Ab initio parametrized polarizable force field for rutile-type SnO2

We report a new, polarizable classical force field for the rutile-type phase of SnO2, casserite. This force field has been parametrized using results from ab initio (density functional theory) calculations as a basis for fitting. The force field was found to provide structural, dynamical and thermodynamic properties of tin oxide that compare well with both ab initio and experimental results at ambient and high pressures. © 2012, Springer

Temperature dependence of alkali-metal rattling dynamics in the β-pyrochlores, AOs2O6 (A = K, Rb, Cs), from MD simulation

We investigate the temperature response of the alkali-metal rattling modes in β-pyrochlores, AOs2O6 (A = K, Rb, Cs), from the results of ab initio molecular dynamics (MD) simulations performed at 20K, 100K and 300K. Our results show that the temperature response of the T1u mode is clearly different from that of the T2g mode for all three pyrochlores. In this regard, two features are of particular note for both K and Rb; (1) the T1u mode exhibits a distinctly stronger softening response with decreasing temperature compared to the T2g mode, and (2) the T1u mode becomes stronger and sharper with decreasing temperature. These two findings suggest that the T1u mode is significantly more anharmonic and sensitive to the cage dynamics than the T2g mode. Examination of the local potentials around the alkali-metal atoms reveals that K has the flattest and most anharmonic potential at all temperatures while Cs exhibits the narrowest potential. The temperature dependence of the local potentials reveals that, for K, the potential at a higher temperature is not a simple extrapolation to higher energy of that at a lower temperature. Instead, we find significant reconstruction of the potential at different temperatures. Finally, we explore the temperature response of the coupling between the alkali metals and find a complex temperature dependence which suggests that the origin of the coupling may be more complex than a pure Coulomb interaction. We also find an unexpected increase in the static disorder of the system at low temperatures for the K and Rb pyrochlores. © 2014, IOP Publishing Lt

Phonon dispersion in superionic copper selenide - observation of soft phonon modes in superionic phase transition.

This paper reports lattice dynamical measurements of Cu1.8Se superionic conductor in the superionic α-phase at ambient temperature. Measurements of phonon dispersion curves were performed with the new triple-axis spectrometer, TAIPAN, at the OPAL reactor. We found that TA [100], TA [111] and TA1 [110] phonon branches demonstrate a decrease in frequency at wavevectors q > 0.5 rather than the flattening observed previously. Results are compared with calculated density functional theoretical calculations showing the presence of unstable soft mode related to ordering of Cu atoms observed in Cu1.8Se at room temperature followed by α–β phase transition at a lower temperature. Superstructure arising from the ordering causes effects similar to the folding of the Brillouin zone, although phonon intensities at new Brillouin zone centres are weak. The coupling of low-energy phonon modes with displacement of mobile ions can explain the strong damping of phonons at q > 0.5 observed in the experiment

Molecular dynamics evidence for alkali-metal rattling in the beta-pyrochlores, AOs(2)O(6) (A = K, Rb, Cs)

We have used ab initio molecular dynamics simulations validated against inelastic neutron scattering data to study alkali-metal dynamics in the ?-pyrochlore osmates AOs2O6 (A=K, Rb, Cs) at 300?K to gain insight into the microscopic nature of rattling dynamics in these materials. Our results provide new evidence at the microscopic level for rattling dynamics: (1)?the elemental magnitude spectra calculated from the MD show a striking dominance by the alkali metals at low energies indicating weak coupling to the cage, (2) the atomic root-mean-square displacements for the alkali metals are significantly larger than for the other atoms, e.g., 25% and 150% larger than O and Os, respectively, in KOs2O6, and (3) motions of the alkali metals are weakly correlated to the dynamics in their immediate environment, e.g.?K in KOs2O6 is 6 times less sensitive to its local environment than Os, indicating weak bonding of the K. There is broadening of the elemental spectra of the alkali metals from Cs to K corresponding to a similar broadening of the local potential around these atoms as determined from potential of mean-force calculations. This feature of the spectra is partly explained by the well-known increase in the relative cage volume with decreasing atomic size of the alkali metal. We find that for the smallest rattler in this series (K) the larger relative cage volume allows this atom freedom to explore a large space inside the cage leading to vibration at a broader range of frequencies, hence a broader spectrum. Thus, since K is considered the best rattler in this series, these findings suggest that a significant feature of a good rattler is the ability to vibrate at several different but closely spaced frequencies. © 2013, IOP Publishing Ltd

The effect of host relaxation and dynamics on guest molecule dynamics in H2/tetrahydrofuran-hydrate

We use ab initio molecular dynamics simulations to obtain classically the effects of H2O cage motions on the potential-energy surface (PES) of encapsulated H2 in the H2/tetrahydrofuran-hydrate system. The significant differences between the PES for the H2 in rigid and flexible cages that we find will influence calculation of the quantum dynamics of the H2. Part of these differences arises from the relaxation of the H2O cage around the classical H2, with a second part arising from the coupling of both translational and rotational motions of H2 with the H2O cage. We find that isotopic substitution of 2H for 1H of the H2O cage affects the coupling, which has implications for experiments that require the use of 2H2O, including inelastic neutron scattering that uses 2H2O cages in order to focus on the H2 guest dynamics. Overall, this work emphasizes the importance of taking into account cage dynamics in any approach used to understand the dynamics of H2 guests in porous framework materials. © 2011, American Chemical Societ

Symmetry of ferroelectric phase of SrTi18O3 determined by ab initio calculations.

Substitution of more than 33% of the naturally abundant 16O in strontium titanate SrTiO3 by 18O causes the system to become ferroelectric at low temperatures. The ferroelectricity has been observed via susceptibility measurements, but to date the details of the ferroelectric phase and the phase transition are unclear. Using ab initio density functional theory and lattice-dynamics calculations, we find that the stable structure of the ferroelectric phase is orthorhombic with Ima2 symmetry. The Ima2 point group is noncentrosymmetric and the proposed structure exhibits an electric dipole moment of (0.57 0 0) eÅ. The Ima2 symmetry is consistent with the limited structural details that are reported using neutron diffraction and Raman spectroscopy. © 2011, American Physical Societ

The crystal structure and methyl group dynamics in the room-temperature and low-temperature phases of lithium acetate dihydrate

Neutron powder diffraction is used to determine the crystal structure of LiCD3COO · 2D2O at 293 and 1.5 K. Lithium acetate dihydrate crystallizes in the Cmmm space group in the room-temperature phase with a = 6.82082(9) Å, b = 10.88842(12) Å, c = 6.59911(7) Å, and Z = 4. The CH3 groups are dynamically disordered, and there are short H bonds between the water molecules and the acetate oxygen atoms. There is a phase transition to a low-temperature modification which has the space group Pman with a = 6.70246(7) Å, b = 10.87932(11) Å, c = 6.56999(7) Å, and Z = 4. The CH3 groups are ordered in this phase, but have large thermal ellipsoids even at 1.5 K. The H atoms of the water molecules also have large thermal ellipsoids even at 1.5 K, but the CH3 group tunneling spectrum shows no evidence for coupling with the motions of the water molecules. © 1996 Academic Press, Inc