A critical evaluation of pressure scales at high temperatures by in situ X-ray diffraction measurements

We conducted multi-anvil experiments at simultaneous high pressures and temperatures using multiple internal pressure standards including Au, Pt, MgO, W, Mo, Pd, and Ag. Extensive synchrotron X-ray diffraction data for Au, Pt, and MgO were collected at pressures up to 28 GPa and temperatures between 300 and 2173 K. We compare pressures calculated from different pressure scales and demonstrate large discrepancies in pressure determination using different pressure standards or different thermal equations of state for the same standard. The comparison allows us to quantitatively determine the differences in pressure using different pressure scales in the high P–T experiments. Using the MgO scale of [J. Geophys. Res. 106 (2001) 515] as a reference pressure scale, new Au and Pt scales are presented that are consistent with the MgO scale. We further examined the validity of the assumption of constant q value (volume dependence of the Grüneisen parameter in the Mie–Grüneisen relation) for the calculations of thermal pressures, and show that an expression of q as a function of temperature and pressure may be necessary to best fit the simultaneous high P–T data.