We investigate the effects of high pressure on the reorientational and vibrational dynamics of me... more We investigate the effects of high pressure on the reorientational and vibrational dynamics of methane molecules embedded in MH-III hydrate-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 CH 4 :D 2 O 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 20GPa. This happens along with a progesssive 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.
We conducted ab initio molecular dynamics simulations of the d phase of the hydrous mineral alumi... more We conducted ab initio molecular dynamics simulations of the d phase of the hydrous mineral aluminium oxide hydroxide (AlOOH) at ambient temperature and high pressure. Nuclear quantum effects were included through a Langevin dynamics in a bath of quantum harmonic oscillators. We confirm that under increasing pressure d-AlOOH undergoes a phase transition from a P2 1 nm structure with asymmetric and disordered O-H bonds to a stiffer Pnnm phase with symmetric hydrogen bonds, which should be stable within the pressure and temperature ranges typical for the Earth's mantle. The transition is initially triggered by proton tunneling, which makes the mean proton position to coincide with the midpoint of the O-O distance, at pressures as low as 10 GPa. However, only at much larger pressures, around 30 GPa as previously found by other calculations, the Pnnm phase with symmetric hydrogen bonds is stable from the classical point of view. The transition is also characterized through the analysis of the H-O stretching modes, which soften considerably and fade out around 10 GPa in the P2 1 nm structure, when thermal and nuclear quantum effects are taken into account in the simulations. At variance, the harmonic picture is not adequate to describe the highly anharmonic effective potential that is seen by the protons at the transition. Finally, we propose that the picture of a dynamical transition to the high-symmetry and proton-centered Pnnm phase, which is brought about by the onset of proton tunneling, could be confirmed by quasi-elastic neutron scattering and vibrational spectroscopy under pressure.
HAL (Le Centre pour la Communication Scientifique Directe), Apr 20, 2018
We investigate the effects of high pressure on the reorientational and vibrational dynamics of me... more We investigate the effects of high pressure on the reorientational and vibrational dynamics of methane molecules embedded in MH-III hydrate-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 CH 4 :D 2 O 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 20GPa. This happens along with a progesssive 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.
We investigate the elementary steps at the microscopic level for proton diffusion in brucite unde... more We investigate the elementary steps at the microscopic level for proton diffusion in brucite under high pressure, which results from a complex interplay between two processes: the O-H reorientations motion around the c axis and O-H covalent bond dissociations. First-principle path-integral molecular dynamics simulations reveal that the increasing pressure tends to lock the former motion, while, in contrast, it activates the latter which is mainly triggered by nuclear quantum effects. These two competing effects therefore give rise to a pressure sweet spot for proton diffusion within the mineral. In brucite Mg(OH) 2 , proton diffusion reaches a maximum for pressures close to 70GPa, while the structurally similar portlandite Ca(OH) 2 never shows proton diffusion within the pressure range and time scale that we explored. We analyze the different behaviors of brucite and portlandite, which might constitute two prototypes for other minerals with the same structure.
Electronic properties of materials are dominated by quantum e↵ects, but nuclei, being much heavie... more Electronic properties of materials are dominated by quantum e↵ects, but nuclei, being much heavier, are usually treated as classical particles. This approximation, although tremendously convenient, is not always valid, even in close to ambient pressure and temperature conditions, especially when light nuclei such as hydrogen are involved. Zero point energy and proton tunneling can be relevant. Isotopic e↵ects, obtained by replacing hydrogen with deuterium, are observed experimentally and are a clear indication of Nuclear Quantum E↵ects (NQE) since mean values obtained through classical statistical physics do not depend on mass. Introducing NQEs into simulations at an acceptable computational cost raises fundamental questions and yields subtle and unexpected results.
Here we present a combined theoretical and experimental study of hydrogen adsorption on Znfunctio... more Here we present a combined theoretical and experimental study of hydrogen adsorption on Znfunctionalized MgO surface. We have systematically compared infrared spectra recorded at increasing hydrogen pressure (10 −5-100 mbar) on rock salt Zn 0.05 Mg 0.95 O and MgO nanocubes. The interpretation of vibrations involving hydrogen was supported by ab initio calculations following the same strategy as for pure MgO. Despite the small Zn concentration, we unambiguously recorded Zn-specific signals. Most interestingly, we find that all chemisorption complexes characteristic of MgO surface reactivity remain highly preserved on Zn 0.05 Mg 0.95 O, while the effect of segregated Zn is strongly localized to its first oxygen neighbors. From the intensity variation of the IR couples when passing from spectra of pure to Zn-modified MgO, we (1) confirm the validity of the adsorption models that we previously proposed for MgO and (2) show that Zn segregates preferentially on low-coordination Mg surface sites (88% and 50%
Paper published as part of the special topic on Collection ARTICLES YOU MAY BE INTERESTED IN Inve... more Paper published as part of the special topic on Collection ARTICLES YOU MAY BE INTERESTED IN Investigating bonding in small silicon-carbon clusters: Exploration of the potential energy surfaces of and using ab initio molecular dynamics
Methane hydrate was recently shown, both experimentally and through simulations, to be stable up ... more Methane hydrate was recently shown, both experimentally and through simulations, to be stable up to the remarkably high pressure of 150 GPa. A new methane hydrate high-pressure (MH-IV) phase, reminiscent of ice at ambient pressure, was described for pressures above approximately 40 GPa. We disentangle here the main contributions to the relative stability of the lower pressure, denoted MH-III and the high-pressure MH-IV structures. Through several simulation techniques, including metadynamics and Path Integral Molecular Dynamics for nuclear quantum effects, we analyze the phase transition mechanism, which implies hydrogen bond breaking and reforming, as well as methane reordering. The transition pathway is far from trivial and the quantum delocalization of the hydrogen nuclei plays a significant role.
The interaction of water molecules and hydroxyl groups with titanium dioxide (TiO 2) surfaces is ... more The interaction of water molecules and hydroxyl groups with titanium dioxide (TiO 2) surfaces is ubiquitous and very important in anatase nanoparticle photocatalytic processes. Infrared spectroscopy, assisted by ab initio calculations of vibrational frequencies, can be a powerful tool to elucidate the mechanisms behind water adsorption. However, a straightforward comparison between measurements and calculations remains a challenging task because of the complexity of the physical phenomena occurring on nanoparticle surfaces. Consequently, severe computational approximations, such as harmonic vibrational ones, are usually employed. In the present work we partially address this complexity issue by overcoming some of the standard approximations used in theoretical simulations and employ the Divide and Conquer Semiclassical Initial Value Representation (DC-SCIVR) molecular dynamics. This method allows to perform simulations of vibrational spectra of large dimensional systems accounting not only for anharmonicities, but also for nuclear quantum effects. We apply this computational method to water and deuterated water adsorbed on the ideal TiO 2 anatase(101) surface, contemplating both the molecular and the dissociated adsorption processes. The results highlight not only the presence of an anharmonic shift of the frequencies in agreement with the experiments, but also complex quantum mechanical spectral signatures induced by the coupling of molecular vibrational modes with the surface ones, which are different in the hydrogenated case from the deuterated one. These couplings are further analyzed by exploiting the mode subdivision performed during the divide and conquer procedure.
The configurations associated with reversible and irreversible adsorption of hydrogen on MgO esca... more The configurations associated with reversible and irreversible adsorption of hydrogen on MgO escape consensus. Here, we report the dissociation of H 2 on MgO nanocubes, which was examined by combining Fourier transform infrared spectroscopy and density functional theory (DFT)-based simulations. We found that the use of ultrahigh vacuum is essential for identifying the very first adsorption stages. Hydrogen pressure was varied from 10 −8 mbar to near ambient, resulting in IR spectra of richer complexity than in current state of the art. Models with oxygen at regular corners (O 3C) and Mg at inverse corners (Mg IC) were identified to be the most reactive and to split H 2 irreversibly already in the lowest pressure regime (P H 2 < 10 −3 mbar). The continuous increase in intensity of the corresponding IR bands (3712/1140 cm −1) in the intermediate range of pressures (10 −3 −1 mbar), along with the appearance of bands at 3605/1225 cm −1 , was demonstrated to stem from cooperative adsorption mechanisms, which could be therefore considered as the main origin of irreversible hydrogen adsorption. At P H 2 > 1 mbar, fully reversible adsorption was shown to occur at O 4C (either on mono-or diatomic steps) and Mg 3C sites. Another OH/MgH couple (3697/1030 cm −1) that became reinforced at high P H 2 but remained stable upon pumping was correlated to O 3C and Mg IC in multiatomic steps. The difference in adsorption and desorption sequences confirmed the proposed cooperative adsorption of H 2 molecules. Our study provides new insights into the mechanisms that can be beneficial for understanding the chemistry of H 2 and other hydrogen-containing molecules, such as CH 4 , on oxide surfaces, but also for the advancement of hydrogen-storage technologies.
Water and ammonia are considered major components of the interiors of the giant icy planets and t... more Water and ammonia are considered major components of the interiors of the giant icy planets and their satellites, which has motivated their exploration under high P-T conditions. Exotic forms of these pure ices have been revealed at extreme (~megabar) pressures, notably symmetric, ionic, and superionic phases. Here we report on an extensive experimental and computational study of the high-pressure properties of the ammonia monohydrate compound forming from an equimolar mixture of water and ammonia. Our experiments demonstrate that relatively mild pressure conditions (7.4 GPa at 300 K) are sufficient to transform ammonia monohydrate from a prototypical hydrogen-bonded crystal into a form where the standard molecular forms of water and ammonia coexist with their ionic counterparts, hydroxide (OH −) and ammonium NH þ 4 À Á ions. Using ab initio atomistic simulations, we explain this surprising coexistence of neutral/charged species as resulting from a topological frustration between local homonuclear and long-ranged heteronuclear ionisation mechanisms.
Journal of Chemical Theory and Computation, Nov 23, 2016
The quantum thermal bath (QTB) has been presented as an alternative to pathintegral based methods... more The quantum thermal bath (QTB) has been presented as an alternative to pathintegral based methods to introduce nuclear quantum effects in molecular dynamics simulations. The method has proved to be efficient, yielding accurate results for various systems. However, the QTB method is prone to zero-point energy leakage (ZPEL) in highly anharmonic systems. This is a well known problem in methods based on classical trajectories where part of the energy of the high frequency modes is transferred to the low frequency modes leading to a wrong energy distribution. In some cases, the ZPEL can have dramatic consequences on the properties of the system. Thus, we investigate the ZPEL by testing the QTB method on selected systems with increasing complexity 1 in order to study the conditions and the parameters that influence the leakage. We also analyze the consequences of the ZPEL on the structural and vibrational properties of the system. We find that the leakage is particularly dependent on the damping coefficient and that increasing its value can reduce, and in some cases, completely remove the ZPEL. When using sufficiently high values for the damping coefficient, the expected energy distribution among the vibrational modes is ensured. In this case, the QTB method gives very encouraging results. In particular, the structural properties are well reproduced. The dynamical properties should be regarded with caution although valuable information can still be extracted from the vibrational spectrum, even for large values of the damping term.
The electronic and atomic structure of several (1 × 1) terminations of the (110) polar orientatio... more The electronic and atomic structure of several (1 × 1) terminations of the (110) polar orientation of SrTiO3 surface are systematically studied by first-principles calculations. The electronic structure of the two stoichiometric SrTiO-and O2-terminations are characterized by marked differences with respect to the bulk, as a consequence of the polarity compensation. In the former, the Fermi level is located at the bottom of the conduction band, while in the latter the formation of a peroxo bond between the two surface oxygens results in a small-gap insulating surface with states in the gap of the bulk projected band structure. We also consider three non stoichiometric terminations with TiO, Sr and O compositions, respectively, in the outermost atomic layer, which automatically allows the surface to be free from any macroscopic polarization. They are all insulating. The cleavage and surface energies of the five terminations are computed and compared, taking into account the influence of the chemical environment as a function of the relative richness in O and Sr. From our calculations it appears that some (110) faces can even compete with the TiO2 and SrO terminations of the (100) cleavage surface: in particular, the (110)-TiO termination is stable in Sr-poor conditions, the (110)-Sr one in simultaneously O-and Sr-rich environments. The available experimental data are compared to the outcomes of our calculations and discussed.
We conducted ab initio molecular dynamics simulations of the δ phase of the hydrous mineral AlOOH... more We conducted ab initio molecular dynamics simulations of the δ phase of the hydrous mineral AlOOH at ambient temperature and high pressure. Nuclear quantum effects were included through a Langevin dynamics in a bath of quantum harmonic oscillators. We confirm that under increasing pressure δ-AlOOH undergoes a phase transition from a P 21nm structure with asymmetric and disordered O-H bonds to a stiffer P nnm phase with symmetric hydrogen bonds, which should be stable within the pressure and temperature ranges typical for the Earth's mantle. The transition is initially triggered by proton tunneling, which makes the mean proton position to coincide with the midpoint of the O-O distance, at pressures as low as 10 GPa. However, only at much larger pressures, around 30 GPa as previously found by other calculations, the P nnm phase with symmetric hydrogen bonds is stable from the classical point of view. The transition is also characterized through the analysis of the H-O stretching modes, which soften considerably and fade out around 10 GPa in the P 21nm structure, when thermal and nuclear quantum effects are taken into account in the simulations. At variance, the harmonic picture is not adequate to describe the highly anharmonic effective potential that is seen by the protons at the transition. Finally, we propose that the picture of a dynamical transition to the high-symmetry and proton-centered P nnm phase, which is brought about by the onset of proton tunneling, could be confirmed by quasi-elastic neutron scattering and vibrational spectroscopy under pressure.
HAL (Le Centre pour la Communication Scientifique Directe), Apr 9, 2003
The electronic and atomic structure of several (1 × 1) terminations of the (110) polar orientatio... more The electronic and atomic structure of several (1 × 1) terminations of the (110) polar orientation of SrTiO3 surface are systematically studied by first-principles calculations. The electronic structure of the two stoichiometric SrTiO-and O2-terminations are characterized by marked differences with respect to the bulk, as a consequence of the polarity compensation. In the former, the Fermi level is located at the bottom of the conduction band, while in the latter the formation of a peroxo bond between the two surface oxygens results in a small-gap insulating surface with states in the gap of the bulk projected band structure. We also consider three non stoichiometric terminations with TiO, Sr and O compositions, respectively, in the outermost atomic layer, which automatically allows the surface to be free from any macroscopic polarization. They are all insulating. The cleavage and surface energies of the five terminations are computed and compared, taking into account the influence of the chemical environment as a function of the relative richness in O and Sr. From our calculations it appears that some (110) faces can even compete with the TiO2 and SrO terminations of the (100) cleavage surface: in particular, the (110)-TiO termination is stable in Sr-poor conditions, the (110)-Sr one in simultaneously O-and Sr-rich environments. The available experimental data are compared to the outcomes of our calculations and discussed.
Bulletin of the American Physical Society, Mar 14, 2016
Submitted for the MAR16 Meeting of The American Physical Society Exploration of the ferroelectric... more Submitted for the MAR16 Meeting of The American Physical Society Exploration of the ferroelectric properties of a new Titaniumbased compound REMI FEDERICCI, ESPCI
The configurations associated with the dissociative adsorption of water on a variety of low-coord... more The configurations associated with the dissociative adsorption of water on a variety of low-coordinated sites of MgO(100) surfaces, including corners, steps, MgO vacancies and kinks on <010> steps have been studied and assigned by combining infrared spectroscopy and ab initio calculations. Three kinds of MgO powders were examined, powders of very high specific surface area prepared by chemical vapor synthesis and well-defined cubic smoke particles obtained by combustion either in 20:80 or 60:40 O2:Ar mixtures, the latter one involving less defects and smaller particles. It appears that an imperative requirement to obtain a precise characterization of the reactive behavior of defects is to keep the samples in ultra-high vacuum conditions and to control the water partial pressure finely.
Anhydrous sodium hydroxide, a common and structurally simple compound, shows spectacular isotope ... more Anhydrous sodium hydroxide, a common and structurally simple compound, shows spectacular isotope effects: NaOD undergoes a first-order transition, which is absent in NaOH. By combining ab initio electronic structure calculations with path integrals, we show that NaOH is an unusual example of a quantum paraelectric: zero-point quantum fluctuations stretch the weak hydrogen bonds (HBs) until they become unstable and break. By strengthening HBs via isotope substitution or applied pressure, the system can be driven down to a broken-symmetry antiferroelectric phase. We also provide a simple quantitative criterion for HB breaking in layered crystals and show that nuclear quantum effects are crucial in paraelectric to ferroelectric transitions in hydrogen-bonded hydroxides.
Nested sampling is an efficient method for calculating Bayesian evidence in data analysis and par... more Nested sampling is an efficient method for calculating Bayesian evidence in data analysis and partition functions of potential energies. It is based on an exploration using a dynamical set of sampling points that evolves to higher values of the sampled function. When several maxima are present, this exploration can be a very difficult task. Different codes implement different strategies. Local maxima are generally treated separately, applying cluster recognition of the sampling points based on machine learning methods. We present here the development and implementation of different search and clustering methods on the nested_fit code. Slice sampling and the uniform search method are added in addition to the random walk already implemented. Three new cluster recognition methods are also developed. The efficiency of the different strategies, in terms of accuracy and number of likelihood calls, is compared considering a series of benchmark tests, including model comparison and a harmon...
We investigate the effects of high pressure on the reorientational and vibrational dynamics of me... more We investigate the effects of high pressure on the reorientational and vibrational dynamics of methane molecules embedded in MH-III hydrate-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 CH 4 :D 2 O 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 20GPa. This happens along with a progesssive 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.
We conducted ab initio molecular dynamics simulations of the d phase of the hydrous mineral alumi... more We conducted ab initio molecular dynamics simulations of the d phase of the hydrous mineral aluminium oxide hydroxide (AlOOH) at ambient temperature and high pressure. Nuclear quantum effects were included through a Langevin dynamics in a bath of quantum harmonic oscillators. We confirm that under increasing pressure d-AlOOH undergoes a phase transition from a P2 1 nm structure with asymmetric and disordered O-H bonds to a stiffer Pnnm phase with symmetric hydrogen bonds, which should be stable within the pressure and temperature ranges typical for the Earth's mantle. The transition is initially triggered by proton tunneling, which makes the mean proton position to coincide with the midpoint of the O-O distance, at pressures as low as 10 GPa. However, only at much larger pressures, around 30 GPa as previously found by other calculations, the Pnnm phase with symmetric hydrogen bonds is stable from the classical point of view. The transition is also characterized through the analysis of the H-O stretching modes, which soften considerably and fade out around 10 GPa in the P2 1 nm structure, when thermal and nuclear quantum effects are taken into account in the simulations. At variance, the harmonic picture is not adequate to describe the highly anharmonic effective potential that is seen by the protons at the transition. Finally, we propose that the picture of a dynamical transition to the high-symmetry and proton-centered Pnnm phase, which is brought about by the onset of proton tunneling, could be confirmed by quasi-elastic neutron scattering and vibrational spectroscopy under pressure.
HAL (Le Centre pour la Communication Scientifique Directe), Apr 20, 2018
We investigate the effects of high pressure on the reorientational and vibrational dynamics of me... more We investigate the effects of high pressure on the reorientational and vibrational dynamics of methane molecules embedded in MH-III hydrate-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 CH 4 :D 2 O 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 20GPa. This happens along with a progesssive 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.
We investigate the elementary steps at the microscopic level for proton diffusion in brucite unde... more We investigate the elementary steps at the microscopic level for proton diffusion in brucite under high pressure, which results from a complex interplay between two processes: the O-H reorientations motion around the c axis and O-H covalent bond dissociations. First-principle path-integral molecular dynamics simulations reveal that the increasing pressure tends to lock the former motion, while, in contrast, it activates the latter which is mainly triggered by nuclear quantum effects. These two competing effects therefore give rise to a pressure sweet spot for proton diffusion within the mineral. In brucite Mg(OH) 2 , proton diffusion reaches a maximum for pressures close to 70GPa, while the structurally similar portlandite Ca(OH) 2 never shows proton diffusion within the pressure range and time scale that we explored. We analyze the different behaviors of brucite and portlandite, which might constitute two prototypes for other minerals with the same structure.
Electronic properties of materials are dominated by quantum e↵ects, but nuclei, being much heavie... more Electronic properties of materials are dominated by quantum e↵ects, but nuclei, being much heavier, are usually treated as classical particles. This approximation, although tremendously convenient, is not always valid, even in close to ambient pressure and temperature conditions, especially when light nuclei such as hydrogen are involved. Zero point energy and proton tunneling can be relevant. Isotopic e↵ects, obtained by replacing hydrogen with deuterium, are observed experimentally and are a clear indication of Nuclear Quantum E↵ects (NQE) since mean values obtained through classical statistical physics do not depend on mass. Introducing NQEs into simulations at an acceptable computational cost raises fundamental questions and yields subtle and unexpected results.
Here we present a combined theoretical and experimental study of hydrogen adsorption on Znfunctio... more Here we present a combined theoretical and experimental study of hydrogen adsorption on Znfunctionalized MgO surface. We have systematically compared infrared spectra recorded at increasing hydrogen pressure (10 −5-100 mbar) on rock salt Zn 0.05 Mg 0.95 O and MgO nanocubes. The interpretation of vibrations involving hydrogen was supported by ab initio calculations following the same strategy as for pure MgO. Despite the small Zn concentration, we unambiguously recorded Zn-specific signals. Most interestingly, we find that all chemisorption complexes characteristic of MgO surface reactivity remain highly preserved on Zn 0.05 Mg 0.95 O, while the effect of segregated Zn is strongly localized to its first oxygen neighbors. From the intensity variation of the IR couples when passing from spectra of pure to Zn-modified MgO, we (1) confirm the validity of the adsorption models that we previously proposed for MgO and (2) show that Zn segregates preferentially on low-coordination Mg surface sites (88% and 50%
Paper published as part of the special topic on Collection ARTICLES YOU MAY BE INTERESTED IN Inve... more Paper published as part of the special topic on Collection ARTICLES YOU MAY BE INTERESTED IN Investigating bonding in small silicon-carbon clusters: Exploration of the potential energy surfaces of and using ab initio molecular dynamics
Methane hydrate was recently shown, both experimentally and through simulations, to be stable up ... more Methane hydrate was recently shown, both experimentally and through simulations, to be stable up to the remarkably high pressure of 150 GPa. A new methane hydrate high-pressure (MH-IV) phase, reminiscent of ice at ambient pressure, was described for pressures above approximately 40 GPa. We disentangle here the main contributions to the relative stability of the lower pressure, denoted MH-III and the high-pressure MH-IV structures. Through several simulation techniques, including metadynamics and Path Integral Molecular Dynamics for nuclear quantum effects, we analyze the phase transition mechanism, which implies hydrogen bond breaking and reforming, as well as methane reordering. The transition pathway is far from trivial and the quantum delocalization of the hydrogen nuclei plays a significant role.
The interaction of water molecules and hydroxyl groups with titanium dioxide (TiO 2) surfaces is ... more The interaction of water molecules and hydroxyl groups with titanium dioxide (TiO 2) surfaces is ubiquitous and very important in anatase nanoparticle photocatalytic processes. Infrared spectroscopy, assisted by ab initio calculations of vibrational frequencies, can be a powerful tool to elucidate the mechanisms behind water adsorption. However, a straightforward comparison between measurements and calculations remains a challenging task because of the complexity of the physical phenomena occurring on nanoparticle surfaces. Consequently, severe computational approximations, such as harmonic vibrational ones, are usually employed. In the present work we partially address this complexity issue by overcoming some of the standard approximations used in theoretical simulations and employ the Divide and Conquer Semiclassical Initial Value Representation (DC-SCIVR) molecular dynamics. This method allows to perform simulations of vibrational spectra of large dimensional systems accounting not only for anharmonicities, but also for nuclear quantum effects. We apply this computational method to water and deuterated water adsorbed on the ideal TiO 2 anatase(101) surface, contemplating both the molecular and the dissociated adsorption processes. The results highlight not only the presence of an anharmonic shift of the frequencies in agreement with the experiments, but also complex quantum mechanical spectral signatures induced by the coupling of molecular vibrational modes with the surface ones, which are different in the hydrogenated case from the deuterated one. These couplings are further analyzed by exploiting the mode subdivision performed during the divide and conquer procedure.
The configurations associated with reversible and irreversible adsorption of hydrogen on MgO esca... more The configurations associated with reversible and irreversible adsorption of hydrogen on MgO escape consensus. Here, we report the dissociation of H 2 on MgO nanocubes, which was examined by combining Fourier transform infrared spectroscopy and density functional theory (DFT)-based simulations. We found that the use of ultrahigh vacuum is essential for identifying the very first adsorption stages. Hydrogen pressure was varied from 10 −8 mbar to near ambient, resulting in IR spectra of richer complexity than in current state of the art. Models with oxygen at regular corners (O 3C) and Mg at inverse corners (Mg IC) were identified to be the most reactive and to split H 2 irreversibly already in the lowest pressure regime (P H 2 < 10 −3 mbar). The continuous increase in intensity of the corresponding IR bands (3712/1140 cm −1) in the intermediate range of pressures (10 −3 −1 mbar), along with the appearance of bands at 3605/1225 cm −1 , was demonstrated to stem from cooperative adsorption mechanisms, which could be therefore considered as the main origin of irreversible hydrogen adsorption. At P H 2 > 1 mbar, fully reversible adsorption was shown to occur at O 4C (either on mono-or diatomic steps) and Mg 3C sites. Another OH/MgH couple (3697/1030 cm −1) that became reinforced at high P H 2 but remained stable upon pumping was correlated to O 3C and Mg IC in multiatomic steps. The difference in adsorption and desorption sequences confirmed the proposed cooperative adsorption of H 2 molecules. Our study provides new insights into the mechanisms that can be beneficial for understanding the chemistry of H 2 and other hydrogen-containing molecules, such as CH 4 , on oxide surfaces, but also for the advancement of hydrogen-storage technologies.
Water and ammonia are considered major components of the interiors of the giant icy planets and t... more Water and ammonia are considered major components of the interiors of the giant icy planets and their satellites, which has motivated their exploration under high P-T conditions. Exotic forms of these pure ices have been revealed at extreme (~megabar) pressures, notably symmetric, ionic, and superionic phases. Here we report on an extensive experimental and computational study of the high-pressure properties of the ammonia monohydrate compound forming from an equimolar mixture of water and ammonia. Our experiments demonstrate that relatively mild pressure conditions (7.4 GPa at 300 K) are sufficient to transform ammonia monohydrate from a prototypical hydrogen-bonded crystal into a form where the standard molecular forms of water and ammonia coexist with their ionic counterparts, hydroxide (OH −) and ammonium NH þ 4 À Á ions. Using ab initio atomistic simulations, we explain this surprising coexistence of neutral/charged species as resulting from a topological frustration between local homonuclear and long-ranged heteronuclear ionisation mechanisms.
Journal of Chemical Theory and Computation, Nov 23, 2016
The quantum thermal bath (QTB) has been presented as an alternative to pathintegral based methods... more The quantum thermal bath (QTB) has been presented as an alternative to pathintegral based methods to introduce nuclear quantum effects in molecular dynamics simulations. The method has proved to be efficient, yielding accurate results for various systems. However, the QTB method is prone to zero-point energy leakage (ZPEL) in highly anharmonic systems. This is a well known problem in methods based on classical trajectories where part of the energy of the high frequency modes is transferred to the low frequency modes leading to a wrong energy distribution. In some cases, the ZPEL can have dramatic consequences on the properties of the system. Thus, we investigate the ZPEL by testing the QTB method on selected systems with increasing complexity 1 in order to study the conditions and the parameters that influence the leakage. We also analyze the consequences of the ZPEL on the structural and vibrational properties of the system. We find that the leakage is particularly dependent on the damping coefficient and that increasing its value can reduce, and in some cases, completely remove the ZPEL. When using sufficiently high values for the damping coefficient, the expected energy distribution among the vibrational modes is ensured. In this case, the QTB method gives very encouraging results. In particular, the structural properties are well reproduced. The dynamical properties should be regarded with caution although valuable information can still be extracted from the vibrational spectrum, even for large values of the damping term.
The electronic and atomic structure of several (1 × 1) terminations of the (110) polar orientatio... more The electronic and atomic structure of several (1 × 1) terminations of the (110) polar orientation of SrTiO3 surface are systematically studied by first-principles calculations. The electronic structure of the two stoichiometric SrTiO-and O2-terminations are characterized by marked differences with respect to the bulk, as a consequence of the polarity compensation. In the former, the Fermi level is located at the bottom of the conduction band, while in the latter the formation of a peroxo bond between the two surface oxygens results in a small-gap insulating surface with states in the gap of the bulk projected band structure. We also consider three non stoichiometric terminations with TiO, Sr and O compositions, respectively, in the outermost atomic layer, which automatically allows the surface to be free from any macroscopic polarization. They are all insulating. The cleavage and surface energies of the five terminations are computed and compared, taking into account the influence of the chemical environment as a function of the relative richness in O and Sr. From our calculations it appears that some (110) faces can even compete with the TiO2 and SrO terminations of the (100) cleavage surface: in particular, the (110)-TiO termination is stable in Sr-poor conditions, the (110)-Sr one in simultaneously O-and Sr-rich environments. The available experimental data are compared to the outcomes of our calculations and discussed.
We conducted ab initio molecular dynamics simulations of the δ phase of the hydrous mineral AlOOH... more We conducted ab initio molecular dynamics simulations of the δ phase of the hydrous mineral AlOOH at ambient temperature and high pressure. Nuclear quantum effects were included through a Langevin dynamics in a bath of quantum harmonic oscillators. We confirm that under increasing pressure δ-AlOOH undergoes a phase transition from a P 21nm structure with asymmetric and disordered O-H bonds to a stiffer P nnm phase with symmetric hydrogen bonds, which should be stable within the pressure and temperature ranges typical for the Earth's mantle. The transition is initially triggered by proton tunneling, which makes the mean proton position to coincide with the midpoint of the O-O distance, at pressures as low as 10 GPa. However, only at much larger pressures, around 30 GPa as previously found by other calculations, the P nnm phase with symmetric hydrogen bonds is stable from the classical point of view. The transition is also characterized through the analysis of the H-O stretching modes, which soften considerably and fade out around 10 GPa in the P 21nm structure, when thermal and nuclear quantum effects are taken into account in the simulations. At variance, the harmonic picture is not adequate to describe the highly anharmonic effective potential that is seen by the protons at the transition. Finally, we propose that the picture of a dynamical transition to the high-symmetry and proton-centered P nnm phase, which is brought about by the onset of proton tunneling, could be confirmed by quasi-elastic neutron scattering and vibrational spectroscopy under pressure.
HAL (Le Centre pour la Communication Scientifique Directe), Apr 9, 2003
The electronic and atomic structure of several (1 × 1) terminations of the (110) polar orientatio... more The electronic and atomic structure of several (1 × 1) terminations of the (110) polar orientation of SrTiO3 surface are systematically studied by first-principles calculations. The electronic structure of the two stoichiometric SrTiO-and O2-terminations are characterized by marked differences with respect to the bulk, as a consequence of the polarity compensation. In the former, the Fermi level is located at the bottom of the conduction band, while in the latter the formation of a peroxo bond between the two surface oxygens results in a small-gap insulating surface with states in the gap of the bulk projected band structure. We also consider three non stoichiometric terminations with TiO, Sr and O compositions, respectively, in the outermost atomic layer, which automatically allows the surface to be free from any macroscopic polarization. They are all insulating. The cleavage and surface energies of the five terminations are computed and compared, taking into account the influence of the chemical environment as a function of the relative richness in O and Sr. From our calculations it appears that some (110) faces can even compete with the TiO2 and SrO terminations of the (100) cleavage surface: in particular, the (110)-TiO termination is stable in Sr-poor conditions, the (110)-Sr one in simultaneously O-and Sr-rich environments. The available experimental data are compared to the outcomes of our calculations and discussed.
Bulletin of the American Physical Society, Mar 14, 2016
Submitted for the MAR16 Meeting of The American Physical Society Exploration of the ferroelectric... more Submitted for the MAR16 Meeting of The American Physical Society Exploration of the ferroelectric properties of a new Titaniumbased compound REMI FEDERICCI, ESPCI
The configurations associated with the dissociative adsorption of water on a variety of low-coord... more The configurations associated with the dissociative adsorption of water on a variety of low-coordinated sites of MgO(100) surfaces, including corners, steps, MgO vacancies and kinks on <010> steps have been studied and assigned by combining infrared spectroscopy and ab initio calculations. Three kinds of MgO powders were examined, powders of very high specific surface area prepared by chemical vapor synthesis and well-defined cubic smoke particles obtained by combustion either in 20:80 or 60:40 O2:Ar mixtures, the latter one involving less defects and smaller particles. It appears that an imperative requirement to obtain a precise characterization of the reactive behavior of defects is to keep the samples in ultra-high vacuum conditions and to control the water partial pressure finely.
Anhydrous sodium hydroxide, a common and structurally simple compound, shows spectacular isotope ... more Anhydrous sodium hydroxide, a common and structurally simple compound, shows spectacular isotope effects: NaOD undergoes a first-order transition, which is absent in NaOH. By combining ab initio electronic structure calculations with path integrals, we show that NaOH is an unusual example of a quantum paraelectric: zero-point quantum fluctuations stretch the weak hydrogen bonds (HBs) until they become unstable and break. By strengthening HBs via isotope substitution or applied pressure, the system can be driven down to a broken-symmetry antiferroelectric phase. We also provide a simple quantitative criterion for HB breaking in layered crystals and show that nuclear quantum effects are crucial in paraelectric to ferroelectric transitions in hydrogen-bonded hydroxides.
Nested sampling is an efficient method for calculating Bayesian evidence in data analysis and par... more Nested sampling is an efficient method for calculating Bayesian evidence in data analysis and partition functions of potential energies. It is based on an exploration using a dynamical set of sampling points that evolves to higher values of the sampled function. When several maxima are present, this exploration can be a very difficult task. Different codes implement different strategies. Local maxima are generally treated separately, applying cluster recognition of the sampling points based on machine learning methods. We present here the development and implementation of different search and clustering methods on the nested_fit code. Slice sampling and the uniform search method are added in addition to the random walk already implemented. Three new cluster recognition methods are also developed. The efficiency of the different strategies, in terms of accuracy and number of likelihood calls, is compared considering a series of benchmark tests, including model comparison and a harmon...
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Papers by Fabio Finocchi