# High pressure decomposition of a sandwich compound

Samuele Fanetti, Sebastiano Romi, Enrico Berretti, Michael Hanfland, Emin Mijit, Frederico Alabarse, Philip Dalladay-Simpson, Federico Gorelli, Roberto Bini, Mario Santoro

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Samuele Fanetti, Sebastiano Romi, Enrico Berretti, Michael Hanfland, Emin Mijit, et al. High pressure decomposition of a sandwich compound. The Journal of Chemical Physics, 2023, 159 (13), pp.134502. 10.1063/5.0167748. hal-05080000

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# High pressure decomposition of a sandwich compound

Samuele Fanetti ; Sebastiano Romi ; Enrico Berretti ; Michael Hanfland ; Emin Mijit ; Frederico Alabarse ; Philip Dalladay-Simpson ; Federico Gorelli ; Roberto Bini ; Mario Santoro

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J. Chem. Phys. 159, 134502 (2023)

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# High pressure decomposition of a sandwich compound

Cite as: J. Chem. Phys. 159, 134502 (2023); doi: 10.1063/5.0167748

Submitted: 14 July 2023 • Accepted: 18 September 2023 •

Published Online: 3 October 2023

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Samuele Fanetti,<fnref n="1" />,<fnref n="2" /> ID
Sebastiano Romi,<fnref n="3" /> ID
Enrico Berretti,<fnref n="1" /> ID
Michael Hanfland,<fnref n="4" /> ID
Emin Mijit,<fnref n="4" /> ID
Frederico Alabarse,<fnref n="5" /> ID
Philip Dalladay-Simpson,<fnref n="6" /> ID
Federico Corelli,<fnref n="2" />,<fnref n="3" />,<fnref n="6" /> ID
Roberto Bini,<fnref n="1" />,<fnref n="2" />,<fnref n="3" />,<fnref n="7" /> ID
and Mario Santoro<fnref n="2" />,<fnref n="3" />,<fnref n="4" /> ID

## AFFILIATIONS

## ABSTRACT

While it is widely recognized that purely organic molecular systems with multiple bonds undergo chemical condensation at sufficiently high pressures (from tenths to tens of GPa), the fate of organometallics at extreme conditions remains largely underexplored. We have investigated the high pressure (up to 41 GPa) chemical transformations in a simple molecular system known as nickelocene, $(C_5H_5)_2Ni$, which serves as a representative example of a class of organometallics called sandwich compounds. Nickelocene decomposed above 13 GPa, at room temperature, while lower pressure thresholds have been observed at higher temperatures (295–573 K). The products were identified as nanocomposite materials, primarily composed of disordered, nickel-rich nanoparticles segregated within an extended, amorphous matrix of hydrogenated carbon (a-C:H). The investigation was conducted by means of diamond anvil cells in combination with optical spectroscopies and microscopy, synchrotron x-ray absorption spectroscopy and diffraction, as well as transmission electron microscopy. Our findings have the potential to stimulate further research into the high-pressure chemical reactivity of organometallics and open up new synthesis routes for the production of metal-based nanoparticles, which find a wide range of applications.

© 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0167748

## I. INTRODUCTION

In recent decades, doubly and triply bonded, purely organic molecular systems subjected to high pressures of several tenths to tens of GPa have been shown to undergo chemical condensation, that is to say oligomerization and polymerization.<fnref n="1" />⁻⁸ This is now a quite general and well assessed behavior for this class of systems, which has been traced back to the reconstruction of the π chemical bonds that occurs at high enough densities for the inter-molecular and intra-molecular distances to become comparable. On the other hand, the high pressure chemical reactivity in organometallic

systems is much less explored. Here, metal ions and organic ligands make the bonds at least partially ionic. This modified bond scheme with respect to purely organic substances may significantly change the high pressure reactivity. For instance, whether chemical condensation or rather decomposition takes place in highly densified organometallics, and how these two fates depend on pressure is certainly something worth investigating. An interesting ensemble in this respect, made of relatively simple molecules, is the class of metallocenes, molecular compounds formed by two cyclopentadienyl anions $(C_5H_5)$ bound to a metal center (M) in the oxidations state II: $(C_5H_5)_2M$. Currently, there are very few studies on metallocenes

<footnote n="1">¹Consiglio Nazionale delle Ricerche - Istituto di Chimica dei Composti OrganoMetallici, CNR-ICCOM, Via Madonna del Piano 10, 50019 Sesto Fiorentino, FI, Italy</footnote>

<footnote n="2">²European Laboratory for Nonlinear Spectroscopy, LENS, via Nello Carrara 1, 50019 Sesto Fiorentino, FI, Italy</footnote>

<footnote n="3">³Consiglio Nazionale delle Ricerche - Istituto Nazionale di Ottica, CNR-INO, Via Nello Carrara 1, 50019 Sesto Fiorentino, FI, Italy</footnote>

<footnote n="4">⁴European Synchrotron Radiation Facility, ESRF, 71 Avenue des Martyrs, CS40220, 38043 Cedex 9 Grenoble, France</footnote>

<footnote n="5">⁵Elettra Sincrotrone Trieste S.C.p.A, in AREA Science Park, 34149 Basovizza TS, Italy</footnote>

<footnote n="6">⁶Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, China</footnote>

<footnote n="7">⁷Dipartimento di Chimica "Ugo Schiff," Università di Firenze, via della Lastruccia 3, 50019 Sesto Fiorentino, FI, Italy</footnote>

<footnote>a) Author to whom correspondence should be addressed: santoro@lens.unifi.it</footnote>
under extreme pressures, limited, as far as we know, to ferrocene, nickelocene and ruthenocene.⁹⁻¹⁸

The high pressure heating of ferrocene, at pressures of 4–8 GPa and temperatures up to 1600 K, led to decompose the molecular substance and to form quasi-spherical, crystalline iron carbide nanoparticles encapsulated in carbon shells, the typical size of which increased from a few nanometer up to 100 nm upon increasing the temperature,⁹⁻¹³ while the systematic fate of the pristine hydrogen does not seem to be entirely clear. These pioneering studies on the high P–T transformation of a metallocene added to the plethora of high-energy synthesis protocols of metal-based nanoparticles from organometallic/hydrocarbon mixtures previously limited to nearly ambient pressure, as discussed in the same works. On the other hand, the room compression of ferrocene to at least 40 GPa, investigated by *in situ* x-ray diffraction (XRD) methods, has been found not to induce chemical reactions of any sort in this substance, which instead was entirely left in its molecular crystal state.¹⁴,¹⁵ Recently, density functional theory calculations, assisted by evolutionary crystal structure prediction algorithms, have found that ferrocene should remain molecular up to 220 GPa whereas polymerization should occur above this pressure.¹⁶ Yet these predictions did not take into account potential decomposition, which could instead occur as indicated by previous results on the pyrolysis of ferrocene at several GPa. However, since in the present work we aimed to investigate potential chemical transformations in metallocenes at pressures of tens of GPa, at room and high temperatures, and test whether polymerization or decomposition will occur, we choose to focus on nickelocene as it should be substantially less chemically stable than ferrocene. On the other hand, high pressure studies on nickelocene are much more limited. High pressure vibrational infrared (IR) absorption spectroscopy investigations on solid nickelocene, along with ferrocene and ruthenocene up to 11–12 GPa, at room temperature, showed that these compounds are chemically stable over this pressure range and provided some insight into the intermolecular interactions between molecules within the unit cell.¹⁷ Subsequent electronic visible absorption measurements at similar pressures were focused on the degree of covalency for these compounds.¹⁸ Recent structural studies on nickelocene in the GPa pressure range investigated the relationship between lattice-strain and molecular conformation and disorder in this compound and compared it with the case of ferrocene.¹⁹

In the following paragraphs, we will present an extensive investigation on the chemical reconstruction of nickelocene at pressures up to 41 GPa, conducted by means of diamond anvil cells (DACs) and six distinct *in situ* materials characterization techniques: IR and Raman spectroscopy, optical microscopy, synchrotron x-ray absorption spectroscopy (XAS) and x-ray diffraction (XRD), and transmission electron microscopy (TEM). Our main outcome is that nickelocene undergoes a progressive pressure induced decomposition above 13 GPa at room T, while we found that this pressure threshold decreased upon increasing temperature up to 573 K. The products of decomposition were recovered at ambient conditions. These materials were found to be nanocomposite materials made by two main components. One component consists of nickel-rich nanoparticles (NiRichNPs). These particles are disordered rather than crystalline, at variance with those obtained from ferrocene at lower pressures and much higher temperatures.⁹⁻¹³ The second component is an extended, amorphous hydrogenated carbon

(a-C:H) matrix, which segregates the nanoparticles. We think our findings could trigger future research on the high pressure chemical reactivity in metallorganics, a potentially large but underexplored field. Last but not least, we have extensively characterized new protocols for the synthesis of metal-based nanoparticles, which are of great interest for a large number of applications such as those in catalysis, biology, biomedicines, electronic and optoelectronic devices among others.¹³,²⁰⁻²²

## II. EXPERIMENTAL METHODS

Polycrystalline nickelocene ($C_{10}H_{10}Ni$, from Sigma Aldrich) was loaded into our DACs²³ at several GPa with no pressure transmitting medium using a glove box, since this substance is not chemically stable in air. The cells were equipped with type IIA diamonds with culet diameter of 250–300 µm and the stainless steel gaskets had holes with an initial diameter and thickness of about 100 and 30 µm, respectively. For the XRD measurements, a gold chip was added as the pressure sensor based on the gold equation of state.²⁴ For optical measurements, pressures were obtained from the pressure shift of the high-frequency edge of the Raman peak of diamond.²⁵

IR absorption spectra were measured by a Fourier transform IR spectrometer (Bruker IFS-120 HR) equipped with a globar lamp, KBR beam splitter, MCT and InSb detectors, and an optical beam condenser based on ellipsoidal mirrors, which provided a natural beam spot size of several hundred microns.²⁶ The frequency resolution was set to 1 cm⁻¹. Raman spectra were measured using a very efficient custom-made micro-Raman set-up²⁷ equipped with a set of Bragg gratings notch filters (spectral bandwidth, FWHM < 0.3 nm), with laser (λ = 660 nm) power limited to a few mW (<1 mW for recovered samples in air), over a spot diameter of 2–3 µm, in order to prevent laser damage as much as possible. In fact, at least at ambient conditions, nickelocene is highly photoreactive. Typical acquisition times were around 5 min in DACs and 25–50 min for recovered samples. The microscope of the Raman set-up was also used for visual observations. Nickel K-edge (8.33 KeV) x-ray absorption spectra (XAS) were measured in energy scanning mode on the recovered samples at the BM23 beam-line of the ESRF, the details of which are reported elsewhere (see Ref. 28 and references therein). Synchrotron angle-dispersive, powder XRD measurements in DACs were performed at two beam-lines: ID15B-ESRF and Xpress-Elettra. We used x-ray wavelengths of 0.495 668 Å (Xpress, wavelength calibration performed via the method of Ref. 29) and 0.410 271 Å (ID15B), large area PILATUS 3S 6M (Xpress) and EIGER2 X CdTe 9M (ID15B) flat panel detectors from DECTRIS, and x-ray spot sizes of 50–80 µm (Xpress) and 10 µm (ID15B). The diffractometer was calibrated using a CeO₂ powder (Xpress) and silicon powder (ID15B) standard and the 2D diffraction patterns were analysed and integrated using Dioptas.³⁰

Transmission Electron Microscopy (TEM) investigations on recovered samples, after the high pressure synthesis, were conducted using a Thermo Fisher TALOS F200X G2 TEM microscope (see also in the supplementary material). High resolution (HR)-TEM images, scanning TEM (STEM) images and Energy Dispersive x-ray emission spectroscopy (EDS) elemental maps were acquired using a 200 kV accelerating voltage. Before the TEM investigation, samples
were thinned using the 30 kV Ga ionic beam of a TESCAN GAIA 3
FIB/SEM device.

### III. RESULTS

#### A. IR absorption spectroscopy

In order to characterize potential chemical modifications in
nickelocene, we first exploited IR vibrational spectroscopy as this
is a very powerful tool for detecting chemical transformations in
molecular materials. In Fig. 1, we report selected IR absorption spec-
tra of nickelocene measured upon increasing pressure to 41 GPa,
at room temperature, and subsequently releasing pressure to the
ambient value. At the starting pressure of 5.8 GPa, the spectrum
shows nothing but the sharp vibrational peaks of molecular nick-
elocene. After that, these peaks exhibit an overall positive pressure
shift. At pressures above 16 GPa, the molecular peaks progressively
weaken while they are substituted, above 20 GPa, by new weak
and broad bands, which progressively intensify and signal a major
chemical transformation of nickelocene into a likely amorphous
material. In particular, a new broad peak appears and intensifies at
around 3000 cm⁻¹, which we easily assign to C–H stretching modes
with C in sp³ hybridization and which consequently shows the dis-
ruption of the π bonds for the new material. Long-term, residual
kinetics were also observed for this transformation by comparing
spectra measured at 27.9 GPa over a time interval of 30 days. Above
this pressure, the sharp molecular peaks were no longer detected,
while the spectrum showed only the broad bands of the product

material, indicating that the transformation was complete and no
other changes occurred up the maximum pressure of 41 GPa. Upon
decreasing pressure down to ambient conditions, the transformation
was not reverted, as shown by the IR spectrum of the new material
which just underwent general band narrowing, but rather moderate.
We note that, simultaneously with the pressure induced changes in
the vibrational IR spectrum, the sample became irreversibly more
and more opaque under visual observations (Fig. 2). The far low-
frequency edge of this strong visible absorption extended into the
mid IR spectral range providing an intense background to the vibra-
tional peaks, which made measurements of the C–H stretching peaks
above 28–30 GPa difficult. Upon reducing pressure to a few GPa,
the background reduced and these peaks were again well detectable,
particularly when a more efficient detector (InSb) than the MCT was
used. Interestingly, for the recovered material we observe two poorly
resolved C–H stretching peaks at 2880 and 2925 cm⁻¹, respectively,
entirely assigned to diamond-like carbon in sp³ hybridization, show-
ing that the hydrocarbon component of the synthesized material is
substantially free from sp² carbon. For comparison, in Fig. 1, we
also report an IR spectrum for benzene-derived amorphous, hydro-
genated carbon (a-C:H), which was obtained under similar high
pressure conditions, as reported in a previous work.³¹ The similar-
ity between the spectrum of the present amorphous hydrocarbon
material and the spectrum of the sample obtained from benzene is
striking, except that the benzene-derived sample contained some sp²
carbon, as shown by a high-frequency shoulder in the C–H stretch-
ing band, positioned above 3000 cm⁻¹. This comparison immedi-
ately qualifies our hydrocarbon form as an a-C:H with almost pure
diamond-like hybridization for carbon. These spectroscopic results
are a first indication that the compressed nickelocene, very likely,
decomposed into an a-C:H form and a segregated, nickel-rich (with
respect to nickelocene) form, which remains to be characterized.
This transformation could be a redox reaction, with the pristine
anionic organic substance and cationic nickel being oxidized and
reduced, respectively.

<figure><img src="image_10.png" /><figcaption>FIG. 1. IR spectra of nickelocene (sample NICK5) measured upon increasing pressure from 5.8 to 41 GPa and subsequently decreasing it to the ambient value, from bottom to top, showing the disappearance of the sharp molecular peaks and the formation of non-molecular broad bands. Correspondingly, C undergoes hybridization changes from sp² to sp³. The upper two blue (recovered) and magenta (6.0 GPa) spectra have been measured with the InSb detector, while for all the others we used the MCT. Orange spectrum (top): amorphous, hydrogenated carbon (a-C:H) obtained from compressed benzene.³¹ Stars indicate peaks of residual, unreacted molecular benzene. Spectra have been vertically shifted for the sake of clarity. The white gap around 2000 cm⁻¹ extends over the spectral region of the two-phonon IR absorption band from diamond anvils.</figcaption></figure>

#### B. Visual observations

In Fig. 2, we report selected pictures of a nickelocene sam-
ple under pressure (NICK6, the same sample used for the Raman
spectroscopy investigation, see below), obtained through a micro-
scope in combined transmitted and reflected light. The pressure was
increased from 2.2 up to 36.9 GPa and then reduced to the ambient
value. The recovered sample, a sliver of which protrudes vertically,
has been photographed in the free standing gasket after having
removed it from the DAC. We see that, while nickelocene is deep
green at ambient conditions, it rapidly and irreversibly becomes fully
opaque with increasing pressure until it can only be observed in
reflected light. At around 20 GPa, the sample exhibits both dark
areas and partially reflecting domains, also depending on the ori-
entation with respect to the incident light, and its appearance does
not further change significantly upon increasing and subsequently
decreasing pressure. The partially reflective and opaque nature of
the newly synthesized material parallels IR spectroscopy in indicat-
ing that nickelocene decomposed and a nickel-rich phase is likely to
have formed, together with some carbon.
<figure><img src="image_11.png" /><figcaption>FIG. 2. Selected pictures of a nickelocene sample (NICK6, diameter of about 100 µm) under pressure, obtained through a microscope in combined transmitted and reflected light. The pressure was increased from 2.2 up to 36.9 GPa and then reduced to the ambient value, from top-left to bottom-right. The recovered sample has been photographed in the free standing gasket; the black circular spot in the bottom side is a damage due to the CW red laser used for Raman spectroscopy which, in one single case, was used at the full power of about 100 mW. The type of light illumination used is consistent across all rows. However, the exposure time of the camera is different and longer in the second and third rows. This adjustment was necessary to effectively accentuate the finer details of the sample, particularly given its notably dark appearance.</figcaption></figure>

### C. Raman spectroscopy

Raman spectroscopy measurements were conducted to complement vibrational IR spectroscopy. In Fig. 3, bottom panel, we report selected Raman spectra of nickelocene under pressure for a sample whose pictures are reported in Fig. 2. Pressure has been increased from 2.2 to 36.9 GPa, and then decreased to the ambient value. At the initial pressure of 2.2 GPa, we observe sharp Stokes and anti-Stokes lattice phonon peaks of the molecular nickelocene crystal around the residual Rayleigh line, below $400 \text{ cm}^{-1}$, and sharp peaks for the internal molecular modes in the 700–1500 $\text{cm}^{-1}$ interval similar to the IR spectra. Upon increasing pressures, rapidly the lattice phonons and the internal mode peaks vanish, already in the pressure range of a few GPa, while they are substituted by broad bands showing a major transformation in the initial molecular crystal toward a likely non-molecular, amorphous material. The new Stokes and anti-Stokes bands positioned below $400 \text{ cm}^{-1}$ clearly show the glass-like nature, i.e. the amorphous character of this material. Incidentally, we note that at several GPa, upon increasing pressure, the formation of a small amount of bulk disordered graphite (D and G peaks) from the molecular nickelocene was inevitably induced by laser irradiation, even using very low laser power densities (these graphite impurities are completely undetectable to direct visual inspection), while above 10 GPa the sample was found to be stable against laser irradiation. At each pressure, we measured the Raman spectrum at a different point and indeed the peaks of bulk disordered graphite were no longer observed above 10 GPa. This marginal photo-induced process is very

unlikely to significantly alter the intrinsic pressure threshold for the decomposition of nickelocene described in this paper. Yet, the large fluorescence background together with the relatively poor S/N ratio of the Raman spectra make Raman investigations not appropriate to identify this threshold with reliable accuracy. In Fig. 3, top panel, we report the spectra of recovered samples from several isothermal compression runs, to similar maximum pressures, at room and high temperatures, up to 482 K. After subtraction of a broad and smooth luminescence background and despite the different synthesis temperatures, the spectra are very similar and they appear to be dominated by the glass-like diffuse Stokes peak below $300 \text{ cm}^{-1}$ and by two broad, partially overlapping bands, in the 1000–1700 $\text{cm}^{-1}$ frequency range, which formed upon reducing pressure below several GPa. While these two bands differ from those of bulk disordered graphite, they are very similar to the spectrum of segregated, disordered graphitic carbon domains observed in $Ni_{1-x}C_x$ nickel carbide thin film nanocomposites,³² except for a frequency shift of a few tens of wavenumbers. We therefore suggest that our two bands also stem from disordered graphitic carbon nano-domains which, in our case, come from the decomposition of nickelocene. This decomposition results into a diamond-like a-C:H matrix (identified by IR spectroscopy), incorporating disordered graphitic carbon nano-domains together with, most likely, a segregated nickel-rich phase.

### D. X-ray absorption spectroscopy

In order to directly identify the possible segregation of nickel in our high pressure processed materials, we measured synchrotron
<figure><img src="image_12.png" /><figcaption>**FIG. 3.** Bottom: selected Raman spectra of nickelocene under pressure (sample pictures in Fig. 2), showing the disappearance of the sharp molecular peaks and the corresponding formation of non-molecular broad bands. Pressure was increased and then decreased from bottom to top. D and G label the peaks of free disordered graphite, which was formed by laser irradiation below 10 GPa, upon increasing pressure. Particularly, the D peak is the very weak and broad band centered roughly at 1350 cm⁻¹, visible only at 5.4 GPa in the high-pressure spectra, on which the sharp, off-scale diamond peak from the anvils overlaps. Top panel: spectra of recovered samples from several isothermal compression runs, at room and high temperatures. Labels indicate the treatment temperature and the maximum pressure for each sample. A broad and smooth luminescence background has been subtracted from these spectra. The orange spectrum has been reproduced from literature.³² In both panels, the spectra have been vertically shifted for the sake of clarity.</figcaption></figure>

nickel K-edge XAS spectra on the ambient pressure recovered samples. Particularly, we aimed to look at possible shifts in the energy position of the absorption edge, signalling potential changes in the oxidation state of nickel with respect to the pristine molecular nickelocene. In Fig. 4, top panel, we report the XAS spectra for three distinct recovered samples after compressions to 35–41 GPa, and labelled as in Fig. 3, compared to those of pristine nickelocene and of a pure nickel foil. Measurements have been performed on samples in the gasket, after having removed the gasket from the DAC.

<figure><img src="image_13.png" /><figcaption>**FIG. 4.** Top panel: nickel K-edge XAS spectra of pristine molecular nickelocene, of three recovered non-molecular samples after isothermal compressions to 35–41 GPa, labelled NICK3, NICK4 and NICK5 as in Fig. 3, and of an elemental nickel foil. Measurements have been performed on samples in the gasket, after having removed the gasket from the DAC. Bottom panel: first derivative of the XAS spectra shown over a reduced energy range.</figcaption></figure>

Pristine nickelocene was pelleted in a typical DAC gasket. All the spectra have been normalized to the same value of the jump in the absorbance across the K-edge and the white line, here rescaled to 1. In the bottom panel, we report the first derivative of the same XAS spectra shown over a reduced energy range, which we used for determining the value of the absorption edge. The spectrum of pristine nickelocene differs from that of elemental nickel in many respects. Particularly, the K-edge in nickelocene is chemically shifted by +7.8 eV with respect to the value of pure elemental nickel: 8332.8 eV, due to the oxidation number of nickel being increased to +2 in the molecular compound. On the other hand, the spectra of the recovered materials differ substantially from that of nickelocene, while agreeing very well with each other. In particular, the K-edge is clearly downshifted. Looking at the first derivative spectra we observe, for these materials, a lowest-energy sharp peak positioned
at 8332.4 eV, which is entirely consistent with the K-edge peak of elemental nickel. Then, at a higher energy of about 4.0 eV, we observe a weak extra-intensity with respect to pure nickel. This spectral feature has intermediate energy between the Ni K-edge peak of elemental nickel and that of nickelocene and may indicate the simultaneous presence, in our processed samples, of some form of nickel with formal intermediate oxidation state such as nickel carbide. In short, the high pressure transformation highlights a redox reaction. In this reaction, the oxidation state of nickel is reduced, which in turn supports the formation of segregated nickel-rich domains of some size, likely incorporating both elemental nickel and nickel carbide local structures. Correspondingly, the pristine anionic hydrocarbon component of the material should be oxidized toward a neutral state, which is consistent with the a-C:H form identified through IR and Raman spectroscopy. Furthermore, we note that the XAS spectra of the recovered samples above the K-edge look to be much smoother than that of elemental nickel, clearly indicating that the nickel-rich domains are disordered.

## E. X-ray diffraction

As a direct structural probe on the high pressure transformation of nickelocene, we performed *in situ* synchrotron XRD measurements along different P–T paths. In Fig. 5, top panel, we report the XRD patterns of nickelocene measured upon increasing pressure up to 35 GPa, at room temperature, with an average pressure step of 0.7 GPa. The Bragg peaks of the pristine crystalline molecular nickelocene progressively weaken and they are hardly detectable above 14 GPa. Particularly, the strongest diffraction peaks observed in the 1.1–1.9 Å⁻¹ interval progressively collapse into a weak and diffuse peak at about 1.35 Å⁻¹, which in turn weakens and finally disappears upon increasing pressure further. On the other hand, at about 13–14 GPa a new diffuse peak appears, weak and with a small-angle, located approximately at 0.6 Å⁻¹, the intensity (Q position) of which increases (decreases) at higher pressures. While we straightforwardly assign the first diffuse peak to amorphous, molecular nickelocene, the small-angle x-ray scattering (SAXS) diffuse peak appears to have a different origin. We suggest tracing this peak to the target size-dependent diffraction produced by nanometer-sized objects that form under pressure. Likely, these objects are the nickel-rich nanoparticles corresponding to the segregated (in a-C:H) amorphous form of nickel, which we introduced above. The Q-position of the SAXS peak is then inversely proportional to the average radius of these particles, while its Q-width reflects the statistical size distribution of the particles. Therefore, our XRD measurements show that nickelocene undergoes a progressive pressure induced amorphization and also, starting at 13–14 GPa, it decomposes, forming NiRichNPs embedded in a-C:H. The decomposition is complete at our maximum pressure. In Fig. 5, bottom left panel, we report the zoomed SAXS peak for a number of recovered products in the gasket, removed from the DAC, synthesized along several isothermal compression runs to close maximum pressures of 34–41 GPa, at room T, 373, 482 and 573 K. An empty gasket, diffuse scattering background has been subtracted from the patterns. Clearly, the higher the synthesis temperature the lower the Q-position of the peak, pointing to larger NiRichNPs.

In the bottom right panel of Fig. 5, we report the full XRD pattern of a typical recovered sample synthesized at room T along with

<figure><img src="image_14.png" /><figcaption>**FIG. 5.** Top panel: XRD patterns (Elettra, $\lambda = 0.495$ 668 Å) of nickelocene measured upon increasing pressure up to 35 GPa, at room T, with an average pressure step of 0.7 GPa. Gold Bragg peaks from the pressure marker are marked by stars. Bottom left panel: XRD patterns (Elettra, $\lambda = 0.495$ 668 Å, and ESRF, $\lambda = 0.410$ 271 Å) of ambient conditions recovered products (removed from the DAC), synthesized along similar isothermal compression runs, at room and high temperatures. An empty gasket diffuse background has been subtracted. The patterns are shown over a reduced Q range and are labelled by the sample name, the maximum pressure and the treatment temperature. Bottom right panel: pair radial distribution function of a recovered sample: NICK5 (see bottom left panel), the original I(Q) pattern of which is shown in the inset.</figcaption></figure>

the pair radial distribution function (PRDF) of it obtained through a procedure described in elsewhere,³³ which also leads to an estimate of the density. The PRDF shows a clear local structures with two well pronounced peaks at 1.51 and 2.50 Å, respectively. The first peak stems from the carbon-carbon bond distances of the carbon matrix hosting the segregated NiRichNPs. The second peak is likely contributed by both the next nearest neighbour carbon-carbon pairs from the carbon matrix and the nearest neighbour Ni-Ni pairs from the nanoparticles. Indeed, the position of the second PRDF peak is very close to the nearest neighbour Ni-Ni distance for elemental crystalline nickel: 2.492 Å.³⁴ It is worth noting that carbon and nickel are expected to contribute with similar intensities to the local structure XRD signal, since the large Z imbalance between them is
approximately compensated by the fact that the number density of carbon is greater than that of nickel by one order of magnitude, at macroscopic scales. Interestingly, the density of the recovered material was found to be about 80% greater than that of pristine nickelocene, which is clearly due to the extended as opposed to molecular nature of the high pressure processed material.

In Fig. 6, top left panel, we report the pressure shift of the Q-position of the SAXS peak for a selected nickelocene-derived extended material with sample name NICK8. In this case, nickelocene was compressed to 6 GPa, at ambient temperature, and then heated to 573 K in about 1 h, while the pressure was dropping to 3 GPa. Under these conditions, we found that the pristine crystalline nickelocene was already partially transformed as shown by a drop in the intensity of the Bragg peaks, and then we observed the complete loss of the crystalline XRD pattern within a few additional minutes. Simultaneously, we observed the formation of the SAXS peak of the non-molecular decomposition product. We then compressed the material to 35 GPa, at 573 K, and subsequently cooled the sample to room temperature and then decompressed it to ambient pressure. Upon compression, we observed a substantial decrease in the Q-position of the SAXS peak, with a maximum slope around 13 GPa. The pressure shift levelled off at around 35 GPa. On decreasing pressure, the peak downshifted more, but to a much less extent. The total drop in the Q-position along the pressure cicle was about 30% of the starting value. Correspondingly, we infer that the NiRichNPs became larger, likely as a result of further nickel clustering. Similar results were obtained at other synthesis temperatures. In

the top right panel of Fig. 6, we report the time evolution for the Q-position and for the intensity of the SAXS peak for a selected sample: NICK9, at constant pressure and temperature. In this case, nickelocene was compressed to 2 GPa and slowly heated by measuring XRD with 10 K temperature steps, until we observed the emergence of just traces of the SAXS peak, at 543 K and 1 GPa. This temperature was then very close to the threshold temperature at 1 GPa for the formation of the decomposition product. Under these P–T conditions, we measured the XRD patterns vs time. We observed an increase (decrease) in the intensity (Q-position) of the peak with a rise time of several tens of minutes, which tended to level off after 400–1000 min. Simultaneously, the Bragg peaks of the pristine nickelocene progressively vanished, and the transformation was almost complete after 1000 min. These measurements show the kinetic of formation for the NiRichNPs, the number and the size of which increase as a function of time due to nickel clustering from the starting metallocene. In the bottom left of Fig. 6, we report the Q-position of the SAXS peaks and the corresponding NiRichNPs average radius *R*, as a function of the synthesis temperature, for the recovered samples reported in the bottom left panel of Fig. 5. The synthesis pressures were close to each other for all these samples. For the evaluation of *R* we first assumed, to the lowest order of approximation, that all the particles are identical spheres of radius *R* and that they are sufficiently diluted in the a-C:H matrix in order for the inter-particle correlations to be neglected. In this case, the scattered intensity vs *Q* can be calculated analytically, resulting in a function which falls off rapidly upon increasing *Q*, with strong oscillations;

<figure><img src="image_15.png" /><figcaption>FIG. 6. Top left panel: pressure shift of the Q-position of the maximum of the SAXS peak for the nickelocene-derived extended material (sample name NICK8). Top right panel: time evolution of the intensity and of the Q-position of the SAXS peak of the non-molecular material for sample NICK9, at constant pressure and temperature, slightly above the P–T reaction threshold. Bottom left panel: Q-position and corresponding nano-particle average radius *R* as a function of the synthesis temperature, for the recovered samples processed along isotherm paths to close maximum pressures: 35–41 GPa, and reported in Fig. 5. Values for the three samples synthesized at ambient T have been averaged. Gray and red shadowed regions: ±*σ*<sub>R</sub> and ±*σ*<sub>Q</sub> intervals around *R* and *Q* (see text). Bottom right panel, kinetic P–T border for the chemical decomposition of nickelocene.</figcaption></figure>
the Q-position for the maxima and the minima of these oscillations
is inversely proportional to the radius of the spheres.³⁵ We then
identified our SAXS peak with the first peak at nonzero Q of this
function, which led us to evaluate R from the relation: R = 5.76/Q.

On the other hand, more realistically, particle radii are likely to be
statistically distributed around R with some standard deviation σR,
and this ditribution will be reflected into a distribution of individ-
ual SAXS peaks, one for each radius value, which will be enveloped

<figure><img src="image_16.png" /><figcaption>FIG. 7. STEM images of a thin lamella of approximate sizes of 10 × 5 × 0.05 µm³ extracted from a typical nickelocene-derived recovered sample, after the high pressure synthesis at 36 GPa and room T. Top left: panoramic view of the lamella with two highlighted regions: A₁ and A₂. Top right: views of a magnified subregion, Map₁, observed within A₁ and of the corresponding K x-ray emission spectral map for nickel (red). Medium left: magnified view of region A₂ with two highlighted subregions: Map₂ and A₃. Medium right: magnified view of region Map₂ together with the corresponding x-ray emission spectral maps for carbon (yellow), platinum (green) and nickel (red). Bottom left: magnified view of A₃ with a highlighted subregion Map₃. Bottom right: views of Map₃ and of a subregion of it, Map₄, together with the corresponding spectral images for nickel.</figcaption></figure>
by the observed SAXS peak. For estimating σR, we evaluated the half
width at half maximum, σQ, of our SAXS peak, over its high Q side,
and then, after differentiation of the previous expression, obtained
σR as: σR = 5.76σQ/Q². The two quantities σR and σQ are reported
in Fig. 6, bottom left panel. In this figure, we see that the NiRich-
NPs are indeed of nanometric size. The average radius increases
from 14.7 to 26.4 Å upon increasing the synthesis temperature over
the 295–573 K interval, very likely as a result of the potentially
increased mass diffusivity, which in turn is expected to enhance the
decomposition of nickelocene and the resulting clustering of nickel.
We then observe values for σR equal to 45%–48% of the average
radius.

Finally, in Fig. 6, bottom right panel, we report the kinetic P–T border for the chemical decomposition of nickelocene, estimated from the first hints of the SAXS peak observed along isothermal and isobaric paths. Clearly, the higher the temperature, the lower the pressure threshold, due to the enhancing effect of thermally activated vibrational and diffusive molecular motions. We note that there is an apparent inconsistency between the reaction pressure threshold observed through IR spectroscopy and XRD, which led us to estimate a value of about 20 and 13 GPa for this threshold, at room temperature, respectively. We attribute this inconsistency to the different sensitivity of the two investigations in detecting the reacted sample. In particularly, XRD was found to be the most sensitive between the two techniques as the decomposition product was identified with a lower pressure threshold through this method. Therefore, we considered the XRD estimate of the reaction pressure threshold to be the most reliable.

F. Transmission electron microscopy

In order to acquire even more direct evidence on the formation
of NiRichNPs from the high pressure decomposition of nickelocene,
a TEM investigation has been performed on one of the recovered
samples after the high-pressure synthesis (room T, 36 GPa). A thin
lamella of approximate sizes of 10 × 5 × 1 µm³ has been extracted
from the surface of the sample and thinned out to reach a final thick-
ness below 50 nm, using a Ga ionic beam (see in supplementary
material). Prior to the lamella carving, a Pt protective layer was
deposited on the sample surface, in order to minimize the degrada-
tion of the surface. The STEM analysis highlighted the presence of
isolate clusters of nanoparticles in the vicinity of the outer (topmost)
border of the sample (Fig. 7, regions A₁ and A₂ and selected sub-
regions). EDS mapping was performed to both assess the chemical
nature of these particles, and to exclude their formation by a depo-
sition of platinum from the protective layer into superficial voids of
the sample. As visible in all the maps of Fig. 7, we were indeed able to
associate a strong Ni signal to the nanoparticles; this signal is much
stronger than the carbon signal in the spots with higher density of
nanoparticles, in line with the presence of a nano-segregated nickel-
rich phase. No Pt signal was reported below the protective layer
(Fig. 7, medium panels, Map2), while minor Ga presence (below 1,
5% at.) was detected in some spots near the border between these
two zones, probably due to inhomogeneous refining of the lamella.
The STEM images also made it possible to estimate the size distri-
bution of the particles, that spans approximately from 5 to 35 nm
in diameter, with an irregular shape which tends to a sphere for
the smallest entities. Noticeably, the smallest diameters agree very

well with those identified by our SAXS observations, which in turn
were limited to sizes smaller than about 10 nm due to the focused
as opposed to collimated nature of the employed synchrotron
x-ray beam. On the other hand, the STEM could also observe much
larger NiRichNPs, which where not captured by the SAXS. HR-TEM
mode was finally used in the attempt to obtain additional struc-
tural information on the nanoparticles. We detected no diffraction
peaks, thus strengthening the hypothesis on the disordered nature of
the nanoparticles, although a certain degree of amorphization could
have perdured from the lamella refining.

IV. DISCUSSION AND CONCLUSIONS

Our combined results from optical spectroscopy and micro-
scopy, synchrotron x-ray absorption spectroscopy and diffrac-
tion, and transmission electron microscopy show that nickelocene
decomposes at high pressures into disordered nickel-rich nanopar-
ticles segregated into an extended, amorphous hydrogenated carbon
(a-C:H) matrix, which in turn stems from the polymerization of the
organic anions of the pristine substance. The transformation is a
redox reaction between the metal and the organic components of
the metallocene. To a large extent, carbon hybridization in the a-
C:H matrix is of the sp³ type, which makes the matrix diamond-like.
The NiRichNPs are made of elemental nickel together with, likely,
nickel carbide, and their diameter values are statistically distributed
in the range of 3–35 nm. We also observed the presence of dis-
ordered, segregated graphitic carbon nano-domains which may, at
least in part, encapsulate the NiRichNPs. Our extensive mapping of
the high P–T chemical phase diagram of nickelocene resulted into
two main results. First, we determined the kinetic P–T border for
the decomposition. This line has a negative slope, clearly showing
that the reaction is enhanced by the atomic thermal motions, and it
also represents a valuable guide for future attempts to optimize the
formation of the metal-rich nanoparticles. Second, we found that
the size of the particles is enhanced at higher temperatures, at the
same maximum pressures, which is an additional evidence of the
enhancing effect of thermal motions. Once compared with the sim-
ilar high temperature-high pressure decomposition of ferrocene at
the maximum pressure where this substance was investigated,⁹⁻¹³
i.e. at around 8 GPa, it results that nickelocene is much less sta-
ble. As a matter of fact, the decomposition of ferrocene occurred
above 750 K, while the studies on it were conducted up to 1573 K,
whereas nickelocene undergoes decomposition already above 450 K,
at the same pressure. The much lower temperatures of our study
also explain why our metal-rich nanoparticles are substantially
amorphous rather than crystalline.

Our study on nickelocene, together with those on ferrocene,
strongly suggests that high pressure decomposition as opposed to
chemical condensation should be a common behaviour among the
metallocenes. On the other hand, we must specify that what decom-
poses here are the metallic moieties from the hydrocarbon moieties
of the substance, whereas the hydrocarbon fractions rather tend to
polymerize as always observed in the pure hydrocarbon substances
so far. Even more generally, the high pressure reaction behav-
ior of metallocenes: decomposition, followed by clustering of the
metal and polymerization of the hydrocarbon moieties, could be a
common behavior for all the organometallic substances. Yet, this
inference remains to be tested on a sufficiently large set of model systems of this sort. Finally, our study may open the way for new synthesis protocols of metal-based nanoparticles from organometallics, with tailored size and properties, of potential interest for applications in different fields such as catalysis, biology, biomedicines, electronics and optoelectronics just to mention a few.

## SUPPLEMENTARY MATERIAL

Supplemental details on TEM procedures.

## ACKNOWLEDGMENTS

We thank LENS for hosting the research. We acknowledge João Elias F. S. Rodrigues for his help during the XAS measurements at the BM23 beamline of ESRF. We then acknowledge the European Synchrotron Radiation Facility and Elettra Sincrotrone Trieste for providing access to the ID15B and Xpress beam-lines, respectively, and for financial support under the Proposal Nos. CH-5992 (ESRF) and 20220094 (Elettra). We thank the Fondazione Ente Cassa di Risparmio di Firenze for the strong support through the SALUS Grant. We also thank “Ente Cassa di Risparmio di Firenze,” Grant No. 2013.0878, and Regione Toscana POR FESR 2014–2020 for the project FELIX (Fotonica ed Elettronica Integrate per l’Industria), Grant No. 6455 and the Italian MIUR funding Fondo Investi-menti (art. 1 comma 1072 legge 205/2017) - Realizzazione, acquisto, ammodernamento, a manutenzione straordinaria delle infrastrutture di ricerca e dei relativi immobili - CNR Progetto N. 40. Acquisto di un microscopio HRTEM ad ultra-alta risoluzione per la caratterizzazione di materiali per la filiera dell'idrogeno, l'energia e la sostenibilità.

## AUTHOR DECLARATIONS

### Conflict of Interest

The authors have no conflicts to disclose.

### Author Contributions

**Samuele Fanetti:** Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Supervision (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal). **Sebastiano Romi:** Conceptualization (supporting); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – review & editing (supporting). **Enrico Berretti:** Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Writing – review & editing (equal). **Michael Hanfland:** Conceptualization (supporting); Data curation (supporting); Formal analysis (supporting); Investigation (supporting); Methodology (supporting). **Emin Mijit:** Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – review & editing (supporting). **Frederico Alabarse:** Conceptualization (equal); Data curation

(supporting); Formal analysis (equal); Investigation (supporting); Methodology (supporting); Writing – review & editing (supporting). **Philip Dalladay-Simpson:** Data curation (supporting); Investigation (supporting); Methodology (supporting); Writing – review & editing (supporting). **Federico Gorelli:** Investigation (supporting); Methodology (supporting). **Roberto Bini:** Conceptualization (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Writing – review & editing (equal). **Mario Santoro:** Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Supervision (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal).

## DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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