RESUMEN
Model materials are precious test cases for elementary theories and provide building blocks for the understanding of more complex cases. Here, we describe the lattice dynamics of the structural phase transition in francisite Cu_{3}Bi(SeO_{3})_{2}O_{2}Cl at 115 K and show that it provides a rare archetype of a transition driven by a soft antipolar phonon mode. In the high-symmetry phase at high temperatures, the soft mode is found at (0,0,0.5) at the Brillouin zone boundary and is measured by inelastic x-ray scattering and thermal diffuse scattering. In the low-symmetry phase, this soft-mode is folded back onto the center of the Brillouin zone as a result of the doubling of the unit cell, and appears as a fully symmetric mode that can be tracked by Raman spectroscopy. On both sides of the transition, the mode energy squared follows a linear behavior over a large temperature range. First-principles calculations reveal that, surprisingly, the flat phonon band calculated for the high-symmetry phase seems incompatible with the displacive character found experimentally. We discuss this unusual behavior in the context of an ideal Kittel model of an antiferroelectric transition.
RESUMEN
In order to resolve inconsistencies encountered in published data for SrSO[Formula: see text], the elasticity and the phase stability of celestite has been studied using thermal diffuse scattering, high pressure powder synchrotron x-ray diffraction, Raman scattering and DFT calculations. The structure of SrSO[Formula: see text] is found to be stable up to 62 GPa at ambient temperature. The preferred values for the components of the elastic stiffness tensor have been determined using x-ray thermal diffuse scattering and are (in GPa): [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text]. The preferred value for the bulk modulus is [Formula: see text] GPa. This work shows that thermal diffuse scattering collected at two temperatures allows the determination of the full elastic tensor of crystals with low space group symmetry.
RESUMEN
We investigate the terahertz dynamics of liquid H2O as a function of pressure along the 450 K isotherm, by coupled quasielastic neutron scattering and inelastic X-ray scattering experiments. The pressure dependence of the single-molecule dynamics is anomalous in terms of both microscopic translation and rotation. In particular, the Stokes-Einstein-Debye equations are shown to be violated in hot water compressed to the GPa regime. The dynamics of the hydrogen bond network is only weakly affected by the pressure variation. The time scale of the structural relaxation driving the collective dynamics increases by a mere factor of 2 along the investigated isotherm, and the structural relaxation strength turns out to be almost pressure independent. Our results point at the persistence of the hydrogen bond network in hot dense water up to ice VII crystallization, thus questioning the long-standing perception that hydrogen bonds are broken in liquid water under the effect of compression.
RESUMEN
An in-vacuum double-phase-plate diffractometer for performing polarization scans combined with resonant X-ray diffraction experiments is presented. The use of two phase plates enables the correction of some of the aberration effects owing to the divergence of the beam and its energy spread. A higher rate of rotated polarization is thus obtained in comparison with a system with only a single retarder. Consequently, thinner phase plates can be used to obtain the required rotated polarization rate. These results are particularly interesting for applications at low energy (e.g. 4 keV) where the absorption owing to the phase plate(s) plays a key role in the feasibility of these experiments. Measurements by means of polarization scans at the uranium M(4) edge on UO(2) enable the contributions of the magnetic and quadrupole ordering in the material to be disentangled.
