Sound Velocities in Iron to 110 Gigapascals

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Science  19 Jan 2001:
Vol. 291, Issue 5503, pp. 468-471
DOI: 10.1126/science.291.5503.468

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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 polycrystalline iron compressed in a diamond anvil cell, revealing an increase of the 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 extrapolate velocities to inner core conditions. Extrapolated longitudinal acoustic wave velocities compared with seismic data suggest an inner core that is 4 to 5% lighter than hcp iron.

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