Raman Spectroscopy of Iron to 152 Gigapascals: Implications for Earth's Inner Core

Abstract

Raman spectra of hexagonal close-packed iron ( ɛ -Fe) have been measured from 15 to 152 gigapascals by using diamond-anvil cells with ultrapure synthetic diamond anvils. The results give a Grüneisen parameter γ 0 = 1.68 (±0.20) and q = 0.7 (±0.5). Phenomenological modeling shows that the Raman-active mode can be approximately correlated with an acoustic phonon and thus provides direct information about the high-pressure elastic properties of iron, which have been controversial. In particular, the C 44 elastic modulus is found to be lower than previous determinations. This leads to changes of about 35% at core pressures for shear wave anisotropies.