Florent Tournus

• PhD
• Université Lyon 1 & CNRS at Claude Bernard University Lyon 1

FeRh alloys in the CsCl-type (B2) chemically ordered phase present an antiferromagnetic to ferromagnetic order transition around 370 K observed in bulk and continuous films but absent in nanoclusters. In...

Size-selected FeRh clusters have been deposited at low energy and under ultra-high vacuum conditions, on a BaTiO3 epitaxial thin film. Using x-ray diffraction in grazing incidence configuration, we have observed the chemical ordering of FeRh nanoparticles into the chemically ordered B2 phase after annealing, while a reciprocal space mapping indicat...

The intrinsic magnetic properties of nanoparticles (NPs) can be accurately determined using highly dispersed NPs in a matrix. In this paper, we study chemically ordered CoPt and FePt NPs with a diameter of 3 nm embedded in an amorphous carbon matrix. Although both alloys exhibit almost the same magnetic and crystallographic properties in the bulk m...

In this paper, we present some specific chemical and magnetic order results obtained on bimetallic FeRh nanoparticles prepared under non-equilibrium conditions using mass-selected Low Energy Cluster Beam Deposition. Clusters around 7 nm incident diameter are in-situ sandwiched between amorphous carbon films before transfer in air, with different su...

In the frame of the 20th Anniversary of the Journal of Nanoparticle Research (JNR), our aim is to start from the historical context twenty, years ago and to give some recent results and perspectives concerning nanomagnets prepared from clusters preformed in the gas phase using the Low Energy Cluster Beam Deposition (LECBD) technique. In this paper,...

Despite Cobalt and Iron–Carbon interactions playing an important role in environmental chemistry activities, major questions are still unresolved on the prevailing mechanism, surface state, required structure as well as the origin of apparent particle size dependence in carbon nanotube synthesis or other heterogeneous catalytic process such as Fisc...

The moiré pattern created by the epitaxy of a graphene sheet on an iridium substrate can be used as a template for the growth of 2D atomic or cluster arrays. We observed for the first time a coherent organization of hard magnetic preformed FePt nanoparticles on the 2D lattice of graphene on Ir(111). Nanoparticles of 2 nm diameter have been mass sel...

In the frame of the 20th Anniversary of the Journal of Nanoparticle Research (JNR), our aim is to start from the historical context 20 years ago and to give some recent results and perspectives concerning nanomagnets prepared from clusters preformed in the gas phase using the low-energy cluster beam deposition (LECBD) technique. In this paper, we f...

Cobalt nanoparticles embedded in copper matrices show strong size effects in the magnetic anisotropy with a non-monotonous dependence on the particle diameter. In this article we discuss quantitative values of the magnetic anisotropy in the frame of two models: in small clusters the surface anisotropy contribution dominates whereas larger particles...

We report on the magnetic properties of Co clusters (around 2.5 nm diameter) embedded in different matrices (C, Au, and Cu). Firstly, we accurately determine the intrinsic magnetic properties of the particles, using highly diluted samples where no interactions are detected, and a procedure relying on the theoretical description of various magnetome...

In this article, we study the thermally activated chemical order transition in FeCo nanoalloys prepared by low energy clusters beam deposition (LECBD). Due to the low Z-contrast (ΔZ = 1) between Fe and Co atoms, a particular strategy based on extended X-ray absorption edge fine structure (EXAFS) experiments and simulations at both Fe and Co K-edges...

In this work, we have focused on the elaboration and characterization of nanostructured L10-CoPt magnetic dot arrays prepared by depositing Co50Pt50 nanoparticles preformed in the gas phase on Si substrates patterned by electron beam lithography (EBL). The MFM observations have revealed an out-of-plane single domain state. The VSM measurements have...

Nanoparticles associating a noble metal and a ferromagnetic metal are appealing from a magneto‐plasmonics point of view, in addition to the problematics of magnetic anisotropy tailoring (interface anisotropy, phase transformation) and of nanoalloy original geometries. Because Co and Au are immiscible in the bulk phase, and since fcc cobalt and gold...

Since CoAg nanoparticles are composed of two immiscible metals, their theoretical structure is predicted to be Co@Ag due to the weaker surface energy of Ag [1]. In the literature these particles are synthesized by chemistry techniques [2‐3], the structure obtained is Co(core)Ag(shell) and has been confirmed by the optical response in UV‐Vis spectro...

Near the point of equiatomic composition, both FeRh and FeCo bulk alloys exhibit a CsCl-type (B2) chemically ordered phase that is related to specific magnetic properties, namely a metamagnetic anti-ferromagnetic/ferromagnetic transition near room temperature for FeRh and a huge magnetic moment for the FeCo soft alloy. In this paper, we present the...

Isothermal remanent magnetization (IRM) combined with Direct current demagnetization (DcD) are powerful tools to qualitatively study the interactions (through the Δm parameter) between magnetic particles in a granular media. For magnetic nanoparticles diluted in a matrix, it is possible to reach a regime where Δm is equal to zero, i.e. where interp...

Strong size-dependent variations of the magnetic anisotropy of embedded cobalt clusters are evidenced quantitatively by combining magnetic experiments and advanced data treatment. The obtained values are discussed in the frame of two theoretical models that demonstrate the decisive role of the shape in larger nanoparticles and the predominant role...

Two-dimensional hexagonal arrays of Pt nanoparticles (1.45 nm diameter) have been obtained by deposition of preformed and size selected Pt80 nanoparticles on graphene. This original self-organization is induced, at room temperature, by the 2D periodic undulation (the moir\'e pattern) of graphene epitaxially grown on the Ir(111) surface. By means of...

In this paper, we present some specific chemical and magnetic order obtained very recently on characteristic bimetallic nanoalloys prepared by mass-selected Low Energy Cluster Beam Deposition (LECBD). We study how the competition between d-atom hybridization, complex structure, morphology and chemical affinity affects their intrinsic magnetic prope...

In this article, we study the intrinsic magnetic properties of diluted FeCo clusters nanoparticles embedded in an inert amorphous carbon matrix. We report an enhancement of the magnetic anisotropy energy (MAE) after annealing demonstrated by superconducting quantum interference device (SQUID) measurements and adjustments. Rutherford backscattering...

Among nanoalloys, Co–Pt type (CoPt or FePt) supported nanostructures are very interesting systems due to the direct link between atom arrangement and magnetic behavior. In addition, these alloys become model systems in the field of nanoalloys, due to the diversity of atom arrangements either present in the bulk state or specific to the nanoscale (c...

By combining high photon flux and chemical selectivity, X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) have been used to study the magnetism of CoPt and FePt clusters before and after their transition to the chemically ordered L10-like phase. Compared to the bulk, we find larger magnetic spin and orbital moments of Fe, C...

Measurements and calculations have shown significant disagreement regarding the sign and variations of the thermal expansion coefficient (TEC) of graphene $\alpha(T)$. Here we report dedicated Raman scattering experiments conducted for graphene monolayers deposited on silicon nitride substrates and over the broad temperature range 150--900~K. The r...

We present a robust and efficient framework to compute isothermal remanent magnetisation (IRM) curves for magnetic nanoparticle assemblies. The assembly is modelled by independent, randomly oriented, uniaxial macrospins and we use a Néel model to take into account the thermal relaxation. A simple analytic expression is established for a single size...

Sub-monolayer thin film morphologies obtained by deposition of size-selected CoxPt1-x clusters on graphite have been analyzed for different values of x. In all cases, the preformed clusters can easily diffuse on the surface and gather to form islands of clusters. By changing the cluster stoichiometry, very different morphologies can be obtained, go...

Highly uniform (8 nm) bare maghemite nanoparticles were pressed into a disc with a volume fraction close to the characteristic filling factor of random-close-packed ensembles of spherical particles. We review the ideal superspin glass behaviour exhibited by this material, including an onset of the absorption component of the ac susceptibility at th...

Nano-hybrid systems have been prepared by UHV deposition of size selected iron─platinum clusters on multi-walled carbon nanotubes. Analyses of the resulting morphologies put into evidence the ability of "large" bare clusters (typically few nanometers in diameter) to diffuse on the nanotube surface. A comparison between cluster behaviors on differen...

The redox behavior of 5 nm Fe-Me alloyed nanoparticles (where Me = Pt, Au, and Rh) was investigated in situ under H2 and O2 atmospheres by near ambient pressure X-ray photoelectron and absorption spectroscopies (NAP-XPS, XAS), together with ex situ transmission electron microscopy (TEM) and XAS spectra simulations. The preparation of well-defined F...

We report on the transition from two-dimensional to one-dimensional (1D) behavior observed in submonolayer deposition on a tubular substrate. Using kinetic Monte Carlo simulations we show that the island density increases when the tube diameter decreases, approaching the 1D behavior. This increase is confirmed by experiments of FePt nanoparticle de...

A recent paper1 examines zero field-cooled/field-cooled (ZFC/FC) susceptibility curves for nanoparticle assemblies with a size distribution. It is explained that the “volume and number weighted distribution are equally valid for the representation of distribution functions in nanoparticle magnetic systems” and the usual modelling approach (abrupt t...

We propose a theoretical framework enabling the simulation of isothermal remanence magnetization (IRM) curves, based on the Stoner-Wohlfarth model combined with the Néel macrospin relaxation time description. We show how low temperature IRM curves, which have many advantages compared to hysteresis loops, can be efficiently computed for realistic as...

A simple single-phase material, a random close-packed (volume fraction 67%) ensemble of highly monodisperse bare maghemite (-Fe2O3) nanoparticles, is shown to exhibit ideal superspin-glass behavior (mimicking that of model spin-glasses), namely, an unprecedentedly sharp onset of the absorption component of the ac susceptibility, narrow memory dips...

We examine the effect of the applied magnetic field amplitude on zero field-cooled/field-cooled (ZFC/FC) curves, through the nonlinear susceptibility of superparamagnetic particles (i.e., at thermondynamic equilibrium, but taking into account the magnetic anisotropy). This nonlinear effect is shown to be the first to manifest itself when going away...

The fundamental question as to the relative importance of interparticle superexchange versus dipolar interaction between oxide magnetic particles in direct physical contact is addressed by examining the magnetic properties of a series of compacted samples comprising identical maghemite particles (8 nm in diameter) coated by nonmagnetic shells (olei...

An annealing-driven transition from a chemically disordered A1-like structure to a chemically ordered L10-like phase has been revealed for size-selected CoPt clusters with diameters from 2 to 4 nm, from experiment to theory confrontation. For chemically ordered particles, atypical lattice-parameter ratios c/a are inferred. This ratio is found to be...

We report the refined quantitative magnetic moments of size-selected 3 nm in diameter CoPt clusters at both Co and Pt-L2, 3 edges. By combining the high photon flux and the chemical selectivity of the synchrotron radiation delivered on ESRF beam lines, we have studied size effect on the local structure and magnetism in CoPt clusters embedded in car...

In sharp contrast to previous studies on FeRh bulk, thin films, and nanoparticles, we report the persistence of ferromagnetic order down to 3 K for size-selected 3.3 nm diameter nanocrystals embedded into an amorphous carbon matrix. The annealed nanoparticles have a B2 structure with alternating atomic Fe and Rh layers. X-ray magnetic dichroism and...

The atomic structure of CoPt and FePt nanoparticles (with a diameter between 2 and 5 nm) has been studied by transmission electron microscopy. The particles have been produced by a laser vaporization cluster source and annealed under vacuum in order to promote chemical ordering. For both alloys, we observe a coexistence of crystalline and multiply...

We have studied size effect on the local structure and magnetism in CoPt clusters embedded in carbon matrix upon annealing in the 2–4 nm diameter range. From Co-K and Pt-L edges extended X-ray absorption fine structure (EXAFS) experiments and simulations, we report the refined quantitative non trivial structure of size-selected 3 nm in diameter CoP...

Experimental ac-susceptibility curves at different frequencies (0.1 Hz ≤ f ≤ 1 kHz) were performed on samples prepared by physical and chemical pathways. By combining the triple fit method and a careful analysis of ac-experimental curves, we demonstrate an unambiguous and consistent determination method of both the magnetic particle size distributi...

We present a method using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) to determine the chemical composition of bi-metallic nanoparticles. This method, which can be applied in a semi-automated way, allows large scale analysis with a statistical number of particles (several hundreds) in a short time. Once a ca...

The magnetic properties of metals are very sensitive to the size of the objects, to their organization when non-isolated, as well as to their chemical content and order when alloyed. In this chapter, first we go through the different magnetic energies in competition and the different length scales of importance in the magnetic behaviour and configu...

We propose a modified Stoner-Wohlfarth model combined with the geometrical approach of the coherent rotation of magnetization for simulating the hysteresis loops of an assembly of magnetic nanoparticles. The temperature and the size distribution are taken into account. This combined model enables the computation of hysteresis loops at low temperatu...

An original physical route is proposed to organize size-selected bimetallic nanoparticles on graphite surfaces. This approach is based on the soft landing of preformed clusters and is demonstrated for bimetallic Au x Pt 1 - x clusters of different compositions. Particle coalescence is avoided thanks to cluster surface reactivity, leading to a self-...

Because of high magnetocrystalline anisotropy in bulk phase, binary nanoalloys from Co 3d and Pt 5d metal are expected to offer the opportunity to obtain room temperature nanomagnets. In this paper, we report a quantitative study combining X-ray absorption and magnetic measurements on well-defined size-selected CoPt clusters assemblies in the 2-4 n...

We report on the magnetic properties of 3 nm cobalt clusters embedded in gold matrix using magnetic investigations. From the analysis of the m_FC, m_ZFC and m ( H ) curves, we determine precisely the Co magnetic diameter Probability Density Function (PDF) in highly diluted samples. We show that even with a 3% vol. concentrations...

We consider diluted assemblies of particles randomly distributed on a surface (2D samples), in a volume (3D samples) or in 2D layers separated with a spacer of thickness t (multilayer samples). Among various considerations on the inter-particle separations, a special interest is given to the nearest-neighbor distance distribution. We also discuss t...

We report on the magnetic properties of cobalt clusters embedded in amorphous carbon using magnetic and structural investigations. From the analysis of the mFC, mZFC, and m(H) curves, we determine the Co magnetic diameter probability density function. An initially magnetically dead interface layer is attributed to a metastable carbide. We found tha...

Spontaneous organisation of preformed size selected platinum and gold clusters on graphite is discussed in terms of reactivity with residual gases. The difference in reactivity between gold and platinum can explain the spontaneous organisation in an high vacuum for platinum and poor vacuum for gold. Contrary to earlier assumptions (Friedel oscillat...

Efforts are currently under way to produce nanoparticle samples with a size dispersion as low as possible, using deposition of preformed clusters on a substrate. However, even for diluted cluster films, contact or coalescence between neighboring clusters cannot be totally avoided due to the random nature of the deposition process. Consequently, the...

Thin films (submonolayer) obtained by deposition of size-selected CoPt clusters on graphite surface have been synthesized. The preformed clusters can easily diffuse on the surface and gather to form "islands" or "bunches" of clusters. By changing the cluster size (in the range of 1.8 to 5.8 nm in diameter), very different morphologies can be obtain...

We investigate the consequence of the dimension reduction on the magnetic anisotropy of FePt and CoPt nanoparticles. Using an extension of the magnetic anisotropy model of N\'eel, we show that, due to a statistical finite size effect, chemically disordered clusters can display a magnetic anisotropy energy (MAE) as high as 0.5\times10^6 J/m3, more t...

Abstract unavailable.

Starting from the theoretical results established in Tournus and Bonet (2010 [1]) to describe ZFC/FC (zero-field cooled/field cooled) susceptibility curves, we examine the limitations of the widely used two states model (or abrupt transition model) where the magnetic particles are supposed to be either fully blocked or fully superparamagnetic. This...

We study a model system made of non-interacting monodomain ferromagnetic nanoparticles, considered as macrospins, with a randomly oriented uniaxial magnetic anisotropy. We derive a simple differential equation governing the magnetic moment evolution in an experimental magnetic susceptibility measurement, at low field and as a function of temperatur...

Ambient pressure photoelectron and absorption spectroscopies were applied under 0.2 mbar of O2 and H2 to establish an unequivocal correlation between the surface oxidation state of extended and nanosized PtCo alloys and the gas-phase environment. Fundamental differences in the electronic structure and reactivity of segregated cobalt oxides were ass...

We have synthesized diluted samples of CoPt clusters and investigated by various techniques the effect of a 2 h anneal at 750 K. Transmission electron microscopy observations have put into evidence L10 chemical ordering without any detectable coalescence upon annealing. Magnetic measurements on CoPt clusters embedded in amorphous carbon have been u...

Even if efforts are currently made to produce nanoparticle samples by deposition of preformed clusters with a size dispersion as low as possible, the incident particle size distribution is necessarily degraded because of the statistical formation of multimers. Here we study diluted Co cluster samples synthesized by mass-selected low energy cluster...

We discuss the possibility of L10 chemical order parameter quantification for an individual particle of CoPt, using transmission electron microscopy. While ``usual'' approaches are found to be unapplicable for small particles (less than 4 nm in diameter), we present a method based on the comparison between an experimental high-resolution image and...

The reduction and oxidation of carbon-supported cobalt nanoparticles (3.50±0.22 nm) and a Co (0001) single crystal was investigated by ambient pressure X-ray photoelectron (APPES) and X-ray absorption (XAS) spectroscopies, applied in situ under 0.2 mbar hydrogen or oxygen atmospheres and at temperatures up to 620 K. It was found that cobalt nanopar...

The formation of thin films by deposition of size-selected platinum and gold clusters on graphite surfaces is analyzed. By precise adjustments of the environment of the clusters, their size, and their nature, the cluster reactivity is evidenced as a parameter to control thin-film morphologies. Thus, by adjusting the kinetics of species adsorption (...

The magnetic anisotropy energy distribution of a diluted assembly of CoPt nanoparticles, size-selected around a diameter of 3 nm, is characterized by superconducting quantum interference device magnetometry. By comparison with unselected CoPt clusters and size-selected Co clusters, we show experimental evidence of a significant anisotropy constant...

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The simultaneous triple adjustment of experimental magnetization curves under different conditions is shown to allow the unambiguous and consistent determination of both the magnetic particle size distribution and anisotropy for granular nanostructures of Co clusters embedded in protective matrices. The importance of interface effects resulting in...

Due to the huge magnetocrystalline anisotropy of bulk CoPt crystallized in the L10 phase, CoPt nanoparticles have been widely studied during the last decade. In order to determine the intrinsic magnetic properties of CoPt clusters, we synthesize benchmark samples: 3 nm diameter CoPt clusters, pre-formed in the gas phase, are embedded in an amorphou...

Various contributions to the magnetic anisotropy energy (MAE) dispersion for an assembly of CoPt nanoparticles are examined, using the empirical Neel anisotropy model. It is shown that, while small shape and composition variations have negligible effects, the statistical distribution of chemical arrangements can be the major source of MAE dispersio...

We report the synthesis and characterization of CoPt nanoparticles, using X-ray magnetic circular dichroism (XMCD) at the Co L2,3 edges. Clusters are produced in ultra-high vacuum conditions, following a physical route, and embedded in non-metallic matrices: MgO and amorphous carbon (a-C). In MgO, Co atoms are partially oxidized, which goes with a...

Novel applications in catalysis rely on the design of tailored nanoarchitectures. In this field, we present a new physical route, based on ultrahigh vacuum deposition of size-selected preformed clusters, leading to self-organization of platinum nanoparticles on the surface. The resulting array of “model” nanoclusters (i.e., size-selected and ligand...

We report the synthesis and characterization of well-defined CoPt clusters with a mean diameter of 3 nm, produced in ultrahigh vacuum conditions following a physical route. Samples made of diluted layers of CoPt clusters embedded in amorphous carbon have been studied by transmission electron microscopy. High-resolution observations have revealed th...

Because of its superior properties silicon carbide is one of the most promising materials for power electronics, hard- and biomaterials. In the solid phase, the electronic and optical properties are controlled by the stacking of double layers of Si and C atoms. In thin films, a change in the stacking order often requires stress, which can be achiev...

Model and real cobalt-platinum alloy clusters are compared in terms of structure, composition and segregation. Canonical and semi grand canonical Metropolis Monte Carlo simulations are performed to model these clusters, using embedded atom (EAM) and modified embedded atom (MEAM) potentials. All of them correctly predict the bulk L12 Co3Pt and CoP...

The π-stacking interaction between various planar organic molecules is investigated within the framework of ab initio calculations. The adsorption of these molecules on the sidewall of the cylindrical carbon structure induces a small binding energy compared to conventional covalent functionalization. Such a weak interaction is found to be only phys...

We have pushed the limits of X-ray absorption spectroscopy under high pressure in order to study the stability of the Ba intercalated silicon clathrate. EXAFS experiments at the Ba K-edge have been performed in the step by step mode up to a pressure of 14.5 GPa using the Paris–Edinburgh cell with sintered diamond anvils. XANES experiments at the Ba...

The in situ x-ray photoelectron spectroscopic and density-functional studies of Si atoms adsorbed on a C 60 film were analyzed. The XPS measurements were performed in situ, in ultrahigh vacuum, on a C 60 film obtained by sublimation and subsequently exposed to a Si-atom beam. It was found that the C 20 isomer was the most favorable, with a binding...

The adsorption of a benzene molecule on carbon nanotubes (CNTs) with various diameters and chiral angles is investigated within the ab initio framework. The physisorption of such an organic molecule is an example of noncovalent functionalization involving π-stacking interactions and corresponding to a weak binding energy. Our calculations show that...

The orientational dependence of the interaction between two C(60) molecules is investigated using ab initio calculations. The binding energy, computed within density functional theory in the local density approximation, is substantially smaller than the one derived from the experimental heat of sublimation of fullerite, which calls into question th...

We investigated the atomic bonding of Si and C60 theoretically (by ab initio calculations within the local density approximation to the density functional theory) and compared the predictions to experimental X-ray absorption results from SiC60 films synthesized by the cluster beam deposition technique. Calculations predicted that Si preferentially...

We investigate the bonding of Si and C60 both theoretically (by ab initio calculations within the local density approximation to the density functional theory) and experimentally. Starting from the laser ablation of Si and C60 followed by a supersonic expansion, we first show that it is possible to obtain small stable (C60)nSim clusters (with n≈m)....

We study both theoretically and experimentally the structure of the doped silicon clathrate II Na x Si 34 . We find that contrary to published works, the sodium atoms do not retain the T d symmetry inside the Si 28 cages and move about 1 Å away from the center of the cage. This displacement, in conjunction with that of a sodium atom in an adjacent...

Composition du jury : M. BROYER, président, professeur au LASIM (Université Lyon 1). J.-C. CHARLIER, rapporteur, chercheur qualifié FNRS au PCPM (Université Catholique de Louvain, Belgique). P. LAGARDE, rapporteur, directeur de recherche CNRS au LURE (Orsay). C. COLLIEX, examinateur, directeur de recherche CNRS au LPS (Orsay). J.-L. HODEAU, examina...

We investigate the binding of Si and C 60 theoretically (by ab initio calculations within the local-density approxi-mation to the density functional theory) and confront the predictions to experimental X-ray absorption results from Si– C 60 films synthesized by the cluster beam deposition technique. The calculations predict that Si preferentially b...

(C60)mNan clusters are produced in a tandem laser vaporization source and analyzed by photoionization and photofragmentation time-of-flight mass spectroscopy. At low sodium coverage, the special behavior of (C60)m=1,2Nan clusters (n £ 6m)(n\leq 6m) is consistent with a significant electron transfer from the first six adsorbed atoms towards each of...

We report on the energetics of C60–Si clusters. By means of ab initio calculations based on the local density approximation to the density functional theory, we have investigated stable and metastable structures of C60–Si, C60–Si–C60, and (C60–Si)2 clusters. In each case, we show that silicon preferentially binds to C60 over a carbon–carbon double...

(C60)nCq+ cationic clusters are produced in a laser vaporization source by quenching the vapors from C60 and graphite targets. They are analyzed in the gas phase by abundance and photofragmentation time-of-flight mass spectroscopy. Among the large number of expected isomers, the present experiments give evidence for the stability of special arrange...

We discuss the high pressure properties of different silicon clathrate structures that we have investigated by means of X-ray diffraction and ab initio calculations. Compressibility, transition pressures or phase transformations are interpreted as a function of the nature of the guest atom intercalation. The compressibility of the clathrate structu...

We report the three-dimensional packing of C60 clusters stabilized by the addition of Si. X-ray absorption spectroscopy reveals that Si atoms are in an unusual environment: between two C60, with ten or more carbon neighbors. According to ab initio calculations, the cohesive energy is about 2 eV per Si atom, much higher than the Van der Waals bindin...

We study the behavior under pressure (up to 35 GPa) of intercalated silicon clathrates, combining x-ray diffraction experiments and ab initio calculations. We show that endohedral doping does not introduce a strong modification of the compressibility of the empty clathrate network and that in particular cases can raise it to values equivalent to th...