The effect of crystallite size and stress condition on the equation of state of nanocrystalline MgO

We performed high-pressure synchrotron x-ray diffraction experiments on nanocrystalline (nc-) MgO compressed both under quasi-hydrostatic and non-hydrostatic conditions in a diamond-anvil cell. Data obtained under hydrostatic conditions show that nc-MgO (average crystallite size of 20nm) is 8-9% more compressible than "bulk" MgO. Analysis of our results collected under non-hydrostatic conditions yields a bulk modulus that is about 27% larger than the one derived from the quasi-hydrostatic compression experiments. Thus, the apparent bulk modulus strongly depends on the experimental stress conditions.