Determination of the elastic properties at high pressure without pressure scale

Pressure and temperature are the key parameters for determining the physical properties of earth materials in the interior of the Earth. In many laboratory studies, the determination of pressure requires pressure-standards , for example, NaCl, Au, or Pt. Recent evidence from the phase boundaries of perovskite-forming reactions suggests that the equations of state for these materials may be in error by as much as 10%. Ultrasonic-base laboratory studies to determine elasticity require measurement of P and S-wave travel times, density and length of sample at the experimental conditions. Combining in-situ X-radiation and ultrasonic measurements, now it is possible to collect all of these parameters simultaneously and directly. This enables pressure scale-free measurements of the equation of state of the sample using a parameterization such as the Birch-Murnaghan equation of state. In addition, pressure at the experimental condition is defined by the sample X-ray volume. Therefore, this technique can be used to re-determine the equations of state for cohabitating pressure standards. We report elasticity data for ferropericlase (Mg,Fe)O with 17 mol% Fe, a potential mantle composition. The ultrasonic velocities have been measured at high pressure and the equation of state has been determined using the pressure-scale free method. Compared with MgO, the bulk modulus and its pressure derivative of (Mg,Fe)O are comparable. But the shear modulus and pressure derivative have been reduced. The pressure suggested by the sample is higher by more than 10% than that calculated from the coexisting NaCl using the Decker pressure scale.