Sound velocities in iron to 110 gigapascals

The dispersion of longitudinal acoustic phonons was measured by inelastic x -ray scattering in the hexagonal closed-packed (hcp) structure of iron from 19 to 110 gigapascals. Phonon dispersion curves were recorded on polycryst alline iron compressed in a diamond anvil cell, revealing an increase of th e Longitudinal wave velocity (V-p) from 7000 to 8800 meters per second. We show that hcp iron follows a Birch law for V-p, which is used to extrapolat e velocities to inner core conditions. Extrapolated Longitudinal acoustic w ave velocities compared with seismic data suggest an inner core that is 4 t o 5% Lighter than hcp iron.