X-ray diffraction measurements of cerium in the vicinity of the isostructural gamma-alpha transition have been performed with high precision and accuracy from room temperature to almost 800 K. The disputed location of the critical point has been found to occur at 1.5+/-0.1 GPa and 480+/-10 K. The data are well fit by the Kondo volume collapse model plus a quasiharmonic representation of the phonons. The resultant free energy is validated against data for the thermodynamic Grüneisen parameter and, beyond the dominant spin-fluctuation contribution, indicates a dramatic change in the lattice Grüneisen parameter across the transition.

Seismic discontinuities in Earth typically arise from structural, chemical, or temperature variations with increasing depth. The pressure-induced iron spin state transition in the lower mantle may influence seismic wave velocities by changing the elasticity of iron-bearing minerals, but no seismological evidence of an anomaly exists. Inelastic x-ray scattering measurements on (Mg(0.83)Fe(0.17))O-ferropericlase at pressures across the spin transition show effects limited to the only shear moduli of the elastic tensor. This explains the absence of deviation in the aggregate seismic velocities and, thus, the lack of a one-dimensional seismic signature of the spin crossover. The spin state transition does, however, influence shear anisotropy of ferropericlase and should contribute to the seismic shear wave anisotropy of the lower mantle.

We have measured the isothermal equation of state (EOS) of osmium to 75 GPa under hydrostatic conditions at room temperature using angle-dispersive x-ray diffraction. A least-squares fit of this data using a third-order Birch-Murnaghan EOS yields an isothermal bulk modulus K0=411+/-6 GPa, showing osmium is more compressible than diamond. Most importantly, we have documented an anomaly in the compressibility around 25 GPa associated with a discontinuity in the first pressure derivative of the c/a ratio. This discontinuity plausibly arises from the collapse of the small hole-ellipsoid in the Fermi surface near the L point.

The five independent elastic moduli of single-crystalline hcp cobalt were determined by inelastic x-ray scattering to 39 GPa and compared to ultrasonic measurements and first principles calculations. In general the agreement is good, in particular, for the evolution of the longitudinal sound velocity in the a-c plane. This confirms the calculations, suggesting that a similar evolution is valid for hcp iron, the main constituent of the Earth's inner core, up to the highest investigated pressure. Our results represent an important benchmark to further refine ab initio calculations.