1. Motivation: More than ninety percent of the Earth’s core by volume is in a liquid state. It is important to study the behavior of liquid iron at high pressures and temperatures for understanding the origin of numerous macroscopic physical properties of the core. The direct experimental study of the structure and density of liquid iron at core conditions is still challenging. We take a different approach by studying the behavior of an amorphous iron at high pressures and at ambient temperature. GeoSoilEnviroCARS The University of Chicago Density Measurements of an Amorphous Iron at High Pressures Guoyin Shen 1, N. Sata 1, V. Prakapenka 1, M. L. Rivers 1, S. R. Sutton 1, J. Oxley 2, K. S. Suslick 2 1) Consortium for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637, USA 2) School of Chemical Sciences, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Sample: The amorphous iron used in this study were prepared from the ultrasonic irradiation of Fe(CO) 5 [1]. Elemental analysis shows it to be >96% iron by weight, with trace amounts of carbon (3%) and oxygen (1%). How do we measure density for amorphous materials? Absorption! I = I 0 exp(-  t) where I the transmitted intensity; I 0 the incident beam intensity;  the absorption coefficient;  the density; t the thickness of a material. The sample configuration is shown in Fig. 1. The issue of absorption coefficient: They are measurable! The issue of thickness: t = ln(I NaCl /I Mo )/(  Mo  Mo -  NaCl  NaCl ) At high pressures, diamond anvils deform, resulting in thickness variations between two anvils. A thickness profile at a certain pressure can be obtained by measuring an intensity profile and a density profile (Fig. 2). For example, the measured thickness variation (blue line) can be more than 2  m at a pressure of ~20 GPa. Fig. 2 Molar volumes: Fig. 3. Molar volumes of the amorphous iron up to 22 GPa together with those of bcc-Fe [3] and hcp-Fe [4]. Errors are estimated from uncertainties in intensity only. The error contribution from crystalline diffraction is small and neglected. The accuracy should be bound to the accuracy of absorption coefficients. Notes: Unable to constrain K’ due to the limited pressure range and large errors. By fixing K’ to be 15, K is 46(2) GPa with V 0 =7.82 cm3/mol (3 rd order BM- EOS). Caution - Extrapolation to core pressures leads to large uncertainties. More measurements at higher pressures are in progress. Structure factors: (data still in process) Fig. 4. Amorphous diffraction at ambient pressure. Fig. 5. Amorphous diffraction at high pressures. References: [1] Suslick et al. Nature, 353, 414 (1991). [2] Shen, et al, Appl. Phys. Lett., September issue (2002). [3] Mao et al. J. Appl. Phys. 38, 272 (1967). [4] Jephcoat et al. JGR, 91, 4677 (1986); Mao et al. JGR, 95, 21737 (1990). Acknowledgment: This work is supported by NSF grant EAR-0001149. Sample configuration: Fe NaCl Mo Fe NaCl Mo (a) (b) Fig. 1. The sample configuration in a diamond anvil cell. (a) X-ray transmission image measured in a two dimensional scan (E=21.996 keV). (b) Two holes with 100  m diameter were drilled at the positions equidistant from the center. The anvil culet size is 500  m. The second hole was loaded with NaCl that serves as a thickness and absorption calibrant [2].