Charge transfer and islands in metal halides-graphite intercalation compounds: New evidence from x-ray diffraction of intercalated Mn Cl2

~

Solid State Communications, Voi.42, No. If, pp.759-762, 1982. Printed in Great Britain.

0038-1098/82/230759-04503.00/0 Pergamon Press Ltd.

CHARGE TRANSFER AND ISLANDS IN METAL HALIDES-GRAPHITE INTERCALATION COMPOUNDS : NEW EVIDENCE FROM X-RAY DIFFRACTION OF INTERCALATED Mn CI 2 F. Baron and S. Flandrois Centre de Recherche Paul Pascal, Universit~ de Bordeaux I, Domaine Universitaire 33405 Talence, France and C. Hauw and J. Gaultier Laboratoire de Cristallographie, Universit~ de Bordeaux I, Domaine Universitaire 33405 Talence, France Received 22 January ]982 by E.F. BERTAUT Single crystal structure studies at room temperature have been made for the first stage Mn CI 2 intercalated graphite. The nominal composition was Cs. 6 Mn C12.~, as deduced from chemical analyses and X-ray diffraction intensities. The data are in agreement with the island model, already proposed for Ni CI 2 intercalation. From the decrease in C-C bond length and comparison with As F 5 compound, it is shown that the charge transfer is determined by chlorine in excess with respect to free metal halide.

1. Introduction

charge transfer from the amount of excess chlorine.

Intercalation of metal chlorides into graphite is not well understood as yet. One of the main problems is the role of chlorine gas needed generally for the reaction to occur. Curious stoichiometries are obtained with an excess of chlorine with respect to free metal chlorides. A related problem is the nature of bonding between graphite and metal halide. A charge transfer is assumed from graphite to metal halide and the extent of this transfer should be connected with the excess of chlorine. A few years ago, we proposed I that intercalated layers are not complete but made of small islands of metal chloride. The presence of islands gives an explanation for the non-integer stoichiometric coefficients from the excess chlorine at the periphery of the islands. This model was presented (for the non-French-speaking communauty) at the Princetown Conference in May 1980 and published in the Proceedings 2 . Nevertheless it was recently published again by another author as a new idea s . In our study of NiCI 2 intercalation 2 we showed for the first time that the in-plane C-C distance is shorter than for pristine graphite, contrary to donor compounds for which it is well known that an expansion of the C-C bond is observed. Assuming that the excess chlorine accounts for the charge transfer, a linear relationship was obtained between the change in C-C bond length and the charge transfer, similar for donor and acceptor compounds. However only one acceptor (NiCI 2) was investigated. To confirm these results it is necessary to extend the measurements to other metal halide compounds. For this purpose well-defined products, in regard to analysis and crystallinity must be prepared. In this paper we present our results about the intercalation of manganese chloride. The crystal structure of the first stage compound has been determined. We will show that the data are in agreement with the assumption of Mn CI 2 islands and the evaluation of the

2. Preparation The intercalation of Mn CI 2 into graphite was first reported by Stumpp and Werner in 1966 ~. They showed that a first stage compound is prepared by heating during several days at about 400°C in sealed tube a mixture of graphite and Mn CI 2 in a chlorine atmosphere. They obtained a ratio metal atoms/C atoms of I : 6.6. We adopted this procedure, but instead of sealed tubes we used a quartz tube connected with a manometer system allowing to monitor the chlorine pressure. Anhydrous manganese chloride from Merck and Madagascar natural graphite (I001000 ~m powder) were used. After reaction the products were washed with diluted HCI to remove the excess metal chloride. For analysis, the samples were oxidized in a Parr bomb, then chlorine was determined by titration. The amount of manganese was obtained by gravimetry from the weight of Mn304 after burning the samples in air. After a few preliminary tests, the reaction temperature was chosen around 500°C. Several samples were prepared with different reaction times : from a few days to two months. In these conditions, first stage compounds were obtained. The average composition deduced from chemical analyses was : C~.s± 0.i Mm CI 2.3± 0.I" A first remark concerns the content in manganese chloride which is 20% higher than in the compounds previously obtained. As usually observed in the intercalation of metal chlorides, there is an excess of chlorine with respect to free manganese chloride. Its value (0.3 ± 0.1) is in good agreement with the lowering of chlorine pressure observed during the intercalation reaction. 3.Crystal Structure of Mn CI 2- Graphite Compounds The structure study of the first stage Mm C12 - intercalated graphite was made at room 759

760

X-RAY DIFFRACTION OF INTERCALATED Mn CI 2

temperature by means of standard single crystal X-ray diffraction techniques. The crystals were chosen among graphite flakes which were almost regular hexagons with a diameter of about 0.5 mm and a thickness of about 0.05 mm. In addition to Weissenberg photographs, quantitative intensity measurements of diffraction peaks and diffusion lines were made on a four-circle automatic Siemens diffractometer, by means of 0 - 2e scans with Cu Ks radiation. The data corresponded to a first stage compound with a period of 9.484 (5) A along the direction perpendicular to the layers. For some crystals, Weissenberg photographs showed in, addition a few weak diffraction spots along C axis corresponding to small domains of second stage material with a period of 12.76 (I) A. A typical photograph is shown in figure I. No (001)- reflection from pristine graphite lattice was discernable in all spectra.

Vol. 42, No. 1.1

~ ~-.124

Figure 2 : Most probable relative orientations of manganese (left) and carbon (right) lattices (projection along C-axis). the diffuse lines due to the Mn lattice, the in-plane coherence length of the manganese chloride appears to have the same order of magnitude. Information concerning the structure of the intercalant layer can be obtained from analysis of intensities of the (OOl)-reflections. Eleven reflections have been considered, among which nine have a non-zero intensity. It is likely to assume that the Mn CI 2 layer has a structure similar to that of pure manganese chloride : each manganese is octahedrally coordinated by chlorine with the octahedral threefold axis along C. Thus the following sequence of layers can be assumed : carbon, chlorine, manganese, chlorine and carbon. Then the diffraction amplitude is : FO01= IfMn+++Pfcl- c o s 2 ~ i z + Q f c c o s ~ l ] e x p ~ )

Figure I : Weissenherg photograph of Mn C12 graphite intercalation compound (zero level).

Reflections other than 001 can be attributed to two independent lattices : carbon and metal halide lattices. The a-p~rameter of the carbon lattice is 2.4522 (3) A, which leads to an in-plane C-C distance of 1.4158 (2) ~, shorter than for pristine graphite 2 (1.4209 (4)o~) and NiCI 2 -graphite compounds 2 (1.4200 (I) A). The a-parameter of the metal halide lattice is 3.694 (2) ~, slightly larger than in free mangenese chloride (3.675 A). As in Ni CI 2 - graphite compounds, a high degree of disorder, reflected on spectra by diffuse streaks, existsbetween successive intercalated layers, and, may be, inside the same intercalated layer. This disorder prevents us from knowing the position of the Mn atoms with respect to the carbon lattice. However the preferential relative orientations of both lattices may be deduced from the reflections corresponding to 1.847 ~ and 2.124 ~ due to metal and carbon lattices respectively (fig. 2). The aaxes of both lattices make an angle of 30 ° . This disorder does not seem to affect the carbon layers. However the coherence length along the C-axis is much smaller than in pristine graphite. From the thickness of the reflections alongothe C*-axis, it can be estimated to about 130 A. Similarly, from the thickness of

where fx is the diffusion factor of X species, z the distance (fraction of unit cell) between manganese and chlorine layers, B the average thermal coefficient, e the Bragg angle for the (OOl)-reflection, % the wavelength, P and Q are the concentrations of chlorine and carbon, resp., for one manganese atom. A mean B-value has been chosen because of the small number of observed reflections. In fact this value is of little importance, because B has only a damping effect on the diffraction amplitudes. Several tests have shown that the most probable value is B = 3.5 ~2. Every other B value modified the reliability index without changing P, Q and z values. The results are : P = 2.46 ± 0.04 Q = 5.63 ±0.05 z = 0.149 ±0.001

3,33 .............

CI

1.41 A .............

Mn Cl

Figure 3 : Schematic side-view of the layers.

Vol. 42, No.

X-RAY DIFFRACTION OF INTERCALATED Mn C12

II

The comparison of observed and calculated structure factors is given in table I. The corresponding reliability index is : R ~

IIFoBsl - IFcalcll

= 0.06

Z IFobs I Table l - Observed and calculated structure factors Reflection

IFobsl

001 002 003 004 005 006 007 0 08 009 0010 001l

IFcalc I

12.71 32.67 31.60 12.28 0.00 41.57 14.28 21.35 3.02 0.00 5.08

15.83 37.47 - 33.03 12.37 0.23 34.47 14.56 17.77 - 4.29 0.93 - 3.59

The composition deduced from X-ray diffraction is then : C5.63 ± 0.05 Mn C12.46 ± 0.04 in good agreement with chemical analyses. The z value gives for the distance between manganese and chlorine layers a value of 1.41 (1) A (fig. 3).

76]

te are never seen on all spectra. Similarly preparation problems cannot be invoked ; the composition obtained is the limiting composition : after a treatment of two months the analyses gave the same results as after a few days. Thus the Mm C12 layers are far from being complete, the filling coefficient being equal to 0.80. This situation is not exceptional but seems to be the rule, as shown in table 2. The filling coefficient of the metal halide layers is given for the metal halide intercalation compounds whose in-plane a-parametershave been determined. For all compounds the intercalate layers do not fill completely the available surface area. In the case of Ni C12 this result has been interpreted I by the formation of small intercalant islands. X-ray diffraction gave for the inplane coherence length of manganese chloride a value of about 130 A. It may be thus assumed that the Mn CI 2 layers are formed ~f islands having in average a diameter of 130 A. Due to the six-fold coordination of manganese atoms, such small island have an excess of chlorine at the periphery. For circular 130 ~ islands the composition can be calculated as Mn Cl2.l.The discrepancy with the value observed (Mn C12.~) could be explained either by Mn vacancies inside the islands, a island shape different from circular, or more likely, some distribution of island diameters. Stoichiometry, Carbon-Carbon Bond Length and Charge Transfer

4. Discussion Stoichiometry and Island Formation The average composition of the best intercalated samples, obtained from chemical analyses and X-ray diffraction intensities, can be given as : Cs. 6 Mn C12. 4. From the a-parameters measured for Mn and C lattices, we can calculate the surface area occupied in the plane of the layers by one Mn atom and one C atom, respectively. We a2 find : SMn = I].817 ~2 and SC = 2.604 A . Thelr ratio SMn/S C = 4.5 gives the theoretical composition of the first stage compound : C~. 5 Mn CI 2 . This difference between observed and theoretical composition is not due to the presence of free graphite : (O01)-reflections of pristine graphi-

Excess chlorine atoms on the island periphery must be the acceptor sites for an eventual charge transfer from graphite to manganese chloride. Magnetic susceptibility measurements 8 gave an effective magnetic moment ~eff = 5.85 ~B, close to the value expected for M n ~ ions (5.92~B). Thus, the composition Cs. 6 Mn C12. ~ implies that 0.4 electronic charge is transferred from 5.6 carbon atoms, i.e. a charge transfer of 0.07 electron per carbon atom. To our knowledge this is the largest value observed for an acceptor compound. As a result, there is a significant decrease of in-plane carbon-carbon bond length : - 0.0051 (3) ~. For donor compounds, we have shown 2 that there is a linear increase of C-C bond lengths with the stoichiometric coefficient of donor metal, i.e. with the content in donor metal which must be roughly proportional to the

Table 2 - Filling coefficient of metal halide layers calculated from observed and theoretical composition

Halide

Observed composition

Theoretical composition

Filling coefficient

Ref. structure

Ni Cl 2

C11.3

C8. 0

0.71

2

Cr Cl 3

C22

CI~.6

0.80

5

Mo CI s

C69

C41.4

0.60

6

Fe CI 3

C6. 9

C6. o

0.60

7

Mn CI 2

Cs. 6

C~. 5

0.80

this work

762

X-RAY DIFFRACTION OF INTERCALATED Mn CI 2

Vol. 42, No. I~

pristine graphite i

1,4200

/~s FS 1,415C

0

MnC2

I

I

0,05

0,1

p

Figure 4 : In-plane C-C bond length of acceptor compounds as a function of the charge transfer P per carbon atom. charge transfer. The acceptor compounds exhibit also a roughly linear decrease of C-C bond length with the charge transfer, as shown in fig. 4. The measured bond lengths are plotted against the charge transfer per carbon atom. For Ni CI 2 and Mn CI 2 the charge transfer is calculated from the excess chlorine. The data for As F s correspond to the second stage compound 9 C16As Fs, whose charge transfer has been determined by several methods : spin susceptibility I° , chemical estimates 11 , reflectivity Iz, magneto-oscillations Is or De Haas-Van Alphen effect 14 . Within

the accuracy, a linear dependence is observed (fig. 4), including As F s compound. This means that the excess chlorine is a measure of the charge transfer. A theory was recently formulated Is to explain the observed change in bond length in terms of charge transfer. A formula was derived allowing to calculate the charge transfer from the variation in bond length. Applying to Mn CI z gives a value of 0.04 electron per carbon atom, in semi-quantitative agreement with experiment.

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