COMPRESSIBILITY OF SIC UP TO 68.4-GPA

A diamond anvil cell was used with synchrotron radiation to measure the compressibility of the 6H polytype of SiC (moissanite) up to 68.4 GPa. NaCl was used as the pressure medium, and Au was used as the pressure calibrant. The effect of pressure gradient was minimized by using a very small x-ray beam (0.015 mm diam), and uniaxial stress was minimized by using three times as much volume of NaCl as SiC. The 101 and 102 peaks of SiC were measured as a function of pressure, and the molar volumes based on them were in excellent agreement. Pressure was calculated from the 111, 200, and 220 peaks of Au. The data when fit to the Birch-Murnaghan equation yield a bulk modulus of K0=230.2+/-4.0 GPa if the K0' is set at 4. These values are in good agreement with earlier unpublished measurements (K0=234.3+/-4.9 GPa; K0'=4) made by us in which NaCl was used as the pressure calibrant. Our compression measurements appear to indicate a stiffer behavior than those based on shock measurements.