Primary Pressure Scale of KCl B2 Phase to the Core‐Mantle Boundary

Abstract

AbstractReliable pressure determination is crucial for high pressure and temperature experiments and meaningful interpretation of their geophysical implications. However, nearly all commonly‐used pressure scales are secondary in nature, meaning their establishments rely on pre‐existing primary shock‐compression‐based pressure scales, which due to their dynamic compression nature, large uncertainty in peak shock temperature estimation and electronic thermal pressure contribution can yield substantial (∼5%) uncertainties at 1 Mbar conditions. To overcome this intrinsic shortcoming, in this study a self‐consistent primary pressure scale of KCl B2 phase was experimentally calibrated up to 85 GPa at ambient temperature using an approach through measuring the acoustic wave velocities and molar volume using Brillouin spectroscopy and Synchrotron X‐ray diffraction. Best fitting of thermoelastic parameters based on our experimental results yields V0 = 32.48 (9) cm3 mol−1, KT0 = 21.33 (70) GPa,  = 4.836 (83), G0 = 16.83 (237) GPa, G′ = 2.147 (115), γ0 = 1.92 (11) and θD0 = 251 (22) K. A KCl B2 phase primary pressure scale based on 3rd order Birch‐Murnaghan equation of state (EOS) is established without relying on any external (shock compressed‐based) pressure scales and further extended also to high temperatures in combination with thermal pressure effect calculated using Mie‒Grüneisen‒Debye model under quasi‐harmonic approximation. Our newly established KCl B2 EOS thus enables accurate pressure determinations at simultaneously high pressure and temperature conditions up to Earth's core‐mantle boundary and can serve as a benchmark for calibrating other secondary pressure scales.