Measurement and analysis of nonhydrostatic lattice strain component in niobium to 145 GPa under various fluid pressure-transmitting media

The d spacings in niobium have been measured to 145 GPa with a diamond anvi l cell using a fluid pressure-transmitting medium [methanol-ethanol-water ( MEW) mixture, or helium]. The conventional geometry, wherein the primary x- ray beam passes parallel to the load axis with image plate, has been used t o record the diffraction patterns. The analysis of the d spacings using the lattice strain equations indicates the presence of nonhydrostatic stress c omponent (with both MEW and He pressure-transmitting media) in the pressure ranges that are well below the freezing pressure of the pressure-transmitt ing medium. A method to correct the measured d spacings for the nonhydrosta tic pressure effect is suggested. This study clearly emphasizes the need to carefully analyze the data for the nonhydrostatic compression effects even if the experiments are performed with fluid pressure-transmitting medium. (C) 2001 American Institute of Physics.