Probing NaCl at High Pressure through Optical Studies and Ab Initio Calculations

The optical constants of sodium chloride in a wide pressure range were determined from the analysis of the reflectance and transmittance spectra of a minute quantity of NaCl powder placed in diamond anvil cells. The so-called "reststrahlen band" dominates the far-infrared reflectance spectra shifting from 150 cm(-1) to 500 cm(-1) at 100 GPa. For the 0-17.5 GPa pressure range, measurements allow accurate determination of both transverse and longitudinal mode frequencies. Higher pressure measurements reveal the B1 -> B2 structural transition around 30 GPa and provide frequencies for the transverse and longitudinal modes. This spectroscopic signature on a sample smaller than 100 mu m using light of a wavelength close to this dimension was observed, thanks to the high brilliance synchrotron source. In addition, ab initio calculations performed for the 0-200 GPa range predict the TO and LO They are validated by the excellent agreement with the experiment. frequencies.