Thermal Conductivity of Complex Materials at High Temperatures and Pressures

Measurements of lattice thermal diffusivity have been obtained on (Mg_0.9Fe_0.1)₂SiO₄ olivine, its high-pressure polymorphs, and of (Mg_0.9Fe_0.1)SiO₃ enstatite at conditions of temperature and pressure. We used two independent techniques: Impulsive Stimulated Light Scattering to 5 GPa and 1273 K and the Ångström method on cylindrical samples in a multianvil apparatus to 20 GPa and 1373 K. Lattice thermal conductivities were calculated from heat capacities and equations of state. Conductivities are consistent with previous results obtained at 1 atm. In olivines conductivity increases at each phase transition from olivine to spinel structures (corresponding to velocity discontinuities at depths of 410 and 520 km in the Earth). Of greatest significance is the fact that for each of these phases, lattice conductivity closely follows a T^-1/2 dependence on temperature T, a prediction of Klemens (1960) for materials containing abundant lattice defects. If such a dependence applies to other silicates and complex crystals, there should be a useful way to estimate conductivities at high temperatures from room temperature measurements.