Interplay between Local Structure and Nuclear Dynamics in Tungstic Acid: A Neutron Scattering Study

Abstract

We provide an exhaustive characterization of structural properties and nuclear dynamics in tungstic acid (WO3·H2O). To this end, we employ neutron and X-ray diffraction (ND and XRD) combined with inelastic neutron scattering (INS) and neutron Compton scattering (NCS) experiments, and we corroborate the analysis with extensive ab initio modeling. The first step in our analysis is the elucidation of the crystal structure based on the refinement of low-temperature powder ND data, extending the knowledge gained from XRD analysis of a mineral specimen of tungstite. These results are confronted with low-temperature INS experiments and zero-temperature phonon calculations. The analysis reveals an inconsistency in the definition of the structure of confined water with respect to crystallographic data, also showing a concomitant failure of the phonon calculations due to a strongly anharmonic confining potential. Extending the computational route toward ab initio MD (AIMD) simulations allows us to probe different structural configurations and provides an improved description of the vibrational dynamics as compared to high-resolution INS experiments, nevertheless, requiring the use of effective classical temperatures. The analysis of both INS and the NCS data reveals a remarkable similarity to the nuclear dynamics earlier reported for water confined in single-wall carbon nanotubes (SWNT), which has been qualitatively described as a new phase of ice. Our analysis reveals a strong two-dimensional hydrogen-bonding network, similar to the shell model for water in SWNT. The reported NCS data show narrowing of the hydrogen momentum distribution with respect to the reference ab initio calculations, indicating a great deal of conformational freedom due to spatial delocalization of protons in the ground state of the system, a clear signature of the quantum character of the nuclei.

Hydrogen Spillover in Tungsten Oxide Bronzes as Observed by Broadband Neutron Spectroscopy

“…The NMD widths of tungsten and oxygen were kept fixed during the fitting at the values calculated from the DFT simulations described in the subsection below. This part of the experimental NCS data analysis was inspired by our previous NCS results in tungstic acid, where the experimental values of NMD widths of oxygen and tungsten practically coincide with the DFT predictions [99]. Moreover, the ratio of the relative scattering intensities (areas under the recoil peaks) of tungsten and oxygen has been fixed using the ratio of their respective total bound scattering cross-section values, weighted by the ratio of the number of tungsten and oxygen atomic species per formula unit in cWO 3 .…”

“…Moreover, the ratio of the relative scattering intensities (areas under the recoil peaks) of tungsten and oxygen has been fixed using the ratio of their respective total bound scattering cross-section values, weighted by the ratio of the number of tungsten and oxygen atomic species per formula unit in cWO 3 . This technique, stoichiometric fixing of scattering intensities, has been frequently applied in NCS data analysis of solid-state systems and molecules [31,[99][100][101][102].…”

Nuclear quantum dynamics in Hexamethylenetetramine and its deuterated counterpart: a DFT-augmented neutron study

“…To account for the temperature effect, the apVDOSs obtained from the HLD simulations were scaled using Boltzmann population factors and then used to compute both the neutron spectroscopic [93][94][95] and diffraction observables [60], as outlined below.…”

“…The details of these simulations are described above, and only INS-relevant details are mentioned here. Following the protocol established in a previous study [93,95,96], the phonon eigenvalues and eigenvectors obtained from the HLD calculation were used as inputs for the calculation of the INS spectra of HMT and DHMT using Oclimax software [97]. In the Oclimax inputs, polycrystalline powder spectra were simulated from HLD in the 0 K limit, assuming an incoherent approximation for both HMT and DHMT and a maximum order of excitation of ten.…”

Combinatorial neutron imaging methods for hydrogenation catalysts

“…found in the literature. 48,55,56,[63][64][65] By applying deuterium, we probe all kinetically accessible hydrogen in the sample, including the thermodynamically irreversibly absorbed hydrogen. The attenuation returns back to zero (the violet arrow in Fig.…”