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

Description

Reliable 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 standards, 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 a novel approach through simultaneously measuring the acoustic wave velocities and molar volume using Brillouin spectroscopy and Synchrotron X-ray diffraction (XRD). Best fitting of thermoelastic parameters based on our experimental results yields V_0 = 32.48 (9) cm3/mole, K_T0 = 21.33 (70) GPa, K_0'= 4.836 (83), G_0 = 16.83 (237) GPa, G^' = 2.147 (115), and γ_0 = 1.92 (11), θ_D0 = 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 (CMB) and can serve as a benchmark for calibrating other secondary pressure scales.

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