How Accurate are High-Pressure Experiments?

A simple polynomial fit to data which is sublinear with pressure gives coefficients which vary strongly with the pressure range of the fit. Converting pressure to a variable which is linear with that measured enables direct comparison of experiments over different pressure ranges. We illustrate this for high-pressure band-gap data in GaAs. Experimental errors such as non-hydrostaticity can occur and be difficult to detect. Comparison of two spectroscopies provides a simple diagnostic. We find a linear shift of the band-gap with the Raman shift under pressure for ZnTe and propose this as a check of pressure conditions. To measure the Raman shift accurately under pressure, resonant Raman is a useful tool, but requires a tunable source. We have developed a tunable, broadband filter for high-pressure work, which rejects Rayleigh scattered light, allowing measurement of Raman shifts as small as 10cm-1.