Spectroscopy at very high pressures. X. Use of ruby R-lines in the estimation of pressure at ambient and at low temperatures

The effect of pressure at low temperature (166.8K) upon the R1 and R2 lines of ruby has been investigated for the first time to establish a calibration for use in high-pressure/low-temperature work. A reinvestigation of the behaviour of R1 and R2 at ambient temperatures and at pressures up to ca 70 kbar supports previous work on the calibration under hydrostatic conditions but shows that in a 4:1 methanol:ethanol mixture conditions become nonhydrostatic at lower pressures (ca 50 kbar) than claimed (ca 100 kbar). The importance of monitoring both R1 and R2 is emphasised as their relative shifts are sensitive indicators of the presence, and approximate magnitude, of non-hydrostatic stress components. It is shown that although changes in half-bandwidths are also associated with development of shear stress, their use in estimating gradients is extremely hazardous and can indicate values greatly in excess of the known shear strength of the material.