69 research outputs found
Structural phase transitions in malononitrile, CH<sub>2</sub>(CN)<sub>2</sub>:crystal structure of the δ phase by neutron powder diffraction, and <i>ab initio </i>calculations of the structures and phonons of the α and δ phases
Negative thermal expansion of cubic silicon dicarbodiimide, Si(NCN)2, studied by ab initio lattice dynamics
Hopping Time Scales and the Phonon-Liquid Electron-Crystal Picture in Thermoelectric Copper Selenide
Diffuse scattering in metallic tin polymorphs
The lattice dynamics of the metallic tin {\beta} and {\gamma} polymorphs has
been studied by a combination of diffuse scattering, inelastic x-ray scattering
and density functional perturbation theory. The non-symmorphic space group of
the {\beta}-tin structure results in unusual asymmetry of thermal diffuse
scattering. Strong resemblance of the diffuse scattering intensity distribution
in {\beta} and {\gamma}-tin were observed, reflecting the structural
relationship between the two phases and revealing the qualitative similarity of
the underlying electronic potential. The strong influence of the electron
subsystem on inter-ionic interactions creates anomalies in the phonon
dispersion relations. All observed features are described in great detail by
density functional perturbation theory for both {\beta}- and {\gamma}-tin at
arbitrary momentum transfers. The combined approach delivers thus a complete
picture of the lattice dynamics in harmonic description
Dimer-mediated cation diffusion in the stoichiometric ionic conductor Li3N
Non-equilibrium molecular dynamics has been used to model cation diffusion in stoichiometric Li3N over the temperature range 50 < T/K < 800. The resulting diffusion coefficients are in excellent agreement with the available experimental data. We present a detailed atomistic account of the diffusion process. Contrary to the conclusions drawn from previous studies, our calculations show that it is unnecessary to invoke the presence of a small concentration of intrinsic defects in order to initiate diffusion. The structure can be considered to consist of alternating layers of composition Li2N and Li. As the temperature increases an increasing number of cations leave the Li2N layers and migrate either to the interlayer space or to the Li layer. Those that move into the interlayer space form Li2 dimers with cations in the Li2N layers and those that move into the neighboring layer form dimers with cations therein. The two types of dimer are aligned parallel and perpendicular to [001], respectively and have lifetimes of ∼3 ps. The vacancies so created facilitate rapid diffusion in the Li2N layers and the interlayer cation motion results in slower diffusion perpendicular to the layers
Computation of diffuse scattering arising from one-phonon excitations in a neutron time-of-flight single-crystal Laue diffraction experiment
Direct phonon excitation in a neutron time-of-flight single-crystal Laue diffraction experiment has been observed in a single crystal of NaCl. At room temperature both phonon emission and excitation leave characteristic features in the diffuse scattering and these are well reproduced using ab initio phonons from density functional theory (DFT). A measurement at 20 K illustrates the effect of thermal population of the phonons, leaving the features corresponding to phonon excitation and strongly suppressing the phonon annihilation. A recipe is given to compute these effects combining DFT results with the geometry of the neutron experiment
New insights into the lattice dynamics of α-quartz
The lattice dynamics of α-quartz has been studied in great details by combining inelastic X-ray scattering (IXS) from single- and polycrystalline samples, 3D mapping of thermal diffuse scattering (TDS) and ab initio calculations. Pronounced features in TDS patterns have been identified and the origin of first peak in vibrational density of states is unambiguously reveale
- …