Lattice dynamics in FeSi measured by inelastic x-ray scattering.

The phonon dispersion of FeSi was measured by inelastic x-ray scattering. The study of its temperature evolution in the range of 100 K-300 K showed that the phonon modes soften to a different extent. The phonons exhibiting specifically strong softening were revealed to belong to the weakly dispersive optical branch. At the same time, the calculations of the lattice dynamics of FeSi suggest that this branch corresponds mainly to the atomic displacements that change the Fe-Fe nearest neighbor distance. It points to the role of the Fe-Fe interaction in the phonon softening.

Raman investigation on the behavior of parasibirskite CaHBO3 at high pressure.

Knowledge about the stability of hydrous borates and borosilicates at high pressures are of critical importance to our understanding on the boron geochemical cycle. Raman spectroscopic measurements of parasibirskite CaHBO3, containing the [BO2(OH)] groups, have been made to pressures up to 5.4GPa. The Raman data show that a progressive structural evolution from ambient pressure to 5.4GPa can be accounted for by the same monoclinic phase P21/m, where the splitting of several Raman bands observed at some pressures is interpreted as the effect of the complex disordering in the H-bond network that has bifurcated H-bonds and ½-occupied H sites. There is no unambiguous evidence for phase transition to the ordered P21 monoclinic phase predicted by first-principles calculations at T=0K (W. Sun et al., Can. Miner., 2011). On the contrary, the disordering of parasibirskite, evidenced by the widening and attenuating Raman spectra, increases markedly at high pressures above 4.5GPa that results in incipient amorphization. Comparison of theoretical (lattice-dynamical) and experimental Raman spectra allows the reliable interpretation of almost all observed bands. The strongest symmetric B-O stretching band v1 at the wavenumber 908cm-1, which is split into a doublet at high pressures, exhibits a shift rate of 4.22cm-1/GPa for the main component.

Lattice dynamics of ß-V2O5: Raman spectroscopic insight into the atomistic structure of a high-pressure vanadium pentoxide polymorph.

We report here the Raman spectrum and lattice dynamics study of a well-crystallized ß-V(2)O(5) material prepared via a high-temperature/high-pressure (HT/HP) route, using α-V(2)O(5) as the precursor. Periodic quantum-chemical density functional theory calculations show good agreement with the experimental results and allow one to assign the observed spectral features to specific vibrational modes in the ß-V(2)O(5) polymorph. Key structure-spectrum relationships are extracted from comparative analysis of the vibrational states of the ß-V(2)O(5) and α-V(2)O(5) structures, and spectral patterns specific to the basic units of the two V(2)O(5) phases are proposed for the first time. Such results open the way for the use of Raman spectroscopy for the structural characterization of vanadium oxide-based host lattices of interest in the field of lithium batteries and help us to greatly understand the atomistic mechanism involved in the α-to-ß phase transition of vanadium pentoxide.