Refined Structure of Metastable Ice XVII from Neutron Diffraction Measurements

The structure of the recently identified metastable ice XVII, obtained by release of hydrogen from the C$_0$ D$_2$O-H$_2$ compound (filled ice), has been accurately measured by neutron powder diffraction. The diffraction pattern is indexed with a hexagonal cell and can be refined with space group $P6_122$ so to obtain accurate values of the oxygen and deuterium positions. The values of the lattice constants at three temperatures between 25 to 100 K are reported, and their behavior is compared with that of ice Ih. Ice XVII is a porous solid that, if exposed to H$_2$ gas, may adsorb a substantial amount of it. Monitoring this effect at a constant temperature of 50 K, we have observed that the two lattice constants show opposite behavior, $a$ increases and $c$ decreases, with the volume showing a linear increase. At temperatures higher than 130 K the metastability of this form of porous ice is lost and the sample transforms into ice Ih

Wavelet imaging of transient energy localization in nonlinear systems at thermal equilibrium: the case study of NaI crystals at high temperature

In this paper we introduce a method to resolve transient excitations in time-frequency space from molecular dynamics simulations. Our technique is based on continuous wavelet transform of velocity time series coupled to a threshold-dependent filtering procedure to isolate excitation events from background noise in a given spectral region. By following in time the center of mass of the reference frequency interval, the data can be easily exploited to investigate the statistics of the burst excitation dynamics, by computing, for instance, the distribution of the burst lifetimes, excitation times, amplitudes and energies. As an illustration of our method, we investigate transient excitations in the gap of NaI crystals at thermal equilibrium at different temperatures. Our results reveal complex ensembles of transient nonlinear bursts in the gap, whose lifetime and excitation rate increase with temperature. The method described in this paper is a powerful tool to investigate transient excitations in many-body systems at thermal equilibrium. Our procedure gives access to both the equilibrium and the kinetics of transient excitation processes, allowing one in principle to reconstruct the full picture of the dynamical process under examination.Comment: Paper to be published in Physical Review B. Talk given by FP at the VI International Symposium on Strong Nonlinear Vibronic and Electronic Interactions in Solids, Tartu (ES), 28 April - 1 May 201

In situ Raman and neutron spectroscopy of complex hydrides

The search of suitable hydrides for hydrogen storage can benefit from Raman and neutron spectroscopy, especially when it is possible to exploit these diagnostic techniques in situ, that is, in controlled conditions of gas pressure and temperature. In this way it is possible to observe and monitor phase transitions or sample chemical decomposition. In our laboratory we have studied the vibrational spectrum of several complex hydride compounds both at cryogenic temperature (15 K) and during or after thermal treatment at high temperature (up to 600 K), in a controlled atmosphere. In particular, in this talk I will present the results of the characterization of bulk samples of Mg(BH4)2 and AlH3 where the proportions of the different phases can be changed by different thermal treatments. Additionally, both Raman and neutron spectra of Na3AlH6, recorded at low temperature, have disclosed the presence of low frequency lattice modes, that have been compared with computational results

Nuclear quantum effects in ab initio dynamics: theory and experiments for lithium imide

Owing to their small mass, hydrogen atoms exhibit strong quantum behavior even at room temperature. Including these effects in first principles calculations is challenging, because of the huge computational effort required by conventional techniques. Here we present the first ab-initio application of a recently-developed stochastic scheme, which allows to approximate nuclear quantum effects inexpensively. The proton momentum distribution of lithium imide, a material of interest for hydrogen storage, was experimentally measured by inelastic neutron scattering experiments and compared with the outcome of quantum thermostatted ab initio dynamics. We obtain favorable agreement between theory and experiments for this purely quantum mechanical property, thereby demonstrating that it is possible to improve the modelling of complex hydrogen-containing materials without additional computational effort

Quantum calculation of inelastic neutron scattering spectra of a hydrogen molecule inside a nanoscale cavity based on rigorous treatment of the coupled translation-rotation dynamics

We present a quantum methodology for the calculation of the inelastic neutron scattering (INS) spectra of an ${\mathrm{H}}_{2}$ molecule confined in a nanoscale cavity. Our approach incorporates the coupled five-dimensional translation-rotation (TR) energy levels and wave functions of the guest molecule. The computed INS spectra are highly realistic and reflect in full the complexity of the coupled TR dynamics on the anisotropic potential energy surfaces of the confining environment. Utilizing this methodology, we simulate the INS spectra of $p$- and $o$-H${}_{2}$ in the small cage of the structure II clathrate hydrate and compare them with the experimental data

Proton Dynamics in Palladium–Silver: An Inelastic Neutron Scattering Investigation

Proton dynamics in Pd77Ag23 membranes is investigated by means of various neutron spectroscopic techniques, namely Quasi Elastic Neutron Scattering, Incoherent Inelastic Neutron Scattering, Neutron Transmission, and Deep Inelastic Neutron Scattering. Measurements carried out at the ISIS spallation neutron source using OSIRIS, MARI and VESUVIO spectrometers were performed at pressures of 1, 2, and 4 bar, and temperatures in the 330–673 K range. The energy interval spanned by the different instruments provides information on the proton dynamics in a time scale ranging from about 102 to 10−4 ps. The main finding is that the macroscopic diffusion process is determined by microscopic jump diffusion. In addition, the vibrational density of states of the H atoms in the metal lattice has been determined for a number of H concentrations and temperatures. These measurements follow a series of neutron diffraction experiments performed on the same sample and thus provide a complementary information for a thorough description of structural and dynamical properties of H-loaded Pd-Ag membranes

VSI@ESS: Case study for a vibrational spectroscopy instrument at the european spallation source

Neutron Vibrational Spectroscopy is a well-established experimental technique where elementary excitations at relatively high frequency are detected via inelastic neutron scattering. This technique attracts a high interest in a large fraction of the scientific community in the fields of chemistry, materials science, physics, and biology, since one of its main applications exploits the large incoherent scattering cross section of the proton with respect to all the other elements, whose dynamics can be spectroscopically detected, even if dissolved in very low concentration in materials composed of much heavier atoms. We have proposed a feasibility study for a Vibrational Spectroscopy Instrument (VSI) at the European Spallation Source ESS. Here, we will summarize the preliminary design calculations and the corresponding McStas simulation results for a possible ToF, Inverted Geometry, VSI beamline

VSI@ESS: Case study for a vibrational spectroscopy instrument at the european spallation source

Neutron Vibrational Spectroscopy is a well-established experimental technique where elementary excitations at relatively high frequency are detected via inelastic neutron scattering. This technique attracts a high interest in a large fraction of the scientific community in the fields of chemistry, materials science, physics, and biology, since one of its main applications exploits the large incoherent scattering cross section of the proton with respect to all the other elements, whose dynamics can be spectroscopically detected, even if dissolved in very low concentration in materials composed of much heavier atoms. We have proposed a feasibility study for a Vibrational Spectroscopy Instrument (VSI) at the European Spallation Source ESS. Here, we will summarize the preliminary design calculations and the corresponding McStas simulation results for a possible ToF, Inverted Geometry, VSI beamline

Exploring ultra-fast proton dynamics in water under a static electric field

We present an experimental investigation of the single-particle dynamics of hydrogen in liquid water and ice subject to static electric field using deep inelastic neutron scattering. The nuclear mean kinetic energy, $\langle E_K\rangle$ , of hydrogen in liquid water at room temperature does not show sensible changes when an electric field of magnitude $10^5\ \text{V/m}$ is turned on. On the contrary, the value of $\langle E_K\rangle$ in ice at 263 K and subject to the same electric field is found to be substantially lower than the reference value for ice Ih at the similar temperature of 271 K and without electric field. This is true both if the electric field is kept on or not while the sample cools from 300 K to 263 K. Concurrent diffraction measurements performed on ice subjected to an electric field show no sizeable structural changes with respect to the expected powder-averaged ice-Ih diffraction pattern