High-pressure X-ray diffraction study on the structure of NaCl melt using synchrotron radiation

Molten NaCl was analyzed by high-pressure X-ray diffraction experiments using synchrotron radiation up to 5 GPa and 1600 °C along the melting curve. The interference function Qi(Q) and the correlation function g(r) were derived from the diffraction data. The first-neighbor distance r₁ is about 2.7 Å and the second-neighbor distance appears around 1.4r₁-1.5r₁. The coordination number, CN, of the nearest neighbor ions increases with pressure from 3.5 at 0.1 MPa to 4.5 at 5 GPa. This is the evidence that the NaCl melt has a B1-like structure with large vacancies over this pressure range and becomes densified by an increase in CN as a result of second neighbor compaction.