Helmut Schober

Formation of ice XII at different conditions

The structural versatility of solid water has been reinforced by the identification of a metastable, crystalline phase called ice XII (ref. 1). It was obtained by cooling liquid water to 260 K at a pressure of 0.55 GPa. We have found ice XII in a completely different region of waters phase diagram and show that it can be formed under different conditions from those previously reported.

Lattice Dynamics To Trigger Low Temperature Oxygen Mobility in Solid Oxide Ion Conductors

SrFeO(2.5) and SrCoO(2.5) are able to intercalate oxygen in a reversible topotactic redox reaction already at room temperature to form the cubic perovskites Sr(Fe,Co)O(3), while CaFeO(2.5) can only be oxidized under extreme conditions. To explain this significant difference in low temperature oxygen mobility, we investigated the homologous SrFeO(2.5) and CaFeO(2.5) by temperature dependent oxygen isotope exchange as well as by inelastic neutron scattering (INS) studies, combined with ab initio (DFT) molecular dynamical calculations. From (18)O/(16)O isotope exchange experiments we proved free oxygen mobility to be realized in SrFeO(x) already below 600 K. We have also evidence that low temperature oxygen mobility relies on the existence of specific, low energy lattice modes, which trigger and amplify oxygen mobility in solids. We interpret the INS data together with the DFT-based molecular dynamical simulation results on SrFeO(2.5) and CaFeO(2.5) in terms of an enhanced, phonon-assisted, low temperature oxygen diffusion for SrFeO(3-x) as a result of the strongly reduced Fe-O-Fe bond strength of the apical oxygen atoms in the FeO(6) octahedra along the stacking axis. This dynamically triggered phenomenon leads to an easy migration of the oxide ions into the open vacancy channels and vice versa. The decisive impact of lattice dynamics, giving rise to structural instabilities in oxygen deficient perovskites, especially with brownmillerite-type structure, is demonstrated, opening new concepts for the design and tailoring of low temperature oxygen ion conductors.

FAST RELAXATION IN A FRAGILE LIQUID UNDER PRESSURE

The incoherent dynamic structure factor of ortho-terphenyl has been measured by neutron time-of-flight and backscattering technique in the pressure range from 0.1 MPa to 240 MPa for temperatures between 301 K and 335 K. Tagged-particle correlations in the compressed liquid decay in two steps. The alpha-relaxation lineshape is independent of pressure, and the relaxation time proportional to viscosity. A kink in the amplitude f_Q(P) reveals the onset of beta relaxation. The beta-relaxation regime can be described by the mode-coupling scaling function; amplitudes and time scales allow a consistent determination of the critical pressure P_c(T). alpha and beta relaxation depend in the same way on the thermodynamic state; close to the mode-coupling cross-over, this dependence can be parametrised by an effective coupling Gamma ~ n*T**{-1/4}.

Ab initio lattice dynamics simulations and inelastic neutron scattering spectra for studying phonons in BaFe 2 As 2 : Effect of structural phase transition, structural relaxation, and magnetic ordering

We have performed extensive ab initio calculations to investigate phonon dynamics and their possible role in superconductivity in

Dynamics of liquid 4He in vycor

{\text{BaFe}}_{2}{\text{As}}_{2}

Lattice dynamics and electron-phonon coupling in transition-metal diborides

and related systems. The calculations are compared to inelastic neutron scattering data that offer improved resolution over published data [Mittal et al., Phys. Rev. B 78, 104514 (2008)], in particular at low frequencies. Effects of structural phase transition and full and/or partial structural relaxations, with and without magnetic ordering, on the calculated vibrational density of states are reported. Phonons are best reproduced using either the relaxed magnetic structures or the experimental cell. Several phonon branches are affected by the subtle structural changes associated with the transition from the tetragonal to the orthorhombic phase. Effects of phonon-induced distortions on the electronic and spin structure have been investigated. It is found that for some vibrational modes, there is a significant change in the electronic distribution and spin populations around the Fermi level. A peak at 20 meV in the experimental data falls into the pseudogap region of the calculation. This was also the case reported in our recent work combined with an empirical parametric calculation [Mittal et al., Phys. Rev. B 78, 104514 (2008)]. The combined evidence for the coupling of electronic and spin degrees of freedom with phonons is relevant to the current interest in superconductivity in

Relationship between phonons and thermal expansion in Zn(CN)2 and Ni(CN)2 from inelastic neutron scattering and ab initio calculations

{\text{BaFe}}_{2}{\text{As}}_{2}

Amorphous polymorphis in ice investigated by inelastic neutron scattering

and related systems.

Phonon spectrum and soft-mode behavior ofMgCNi3

Using inelastic neutron scattering, we have observed well-defined phonon-roton ( p-r) excitations in superfluid 4He in Vycor over a wide wave-vector range, 0.3</=Q</=2.15 A(-1). The p-r energies and lifetimes at all temperatures are the same as in bulk liquid 4He. However, the weight of the single p-r component does not scale with the superfluid fraction rho(S)(T)/rho as it does in the bulk. In particular, we observe a p-r excitation above T(c) = 1.952 K, where rho(s)(T) = 0 in Vycor. This suggests, if the p-r excitation intensity scales with the Bose condensate, that there is a separation of the Bose-Einstein condensation temperature and the superfluid transition temperature T(c) of 4He in Vycor.

On the heterogeneous character of water's amorphous polymorphism

The phonon density of states of transition metal diborides