N Rey

Pierre Toulemonde

D Machon

L Duclaux

S Le Floch

-M Flank

P Lagarde

W A Chrichton

V Pischedda

J P Itié

A.-M Flank

M Mezouar

Th Strässle

D Sheptyakov

G Montagnac

A San-Miguel
email: alfonso.san.miguel@lpmcn.univ-lyon1.fr

M. S. Dresselhaus G Dresselhaus

T E Weller

M Ellerby

S S Saxena

R P Smith

Nature Physics N T Skipper

N Emery

M. d C Herold

' Astuto

V Garcia

C Bellin

High pressure behavior of CsC 8 graphite intercalation compound

Keywords:

High pressure behavior of CsC8 graphite intercalation compound

I. INTRODUCTION

Graphite intercalation compounds (GICs), well-known basic two-dimensional systems, have been extensively studied as template layered materials, allowing the intercalation of numerous different chemical species (≫ 100) 1 . A recent renew of interest on GICs has been motivated by the discovery of the highest superconducting critical temperature T c of this family of materials in CaC 6 (11.5 K) and YbC 6 (6.5 K) 2,3 leading to subsequent experimental 4,5,6,7,8,9 and theoretical 10,11,12,13,14,15 efforts.

Pressure is a crucial thermodynamic parameter in the understanding of electronic and structural properties of these low dimensional guest-host systems. Pressure effects include the enhancement of the host-guest interactions. In addition, under high pressure (HP) and high temperature (HT), GICs can constitute precursors for the formation of new carbon intercalated materials such as carbon clathrates 16 , which could show promising superconductivity 17 and mechanical 18 properties.

In GICs, several structures associated with the layered topology like staging, in-plane decoration or stacking are observed. One of the most remarkable structural features is the staging phenomenon occurring at longrange order 1 . Staging refers to the one-dimensional periodic alternation of filled and empty graphitic galleries. Stage-n is defined as the structure in which intercalates are accommodated regularly in every n-th graphite layers. Thus for stage-1, all the graphitic galleries are periodically occupied by the intercalate layers. The repeat distance I c is defined as the distance along the c-axis between two successive intercalated layers. Important structural features are distinguished according to the value of the stage index-n. Two different decorations of intercalated layers commensurate to the graphite lattice for the stage-1 compounds have been observed: an hexagonal 2×2 superlattice (Rb, Cs and K) and an hexagonal √ 3 × √ 3 superlattice (Li, Ca, Yb and others). In higher stage compounds, the metal layer arrangement can lead to an incommensurate modulation with respect to the graphite layers or can be even similar to a liquid -like behavior 19,20 .

Among the donor compounds, the family of K-GICs has been used as model for the description of pressureinduced transitions. The data from different studies were limited to relatively low values of pressure (<3 GPa). In the stage-1 KC 8 , staging transitions occurred with the presence of a stage-2 and a fractional stage-3/2 in the 0-3.0 GPa range 21 . Bloch et al observed a √ 3 × √ 3 superlattice in the same pressure range 22 . These structural changes in the decoration and in the staging are accompanied with an anomaly in the resistivity at 1.5 GPa 23 . No staging transitions were found in CsC 8 and RbC 8 up to 1 GPa 24 although there were observed pressure-induced anomalies in the resistivity of these two compounds at 2.0 GPa. In the stage-1 CaC 6 , pressure induces the enhancement of superconductivity and near 8 GPa a structural instability was found when the critical temperature T c suddenly drops 25 . Similar behavior of T c in YbC 6 is observed at 1.8 GPa 26 .

For "diluted" intercalation, i.e. n ≥2, staging transitions n → n+1 or n+2 were observed for moderate pres-sures 0.5-1.0 GPa. First staging transition from a stage-2 to stage-3 was evidenced in KC 24 by Fuerst et al 27 . This work has been extended to higher stage K-GIC and to high-stage Rb and Cs GICs by Wada 24 .

CsC 8 is a good candidate to investigate the size effect on the guest-host interaction with pressure and to give new insight into the high-pressure properties and stability of this important class of materials. The total lack of information for P>2 GPa prompted us to examine this compound at higher pressure. We have hereby explored the high-pressure evolution up to 32 GPa of CsC 8 . Information on the Cs sub-network has been obtained using X-ray diffraction thanks to the high Z of the guest species. On the other side, neutron diffraction appears as more sensitive to the carbon sub-network. Finally, local probes that constitute Raman and X-ray absorption spectroscopies are an excellent complement of the longrange order technique that is diffraction. This multitechnique approach is a key point for the understanding of the high pressure behavior of this highly reactive and complex ordered material. In particular, we have investigated the pressure-induced effects such as charge transfer between the guest and host species, the structural modifications in the Cs and the graphitic sub-networks.

II. EXPERIMENTAL ENVIRONMENT

A. Synthesis route and structural characterization

CsC 8 powder samples were synthesized from a high purity stoichiometric mixture of powder graphite (Aldrich, <20 µm) and Cs metal melt (Alfa Aesar, 99.98%) in an argon filled glove-box 28 . A 48h static vacuum annealing of the sample sealed in an ampoule allowed to achieve the intercalation process. Brown/reddish typical color of the stage-1 heavy alkali GICs was observed after the treatment. Figure 1 shows the neutron powder diffraction pattern of our CsC 8 powder measured at ambient conditions together with the residual of the Rietveld refinement using the Fullprof software 29 . Our structural model includes an anisotropic size broadening effect. A better quality refinement was found when the Cs atoms are in the 3d Wyckoff position (χ 2 =2.73) rather than in the 3b position (χ 2 =3.74) as Guérard et al suggested 30 . A 3b position would imply a ααα stacking sequence for the Cs atoms whereas the stacking sequence for this compound is supposed to be αβγ which can be reproduced with the 3d setting. The structural parameters obtained from the refined model are presented in Table I.

B. High-pressure diffraction

Experimental procedure

We have collected both high pressure X-ray and neutron powder diffraction data up to 14 GPa and 8.9 GPa respectively. Angle-dispersive X-ray diffraction (XRD) at the ID27 beamlime of the European Synchrotron Radiation Facility (ESRF) was performed using a monochromatic radiation (λ=0.26472 Å) and a diamond anvil cell (DAC). The powder sample was loaded inside a 150 µm hole of a steel gasket with heavy mineral oil as pressure transmitting medium (PTM) and placed between the anvils of the DAC with diamond culets of 600 µm.

Due to the high reactivity of the sample in the presence of oxygen and humidity, all loadings of the DAC were made inside an argon filled glove-box. The heavy mineral oil PTM ensures a further protection of the sample. A ccd detector was used to collect the high-pressure diffraction images. Calibration of the sample-detector distance and the tilt angle were done with a Si-diamond powder standard. The two-dimensional image files were azimutally integrated with the fit2D software yielding to one-dimensional intensity versus 2θ 31 . The pressure was determined with the ruby fluorescence method 32 . In situ angle-dispersive neutron powder diffraction (NPD) experiments were carried out at the HRPT diffractometer 33 located at the Swiss spalliation source (SINQ) using a VX5 Paris-Edinburgh Press 34 in a radial configuration. In this geometry, the incident beam (monochromatic radiation of λ=1.494 Å) and the diffracted beam pass through the TiZr gasket in the equatorial plane of the anvils. Neutron absorbing cubic boron-nitride (BN) anvils and null-scattering TiZr alloy gaskets were used in order to omit any signal from the pressure cell. Full details of the experimental setup are described in Ref. 35.

The assembly of the gasket and the loading of the press were completed inside an helium glove-box. Dried NaCl was used as pressure marker.

X-ray diffraction results

High pressure XRD of CsC 8 was performed up to 14.0 GPa. A sequence of selected diffraction patterns is displayed in Figure 2. The first pattern recorded at 0.2 GPa matches the hexagonal ambient structure. No traces of oxide or hydroxide have been found. Inhomogeneous grain size distribution, preferential orientation effects and the identified stacking faults render a Rietveld refinement or even a Le Bail refinement extremely complex. The very low intensity observed for the (003) and the (009) peaks makes difficult to detect their positions. Only the (006) line provides us a direct reliable information about the c-axis compressibility. Gradual transformations under increasing pressure is the most eye-catching feature as well as the overall decrease of intensity of the diffraction patterns. At 1.2 GPa, we observe the coexistence of the hexagonal phase with a new phase. This new phase is clearly established at 2.0 GPa. At 3.5 GPa, new changes in the diffraction pattern evidence a new phase transformation. Furthermore, we notice that the number of peaks of this last high pressure phase is lower than in the preceding phases, which could be associated to a higher cell symmetry or to a smaller lattice. As the pressure increases from 7.3 GPa to 14 GPa, the diffraction line broadening becomes more and more pronounced, and the profile becomes highly asymmetric.

Let us turn to the compressibility of the starting hexagonal phase at low pressures. Figure 3 shows the lattice parameters evolution with pressure as obtained from our XRD experiment. The compressibility along the c-axis was found to be slightly lower than the one measured by Wada 24 . The difference must be moderated by considering the error bars and the use of different starting graphite samples for the two experiments. Wada used singe crystal graphite while we used powder graphite. We deduced a linear compressibility κ c of 1.079×10 -2 GPa -1 corresponding to a linear bulk modulus B c of 93 GPa III).

(see table IV). The a lattice parameter was found to be very rigid in this pressure range. We estimated the bulk modulus B v =94 GPa, close to the linear inter-plane bulk modulus.

Different attempts to determine the crystal structures of the high pressure phases appearing above 1.2 GPa and above 3.5 GPa were carried out. The presence of a diffraction line at very low angle ∼2 • corresponding to a d-spacing of 7.69 Å prompted us to look for a larger crystalline cell assuming a new stacking. We propose the space group C222 as a possible candidate for the first HP phase. This choice corresponds to a space group search procedure with the Crysfyre Suite 36 and Checkcell 37 softwares. Among the possible space groups obtained, C222 reproduced best the experimental data. The corresponding orthorhombic lattice is described in term of a supercell with parameters fitted within a Le Bail refinement (Table II). The second HP phase, observed between 3.5 and 8.0 GPa was found to be very close to the orthorhombic cell defined in RbC metastable phase of Cs GICs. We used the same space group Fddd and Wyckoff positions as RbC 8 for the refinement model. This supposes a change of the stacking from αβγ to αβγδ with a corresponding I c =c/4=5.72 Å (a=4.39 Å b=9.20 Å and c=22.87 Å) (Table II). Figure 4 shows the Le Bail refinement and residuals for the two HP phases using the GSAS analysis software 40 . The two refinements are not completely satisfying: in the 4.8 GPa refinement, essentially broadening is not taken into account as we can see. And in the 2.0 GPa refinement, the position of the peak located around 2 • is not well reproduced. In order to check the validity of our proposed lattices and to go deeper into the structural determination, i.e. to determine the atomic positions, better complementary and/or quality data are needed.

Neutron powder diffraction results

We consider now the neutron diffraction data. In contrast to the X-ray case, where diffraction is dominated by the scattering cross-section of Cs, neutron diffraction is expected to provide a better information for the carbon sub-network because of the better balance between the scattering cross sections of carbon (σ coh =5.551 barns) and Cs (σ coh =3.690 barns). Due to the characteristics of the HP device, the maximal pressure was limited to 8.9 GPa. Three different experiments have been done: sample with NaCl (experiment no.1), sample with mineral oil (experiment no.2) and sample without any PTM (experiment no.3). The reversibility of the high pressure transformation was obtained in all the three cases after complete pressure release from 8.9 GPa as shown in Figure 5 for the first experiment. In the second experiment, increased incoherent background signal from the oil did not allow us to collect good quality data when increasing pressure. The pressure evolution of the c and a axis of CsC 8 from the first experiment is displayed in Figure 6. By using the pressure calibration curve of the first experiment as the pressure scale of the 3rd exp., its analysis leads to the same pressure dependence of the lattice parameters than for the 1st one. The compressibility of CsC 8 in the basal direction matches the compressibility of the graphite a-axis whereas the presence of intercalated Cs atoms produces a stiffening of the c-axis. In spite of the poor signal to noise ratio, appreciable changes in the intensity ratios between the (003) and ( 006) lines (see inset of Fig. 1) and in the broadening of the lines of the neutron data tend to support the observed transitions by XRD corresponding to the I→II transition around 1 GPa and maybe III→IV transition around 8 GPa. Contrarily to the XRD experiments, no evident sign of phase transformation was found between 1.9 GPa and 8.9 GPa. We only noticed the presence of two very lowintensity peaks, around around 11 • (7.8 Å) and 22-24 • (3.6-3.9 Å) respectively (see stars in Fig. 5) respectively, when increasing pressure and which remain detectable after releasing the pressure to zero. The origin of these peaks is not clearly understood but no significant shift with pressure associated to these peaks is observed. Consequently, they cannot correspond to the (00ℓ) lines of a stage-3/2 as it was previously reported in KC 8 21,42 . In addition, the extrapolated lattice parameters of the superdense CsC 4 phase (using the c-and a-axis compressibilities of Fig. 6) observed by Nalimova et al around 0.5-1 GPa 39 cannot either explain these very weak peaks. Hence, they could result to the presence of a minority phase corresponding to an impurity or they could come from the HP environment.

Combining X-ray and neutron diffraction is shown to be relevant to understand the behavior of CsC 8 under pressure. Differences between XRD and NPD results should be then attributed to two facts. First, XRD is essentially sensitive to the Cs layer order while NPD patterns are more dominated by the C atoms contribution. Secondly, the scattering geometries are different for the NPD (radial geometry) and XRD (axial geometry) experiments. It seems that due to the preferential orientation effect occurring with pressure in this lamellar compound, as it was observed for graphite 43 , the radial geometry is better adapted to follow the (00ℓ) lines. At least two clear transitions have been evidenced at 2.0 GPa and 4.8 GPa by XRD associated to changes in the decoration of the Cs layer and in the stacking. On the other side, the NPD data show us that such transitions do not imply a structural change in the graphitic (host) sub-network (Fig. 5). We observe a stiffening of the c-axis in CsC 8 in comparison with graphite. Hence, neutrons reveal that under high pressure, the initial average structure of CsC 8 initial phase is not strongly modified.

C. Raman spectroscopy

The Raman spectra were collected with a LabRam HR800 spectrometer from Jobin-Yvon equipped with a ccd detector in the backscattering geometry. Spectra were excited using 632.8 nm radiation from an He-Ne air-cooled laser, Spectra Physics TM . Low power radiation from the laser source (few mW) was tuned to avoid de-intercalation effects caused by the local heating 44,45 . The laser beam was focused into the DAC through a Mitutoyo 50× objective down to a laser spotsize of approximatively 2 µm. The DAC was loaded inside a glove box to prevent the contamination of the highly sensitive sample. Stainless steel gaskets of 30 µm thickness were drilled to 135 µm hole diameter. Heavy mineral oil was used as a PTM in the pressure chamber. Ruby chips served for pressure calibration. The CsC 8 Raman spectrum at ambient pressure is shown in Figure 7. Characteristic features are a highfrequency broad structure (at approximatively 1500 cm -1 ) and an low-frequency structure (near 560 cm -1 ) in agreement with previous studies 44,45 . Intercalation of donor species like the alkali leads to a metallic behavior instead of the semi-metallic (graphite) character. This metallic character is more pronounced for rich alkali/C atoms compositions, typically for stage-1 compounds as in our case, making difficult the detection of the Raman scattering signal. Two first-order active Raman modes can be observed for pristine graphite, at 42 cm -1 and at 1582 cm -1 . In GICs, the guest intercalate alkali layers act like a perturbation on the graphite modes thus leading to a shift in frequency 46 and turning on new active Raman modes. The striking high frequency broad asymmetric line is interpreted as a Breit-Wigner-Fano (BWF) resonance 47 . This BWF line shape arises from an interaction of an electronic continuum with in-plane carbonlayer vibration corresponding to the Γ-point mode of pristine graphite. However, the origin of the singular structure in the vicinity of 560 cm -1 which is in fact a triplet 48 rather than a doublet still remains unclear. This special feature was ascribed to a zone folding of M 1g modes into the Γ-point for the (2 × 2) superlattice corresponding to the MC 8 formula 45 whereas the triplet was well accounted by disorder-induced scattering 48 .

Figure 8 shows the behavior of the low-frequency domain from 0.1 to 3.2 GPa. The first spectra collected in the DAC is in perfect agreement with the signal at ambient pressure reported in Figure 7(a). The signalto-noise ratio becomes of better quality as the pressure increases and we observe a better resolved triplet at 1.4 GPa. Then, the triplet gradually disappears above 1.4 GPa and cannot be detected beyond 3.2 GPa within our incertitude.

During the compression study, several changes of the high frequency mode have been observed. Selected high frequency Raman spectra of the CsC 8 compound taken at different pressures are presented in Figure 9 where the pronounced asymmetric broad shape associated to the BWF resonance is also shown.

In the low pressure region 1.4-2.1 GPa, the complete disappearance of the low-frequency triplet is accompanied by a large shift of the BWF resonance from around 1500 cm -1 to 1590 cm -1 (Fig. 10). At 10.5 GPa, another peak appears close to the first one but slightly upshifted resulting in a doublet. Above 16 GPa and up to 32 GPa, it is extremely difficult to dissociate the two peaks, due to their large broadening.

Upshifted Raman frequencies from the graphite mode (1582 cm -1 ), visible above 10.5 GPa (Fig. 9), have been also observed in the case of diluted Cs-GICs. At ambient FIG. 9: High-pressure evolution of the high-frequency Raman spectra of CsC8 at room temperature. From 0 to 8.2 GPa Raman spectra were collected using a 1800-line grating and a 600-line grating wase used from 10.5 to 32.0 GPa. The background is strengthened with Raman contribution of the oil.

pressure, the stage-2 compound CsC 24 exhibits a Raman peak at 1598 cm -144, 45 . In higher diluted compound like the CsC 36 stage-3, a doublet (1604-1579 cm -1 ) is observed at ambient pressure 44,45 . The lower frequency of this doublet is related to interior graphitic layers whereas the upper frequency is ascribed to the bounding graphitic layers. The high frequency doublet is systematically present for n>2 when the screening of the interior graphite layer by the bounding layer is not efficient anymore. The pressure behavior of the E 2g graphite mode of graphite from Ref. 49 is also shown in Fig. 10. We remark that the peak appearing in the CsC 8 spectrum at 2.1 GPa around 1585 cm -1 corresponds with the evolution of the graphite E 2g peak and therefore could be associated to the interior graphitic layers mode observed in stage-n with n > 2. Consequently, the higher frequency of the doublet could be associated to the bounding layers mode. When the pressure is released from 32 GPa to 3.5 GPa, a reversible transformation accompanied with an hysteresis is observed (Fig. 10). Due to technical reasons, we were not able to collect a Raman spectra at ambient pressure in the DAC. The initial Raman spec- trum is recovered when the pressure is totally released from a maximum pressure of 8.2 GPa (Fig. 11), indicating the reversible back-transformation to the ambient phase of CsC 8 as previously observed in our NPD experiments here reported. Thus, assuming de-intercalation process involving diffusion of the alkali atoms outside the graphitic host above 2 GPa, is incompatible with the observation of the CsC 8 low and high-frequencies features after the complete decompression.

D. X-ray absorption spectroscopy

We carried out a X-ray absorption spectroscopy (XAS) experiment at the Cs L 1 ,L 2,3 -edges on the microfocus Lucia beamline at SLS (Swiss Light Source, PSI, Villigen) under high pressure up to 14.5 GPa. Further detailed description of the beam-line set-up combined with high pressure are fully described in Ref. 50. We worked in the energy range of 5-5.8 keV in the transmission configuration. DAC with stainless steel gaskets and hollowed diamonds were used to minimize X-ray absorption from the environment 51 . The sample was loaded with mineral heavy oil as PTM in a glove-box. Pressure was measured with the ruby fluorescence method. The three X-ray absorption Cs edges were collected using the same set-up. In order to reduce the presence of glitches originated from the single crystal diamond anvils, the cell was properly oriented.

XAS is a powerful tool for the study of materials under very high pressure conditions 52 . In particular, it allows to obtain both electronic and structural informations of the local environment of atoms (in our case the Cs atoms). The Cs L 2,3 -edges enable the study mainly of d-states whereas the L 1 -edge p-final states are involved, as given by the dipolar selection rules. Information on guest-host hybridization or charge-transfer under pressure will be made available through their study. EXAFS oscillations were obtained for the L 3 -edge, allowing, as it will be fur-ther discussed, to obtain qualitative information on the local structure evolution with pressure.

Figure 12 shows the normalized Cs L 1 and L 3 XANES evolution as function of pressure. The L 2 -edge is not shown due to a lower signal-to-noise ratio and the similarity with the L 3 -edge. For both Cs L 1 -and L 3 -edges, we observe a progressive evolution of XANES resonances with pressure. No sign of phase transition can be directly derived from XANES. We followed the inflexion point for the different edges to monitor their energy position as a function of pressure. The absolute error bars in energy were too large to observe a significant evolution in the 0.9-14.5 GPa range. Nevertheless the accuracy of measurements was high enough to study the pressure dependence of the relative position between the L 1 -and L 3 -edges at each pressure point. In Fig. 13 we plot the difference of energy position between the L 1 -and L 3edges as a function of pressure. An abrupt jump of approximatively 1 eV occurs between 7 and 10 GPa. This sudden change can be related to charge transfer effects between guest Cs atoms and the graphite host. Selection rules impose that this changes concern p-states and/or dstates. However there is no further information allowing to establish the relative weight of charge transfer between the p-and d -channels.

The Cs L 3 -edge EXAFS oscillations were extracted from the absorption spectra with the standard AUTOBK procedure 53 . In Figure 14 are shown the obtained EX- AFS oscillations which are expressed in terms of the photoelectron wave number k = 1/( 2m(E -E 0 )) where m is the electron mass and E 0 is the absoprtion edge position. The limited extent of the signal originates from dynamic or static disorder as well as in the low-Z values of the main photoelectron scatterers (C atoms). In spite of this, we can appreciate important changes in the amplitude of EXAFS oscillations in the region between 2.5 and 4.5 Å-1 . These changes correspond to a sudden increase of the amplitude of the signal that takes place between 7 GPa and 10 GPa and that can be associated with the changes in the relative position of the absorption edges (Fig. 13).

Due to the small k-range window for the EXAFS os- cillations and the presence of disorder in the material, we have restricted the quantitative analysis to the first shell of carbon neighbors around the absorbing Cs atoms. We used the ab initio feff 8.0 code 54 (based on Hedin-Lundqvist exchange potential) to generate the scattering functions (amplitude and phase shift of the photoabsorbed and backscattered atoms). The EXAFS oscillations corresponding to the first shell Cs-C was Fourier filtered and fitted with the first single diffusion path using the feffit program 55 . In addition to the Cs-C first neighbors distance (Fig. 15(a)) and the associated pseudo Debye-Waller factor, we considered also the third cumulant as the only free parameters in our fitted EX-AFS expression. We used the cumulant expression implemented in the feffit program based on an anharmonic effective pair potential. The observed change in magnitude of the EXAFS oscillations translates into a modification of the pseudo Debye-Waller contribution as shown in Fig. 15(b). Nevertheless, adding local disorder effect (thermal or structural) with the C 3 cumulant allows to reproduce the graphite inter-plane distance in a good agreement with our other data from XRD and NPD (Fig. 16).

III. DISCUSSION

As it is shown in Table III, we can distinguish four domains of stability characterized by modifications in the crystalline structure (as determined by XRD, NPD or by Raman spectroscopy) or by modifications of the electronic structure (evidenced either by XAS or Raman spectroscopy) of CsC 8 . The most remarkable facts are the smooth evolution both of the c lattice parameters obtained by NPD up to 9 GPa and of the Cs-C distances given by EXAFS up to 14.6 GPa even if the later case the incertitudes remain very large. The high sensitivity of XRD to the Cs atoms ordering with respect to NPD let us conclude that the two first phase transitions up to 14.0 GPa are related to slight modifications of the arrangement of Cs atoms, which involve the modification of the correlation between the different layers. In fact, the best fits to the XRD data at 2.0 GPa was performed with a very large unit cell (a=5.24 Å b=6.02 Å and c=56.75 Å, SG C222, No 21) which in addition is not commensurate with the graphitic sub-network.

The I→II phase transition is particularly clear as it is well defined and both observed by XRD and by Raman spectroscopy. The confrontation of XRD and NPD data points out a modification of Cs decoration without changes in the graphitic plane stacking, because in spite of their relative intensity changes, (00ℓ) reflections remain under the whole compression. The important change in the high-frequency Raman peak position at the I→II phase transition could be interpreted as a stage modification. Nevertheless, this appears as contradicting with the NPD data. A possible interpretation is that Raman spectroscopy being more surface sensitive in this metallic compound, staging transitions could be observable at the grain surface. The II→III phase transition, as was already discussed, has only been detected through XRD and it may be accompanied by the presence of an intermediate phase. As for phase II, phase III seems to involve a very large crystalline lattice as suggested by the appearance of low-angle peaks. NPD does not show any difference between phase I, II and III, so again CsC 8 -III is most probably associated to changes in the decoration of the Cs sub-network. The last phase transition, III→IV around 8 GPa, is related to changes of the electronic structure, that could imply a stage transition. It is clearly observed by XAS with the associated 1 eV shift between the Cs L 2 and L 3 absorption edges, and by Raman spectroscopy with an interior graphitic layers mode and a bounding layers mode, characteristic of a highstage n >2. This transition occurs in all the sample since XAS is a bulk probe. We shall again mention that the observation of a doublet in the Raman signal from around 10 GPa involves the grain surface of the metallic sample. Furthermore, we cannot exclude that the doublet result from the local laser heating. As we do not have NPD data above 9 GPa and fits of the XRD data in this region are not reliable, we cannot assure the persistence of the stage-1. Other structural changes associated to this last transition cannot be excluded. Indeed, n → n+1 staging transition implies the presence of an additional phase to maintain the density of Cs atoms.

Let us now turn to the compressibility of the structure. Compressibility of phase I, characterized either by the volume bulk modulus or by the linear bulk modulus of the c-axis is given in Table IV. Our values are compared with other values from the literature which propose higher compressibilities. As mentioned, the neutron diffraction data (Fig. 5) show a smooth variation of the graphitic-like cell parameters with pressure that do not appear to be affected by any clear phase transitions within the calculated incertitude. With a Murnaghan fit up to the highest pressure measured in the NPD experiment, i.e. 8.9 GPa, we obtain a bulk modulus equal to 72 GPa, which, as can be seen in Table IV, is closer to the value published by Wada 24 .

In addition, we deduced the graphitic inter-plane compressibility from the projection of the Cs-C distance on the c axis by EXAFS. Taking into account an anharmonic correction through the third cumulant related to this distance allows to provide a reasonable agreement with the compressibility obtained by the diffraction experiments within the incertitudes (Fig. 16). Introduction of disorder variations in the EXAFS expression to fit as good as possible the diffraction data is a consequence of the changes in the Cs atoms decoration observed in the XRD patterns. We observe in Figure 15(b) from the pseudo Debye-Waller factor evolution that compression leads to a progressive reduction of the static or/and dynamic disorder of the local Cs structure.

IV. CONCLUDING REMARKS

Combining different techniques, we have studied the pressure behavior of the guest and host sub-networks and the guest-host interaction of the CsC 8 intercalated graphite in the 0-32 GPa range. Three pressure-induced transformations have been clearly observed. The high reactive nature of the sample coupled with its ill order nature, made the use of complementary experimental techniques a necessity. The combination of these techniques, in which different selectivities apply, has been essential for the understanding of the observed transitions. We have given evidences of evolutions in both the electronic structure (using Raman and XAS spectroscopies) and in the crystallographic structure (XRD and NPD) occurring in the studied pressure range. The main observed facts can be summarized as follow: (i) anisotropic compressibility, (ii) stiffening of the c-axis in CsC 8 in comparison to graphite up to 8.9 GPa due to the presence of intercalated Cs atoms, (iii) subtle rearrangements of the Cs atoms layer lead to a complex lattice, (iv) a re-organization of the stacking probably occurs (from hexagonal αβγ to orthorhombic αβγδ at 4.8 GPa for example) and (v) hybridization between Cs atoms and graphite layers could explain the charge transfer observed in XAS around 8 GPa. Finally, with exception of the appearance of an impurity phase, it seems that these transformations are reversible at least up to ∼8 GPa, indicating that they do not result from a deintercalation process.

  FIG.1: Ambient pressure and temperature neutron powder diffraction pattern of CsC8 fitted within a Rietveld refinement. The pattern was collected in a vanadium container. The inset shows the relative intensity of the (003) and (006) lines with pressure. Open symbol for decompression at zero pressure.

  FIG.2: Selected X-ray diffraction patterns of the CsC8 compound as a function of pressure. Pressure in GPa is given at the right side of each curve. The first pressure pattern corresponds to the hexagonal structure (CsC8-I). The sample was confined in oil as pressure transmitting medium. Intensity was normalized for all patterns to the same counting time. Stars (⋆), plus (+) and crosses (×) symbols show the appearance of new Bragg peaks corresponding to CsC8-II, CsC8-III and CsC8-IV phases respectively (see TableIII).

FIG. 3 :

 3 FIG.3: Compressibility of the hexagonal CsC8 lattice parameters (solid symbols) derived from our X-ray diffraction experiment and of the c-axis reported by Wada (open symbols show two sets of data)24 .

FIG. 5 :

 5 FIG. 5: Neutron powder diffraction patterns of CsC8 as a function of pressure with NaCl (+) as pressure transmitting medium and pressure marker. The two stars (⋆) indicate additional peaks (see text). The ambient diffraction pattern was taken in a vanadium container. The released pressure diffraction pattern was recorded in the press after the HP cycle.

FIG. 6 :

 6 FIG. 6: Evolution of the lattice parameters of CsC8 (filled symbols) from NPD as a function of pressure compared to the graphite ones (dashed lines) from Ref. 41. The errors bars are provided from the Rietveld refinements.

FIG. 7 :

 7 FIG. 7: High-(b) and low-(a) frequencies Raman signal of CsC8 at ambient pressure and room temperature with a 632.8 nm laser excitation.

FIG. 8 :

 8 FIG. 8: Low-frequency Raman spectra of CsC8 as a function of pressure. The last spectrum was collected after a HP cycle to 8.2 GPa. The star indicates the weak component consistent with the triplet observed by Caswell and Solin 48 .

FIG. 10 :FIG. 11 :

 1011 FIG. 10: High-frequency Raman region peak positions of CsC8 versus pressure. The straight line indicates the evolution of the E2g graphite mode with pressure from Ref. 49. Filled circles represent the compression whereas open circles show the decompression of the sample.

FIG. 12 :

 12 FIG. 12: XANES evolution with pressure at both L1-edge (a) and L3-edge (b) of CsC8 from our experiment at SLS.

FIG. 13 :

 13 FIG.13: Difference between the energy of the L1-and L3-Cs edges in CsC8 as a function of pressure.

FIG. 14 :

 14 FIG.14: Extracted Cs L3-edge EXAFS oscillations as a function of pressure. The arrow shows increasing pressures from 0.9 to 14.6 GPa. Dotted lines corresponds to the low pressure phase and continuous lines to the high pressure phase (see text).

FIG. 15 :

 15 FIG. 15: Pressure-evolution of the relative Cs-C first neighbors distance (a) and the pseudo-Debye Waller σ 2 (b). The EXAFS signal was k 2 weighted and Fourier transformed [2.5, 6.0] Å-1 to real space and finally fitted in the range [1, 3] Å.

TABLE I :

 I Structural parameters of the hexagonal structure of CsC8 at ambient conditions obtained from a Rietveld refinement. The corresponding stacking is AαAβAγ. Thus, the c-axis lattice parameter corresponds to 3Ic.

	Space group, No	P6222, 180
	Lattice constants ( Å)	a= 4.9601, c=17.847
	Repeat distance ( Å)	Ic=c/3=5.949
	Atoms	Wyckoff positions
	C	12k (1/6, 1/3, 1/3)
	C	6i (5/6, 2/3, 0)
	C	6i (2/3, 1/3, 0)
	Cs	3d (1/2, 0, 1/2)

TABLE II :

 II Structural parameters of the possible candidate phases of CsC8 appearing at 2-3 GPa and 4.8 GPa.

		2.0 GPa	4.8 GPa
	Space group, No	C222, 21	Fddd, 70
	Lattice constants ( Å)	α = β = γ= 90 •
	a	5.24	4.39
	b	6.02	9.20
	c	56.75	22.87

TABLE III :

 III Summary of the principal changes observed in the CsC8 compound as a function of pressure.

		P-stability (GPa)	Observed structures	Most significative changes
	CsC8-I	0.0-1.2	P6222	Linear compressibility
			stage-1 (αβγ)
	CsC8-II	1.2-3.5	C222 a	Change in the Cs decoration accompanied with small displace-
				ments and strong modification of the Raman signal.
			stage-1 + ?	Abrupt change in the relative intensity of (003) and (006),
				strong broadening of the lines in NPD.
	CsC8-III	3.5-8.0	Fddd a	Changes in the Cs decoration (slight displacements), in the
				stacking (αβγδ) and possible intermediate structure between
				II-III.
			stage-1 + ?	Broadening of the diffraction lines in NPD.
	CsC8-IV	8.0-32.0	stage-n (n >2)	Electronic changes observed by EXAFS and Raman at the
				transition.
			+ other phase?	Significant broadening of the diffraction lines and very low
				intensity signal in XRD. No evidence of the presence of the
				stage-1. Doublet observed in the Raman signal.

a Proposed structures

TABLE IV :

 IV Axial compressibility and bulk modulus of CsC8. Incertitudes for our values are of 10%. FIG. 16: Dependence of the CsC8 graphitic inter-plane distance with pressure combining both diffraction (XRD and NPD) datas indicated by empty and solid circles and local spectroscopy (EXAFS) datas indicated by solid squares. Crosses from 0 to 1.1 GPa correspond to the data of Wada 24 . The compressibility of graphite (solid line) from Ref. 41 is plotted for comparison.

			1.00
			0.98
			0.96
			0.94
	c/c	0	0.92
			0.90
			0.88
			0.86
			0	2	4	6	8	10	12	14	16
						Pressure (GPa)
										Techniques	Bc (GPa) B ′ c Bv (GPa) B ′ v	P-range
										XRD this work a	93	0	94	0 0-1.1 GPa
										INS b	58	ambient
										XRD c	64	0-1.1 GPa
										NPD this work	72	8	70	6 0-8.9 GPa

a Linear fit. b Inelastic Neutron Scattering (INS) from reference 56 . c Reference 24 .

Acknowledgments

This work was supported by the French Ministère Délégué la Recherche et aux Nouvelles Technologies and the CNRS through grants "ACI-2003 n. NR0122" and "ACI-2003 n. JC2077". We thank gratefully H. Feret (LPMCN, France) for his assistance and technical support. This research project has been supported by the European Commission under the 6th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract no: RII3-CT-2004-506008. This work is based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland.


Robert Waweru

Robert Waweru Challenges Faced

CHALLENGES FACED IN THE RESETTLEMENT OF INTERNALLY DISPLACED PERSONS IN KENYA

Introduction

This paper examines the initiatives that have been undertaken by the government of Kenya from 2008 to 2016 to address the plight of Internally Displaced Persons (IDPs) after the 2007/08 postelection violence (PEV). The violence erupted after the announcement of Mwai Kibaki as the winner of the presidential election against Raila Odinga. 1 Odinga disputed the election outcome, claiming that electoral fraud had taken place and called upon his supporters for mass action. The demonstrations quickly degenerated into politically instigated ethnic violence that was witnessed across the country and was most severe in urban areas such as Nairobi, Naivasha and Kisumu, where supporters of the two candidates engaged in attacks and counter attacks (KNCHR 2008).

The Rift Valley was also an epicenter of violence where attacks were mainly directed at the Kikuyu ethnic group for displacement. By 28th February 2008, when Kibaki and Odinga signed the Principles of Partnership after successful mediation by Kofi Annan, over 1300 Kenyans had lost their lives and 650,000 more had been displaced from their homes and their properties destroyed. 310,000 of the displaced persons integrated among communities while the other 350,000 sought refuge in 118 IDP camps across the country (Muluka 2010: 53). The first initiative, Operation Rudi Nyumbani (Return Home), was the immediate effort made by the Grand Coalition government formed to end the violence with Kibaki as President and Odinga as Prime Minister.

The second initiative, Operation Ujirani Mwema (Good Neighborliness) and Operation Tujenge Pamoja (Build Together) meant to ensure safe and peaceful environment for the returning displaced persons. Third was the provision of Ksh 10,000 and Ksh 25,000 start-up and reconstructing funds to support the returning IDPs in re-starting their livelihoods. However, not all IDPs were displaced from farms: a significant number were displaced from urban areas and therefore had nowhere to return to. Moreover, not all IDPs were willing to return to places they had been displaced from.

They had to be resettled on government purchased lands in several parts of the country including Rongai, Nakuru County, where this study was carried out because of the large number of IDPs that were resettled in the area. According to the government, it seemed convenient for resettlement because of logistical reasons and the availability of large and cheap land. However, I was curious to find out what challenges the resettlement would face given the fact that the host community in this area, the Kalenjin, was the same ethnic community that was responsible for the displacement of many IDPs in the first place. This paper conducts an analysis of the socio-cultural, political, economic and logistical challenges that faced the resettlement process and offer recommendations on how these challenges could have been avoided. It also presents the views of stakeholders in the resettlement process, i.e the IDPs, the government through the National Coordination Consultative Committee on IDPs and the Provincial Administration as well as Civil Society Organizations (CSOs) such as the Kenya Human Rights Commission.

Inadequate funds

After the 2007/08 PEV and the formation of the Grand Coalition Government, the Ministry of Special Programmes (MoSP) was tasked with the mandate of responding to the plight of the 663,921 IDPs (Kamungi 2009: 355). 2 Lack of suffi cient funding is the greatest challenge the government faced in its effort to address the plight of the IDPs. By December 2009, 24,709 IDPs households had not received the Ksh 10,000 start-up capital and 55,725 households had not received the Ksh 25,000 for the reconstruction (KNCHR 2009: 5). As of July 2010, the government had disbursed Ksh 2,380,170,000 to IDPs and still required an additional 1,161,390,000 to address the cash disbursement. By July 2010, out of the 6802 IDPs households who had land to return to, only 789 households had benefited from the 21/4 acres of land resettlement (GoK, 2010). By the end of 2016, the government had provided the Ksh 10,000 start-up capital to 170,349 households and the Ksh 25,000 reconstruction funds to 37,843 households. A total of 122,245 persons had also benefited from the psychosocio support initiatives. A further 8,784 households been resettled on 20,630 acres of land purchased by the government. 3 The cash disbursement process was however criticized by both the IDPs and Civil Society as a process that lacked transparency and accountability. There were allegations of some government offi cials misappropriated cash meant for the benefit of IDPs. 4 Consequently, a majority of IDPs received either the Ksh 10,000 or the Ksh 25,000 with some receiving nothing at all. A beneficiary accounts: "In my household, my wife, my son and I received Ksh 10,000 each. However, I am the only one who received the Ksh 25,000. The others did not receive the Ksh 25,000". 5 On the other end, some IDPs received multiple payments by registering multiple times in multiple places.

Poor records management of IDPs

As mentioned earlier, lack of proper records of IDPs was also a major obstacle in the implementation of these initiatives, especially early on in the process. Before 2007/08, internal displacement was not very common in the country and the government had not been keen in keeping an updated record of displaced persons. Consequently, after 2007/8, the government did not have any clear records on who was an IDP and who was not. Therefore, when the government tried to address the plight of IDPs, it faced a very serious challenge of identifying genuine IDPs and fraudsters. A local Provincial Administrator said: "The process of identifying genuine IDPs should have started at the onset of the resettlement program. The government should have involved the provincial administration to determine who a genuine IDP is and who is not". 6 The area Chief also reiterated: "We endorsed over twenty lists of benefi ciaries and we don't even know which one was used to compensate people. Sometimes individuals who should have benefi ted should not have while some genuine IDPs who should have benefited did not". 7 Even the National Consultative Coordination Committee on Internally Displaced Persons (NCCCIDP), the national government agency formed in 2013 to coordinate the resettlement of IDPs, admits that at the beginning of the resettlement process in 2008, no clear records were used during the compensation of the IDPs. 2 In the Grand Coalition Government, the MoSP was a Kibaki/PNU Ministry headed by Naomi Shaban. However, the Prime Minister Odinga was in charge of supervising all the ministries and therefore the resettlement was a collective government effort. 3 Interview with a National Government Offi cial, 22/11/2016, Nairobi. 4 Karanja Njoroge and Kenan Miruka, "How offi cials used IDPs' names to siphon Sh200m meant for victims", The Standard, 12/1/210. Accessed October 5th 2017, online: https://www.standardmedia.co.ke/article/2000000781/how-offi cials-used-idps-names-to-siphon-sh200m-meant-for-victims 5 Interview with Dominic Otieno, a resettled IDP, 30/09/2017, Makongeni Farm. 6 Interview with a Senior Provincial Administration Offi cer, 22/06/2016, Rongai Sub County Offi ces. 7 Interview with a Junior Provincial Administrator Offi cer, 22/06/2016, Rongai Sub County Offi ces.

Alleged inequality in the resettlement process

There is a widespread perception that the government, both the Kibaki/Odinga's and the Uhuru/Ruto's discriminated against IDPs from other parts of the country in the resettlement process by paying more attention on IDPs from the Kikuyu community displaced from the Rift Valley. [START_REF] Bwayo | IDPs cite discrimination in resettlement plan[END_REF] This claim is even held by resettled IDPs who still maintain that they did not receive similar attention as other IDPs. A Luo IDP complained: "With majority of our people [Luos], when the fracas erupted they went home [Nyanza]. They therefore were not compensated. Us who were compensated are just a drop in the ocean. For example, my brother lost all his property here and there is nothing he got. If you go to where Kikuyu's were allocated land, you will see how we were neglected. Honestly speaking it was much skewed". [START_REF] Interview | a resettled IDP[END_REF] Patrick Njagi, the chief executive offi cer of National Consultative Coordination Committee on IDPs, was however quick to dismisses such an argument and maintained that the fi rst priority was given to the plight of IDPs who were in the camps, facing a dire humanitarian crisis. Integrated IDPs, those who had gone back and integrated within their communities were to be addressed in subsequent programs.

Failure to involve and consult with stakeholders

A key provision in all legal frameworks that protect IDPs, such as the United Nations Guiding Principles on Internal Displacement and the Kenya IDP Act of 2012, provides that IDPs should be consulted on any programs undertaken by authorities to address their plight. The government of Kenya has however faced a lot of criticism from IDPs, CSOs and other stakeholders who accused the government of implementing resettlement programs without engaging them in any adequate and meaningful consultation. A resettled IDP complained of government of unilateralism: "People should be consulted on where they should be resettled. Our resettlement in that place was somehow forceful. How can you be taken to a place you have never seen? You should be consulted and even counseled that the place (...) (...) you are going to be resettled is pathetic. I was resettled in Majani Mingi but I decided to come back here [Njoro]. Majani Mingi is a very dry area, arid. It is very rocky, you cannot even till it. You can't even sell it. It is useless, there is no value addition that you can do on it. In fact, majority of the people resettled there are currently not living there because you cannot till the land. There is no water, there is no rain and no amenities". 10 A Human Rights Offi cer advises: "The Kenyan Constitution provides for public participation. IDPs are your target audience, if you don't consult them on a regular basis on how best to address their issues how then do you address their problems?". 11 Mr Patrick Njagi from the NCCCIDP however dismisses such arguments and maintains that it is not practical to resettle all the IDPs in places where each one of them would want. The government urges the resettled IDPs to be thankful for all the efforts the government has undertaken to address their plight instead of always complaining. According to Mr. Njagi, the government has done its best to help the IDPs at the prevailing circumstances and despite all the challenges. A National Government Official remarked: "Some of those persons complaining that they were not consulted and that the land they got is not good did not even own land in the first place. Instead of being grateful for the piece of land which they can sell and get money to do other business, they are complaining. It is a big challenge (...)

A house belonging to a resettled IDP in Makongeni but has since been abandoned @ Robert Waweru (...) getting big, fertile and arable land because the process of buying land is based on a willing buyer willing seller Principle". [START_REF] Interview | Chief Executive Offi cer[END_REF]

Lack of social amenities and basic infrastructure

IDPs resettled in Rongai, Nakuru, argued that the government cannot claim to have resettled them. They have the feeling they were just dumped in a place with no roads, schools, hospitals, markets, security, water or any other form of social amenity.

The most pressing challenge is the lack of clean water. The only source of water is an open dam that was dug by the government, which is not clean and safe for human consumption.

Hostility between the IDPs and the host community

The resettlement of IDPs in many parts of the country faced stiff resistance by the local communities who rejected the resettlement of new communities in their areas. [START_REF] Nyaboke | Kenya: Burnt Forest Villagers Resist IDPs Resettlement[END_REF] The script was not any different in Rongai, as the resettlement provoked hostility from the local community who were mainly laborers working in the sisal farms that had been purchased to resettle IDPs.

The local communities argued that the government could not resettle people from other regions such as Western and Nyanza while there were landless people residing in the area. An IDP resettled in Rongai narrates: "They were not accommodative, they became very aggressive towards us until the government decided to allocate some of them land to appease them. They did not want us there, even now they don't want us there. In fact, there is a friend of mine who planted maize and the maize somehow did well. He was a Luhya. At night, a host neighbor deliberately grazed his cattle on all the maize and there is nothing the farmer could do. You cannot raise any issue because you are in a foreign land. We are voiceless there." [START_REF] With | a resettled IDP[END_REF] To avoid the problem of hostility from local communities, in 2013 the NCCC on IDPs designed a formula that would see the local communities also get a share of the land. In Rongai, they came up with a 60%-40% formula where the IDPs were to get a 60% share of the land while the local community was to get a 40% share of the land. Mr. Njagi however downplays any such hostility and argues that it is not an attempt by the local community to resist the resettlement of new communities in their areas but that the local host communities also require land: "I cannot call it hostility. What happens is that, there are landless people in every community. Therefore, when they see you bringing people to resettle, they also demand to be resettled too because they too do not have land. It is not an issue of hostilities between communities. What we have done is work on a formula where as we resettle the IDPs we also give a portion of the land to the landless people". [START_REF] Interview | Chief Executive Offi cer[END_REF]

A girl draws water from an open dam, the only source of water in Makongeni farm @ Robert Waweru

The piece of land set aside for the construction of social amenities in Makongeni but none has been established yet @ Robert Waweru

Politicization of the resettlement process

Politicization of the resettlement process was a big challenge to effective implementation of the resettlement program across the country. Kenyan politics and voting is conducted along ethnic lines that matches with political contests. Therefore, the resettlement of any group of people in a given place alters the ethnic composition of that area and consequently affects the voting patterns, unless the IDPs being resettled are of the same ethnic group as the host ethnic group. Politicians either supported or opposed the resettlement of IDPs in various areas depending on what impact the resettlement would have to their political interests. In Nakuru, a Kalenjin politician would oppose Kikuyu IDPs to be resettled in his/her area because they might vote for a Kikuyu politician. However, if the IDPs were Kalenjin, Kalenjin leaders would support the process because resettlement might increase his/her votes. Where they thought IDPs resettlement would be benefi cial to them, local leaders would support the process, but where they thought the resettlement would threaten their political interests, they would oppose often by inciting the host communities against accepting the settlement of IDPs in their areas. [START_REF] Siele | The Right To information for Internally Displaced Persons in Kenya[END_REF]

Conclusion

While the government of Kenya said it "tried its best" to address the plight of IDPs, the implementation of the resettlement initiatives has encountered numerous challenges that negatively affected the effectiveness of the process. This article postulates that the resettlement of IDPs is often a very complicated process that requires proper planning before the implementation of any program intended to provide lasting solutions to IDPs. Consultation and inclusion of the IDPs is key to the success of any resettlement process. The special needs of IDPs do not necessarily disappear immediately after the resettlement. Resettling authorities and other stakeholders should follow up on IDPs because they continue to face diffi culties in establishing livelihoods and lack of social amenities such as schools, hospitals and infrastructure particularly pose major challenges. After the numerous challenges experienced with land resettlement, the government of Uhuru Kenyatta subsequently opted to give cash to IDPs for selfresettlement. Eventually from 2016, the government is compensating integrated IDPs, but many structural challenges are yet to be addressed.

Robert Waweru is an alumnus of University of Nairobi holding a Masters Degree in Armed Confl ict and Peace Studies and a B.A in Political Science and Confl ict & Peace Studies from the same Institution. This paper presents part of his Masters Dissertation's fi ndings that explored the resettlement of IDPs in Kenya and the challenges that were encountered in the resettlement process.

1

  Kibaki of Party of National Unity (PNU) was a Kikuyu while Odinga of Orange Democratic Movement (ODM) was a Luo who enjoyed the support of other ethnic groups like the Kalenjins (Klopp & Kamungi, 2007/2008: 2). More specifi cally on the PEV and the IDPs, see Kamungi & Klopp, 2007: 52-53.