Hydrostaticity of pressure-transmitting media for high pressure infrared spectroscopy

Ensuring hydrostatic conditions in high pressure experiments is of utmost importance for obtaining reproducible and comparable dataset. Here, we investigate the degree of hydrostaticity of four pressure-transmitting media (PTM), three alkali halides and polyethylene, commonly used in infrared (IR) spectroscopy using multiple rubies as pressure sensors within a diamond anvil cell (DAC) pressure cavity. We demonstrate that the pressure standard deviation is always below 0.3 GPa at pressures below 10 GPa and that sodium chloride (NaCl) provides the best quasi-hydrostatic conditions up to 21 GPa. In the 10-20 GPa pressure range, NaCl, cesium iodide (CsI), and polyethylene perform significantly better than 4:1 ethanol-methanol or silicone oil - widely used PTM in high pressure experiments. These results will enable a more reliable and quantitative comparison between different experiments where the PTM investigated here are used.