Prior studies showed that the LS equation reasonably describes the thermal conductivity of H 2O ice VII 3 and MgO 16 within a volume compression range of ~ 33% and ~ 25%, respectively, at room temperature. The LS equation was formulated for pure, isotropic dielectric crystals where the anharmonic three phonon scattering between acoustic modes was assumed to be the predominant scattering mechanism for thermal transport 14, 15. For instance, Leibfried-Schlömann (LS) equation is a simple physical model commonly used to predict the P–T dependence of the thermal conductivity of a material. In addition to the useful applications to high-pressure experiments, the evolution of NaCl’s thermal conductivity and sound velocity with extreme conditions serves as excellent platforms to test the validity of a theory for thermal transport and lattice dynamics, respectively, shedding lights to their fundamental physical mechanisms. It is in a face-centered cubic structure (six-coordinated, B1 phase) at ambient conditions and transforms to a body-centered cubic structure (eight-coordinated, CsCl-B2 phase) upon compression to ~ 27–30 GPa 12, 13 at room temperature. NaCl is a simple, prototypical ionic crystal. Practically, in high P–T DAC experiments, NaCl is a commonly used pressure medium and thermal insulation layer therefore, knowledge of the thermal conductivity of NaCl at high P–T conditions is critically needed in order to accurately model the heat transfer and temperature distribution within the DAC. In these measurements, the thermal conductivity of a sample of interest within the DAC is typically derived by comparing experimental data related to the heat diffusion rate through the sample with numerical calculations by a thermal model where the thermal conductivity of the pressure transmitting medium is one of the key parameters. Recent advances in the combination of time-resolved optical techniques with a diamond-anvil cell (DAC) 8, 10, 11 have enabled better determination of materials’ thermal conductivity under extreme conditions. Precise measurements of materials’ thermal conductivity under extreme P–T conditions, however, have long been challenging, primarily due to the difficulties in the experimental techniques, such as an accurate characterization of the temperature profile within a small high-pressure chamber. Understanding how the extreme environments, such as pressure ( P) and temperature ( T), influence the thermal conductivity of a material is of fundamental importance for condensed matter physics 1, 2, 3, 4 and geosciences 5, 6, 7, 8, 9. Our findings offer critical insights into the dominant physical mechanism of phonon transport in NaCl, as well as important data that significantly enhance the accuracy of modeling the spatiotemporal evolution of temperature within an NaCl-loaded DAC. The compressional velocities of NaCl-B1 and B2 phase both scale approximately linearly with density, indicating the applicability of Birch’s law to NaCl within the density range we study. The high P–T thermal conductivity of NaCl enables us to confirm the validity of Leibfried-Schlömann equation, a commonly used model for the P–T dependence of thermal conductivity, over a large compression range (~ 35% volume compression in NaCl-B1 phase, followed by ~ 20% compression in the polymorphic B2 phase).
Using an externally-heated DAC, we further show that thermal conductivity of NaCl-B1 phase follows a typical T −1 dependence. Here we couple ultrafast optical pump-probe methods with the DAC to study thermal conductivity and compressional velocity of NaCl in B1 and B2 phase to 66 GPa at room temperature. Its thermal conductivity at high pressure–temperature ( P–T) conditions is a critical parameter to model heat conduction and temperature distribution within an NaCl-loaded DAC.
Sodium chloride (NaCl) is an important, commonly used pressure medium and pressure calibrant in diamond-anvil cell (DAC) experiments.