Strength of magnesium oxide under high pressure: evidence for the grain-size dependence

X-ray diffraction patterns from magnesium oxide compressed in a diamond anvil cell up to 55 GPa have been recorded and the differential stress (a measure of compressive strength) and grain-size (crystallite size) determined as a function of pressure from the line-width analysis. The strength agrees well with the uniaxial stress component (another measure of compressive strength) derived earlier from the line-shift data. The strength increases while the crystallite size decreases steeply as the pressure is raised from ambient to ∼10 GPa. The increase in strength is much smaller at higher pressures. The strength-pressure data are explained by combining the grain-size dependence of strength and the shear-modulus scaling law. The dependence of strength on grain-size has not been considered in the past in the discussion of high-pressure strength data.