A review of methods to determine the pressure-volume-temperature equation of state and refractive index of disordered materials in the diamond anvil cell

The pressure-volume-temperature (PVT) equation of state (EOS) of crystalline solids can be measured routinely in the diamond anvil high pressure cell (DAC) using synchrotron X-ray diffraction combined with laser heating or resistive heating to access high temperatures. In contrast, measuring the PVT EOS of disordered materials (fluids and amorphous solids) in the DAC is experimentally challenging and EOS determination by extrapolation of low pressure data is risky. Here we review the different approaches that have been utilised to experimentally measure the PVT EOS of disordered materials in the DAC, before focussing on what we believe is the most promising approach: Measuring the volume change of the DAC sample chamber upon compression. This can be achieved by probing the sample with X-rays or visible light. The use of X-rays is arguably the most accurate approach possible, but is only available at a handful of synchrotron light sources around the world. The use of visible light requires a determination of the sample refractive index, however this is an important scientific problem in it's own right as the refractive index can provide much additional information about the sample. We expound, using real data, aspects of these problems that are not widely discussed in the scientific literature. To conclude, we expand the discussion to include absorbing materials and the difficulties involved in measurement of the imaginary part of the refractive index in these materials.