Grain boundary effect on the compressibility of diamond and a possible transition in diamond grain boundary

Equation of state of diamond powder with different average grain sizes was investigated using in situ synchrotron x-ray diffraction. Comparison of compression curves was made for two samples with average grain size of 50nm and 100nm. Experiments were conducted at room temperature and high pressures up to 50 GPa. Fitting the compression data in the full pressure range into the third order Birch-Murnaghan equation of state yields bulk modulus (K) and its pressure derivative (K’) of 392 GPa and 5.3 for 50nm sample and 398GPa and 4.5 for 100nm sample respectively. Using a simplified core-shell grain model, this result indicates that the grain boundary has an effective bulk modulus of 54 GPa. This value is similar to that observed for carbon nanotube validating the recent theoretical diamond surface modeling. Differential analysis of the compression cures demonstrates clear relative compressibility change at the pressure about 20 GPa. When fit the compression data below and above this pressure separately, the effect of grain size on bulk modulus reverses in the pressure range above 20 GPa. This observation indicates a possible transition of grain boundary structure, likely from sp² hybridization at the surface towards sp³ like orbital structure which behaves alike the inner crystal.