-plane atomic organization of these phases. In these different assemblies, the (hk0) diffracted Bragg reflexions are selected by correct positioning of sample with respect to direct X-ray beam during diffraction. Figures from 11 to 17 display the equatorial stratum image of poly- and mono-crystal samples according to the corresponding diffraction device. In tables 6, 7 and 8 are given the hk0 indices of three examples of unit cells encountered with these phases.

Looking at all of these crystal structures data, some common remarks can be issued:

- Existence of various in-plane atomic organizations of the intercalated alloy according to the nature of the examined phase. Thus, the α-type phases (and 1β-K-As phase) whose chemical formula could be written asMAsxC4s (M = K, Rb and Cs, s = stage), and which are characterized by the smallest inter-planar distances, and which are often formed after a short duration time reaction, arequasi-randomly in-plane ordered. As is shown on figures 11 and 12, the largely stretched hk0 spots of the X-ray synchrotron Laue image (figure 11) and the complex aspect of 00l X-ray diffractogram of the first stage -K-As phase (figure 12a) indicate that intercalate is not well crystallized into these phases resulting in a poorly ordered framework type structure.