First-principles study of the lattice thermal conductivity of MgSiO<sub>3</sub> akimotoite in the mantle transition zone

The lattice thermal conductivity (<inline-formula><span class="inline-formula-span">\kappa _\rmlatt</span><img text_id='M1' class='formula-img' style='display:none;' src='RA566-zhangli-F_M1.jpg'><alternatives></alternatives></inline-formula>) of mantle minerals plays a crucial role in the heat flow and temperature distribution within the Earth. MgSiO₃ akimotoite is stable at the bottom of the mantle transition zone; it transitions to MgSiO₃ perovskite (MgPv). In this work, we carry out a study of the <inline-formula><span class="inline-formula-span">\kappa _\rmlatt</span><img text_id='M1' class='formula-img' style='display:none;' src='RA566-zhangli-F_M1.jpg'><alternatives></alternatives></inline-formula> of MgSiO₃ akimotoite for pressures up to 25 GPa and temperatures up to 2500 K, based on first-principles calculations combined with lattice dynamics theory. At 300 K and 25 GPa, the <inline-formula><span class="inline-formula-span">\kappa _\rmlatt</span><img text_id='M1' class='formula-img' style='display:none;' src='RA566-zhangli-F_M1.jpg'><alternatives></alternatives></inline-formula> of MgSiO₃ akimotoite is 37.66 W m^–1 K^–1, larger than that of MgPv (13.46 W m^–1 K^–1), which implies that the phase transition explains the reduction in <inline-formula><span class="inline-formula-span">\kappa _\rmlatt</span><img text_id='M1' class='formula-img' style='display:none;' src='RA566-zhangli-F_M1.jpg'><alternatives></alternatives></inline-formula>. At 300 K, the pressure dependence of <inline-formula><span class="inline-formula-span">\kappa _\rmlatt</span><img text_id='M1' class='formula-img' style='display:none;' src='RA566-zhangli-F_M1.jpg'><alternatives></alternatives></inline-formula> is 0.68 W m^–1 K^–1 GPa^-1, stronger than that of MgPv (0.48 W m^–1 K^–1 GPa^-1). The azimuthal anisotropy in <inline-formula><span class="inline-formula-span">\kappa _\rmlatt</span><img text_id='M1' class='formula-img' style='display:none;' src='RA566-zhangli-F_M1.jpg'><alternatives></alternatives></inline-formula>of MgSiO₃ akimotoite decreases from 45.5% at 0 GPa to 28.94% at 25 GPa, while the variation trend is opposite to that of MgPv. In MgSiO₃ akimotoite, Fe incorporating in the mineral leads to a decrease in <inline-formula><span class="inline-formula-span">\kappa _\rmlatt</span><img text_id='M1' class='formula-img' style='display:none;' src='RA566-zhangli-F_M1.jpg'><alternatives></alternatives></inline-formula> and an increase in azimuthal anisotropy. Along the geotherm, the <inline-formula><span class="inline-formula-span">\kappa _\rmlatt</span><img text_id='M1' class='formula-img' style='display:none;' src='RA566-zhangli-F_M1.jpg'><alternatives></alternatives></inline-formula> of MgSiO₃ akimotoite is lower than that of ringwoodite, which would suggest that MgSiO₃ akimotoite slows down heat conduction at the bottom of mantle transition zone. These findings are useful for determining the thermal structure of, and understanding heat transfer in, the interior of the Earth.