Publications

2026

1. High-pressure synthesis of an arsenopyrite-type polymorph of ReS₂ recoverable to ambient conditions

   Umbertoluca Ranieri, Simone Di Cataldo, James Spender, and Dominique Laniel

   Phys. Rev. Materials, Mar 2026

   Abs DOI

   Transition-metal dichalcogenides have attracted a great deal of attention in the context of two-dimensional materials because of their electronic properties, derived from their layered crystal structures, as well as their exfoliability. Surprisingly, the combination of high pressure and high temperature has been rarely exploited in the study of these systems, although it is expected to be an efficient way of inducing the formation of novel polymorphs. Here, rhenium and carbon disulfide were observed to react at a pressure of 54 GPa in a laser-heated diamond anvil cell to form a hitherto unknown polymorph of rhenium disulfide, denoted as 𝑚⁢𝑃⁢12−ReS2 in the Pearson notation. Its crystal structure was solved and refined using synchrotron single-crystal x-ray diffraction data, revealing that 𝑚⁢𝑃⁢12−ReS2 adopts the arsenopyrite structure type (space group 𝑃⁢21/𝑐). The structure is characterized by an extended three-dimensional framework of corner- and edge-sharing distorted ReS6 octahedra. Raman spectroscopy data further confirm the formation and structure of 𝑚⁢𝑃⁢12−ReS2. Remarkably, upon decompression, 𝑚⁢𝑃⁢12−ReS2 was recovered to ambient conditions and found to be stable in air. Thermodynamic, electronic, and bonding properties of the new phase were also studied computationally within the framework of density functional theory. From these, 𝑚⁢𝑃⁢12−ReS2 is found to be semimetallic at 0 GPa, and to have a lower enthalpy than the previously known ReS2 polymorph from ∼0.5 GPa onwards. The discovery of this new compound warrants further investigations of its physical properties, and may open new possibilities for applications.

2. High-pressure formation and characterization of a boron carbide polymorph featuring bent C–B–C chains

   Akun Liang, Umbertoluca Ranieri, James Spender, Charles Lamb, Sarah Bolton, Bernhard Massani, Ryan Stewart McWilliams, Timofey Fedotenko, Konstantin Glazyrin, Nico Giordano, Jonathan Wright, Florian Trybel, and Dominique Laniel

   Phys. Rev. B, Mar 2026

   Abs DOI

   Boron carbide is a material of choice for multiple industries, e.g., aerospace, as a lightweight structural ceramic due to its high hardness, high melting temperature, and low density. However, its mechanical properties have been observed to radically degrade under shockwave compression, presumably as a consequence of stress-induced phase transitions resulting in its partial amorphization. So far, the physical mechanism underpinning this behavior remains unclear. Here, we report a pressure-induced phase transition in boron carbide occurring between 78 and 90 GPa during static compression in diamond anvil cells, both at room temperature and after quenching from high temperatures. The crystal structure of the new phase was solved and refined via synchrotron single-crystal x-ray diffraction measurements and further investigated by Raman spectroscopy as well as density functional theory calculations. The discovered high-pressure polymorph, 𝑚𝐶⁢60−B13⁢C2, has strong resemblance with the known ambient conditions phase, ℎ𝑅⁢45−B13⁢C2, with the important distinction that the linear C–B–C chain linking B12 icosahedra in ℎ𝑅⁢45−B13⁢C2 is bent in 𝑚𝐶⁢60−B13⁢C2. Such bending of the C–B–C chain has been hypothesized as key to explain boron carbide’s drop in strength, phase transitions, and amorphization. The observed reversibility of the phase transition, as well as the formation of covalent bonds between B12 icosahedra and the bent C–B–C chains, are assessed to decipher the C–B–C chain’s bending importance on the amorphization and mechanical properties of boron carbide.

3. High-Pressure Synthesis of the First Thermodynamically Stable Silver Nitride, AgN₅

   Akun Liang, Henricus R. A. Ten Eikelder, Umbertoluca Ranieri, James Spender, Bernhard Massani, Timofey Fedotenko, Konstantin Glazyrin, Nico Giordano, Eleanor Lawrence Bright, Jonathan Wright, Lan-Ting Shi, Florian Trybel, and Dominique Laniel

   JACS Au, Jan 2026

   Abs DOI

   Being a noble metal, silver is known for its chemical inertness. Molecular nitrogen, due to its extremely strong covalent triple bond, is also typically considered unreactive. It is thus unsurprising that no credible report on the formation of a thermodynamically stable silver and nitrogen compound exists. In this study, we report the synthesis of silver pentazolate (AgN5), achieved through the direct reaction of elemental silver with molecular nitrogen at a pressure of 118(3) GPa and a temperature of 2000(200) K. The crystal structure of AgN5 was determined from synchrotron single-crystal X-ray diffraction (SCXRD) data, revealing it to be comprised of cyclo-N5– anions. Remarkably, this solid’s structure does not correspond to any of the silver nitrides previously predicted. Moreover, density functional theory (DFT)-based enthalpy convex hull calculations demonstrate that this AgN5 compound is the only thermodynamically stable Ag–N solid between 10 and 120 GPa while also providing information on its phonon and electron band structures, including its electronic band gap. Both DFT calculations and SCXRD experimental data yield insights into the stability pressure range of AgN5 upon decompression. This study provides yet another example of the capability of high pressure and high temperature to facilitate unprecedented chemical reactions between elements often assumed to be inert, in turn enabling the formation of novel nitrogen-rich compounds.

2025

1. Hydrogen bond symmetrization in high-pressure ice clathrates

   Lorenzo Monacelli, Maria Rescigno, Alasdair Nicholls, Umbertoluca Ranieri, Simone Di Cataldo, and Livia Eleonora Bove

   Phys. Rev. B, Dec 2025

   Abs DOI

   Hydrogen bond symmetrization is a fundamental pressure-induced transformation in which the distinction between donor and acceptor sites vanishes, resulting in a symmetric hydrogen-bond network. While extensively studied in pure ice—most notably during the ice VII to ice X transition—this phenomenon remains less well characterized in hydrogen hydrates. In this work, we investigate hydrogen bond symmetrization in the high-pressure phases of hydrogen hydrates (H2−H2⁡O and D2−D2⁢O) through a combined approach of Raman spectroscopy and first-principles quantum atomistic simulations. We focus on the C2 and C3 filled-ice phases, using both hydrogenated and deuterated water frameworks. Our results reveal that quantum fluctuations and the interaction between the encaged H2 molecules and the host lattice play a crucial role in driving the symmetrization process. Remarkably, we find that in both C2 and C3 phases hydrogen bond symmetrization occurs via a continuous crossover at significantly lower pressures than in pure ice, without any change in the overall crystal symmetry. These findings provide insight into the quantum-driven mechanisms of bond symmetrization in complex hydrogen-bonded systems under extreme conditions.

2. High-Pressure Synthesis of Two Pb₃(C₃N₆) Polymorphs Featuring Fully Deprotonated [C₃N₆]^6– Melaminate Anions

   Umbertoluca Ranieri, Akun Liang, Charles Lamb, James Spender, Sarah Bolton, Andrey Aslandukov, Bernhard Massani, Ferenc Tasnádi, Pierre-Olivier Autran, James A. D. Ball, Angelika D. Rosa, Bihan Wang, Florian Trybel, and Dominique Laniel

   JACS, Oct 2025

   Abs DOI

   Graphitic carbon nitride (g-C3N4), melamine, and their derivatives are of significant interest due to their diverse structural properties and potential applications in catalysis, energy storage, and materials science. In this study, two new compounds, both with the Pb3(C3N6) chemical formula, were synthesized at pressures of 40(2) to 48(2) GPa through a direct reaction between Pb, C and N2, or Pb and C6N4, in laser-heated diamond anvil cells. Their crystal structures were solved and refined using synchrotron single-crystal X-ray diffraction. The two Pb3(C3N6) polymorphs, namely tP48-Pb3(C3N6) and hP72-Pb3(C3N6), crystallize in the noncentrosymmetric space groups P4̅21m and P6122, respectively. Both solids contain the previously unknown hydrogen-free [C3N6]6– melaminate anion and were experimentally found recoverable to ambient conditions. Density functional theory calculations provide further insights into the structural and electronic properties, and reveal that both compounds are direct narrow band gap semiconductors.

3. Hydrogen hydrates under extreme conditions: Insights into high-pressure phases and implications for planetary interiors

   L. Andriambariarijaona, T. Poręba, S. Di Cataldo, R. Gaal, U. Ranieri, M. Santoro, T. C. Hansen, M. Mezouar, G. Tobie, and L. E. Bove

   Phys. Rev. B, Jun 2025

   Abs DOI

   Hydrogen hydrates, formed from hydrogen and water under high-pressure and low-temperature conditions, exhibit a rich phase behavior governed by intricate intermolecular interactions. Their structural evolution under extreme conditions provides a unique platform to study the interplay between hydrogen bonding, lattice dynamics, and guest-host interactions. This study investigates the complex phase diagram of hydrogen hydrate up to 50 GPa using angle-resolved neutron and x-ray diffraction techniques. We focus on the behavior of key phases: C1, C2, and C3, examining their crystallographic transformations, elastic properties, and pressure-induced structural deformations. Our findings provide critical insights into phase transitions, stability regimes, and the fundamental physics of molecular interactions in dense hydrogen-water systems, enhancing our understanding of their thermodynamic and mechanical properties under extreme conditions. These data are also essential to quantify the role of hydrogen on the thermal state and chemical evolution of water-rich worlds of various sizes.

4. Implications of high-pressure oxygen hydrates on radiolytic oxygen in Jovian icy moons

   Mungo Frost, Mikhail A. Kuzovnikov, Philip Dalladay-Simpson, Ross T. Howie, John S. Loveday, Umbertoluca Ranieri, and Eugene Gregoryanz

   Comm. Chemistry, Apr 2025

   Abs DOI

   Various icy moons, such as Europa and Ganymede, have thin oxygen atmospheres and exhibit spectral features attributed to oxygen held in their surface ices. The oxygen forms from the radiolysis of water. The interiors of these bodies are subject to high pressures and it is not known how deep into icy moons oxygen-bearing ices can penetrate, or the structures formed by the oxygen–water system at high pressure. Here, we show that oxygen hydrates are stable to 2.6 GPa, allowing them to penetrate deep into icy moons, both above and below proposed sub-surface liquid-water oceans. Similarities between oxygen and hydrogen hydrates indicate potentially enhanced recombination rates, transforming them back into water and offering a resolution to the discrepancy between predicted and measured radiolysis rates. In addition to the low-pressure CS-II clathrate, our results find three high-pressure phases in the oxygen–water system: an ST clathrate, a C0 hydrate, and a filled ice isomorphous with methane hydrate III. This shows a vast storage potential for molecular oxygen in icy moons and indicates that Europa could still be absorbing oxygen into its crustal ice.

5. Observation of plastic ice VII by quasi-elastic neutron scattering

   Maria Rescigno, Alberto Toffano, Umbertoluca Ranieri, Leon Andriambariarijaona, Richard Gaal, Stefan Klotz, Michael Marek Koza, Jacques Ollivier, Fausto Martelli, John Russo, Francesco Sciortino, Jose Teixeira, and Livia Eleonora Bove

   Nature, Apr 2025

   Abs DOI

   Water is the third most abundant molecule in the universe and a key component in the interiors of icy moons, giant planets and Uranus- and Neptune-like exoplanets^1,2,3. Owing to its distinct molecular structure and flexible hydrogen bonds that readily adapt to a wide range of pressures and temperatures, water forms numerous crystalline and amorphous phases^4,5,6. Most relevant for the high pressures and temperatures of planetary interiors is ice VII (ref. 4), and simulations have identified along its melting curve the existence of a so-called plastic phase^{7,8,9,10,11,12} in which individual molecules occupy fixed positions as in a solid yet are able to rotate as in a liquid. Such plastic ice has not yet been directly observed in experiments. Here we present quasi-elastic neutron scattering measurements, conducted at temperatures between 450 and 600 K and pressures up to 6 GPa, that reveal the existence of a body-centred cubic structure, as found in ice VII, with water molecules showing picosecond rotational dynamics typical for liquid water. Comparison with molecular dynamics simulations indicates that this plastic ice VII does not conform to a free rotor phase but rather shows rapid orientational jumps, as observed in jump-rotor plastic crystals^13,14. We anticipate that our observation of plastic ice VII will affect our understanding of the geodynamics of icy planets and the differentiation processes of large icy moons.

6. High‐Pressure Synthesis of oP28‐C₃N₄ Recoverable to Ambient Conditions

   Dominique Laniel, Florian Trybel, Wenju Zhou, Andrey Aslandukov, James Spender, Ferenc Tasnádi, Timofey Fedotenko, Umbertoluca Ranieri, Akun Liang, Alena Aslandukova, Fariia I. Akbar, Yuqing Yin, Stella Chariton, Anna Pakhomova, Gaston Garbarino, Mohamed Mezouar, Michael Hanfland, Vitali Prakapenka, Igor A. Abrikosov, Leonid Dubrovinsky, and Natalia Dubrovinskaia

   Adv. Funct. Materials, Mar 2025

   Abs DOI

   The thermodynamic parameter pressure is ideal for producing novel ultraincompressible and superhard materials as it promotes the formation of polymeric frameworks and higher atomic coordination. In this regard, carbon and nitrogen are particularly attractive elements as they can produce extended arrangements of strong covalent bonds. In this study, a previously unobserved C3N4 polymorph, denoted as oP28-C3N4 (Pnnm, #58), is synthesized at pressures between 73 and 104 GPa in laser-heated diamond anvil cells and found recoverable to ambient conditions and stable in air. The crystal structure of oP28-C3N4, comprised of corner-sharing CN4 tetrahedra, is solved and refined using synchrotron single-crystal X-ray diffraction. With a bulk modulus of 334(3) GPa deduced from experimental data, the compound is highly incompressible. Based on macroscopic and microscopic calculations, its hardness may achieve 47.5 or 79.7 GPa, respectively, making it a superhard material. Incompressibility of CN4 tetrahedra in all experimentally observed C3N4 polymorphs is found to be greater than that of the CC4 and BN4 tetrahedra forming the structures of diamond and cubic boron nitride. Density functional theory calculations provide further insight into the electronic, vibrational, and mechanical properties of oP28-C3N4, as well as their stability relative to other C─N phases.

2024

1. Giant Splitting of the Hydrogen Rotational Eigenenergies in the C2 Filled Ice

   Simone Di Cataldo, Maria Rescigno, Lorenzo Monacelli, Umbertoluca Ranieri, Richard Gaal, Stefan Klotz, Jacques Ollivier, Michael Marek Koza, Cristiano De Michele, and Livia Eleonora Bove

   Phys. Rev. Lett., Dec 2024

   Abs DOI

   Hydrogen hydrates exhibit a rich phase diagram influenced by both pressure and temperature, with the so-called C2 phase emerging prominently above 2.5 GPa. In this phase, hydrogen molecules are densely packed within a cubic icelike lattice and the interaction with the surrounding water molecules profoundly affects their quantum rotational dynamics. Herein, we delve into this intricate interplay by directly solving the Schrödinger’s equation for a quantum H2 rotor in the C2 crystal field at finite temperature, generated through density functional theory. Our calculations reveal a giant energy splitting relative to the magnetic quantum number of ±3.2  meV for 𝑙=1. Employing inelastic neutron scattering, we experimentally measure the energy levels of H2 within the C2 phase at 6.0 and 3.4 GPa and low temperatures, finding good agreement with our theoretical predictions. These findings underscore the pivotal role of hydrogen-water interactions in dictating the rotational behavior of the hydrogen molecules within the C2 phase and indicate heightened van der Waals interactions compared to other hydrogen hydrates.

2. Structural phase transition in NH₄F under extreme pressure conditions

   Umbertoluca Ranieri, Christophe Bellin, Lewis J. Conway, Richard Gaal, John S. Loveday, Andreas Hermann, Abhay Shukla, and Livia E. Bove

   Comm. Chemistry, Sep 2024

   Abs DOI

   Ammonium fluoride (NH₄F) exhibits a variety of crystalline phases depending on temperature and pressure. By employing Raman spectroscopy and synchrotron X-ray diffraction beyond megabar pressures (up to 140 GPa), we have here observed a novel dense solid phase of NH₄F, characterised by the tetragonal P4/nmm structure also observed in other ammonium halides under less extreme pressure conditions, typically a few GPa. Using detailed ab-initio calculations and reevaluating earlier theoretical models pertaining to other ammonium halides, we examine the microscopic mechanisms underlying the transition from the low-pressure cubic phase (P-43m) to the newly identified high-pressure tetragonal phase (P4/nmm). Notably, NH₄F exhibits distinctive properties compared to its counterparts, resulting in a significantly broader pressure range over which this transition unfolds, facilitating the identification of its various stages. Our analysis points to a synergistic interplay driving the transition to the P4/nmm phase, which we name phase VIII. At intermediate pressures (around 40 GPa), a displacive transition of fluorine ions initiates a tetragonal distortion of the cubic phase. Subsequently, at higher pressures (around 115 GPa), every second ammonium ion undergoes a rotational shift, adopting an anti-tetrahedral arrangement. This coupled effect orchestrates the transition process, leading to the formation of the tetragonal phase.

3. Synthesis of LaCN₃, TbCN₃, CeCN₅, and TbCN₅ Polycarbonitrides at Megabar Pressures

   Andrey Aslandukov, Akun Liang, Amanda Ehn, Florian Trybel, Yuqing Yin, Alena Aslandukova, Fariia I. Akbar, Umbertoluca Ranieri, James Spender, Ross T. Howie, Eleanor Lawrence Bright, Jonathan Wright, Michael Hanfland, Gaston Garbarino, Mohamed Mezouar, Timofey Fedotenko, Igor A. Abrikosov, Natalia Dubrovinskaia, Leonid Dubrovinsky, and Dominique Laniel

   JACS, Jun 2024

   Abs DOI

   Inorganic ternary metal–C–N compounds with covalently bonded C–N anions encompass important classes of solids such as cyanides and carbodiimides, well known at ambient conditions and composed of [CN]− and [CN2]2– anions, as well as the high-pressure formed guanidinates featuring [CN3]5– anion. At still higher pressures, carbon is expected to be 4-fold coordinated by nitrogen atoms, but hitherto, such CN4-built anions are missing. In this study, four polycarbonitride compounds (LaCN3, TbCN3, CeCN5, and TbCN5) are synthesized in laser-heated diamond anvil cells at pressures between 90 and 111 GPa. Synchrotron single-crystal X-ray diffraction (SCXRD) reveals that their crystal structures are built of a previously unobserved anionic single-bonded carbon–nitrogen three-dimensional (3D) framework consisting of CN4 tetrahedra connected via di- or oligo-nitrogen linkers. A crystal-chemical analysis demonstrates that these polycarbonitride compounds have similarities to lanthanide silicon phosphides. Decompression experiments reveal the existence of LaCN3 and CeCN5 compounds over a very large pressure range. Density functional theory (DFT) supports these discoveries and provides further insight into the stability and physical properties of the synthesized compounds.

4. Crossover from gas-like to liquid-like molecular diffusion in a simple supercritical fluid

   Umbertoluca Ranieri, Ferdinando Formisano, Federico A. Gorelli, Mario Santoro, Michael Marek Koza, Alessio De Francesco, and Livia E. Bove

   Nature Comm., May 2024

   Abs DOI

   According to textbooks, no physical observable can be discerned allowing to distinguish a liquid from a gas beyond the critical point. Yet, several proposals have been put forward challenging this view and various transition boundaries between a gas-like and a liquid-like behaviour, including the so-called Widom and Frenkel lines, and percolation line, have been suggested to delineate the supercritical state space. Here we report observation of a crossover from gas-like (Gaussian) to liquid-like (Lorentzian) self-dynamic structure factor by incoherent quasi-elastic neutron scattering measurements on supercritical fluid methane as a function of pressure, along the 200 K isotherm. The molecular self-diffusion coefficient was derived from the best Gaussian (at low pressures) or Lorentzian (at high pressures) fits to the neutron spectra. The Gaussian-to-Lorentzian crossover is progressive and takes place at about the Widom line intercept (59 bar). At considerably higher pressures, a liquid-like jump diffusion mechanism properly describes the supercritical fluid on both sides of the Frenkel line. The present observation of a gas-like to liquid-like crossover in the self dynamics of a simple supercritical fluid confirms emerging views on the unexpectedly complex physics of the supercritical state, and could have planet-wide implications and possible industrial applications in green chemistry.

5. High‐Pressure Synthesis of Ultra‐Incompressible, Hard and Superconducting Tungsten Nitrides

   Akun Liang, Israel Osmond, Georg Krach, Lan‐Ting Shi, Lukas Brüning, Umbertoluca Ranieri, James Spender, Ferenc Tasnadi, Bernhard Massani, Callum R. Stevens, Ryan Stewart McWilliams, Eleanor Lawrence Bright, Nico Giordano, Samuel Gallego‐Parra, Yuqing Yin, Andrey Aslandukov, Fariia Iasmin Akbar, Eugene Gregoryanz, Andrew Huxley, Miriam Peña‐Alvarez, Jian‐Guo Si, Wolfgang Schnick, Maxim Bykov, Florian Trybel, and Dominique Laniel

   Adv. Funct. Materials, May 2024

   Abs DOI

   Transition metal nitrides, particularly those of 5d metals, are known for their outstanding properties, often relevant for industrial applications. Among these metal elements, tungsten is especially attractive given its low cost. In this high-pressure investigation of the W–N system, two novel ultra-incompressible tungsten nitride superconductors, namely W2N3 and W3N5, are successfully synthesized at 35 and 56 GPa, respectively, through a direct reaction between N2 and W in laser-heated diamond anvil cells. Their crystal structure is determined using synchrotron single-crystal X-ray diffraction. While the W2N3 solid’s sole constituting nitrogen species are N3- units, W3N5 features both discrete N3- as well as N24- pernitride anions. The bulk modulus of W2N3 and W3N5 is experimentally determined to be 380(3) and 406(7) GPa, and their ultra-incompressible behavior is rationalized by their constituting WN7 polyhedra and their linkages. Importantly, both W2N3 and W3N5 are recoverable to ambient conditions and stable in air. Density functional theory calculations reveal W2N3 and W3N5 to have a Vickers hardness of 30 and 34 GPa, and superconducting transition temperatures at ambient pressure (50 GPa) of 11.6 K (9.8 K) and 9.4 K (7.2 K), respectively. Additionally, transport measurements performed at 50 GPa on W2N3 corroborate with the calculations.

6. Large-cage occupation and quantum dynamics of hydrogen molecules in sII clathrate hydrates

   Umbertoluca Ranieri, Leonardo Del Rosso, Livia Eleonora Bove, Milva Celli, Daniele Colognesi, Richard Gaal, Thomas C. Hansen, Michael Marek Koza, and Lorenzo Ulivi

   J. Chem. Phys., Apr 2024

   Abs DOI

   Hydrogen clathrate hydrates are ice-like crystalline substances in which hydrogen molecules are trapped inside polyhedral cages formed by the water molecules. Small cages can host only a single H2 molecule, while each large cage can be occupied by up to four H2 molecules. Here, we present a neutron scattering study on the structure of the sII hydrogen clathrate hydrate and on the low-temperature dynamics of the hydrogen molecules trapped in its large cages, as a function of the gas content in the samples. We observe spectral features at low energy transfer (between 1 and 3 meV), and we show that they can be successfully assigned to the rattling motion of a single hydrogen molecule occupying a large water cage. These inelastic bands remarkably lose their intensity with increasing the hydrogen filling, consistently with the fact that the probability of single occupation (as opposed to multiple occupation) increases as the hydrogen content in the sample gets lower. The spectral intensity of the H2 rattling bands is studied as a function of the momentum transfer for partially emptied samples and compared with three distinct quantum models for a single H2 molecule in a large cage: (i) the exact solution of the Schrödinger equation for a well-assessed semiempirical force field, (ii) a particle trapped in a rigid sphere, and (iii) an isotropic three-dimensional harmonic oscillator. The first model provides good agreement between calculations and experimental data, while the last two only reproduce their qualitative trend. Finally, the radial wavefunctions of the three aforementioned models, as well as their potential surfaces, are presented and discussed.

7. Simple Molecules under High‐Pressure and High‐Temperature Conditions: Synthesis and Characterization of α‐ and β‐C(NH)₂ with Fully sp³‐Hybridized Carbon

   Thaddäus J. Koller, Siyu Jin, Viktoria Krol, Sebastian J. Ambach, Umbertoluca Ranieri, Saiana Khandarkhaeva, James Spender, Stewart McWilliams, Florian Trybel, Nico Giordano, Tomasz Poreba, Mohamed Mezouar, Xiaoyu Kuang, Cheng Lu, Leonid Dubrovinsky, Natalia Dubrovinskaia, Andreas Hermann, Wolfgang Schnick, and Dominique Laniel

   Angewandte Chemie, Feb 2024

   Abs DOI

   The elements hydrogen, carbon, and nitrogen are among the most abundant in the solar system. Still, little is known about the ternary compounds these elements can form under the high-pressure and high-temperature conditions found in the outer planets’ interiors. These materials are also of significant research interest since they are predicted to feature many desirable properties such as high thermal conductivity and hardness due to strong covalent bonding networks. In this study, the high-pressure high-temperature reaction behavior of malononitrile H2C(CN)2, dicyandiamide (H2N)2C=NCN, and melamine (C3N3)(NH2)3 was investigated in laser-heated diamond anvil cells. Two previously unknown compounds, namely α-C(NH)2 and β-C(NH)2, have been synthesized and found to have fully sp3-hybridized carbon atoms. α-C(NH)2 crystallizes in a distorted β-cristobalite structure, while β-C(NH)2 is built from previously unknown imide-bridged 2,4,6,8,9,10-hexaazaadamantane units, which form two independent interpenetrating diamond-like networks. Their stability domains and compressibility were studied, for which supporting density functional theory calculations were performed.

2023

1. Observation of the most H₂-dense filled ice under high pressure

   Umbertoluca Ranieri, Simone Di Cataldo, Maria Rescigno, Lorenzo Monacelli, Richard Gaal, Mario Santoro, Leon Andriambariarijaona, Paraskevas Parisiades, Cristiano De Michele, and Livia Eleonora Bove

   Proc. Natl. Acad. Sci. U.S.A., Dec 2023

   Abs DOI

   Hydrogen hydrates are among the basic constituents of our solar system’s outer planets, some of their moons, as well Neptune-like exo-planets. The details of their high-pressure phases and their thermodynamic conditions of formation and stability are fundamental information for establishing the presence of hydrogen hydrates in the interior of those celestial bodies, for example, against the presence of the pure components (water ice and molecular hydrogen). Here, we report a synthesis path and experimental observation, by X-ray diffraction and Raman spectroscopy measurements, of the most H2-dense phase of hydrogen hydrate so far reported, namely the compound 3 (or C3). The detailed characterisation of this hydrogen-filled ice, based on the crystal structure of cubic ice I (ice Ic), is performed by comparing the experimental observations with first-principles calculations based on density functional theory and the stochastic self-consistent harmonic approximation. We observe that the extreme (up to 90 GPa and likely beyond) pressure stability of this hydrate phase is due to the close-packed geometry of the hydrogen molecules caged in the ice Ic skeleton.

2. Structure determination of ζ-N₂ from single-crystal X-ray diffraction and theoretical suggestion for the formation of amorphous nitrogen

   Dominique Laniel, Florian Trybel, Andrey Aslandukov, James Spender, Umbertoluca Ranieri, Timofey Fedotenko, Konstantin Glazyrin, Eleanor Lawrence Bright, Stella Chariton, Vitali B. Prakapenka, Igor A. Abrikosov, Leonid Dubrovinsky, and Natalia Dubrovinskaia

   Nature Comm., Oct 2023

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   The allotropy of solid molecular nitrogen is the consequence of a complex interplay between fundamental intermolecular as well as intramolecular interactions. Understanding the underlying physical mechanisms hinges on knowledge of the crystal structures of these molecular phases. That is especially true for ζ-N2, key to shed light on nitrogen’s polymerization. Here, we perform single-crystal X-ray diffraction on laser-heated N2 samples at 54, 63, 70 and 86 GPa and solve and refine the hitherto unknown structure of ζ-N2. In its monoclinic unit cell (space group C2/c), 16 N2 molecules are arranged in a configuration similar to that of ε-N2. The structure model provides an explanation for the previously identified Raman and infrared lattice and vibrational modes of ζ-N2. Density functional theory calculations give an insight into the gradual delocalization of electronic density from intramolecular bonds to intermolecular space and suggest a possible pathway towards nitrogen’s polymerization.

3. Low-Temperature Dynamics of Water Confined in Unidirectional Hydrophilic Zeolite Nanopores

   Maria Rescigno, Matilde Lucioli, Frederico G. Alabarse, Umbertoluca Ranieri, Bernhard Frick, Benoit Coasne, and Livia E. Bove

   J. Phys. Chem. B, May 2023

   Abs DOI

   The dynamical properties of water molecules confined in the unidirectional hydrophilic nanopores of AlPO4-54 are investigated with quasi-elastic neutron scattering as a function of temperature down to 118 K. AlPO4-54 has among the largest pores known for aluminophosphates and zeolites (about 1.3 nm), though they are small enough to prevent water crystallization due to the high degree of confinement. Water molecular diffusion into the pore is here measured down to 258 K. Diffusion is slower than in bulk water and has an activation energy of Ea = (20.8 ± 2.8) kJ/mol, in agreement with previous studies on similar confining media. Surprisingly, local hydrogen dynamics associated with water reorientation is measured down to temperatures (118 K), i.e., well below the expected glass transition temperature of bulk water. The reorientational time scale shows the well-known non-Arrhenius behavior down to the freezing of water mass diffusion, while it shows a feeble temperature dependence below. This fast local dynamics, of the order of fractions of nanoseconds, is believed to take place in the dense, highly disordered amorphous water occupying the pore center, indicating its possible plastic nature.

2022

1. Temperature- and pressure-dependence of the hydrogen bond network in plastic ice VII

   Alberto Toffano, John Russo, Maria Rescigno, Umbertoluca Ranieri, Livia E. Bove, and Fausto Martelli

   J, Chem. Phys., Sep 2022

   Abs DOI

   We model, via classical molecular dynamics simulations, the plastic phase of ice VII across a wide range of the phase diagram of interest for planetary investigations. Although structural and dynamical properties of plastic ice VII are mostly independent on the thermodynamic conditions, the hydrogen bond network (HBN) acquires a diverse spectrum of topologies distinctly different from that of liquid water and of ice VII simulated at the same pressure. We observe that the HBN topology of plastic ice carries some degree of similarity with the crystal phase, stronger at thermodynamic conditions proximal to ice VII, and gradually lessening when approaching the liquid state. Our results enrich our understanding of the properties of water at high pressure and high temperature and may help in rationalizing the geology of water-rich planets.

2. Formation and Stability of Dense Methane-Hydrogen Compounds

   Umbertoluca Ranieri, Lewis J. Conway, Mary-Ellen Donnelly, Huixin Hu, Mengnan Wang, Philip Dalladay-Simpson, Miriam Peña-Alvarez, Eugene Gregoryanz, Andreas Hermann, and Ross T. Howie

   Phys. Rev. Lett., May 2022

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   Through a series of x-ray diffraction, optical spectroscopy diamond anvil cell experiments, combined with density functional theory calculations, we explore the dense CH4−H2 system. We find that pressures as low as 4.8 GPa can stabilize CH4⁢(H2)2 and (CH4)2⁢H2, with the latter exhibiting extreme hardening of the intramolecular vibrational mode of H2 units within the structure. On further compression, a unique structural composition, (CH4)3⁢(H2)25, emerges. This novel structure holds a vast amount of molecular hydrogen and represents the first compound to surpass 50 wt % H2. These compounds, stabilized by nuclear quantum effects, persist over a broad pressure regime, exceeding 160 GPa.

2021

1. Diffusion in dense supercritical methane from quasi-elastic neutron scattering measurements

   Umbertoluca Ranieri, Stefan Klotz, Richard Gaal, Michael Marek Koza, and Livia E. Bove

   Nature Comm., Mar 2021

   Abs DOI

   Methane, the principal component of natural gas, is an important energy source and raw material for chemical reactions. It also plays a significant role in planetary physics, being one of the major constituents of giant planets. Here, we report measurements of the molecular self-diffusion coefficient of dense supercritical CH4 reaching the freezing pressure. We find that the high-pressure behaviour of the self-diffusion coefficient measured by quasi-elastic neutron scattering at 300 K departs from that expected for a dense fluid of hard spheres and suggests a density-dependent molecular diameter. Breakdown of the Stokes–Einstein–Sutherland relation is observed and the experimental results suggest the existence of another scaling between self-diffusion coefficient D and shear viscosity η, in such a way that Dη/ρ=constant at constant temperature, with ρ the density. These findings underpin the lack of a simple model for dense fluids including the pressure dependence of their transport properties.

2019

1. Observation of methane filled hexagonal ice stable up to 150 GPa

   Sofiane Schaack, Umbertoluca Ranieri, Philippe Depondt, Richard Gaal, Werner F. Kuhs, Philippe Gillet, Fabio Finocchi, and Livia E. Bove

   Proc. Natl. Acad. Sci. U.S.A., Aug 2019

   Abs DOI

   Gas hydrates consist of hydrogen-bonded water frameworks enclosing guest gas molecules and have been the focus of intense research for almost 40 y, both for their fundamental role in the understanding of hydrophobic interactions and for gas storage and energy-related applications. The stable structure of methane hydrate above 2 GPa, where CH4 molecules are located within H2O or D2O channels, is referred to as methane hydrate III (MH-III). The stability limit of MH-III and the existence of a new high-pressure phase above 40 to 50 GPa, although recently conjectured, remain unsolved to date. We report evidence for a further high-pressure, room-temperature phase of the CH4–D2O hydrate, based on Raman spectroscopy in diamond anvil cell and ab initio molecular dynamics simulations including nuclear quantum effects. Our results reveal that a methane hydrate IV (MH-IV) structure, where the D2O network is isomorphic with ice Ih, forms at ∼40 GPa and remains stable up to 150 GPa at least. Our proposed MH-IV structure is fully consistent with previous unresolved X-ray diffraction patterns at 55 GPa [T. Tanaka et al., J. Chem. Phys. 139, 104701 (2013)]. The MH-III –> MH-IV transition mechanism, as suggested by the simulations, is complex. The MH-IV structure, where methane molecules intercalate the tetrahedral network of hexagonal ice, represents the highest-pressure gas hydrate known up to now. Repulsive interactions between methane and water dominate at the very high pressure probed here and the tetrahedral topology outperforms other possible arrangements in terms of space filling.

2. Salt- and gas-filled ices under planetary conditions

   Livia E. Bove and Umbertoluca Ranieri

   Phil. Trans. R. Soc. A., Jun 2019

   Abs DOI

   In recent years, evidence has emerged that solid water can contain substantial amounts of guest species, such as small gas molecules—in gas hydrate structures—or ions—in salty ice structures—and that these ‘filled’ ice structures can be stable under pressures of tens of Gigapascals and temperatures of hundreds of Kelvins. The inclusion of guest species can strongly modify the density, vibrational, diffusive and conductivity properties of ice under high pressure, and promote novel exotic properties. In this review, we discuss our experimental findings and molecular dynamics simulation results on the structures formed by salt- and gas-filled ices, their unusual properties, and the unexpected dynamical phenomena observed under pressure and temperature conditions relevant for planetary interiors modelling.

3. Quantum Dynamics of H₂ and D₂ Confined in Hydrate Structures as a Function of Pressure and Temperature

   Umbertoluca Ranieri, Michael Marek Koza, Werner F. Kuhs, Richard Gaal, Stefan Klotz, Andrzej Falenty, Dirk Wallacher, Jacques Ollivier, Philippe Gillet, and Livia E. Bove

   J. Phys. Chem. C, Jan 2019

   Abs DOI

   We present an extensive study of the quantum dynamics of molecular hydrogen trapped within the nanocavities of two hydrate structures at low temperatures, namely, clathrate structure II and filled ice structure C1. By inelastic neutron scattering measurements, we investigate a (i) simple H2 hydrate in clathrate structure II at ambient and high pressure and different temperatures, (ii) binary He–H2 hydrate in clathrate structure II at high pressure and different temperatures, (iii) simple D2 hydrate in clathrate structure II at ambient pressure and different temperatures, and (iv) simple H2 hydrate in structure C1 at high pressure and different temperatures. Molecular quantum rotations and translations, as well as combinations of these are identified in the spectra for hydrogen molecules in the small and large cages of clathrate structure II and in the channels of structure C1 and their energy is provided.

2018

1. Orientational Ordering, Locking-in, and Distortion of CH₄ Molecules in Methane Hydrate III under High Pressure

   Sofiane Schaack, Umbertoluca Ranieri, Philippe Depondt, Richard Gaal, Werner F. Kuhs, Andrzej Falenty, Philippe Gillet, Fabio Finocchi, and Livia E. Bove

   J. Phys. Chem. C, May 2018

   Abs DOI

   We investigate the effects of high pressure on the reorientational and vibrational dynamics of methane molecules embedded in methane hydrate III—the stable form of methane for pressures above 2 GPa at room temperature—by combining high-pressure Raman spectroscopy with ab initio simulations including nuclear quantum effects. We observe a clear evolution of the system from a gas-filled ice structure, where methane molecules occupy the channels of the ice skeleton and rotate almost freely, to a CH4:D2O compound where methane rotations are hindered, and methane and water dynamics are tightly coupled. The gradual orientational ordering of the guest molecules results in a complete locking-in at approximately 20 GPa. This happens along with a progressive distortion of the guest molecules. Finally, as pressure increases beyond 20 GPa, the system enters a strong mode coupling regime where methane guests and water hosts dynamics are intimately paired.

2017

1. Fast methane diffusion at the interface of two clathrate structures

   Umbertoluca Ranieri, Michael Marek Koza, Werner F. Kuhs, Stefan Klotz, Andrzej Falenty, Philippe Gillet, and Livia E. Bove

   Nature Comm., Oct 2017

   Abs DOI

   Methane hydrates naturally form on Earth and in the interiors of some icy bodies of the Universe, and are also expected to play a paramount role in future energy and environmental technologies. Here we report experimental observation of an extremely fast methane diffusion at the interface of the two most common clathrate hydrate structures, namely clathrate structures I and II. Methane translational diffusion—measured by quasielastic neutron scattering at 0.8 GPa—is faster than that expected in pure supercritical methane at comparable pressure and temperature. This phenomenon could be an effect of strong confinement or of methane aggregation in the form of micro-nanobubbles at the interface of the two structures. Our results could have implications for understanding the replacement kinetics during sI–sII conversion in gas exchange experiments and for establishing the methane mobility in methane hydrates embedded in the cryosphere of large icy bodies in the Universe.

2016

1. Dynamical Crossover in Hot Dense Water: The Hydrogen Bond Role

   Umbertoluca Ranieri, Paola Giura, Federico A. Gorelli, Mario Santoro, Stefan Klotz, Philippe Gillet, Luigi Paolasini, Michael Marek Koza, and Livia E. Bove

   J. Phys. Chem. B, Sep 2016

   Abs DOI

   We investigate the terahertz dynamics of liquid H2O as a function of pressure along the 450 K isotherm, by coupled quasielastic neutron scattering and inelastic X-ray scattering experiments. The pressure dependence of the single-molecule dynamics is anomalous in terms of both microscopic translation and rotation. In particular, the Stokes–Einstein-Debye equations are shown to be violated in hot water compressed to the GPa regime. The dynamics of the hydrogen bond network is only weakly affected by the pressure variation. The time scale of the structural relaxation driving the collective dynamics increases by a mere factor of 2 along the investigated isotherm, and the structural relaxation strength turns out to be almost pressure independent. Our results point at the persistence of the hydrogen bond network in hot dense water up to ice VII crystallization, thus questioning the long-standing perception that hydrogen bonds are broken in liquid water under the effect of compression.