Abstract:

The alkali halides are often used as optically-transparent pressure-transmitting media and thermal insulators in laser-heated diamond anvil cell experiments. High P–T equations of state for these materials would allow them to be used simultaneously as sensitive in situ pressure markers, making sample preparation and data analysis simpler. KCl is especially useful for this application because its high melting point, crystallographic simplicity, and low electron density are ideal for use in high-temperature X-ray diffraction experiments. However, the high-temperature static equation of state data for this material is limited in pressure to 8 GPa. As experiments routinely exceed 100 GPa in pressure and thousands of Kelvin, it is necessary for the equation of state of KCl to be determined experimentally at these conditions if it is to be used as an equation of state calibrant in the future. This work combines new high-pressure, high-temperature data of the B2 phase of KCl (up to 167 GPa and 2400 K) with the previously published room-temperature data of Dewaele et al. (2012) to produce a Mie-Grüneisen-Debye thermal equation of state for this material. The room temperature equation of state parameters are similar to those reported previously: V₀ = 32.0(3) cm³/mole, K₀ = 24(1) GPa and K₀' = 4.56(5). The thermal parameters, ${\gamma }^{}$₀ and q, are 2.9(4) and 1.0(1), respectively. While a $q$ of 1 is expected, the ${\gamma }^{}$₀ is higher than expected from previous calculations, lower pressure experimental data, and the available shockwave data. Thus, previously-reported equations of state underestimate the pressure of KCl at high temperatures.