Nuclear Inelastic X-Ray Scattering of FeO to 48 GPa

The partial density of vibrational states has been measured for Fe in compressed FeO (w\"ustite) using nuclear resonant inelastic x-ray scattering. Substantial changes have been observed in the overall shape of the density of states close to the magnetic transiton around 20 GPa from the paramagnetic (low pressure) to the antiferromagnetic (high pressure) state. Our data indicate a substantial softening of the aggregate sound velocities far below the transition, starting between 5 and 10 GPa. This is consistent with recent radial x-ray diffraction measurements of the elastic constants in FeO. The results indicate that strong magnetoelastic coupling in FeO is the driving force behind the changes in the phonon spectrum of FeO.Comment: 4 pages, 4 figure

Phonons and related properties of extended systems from density-functional perturbation theory

This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudo-potential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long wave-length vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.Comment: 52 pages, 14 figures, submitted to Review of Modern Physic

The pressure dependence of the Ta

The pressure dependence of the $\rm TA_1[110]$ phonon frequencies in ordered $\rm Fe_3Pt$ was studied by inelastic neutron scattering up to 7 GPa. Very large frequency shifts ($\sim 50\%$) were observed in this pressure range. We found that the mode Grüneisen parameters remain very large ($\sim 8$) for pressures exceeding the critical pressure of about 3 GPa, which drives ordered $\rm Fe_3Pt$ from a high-spin to a low-spin state. This indicates that the unusually large Grüneisen parameters are probably not a magnetoelastic effect, but rather have to be associated with the martensitic instability