Negative differential stresses in niobium: Analysis of x-ray measured pressure-volume data

The macro-stresses that develop in a sample under nonhydrostatic compression in a diamond anvil cell (DAC) are axially symmetric about the load axis, the axial stress component being larger than the radial stress component. The difference between the two, termed differential stress t, is a positive quantity. The lattice strain theory (LST) suggests that the measured unit cell volume using conventional x-ray diffraction geometry of the DAC is overestimated in the presence of a positive t. In an earlier study on niobium, one set of x-ray measured compression data was found to yield negative t. It is suggested that the inward collapse of the gasket leads to negative t. It is shown that the volume-effect produced by negative t is also in accordance with the existing LST.