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A detailed study of the thermal behaviour of atomic motions in the organic ferroelectric croconic acid is presented in the temperature range 5–300 K. Using high-resolution inelastic neutron scattering and first-principles electronic-structure calculations within the framework of density functional theory and a quasiharmonic phonon description of the material, we find that the frequencies of the well defined doublet in inelastic neutron scattering spectra associated with out-of-plane motions of hydrogen-bonded protons decrease monotonically with temperature indicating weakening of these bonding motifs and enhancement of proton motions. Theoretical mean-square displacements for these proton motions are within 5% of experimental values. A detailed analysis of this observable shows that it is unlikely that there is a facile proton transfer along the direction of ferroelectric polarization in the absence of an applied electric field. Calculations predict constrained thermal motion of proton along crystallographic lattice direction c retaining the hydrogen bond motif of the crystal at high temperature. Using the Berry-phase method, we have also calculated the spontaneous polarization of temperature dependent cell structures, and find that our computational model provides a satisfactory description of the anomalous and so far unexplained rise in bulk electric polarization with temperature. Correlating the thermal motion induced lattice strain with temperature dependent spontaneous polarizations, we conclude that increasing thermal strain with temperatures combined with constrained thermal motion along the hydrogen bond motif are responsible of this increase in ferroelectricity at high temperature. [ABSTRACT FROM AUTHOR]

We address the question as to whether the melting of chemically substituted fullerenes is driven by the dynamics of the fullerene moiety (the head) or the substituted sub-unit (the tail). To this end, we have performed quasielastic neutron-scattering experiments and classical molecular-dynamics simulations as a function of temperature on the prototypical fullerene derivative phenyl-C61-butyric acid methyl ester. To enable a direct and quantitative comparison between experimental and simulation data, dynamic structure factors for the latter have been calculated from atomic trajectories and further convolved with the known instrument response. A detailed analysis of the energy- and momentum-transfer dependence of this observable in the quasielastic regime shows that melting is entirely driven by temperature-activated tail motions. We also provide quantitative estimates of the activation energy for this process as the material first enters a plastic-crystalline phase, followed by the emergence of a genuine liquid at higher temperatures. [ABSTRACT FROM AUTHOR]

First-principles molecular dynamics simulations and neutron-scattering experiments have been employed to investigate the structure and underlying vibrational motions in croconic acid as a function of temperature over the range 4–400 K. Calculated hydroxyl-bond distances were within 4% of the experimentally determined bond lengths. Temperature-dependent structures have been explored using large-scale molecular dynamics simulations. From the calculated radial distribution functions, it is found that medium-range order associated with O…H and O…O correlations are affected by an increase in temperature, yet the characteristic long-range layered structure of this material remains unaltered. Hydrogen-bond anharmonicity has been assessed from the molecular dynamics simulations, showing a red shift of ca. 50 cm−1 of the O–H stretch frequency relative to quasi-harmonic results. This shift shows the importance of anharmonic corrections on hydrogen bonds in solid croconic acid. [ABSTRACT FROM AUTHOR]

High surface area (HS) AlF3samples have been examined by X-ray photoelectron spectroscopy (XPS). The experimentally observed binding energy (BE) shifts were analysed by reference to core level BEs obtained from ab initiototal energy calculations on a range of different, clean and hydroxylated α- and β-AlF3surfaces. Examination of the two components visible in the Al 2p emission indicates that surface Al3눫 can, depending on the local geometric structure, contribute to both a high BE peak at 77.0 eV and a low BE peak at 76.1 eV. Consequently, the areas under the peaks do not quantitatively correlate with surface area or Lewis acidity. However, a significant correlation between the number of surface Al centres with dangling F or OH groups and the appearance of an Al 2p emission component at a BE lower than in the α-AlF3bulk is predicted. The experimental F 1s emission data indicate that dangling F species are essentially absent. Examination of the O 1s emission suggests that HS AlF3handled at room temperature under any practical laboratory conditions, including glovebox environments, probably contains intrinsically a significant amount of OH groups and adsorbed water, which results in the covering of AlF3surfaces by dangling or bridging OH groups. These Brønsted acid species must be removed by treatment at higher temperature before HS AlF3reagents can fully develop their Lewis acidity. [ABSTRACT FROM AUTHOR]