High-pressure high-temperature behavior of iron silicide (Fe5Si3) to 58 GPa and 2400K

Abstract:

Silicon is an important candidate for the light element in the outer core. Here we present new measurements of the equation-of-state, thermal expansion, melting temperature, and thermal conductivity of iron silicide (Fe₅Si₃) at high pressures and temperatures. We performed a series of X-ray diffraction experiments in the laser-heated diamond anvil cell on Fe₅Si₃ at ALS beamline 12.2.2. Diffraction patterns and temperature-versus-laser power curves were measured in situ at pressures up to 58 GPa and temperatures up to 2300 K. In one set of experiments both NaCl and Ne were used as the pressure transmitting, thermal insulator and pressure calibrant. In a second set of experiments, only NaCl was present and served those three purposes. The measurements yield a new thermoelastic equation of state for Fe₅Si₃, including bulk modulus, high-pressure thermal expansion, and the Grüneisen parameter. In addition, we have determined a lower bound on the melting behavior up to 58 GPa. This information helps constrain compositionally-sensitive models describing the density, compressibility, and dynamics of Earth’s core. The temperature-versus-laser power measurements provide information about the heat flow environment in the diamond anvil cell. A comparison of the temperature-versus-laser power measurements for pure iron and Fe₅Si₃ yields a measure of how the presence of Si influences the thermal conductivity of iron at high pressures and temperatures. Our measurements also show a jump in thermal conductivity of NaCl across the B1- B2 phase transition. This information is important for interpreting thermal conductivity values in the present work and also has broader implications for experimental design and data interpretation in laser-heated diamond anvil cell experiments.